Recent advances in 1,10-phenanthroline ligands for chemosensing of cations and anions

Abstract

This review encompasses and highlights recent developments of 1,10-phenanthroline (phen) ligands behaving as a customized moiety for assisting in recognition and sensing of cations and anions. Derivatization at various positions of phen (5,6-, 2,9-, 3,8- and 4,7-) results in ligands capable of acting as chemosensors for cations and anions. The perturbation induced in the energy levels of highly conjugated phen on substitution leads to detectable and measurable changes in the photophysical (UV-vis and fluorescence) properties of the system that serve as a tool for sensing cations and anions relevant to both environmental and biological systems. Hence phen yields a library of derivatized compounds which using the principles of coordination chemistry function as sensors for specific analytes (cations and anions). A database of various phenanthroline-derived receptors serving as chemosensors for various analytes in different solvents is presented and correlation of results can be used in the future for tuning photophysical properties, either by changing the substituted groups or their positions, so that the nearest desirable properties can be achieved for applications in biological and environmental sciences.

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1. Introduction

Ligand preorganization plays an important role in ligand design for biomedical, biological, environmental and sensing applications.¹ Due to high levels of ligand preorganization in macrocyclic ligands such as in crown ethers and cryptands, their metal complexes have a higher thermodynamic stability in addition to a higher metal-ion selectivity.^2–4 However, their lengthier syntheses and greater expense make them commercially less usable. Preorganized non-macrocyclic ligands overcome these problems enhancing their applications in cation/anion recognition. Amongst various types of preorganized non-macrocyclic ligands, phen encompasses a number of appealing structural and chemical properties. Its two aromatic nitrogen atoms act cooperatively in guest binding making it an entropically better chelating molecule than other preorganized non-macrocyclic ligands.^5,6 The distance of 2.5 Å between the nitrogen atoms and a dipole moment of 3.64 D enable it to act as a bidentate ligand to form five membered chelate rings with either a metal or hydrogen ion. The relative inertness of phen towards chemical reaction other than salt formation and chelation is a significant asset in its analytical applications. Longuet–Higgens and Coulsen established a correlation between anionoid and cationoid reactions and evaluated the electron distributions in the phen molecule.⁷ Many phen derivatives have been tailored to yield fascinating molecular architectures such as catenanes, rotaxanes and knots.⁸ Moreover, the planar structure of phen prompts intercalation or groove binding with DNA or RNA.^9,10 They have also found many applications for making optoelectronic devices, herbicides, analytical probes and pharmaceuticals.⁵

The most active positions of phen towards nucleophilic reagents are the 2,9 and 4,7-positions; while, electrophilic reagents preferentially attack positions 5,6 or 3,8 where electron densities are higher.⁷ Therefore, based on the substitution pattern of various ring positions in the phen nucleus, a wide range of tailor made preorganized ligands can be designed. Though, the parent phen absorbs in the ultraviolet spectral region of 200–320 nm and displays a short emission wavelength (λ_em ≈ 360 nm) with a low fluorescence quantum yield (Φ_F ≤ 0.01),^5,11,12 it still behaves as an ideal candidate for cation/anion recognition as the photophysical properties of phen can be well tuned according to the substitution pattern of various functional groups. 1,10-Phenanthroline and its derivatives possess two very close-lying molecular orbitals, a₂(χ) and b₁(ψ) that can accommodate excess negative charge which is either added from outside or created by excitation from high lying occupied metal based orbitals. HMO perturbation calculations reveal the trend for preference for the a₂(χ) over the b₁(ψ) π* orbital as lowest unoccupied MO in the sequence of 5,6-dap > 2,9-dap > 4,7-dap > phen > 3,8-dap (Fig. 1).¹³

Fig. 1 Two very close-lying molecular orbitals, a₂(χ) and b₁(ψ) of 1,10-phenanthroline and its derivatives.¹³

The substitution pattern effects the distribution of the electronic levels and the introduction of appropriate substituents can even increase the energy gap between the otherwise closely spaced π–π* (strongly emissive, radiative decay pathway) and n–π* states (poorly emissive, non radiative decay pathway) responsible for weak emission of phen ligand.^5,6 Consequently there is a marked difference in the combinatorial properties (photophysical, theoretical, electrochemical and ¹H-NMR studies) of the differently substituted phen derivatives. The non-dependence of photophysical properties of phen on solvent polarity add to its advantage as an analytical reagent.¹⁴ Moreover, phenanthrolines owing to their low lying π* antibonding orbitals can readily complex with the reducing metal cations hence generating low energy metal-to-ligand-charge-transfer electronic states (MLCT).^5,15

The previous reviews on phen have either discussed the effect of protonation on phen and the study of structural properties of coordination compounds with transition metal ions⁵ or the broad overview of both synthesis and properties of phen ligand and limited discussion on its chemosensing properties towards cations/anions.¹⁵ Two other reviews have described recognition studies of only metal ions with various polypyridine possessing ligands, amongst which one is phen.^1,16 However, this review is an extensive report on a detailed study and the mechanism of chemosensing properties of phen towards cations and anions depending upon its variable substitution patterns.

2. Chemosensors based on metal complexes of 1,10-phenanthroline ligands

The free nitrogens of phen in imidazole derivatives have further been used for the synthesis of various metal complexes such as Ru(II), Re(I), Eu(III) and Ir(III). These luminescent metal complexes of phen are unique in terms of chemical stability, are good visible light absorbers, exhibit relatively intense and long lived luminescence and have reversible redox behaviour both in the ground and excited states.

2.1 Chemosensors based on Ru(II) complexes of 1,10-phenanthroline ligands

Ru(II) polypyridyl complexes possess intense polarised luminescence, large Stokes shifts, high chemical and photostability, low-energy absorption and relatively long lifetimes^17–19 making them promising candidates for construction of luminescent sensors and photoactive molecular-scale devices.²⁰ Moreover, Ru(phen)₃²⁺ intercalates into the grooves of double strand DNA (dsDNA) with high affinity making it a good electrochemiluminescence (ECL) reagent.^21,22

2.1.1 Cations. Schmittel and co-workers²³ synthesized different Ru(II) phen complexes possessing variable azacrown ether units attached via 4,7-positions of the phen (1a–1d) for their use as molecular sensors for ECL, luminescence and redox measurements. The ruthenium trisphen complexes (1b–1d) entailed large shifts in the redox potential (up to 370 mV) and in the emission spectra (up to 87 nm) upon addition of Ba²⁺ ions (Table 1).

Table 1 The redox potential and emission changes in complexes 1 upon Ba²⁺ complexation

Complex	ΔEp with Ba^2+	λmax (em)/nm	λmax (em)/nm with Ba^2+
1a	—	600	600
1b	170 mV	672	627
1c	270 mV	730	643
1d	370 mV	682	642

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The emission intensity increased for all the complexes (maximum for 1d being nearly 3 fold), because the lone pair of the nitrogen of the aza crown moiety reduced the reductive electron transfer quenching of the excited ruthenium state resulting in emission enhancement. An anomaly observed in these crowded azacrown ether ruthenium complexes suggested the formation of two different species in 1c and 1d, one of which may have a sandwich structure. Complex 1b gave detection of different metal ions with distinct detection methods i.e. UV-vis, redox, photoluminescence (PL) and ECL (Scheme 1).²⁴

Scheme 1 Detection of different metal ions by 1b with distinct detection methods.

Cyclopentadithiophene (CPDT) appended phen Ru(II) complexes (2a and 2b) were electropolymerized on different electrodes [Pt and ITO (indium tin oxide)] to generate stable thin films and thus, detect different metal ions (Scheme 2).^25,26 The electropolymerization of 2a did not succeed despite applying a variety of conditions, different substrates and various monomer concentrations due to the large steric hindrance exerted by the four bulky azacrown ethers at each phen ligand. Less steric hindrance of azacrown ether units and double electropolymerisable CPDT units present in 2b as compared to 2a made possible its direct electropolymerisation on the Pt electrode. The polymer of 2b film on Pt showed the most pronounced changes with Pb²⁺ (∼160 mV) and Ba²⁺ (∼140 mV).

Scheme 2 Detection of different metal ions by 2b with distinct detection methods.

Invoking the principle of amide side chain(s) after anion binding for PL change and thus sensing, Schmittel and co-workers synthesized a single molecular dual analyte sensor (3) based on the bis heteroleptic Ru(II) complex.²⁷ Complex 3 was non-emissive due to its structural flexibility at the amide benzothiazole unit in 3 leading to increased non-radiative decay from the MLCT excited state. Addition of Hg²⁺ ions resulted in a 24 fold increase of the PL band present at 648 nm accompanied by its 12 nm red shift [limit of detection (LOD) = 118 nm]. On the other hand, Ag⁺ ions addition blue shifted the 648 nm to 630 nm with a simultaneous increase in its PL intensity by 33 times resulting in bright orange red luminescence (LOD = 112 nm) (Fig. 2). Both Hg²⁺ and Ag⁺ being highly thiophilic in nature preferred S linkage over N and O atoms for complexation.

Fig. 2 PL spectra of 3 (in figure mentioned as 1) upon addition of various other metal ions (30 equiv.) in 0.1 M HEPES buffer/CH₃CN (v/v 1 : 1, pH 7.4).²⁷

Selective Cu²⁺-binding induced luminescence quenching stemming from the paramagnetic nature of Cu²⁺ has been observed in receptors 4 and 5 based on the Ru(II) complex.^28–30 On adding Cu²⁺ ions to a solution of 4, the luminescence signal at 585 nm decreased by 94.5% due to its binding to phenol O and imidazole N atoms. The luminescence of 5 at 620 nm (pH = 7) was quenched upon addition of Cu²⁺ and Ni²⁺ only because of binding to amide donors at physiological pH. However, at pH = 5, Cu²⁺ was the only metal ion that led to significant quenching.

Paul et al. demonstrated that the steric crowding and the intramolecular interactions in 6 twisted the crown rings and provided effectively less cavity size available to the guest ions than in the planar conformation.³¹ Thus, although the size of the aza-crown in 6 matched well with the K⁺ ion diameter, none of the results indicated K⁺ binding (Table 2). Rather, cations smaller in size than K⁺ formed stable complexes in the order of Pb²⁺ > Na⁺ > Cu²⁺ > Hg²⁺.

Table 2 Differential emission changes in complexes 6a and 6b with addition of different metal ions

Complex	λem (nm)	Metal ion	% quenching	Ks (10^−2 M^−1)	Cathodic shift (mV)
6a	627	Pb^2+	75–80%	49.15	∼128
		Cu^2+	90–97%	2.69	—
		Na^+	25–40%	5.90	∼88
		Hg^2+	25–40%	9.95	—
6b	617	Pb^2+	75–80%	99.72	∼128
		Cu^2+	90–97%	4.19	—
		Na^+	25–40%	8.81	∼88
		Hg^2+	25–40%	12.12	—

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Li et al. utilized the quencher displacement technique for sensing Cr³⁺ ions by luminescent chemosensor 7 using Cu²⁺ as a metal quencher.³²

Li et al. studied the turn-on fluorescence response in 7 via the metal displacement approach as shown in Fig. 3. The emission of 7 at 590 nm was appreciably quenched by Cu²⁺ and was not interfered by the presence of other ions except Cr³⁺. Both Cu²⁺ and Cr³⁺ bonded with 7 in a 1 : 3 (L : M) stoichiometric ratio. The formation constants for Cu²⁺ (10¹⁵ M⁻³) and Cr³⁺ (10¹⁸ M⁻³) revealed that Cr³⁺ had a stronger binding preference for 7 than Cu²⁺. The stoichiometric addition of Cr³⁺ into Cu²⁺–7 restored the fluorescence; while Cr⁶⁺ did not give this effect.

Fig. 3 Illustration of the transformation from a nonselective fluorogenic receptor (P₁) to a selective Off–On FCS (P₂) for metal cations through quencher displacement. K₁, K₂, and K₃ represent the binding constants for the interference (M₁), quencher (M₂), and target (M₃) metal cations, respectively; K₁ < K₂ < K₃.³²

Liu and his co-workers envisioned that Hg²⁺-triggered transformation of the chemodosimeter (8) brought a change in the photophysical properties of the phosphorescent material (8).³³ The emission intensity of 8 at 610 nm increased 4-fold with addition of Hg²⁺ in HEPES buffer [4-(2-hydroxyethyl)-1-piperazine ethanesulfonic acid], enabling a LOD (limit of detection) of Hg²⁺ as low as 5.4 nm. This change was attributed to Hg²⁺-promoted desulfurization and intramolecular cyclic guanylation of thiourea reaction into the luminophore (Scheme 3).

Scheme 3 Structure change of chemodosimeter 8.

Thummel et al. synthesized different mononuclear Ru(II) complexes of bi-1,10-phenanthrolines (9a–9d) for differential sensing of various metal ions.³⁴ Upon addition of Zn²⁺ to a MeOH (methanol) solution of 9a–9d, maximum changes in the emission spectra were observed with 9c, where the intensity of emission at 636 nm increased continuously with addition of Zn²⁺ accompanied by a bathochromic shift of 22 nm. Here, Zn²⁺ addition intensified the extent of delocalization over the bridging ligand, providing an extended π*-orbital available for the promoted electron, resulting in both enhancement and red shift of the emission maxima. Addition of Cd²⁺ and Hg²⁺ led to similar changes as that for Zn²⁺ but with lower association constants. However, addition of Co²⁺, Cu²⁺, Ni²⁺ and Mn²⁺ quenched the emission intensity of 9c due to electron transfer from the excited triplet state of Ru(II) to metal ion. The emission intensities of both 9b and 9d decreased on adding Zn²⁺ with a simultaneous red shift by 16 (from 641 to 657 nm) and 10 nm (from 636 to 646 nm), respectively. However, addition of Zn²⁺ to 9a led to negligible changes in the emission spectra. The binding constant values pointed to the stronger binding of 9b in comparison to 9c and 9d due to easy accessibility of unbound phen to the added metal ion owing to free rotation about 3,3′-bond in 9b as compared to bridged complexes 9c and 9d.

Palomares et al.³⁵ demonstrated varying sensitivities and extent of accessibility of thiocyanate ligands of two diastereoisomers (10a and 10b) for sensing Hg²⁺ ions. Solutions of both 10a and 10b gave a red coloured solution in ethanol displaying an absorption band at 511 nm. The introduction of Hg²⁺ ions blue shifted the absorption band to 465 nm along with a color change from red to orange. The changes were attributed to a decreased electron density on the Ru²⁺ metal ion owing to linkage of mercury to the thiocyanate groups. However, the complex 10b required four times the concentration of Hg²⁺ to produce changes similar to that observed with 10a.

Yam et al. reported the involvement of lone pair electrons of the aza nitrogen atoms of the appended aza-crown of 11f in cation binding, hence, inhibiting the intramolecular charge transfer (ICT) transition from the aza nitrogen to the phen unit.³⁶ Other Ru(II) complexes (11a–11e) with appended crown ether ligands showed negligible changes in the absorption or fluorescence spectrum (Table 3). However, the emission changes of 11f were attributed to the inability of nitrogen atoms of the crown moiety to quench the emissive MLCT state by PET (photoinduced electron transfer) upon coordination with metal ions. Also, different electrochemical excitations (annihilation, oxidative–reduction and reductive–oxidation) were used to generate the luminescent excited emissive states and the cation-binding effects on the ECL behaviour were studied.³⁷

Table 3 Emission, absorption and ECL changes observed in 11a and 11f with addition of metal ions

Complex	Metal ion	Emission changes process (log Ks)	UV-vis log Ks	Annihilation ECL intensity	Ox-Red ECL intensity	Red-Ox ECL intensity
11a	Hg^2+	Quenching (4.52)	—
11f	Hg^2+	Enhancement (7.76)	7.84	—	—	—
	Cd^2+	Enhancement (4.61)	4.54	—	—	—
	Zn^2+	Enhancement (4.38)	4.34	—	—	Increased
	Ba^2+	Enhancement (3.94)	3.85	Increased	—	—
	Ca^2+	Enhancement (3.60)	3.52	Increased	—	Increased
	Mg^2+	Enhancement (3.76)	3.66	Increased	—	Increased

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ECL of 11f, generated via an oxidative-reduction route [TPrA(tri-n-propylamine) as a coreactant] was not altered by the addition of alkaline-earth and transition-metal ions, rather, TPrA suppressed luminescence enhancement of the complex on adding metal ions due to its competing nature for binding to cations.

The cross-coupling Suzuki methodologies in a “building block” approach have been used for the synthesis of complex 12 incorporating both bidentate phen and terdendate terpyridine (tpy) units.²⁰ Addition of paramagnetic Cu²⁺, Co²⁺, Fe²⁺ and Ni²⁺ ions to a solution of 12 resulted in a decreased fluorescence intensity. However, an increase in intensity and a red shift of the MLCT emission was observed with addition of Zn²⁺ ions. This change was attributed to an increase in the delocalization of the electronic density over the bridging ligand in 12–Zn²⁺ complex, which resulted in the stabilization of the lowest energy ³MLCT state, causing red-shift.

Cheng et al. synthesized dinuclear Ru(II) complexes (13a–13c) containing heterocyclic rings possessing vacant coordination sites for binding to cations.^38,39 The optical changes shown by 13a–13c are given in Table 4.

Table 4 Differential emission changes induced in complex 13 with addition of cations and anions

	λem (nm)	M^n+ (fluorescence change)^a	A^− (fluorescence change)^b	M^n+ (fluorescence change)^b	log Kass	Detection limit mol L^−1
a Solvent system: CH3CN : H2O (1 : 1, v/v).b Solvent system: CH3CN.c Not determined.
13a	609	Cu^2+ (quenching)	—	—	4.74 (Cu^2+)	5.78 × 10^−7 (Cu^2+)
13b	609	Cu^2+ (enhancement)	—	—	^c	^c
13c	614	Cu^2+ (quenching)	F^−, AcO^− (quenching), H2PO4^− (enhancement)	Mn^2+, Co^2+, Cu^2+ (quenching), Zn^2+, Cd^2+, Hg^2+ (enhancement)	6.04 (F^−), 6.07 (AcO^−), 5.90 (H2PO4^−), 7.07 (Zn^2+), 6.68 (Cd^2+), 6.76 (Mn^2+), 11.43 (Cu^2+),^a 6.22 (Cu^2+)^b	4.53 × 10^−7 (Cu^2+),^a 3.33 × 10^−8 (Cu^2+)^b

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The emission quenching observed in 13a and 13c with only Cu²⁺ ions was due to an energy transfer process. On the other hand, the fluorescence intensity of complex 13b was enhanced by several transition metal ions (maximum change with Cu²⁺) due to the non-availability of the lone pair of electrons of the sulphur atom for the PET process owing to its involvement in the coordinative interaction. Chao et al. employed 13c as a sensing probe for the detection of Cu²⁺ in living cells and zebrafish.¹⁷

Baitalik et al. demonstrated F⁻/AcO⁻ induced deprotonation of imidazole NH protons of 13c in CH₃CN, causing an intramolecular PET from the deprotonated imidazole moiety to the excited state Ru(II) center and resulting in fluorescence quenching.⁴⁰ In sharp contrast, ∼2-fold emission enhancement was observed by addition of H₂PO₄⁻ ions due to rigidity induced in 13c–H₂PO₄⁻ complex because of H-bonding interactions between 13c and imidazole N–H, leading to luminescence enhancement (Fig. 4).

Fig. 4 Changes in PL spectra of 13c upon addition of various anions added as their TBA salts (Reproduced from ref. 40 with permission from the ©Royal Society of Chemistry).

The interaction of Ru(II) complexes with DNA is a well known phenomenon and can take place via several possible modes. Some Ru(II) polypyridyl complexes intercalate into the bases of the duplex of DNA, resulting in luminescence enhancement of the complexes. Rajput et al.⁴¹ reported intercalation of dimetallic Ru(II) complex between DNA bases in a threading fashion resulting in fluorescence enhancement. Turro and co-workers⁴² described electrostatic binding between DNA and Ru(II) complex responsible for using them as a DNA light switch. Different phen–Ru(II) complexes intercalate into the bases of ctDNA (calf-thymus DNA) duplex, leading to an increase in the luminescence intensity. Introduction of metal ion to the intercalated phen–Ru(II)–ctDNA complex again decreases the luminescence due to a weakening of the electrostatic attraction between the phen-Ru(II) complex and ctDNA and facilitated electrostatic interaction between the positive charge of the metal ion and the phosphate of the ctDNA backbone (Scheme 4). The sequential addition of metal ion and then ctDNA to solutions of phen–Ru(II) complex results in an initial quenching and then, enhancement of the luminescence.

Scheme 4 Schematic representation of the interaction of ctDNA and/Mⁿ⁺ with Ru²⁺ complexes.

On the contrary, addition of Cu²⁺ ions to the 14a–ctDNA system caused only negligible changes in the luminescence or absorption spectrum, which pointed to the fact that the middle phen chelating unit of 14a was located deeply in between the hydrophobic DNA base pairs, making access of Cu²⁺ ions difficult.⁴³

The complexes 14b, 14c, 16a and 16b also gave luminescence changes with Cu²⁺ ions and ctDNA;^44–47 however, 15a–ctDNA and 15b–ctDNA were selective towards Co²⁺ and Ag⁺, respectively.^48,49 The decreased emission intensity can be revived in both cases by adding EDTA (ethylenediamine tetraacetic acid) owing to a strong chelation of metal ions with EDTA.

Yao et al. detected Hg²⁺ by making a three component assembly of Ru(II) complex 16c, graphene oxide (GO) and dsDNA.⁵⁰ The adsorption of 16c on the surface of GO occurred due to formation of a charge-transfer (CT) (GO–Ru) complex. The fluorescence emission of 16c at 605 nm (λ_ex = 455 nm) was quenched almost completely by GO due to a strong interaction between them. GO surface adsorbed T(Thymine)-rich ssDNA (single-strand DNA) through π–π stacking, restoring the fluorescence intensity of 16c by 42%. However, on subsequent introduction of Hg²⁺ into the system, T-rich ssDNA is coordinated to Hg²⁺ due to the strong affinity of thymine base with Hg²⁺, resulting in the formation of T–Hg²⁺–T coordination bonds and detachment from the GO surface. The complex 16c then intercalated into the T–Hg²⁺–T DNA base pairs, resulting in a sharp increase in the fluorescence intensity (152%).

Ru(phen)₃²⁺ has also been used to develop sandwich ECL biosensors through its intercalation into the grooves of dsDNA that can capture multiple target molecules and signal probes on the surface of gold electrodes upon DNA hybridization. A functional oligonucleotide is so designed that it contains two functional sections; an intermolecular duplex for intercalation of Ru(phen)₃²⁺, and a pyrimidine base pair section for specific metal ion binding (Scheme 5).

Scheme 5 Intercalation of Ru(Phen)₃²⁺ into the grooves of dsDNA responsible for ECL changes.

Yin et al. described two such ECL biosensors for Hg²⁺ or Ag⁺ ion sensing.⁵¹ Upon addition of Hg²⁺ and Ru(phen)₃²⁺ to functional oligonucleotide solution, the T–Hg²⁺–T complex formed and then, Ru(phen)₃²⁺ was introduced onto the electrode surface. ECL emission related to the Hg²⁺ concentration was observed when the voltage of the modified electrode was swept from 0 to 1.25 V with TPrA as co-reactant. A similar concept was adopted to determine Ag⁺ ion content when a C–Ag⁺–C (Cytosine–Ag⁺–Cytosine) complex was formed.⁵² Xu and co-workers also used the same strategy for making supersandwich ECL assays based on T–Hg²⁺–T coordination and the intercalation of Ru(phen)₃²⁺ for Hg²⁺ recognition.⁵³ The only difference was the use of three ssDNA’s as compared to one dsDNA.

2.1.2 Anions. For anion recognition, the Ru(II) polypyridine moiety has an advantage over organic molecules in that the positive charge on the metal ion makes an electrostatic interaction with the negatively charged species leading to a strong host–guest interaction.⁵⁴

Khanmohammadi et al. detected F⁻ and AcO⁻ ions in commercially available mouthwash and vinegar using azo-coupled salicylaldehyde imidazole ligand (17) and its Ru(II) complex (17-Ru²⁺).⁵⁵ The addition of F⁻, AcO⁻ and H₂PO₄⁻ ions to the DMSO solution of 17/17-Ru²⁺ decreased the intensity of 355 nm bands with the simultaneous appearance of a new band at ∼550 nm responsible for the color change from light purple to deep purple/from light orange to dark brown, respectively. In the emission spectra, the weak emission band of 17 and 17-Ru²⁺ present at 420 nm (λ_ex = 355 nm) and 735 nm (λ_ex = 460 nm), respectively, enhanced upon addition of anions (F⁻, AcO⁻ and H₂PO₄⁻).

Ye and co-workers⁵⁶ incorporated the concept of multipoint hydrogen bonding interactions leading to increased anion affinity and better water solubility for studying the effect of anion binding with different Ru(II) complexes (18 and 19).

Amongst various anions, addition of CN⁻ anion to 18 and 19 in water at physiological conditions resulted in changes in emission properties via formation of multiple hydrogen bonding interaction (Table 5). The emission spectra were sensitive to pH owing to protonation or deprotonation of the imidazole ring. The binding constant of 19 was 2.5 times larger than that of 18 and detection limit of 19 was 20 times lower than that of 18. This was attributed to the C-shape acyclic cavity furnished by 19 with multiple hydrogen bonding interactions towards CN⁻ (Fig. 5).

Table 5 Absorption and emission changes in 18 and 19 upon addition of different anions

	λab (nm)	λab nm (F^−, AcO^−)	λab nm (CN^−)	λem (nm)	Fl. change (F^−, AcO^−)	Fl. change (F^−, AcO^−)	Fl. change (H2PO4^−)	Fl. change (CN^−)	log Kass^a
a Solvent system: H2O (pH 7.50).b CH3CN.c Not determined.
18^a	458	—	^c	593	—	—	—	Quenching	2.94
19^a	461	—	^c	593	Quenching	—	—	Quenching	2.53
19^b	458	520	^c	612	^c	Quenching	Enhancement	—	^c

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Fig. 5 Optimized structure of 19-CN⁻ using the B3LYP method and the 6-31G* basis set for hydrogen, carbon, and nitrogen atoms and the SDD energy-consistent pseudopotentials for ruthenium.⁵⁶

However, Baitalik et al. studied the optical and redox changes of 19 with various anions in CH₃CN (Table 5).⁵⁷ Protonation of 19 in the excited state was responsible for emission enhancement with H₂PO₄⁻ addition. The differential optical changes pointed to the deprotonation of imidazole N–H by excess of F⁻ and AcO⁻ ions; while with H₂PO₄⁻, the most probable mode of interaction was hydrogen bonding of H₂PO₄⁻ with the imidazolyl N–H protons.

The acidity of imidazolyl NH proton has been increased on coordinating with metal ions as in 21a and 21b as evident from their higher binding constant values (2–3 orders greater) with F⁻, CN⁻ and AcO⁻ compared to that with free ligand without metal complex (20).⁵⁸ The emission intensity of the peak of 21a at 750 nm increased in intensity with a blue shift of 5 nm upon addition of F⁻, CN⁻ and AcO⁻ only, indicating an enhancement of the radiative decay. However, addition of F⁻, CN⁻ and AcO⁻ resulted in a red shifting of the peak of 21b at 722 nm to 752 nm accompanied by significant quenching in its intensity, indicating a lowering in energy of the excited state and enhancement of non-radiative decay (Table 6).

Table 6 Absorption and emission changes in 20–23 upon addition of different anions

	Color	λab (nm)	Color (λab nm) F^−, AcO^−	Color (λab nm) H2PO4^−	λem (nm)	Fl. change F^−, AcO^−	Fl. change H2PO4^−	log Kass
a Not determined.b Not observed.
20	—	345	— (400)	— (400)	460	Quenching	Quenching	4.70 (F^−), 3.84 (AcO^−), 3.29 (H2PO4^−)
21a	—	494, 462	— (502, 467)	^a	750	Enhancement	^a	6.49 (F^−), 6.48 (AcO^−)^a
21b	—	495, 436	— (500, 438)	^a	722	Quenching	^a	6.45 (F^−), 6.43 (AcO^−)^a
22	Pale-yellow	—	^b	Red-purple (575)	608	Quenching	Enhancement	4.52 (F^−), 4.57 (AcO^−), 9.88 (H2PO4^−)
23	Light-yellow	423, 425	Deep-yellow (432)	^b	608	Quenching	Enhancement	4.74 (F^−), 5.11 (AcO^−), 10.17 (H2PO4^−)

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Wang and co-workers synthesized the Ru(II) complex of 20 (22)⁵⁹ and then a further Re complex of 22 (23)⁶⁰ and reported a similar type of differential optical changes with F⁻, AcO⁻ and H₂PO₄⁻ ions as observed by Baitalik et al. (Table 6). The complex 23 also acted as a pH induced “off–on–off” luminescent switch with one of the switching actions being driven by pH variations over the pH range of 5.3–8.0.

Liu et al. showed different fluorescence changes with the addition of different anions to calix[4]arene possessing phen derivative (24) and its Ru(II) complex (24-Ru²⁺).⁶¹ Upon addition of F⁻ and AcO⁻ ions to a solution of 24, the emission intensity of the 460 nm band decreased with the appearance of new peaks at 537 and 510 nm. The new peaks pointed to anion induced deprotonation of imidazole N–H. Presence of Cl⁻, Br⁻, HSO₄⁻ and I⁻ affected the spectrum negligibly. On the other hand, the addition of Cl⁻, Br⁻, I⁻ and HSO₄⁻ to the DMF solution of 24-Ru²⁺, caused fluorescence enhancement due to partial neutralization of positive charge of Ru(II) by the free anions, decreasing the PET effect from the nitrogen lone pairs to the Ru(II) center. On the other hand, F⁻ and AcO⁻ addition led to fluorescence quenching attributed to the favourable intramolecular PET that arose due to deprotonated 24.

The substitution of the indole group on the Ru²⁺ complex of phen (25)⁶² increased the selectivity and sensitivity towards AcO⁻ as compared to other anions such as F⁻, Cl⁻, Br⁻, I⁻, H₂PO₄⁻ and NO₃⁻ ions. Successive addition of AcO⁻ to a DMSO (dimethyl sulfoxide) solution of 25 resulted in a gradual decrease in the band at 291 nm (due to π → π*) and 462 nm (due to MLCT band) with remarkable enhancements in the band at 346 nm and the wave valley at 550 nm. These changes were accompanied by emergence of a new peak at 430 nm and a color change from yellow to orange. Addition of F⁻ to the solution of 25 gave similar UV-vis spectral changes to those noted for AcO⁻, but, the amplitude of the absorption changes observed with F⁻ were less than that observed with AcO⁻. Upon excitation at 462 nm, the complex 25 in DMSO strongly emitted at 618 nm. Addition of both AcO⁻ and F⁻ to the solution of 25 resulted in quenching of the fluorescence intensity with emission on–off intensity ratios of 19 and 6, respectively.

Changing the metal core in phen based metal complexes has a direct effect on their optical properties as evidenced in complexes 26a–26c.⁶³ The complex 26a showed two strong absorption bands at 284 nm and 368 nm. Upon addition of F⁻ ion to a DMSO solution of complex 26a, the band at 284 nm showed a 14 nm bathochromic shift with no band shift for the peak at 368 nm and a simultaneous appearance of a new band at 512 nm giving rise to a color change from yellow to pink. To clarify the interaction of 26a with F⁻, methylation of NH on the imidazole ring was performed. However this methylated complex showed no change upon addition of F⁻, Cl⁻, Br⁻, I⁻, HSO₄⁻, H₂PO₄⁻, CH₃COO⁻ and ClO₄⁻ [10 equivalent (equiv.)] in DMSO. This demonstrated that F⁻ interacted with 26a by deprotonating imidazole N–H. Complexes 26b and 26c did not show any UV-vis or color changes, rather, their emission properties were changed upon addition of F⁻ ions. The weak fluorescence band observed at 489 in the cases of 26b and 26c was enhanced selectively for F⁻ as compared to other ions.

Different research groups pointed to the hydrogen bonding interactions of AcO⁻, F⁻, H₂PO₄⁻ with the imidazole N–H of Ru(II) complexes or anion induced deprotonation of imidazole N–H in extreme cases. Due to deprotonation/hydrogen bonding interactions, dramatic color and fluorescence changes have been observed in different Ru(II) complexes (27–37) (Table 7).

Table 7 Absorption and emission changes in 27–37 upon addition of different anions

	Color and λab change F^−, AcO^−, H2PO4^−	Color and λab change Cl^−, Br^−, I^−	λem (process) F^−, AcO^−, H2PO4^−	λem (process) Cl^−, I^−, Br^−	log Kass
a Not determined.b More drastic changes with AcO^− compared to F^− and H2PO4^−.c More drastic changes with F^− and AcO^− compared to F^−
27	Yellow to red (386 → 315, 476)	—, (386 → 332)	566 (quenching)	566 (quenching)	>6 (AcO^−), 4.15 (F^−), 4.65 (H2PO4^−), 2.04 (Cl^−), 1.20 (Br^−)
27–Ru^2+	—, 364, 460 → 460	—, 364, 460 → 350–500	523, 610 (enhancement)	523, 610 (quenching)	>6 (AcO^−), 5.20 (F^−), 5.97 (H2PO4^−), 2.80 (Cl^−), 1.75 (Br^−)
28	Yellow to green,^b 434 → 622^b	—	^a	^a	4.59 (AcO^−),
28–Ru^2+	Purple to green,^b 430, 463 → 696^b	—	^a	^a	5.95 (AcO^−)
29	Purple to green, 416, 457 → 645	—	^a	^a	3.88 (AcO^−), 2.87 (H2PO4^−)
30	—, 286, 440	—	613 (quenching)	—	4.69 (AcO^−), 4.35 (F^−), 4.40 (H2PO4^−)
31a	—	—	605 (quenching)	—	4.28 (AcO^−), 4.07 (F^−), 4.08 (H2PO4^−)
31b	286, 452				4.10 (AcO^−), 3.86 (AcO^−), 3.50 (H2PO4^−)
31c					4.21 (AcO^−), 4.25 (F^−), 4.16 (H2PO4^−)
32a	Yellow → orange → red-brown, 340, 455 → 398, 580, 545	—	598 (quenching)	—	5.27 (AcO^−), 5.34 (F^−), 4.93 (H2PO4^−)
32b	Yellow → orange-red, 343 → 372, 455–590	—	616 (quenching)	—	4.79 (AcO^−), 4.81 (F^−), 4.14 (H2PO4^−)
33	Orange → blue-violet, 475 → 580 (F^− and H2PO4^−)	—	630 (enhancement)	—	6.23 (F^−)
34a	Orange-red → red^c	—	^a	^a	4.13 (AcO^−), 3.94 (F^−), 2.77 (H2PO4^−)
34b	340, 475 → 380, 500^c				4.81 (AcO^−), 4.65 (F^−), 3.58 (H2PO4^−)
34c	Color deepens^c	—	^a	^a	4.75 (AcO^−), 5.14 (F^−), 4.74 (H2PO4^−)
34d	Abs. at 471 increases^c				5.03 (AcO^−), 5.38 (F^−), 4.94 (H2PO4^−)
35a	Color deepens^c	—	^a	^a	4.04 (AcO^−), 4.37 (F^−), 3.26 (H2PO4^−)
35b	Abs. at 450, 470 increases^c				3.66 (AcO^−), 3.97 (F^−), 1.6 (H2PO4^−)
	Abs. at 420 decreases^c
36	—, 493 → 534^c	—	^a	^a	6.25 (AcO^−), 6.11 (F^−), 5.46 (H2PO4^−)
37	Orange-red → red,^c 340, 475 → 375, 525^c	—	542 (enhancement)		4.81 (AcO^−), 4.65 (F^−), 3.58 (H2PO4^−)

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Lin et al. showed the binding affinity of different Ru(II) complexes (27–29) possessing a varied number of e⁻-withdrawing –NO₂ groups towards anions (Table 7).^64–66 Although complex 29 possessed two –NO₂ groups, even then, it showed only hydrogen bonding interactions with anions, rather than deprotonation of the imidazole N–H. This was because of formation of two stable six membered rings due to intramolecular hydrogen bonding between NO₂ and –NH, which lowered the acidity of –NH in 29 and thus, ruling out the possibility of deprotonation (Fig. 6).

Fig. 6 Intramolecular H-bonding between –NO₂ and –NH groups of 29–F⁻ complex.

The complexes 30 and 31 exhibited substantial quenching in emission intensity with F⁻, H₂PO₄⁻ and AcO⁻ (65–80%),⁵⁴ because the strong basicity of the anions enhanced electron density on NH and NH₂ which subsequently propagated to the metal bound phen ring causing an increase in intramolecular quenching in the emission intensity.

Das and co-workers described the effect of the presence of an e⁻-withdrawing functionality on the relative acidity of the H atom of the NH of the urea moiety and its consequential effect on the anion binding affinity in Ru(II) complexes (32a)⁶⁷ and (32b).⁶⁸ On addition of 1.2 equiv. of anions (A⁻ = F⁻, AcO⁻, H₂PO₄⁻), the absorption band present at 340 and 455 nm red shifted to 398 and 588 nm, respectively. Moreover, a new absorption band at 545 nm developed with a simultaneous increase in the absorption intensity at 440 nm. On addition of higher equiv. of A⁻, a new isosbestic point was observed at 300 nm (Fig. 7). Different optical studies supported for the involvement of two different equilibrium processes: the first one being the H-bonded 1 : 1 adduct formation (32a–A⁻ for [A⁻] < 1.5 mol equiv.) and the second one being the deprotonation equilibrium involving the deprotonation of NH_urea, adjacent to phen (for [A⁻] > 1.5 mol equiv.). The differential optical changes observed in 32a and 32b (Table 7) suggested that the strong e⁻-withdrawing effect of the –NO₂ functionality had an influence on the acidity of the –NH_urea proton and thus, its ability towards different anions and deprotonation phenomenon.

Fig. 7 UV-vis titration of 32a (2.0 × 10⁻⁵ M) in acetonitrile solution with F⁻ (2.0 × 10⁻⁶ M to 8.0 × 10⁻⁵ M).⁶⁷

Duan and co-workers studied F⁻ induced tautomerization of quinonehydrazone to azophenol in complex (33) and made easy-to-prepare test paper for detecting ∼10 ppm of F⁻ ions (Scheme 6).^69,70 Other quinoxaline possessing Ru(II) phen complexes (34–37) also gave prominent changes with F⁻ and AcO⁻ ions as compared to H₂PO₄⁻ ions.^71–74 The binding constants of Ru(II) complexes (34a–34b) were higher than Co(II) complexes (34c–34d) as Co(II) complexes involved d–π feed back bonding responsible for intensifying the negative charge on the amide groups.

Scheme 6 F⁻ promoted tautomerization of quinonehydrazone (33) to azophenol.

Sessler et al. synthesized fused a dipyrrolylquinoxaline (dpq) phen derivative (38) and its corresponding Ru(II) (39a) and Co(III) (39b) complexes.⁷⁵ The quinoxaline backbone in 39a and 39b exerted e⁻ withdrawing effects, increasing the acidity of the pyrrole N–H, hence the binding affinity for the F⁻ ion (Table 8). The anion binding affinity and constants (M⁻¹) for 38, 39a and 39b followed the order of 39b (540 000) > 39a (120 000) > 38 (440) which could be explained on the basis of increasing cationic charges in the same order. The metal-free phen derivative 38 exhibited a low affinity for F⁻ due to the additional electron density donated to the dpq NH binding functionality from the nitrogen rich phen. Another similar dpq possessing Ru(II) complex (40),⁷⁶ similar to 39a, showed affinity towards CN⁻ and F⁻ ions. Schmittel et al. invoked the well-known cyanohydrin formation at the aldehyde group for selective luminescence detection of CN⁻ using Ru(II) complexes (41a) and (41b) (Scheme 7).⁷⁷

Table 8 Absorption and emission changes in 39–41 upon addition of different anions

	Color and λab change F^−	Color and λab change CN^−	λem (process) F^−	λem (process) CN^−	log Kass
a Not determined.
39b	Red-pink → pale-purple, 323, 525 → 652	Not determined	Not determined	Not determined	4.73 (F^−), ^a (CN^−)
40	—, Abs. decreases at 465	—, Abs. decreases at 465	595 (quenching)	595 (quenching)	5.80 (F^−), 5.63 (CN^−)
41a	—	Orange-red → yellow, 287, 370–600 → 435, 450			(F^−), 8.56 (CN^−)^a
41b	—	Orange-red → yellow, 264, 473 → 435, 447	732 → 614 (enhancement)	732 → 624 (enhancement)	(F^−), 9.14 (CN^−)^a

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Scheme 7 Cyanohydrin formation in 41b.

The complexation of the triazole pyridine unit with the Ru(II) centre in (42)⁷⁸ served the dual purpose of making a triazole C–H a more efficient hydrogen bond donor as well as a sensitive anion sensor in CH₃CN. The UV-vis spectrum of 42 showed absorption bands at 262 nm (π–π*), 463 nm and 445 nm (MLCT) in CH₃CN. Upon addition of various anions to 40 μM solution of 42, the MLCT absorbance band at 445 nm red shifted only in the presence of H₂PO₄⁻ and HP₂O₇³⁻. Excitation of 42 at 403 nm resulted in broad centered weak luminescence at 590 nm. Enhancement of the PL emission intensity of 42 along with the 10 nm red shift of the emission maxima was observed in the presence of H₂PO₄⁻/HP₂O₇³⁻ selectively.

By combining the redox activity of the ferrocene moiety with the strong hydrogen bonding ability of the imidazole ring and the photoactive behaviour of the phen ring, the chemosensor (43) and its Ru(II) complex (43–Ru²⁺) were synthesized by Molina et al. for multichannel detection of ADP (adenosine diphosphate), ATP (adenosine triphosphate) and HP₂O₇³⁻ ions.⁷⁹ The UV-vis spectrum of 43 in CH₃CN exhibited strong absorption bands at 256 and 279 nm (π–π*). Addition of HP₂O₇³⁻ ion resulted in red-shifting by ∼13 nm with the emergence of a new band at 319 nm with a color change from yellow to orange. On the other hand, the intensity of 256 nm increased with the disappearance of the 279 nm band and a growing of a shoulder peak at 319 nm with the addition of ATP and ADP to 43. This caused a color change from yellow to pink. Addition of HP₂O₇³⁻, F⁻, ATP and ADP anions also resulted in chelation enhanced fluorescence (CHEF) at 453 nm (λ_ex = 390 nm). All the results suggested that interaction between 43 with ATP and ADP was mainly due to hydrogen-bonded complex formation; while HP₂O₇³⁻ interacted with 43 involving both formation of the hydrogen-bonded complex and deprotonation. On the contrary, the 43–Ru²⁺ complex selectively recognized Cl⁻ ions over other anions. Cathodic redox shift of −80 mV of the Fe(II)/Fe(III) redox couple was observed with the addition of Cl⁻ ions keeping the oxidation wave of Ru(II) center unchanged. Remarkably, only Cl⁻ ions increased the integrated fluorescence intensity in a 30-fold and 43-fold increase in quantum yield.

Luminescent complexes of Re(I) and Ru(II) with appended macrocyclic groups (18-crown-6, 24-crown-8 and two carboxamide units) derived from 5,6-dihydroxy-phenanthroline (44) and (45) have been used for both cation and anion binding.^80,81 Macrocyclic units, being crown ethers in all the complexes, were expected to bind with K⁺ ions. However, due to the poor e⁻-donating ability of two oxygen atoms, which are directly attached to e⁻-withdrawing Ru(II) phen unit, the K⁺ binding constant values were very low. The fluorescence changes observed with the addition of different cations and anions have been summarized in Table 9. It was observed that binding of Ba²⁺ to 45a was weaker than to 45b. This was due to e⁻-deficient and poor donors (–O) present in 45a. In 45b, the macrocyclic unit was too large for all eight –O atoms to be required for Ba²⁺ binding, consequently leading to a higher binding constant.

Table 9 Absorption and emission changes in 44–45 upon addition of different anions

	λem	Fl. process (Ba^2+)	Fl. process (Cu^2+)	Fl. process (F^−)	Fl. process (H2PO4^−)	Stoichiometry	log Kass
a Not determined.
44c	609	—	—	Quenching	Enhancement	1 : 2 (H2PO4^−), F^−^a	F^−,^a H2PO4^− (6.27, 5.89)
44d	604	—	—	Quenching	Enhancement	1 : 1 (F^−, H2PO4^−)	F^− (3.14), H2PO4^− (4.53)
44e	603	—	Quenching	Quenching	Quenching	1 : 1 (F^−, H2PO4^−, Cu^2+)	F^−(2.45), H2PO4^− (4.65), Cu^2+ (3.30)
45a	606	Quenching	—	—	—	1 : 1 (Ba^2+)	(Ba^2+) 4.65
45b	606	Quenching	—	—	—	1 : 1 (Ba^2+)	(Ba^2+) 5.11
45c	608	—	—	Quenching	—	—	F^−^a

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The complexes 44c–44e and 45c possessing amidic NH showed significant changes in their luminescence behaviour on addition of F⁻ or H₂PO₄⁻ ions. The multiple hydrogen bonding interactions responsible for binding two H₂PO₄⁻ ions generated a cyclic rigid hydrogen-bonded structure with 44c/44d and prevented the quenching of the ³MLCT excited state and hence, led to an enhancement in emission intensity. Addition of F⁻ to 44c and 45c quenched the emission intensity completely with the color of the solution changing to nearly black. The F⁻ ion addition caused deprotonation of the carboxamide NH and led to a new band at 600 nm in the visible region responsible for the color change.

2.2 Chemosensors based on Ln(III) complexes of 1,10-phenanthroline ligands

Trivalent lanthanide ions (Eu, Tb) inherently suffer from weak light absorption capabilities because their f–f transitions are Laporte forbidden resulting in molar absorption coefficients (ε) smaller than 10 L mol⁻¹ cm⁻¹ for most absorption coefficients; whereas organic chromophores have values 5000 times larger. Thus, organic lanthanide complexes are good candidates as optical indicators for analytes owing to their peculiar photophysical properties such as large Stokes shift, narrow emission bands and long excited state lifetimes. Furthermore the emission color depends only on the lanthanide ion and is independent of the environment of a given lanthanide ion. Counter ions can coordinate to lanthanide ions in coordinating unsaturated ligands to fulfil the coordination requirements of lanthanide ions.^82,83

2.2.1 Cations. Gunnlaugsson, Wang and Hou et al. explored the use of mixed f–d metal ions in the synthesis of supramolecular assemblies in which the luminescence arising from the lanthanides was directly modulated by the presence of the d-metal ions. The interaction of d-metal ion reduced the antenna effect of the ligand responsible for f–f transition of Eu³⁺ ions in 46–50, resulting in quenching of the luminescence intensity (Table 10).^84–90

Table 10 Absorption and emission changes in 46–50 upon addition of different metal ions

	λab change Fe^2+	λab change Fe^3+	λab change Cu^2+	λem (process) Fe^2+	λem (process) Fe^3+	λem (process) Cu^2+	Stoichiometry M : L	log Kass
46	266 → 276, 518	—	266 → 276	430 (quenching)	—	430 (quenching)	1 : 3	ML(6.58), ML2(11.35), 16.03(ML3)–Cu^2+, ML(6.14), ML2(11.56), ML3(16.56)–Fe^2+
47	275 → 535	—	—	590, 618, 695 (quenching)	—	—	1 : 3	ML3(22.0)–Fe^2+
48	—	—	—	—	590, 618, 695 (quenching)	—	1 : 1	ML(3.93)
49	—	—	—	—	—	590, 618, 695 (quenching)	1 : 1	ML(3.06)
50a	—	—	333 → 305	—	—	585, 591, 617 (quenching)	1 : 1	ML(3.75)

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A cationic Eu(III) based cyclen (1,4,7,10-tetraazacyclododecane) complex based on the phen moiety (46) acted as a receptor for H⁺/Cu²⁺/Co²⁺/Fe²⁺ ions (Fig. 8).^84,86 In the physiological pH range, Eu(III) emission was ‘switched on’ while it ‘switched off’ in both highly acidic and alkaline pH generating a bell shaped pH-emission curve with pK_a’s of 3.8(±0.1) and 8.1(±0.1).⁸⁴ The phen complex (47) detected Fe²⁺ ions in aqueous buffered solution (pH = 7.4) through displacement assay, which showed quenching of Eu(III) emission on adding Fe²⁺ evidencing the formation of Fe·(phen)₃ as a displacement generated non-emissive Eu(III) cyclen complex.⁸⁷

Fig. 8 The quenching of the Eu(III) emission of 46 upon titration with Cu(II) at pH 7.4 (Reproduced from ref. 84 with permission from the ©Royal Society of Chemistry).

Two phen based ternary europium complexes (50a) and (50b),⁹⁰ differing by only –COOH substituents gave differential changes with Cu²⁺ ions. Complex 50a with a 4-substituted carboxyl group behaved as “on/off” switch for Cu²⁺ luminescence, while, complex 50b without the carboxyl group did not exhibit any absorption or emission changes in the presence of Cu²⁺ ascertaining interaction of carboxyl group and Cu²⁺ during the recognition process.

Based on the fact that the introduction of appropriate amounts of Zn²⁺ in Ln-Schiff base complexes could enhance the luminescence of the lanthanide complexes,^91–94 Zhou et al. synthesized two such Eu²⁺-complexes (51a and 51b)⁹⁵ to determine the trace amounts of Eu³⁺ and Zn²⁺. Introduction of phen to 51 and further adding Zn²⁺ to 51-Phen system increased the PL intensity by 20–22 and 26.4–60 times, respectively. The calibration curves of Eu³⁺ and Zn²⁺ were used for the determination of trace amounts of Eu³⁺ and Zn²⁺. Based on this, the trace amounts of Eu³⁺ was determined in synthetic rare earth oxide mixture and high purity La₂O₃ matrix while that of Zn²⁺ in high purity Mg(NO₃)2.6H₂O matrix, hence, ascertaining the practical applicability of the system.⁹¹

A nucleotide/Tb³⁺ coordination polymer formed by self assembly of biomolecule nucleotide (ADP) upon coordination with phen gives rise to a luminescent nanosensor (52)⁹⁶ for selectively sensing Fe²⁺ ions. Otherwise weakly emissive Tb³⁺–ADP became highly fluorescent upon coordination with phen (32 folds increase) and displayed emission peaks at 488, 545, 584 and 619 nm (Fig. 9). The fluorescence emission of 52 was a maximum when the ratio of Tb³⁺ to Phen was 1 : 1. Upon adding increasing concentrations of Fe²⁺ to 52, the fluorescent intensity decreased appreciably owing to the formation of a more stable complex Fe(phen)₃. The absorption of 52 was observed at 265 nm in the UV-vis spectrum, however, a new peak emerged at 512 nm with added Fe²⁺ in addition to a broad peak in the 360–580 nm range.

Fig. 9 Emission spectra of Tb-ADP (a), Tb-ADP-Phen (b) and Tb-ADP-Phen in the presence of Fe²⁺ (20 mM) (c) under a 275 nm exciting light (Reproduced from ref. 96 with permission from the ©Royal Society of Chemistry).

2.2.2 Anions. Lanthanide complexes are markedly oxophilic and positively charged, thereby, providing strong electrostatic interactions with anions. Furthermore, it is also known that weakly-coordinated ligands can be displaced by stronger ones. Generally, there are two ways of modulating the emission property of lanthanide complexes by anions. Firstly, water molecules bound to a lanthanide center could be displaced by anions leading to enhanced luminescent intensity; the other one is the displacement of ligands by anions resulting in the increase or decrease of luminescent intensity.⁸³

Wang and co-workers invoked the use of Eu(III) thenoyltrifluoroacetonate (TTA)/Eu(III) tris (dibenzoylmethane) (DBM) complex as the initial building block to attach it to derivatized phen ligand to retain high luminescence intensity and also to meet the requirement of a high coordination number (C.N) of Eu(III) ion (53–55). All luminescent Eu(III) complexes showed emission peaks in the red region at 583, 590, 615, 650 and 700 nm (λ_ex = 280 or 350 nm), which underwent quenching with addition of different anions, giving rise to varied luminescence colors (Table 11).^82,97–101 The changes with F⁻ and AcO⁻ were attributed to the hydrogen bonding interactions of the F⁻/AcO⁻ with NH of the receptor. The addition of H₂PO₄⁻ decreased the emission of Eu³⁺ ions with a simultaneous increase in the broad band of phen in the blue region which was attributed to the displacement of the phen ligand by H₂PO₄⁻, hence, displaying phen’s own blue emission (Fig. 10).

Table 11 Absorption and emission changes in 53–57 upon addition of different anions

	λab & color change (F^−)	λab & color change (AcO^−)	λab & color change (H2PO4^−)	Fl. Color (F^−)	Fl. Color (AcO^−)	Fl. color (H2PO4^−)	Number of equivalents required for complete change
a Not determined.
53a	274, 341 → 294, colorless → deep-yellow	274, 341 → 294, colorless → deep-yellow	^a	Blue-green	Yellow-red	—	1 (F^−), 2 (AcO^−)
53b	281, 340 → 295, colorless → light-yellow	281, 340 → 295, colorless → light-yellow	^a	Dark green	Pale-blue	—	1 (F^−), 1 (AcO^−)
53c	283, 346 → 294	283 346 → 294	^a	Purple	Green	Blue	5–10 (F^−), 5–10 (AcO^−)
54a	303, 335 → 303, green → deepened	^a	303, 335 → 303, green → dark	Green	^a	Green	2–5 (F^−), 2–5 (H2PO4^−)
54b	290, 353 → 342, yellow → deepened	^a	290, 353 → 342, yellow → deepened	Blue	^a	Blue	2–5 (F^−), 0.2–0.5 (H2PO4^−)
55	270, 350 → increase	270, 350 → increase	^a	Blue	Red	^a	1–5 (F^−), 1–5 (AcO^−)
56	280 → increase	280 → increase	^a	Green-blue	Green-blue	^a	1–10 (F^−), 1–10 (AcO^−)
57	318, 337, 354 → 425, colorless → pale-yellow	^a	^a	Transparent	^a	^a	2 (F^−)

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Fig. 10 Luminescent Color changes observed in ternary Eu(III) complex-based luminescent sensors upon addition of various anions.⁸²

Upon embedding 53a/53c into the polymeric hosts, the thick films like PMMA [poly(methyl methacrylate)] were formed on the glass substrate, which increased the quantum yield of Eu(III) ions and energy transfer efficiency.

In complex 54a, incomplete energy transfer from the ligand to Eu(III) ions occurred, as a result of which 54a gave yellow emission. The hydrogen bonding and competitive coordination interactions between anion and 54a/54b were responsible for the sensing process.

Both complexes 55 and 56 exhibited “on–off” changes after addition of F⁻. On the other hand, AcO⁻ ions, being larger in size than F⁻, were found suitable for filling the remaining coordinating sites or replace one or two coordinated water molecules. Thus, the –OH oscillators, which acted as quenchers, were removed and the emission intensity was enhanced dramatically.

The quantum yield of 57 was less than other such reported Eu(III) complexes because of unsaturated coordination around the metal center arising due to less photosensitization of ligand in 57 to the Eu(III) center. A colorless solution of 57 in DMSO showed color, absorption and fluorescence changes only with addition of F⁻ ions amongst Cl⁻, Br⁻, I⁻, ClO₄⁻ ions, attributed to a large hydrogen bonding tendency of F⁻ ion owing to its small size and high charge density (Table 12).

Table 12 PL and ECL changes in 76–77 upon addition of various metal ions

	M^n+	76a	77a	77b
PL changes	Ba^2+	8-Fold Enhancement, 519, 547 → 590	—
	Ca^2+	—	—
	Cu^2+	Quenched	—
	Pb^2+	No intensity change 519, 547 → 580	—
	Hg^2+	—	80% quenching, 527, 559 → 595	Quenching
	Ag^+	—	3.4-Fold enhancement, 527, 559 → 595	0.3-Fold enhancement
ECL changes	Ba^2+	9-Fold enhancement, 520, 553 → 590	—	—
	Ca^2+	3-Fold enhancement, no red shift	—	—
	Hg^2+	—	3-Fold enhancement, 529, 559 → 575	—
	Ag^+	—	9-Fold enhancement, 529, 559 → 594	—

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The Eu(III) complexes possessing phen as the second ligand (58–59)¹⁰² gave different luminescence changes upon addition of AcO⁻ ion when the first ligand changes form TTA (in 58) to DBM (in 59). The DMSO solution of 58 and 59 gave transparent solutions which turned light yellow for 58c and 59c along with the disappearance of the 280 nm band and a 7 nm bathochromic shift of the 353 nm band, upon selective addition of AcO⁻ ions. Complexes 58a–b and 59a–b exhibited absorption bands around 281–282 and 327–344 nm which did not undergo any changes in the λ_max value or intensity with the addition of different anions, including AcO⁻ ion. In complexes 58, the intense broad bands between 250 nm and 400 nm dominated the large portions of excitation spectra of these complexes. The intensity of peaks (250–400 nm) decreased with no change in the peak positions. However, in the case of the complexes 59, the emission peaks observed in the range 250–300 nm and 300–425 nm grew in intensity (nearly 5 folds) with incremental addition of AcO⁻ ions (0–3 equiv.), suggesting strong hydrogen bonding interaction between imidazolyl NH groups, aromatic and phen ring’s protons with AcO⁻ of all the complexes.

The Eu(III) complexes of phen based hexadentate ligands (60a) and (60b)¹⁰³ bearing semicarbazone or imine moieties showed different emission responses. The luminescence spectrum of 60a revealed only ligand centered (LC) emission and metal-induced emission was not active. However, the complex 60b showed metal centered (MC) emission, which might be due to the substitution of the quencher NH group by a sp³ carbon atom. Amongst various anions, selective addition of H₂PO₄²⁻ and ATP to a solution of 60 caused quenching of the emission band.

Another Eu(III) complex possessing two TTA units as the first ligands also gave absorption and fluorescence changes with F⁻, AcO⁻ and H₂PO₄⁻ attributed to strong H bonding interactions between the imidazole ring of (58) with said anions.¹⁰⁴ Moreover, free radical copolymerization of 58 with N-isopropyl acrylamide in DMF resulted in formation of hydrogel which showed excellent selectivity for H₂PO₄⁻ (LOD of 10⁻⁵ M) over F⁻ and AcO⁻ in pure water.

Li and co-workers found that the values of the quantum yields were inversely related to the extent of conjugation in the Eu(III) complexes 59 and 60.¹⁰⁵ The increase in the conjugation increased the non-emissive ratio of the ligands in comparison to the emissive ratio. With increasing conjugation, the triplet state (T₁) energy which was to be transferred to the Eu(III) center to cause MC f–f transition, was rolled back due to a lower energy gap between the Eu(III) center and T₁, inhibiting the f–f transition and hence, lowering the emissive performance (Scheme 8). Amongst all the complexes screened for various anions, complexes 59d and 60d exhibited more sensitive emission changes with the AcO⁻ ion. The emission intensity increased continuously with increased added concentration of AcO⁻ ions, displaying an emission “Off–On” effect. This clearly stated that AcO⁻ ions decreased the energy roll back restoring the MC f–f transition leading to an enhancement of emission intensity. The Eu(III) emission was also found to be sensitive to molecular oxygen through a back energy transfer mechanism. A similar type of energy transfer roll back was observed by Jiang et al. in 2013 in complex 61,⁸³ which recognized AcO⁻ over other anions by the increase of luminescence intensity.

Scheme 8 A potential sensing mechanism in Eu(III) complexes.

Li et al. synthesized Tb(III) complex Tb(PMIP)₃(PhCN) [PMIP is 4-isobutyl-3-methyl-5-pyrazolone and PhCN is pyrazino[2,3-f][1,10-phenanthroline-2,3-dicarbonitrile] (62), in which back-energy transfer from the excited triplet state of Tb³⁺ ion and PMIP to the triplet state of PhCN was operational.¹⁰⁶ Tb³⁺ ion has generally a high C.N of nine in its complexes, but, exhibited a C.N of 8 in 62, indicating its tendency to bind one anion to reach up to a C.N of 9, hence an increase in the fluorescence emission was observed on adding F⁻ ion. F⁻ ion having a strong binding ability replaced both a solvent molecule and a PhCN molecule that were weakly coordinated to Tb³⁺ in the complex 62, making the C.N of Tb³⁺ to 9 and prohibiting the back-energy transfer process. However, further addition of F⁻ quenched the emission which was possibly due to replacement of PMIP ligands by F⁻ ions, inhibiting the sensitization of Tb³⁺ ions by PMIP, hence reducing the fluorescent quantum yield of the complex.

The reaction of 5-amino-1,10-phenanthroline with trifluoro-p-tolyl isocyanate in CHCl₃ resulted in formation of urea possessing phen ligand, which upon further reaction with Eu(CF₃SO₃)₃ in CH₃CN resulted in formation of complex 63.¹⁰⁷ The absorption and emission changes observed upon addition of Eu(III) pointed to the formation of 1 : 3 stoichiometric complex in solution. Addition of AcO⁻ ions resulted in formation of four isosbestic points at 256, 280, 303 and 322 nm, attributed to the interaction of the anion with the three urea moieties. Addition of F⁻, Cl⁻, Br⁻ and H₂PO₄⁻ ions gave the same absorption changes, but, in different binding modes. The emission spectrum was perturbed with addition of AcO⁻, H₂PO₄⁻, Cl⁻, Br⁻ and I⁻ ions with fully quenched Eu(III) emission in the case of AcO⁻ and H₂PO₄⁻. On the contrary, titration of F⁻ gave rise to initial enhancement in the Eu(III) emission up to 1 equiv., followed by fluorescence quenching. These different fluorescence changes were assigned to dual binding modes involving initial binding of F⁻ to Eu(III), with concomitant removal of one solvent molecule.

2.3 Chemosensors based ON Ir(III) complexes of 1,10-phenanthroline ligands

The octahedral d⁶ Ir(III) complexes have interesting properties like decent thermal stability, intense luminescence at ambient temperature, larger Stokes shifts, long excited-state lifetimes, prominent color tunability and emission/absorption wavelengths across the entire visible light region that can be modulated by changing the auxiliary ligands.^108–112

2.3.1 Cations. You and Ma et al. synthesized different Ir(III) complexes (67–72) possessing a Zn²⁺-specific di(2-picolyl)amine (DPA) receptor and compared their phosphorescent properties with complexes devoid of a DPA unit.^113–117 In all the complexes, PL enhancement with varied red-shift was observed. This was due to the occurrence of nonradiative PET from the DPA to the photoexcited Ir(IV) species in all the complexes, while coordination of Zn²⁺ suppressed the PET by lowering the oxidation potential of the sensor resulting in the PL changes.¹¹⁶ Complex 67b exhibited dual emission in the blue (461 nm) and yellow (528 nm) regions. Continuous addition of Zn²⁺ ions to an CH₃CN solution of 67b “turned on” the yellow phosphorescence in a ratiometric manner. However, complex 67a, which was not substituted by DPA at the 4-position of the phen ligand, did not show such changes on Zn²⁺ addition. Complex 67b was also used as a phosphorescent turn-on probe for tracing mobile zinc in live a549 cells and for other bioimaging applications.¹¹³

A similar analogue 67c¹¹⁴ possessing a Cr³⁺ responsive bis(2-(2-(methylthio)ethylthio)ethyl)amino(BTTA) receptor introduced at the 4-position of the phen ligand displayed weak green phosphorescence emission at 515 nm. Addition of Cr³⁺ ion to a solution of 67c caused rapid double phosphorescence ratiometric change with a 8-fold turn-on and 42 nm red-shift. The first reversible phosphorescence change was due to modulation of PET and ILCT (intra-ligand-charge-transfer) transitions leading to binding of Cr³⁺ with BTTA, as established by steady-state and time-resolved photophysical experiments. Then Cr³⁺ evoked a biomimetic oxidation reaction by activating O₂ to cleave Cr(BTTA) ionophore, leading to a second phosphorescence ratiometric response.

Amongst various added cations, the emission intensity of complexes 68a–68d was increased by about 1.2–5.4 fold only with Zn²⁺ ions.¹¹⁵ However, no observable changes were seen in the DPA free 69a–69d complexes. Though Fe²⁺, Cd²⁺ and Pb²⁺ ions also induced the emission enhancement of complexes 68a–68d, but, changes were quite less as compared to Zn²⁺ addition.

However, the phosphorescence spectrum of 70a–70c displayed a “turn on” response with Zn²⁺ addition, while, complexes 70e and 71 exhibited a weak “turn-off” response.¹¹⁶ The luminescence intensity of 72 was enhanced upon titration with Zn²⁺ in tris buffer associated with a bathochromic shift of 60 nm.¹¹⁷ The changes due to Zn²⁺ presence were visible to the naked eye under UV illumination resulting in a luminescence color change from blue to green.

An Ir(III) complex possessing a substituted urea derivative (73a) showed luminescence quenching upon addition of Cu²⁺ ions.¹¹⁸ Upon excitation at 365 nm in CH₃OH : H₂O (1 : 99), 73a gave MLCT luminescence centered in the orange region at 600 nm. Almost complete emission quenching was observed only by the addition of Cu²⁺. The nitrogen and oxygen on amide, nitrogen on pyridine were the most likely coordination sites of 73a for Cu²⁺ coordination as depicted by their electrospray mass spectrum (Fig. 11).

Fig. 11 Electrospray mass spectrum of 73a with 2 equiv. of Cu²⁺.¹¹⁸

On the basis of familiar Hg²⁺-promoted desulfurization and intramolecular cyclic guanylation of thiourea reaction, Ir(III) complex 73b has been developed by Tang et al.¹¹² The phosphorescence spectrum of complex 73b exhibited weak emission band at 560 nm. With incremental addition of Hg²⁺ ions, the intensity of 560 nm emission band increased linearly (6-folds). Also, the NMR changes upon Hg²⁺ addition like disappearance and downfield shift of N–H peaks of thiourea group of 73b, downfield shift and splitting of one of the methylene groups supported the Hg²⁺-induced desulfurization reaction and cyclization of the chemodosimeter 73b to form a rigid structure (Scheme 9). Other Ir(III) complexes (73c–73k)¹¹⁹ showed only luminescence quenching of green to orange luminescence with addition of OAc⁻, F⁻ and H₂PO₄⁻ anions, irrespective of the presence of various cyclometalating ligands and substituents of the thiourea moiety.

Scheme 9 Cyclization of 73b upon Hg²⁺ addition.

A similar type of chemodosimetric approach in which Cu²⁺ ions caused the rhodamine moiety to undergo a conformational change from a spirolactam form to a ring-opened amide form has been studied by Wang et al. in Ir(III) complexes (74a–74c) (Scheme 10).¹²⁰ The UV-vis absorption spectra of complexes 74a–74c displayed no significant absorption in the region of 400–600 nm. However, addition of Cu²⁺ ions to complexes 74a–74c generated a new absorption band at 555 nm with a 50 fold increase in absorption signal, causing a naked-eye color change from colorless to pink.

Scheme 10 Cyclization of a spirolactam form to a ring opened amide upon addition of Cu²⁺.

The reaction of 5,6-diamino-1,10-phenanthroline and salicylaldehyde in dry CH₃OH resulted in formation of a di-Schiff base, whose Ir(III) complex (75)¹⁰⁸ underwent a color change from colorless to yellow upon addition of Cu²⁺ ions. The fluorescence intensity of complex 75 in CH₃CN appeared as an intense orange emission at 560 nm (ϕ = 0.39) (λ_ex = 355 nm). The band quenched completely (ϕ = 0.0031) with selective addition of Cu²⁺ ions. Electron transfer from metal center to ligand influenced the photophysical properties of 75.

Schmittel et al. synthesized different aza crown ether appended Ir(III) complexes (76a) and (77a) that showed ECL changes with addition of metal ions and these properties were compared to structurally analogous Ru(II) complexes 1b and 77b.^24,121,122 As revealed in Table 12, the metal ion binding to the aza crown ether sites of ligands 76 and 77 induced much more intense influence on Ir(III) complexes (76a and 77b) than on Ru(II) complexes (1b and 77b) and also, the Ir(III) complexes exhibited a red-shift of the emission wavelength depending on the metal cation, while the Ru(II) complexes did not display any shift. This divergence in the behaviour of Ir(III) and Ru(II) complexes was supported by DFT calculations that revealed the azacrown ether phen part to be LUMO (lowest unoccupied molecular orbital) in Ir(III) complexes, but is part of HOMO (highest occupied molecular orbital) in Ru(II) complexes. As a result, the e⁻-transfer equilibria of the oxidative-reduction ECL process led to preferential formation of metal free excited Ru(II) systems i.e. only 1b* and 77b*. On the contrary, oxidation of Ir(III) complexes 76a and 77a involved the iridium phenylquinoline part, which was not invoked in metal-ion binding and thus, remained coordinated to metal ions in the excited state also. The PL enhancement with Ag⁺ was due to weakening of the e⁻-donating ability of the nitrogen of the aza crown ether moiety upon complexation with Ag⁺ ions. However, the PL changes of 77 upon Hg²⁺ addition was a combined result of emission-enhancing effect due to decreased nitrogen e⁻-donating ability upon Hg²⁺ binding and e⁻-or energy-transfer quenching effects of unbound Hg²⁺ ions.

2.3.2 Anions. The weak luminescence properties of bis(ferrocenyl)phenanthroline Ir(III) complex (78)¹²³ were increased by a factor of ∼130 at λ_em 580 nm with addition of CN⁻ ions, only due to its attack on the Ir(III) center of 78, leading to a displacement of phen ligand. In contrast, the ECL channel was exploited to selectively monitor F⁻ by >60-fold enhancement. This was due to strong ferrocenium–F⁻ interaction preventing reduction at the ferrocenium unit of 78²⁺, allowing e⁻-transfer from the TPrA radical to the LUMO of the oxidized iridium unit rather than to the F⁻–ferrocenium unit. Thus, formed *78⁺–F⁻ was luminescent due to its *Ir(III) excited state and lack of intramolecular e⁻-transfer quenching.

Huang et al. studied the tuning of the photophysical and electrochemical properties of various Ir(III) complexes (79a–79c)¹²⁴ containing different imidazo[4,5-f]-1,10-phenanthroline derivatives by the addition of anions and protons. Upon addition of F⁻, AcO⁻ and H₂PO₄⁻ to CH₂Cl₂ solution of 79a–79c, the emission was quenched completely and their solution colors changed from yellow–green to brown. UV-vis spectra showed that upon addition of F⁻, AcO⁻ and H₂PO₄⁻ ions, the absorbance bands of 79a–79c at 277 nm and 404 nm gradually decreased and three new bands at 300, 338 (IL π–π*) and 476 nm (MLCT) appeared. The interaction of NH in the imidazolyl group of the complexes with said anions was responsible for significant optical variations. Moreover, addition of CF₃COOH into CH₃CN solutions of 79a–79c resulted in an emission color change from yellow to red.

Other such Ir(III) complexes (80a–80d)¹²⁵ possessing highly e⁻-withdrawing substituents at desired positions also gave absorption and fluorescent changes with addition of F⁻, CH₃COO⁻ and H₂PO₄⁻ ion, in a manner similar to that observed in complexes 79.

2.4 Chemosensors based ON Re(I) complexes of 1,10-phenanthroline ligands

The MLCT of luminescent Re(I) tricarbonyl diamine complexes are well documented to show large responsive changes towards host–guest complexation.^126–130

2.4.1 Cations. Yam et al. synthesized different Re(I) diimine complexes (81) that gave differential optical changes with Hg²⁺, Ag⁺ and Pb²⁺ ions depending on the size of the crown ether cavity and the degree of hardness/softness of the donor atoms in the crown ether (Table 13).¹³¹ The CH₃CN solutions of complexes (81a–81d) displayed absorption bands at 250, 290 (IL π–π* bands) and 390 nm (MLCT band). The binding constant of 81d with Hg²⁺ was less than that of 81a due to a larger cavity size and less soft atoms in 81d as compared to 81a. The complexes 81a–81d displayed an emission band at ∼555 nm that was enhanced and red-shifted to varying degrees upon addition of Hg²⁺, Ag⁺ and Pb²⁺ ions. These emission changes were attributed to metal ion coordination to the crown cavity, that reduced the quenching ability of donor atoms by PET (led to enhancement) and stabilized the π* orbital of phen by decreasing its σ-donating ability (responsible for the red shift).

Table 13 log K_s of 81a–81d for metal ions in CH₃CN

	log Ks with Hg^2+	log Ks with Pb^2+	log Ks with Ag^+
a The spectral changes were too small for an accurate determination.
81a	7.20 + 0.08	^a	^a
81b	^a	3.26 + 0.01	^a
81c	log K1 = 5.93 + 0.15; log K2 = 5.33 + 0.55	3.83 + 0.01	^a
81d	5.46 + 0.06	3.02 + 0.02	3.49 + 0.04

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Re(I) polypyridine complexes containing the tyramine-derived 2,2′-dipicolylamine (DPAT) unit (82a–82c)¹³² showed observable optical changes with Zn²⁺ and Cd²⁺ ions. An increase in emission intensity (I/I₀ ∼ 1.8 to 3.9) and emission lifetime (τ/τ₀ of ∼1.4 to 1.6) was observed on adding Zn²⁺ and Cd²⁺ to complexes 82a–82c. Coordination of the metal ions to the DPAT unit via its amine inhibited self-quenching of the complexes, resulting in emission enhancement and lifetime extension.

Axially functionalized Re(I) complexes (83a–83c)¹³³ were synthesized by Pope et al. using a reductive amination method utilising 4-aminomethylpyridine and various carboxaldehyde derivatives for recognizing specific analytes and modulating visible phosphorescence output. Addition of Zn²⁺ and Cu²⁺ ions to 83a and 83b induced, respective, increases and decreases of the emission intensity. The complex 83c showed a more pronounced change with Hg²⁺ ions as compared to Zn²⁺ or Cu²⁺ ions. The characteristic ³MLCT luminescence lifetime of the emission suggested varying degrees of excited state quenching, depending on the nature of the axial ligand, enabling different complexes (83a–83c) to allow detection of a given cation in a competing ionic mixture.

2.4.2 Anions. Based on the strategy that incorporation of an appropriate chromophore into a luminophore with emission at an energy region, where the absorption of the chromophore would change upon anion binding, Lo et al. synthesized and studied anion-binding properties of the Re(I) polypyridine complex (84a)¹³⁴ bearing a thiourea-anthraquinone moiety and its anthraquinone free analogue (84b). The absorption spectra of 84a and 84b in CH₃CN displayed intense absorption bands in the range 249–299 nm with a less intense absorption shoulder at 362 nm and a low energy band at 435 nm observed for 84a only. The addition of F⁻, AcO⁻ or H₂PO₄⁻ to a yellow solution of 84a resulted in a bathochromic shift of the 435 nm band to 505 nm accompanied by a color change to purple, owing to binding of F⁻ to thiourea hydrogen atoms. Though 84b also interacted with F⁻ in a manner parallel to 84a, due to a lack of chromophore (anthraquinone), the spectral changes were not induced. The emission intensities of both 84a and 84b were quenched upon addition of F⁻, AcO⁻ and H₂PO₄⁻ ions with the red shift of the 535 nm band to 596 nm for 84a only.

Mono- and di-nuclear Re(I) complexes (85a) and (85b) bearing thiourea H bonding unit(s) showed binding with F⁻, AcO⁻, and H₂PO₄⁻ ions in a 1 : 1 and 1 : 2 stoichiometric manner, respectively.¹³⁵ Addition of F⁻, AcO⁻, and H₂PO₄⁻ ions to a solution of 85a and 85b caused a gradual increase in absorption at 350 and 440 nm. The ¹H NMR titrations showed that addition of the said anions to both complexes caused broadening of the signals initially, and then a quick disappearance of the two NH signals (∼10.25 ppm). The Re(I) centre, being an excellent e⁻- withdrawing group, acidified the NH group and the extended π conjugation of the phen stabilizing the deprotonated form of the receptor by a delocalization of charge.

Incorporation of the triarylboron moiety through an alkynyl ligand to the Re(I) complexes (86a–86c)¹²⁶ perturbed the optical properties of the MLCT excited state of these complexes and recognized F⁻ ions. Addition of F⁻ ions to solutions of 86a–86c caused only a decrease in the absorption intensity of low-energy band along with a naked eye color change. The emission spectra of 86a–86c showed a diminution of the emission intensities with addition of F⁻ ions. This quenching was attributed to coordination of F⁻ ion to the empty p orbital of the boron atom. This coordination blocked the ICT and interrupted the π conjugation extended through to the boron atom, leading to phosphorescence quenching.

2.5 Chemosensors based on other metal complexes of 1,10-phenanthroline ligands

Benzo-[15]-crown-5-phenanthroline Pt(II) complexes (87a–87c)¹³⁶ selectively detected Ca²⁺ ions upon addition of various alkaline earth metal ions. Complexes 87a and 87b exhibited absorption bands in the range of 350 nm and 400–500 nm, whose intensity decreased along with the emergence of a new band at a longer wavelength (∼480 nm) with addition of Ca²⁺ ions. The emission spectra of 87a and 87b showed emission peaks in the range of 430–550 and 570 nm, respectively, whose emission intensity decreased accompanied by a bathochromic shift of 16 nm (λ_ex = 365 nm). Here, the benzo-[15]-crown-5 moiety intensified the e⁻-accepting ability of the phen ligand and decreased the energy of LUMO (π*). The role of the crown-ether unit in 87a and 87b in recognition of Ca²⁺ was ascertained by the absence of any optical changes observed in the spectra of the crown ether-free phen platinum complex 87c.

The binuclear metal complex (88)¹³⁷ possessing indole-3-acetic acid and phen units undergoes optical changes with addition of Pb²⁺ amongst Na⁺, K⁺, Mg²⁺, Ca²⁺, Mn²⁺, Cr³⁺, Fe³⁺, Ni²⁺, Cd²⁺, Co²⁺, Cu²⁺, Hg²⁺, Ag⁺, Pb²⁺ in DMF/H₂O (1 : 9 v/v). The emission band of 88 was observed at 345 nm which underwent nominal changes with addition of the aforementioned cations. However, a new emission peak emerged at 602 nm with selective addition of Pb²⁺ ions, which was attributed to the cation exchange reaction of Pb²⁺ ions with Cd²⁺ ions and interaction of Pb²⁺ ions with indole groups, inducing fluorescent enhancement.

3. Chemosensors based on 5-mono- or 5,6-disubstituted derivatives of 1,10-phenanthroline ligands

Phen is known to form complexes with metal ions that can intensely absorb light and transfer the energy to the metal center enabling phen derivatives to act as attractive chemosensors.^138–143 Substituted imidazole-[4,5-f]-1,10-phenanthroline derivatives utilize two nitrogen atoms of phen and the other two nitrogen atoms of the imidazole unit to act as metal ion binding sites. The protonation and deprotonation of the imidazole moiety due to acid–base addition also induces large energy perturbations.

3.1 Cations

A podand possessing imidazo-[4,5-f]-1,10-phenanthroline groups at both terminals (89) and its Ru(II) complex (90) led to different fluorescence changes with addition of Li⁺, Na⁺, K⁺, Ca²⁺, Ba²⁺, Mg²⁺ ions.¹⁴⁴ With the addition of the said cations, the fluorescence emission intensity of DMF solution of 89 decreased accompanied by different red shifts, contrasting the increased emission intensity with no red shifts for 90 under comparable conditions. Due to good complementarity between the pseudocavity of the podand and the ionic radii of Mg²⁺, a large decrease of monomer emission and corresponding increase in excimer emission at 503 nm (λ_ex = 285 nm) was observed in the fluorescence spectra of 90 only upon binding with Mg²⁺ as intramolecular excimer formation is prevented in 90 due to enlarged terminal groups on Ru coordination. ICT occurred from the amino group of imidazole (donor) to the oxygen atom attached to the benzene ring (acceptor) as the presence of a benzene ring reduced the e⁻-donating character of oxygen atoms attached to them, making them conversely change into the ‘acceptor’ compared with the amino group in imidazole, hence producing a red shift of the emission spectra. However, the intramolecular PET from oxygen lone pairs to the Ru(II) center was suppressed upon metal complexation in 90, causing fluorescent enhancement.

Relying on the anticipation that coordination of chemosensors with metal ions would provoke enhanced ICT assigned to IL π–π* transitions, Wang et al. synthesized chemosensors (91a)¹⁴⁵ and (91b)¹⁴⁶ for selective detection of Co²⁺ and K⁺/Co²⁺, respectively in DMF-buffered solution. The changes in the UV-vis and fluorescence spectra of 91a and 91b with Co²⁺ and K⁺/Co²⁺ as shown in the Table 14 well established the sensing behaviour of 91a and 91b.

Table 14 Absorption and emission changes in 91a and 91b with addition of Co²⁺ and K⁺ ions

	λab changes (nm) Co^2+	λab changes (nm) K^+	λem changes (nm) (process) Co^2+	λem changes (nm) (process) K^+	Stoichiometry M : L	log Kass
a Not determined.
91a	318, 327 → 350	—	441 nm quenching	—	2 : 1 (Co^2+)	ML (4.75), M2L (4.07)
91b	308, 336 → increase, 336 → 348	308, 336 → increase, 336 → 330	442 nm quenching	442 nm enhancement	2 : 1 (Co^2+), 1 : 1 (K^+)	^a

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The binding stoichiometry of 2 : 1 with Co²⁺ of both 91a and 91b corresponded to binding of one Co²⁺ to two N’s of phen, while the other coordinates to two N’s of the imidazole–pyridine rings and imidazole–thiophene rings, respectively. K⁺, however coordinated in a 1 : 1 stoichiometry involving phen N’s only (Scheme 11). A similar phenol–substituted imidazole–derivative (91c)¹⁴⁷ has also been reported by Jayabharathi et al. for selective complexation with Co²⁺ in a 1 : 1 DCM/H₂O mixture resulting in a decreased fluorescence intensity.

Scheme 11 Binding mode of Co²⁺ and K⁺ with 91b.

Hybrid of tpy and phen (91d)¹⁴⁸ acted as a multifunctional optical sensor for multiple analytes in CH₃CN/DMF (43 : 1, v/v). The chemosensor 91d displayed absorption bands at 282 and 339 nm in the UV-vis spectrum. Upon addition of F⁻ ions to 91d, the hydrogen bonding interactions of N–H of imidazole with F⁻ ions become operational via a two step mechanism of protonation–deprotonation resulting in a red shifted CT band at 397 nm changing the colorless solution of 91d to yellow. The F⁻ addition quenched the emission intensity of 91d at 446 nm (77%). However, with Fe²⁺, the colorless solution changed to magenta due to the MLCT band at 575 nm accompanied by a decreased absorbance at 339 nm. 91d remained silent towards Fe³⁺ addition and hence is capable of distinguishing two oxidative species unlike other terpyridine containing chemosensors. The intensity of the emission peak at 446 nm was quenched and showed red shifts of 88 nm and 76 nm with the addition of Zn²⁺ ions and Cd²⁺ ions, respectively.

The boronic acid unit possessing imidazo-phen scaffold (91e)¹⁴⁹ induced chromogenic and fluorogenic responses to Fe²⁺ and Zn²⁺ ions, respectively. The broad absorption band of 91e between 275–350 nm red-shifted to 528 nm, resulting in a color change to red with addition of Fe²⁺ ions, only. Other competitive transition metal ions do not give any absorption or color changes. On the other hand, binding of Zn²⁺ ions to 91e caused a fluorescence enhancement at 498 nm due to the blocking of PET between imidazo-phen and phenyl boronic acid units.

The reaction of 2-p-tolyl-imidazo[4,5-f]-1,10-phenanthroline with SeO₂ in dioxane solution yielded (92)¹⁵⁰ in which one of the nitrogens becomes protonated. Two water molecules were also crystallized along with 92 and act as hydrogen bond donors and acceptors along with improving the aqueous solubility. The otherwise quenched luminescence intensity of 92 (due to the vibration of O–H of water molecules) was dramatically enhanced upon titration of Zn²⁺, resulting in green colored fluorescence. Other metal ions such as Na⁺, K⁺, Mg²⁺, Ca²⁺, Mn²⁺ and Cr³⁺ do not give any response even at a 40-fold excess concentration. However, some paramagnetic metal ions (Fe³⁺, Cu²⁺, Co²⁺, Ni²⁺) quenched the fluorescence. Confocal fluorescence microscopy confirmed that 92 could be used for monitoring Zn²⁺ in living cells.

Li and Zhu et al. have reported dipyrido[3,2-a: 2′,3-c]phenazine (dppz) containing phen based conjugated polymers (93, 94)^151,152 synthesized by the Suzuki coupling reaction as selective chemosensors for Ni²⁺ and Cu²⁺, respectively. Table 15 reveals the photophysical changes induced in 93 and 94 upon addition of Ni²⁺ and Cu²⁺, respectively. The rigid planar structure of the dppz unit prevented metal ion induced conformational change of the polymer backbone. No observable changes were seen in the UV-vis and fluorescence spectrum of the phenanthrene analogue of 93 upon Ni²⁺ addition establishing the coordination of Ni²⁺ to phen N’s while ¹H-NMR spectral studies confirm coordination of Cu²⁺ to N’s of dppz in 94. Similarly Nair et al. also reported phen based fluorescent chemosensors (95a and 95b) for sensing Ni²⁺ and Cu²⁺, synthesised by condensation of a diamine with a diketone.¹⁵³ A complete quenching of the emission bands of 95a and 95b, respectively, at 513 and 503 nm was observed in the presence of Ni²⁺ and Cu²⁺ ions. The chemosensor 95a could detect 0.4 ppm of Ni²⁺ (K_a = 2.51 × 10⁷ M) and 0.35 ppm of Cu²⁺ (K_a = 8.96 × 10⁶) and in the case of 95b, the detection limits were 0.2 ppm (K_a = 8.13 × 10⁶ M) for Cu²⁺ and 0.4 ppm (K_a = 3.75 × 10⁶ M) for Ni²⁺ ions.

Table 15 Absorption and emission changes in 92 and 93 upon addition of Ni²⁺ and Cu²⁺ ions

	λab changes (nm) (Ni^2+)	λab (nm) changes (Cu^2+)	λem changes (nm) (process) (Ni^2+)	λem changes (nm) (process) (Cu^2+)
a Solvent system: THF.b Solvent system: CHCl3: C2H5OH (80 : 20, v/v).
92^a	(253 → 266), (320 → 332)	—	614 quenching (50%)	614 quenching (75%)
93^b	—	(350 → 400) → (400–477)	—	556 quenching (100%)

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Other simple dppz possessing phen based fluorescent chemosensors (96 and 97),^154,155 differing only by the presence/absence of phenyl ring recognized Hg²⁺ and Zn²⁺ ions, respectively in DMF buffered solution. The chemosensor 96 gave enhanced fluorescence with addition of Hg²⁺ ions, attributed to the formation of the 96–Hg²⁺ complex with a stability constant of 3.7 × 10⁵ M⁻¹. On the other hand, chemosensor 97 differing only by the absence of a phenyl ring selectively recognized Zn²⁺ ions, with a relative fluorescence intensity four times that of any other metal ion.

Hudhomme et al. described the synthesis of fused extended tetrathiafulvalene–dipyridoquinoxaline (TTF–dpq) (98)¹⁵⁶ dyad using the Horner–Wadsworth–Emmons olefination method, which acted as a dual redox and colorimetric sensor for Fe²⁺ amongst Ni²⁺, Ca²⁺, Pb²⁺, Zn²⁺ ions. Upon addition of Fe(ClO₄)₂ to DCM solution of 98, the IL band at 470 nm decreased with the emergence of two new bands at 492 nm (MLCT d-π*) and 600 nm (ILCT). The ILCT band occurred due to the transition from donor tetrathiafulvalene to acceptor dipyridoquinoxaline unit. A quasi reversible 2e⁻ oxidation wave at E_ox = +0.56 V showed a positive shift upon progressive addition of aliquots of Fe²⁺ ions to a CH₂Cl₂ solution of 98, as the redox potential changed appreciably upon cation coordination with an anodic shift owing to difficult oxidation of the sensor–cation complex.

A ferrocene-imidazophenazine dyad based chemosensor (99)¹⁵⁷ showed its ability to sense Hg²⁺ via three different channels (electrochemical, absorption and emission) as revealed in Table 16.

Table 16 Three channel response of dyad 97 to Hg²⁺ in CH₃CN

	Redox changes (V)	Absorption (λab) changes (nm)	Emission (λem) changes (nm) (process)
99–Hg^2+	E1/2(ox) = 0.52–0.72	323, 385 → 362, 485	512 → 544 enhancement (CHEF = 165)

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Both spectral changes and electrochemical results of 99 with anions (AcO⁻, H₂PO₄⁻, F⁻, HP₂O₇³⁻, Cl⁻, Br⁻, NO₃⁻ and HSO₄⁻) revealed the formation of a hydrogen bonded complex and deprotonation with AcO⁻ and H₂PO₄⁻ while only deprotonation occurred with HP₂O₇³⁻ and F⁻ ions.

An amidic functionality possessing dye, belonging to the class of PET sensors Phen Green™SK (100)¹⁵⁸ was used to form complexes with Cu⁺ and Cu²⁺ ions. On varying the Cu⁺ concentrations (0–10 μM) in the presence of 0.5 M NaCl, the fluorescence intensity was quenched until saturation (2 μM), leading to a binding constant of 10^11.38. The optimized experimental conditions avoided oxidation of Cu⁺ to Cu²⁺ and EDTA addition (competing ligand) eliminated interference of Cu²⁺ in the quenching process giving a conditional stability constant of 100 with Cu²⁺ as 10^9.8. Another amidic functionality based chemosensor (101)¹⁵⁹ has been reported by Shah et al. to sense and distinguish Fe³⁺ colorimetrically (colorless to light pink) from other chemically similar ions Co²⁺, Cr³⁺, Cu²⁺, Mn²⁺, Ni²⁺, Ag²⁺, Zn²⁺ in an aqueous medium.

Liu and Jung et al. synthesized and studied phen containing Cu²⁺ selective fluorescent chemosensors (102 and 103)^140,160 that worked on the principle of reverse PET when Cu²⁺ bound to the nitrogen atoms of the phen unit behaved as a PET donor. 102 was synthesized by reaction of 5-amino-1,10-phenanthroline with 2-(3-isocyanato-propionyloxy)ethyl methacrylate. Emulsion polymerization of N-isopropylacrylamide in the presence of anionic surfactant N,N′-methylenebis(acrylamide) and 102 at neutral pH resulted in the formation of near-monodisperse Cu²⁺-sensing microgels. In contrast, 103 was adsorbed onto functionalized silica nanotube (FSNT) by a sol–gel reaction. The fluorescence intensity of both 102 and 103 at 452 and 451 nm, respectively, was quenched upon addition of Cu²⁺ ions in aqueous medium and CH₃CN, respectively. When microgels were heated above 32 °C, an ∼33% increase in the fluorescence intensity was observed due to their volume phase transfer (Scheme 12). The quenching efficiency of Cu²⁺ ions to microgels was considerably improved via thermo-induced microgel collapse, by which LOD values were enhanced from ∼28 nm at 20 °C to ∼8 nm at 40 °C. The adsorption of 103 and Cu²⁺ selectivity was also explored with functionalized mesoporous silica (FMS) and functionalized silica nanoparticles (FSNP-15) with an immobilized phen moiety in 2008 in H₂O/CH₃CN (8 : 2, v/v) system at pH 7.¹⁶¹ The varied Cu²⁺ adsorption capacities of FSNT (75%), FMS (52%) and FSNP-15 (36%) was dependent on the shape, surface area and extent of dispersion of supporting nanomaterials in the solvents.

Scheme 12 Detection mechanism of microgel-based Cu²⁺-chemosensors via thermo-induced microgel collapse.¹⁶⁰

Wang and co-workers also formed an organic–inorganic amalgamated fluorescent material (104)¹³⁸ bearing 2-substituted imidazole-[4,5-f]-1,10-phenanthroline derivative immobilized on a silica surface by a sol–gel reaction, which showed excellent selectivity for Pb²⁺ ions amongst several investigated metal ions. The adsorption and desorption of metal ions on amalgamated material varied with pH moderation of the phen ligand hence making it suitable as an off–on–off sensor.

An excited state intramolecular proton transfer (ESIPT) enabled phen diimino phenol (105)¹⁶² selectively recognized Hg²⁺ ions in aqueous medium, while showed colorimetric F⁻ ion detection (light yellow to pink) in non aqueous medium (CH₃CN). In the fluorescence spectrum, upon Hg²⁺ addition, a weak band at 466 nm (due to excited enol) remains unaltered while emission at 617 nm (due to excited keto) intensified accompanied by a red shift signifying that complexation of 105 with Hg²⁺ ions facilitated the formation of the keto tautomer due to achieving the best fit according to the size of the cavity available in the keto form rather than the enol form. Thus, Hg²⁺ specifically controls the extent of tautomerism through ESIPT, ultimately favouring the keto form. The phenanthroline N’s have been proposed to possibly increase the acidity of phenolic hydrogen for ESIPT rather than being directly involved in the coordination.

Hydrazone based chemosensors (106a) and (106b)¹⁶³ showed selective binding to Pb²⁺ ions even at the micromolar level, enabling naked eye color change from yellow to red. The UV-vis spectra of 106a revealed an absorption band at 352 nm in CH₃CN/H₂O (9 : 1, v/v) (pH 7.4), whose intensity decreased with the concomitant appearance of a new peak at 483 nm upon addition of Pb²⁺ ions. The azo form of the receptor generated from the hydrazo moiety was probably responsible for generation of the red shift and the color change. Nitrogen atoms of the phen core probably do not get involved in the binding process due to protonation of the phen ring nitrogen atoms that stabilized the azo form with the hydrogen atom attached to it. The emission maxima of 106a at 395 nm (λ_exc = 352 nm) shifted to 401 nm displaying the CHEF effect, only upon Pb²⁺ ion addition. Receptor 106b also displayed similar color and optical changes with Pb²⁺ ions, but at a much higher concentration compared to that of 106a.

Advanced carbon surfaces were created by grafting and polymerization of phen derivatives (poly-107a and poly-107b) on the glassy carbon (GC) electrode as well as GC electrodes modified by electrochemically reduced forms of poly-107a and poly-107b, which determined Cu²⁺ ions using differential pulse voltammetry.¹⁶⁴ The interaction of Cu²⁺ ions was studied with the modified surfaces to ensure the formation of polymeric layers on the GC electrode under optimized conditions (BR buffer, pH = 5, 10 mM of Cu²⁺, 1 h incubation). Electrochemical reduction of poly-107a and poly-107b lowered the capacity to bind Cu²⁺ due to their positively charged electrode surfaces. However, the presence of nitro groups in poly-107a and poly-107b made the electrode surface negative and enhanced their tendency to bind Cu²⁺ ions. The LOD values of Cu²⁺ ions by poly-107a/GC and poly-107b/GC were evaluated to be 0.8 × 10⁻¹⁰ M and 0.2 × 10⁻¹⁰ M, respectively.

A similar concept has been used by Kumar and co-workers¹⁶⁵ to electro-oxidize phen in aqueous medium as electro-oxidation potential of phen was much higher than the water decomposition potential, making its electro-oxidation practically difficult in aqueous medium. This was performed by adsorbing the phen derivative, (108), on a multiwalled carbon nanotube (MWCNT) and modified glassy carbon electrode (GCE) surface in an aqueous medium (phosphate buffer, pH = 7), resulting in formation of GCE-MWCNT-108. Electrochemical oxidation of phen was possible due to strong π–π interactions between 108 and MWCNT and limited diffusion of 108 on the surface of MWCNT. GCE-MWCNT-108 showed selective recognition of Cu²⁺ ions. This resulted in formation of the GCE-MWCNT-108–Cu²⁺ complex that displayed an obvious hydrogen peroxide electrocatalytic signal. The formation of GCE-MWCNT-108–Cu²⁺ system was ascertained by SEM, TEM, XPS. The model compound 9,10-phenanthrenequinone (without nitrogens) did not show hydrogen peroxide electrocatalytic signal, indicating the binding of Cu²⁺ to N’s of phen in GCE-MWCNT-108–Cu²⁺. Moreover 2,2′-bipyridine, the heterocyclic analogue of phen was non-responsive to adsorption on MWCNT-GCE, indicating selective recognition of phen ligand by MWCNT-GCE by electro-oxidation.

A photoswitching system based on two photochromic dithienylethenes with imidazole containing substituents (109a and 109b)^166,167 was realized for multi-addressable information transmission at the unimolecular platform (Scheme 13). The absorption spectra of 109a¹⁶⁶ and 109b¹⁶⁷ exhibited sharp absorption within 280–285 nm, which upon photoradiation, converted into their closed ring isomers with absorption maxima lying in the range of 600–620 nm and the color of the solution changed from colorless to blue. Addition of Cu²⁺ ions to solutions of 109a and 109b resulted in a blue shifting of the absorption maxima by 85 nm, only in the case of 109a. However, with both 109a and 109b, an ∼95% decrease of absorption intensity along with a color change from blue to colorless was observed. Amongst various tested ions (Fe³⁺, Cu²⁺, K⁺, Mg²⁺, Ca²⁺, Ba²⁺, Pb²⁺, Zn²⁺, Hg²⁺, Co²⁺, Cd²⁺, Ni²⁺, Mn²⁺, Al³⁺, Sr²⁺, Cr³⁺, Fe³⁺, Sn²⁺) fluorescence of both 109a and 109b was significantly enhanced by Ca²⁺, Mg²⁺ and Sr²⁺ ions. The receptor 109b showed an emission peak at 454 nm (λ_ex = 363 nm), whose intensity increased dramatically (14 fold) accompanied by a 33 nm bathochromic shift of 454 nm band (487 nm) only upon addition of Ca²⁺ ions, resulting in a color change from dark to bright cyan turning “on” the “off” the fluorescence. Fluorescence of 109b–Ca²⁺ was also quenched upon irradiation with UV-vis light due to the formation of a non-fluorescent closed ring isomer. Photo-induced electron transfer was responsible for fluorescence enhancement. Modulation of the emission intensity of 109a and 109b was possible by either light or chemical stimuli, hence, combinational logic circuits with four inputs In₁ (297 nm UV light), In₂ (>500 nm visible light), In₃ (Ca²⁺) and In₄ (EDTA) and one output (emission intensity at 444/487 nm) was also realized.

Scheme 13 Photoswitching between open and ring closed systems with UV and visible light in dithienylethenes.

3.2 Anions

In anion recognizing sensor systems, the imidazole unit has been widely utilized with both (C–H)^168–171 or (N–H)^172,173 as recognition sites. The acidity of the NH group of the imidazole group can be easily tuned by modulating the electronic properties of the adjacent substituents in such a way that anion recognition may proceed either through hydrogen bonding or deprotonation.^174–178 The significant absorption and fluorescent changes in anion chemosensors (110–112) are summarized in Table 17.

Table 17 Absorption and emission changes in 110–112 upon adding various anions

	Color and λab (nm) change F^−, AcO^−, H2PO4^−	Color and λab (nm) change Cl^−, Br^−, I^−	λem changes (nm) (process) F^−, AcO^−, H2PO4^−	λem changes (nm) (process) Cl^−, Br^−, I^−	log Ka in the order F^−, AcO^−, H2PO4^−, Cl^−, Br^−, I^−
a Solvent system: DMSO.b CH3CN.c Not determined.
110^a	Transparent → light yellow, 283, 320 → decrease	—, —	443 → 523, quenching (F^−, AcO^−)	—	6.65 (F^−), 6.31 (AcO^−)
111^a	Pale yellow → yellow, 310, 329, 346 nm → increase	325, 400 nm → increase (Cl^−)	^c	^c	2.1 (F^−), 3.1 (AcO^−), 2.5 (H2PO4^−), 1.8 (Cl^−), 1.09 (Br^−)
112^b	266 → 270, 320 → 325 (F^−), 266, 320 → increase (H2PO4^−, AcO^−)	266, 320 → increase (Cl^−)	422, quenching	422 → 428 (Cl^−), enhancement	5.19 (AcO^−), 4.35 (H2PO4^−), 3.84 (Cl^−)

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The imidazole NH of three-arm phen derivative (110)¹⁷⁴ exhibited hydrogen bonding interactions with AcO⁻ and at lower F⁻ ion concentrations while at its higher concentrations, a new green emitting peak at 523 nm corresponded to a strong basic behaviour of F⁻ ions hence, showing high affinity for H⁺ leading to deprotonation (HF₂⁻ is formed). The effect was not observed for AcO⁻ ions.

The AcO⁻, F⁻ and H₂PO₄⁻ anions responded differently from Cl⁻, Br⁻ and I⁻ towards spectral changes for receptor (111).^{179 1}H NMR titrations and absorption changes of 111 with AcO⁻, F⁻ and H₂PO₄⁻ anions pointed to deprotonation of imidazole NH while new absorption bands observed at 325 and 400 nm on Cl⁻ addition reflected the hydrogen bonding interaction of H* of phen with Cl⁻, thus making a supramolecular complex as shown by 111–Cl⁻ complex. Gupta and co-workers made an ion-selective membrane based on 111¹⁸⁰ that served as a potentiometric sensor for Cl⁻ ion in PVC (poly-vinyl chloride) matrix in a similar manner as proposed for the 111–Cl⁻ complex with a LOD value of 1 × 10⁻⁸ M.

A phen based urea ligand 112¹⁸¹ reported by Gunnlaugsson and co-workers showed hydrogen bonding interactions with AcO⁻, H₂PO₄⁻ and Cl⁻ ions while deprotonation of NH of the urea group with F⁻ ion. The fluorescence quenching was attributed to activation of electron transfer quenching of the phen excited state from the electron rich anion receptor (anion promoted PET quenching). However, a minor twist of the urea moiety by spherical Cl⁻ ion in addition to a reduction in the PET quenching effect of the phen fluorophore by trifluoro-p-tolyl led to an increased fluorescence intensity combined with a red shift. The initial additions of Cl⁻ to 112 resulted in self assembly with a Cl-(112)₂ type of complex; while formation of a 1 : 1 complex dominated thereafter.

4. Chemosensors based on 2-mono- or 2,9-disubstituted derivatives of 1,10-phenanthroline ligands

4.1 Cations

Bencini et al. synthesized a series of polyamine macrocycles (113–118) containing a phen unit as an integral part of the cyclic framework by reaction of tosylated polyamine with 2,9-dibromomethyl-1,10-phenanthroline by modification of the Richman and Atkins’ method. Different macrocyclic polyamines containing phen units have been used as chemosensors for binding H⁺ and metal ions in solution. Phen being a large and rigid aromatic unit, gives rise to a stiffened macrocyclic structure and thus form more stable complexes than saturated polyamine ligands containing the same number of nitrogen donors. Protonation and potentiometric studies have been carried out on the interaction of polyamine macrocycles 113a–113c with Pb²⁺ and Zn²⁺ ions.^182–184 The macrocycles 113a–113c formed stable water soluble 1 : 1 Pb²⁺ complexes whose stability decreased from 113a to 113c due to a marked decrease of the enthalpic contribution to the formation of complexes. The absorption spectrum of 113a–113c displayed a sharp band at ∼273 nm, whose intensity decreased with the addition of Pb²⁺ ions until a 1 : 1 molar ratio was reached. Due to the stiffened structure of 113a–113c, both the benzylic nitrogens are in the endo conformation and in close proximity to the phen core, so they do not get involved in coordination of Zn²⁺ ions. Because of the interaction of the lone pairs of the benzylic nitrogen atoms with the phen group, the fluorescence emission of free 113a–113c was quenched. By protonation or Zn²⁺ complexation, the benzylic nitrogen atoms were deactivated, thus, avoiding a quenching effect. On moving from 113a to 113c, the stability of their Zn²⁺ complexes decreased. This indicated that a larger number of uncoordinated donor atoms formed larger, less stable chelate rings and thus, reduced the complex stability. On the other hand, an increasing number of uncoordinated amine groups favoured the formation of protonated complex species and allowed 113c to have enough free donor atoms to bind a further metal ion. Macrocycle 113a also showed fluorescence quenching with the addition of Cd²⁺ and Hg²⁺ ions,¹⁸⁵ either due to a heavy atom effect or to a weak involvement of the benzylic nitrogens in metal coordination.

Moreover, the formation constants of mono- and bi-nuclear complexes of 113a with Cu²⁺ were also determined by potentiometric measurements in aqueous solution.¹⁸⁶ Mononuclear complexes were prevalent in solutions containing 113a and Cu²⁺ in a molar ratio less than 1, indicating the involvement of a phen moiety in metal coordination. However, with a 113a : Cu²⁺ molar ratio more than 1, binuclear complexes were formed, indicating that the second metal was bound to nitrogens of the pentaamine chain (Scheme 14). The macrocycle 114 formed a complex with Cu²⁺ in a similar manner as formed by 113a. However, in the binuclear 114 : Cu²⁺ complex, the piperazine rings led to inversion from the chair to a less energetically favoured boat conformation.

Scheme 14 Proposed coordination modes of Cu²⁺ in (a) [Cu(113a)]²⁺ and (b) [Cu₂(113a)]⁴⁺ complexes.

Coordination of appendant anthracene functionality in 113d gave rise to Zn²⁺-induced “on–off” switching of the exciplex emission.^187,188 Potentiometric measurements of 113d : Zn²⁺ (1 : 1) solutions showed the formation of stable [Zn − 113d]²⁺ [Zn − H − 113d],³⁺ [Zn − H₂ − 113d],⁴⁺, [Zn − 113d(OH)]⁺, [Zn − 113d(OH)₂], complexes. The fluorescence emission spectra of 113d with Zn²⁺ ions showed an emission band at 418 nm due to coordination of Zn²⁺ ions to the amine groups which precluded the e⁻-transfer quenching. The excitation spectrum obtained at 600 nm was ascribed to an intramolecular π-stacking complex in the excited state, involving phen and anthracene.

The macrocycle 113e, possessing an ethylamino group as a pendant arm attached to 113a, showed that the attachment of an aminoalkyl side arm has affected both the coordination as well as photophysical properties of its Zn²⁺, Cd²⁺ and Hg²⁺ complexes.¹⁸⁵ The Zn²⁺, Cd²⁺ and Hg²⁺ complexes of 113a were non-emissive. On the contrary, insertion of an ethylamino side arm on 113a led to increased stability of M–113e complexes, accompanied by an increased tendency to form mono-protonated [M − 113e − H]³⁺ complexes. Upon mono-protonation, the involvement of the benzylic amine groups in metal coordination was enhanced as also indicated by an increase in fluorescence intensity upon protonation of non-emissive [M − 113e]²⁺ complexes (Scheme 15).

Scheme 15 Sketches of the proposed metal coordination environments in the [M − (113e)]²⁺ and [M − (113e) − H]³⁺ complexes.

Zn²⁺ coordination by the macrocyclic ligand 116 showed less stability than the open chain ligand 115, possessing the same number of nitrogen donors for metal coordination as possessed by 115, due to the replacement of ethylenic chains linking the amine groups in 115 with propylenic ones in 116.¹⁸⁹ The protonated Zn²⁺ complexes possessed two potential sites for cooperative anion binding i.e. the metal ion and the charged ammonium groups. However, complex [Zn − 115]²⁺ showed less affinity towards anion i.e. ATP as compared to complex [Zn − 116]²⁺ due to the saturated coordination sphere of the metal in [Zn − 115]²⁺, which reduced their binding ability towards ATP. In the ATP adducts with [Zn − 115/116]²⁺ complexes, the nucleotide interacted with the metal via the terminal P_γ phosphate group. In addition to coordination bonds between Zn²⁺ and the phosphate groups, the formation of salt bridges between the ammonium functions of the ligand and the anionic phosphate chain of ATP also contributed to the complex stability.

Metal coordination by116¹⁹⁰ resulted in either a decrease (in the case of Zn²⁺, Cd²⁺ and Pb²⁺ complexation) or an increase (in the case of Cu²⁺) of the absorbance of its UV band at 270 nm. Replacing the phen unit from 116 by bipyridine made the ligand able to selectively coordinate Cd²⁺ over Zn²⁺ and Pb²⁺ ions due to the presence of a tetradentate cavity composed of the less basic heteroaromatic and benzylic nitrogen atoms. On the contrary, the rigidity of phen in 116 did not allow the simultaneous involvement of both the heteroaromatic and the adjacent benzylic nitrogen atoms avoiding formation of the tetradentate cavity. This also made Zn²⁺ and Cd²⁺ complexes of 116 non-emissive.

The macrocyclic ligand (117) containing a triamine aliphatic chain linking 2,9-positions of phen and its derivative, 118, composed by two 117 moieties connected by ethylenic bridge showed differential coordination properties towards Cu²⁺, Zn²⁺, Cd²⁺, Pb²⁺ and Hg²⁺ ions.¹⁹¹ 117 formed 1 : 1 metal complexes, while 118 gave both mono- and di-nuclear complexes. Addition of Cu²⁺ to the aqueous solution of 117 resulted in an increased absorbance at 273 nm up to a 1 : 1 Cu²⁺ : 117 molar ratio. Similar changes in absorption spectra were observed with addition of the aforementioned metal ions. The formation of all the said metal ion complexes were accompanied by a marked quenching of the fluorescence emission. As 118 was composed by two equal macrocyclic binding units, so, it formed both mono- and di-nuclear complexes with the aforementioned metal ions. In the mononuclear 118 complexes, the metal ion was sandwiched between two cyclic moieties. Due to the presence of two metal ions and two hydrophobic units within 117, it also showed hydrolytic activity.

Pina and co-workers synthesized the macrocyclic receptors (119–121) containing two phen chromophores that gave potentiometric and fluorescence changes with H⁺ and metal ions. The fluorescence spectrum of 119 displayed pH dependent emission peaks at 323 (due to the naphthalene moiety), 366 (due to the phen moiety) and 460 nm.¹⁹² The peak at 460 nm was ascribed to excimer formation between two phen units due to the bending movement of pendent arm induced interaction of the two phen units. The fluorescence excitation spectra of 119 in pH range 2–11 revealed the formation of the excimer band only at pH >4 as, under highly acidic conditions, complete protonation of the polyamine N’s increased the rigidity of the complex to an extent that inhibited the bending movement and hence, excimer formation. The increase of emission intensity of the 460 nm band at pH >4 coincided with the decrease of 366 nm emission intensity; while naphthalene emission at 323 nm showed trivial changes up to pH 6. On the other hand, the excimer emission showed a maximum increase for 1 : 1 Zn/119 ratio and titration of pH dependent fluorescence of 119 with Zn²⁺ showed an excimer peak at 460 nm at pH >3.2 i.e. appearing early than in the case with only 119 (at pH > 4). Moreover the enhancement of the excimer intensity was also more for the Zn²⁺–119 complex than only 119 at identical pH values indicating that Zn²⁺ coordination intensified the interaction of two phen units as all the nitrogens of 119 were either protonated or coordinated to Zn²⁺. On the other hand, Cu²⁺ coordination quenched the emission spectra on the whole including excimer emission, accompanied by a 10 nm blue shift of the 460 nm peak at pH >4.

Another macrocyclic ligand 120¹⁹³ was synthesized by a 2 : 2 reaction of 1,10-phenanthroline-4,7-dialdehyde with 1,3-diaminopropane, followed by reduction with NaBH₄. In 120, two phen groups were linked together in such a way that the heteroaromatic nitrogen atoms pointed outside the ligand cavity and provided hindrance to the chelating ability of two 1,3-diaminopropane chains. At pH 10, 120 formed both mono- and di-nuclear complexes with Zn²⁺ ions, whose ratios varied according to Zn²⁺ : 120 molar ratios. In the solution phase, the addition of Zn²⁺ ions to 120 at pH 10 resulted in a decreased absorbance of the 230 nm band and the red-shift of the 270 nm one to 275 nm, until a Zn²⁺ : 120 molar ratio of 1 : 2 was reached. The Zn²⁺–120 complex was also pH dependent. On increasing the solution pH, the emission at 365 nm was increased due to the formation of the [Zn − H − 120]³⁺ complex until pH 3.5 and then started decreasing with the formation of the less emissive [Zn − H₂ − 120]⁴⁺ and [Zn − H₄ − 120]⁶⁺ complexes responsible for chelation enhanced fluorescence quenching (CHEQ).

However, the macrocycle 121¹⁹⁴ possessing one phen and two naphthalene units showed selectivity order for the binding of 121 with metal ions as Zn²⁺ over Pb²⁺ ⋙ Cd²⁺ over Pb²⁺ > Zn²⁺ over Cd²⁺ (Fig. 12). At pH 2.33, 121 (metal free form) gave emission bands at 325 and 365 nm due to the naphthalene and phen fluorophores, respectively. Increasing the pH of the solution of 121, complexation of Zn²⁺/Cd²⁺ caused complete quenching of phen emission (365 nm); while the naphthalene emission (325 nm) was only partially quenched along with formation of a non-structured and red-shifted broad band at ∼460 nm.

Fig. 12 Selectivity order for the binding of 121 with Zn²⁺, Pb²⁺ and Cd²⁺ ions.¹⁹⁴

This non-structured band was ascribed to exciplex formation due to an intramolecular π-stacking complex in the excited state involving phen and naphthalene ligands. In the case of Cd²⁺ coordination, naphthalene fluorescence was almost completely quenched above pH 8; while Pb²⁺ complex was completely quenched.

Similar to 120, another receptor consisting of the phen group functionalized with diamine chains (122) undergoes protonation and in the absence or presence of Zn²⁺, displays poly-recognition to the bound ATP by multiple supramolecular interactions, including coordination, stacking, H-bonding, and ion-pairing interactions and possible ion-π donor, hydrophobic/hydrophilic and van der Waals’ interactions. Owing to the different basicity and steric effect of the receptors, the Zn²⁺ complexes of 122–123¹⁹⁵ had different coordination structures and stability. The molar complexes of ATP with 122 and 123 were formed in weakly alkaline to slightly acidic solution due to electrostatic interactions by the ammonium cations with the phosphate anions as well as due to H-bonding between the protons on the ammonium and the phosphate oxygen. However, introduction of Zn²⁺ into the 122/123–ATP system provided additional stability to the mixed-ligand interactions by which the ternary Zn²⁺–122/123–ATP complex was formed in the entire studied pH range. In the ternary complex, the tendency for the phosphate chain to receive a proton and metal ion increased, which facilitated the cleavage of the phosphate chain of the nucleotide.

Hancock et al. for the first time, showed that a high level of preorganization and significant stabilization of complexes of different metal ions of a tetradentate ligand (124)¹⁹⁶ was attributed to the rigidity of its phen based backbone. The formation constant (log K) for 124 with different metal ions such as Mg²⁺ (3.53), Ca²⁺ (7.3), Sr²⁺ (5.61), Ba²⁺ (5.43), La³⁺ (13.5), Gd³⁺ (16.1), Zn²⁺ (11.0), Cd²⁺ (12.8), Pb²⁺ (11.4) and Cu²⁺ (12.8) indicated the selectivity of 124 towards larger metal ions such as Ca²⁺, Cd²⁺, Gd³⁺, with an ionic radius of about 1.0 Å. The extended aromatic system of another phen possessing ligand (125)¹⁹⁷ provided a good combination of coordinating groups and fluorophores. Ligand 125 formed five membered chelate rings and favoured complexation with metal ions having an ionic radius ∼1 Å. Hence Cd²⁺ with an ionic radius of 0.96 Å exhibited a strong CHEF effect with 125 and could be detected up to concentrations of 10⁻⁹ M. No fluorescence change was observed with Zn²⁺, Hg²⁺ and Pb²⁺, however, Ca²⁺ and Na⁺ having a similar size were capable of producing a CHEF effect but required higher concentrations for achieving the same change. This was due to a much lower thermodynamic stability of Ca²⁺/Na⁺ complexes of 124.

Naruta and co-workers synthesized phen-embedded porphyrin analogues (126a–126d) by McDonald [2 + 2] type condensations between phen vinyl ester and the corresponding aryl-dipyrromethanes. The structural features such as non-planar macrocyclic geometry, a single acidic proton and a smaller coordination sphere, facilitated complexation of 126 with Mg²⁺ ions. Upon Mg²⁺ addition, a reddish orange CH₃CN solution of 126a,¹⁹⁸ changed to purple accompanied by a red shift of the visible band from 489 nm to 532 nm and 565 nm with increased intensity. Weakly fluorescent 126a (λ_max = 572 nm, Φ = 0.003) displayed a stronger red shifted fluorescence emission (λ_max = 639 nm, Φ = 0.015, 16 fold enhancement) with slight quenching of the original emission upon complexation with Mg²⁺. Other analogues 126b–126d²⁰⁰ showed similar responses as shown by 126a towards Mg²⁺. Detailed EPR and TA spectroscopic studies of the photoinduced charge separated species in Mg²⁺–126 pointing to the fact that an internal d-PET process from the dipyrrin subunit to the phen moiety was responsible for different emissive properties of Mg²⁺–126 complexes.¹⁹⁹

A chemosensor (127),²⁰⁰ whose design combined the Mg²⁺ selective porphyrin-like macrocycle and the enhanced metal ion selectivity of 2,9-dipyridyl-1,10-phenanthroline unit, showed Zn²⁺ specific fluorescence enhancement (10 folds) due to restricted free rotation of the aryl–alkene bond. The e⁻-donating ability of N of imidazolone moieties decrease upon addition of Zn²⁺ to a solution of 127, hence blue shifting the emission from 506 to 485 nm.

A chemosensor utilizing terphenyl-phen macrocyclic Schiff base as a scaffold (128) demonstrated selective optical changes with Zn²⁺ ions in THF (Scheme 16).²⁰¹ The 374 nm band of 128 showed a 56 nm bathochromic shift with interaction of Zn²⁺ ions due to ICT from the phen moiety to the imino units while the fluorescent changes correspond to a modulation of PET caused by Zn²⁺ coordination.

Scheme 16 Fluorescent changes in 128 upon interaction with Zn²⁺ ions.

Addition of anions (F⁻, AcO⁻ and H₂PO₄⁻) to 128.Zn led to a decomplexation of Zn²⁺ ions, hence decreasing the fluorescence intensity. However, upon additional addition of Zn²⁺ ions to a solution containing 128–Zn²⁺–AcO⁻/128–Zn²⁺–F⁻/128–Zn²⁺–H₂PO₄⁻, the fluorescence intensity revived in the case of F⁻/AcO⁻ due to the binding of Zn²⁺ ions with the nitrogen atoms of imine and phen ligands. However, due to the presence of e⁻-rich oxygen atoms of H₂PO₄⁻ in the 128–Zn²⁺–H₂PO₄⁻ complex, the binding preference for Zn²⁺ becomes shifted to the oxygen atoms of the methoxy groups leading to a blue-shifted band.

Xu et al. synthesised phen Schiff bases (129a and 129b)²⁰² bearing chiral amino alcohol groups whose fluorescence intensity was appreciably quenched by addition of Eu³⁺ cation in aqueous medium. 129b, due to its steric bulk, showed very little change with addition of the Eu³⁺ cation. Amongst all the tested chiral α-carboxylate anions (D- or L-alanine, phenylalanine, mandelate, tartrate, D- or L-malate), different levels of fluorescence recovery were found only when the isomers of D- or L-malate anion were added to the solution of the 129a–Eu³⁺ complex. The preorganised structure of 129a–Eu³⁺ and the space complementary between the sensor and the anion guest resulted in selective recognition for malate ions.

Knowing the high thermodynamic and kinetic stability of lanthanide complexes of 1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid, it was attached to the phen chromophore by Quici et al. (130)²⁰³ to recognize Ln³⁺ ions such as Eu³⁺, Gd³⁺, Tb³⁺ and Yb³⁺ in aqueous medium. The band position changed with decreased absorption intensity (272–280 nm) for Eu³⁺, Gd³⁺ and Tb³⁺. The fluorescence intensity of 130 decreased on coordination with Gd³⁺ due to the heavy atom effect introduced by the Gd metal center. On the contrary, the rigidity and spatial arrangement of phen inhibited the approach of water molecules in the first coordination sphere of the metal ion and is thus, responsible for the increased luminescence of Eu³⁺ and Tb³⁺ complexes in water.

Click chemistry was involved for synthesizing the Zn²⁺ selective water soluble phen bridged bis (β-cyclodextrin) chemosensor (131).²⁰⁴ An inclusion complex was then formed between β-cyclodextrin and the admantyl skeleton (owing to strong association). The Zn²⁺ sensing abilities of 131 and its complex with 1-admantanecarboxylic acid sodium salt (131–AdCA) are compared in Table 18.

Table 18 Comparison of sensing ability of 131 and 131–AdCA towards Zn²⁺

	λem changes (nm) (process)	log Kass	LOD
131	377 → 385 (enhancement)	5.95	4.92 × 10^−7 M
131–AdCA	377 → 385 (enhancement)	7.74	3.38 × 10^−7 M

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The enhanced fluorescence of 131–Zn²⁺ complex/131–AdCA–Zn²⁺ complex was attributed to inhibition of the PET process and the favourable size/shape match between the radius of Zn²⁺ and the cavity formed by the cooperative coordination of phen and triazole units. A larger log K value and lower detection limit of the 131–AdCA system towards Zn²⁺ make it superior as a sensing system compared with 131.

Kaur et al. synthesized a simple tailor made amidic functionality based on the phen unit (132) that gave differential adsorption changes with metal ions.²⁰⁵ In a CH₃CN solution of 132a, addition of Cd²⁺ and Zn²⁺ ions resulted in a bathochromic shift of ∼65 nm (270 to 335 nm), whereas Cu²⁺ ions resulted in a hyperchromic effect with only an ∼15 nm bathochromic shift (270 to 286 nm). These differential changes observed with Cd²⁺ and Cu²⁺ ion addition enabled the chemosensor 132a to construct IMLICATION and TRANSFER logic gates. The observed binding constant values were found to be in the order of Cd²⁺ (1.78 × 10⁵) > Zn²⁺ (2.64 × 10⁴) > Cu²⁺ (2.11 × 10⁴), which is consistent with reported 2,9-disubstituted-1,10 phenanthroline derivatives favouring complexation with metal ions having a radius of ∼1.0 Å.^196,197,206 However, the presence of the picolyl group in place of the methoxybenzene ring enabled 132b²⁰⁷ to give changes to both Cu²⁺/Zn²⁺ and I⁻ ions (Fig. 13). Addition of Cu²⁺ to a solution of 132b resulted in a complete quenching of the emission bands at 351 and 368 nm, whereas Zn²⁺ addition caused significant quenching at both wavelengths along-with red shifting of the fluorescence spectrum because of oxidative PET. Amongst various anions, addition of only I⁻ ions showed a turn-off behaviour due to the compatible size of the pseudocavity formed by 132b.

Fig. 13 Fluorescence emission responses of 132b (10 μM) with different metal ions (100 equiv.) at 351 and 368 nm.²⁰⁷

4.2 Anions

In general, for the recognition of anions, –NH fragments of carbonylamides, sulphonamides, ureas, thioureas and pyrroles have been used.^208–212 Also, the N–H fragment of a sensor can further be polarized by introducing e⁻-withdrawing substituents or positively charged groups on the sensor framework.

The anion chemosensors synthesised and reported by several groups^{208,213–222} point towards hydrogen bonding interactions/deprotonation of acidic N–H with the added anions (F⁻, Cl⁻, Br⁻, I⁻, AcO⁻, H₂PO₄⁻, OH⁻) that can be evidenced by a change in the absorption intensity and red shift of the absorption band in most of the cases producing a clear color change. The photophysical results of chemosensors 133–137 (Table 19) set a general trend of the processes involved in the recognition of the said anions.

Table 19 Absorption and emission changes in 133–137 upon addition of different anions in DMSO

	Color and λab (nm) change F^−, AcO^−, H2PO4^−	Color and λab (nm) change Cl^−, Br^−, I^−	λem changes (nm) (process) F^−, AcO^−, H2PO4^−	λem changes (nm) (process) Cl^−, I^−, Br^−	log Kass
a Not determined.
133	—, 350 → 575	—	Enhancement 542 nm	—	3.94 (F^−), 3.83 (AcO^−), 3.63 (H2PO4^−)
134	Light yellow → orange, 340 → 400, 475	—	Enhancement 430 nm	—	4.81 (F^−), 4.46 (AcO^−), 5.22 (H2PO4^−)
135a	Yellow → purple, 470 → 550	—	^a	^a	4.95 (F^−), 5.86 (AcO^−), 4.85 (H2PO4^−)
135b	Yellow → green, 470 → 613 (AcO^−)	—	^a	^a	5.48 (AcO^−)
135c	—	—	—	—	^a
136	Light yellow → orange, 373 → 457 (AcO^−, H2PO4^−)	—	Enhancement 405 nm	—	3.99 (AcO^−), 2.58 (H2PO4^−)
137a	Colorless → yellow, 336, 351 → 422	Colorless → yellow, 336, 351 → 422	^a	^a	6.75 (F^−), 5.29 (AcO^−), 5.52 (Cl^−), 4.09 (I^−), 5.27 (Br^−)
137b	Colorless → yellow, 360 → 423	—	^a	^a	3.79 (F^−), 3.73 (AcO^−), 2.98 (H2PO4^−)
137c	^a	^a	Enhancement 377, 397 (F^−) 377, 397 → 450(AcO^−)	Enhancement 377, 397 (Br^−)	3.86 (F^−), 2.43 (AcO^−) 3.42 (Br^−)
137d	^a	^a	Enhancement 377, 396 (F^−, AcO^−)	Enhancement 377, 396 (Cl^−, Br^−)	3.68 (F^−), 2.36 (AcO^−), 3.25 (Cl^−), 4.13 (Br^−)
137e	^a	^a	465 Quenching (F^−, AcO^−)	465 Enhancement (Cl^−)	4.92 (F^−), 4.51 (AcO^−), 3.93 (Cl^−), 2.14 (I^−), 2.56 (Br^−)
137f	Yellow → red, 370 → 525	—	^a	^a	4.22 (F^−), 4.58 (AcO^−), 4.01 (H2PO4^−)

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The enhancement in the fluorescence intensity (133, 134, 136, 137c, 137d) was attributed to the inhibition of the PET phenomenon except in 137e where fluorescence intensity increased with Cl⁻ ions while it decreased with AcO⁻ and F⁻ ions owing to restricted rotation of benzimidazole groups due to hydrogen bonding interactions between Cl⁻ and N–H and deprotonation of N–H of benzimidazole groups in addition to PET, respectively (Scheme 17).

Scheme 17 Proposed binding mode of 137e with F⁻, AcO⁻, Cl⁻, Br⁻ and I⁻ in solution.

The binding affinity for various anions, hence the binding constants varied on the basis of angles and configuration provided by the cavity of the chemosensor and the correlation between the acidity and basicity of the N–H and added anion, respectively.

An imidazolium receptor (138)²²³ containing two anthracene moieties displayed a selective PET quenching effect and a unique excimer peak at 485 nm (λ_ex. 368 nm) upon addition of H₂PO₄⁻ ion, only. This suggested a close location of two anthracene groups due to the H-bonding between H₂PO₄⁻ and the two imidazolium moieties which induced excimer formation. Another phen dialdehyde based chemosensor 139²²⁴ selectively recognized large I⁻ ion by creating a compatible rigid cavity for the binding of I⁻ ion.

The chemosensor (140)²²⁵ has been designed in such a way that a specific analyte promoted certain chemical reactions detaching the e⁻-donating group to rejuvenate its original fluorescence intensity. Upon addition of sodium hypochlorite as the source of OCl⁻ in aqueous medium, the absorption band of 140 at 395 nm gradually weakened accompanied by a visually detectable distinct color change from yellow to colorless; while the yellow color of 140 remained unchanged in the presence of competing species such as H₂O₂, HS⁻, NO₃⁻, NO₂⁻, hydrazine, CN⁻, F⁻ and Cl⁻ ions. The increased fluorescence intensity (λ_em = 447 nm, 10 folds) with blue shift to 420 nm could be attributed to OCl⁻ catalyzed de-diaminomaleonitrile reaction of 140 (Scheme 18), by which the ICT mechanism was interrupted due to the breaking of donor and acceptor linkages and consequently, fluorescence was turned on through the generation of the phen dialdehyde.

Scheme 18 Proposed mechanism of sensing of hypochlorite by 140.

Anslyn et al. synthesised a metallated fluorescent chemosensor (141–Cu²⁺) based on a Cu²⁺ bound phen attached to a bis(aminoimidazolium) receptor for sensing citrate.²²⁶ The emission of non-metallated 141 at 365 nm was quenched on adding Cu²⁺ ions, but was revived on incremental additions of citrate. To ascertain the roles of both Cu²⁺ and guanidinium groups (citrate sensor), another compound similar to 141 but without guanidinium groups, was synthesised that displayed negligible change with citrate additions. Moreover non-metallated 141 remained silent upon adding citrate, evidencing Cu²⁺ binding as a trigger to citrate sensing by creating an additional binding site for citrate. The sensing assay was applied to citrate detection at the micromolar level in beverages.

The (142–Cu²⁺) complex¹⁴³ recognized Cl⁻ over other halide anions in CH₃CN. The determining role of CH₃CN in Cl⁻ recognition was further ascertained by performing a similar titration in CH₂Cl₂, where a 2 : 1 complex was formed between 142 and Cu²⁺ leaving no possible coordination site for Cl⁻ binding. The complex 142–Cu²⁺ was highly non-emissive owing to the PET process, but addition of Cl⁻ ion revived the intensity by nearly 70%. In 142–Cu²⁺ complex, Cu²⁺ coordinated to two N’s of phen; while the rest of the coordination sites were occupied by CH₃CN molecules. Addition of Cl⁻ caused replacement of two CH₃CN molecules by two Cl⁻ ions as evident from the ¹H-NMR experiments. This suppressed the PET process between Cu²⁺ and 142 and hence, caused an enhancement of the fluorescence.

In order to favour the formation of small anion binding sites following metal complex assembly, an urea-based anion sensor possessing trifluoromethylphenyl group (143)²²⁷ was synthesized in order to induce enhanced CT transitions. Addition of Cu⁺ to a solution of 143 resulted in a MLCT band at 434 nm with red–purple color due to formation of the [Cu^I(143)₂]⁺ complex. The single X-ray structure of [Cu^I(143)₂]⁺ complex showed its geometrical arrangement in such a way that two facing urea subunits gave tetrafurcate H-bond interaction with Cl⁻, AcO⁻, F⁻ and H₂PO₄⁻ ions. Addition of Cl⁻, AcO⁻, F⁻ and H₂PO₄⁻ to a solution of 143/[Cu^I(143)₂]⁺ complex gave rise to 1 : 1 (in case of Cl⁻ and AcO⁻) and 1 : 2 (anion : ligand in the case of F⁻ and H₂PO₄⁻) (Fig. 14). However, on further addition of F⁻ and H₂PO₄⁻ to 143, the 1 : 2 complex decomposed to give two 1 : 1 [A–143]⁻ complexes due to either more basicity or ability to form tetrafurcate interactions. Here, the use of Cu⁺ metal ion imparted versatility to the receptor, which switched from a locked arrangement, suitable for sensing F⁻ and H₂PO₄⁻ ions, to an unlocked arrangement which favoured interaction with the Y-shaped AcO⁻ ion.

Fig. 14 Two facing urea subunits of 143–Cu²⁺ complex giving tetrafurcate H-bonding interaction with Cl⁻.²²⁷

5. Chemosensors based on 4-mono- or 4,7-disubstituted derivatives of 1,10-phenanthroline ligands

5.1 Cations

Simple modification from methyl to 4-dimethylaminobenzaldehyde group in boradiazaindacenes (BODIPYs) containing phen probes (144a and 144b)^228,229 changed the selectivity from Cd²⁺ to Zn²⁺, although both these analytes belong to the same group of the periodic table. Knoevenagel-type condensation of 144a with 4-dimethylaminobenzaldehyde afforded 144b which gave absorption and fluorescence changes with only Zn²⁺ ions. Upon Zn²⁺ addition, the absorption intensity of bands of 144b at 234, 264, 320, 374 and 524 nm decreased with simultaneous red shifts of 15 nm of the band at 629 nm, hence enabling colorimetric detection of Zn²⁺ ions (grape to purple). A broad emission peak of 144b at 549 nm (λ_ex 510 nm) quenched (through oxidative PET) upon addition of Zn²⁺ ions only.

Receptor (145)²³⁰ containing both a polyamine scorpiand-like macrocycle and a phen moiety, whose relative disposition prevented their simultaneous coordination to a single Zn²⁺ ion, allowing movement of Zn²⁺ ions between these two coordinating sites (Scheme 19). Since the ligand involved aliphatic amines of polyamine and other protonation prone nitrogens, the absorption and fluorescence emission spectra of 145 were pH dependent. A little increase in the absorption and major decrease in the emission intensity were observed at pH > 6 and were attributed to the PET process that was prevented by protonation. Initial addition of Zn²⁺ to 145 (pH = 8) resulted in red shifting of the spectrum and increased the absorption intensity owing to fast coordination of Zn²⁺ to phen (k_obs = 2 × 10⁶). The second step (for equilibrium) of movement of Zn²⁺ in the macrocyclic site and rearrangement to coordinate to pendant arm was slow resulting in an absorption closer to that of the free ligand. For 1 : 1 complexation, Zn²⁺ ions resided in the macrocyclic cavity, while for a 2 : 1 (Zn²⁺ : 145) ratio both the macrocyclic cavity and phen units, were coordinated to Zn²⁺ ions leading to a high emission intensity. The Zn²⁺ binding was slower at low pH values due to competing proton binding.

Scheme 19 Mechanism proposed for Zn²⁺ coordination with 145.

5.2 Anions

The presence of two –C=O groups in 146²³¹ enhanced the acidity of two NH protons of phen to such an extent that it gave ratiometric sensing of F⁻ in aqueous CH₃CN. Upon addition of F⁻, the absorption peak at 342 nm was red shifted to 353 nm while a new emission band ascribed to the 146–F⁻ complex appeared at 450 nm (λ_ex 303 nm) in the fluorescence spectrum. The intensity of 450 nm drastically enhanced with increased F⁻ ion concentration, while the intensity of the original emission at 352 nm gradually decreased. At a concentration of H₂O in CH₃CN up to 20%, 146 retained its capability of ratiometric sensing of F⁻ ions.

6. Chemosensors based on 3,8-disubstituted derivatives of 1,10-phenanthroline ligands

A cell-compatible fluorescent chemosensor (147)^232,233 possessing e⁻-donating effects of two substituted thiophene groups at the 3- and 8-positions of phen showed distinctive fluorescent enhancement on complexation with Zn²⁺ ions. Upon Zn²⁺ addition to 147a, the 375 nm absorption band and 422 nm emission band were both red shifted to 390 and 461 nm, respectively. A dramatic CHEF effect (51-fold) could be attributed to the 1 : 2 stochiometric complex of 147a with Zn²⁺ with an association equilibrium constant of 4.9 × 10¹¹ m⁻² over a broad pH range. Although chemosensors 147b and 147c displayed fluorescent changes similar to 147a on complexation with Zn²⁺ ions (owing to extension of π systems due to introduction of e⁻ donating thiophene derivatives at the 3 and 8 positions of phen) 147a showed a stronger sensing ability at comparatively longer emission wavelengths due to a lower steric hindrance of the 5-methyl groups on the thiophene rings. A similar CHEF effect was observed with Cd²⁺ ions along with a hypsochromic shift of ∼11 nm.²³³ On moving from 147a to 147d,²³² the stoichiometry of Zn²⁺ complexation changed to 1 : 1 with the emission band showing a CHEF effect along with a bathochromic shift of ∼59 nm.

The conjugated polymers (148) and (149)^234,235 showed higher sensing sensitivity towards metal ions in comparison to other similar analogues possessing a twisted bipyridine showing the effect of rigidity of molecular recognition sites on metal ion sensing. Polymers 148 and 149 were prepared by Suzuki coupling reaction between monomers 3,8-dibromo-1,10-phenanthroline and substituted fluorene or 1,4-dialkoxybenezene units. Different extents of absorption red shifts (2–34 nm) and fluorescence quenching were exhibited by the addition of different cations to 148a. Only addition of Zn²⁺ induced a new emission peak, attributed to Zn²⁺-induced π–π* transition. The other polymers 148b and 149 also responded to H⁺ and Mⁿ⁺ with changes in the UV-vis and PL spectra. Addition of Co²⁺, Ni²⁺, Cu²⁺ and Pd²⁺ ions led to a large red-shift of λ_em by ∼20–50 nm and complete quenching of PL due to an energy transfer from the photoactivated π-conjugated polymer main chain to the metal complex part.

The electroactive precursor (150)²³⁶ underwent oxidative coupling resulting in polymerization directly onto the ITO (indium tin oxide) electrode and gave selective and sensitive detection of Fe³⁺. On electropolymerisation (EP), the otherwise blue emission of 150 in THF solution, showed a bathochromic shift of 25 nm due to the resulting cross-linked network structures of EP films. The fluorescence intensity of EP films was drastically quenched with Fe³⁺ ions only. The strong metal-chelated properties of the planar phen ring in 150 and the intrinsic porous cross-linked network microstructure of EP films were responsible for a better performance of EP films. 150 was also coated onto a glassy carbon electrode (150–GCE) by the EP method for the electrochemical detection of Pb²⁺.²³⁷ The CV of the bare 150–GCE was nearly linear in the absence of Pb²⁺ ions. After exposure to Pb²⁺, a significant oxidation peak and reduction peak appeared in the CV curves, allowing detection of 1.33 × 10⁻¹¹ M of Pb²⁺ under optimal conditions.

Stang and co-workers synthesized a phen containing molecule which self-assembled via an organometallic clip into a supramolecular rectangle (151),²³⁸ that was sensitive to micromolar concentrations of Ni²⁺, Cd²⁺ and Cr³⁺ ions. The coupling of 3,8-diethynyl-1,10-phen with 2.3 equiv. of 4-iodopyridine in the presence of Pd(0)/CuI catalysts provided ditopic pyridine, whose further reaction with 1 equiv. of 1,8-bis(trans-Pt(PEt₃)₂(NO₃))anthracene in an acetone/H₂O mixture resulted in the formation of the supramolecular rectangle 151. A methanolic solution of 151 exhibited absorbance bands at 350, 280 and 230 nm. Addition of Ni²⁺, Cd²⁺ or Cr³⁺ to solutions of 151 in methanol induced dramatic changes in the UV-vis spectrum with binding constant values of 2.01 × 10⁷, 3.39 × 10⁴ and 7.53 × 10³ M⁻¹, respectively.

7. Chemosensors based on non-derivatized 1,10-phenanthroline ligands

A potentiometric sensor incorporating a liquid polymeric membrane consisting of phenH⁺-tetra-phenylborate, nitrophenyl octyl ether and poly(vinyl chloride) as ion exchanger, plasticizer and polymeric support has been used for monitoring the potentiometric titration of Hg²⁺ and Cu²⁺ ions with phen solution in the presence of citrate and acetate buffers of pH 4.2, respectively.²³⁹ The sensor being sensitive to phenH⁺ concentrations showed fast potential responses with phenH⁺ conc. of 10⁻³ to 10⁻⁵ M by which a ∼95% final steady potential was attained in 5–15 s. The ability of Hg²⁺ and Cu²⁺ to form insoluble phen complexes in the presence of Cl⁻ and SCN⁻, respectively, made their determination feasible by direct potentiometric titration. This sensor showed sharp end points in the 60–80 mV range leading to a 1 : 1 stoichiometric complex and could detect Hg²⁺ up to a minimum concentration of 10 mg L⁻¹. The necessity of Cl⁻ for Hg²⁺ detection was ascertained by the absence of a potentiometric response with Hg²⁺ nitrate and acetate. Similarly, for Cu²⁺ in acetate buffer (pH = 4.2) sharp inflection breaks were observed at 180 mV corresponding to complex ratios of 1 : 2 : 1 Cu²⁺ : SCN⁻ : phen. No inflection points were seen without SCN⁻ and an excess of SCN⁻ did not interfere the titration. The Cu²⁺ was detectable at a min. conc. of 0.6 mg L⁻¹. The sensor was used for quantification of Hg²⁺ and Cu²⁺ content in several samples such as pharmaceuticals, rocks and tea leaves.

A visual strip for sensing Fe²⁺ was synthesized by UV-radiation induced graft polymerization of acrylamide monomer in microporous poly(propylene)base. The in situ polymerization of acrylamide was performed in the presence of phen for physical immobilization of the Fe²⁺ selective reagent (152) (Fig. 15).²⁴⁰ The strip sensor was applied to Fe²⁺ determinations in various samples of natural water and fruit juices with a LOD value of 1 ng mL⁻¹. The ivory coloured membrane turned orange red on interaction with Fe²⁺, which is known to form a bright orange–red colored complex with phen along with a broad absorption band in the range of 400–600 nm with a maximum at 515 nm.

Fig. 15 Mechanism of polymerization of acrylamide in the presence of phen for physical immobilization of Fe²⁺.²⁴⁰

A CdTe-phen quantum dots based sensor (153)²⁴¹ distinguished Cd²⁺ ions from closely related Zn²⁺ ions which had a similar configuration and properties as Cd²⁺ ions. The green PL of the CdTe quantum dots diminished on adding phen with a simultaneous red shift of 5 nm due to strong interactions between the CdTe quantum dots and phen. Cd²⁺ alone had no effect on the PL of CdTe QDs over a wide range of concentration, but, the increased addition of Cd²⁺ ions to CdTe QD-phen recovered the original PL intensity of QDs (2.5 fold) accompanied by a color change from dark to bright green until the molar ratio of added Cd²⁺ to the phen ligands was 1 : 2. The PL switch-on was attributed to the detachment of phen by the formation of [Cd(Phen)₂(H₂O)]²⁺ in the solution (Fig. 16).

Fig. 16 (A) PL spectra representing the quenching of CdTe QDs by phen and PL spectra of phen and [Cd(Phen)2(H₂O)₂]²⁺. Inset: PHT mechanism based on energy levels of green CdTe QDs versus phen. (B) PL spectra representing the restoration of the CdTe QD-phen sensor by Cd²⁺.²⁴¹

The chemiluminescence (CL) detection of Cu²⁺ in alkaline buffer solution was demonstrated by Chen et al. using a PDMS (polydimethylsiloxane) microfluidic chip.²⁴² The Cu²⁺ catalysed oxidation of phen by H₂O₂ emitted light (CL) that was measured and hence, Cu²⁺ was detected in water samples with a LOD value of 9.2 × 10⁻⁹ mol L⁻¹.

8. Conclusions

A comprehensive discussion on phenanthroline derived chemosensors already developed is presented to highlight what is needed by the next generation of approaches to develop practically applicable chemosensors. We have thoroughly reviewed the use of phen based ligands for chemosensing of cations and anions using absorption and/or fluorescence spectroscopic techniques. Insertion of a rigid phen within a cyclic or acyclic structure imparts a high degree of pre-organization to the derived ligands that results in the detection of both environmentally and biologically important analytes (cations and anions) via detectable and measurable changes in their optical properties. The metal complexes of phen [Ru(II), Ln(III), Ir(III), Re(I)] dominate in the categories of phen based chemosensors owing to their high stability and intense long lived luminescence. This review is intended to provide extensive assistance to research groups exploring the coordination ability and sensing properties of phenanthroline.

Abbreviations

Phen,1	10-Phenanthroline
MLCT	Metal-to-ligand-charge-transfer
dsDNA	Double strand DNA
ECL	Electrochemiluminescence
PET	Photoinduced electron transfer
TPrA	Tri-n-propylamine
LC	Ligand centered
PL	Photoluminescent
LOD	Limit of detection
equiv.	Equivalent
MMCT	Metal–metal charge transfer
ctDNA	Calf-thymus DNA
EDTA	Ethylenediaminetetraacetic acid
CT	Charge transfer
ssDNA	Single strand DNA
dpq	Dipyrrolylquinoxaline
ADP	Adenosine diphosphate
ATP	Adenosine triphosphate
IL	Intra-ligand
CHEF	Chelation enhanced fluorescence
C.N	Coordination number
TTA	Thenoyl trifluoroacetonate
DBM	Tris(dibenzoylmethane)
MC	Metal centered
DPA	Di(2-picolyl amine)
ILCT	Intra-ligand charge transfer
Dppz	Dipyrido[3,2-a:2′,3-c]phenazine
DMSO	Dimethyl sulfoxide
MeOH	Methanol
CH3CN	Acetonitrile
DMF	Dimethyl formaamide
CH2Cl2	Dichloro methane
THF	Tetrahydrofuran
CHCl3	Chloroform
C2H5OH	Ethanol

Acknowledgements

We thank DST–SERB for the financial assistance of the project (Grant no. SR/FT/CS-36/2011).

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