A robust fluorescent chemosensor for aluminium ion detection based on a Schiff base ligand with an azo arm and application in a molecular logic gate

Abstract

In this present work we have reported the synthesis and structural characterisations of a N₂O₂ donor Schiff base chemosensor with an azo arm (H₂L). Various spectroscopic tools like single crystal X-ray, NMR, UV-vis, FTIR, ESI-mass spectrometry etc. have been deployed to develop the present work. In recent years a number of azo derived chemosensors have been reported by different research groups. This is first time we are reporting the design and properties of an azo derived chemosensor (H₂L) for the detection of aluminium ions in semi aqueous medium. It has been found that it selectively senses Al³⁺ ions in semi aqueous solution. Here, the sensing process is mainly based on a chelation enhanced fluorescence process (CHEF). It has very high selectivity over other metal ions and anions. A detailed literature survey has been carried out and compared with this work. It has an appreciably low detection limit i.e. 6.93 nM. ¹H NMR titration was carried out to support the plausible complexation process. 1 : 1 stoichiometry binding between the chemosensor and Al³⁺ ions has been confirmed from Job's plot. An inhibition molecular logic gate has been constructed using chemosensor (H₂L), where Al³⁺ and EDTA act as inputs and fluorescence emission is the output. The structural and electronic parameters of the chemosensor (H₂L) and complex [AL(L)]NO₃ have been studied in detail using theoretical tools like DFT and TDDFT.

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Introduction

The design and construction of powerful and highly selective fluorescent chemosensors have attracted considerable attention in biomimetic chemistry.¹ Among various heavy metal ions aluminium is an essential element due to its abundance and use in every sphere of life in utensils, electronic and electrical components of different gadgets, building equipment, different packaging items, water treatment, food additives, pharmaceutical products, occupational dust etc. Aluminum is a known neurotoxin to organisms¹ and is believed to cause Alzheimer's disease,² osteomalacia³ and breast cancer.⁴ Toxicity, due to the presence of excess aluminium, in human health may arise as it inhibits several essential elements of similar size and charge like Mg²⁺, Ca²⁺ and Fe³⁺. It is also responsible for retarded growth of plants⁵ and oxidative damage of the cell membrane.⁶ In spite of its many drawbacks, the enormous use of Al in daily life causes accumulation of Al³⁺ and hence toxicity towards human health and the environment. The WHO recommend an average weekly human body dietary intake of Al³⁺ of around 7 mg kg⁻¹ body weight.⁷ Thus, it is of utmost urgency to detect Al³⁺ ions in the environment in trace levels i.e. in ppb, ppm or nano levels. Unfortunately, the determination of Al³⁺ is complicated mainly due to its poor coordination ability, strong tendency to hydration, and lack of suitable spectroscopic characteristics.⁸ Some analytical methods are available for detection of Al³⁺ such as graphite furnace atomic absorption spectrometry, inductively coupled plasma atomic emission spectrometry, atomic absorption spectra,⁹ electrochemistry,¹⁰ mass spectrometry,¹¹ and ²⁷Al NMR technology.¹² Most of them are expensive and time consuming. Optical detection particularly fluorescence spectroscopic technique is advantageous over the other techniques because of its operational simplicity, low cost, low detection limit, real-time detection, selectivity, time saving and environmental friendly property. Therefore design and synthesis of new chemosensor for the detection of Al³⁺ has received considerable attention. Recently various research groups have reported different organic probes which can selectively detect Al³⁺.^13,14 Although example of such Al³⁺ detecting chemosensors are still less compared with other metal ions detecting chemosensors like Zn²⁺ mainly due to less coordinating ability of Al³⁺ compared to other metal ions. Therefore, more improvement in this field is still required. Current literature survey (Table S1†) reveals that preparation of most of the chemosensors for detection of Al³⁺ involves expensive stating materials, multiple reaction steps, use of different solvent mixtures and maintenance of drastic reaction conditions i.e. very high or low temperature, presence of catalyst etc. G. Das and et al. in a recent work¹⁴ⁿ have reported an Al³⁺ sensing organic probe viz. (E)-N′-((E)-3-(4-(dimethylamino)phenyl)allylidene)picolinohydrazide using starting materials picolinohydrazide and 4-(dimethylamino)cinnamaldehyde. Preparation of picolinohydrazide involves multiple steps. Preparation of another chemosensor viz. 2-((naphthalen-6-yl)methylthio)ethanol involves inert atmosphere and finally the product has been purified through column chromatography with 65% yield.^14e S. Goswami and coworkers have reported a spirobenzopyran-quinoline (SBPQ) based sensor which selectively detects Al³⁺ along with Fe³⁺ and Cr³⁺. Preparation of this type of chemosensors also involves multiple steps.^14o It is also important to mention that 2-hydroxynaphthaldehyde based organic probes are mostly used for fluorescence sensing of Al³⁺.^14p

Different mechanistic pathway like intramolecular charge transfer (ICT),¹⁵ photo induced electron transfer (PET),¹⁶ chelation-enhanced fluorescence (CHEF),¹⁷ metal–ligand charge transfer (MLCT),¹⁸ excimer/exciplex formation, imine isomerization,¹⁹ intermolecular hydrogen bonding,²⁰ excited-state intramolecular proton transfer,²¹ displacement approach,²² and fluorescence resonance energy transfer²³ are adopted to explain the action of different fluorescence probe. Among different types of chemosensors, Schiff bases incorporated with different functionalized moiety are widely used mainly due to their ease of synthesis and low cost.

In this work an azo based salen-type of Schiff base ligand (E)-6,6′-((1E,1′E)-(propane-1,3-diylbis(azanylylidene))bis(methanylylidene))bis(2-methoxy-4-((E)-phenyldiazenyl)phenol) (H₂L) is structurally characterized and it exhibit high selectivity towards Al³⁺ (3.31 × 10³ M⁻¹) with low detection limit (6.93 nM). Azo derivatives have prevalent use in different fields like pharmaceuticals, optical data storage, non-linear optics, photo switching devices, textile industry dye-sensitized solar cells and they also act as fluorescent chemosensors.^24–30 Our work is important and novel in this aspect because although various azo containing chemosensors are reported in literature but to the best of our knowledge in this work we are first time reporting one such sensor which can selectively detect Al³⁺. Our organic probe possesses some extra advantages. We have prepared H₂L using easily available starting materials. The azoaldehyde was prepared from well known easy diazotization process with very high yield. The azoaldehyde upon reaction with a simple diamine produced H₂L with >90% yield. We have successfully isolated both the azoaldehyde and Schiff base H₂L in highly crystalline form. The ligand is characterized by different techniques including X-ray crystallography. The composition of the Al³⁺–ligand complex (complex 1) has been established by different spectroscopic data like IR, Mass, Job's plot and ¹H NMR. The DFT computation of optimized geometry of H₂L and the complex [Al(L)]⁺ has been used to support the electronic spectral properties.

Experimental

Materials and physical measurements

All reagent or analytical grade chemicals and solvents were purchased from commercial sources and used without further purification. Elemental analysis for C, H and N was carried out using a Perkin-Elmer 240C elemental analyzer. Infrared spectra (400–4000 cm⁻¹) were recorded from KBr pellets on a Nicolet Magna IR 750 series-II FTIR spectrophotometer. Absorption spectra were measured using a UV-2450 spectrophotometer (Shimadzu) with a 1 cm-path-length quartz cell. Measurements of NMR spectra were conducted using a Bruker 300 spectrometer in DMSO-d₆ and CDCl₃ respectively. Emission was examined by LS 55 Perkin-Elmer spectrofluorimeter at room temperature (298 K) in HEPES buffer at pH = 7.4 solution under degassed condition. Fluorescence lifetimes were measured using a time-resolved spectrofluorimeter from IBH, UK.

Synthesis of (E)-5-(2-phenyldiazenyl)-2-hydroxy-3-methoxybenzaldehyde

4 mL conc. HCl was added to 15 mL water and kept in an ice bath keeping the temperature 0 °C. To it, aniline (4.0 mmol, 0.372 g) was added. To this mixture an aqueous solution of sodium nitrite (6.0 mmol, 0.414 g) was drop wise added over a time of 30 min. The mixture was stirred for 1 h at 0 °C. Then the resultant solution was added to the alkaline o-vanillin (4.8 mmol, 0.732 g) which is kept in another ice bath. The mixture was further stirred for 1 h at 0 °C. The solution was neutralised using dilute HCl and pH was maintained around 7.0. The mixture was extracted with chloroform and evaporated to result orange colored crystals of the aldehyde.

Yield: 0.953 g (94%). Anal. calc. for C₁₄H₁₂N₂O₃: C 64.65%; H 5.79%; N 16.34%. Found: C 64.36%; H 5.31%; N 16.17%. IR (cm⁻¹, KBr): ν(N=N) 1459 s; ν(C–H) 762 s. ESI-MS (positive) in MeOH: the base peak was detected at m/z = 279.00, corresponding to [M + 23]⁺.

¹H NMR (CDCl₃, 300 MHz) δ ppm: 4.01 (–OCH₃) (s, 3H), 7.25–7.35 (Ar-H) (m, 2H), 7.74–7.86 (Ar-H) (m, 5H), 10.02 (–CH=O) (s, 1H), 11.05 (–OH) (bs, 1H).

Synthesis of Schiff base ligand (H₂L)

A mixture of (E)-5-(2-phenyldiazenyl)-2-hydroxy-3-methoxybenzaldehyde (2.0 mmol) and 1,3-diaminopropane (1.0 mmol, 0.074 g) was heated to reflux for 4 h in methanol–chloroform solvent mixture (1 : 1, v/v). Upon slow evaporation of the solvent mixture deep red colored crystals were obtained.

Yield: 0.5282 g (96%). Anal. calc. for C₂₉H₂₆N₆O₂: C 67.62%; H 5.49%; N 15.26%. Found: C 66.86%; H 5.31%; N 14.97%. IR (cm⁻¹, KBr): ν(C=N) 1648 s; ν(N=N) 1455 s; ν(C–H) 764 s. ESI-MS (positive) in MeOH: the base peak was detected at m/z = 551.21, corresponding to [M + 1]⁺. UV-vis, λ_max (nm), (ε (dm³ mol⁻¹ cm⁻¹)) in acetonitrile: 290 (4533) and 388 (5736).

¹H NMR (CDCl₃, 300 MHz) δ ppm: 2.25 (–CH₂) (m, 1H), 3.82–3.84 (–CH₂) (bs, 2H), 4.01 (–OCH₃) (s, 3H), 7.42–7.90 (Ar-H) (m, 7H), 8.43 (–CH=N) (s, 1H).

¹H NMR (DMSO-d₆, 300 MHz) δ ppm: 2.08 (–CH₂) (m, 2H), 3.74–3.82 (–CH₂, –OCH₃) (bs, 10H), 7.30–7.48 (Ar-H) (m, 8H), 7.66–7.74 (Ar-H) (m, 6H), 8.66 (–CH=N) (s, 2H).

¹³C NMR (DMSO-d₆, 75 MHz) δ ppm: 31.46 (–CH₂), 56.34 (–CH₂), 118.22, 122.58, 126.93, 127.41, 129.08 and 130.47 (Ar-C), 165.38 (–CH=N).

Synthesis of [Al(L)](NO₃) complex

A 2 mL methanolic solution of aluminium nitrate nonahydrate (1.0 mmol, 0.375 g) was added drop wise to 20 mL methanolic solution of H₂L (1.0 mmol, 0.551 g) followed by addition of triethylamine (2.0 mmol, ∼0.4 mL) and the resultant reaction mixture was stirred for 4 h under nitrogen atmosphere. Then the resultant mixture was dried to powdered form and further characterizations were carried out.

Yield: 0.6246 g (98%). Anal. calc. for AlC₂₉H₂₄N₇O₅: C 67.62%; H 5.49%; N 15.26%. Found: C 66.86%; H 5.31%; N 14.97%. IR (cm⁻¹, KBr): ν(C=N) 1646 s; ν(N=N) 1545 s; ν(C–H) 770 s. ESI-MS (positive) in MeOH: the base peak was detected at m/z = 575.25, corresponding to [Al(L)]⁺. UV-vis, λ_max (nm), (ε (dm³ mol⁻¹ cm⁻¹)) in acetonitrile: 286 (5083) and 375 (5949).

¹H NMR (DMSO-d₆, 300 MHz) δ ppm: 2.16 (–CH₂) (bs, 2H), 3.89–3.95 (–CH₂, –OCH₃) (bs, 10H), 7.54–7.81 (Ar-H) (m, 14H), 9.24 (–CH=N) (s, 2H).

X-ray crystallography

Single crystal X-ray data of azoaldehyde and Schiff base ligand (H₂L) were collected on a Bruker SMART APEX-II CCD diffractometer using graphite monochromated Mo Kα radiation (λ = 0.71073 Å) at room temperature. Data processing, structure solution, and refinement were performed using Bruker Apex-II suite program. All available reflections 2θ_max were harvested and corrected for Lorentz and polarization factors with Bruker SAINT plus.³¹ Reflections were then corrected for absorption, inter-frame scaling, and other systematic errors with SADABS.³² The structures were solved by the direct methods and refined by means of full matrix least-square technique based on F² with SHELX-1997 and SHELX-2013 software package.³³ All the non-hydrogen atoms were refined with anisotropic thermal parameters. C–H hydrogen atoms were inserted at geometrical positions with U_iso = 1/2U_eq. to those they are attached. Crystal data and details of data collection and refinement are summarized in Table 1.

Table 1 Crystal parameters and selected refinement details for H₂L

Compound	Azoaldehyde	H2L
Empirical formula	C14H12N2O3	C31H28N6O4
Formula weight	256.26	548.59
Temperature (K)	155(2)	150(2)
Crystal system	Monoclinic	Triclinic
Space group	Cc	p-1
a (Å)	10.7683(4)	8.648(2)
b (Å)	13.4401(5)	11.051(3)
c (Å)	8.4892(3)	13.816(4)
α (°)	90.00	91.833(9)
β (°)	93.755(2)	97.710(9)
γ (°)	90.00	92.902(8)
Volume (Å^3)	1225.98(8)	1305.8(6)
Z	4	2
Dcalc (g cm^−3)	1.388	1.395
Absorption coefficient (mm^−1)	0.100	0.095
F(000)	536	576
θ range for data collection (°)	2.43–26.40	2.326–23.559
Reflections collected	4910	12 084
Independent reflections/Rint	2204/0.0313	3806/0.0726
Observed reflections [I > 2σ(I)]	1980	2308
Data/restraints/parameters	2204/2/175	3806/0/373
Goodness-of-fit on F^2	1.029	1.752
Final indices [I > 2σ(I)]	R1 = 0.0331	R1 = 0.1611
	wR2 = 0.0812	wR2 = 0.4280
R indices (all data)	R1 = 0.0378	R1 = 0.2089
	wR2 = 0.0842	wR2 = 0.4619
Largest diff. peak/hole (e Å^−3)	0.150/−0.120	0.968/−0.620

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Computational method

All computations were performed using the GAUSSIAN09 (G09)³⁴ software package. Coordinates obtained from single crystal X-ray data were used for optimization of structure of ligand (H₂L). For optimization we used the density functional theory method at the B3LYP level^35,36 and the standard 6-31+G(d) basis set for C, H, N and O atoms^37,38 and the lanL2DZ effective potential (ECP) set of Hay and Wadt^39–41 for aluminum atom have been chosen for optimization.

TDDFT calculation was performed with the optimized geometry to ensure only positive eigen values. Time-dependent density functional theory (TDDFT)^42–44 was performed using conductor-like polarizable continuum model (CPCM)^44–47 and the same B3LYP level and basis sets in aqueous solvent system. GAUSSSUM⁴⁸ was used to calculate the fractional contributions of various groups to each molecular orbital.

Results and discussion

Synthesis and characterization

The azoaldehyde 5-(2-phenyldiazenyl)-2-hydroxy-3-methoxybenzaldehyde is synthesized by first diazotization of primary aromatic amine (aniline) followed by coupling with an aromatic alcohol (o-vanillin). Azoaldehyde reacted with 1,3-diamino propane in 2 : 1 molar ratio at ambient temperature in methanol to generate the chemosensor H₂L (Scheme 1). The yield of both the aldehyde and H₂L is >90%. Both of them are crystallized from slow evaporation of methanol–chloroform (3 : 5) mixture. They are well characterized by ¹H NMR, ¹³C NMR, mass and IR spectroscopy. In IR spectrum, azo (N=N) band of both azoaldehyde and H₂L appear at 1459 cm⁻¹ and 1455 cm⁻¹ respectively (Fig. S1†). Other important stretching vibrations of H₂L are 1648 (s, ν(C=N)); 3100 cm⁻¹ (ν(–OH)); 764 cm⁻¹ (ν(C–H)) respectively. Similar characteristic stretching frequencies of [Al(L)]NO₃ are obtained at 1646 (s, ν(C=N)); 1530 cm⁻¹ (ν(N=N)); 770 cm⁻¹ (ν(C–H)), respectively (Fig. S2†). In complex 1, nitrate ion is present as a counter anion. The characteristic stretching frequencies are obtained at 765 cm⁻¹ (planar rock), 815 cm⁻¹ (NO, deformation) and 1375 cm⁻¹ (NO, asymmetric stretch), respectively, which are comparable with that of previously reported literature values.⁴⁹ In ¹H NMR spectrum of azoaldehyde (CDCl₃ solvent), aldehyde proton (–CH=O) appeared at 10.02 ppm whereas –OCH₃ and phenolic –OH protons appeared at 4.01 ppm and 11.05 ppm respectively. Azoaldehyde contains two aromatic rings. Aromatic protons came as multiplate at two different regions i.e. from 7.25 to 7.35 ppm and from 7.74 to 7.86 ppm respectively. In case of the H₂L (CDCl₃ solvent), aromatic protons appeared around 7.74–7.30 ppm whereas azomethine proton, –OCH₃ protons and aliphatic protons appeared at 8.43, 4.01, 3.95 and 2.25 ppm respectively. Here, phenolic –OH proton is so labile that it becomes very difficult to arrest in both CDCl₃ and DMSO-d₆ solvent system. In ¹³C spectra of H₂L (DMSO-d₆ solvent), aliphatic carbon signals appeared at 31.46 ppm and 56.34 ppm respectively, whereas aromatic carbons appeared in the region of 118 ppm to 130 ppm. The signal at 165 ppm is the characteristic signal for imine carbon ([double bond splayed left]C=N) (Fig. S3–S6†). Mass spectral analysis of starting azoaldehyde and H₂L exhibit m/z peaks at 279.00 and 551.21, respectively (Fig. S7–S9†). Mass spectrum of H₂L in the presence of Al³⁺ shows peak m/z at 575.25 which is corresponding to [Al(L)]⁺ (Fig. S10†).

Scheme 1 The route to the syntheses of H₂L.

The structure of both azoaldehyde and H₂L have been established by single crystal X-ray diffraction measurement. Crystals of both the organic compound are obtained from slow evaporation of CHCl₃–CH₃OH solvent mixture. The ORTEP plot of azoaldehyde and H₂L are shown in Fig. 1 and 2 respectively and the selected bond parameters are listed in Table 2. The azoaldehyde crystallizes with a monoclinic space group Cc. It has almost planar structure where the azo nitrogen atoms and the aromatic ring containing –CHO, –OCH₃, –OH groups belong in the same plane. The remaining aromatic ring deviates from the plane to a small extent (10.71°). The N=N and C=O (–CHO) bond distances are 1.252 Å and 1.224 Å respectively which are similar with other reported values.⁵⁰ Aromatic C–C bond distances observed around 1.368–1.404 Å. In solid state it form 2D network through intermolecular hydrogen bonding between the phenoxy O-atom and aromatic hydrogen atom and edge to edge π–π stacking interaction between two phenyl rings. The distances are 2.704 Å and 3.70 Å respectively (Fig. S11†).

Fig. 1 Ortep view of azoaldehyde. Atoms are shown as 30% thermal ellipsoids. H atoms are omitted for clarity.

Fig. 2 Ortep view of the ligand (H₂L). Atoms are shown as 30% thermal ellipsoids. H atoms are omitted for clarity.

Table 2 Selected bond lengths (Å) and bond angles (°) for azoaldehyde and H₂L

Azoaldehyde			H2L
	X-ray	Calculated		X-ray	Calculated
C3–O2	1.340	1.3450	N1–N2	1.22(1)	1.2645
C8–O1	1.224	1.2264	N5–N6	1.267(9)	1.2647
N1–N2	1.252	1.2611	C15–N3	1.29(1)	1.2853
C2–O3	1.362	1.3631	C17–N4	1.28(1)	1.2845
			C15–C16–C17	114.4(7)	115.79
			C1–O1	1.259	1.275
			C2–O2	1.286	1.297

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The Schiff base ligand (H₂L) crystallizes with a triclinic space group P1̄. The azo –N=N– bond distances are 1.217 Å and 1.268 Å respectively. The average C=N– bond distance is 1.287 Å. The C–C bond distances of the phenyl rings found around 1.342–1.472 Å, whereas aliphatic C–C bond distances appear around 1.511–1.531 Å. The propylenic part of the Schiff base, N3–C15–C16–C17–N4, is to some extent puckered due to the sp³ hybridization of the saturated portion of the ligand. The bond angle (C15–C16–C17), 114.35(6)° deviates appreciably from its ideal value. OH group of the both of phenyl ring are placed 180° apart from each other to minimize the dipole–dipole interaction. In the Schiff base ligand each azo nitrogen atoms, imine atoms and the aromatic ring containing –CHO, –OCH₃, –OH groups belong in the same plane. The remaining aromatic ring deviates from the plane significantly by an angle of 83.03°. The molecule dimerizes in solid state through intermolecular H-bonding between the two –OH groups as well as between the –OH group and imine nitrogen atom. –O–H⋯O bond distance is 3.879 Å whereas –O–H⋯N (imine) bond distance is 2.357 Å (Fig. S12†).

Structural parameters are again calculated from optimized structure using DFT where bond distances are marginally elongated by 0.0023–0.0435 Å and bond angles varies within the range of 0.05–1.39° (Table 2) compare with the X-ray crystallographic data. Therefore, the optimized results are helpful to explain the electronic structure and electronic properties of the ligand H₂L.

Absorption study

The UV-vis spectrum of the chemosensor H₂L was recorded at 25 °C in aqueous buffer–acetonitrile solution (1 : 100 v/v, HEPES buffer at pH 7.4) which exhibited well-defined bands at 290 and 388 nm respectively. In order to study the binding property of H₂L toward Al³⁺ ion, UV-vis spectra of H₂L (10 μM) in the presence of various concentrations of Al³⁺ (0–10 μM) were recorded at room temperature, as shown in Fig. 3. It has been observed that old peaks disappear and two new absorption bands emerge at 286 and 375 nm respectively, and its absorbance gradually increases with the gradual addition of Al³⁺ and it gets saturated upon addition of 1.0 equivalents of Al³⁺ keeping the concentration of H₂L fixed at 10 μM. These observations indicate the coordination of H₂L with one eq. of Al³⁺ (Fig. 3).

Fig. 3 Absorption titration of H₂L (10 μM) with gradual addition of Al³⁺, 0–10 μM in MeCN/HEPES buffer at pH 7.4.

Al³⁺ ion sensing by fluorescence studies

The fluorescence property of H₂L was also investigated in HEPES buffer solution at pH 7.4 at room temperature. The ligand, H₂L, emits weakly at 510 nm when excited at 388 nm and the fluorescence quantum yield is (Φ = H₂L) 0.00939. Upon excitation at 290 nm we observed similar emission at 510 nm. Low fluorescence intensity of free H₂L probably due to photo induced electron transfer process (PET) caused by electron delocalization from the two phenoxido oxygen atoms to the π-conjugated system of two aromatic rings, [double bond splayed left]C=N– group and azo (–N=N–) group. Here presence of extended delocalization through azo group (–N=N–) initiates appearance of fluorescence pick at 510 nm. After gradual addition of Al³⁺ ion with various concentrations (0–10 μM), a significant changes in emission spectra have been noticed. In presence of metal ion the emission band of H₂L is blue shifted to 478 nm. This result also confirms a high sensitivity of the receptor towards Al³⁺. A plot of fluorescence intensities at 478 nm (I₄₇₈) vs. concentration of aluminum has been given in Fig. 4: inset. It shows that sensing character of H₂L (I₄₇₈) increases with the increasing concentration of Al³⁺ and a clear bend of the curve was observed at 1.0 equivalent of added Al³⁺ which prove 1 : 1 stoichiometry of the [Al(L)]⁺. Such type of sigmoid curve reflects nature of interaction between the organic probe and Al³⁺ ion. Upon addition of 1–3 μM of Al³⁺ to 10 μM solution of chemosensor, very small change of fluorescence intensity of the probe at wavelength 478 nm has been observed. Upon gradual addition of Al³⁺ from 4–9 μM fluorescence intensity increases from 300–1400 a.u. which indicates strong interaction between Schiff base ligand and Al³⁺. Maximum increase of fluorescence intensity has been observed up to 10 μM addition of Al³⁺. This clearly indicates a 1 : 1 binding between H₂L and Al³⁺. The curve became a plateau with further addition of Al³⁺, when further increment in fluorescence intensity has not been observed.

Fig. 4 Fluorescence titration of H₂L (10 μM) in HEPES buffer at pH = 7.4 by gradual addition of Al³⁺ (0–10 μM) with λ_em = 478 nm (5/5 slit). Inset: non-linear plot of fluorescence intensity vs. concentration of Al³⁺ ion.

Enhancement in fluorescence intensity of H₂L in presence of Al³⁺ probably due to elimination of photoinduced electron transfer (PET) in free H₂L followed by chelation enhancement effect (CHEF) through the co-ordination of azomethine-N and phenolic-O to metal ion (Scheme 2). Hence, upon complexation a large CHEF effect is observed because of the rigid framework and thereby obstructs the PET process. Thus, upon coordination with Al³⁺ ion, the PET process within the ligand system would become weak by reduction of either the electron-accepting ability of the π-conjugated system of two aromatic rings, [double bond splayed left]C=N– and azo (–N=N–) groups or the electron-donating ability of the phenoxido oxygen atoms.

Scheme 2 Schematic diagram for photoinduced electron transfer (PET) in free H₂L and chelation enhanced fluorescence effect (CHEF) in [Al(L)]⁺.

Fluorescence intensity of H₂L has been examined with various cations (Cd²⁺, Co²⁺, Cr³⁺, Cu²⁺, Fe³⁺, Hg²⁺, K⁺, Mn²⁺, Na⁺, Ni²⁺, Sn²⁺, Zn²⁺, Ag⁺, Fe²⁺ and Ca²⁺) using their nitrate salt (Fig. S13†). Fig. S13† shows that only Al³⁺ can induce significant enhancement of fluorescence intensity. Interestingly, upon addition of different metal ions the intensity of the fluorescence either remains unchanged or weakened. In presence of Co²⁺, Cr³⁺, Cu²⁺, K⁺, Mn²⁺, Na⁺, Ni²⁺, Fe²⁺ and Ca²⁺ metal ions emission intensity remain almost unchanged and in presence of Fe³⁺, Cd²⁺, Sn²⁺, Ag⁺, Hg²⁺, and Zn²⁺ metal ions emission intensity slightly increased (Fig. S13†). A competition assay of H₂L in the presence of Al³⁺ and other metal ions have also been studied and presented in Fig. 5, which prove that fluorescence enhancement due to Al³⁺ nullify the possible interference of other metal ions. Upon addition of different anions viz. S₂O₃²⁻, S²⁻, SO₃²⁻, SO₄²⁻, SCN⁻, N₃⁻, AsO₄³⁻, PO₄³⁻, ClO₄⁻, AcO⁻, Cl⁻, NO₂⁻, NO₃⁻ and SCN⁻ in HEPES buffer at pH 7.4 (Fig. 6) chemosensor H₂L showed no significant fluorescence enhancement. Thus, the probe is an excellent example of fluorescence chemosensor towards Al³⁺ in the presence of different metal ions.

Fig. 5 Relative fluorescence intensity profile of [H₂L–Al³⁺] system in the presence of different cations in HEPES buffer at pH 7.4. H₂L (10 μM) + Al³⁺ (10 μM) + Mⁿ⁺ (500 μM), where Mⁿ⁺ = ((1) – Cd²⁺, (2) – Co²⁺, (3) – Cr³⁺, (4) – Cu²⁺, (5) – Fe³⁺, (6) – Hg²⁺, (7) – K⁺, (8) – Mn²⁺, (9) – Na⁺, (10) – Ni²⁺, (11) – Sn²⁺, (12) – Zn²⁺, (13) – Ag⁺, (14) – Fe²⁺, (15) – Ca²⁺).

Fig. 6 Relative fluorescence intensity profile of H₂L (10 μM) in the presence of various common anions (50 μM) in HEPES buffer at pH 7.4. (1) – H₂L + Al³⁺, (2) – 14H₂L + anions, anions = (2) – S₂O₃²⁻, (3) – S²⁻, (4) – SO₃²⁻, (5) – SO₄²⁻, (6) – SCN⁻, (7) – N₃⁻, (8) – AsO₄³⁻, (9) – PO₄³⁻, (10) – ClO₄⁻, (11) – OAc⁻, (12) – Cl⁻, (13) – NO₂⁻, (14) – NO₃⁻.

To find out the binding ability of our chemosensor H₂L with Al³⁺ ions, binding constant was calculated using Benesi–Hildebrand equation (eqn (1)) involving fluorescence titration curve.

where, F_max, F₀ and F_x are fluorescence intensities of H₂L in the presence of Al³⁺ at saturation, free H₂L and any intermediate Al³⁺ concentration at λ_max = 478 nm. K is the dissociation constant of the complex. Concentration of Al³⁺ ions is represented by C and slop n is measured with Hill plot. Then binding constant (K_a) of the complex has been determined using the relation, K_a = 1/K_d. A plot of vs. provides the apparent binding constant value as 3.31 × 10³ M⁻¹ (Fig. S14†) (n = 1.0).^14e The Job's plot suggests that the ligand H₂L forms 1 : 1 complex with Al³⁺ ions (Fig. S15 and S16†). To determine the binding stoichiometry of H₂L and Al³⁺ fluorescence intensity is plotted against different mole fractions of Al³⁺ while volume of solution has remained constant (Fig. S16†). Maxima in this plot has been obtained at 0.5 mole fraction, which suggests about 1 : 1 complex formation of H₂L and Al³⁺. The formation of the complex further confirmed from ESI mass spectroscopy. Mass spectrum of H₂L in the presence of Al³⁺ shows peak m/z at 575.25 which is corresponding to cationic part of the complex 1 [Al(L)]⁺ (Fig. S10†). The simulated pattern shows well agreement with that of experimental values (Fig. S17†). Experimental findings (absorption spectra and fluorescence spectra of Al³⁺ and other cations and various anions with chemosensor H₂L) clearly revealed that H₂L has strong affinity towards Al³⁺. Here Al³⁺ acts as strong Lewis acid which can simply binds with two phenoxido oxygen atoms and two imine nitrogen atoms of the Schiff base ligand. Thus the chemosensor acts as a tetradentate N₂O₂ donor ligand and form a distorted tetrahedral complex with Al³⁺. Comparable size of the inner-cavity of the chemosensor and Al³⁺ i.e. suitable size and high charge density of Al³⁺ permits strong interaction between H₂L and the metal ion.

Fluorescence decay behavior of H₂L and complex 1 has been studied. Decay curves have been given in ESI† (Fig. 7). Fluorescence life time of H₂L is found to quite low (1.324 ns). Fluorescence life time of H₂L in presence of one eq. of Al³⁺, increases up to 2.058 ns. Quantum yields of H₂L and complex 1 are calculated using eqn (2) and found to be very low. These have been measured to be 0.00939 and 0.2133, respectively (Table S1†). It is to be noted that although quantum yield of the complex is low but it is almost ∼22 folds greater compared to free H₂L.

where F_x, F_s are the wavelength integrated emission intensities of the samples and reference (here reference is coumarine); A_s, A_x are the optical densities at their corresponding wavelengths of excitation.

Fig. 7 Time-resolved fluorescence decay curves (logarithm of normalized intensity vs. time in ns) of H₂L in the absence () and presence () of Al³⁺ ion, () indicates decay curve for the scattered (λ_ex = 478 nm).

Free ligand upon excitation gives peak at 510 nm. In presence of Al³⁺ there is a slight blue shift ∼32 nm of intensity and emission peak observed at 478 nm. Upon gradual addition of Al³⁺ to H₂L a steady increase in emission intensity at 478 nm has been observed.

Sensitivity of H₂L towards Al³⁺ has been checked by determining limit of detection (LOD) value. The detection limit of the chemosensor was calculated using 3σ method⁵¹ and it is found to be 6.93 × 10⁻⁹ mol L⁻¹.^14e Low LOD value clearly indicates high sensitivity of H₂L towards Al³⁺ ion. In Table S1† some recently published chemosensors for Al³⁺ ion has been reported along with their LOD values.^{14a,e,g,i–o} The reported chemosensor has some advantages over the others while it has some draw backs also. Our probe is synthesized by easy single step Schiff base condensation process. LOD value is significantly low but not the lowest among all.

We also examined fluorescence intensity of H₂L in absence and presence of Al³⁺ ion at various pH values. We have maintained the pH range from 3 to 10. The solution concentration of both H₂L and Al³⁺ is 10 μM respectively. It has been observed that the chemosensor in free condition exhibit very little fluorescence in the acidic condition (pH = 3 to pH = 7). Fluorescence intensity remains unchanged within this pH range. Upon increasing pH from 7 to 10 a slight increase in fluorescence intensity has been observed. In presence of Al³⁺, in acidic condition (pH = 3 to pH = 5) fluorescence intensity of the chemosensor remain unchanged due to protonation of phenolic hydroxyl group of the ligand, therefore nullifying chelation ability with Al³⁺. Al³⁺ ion sensing abilities were exhibited by the ligand when pH was increased from 5.0 to 8.0. Thus, H₂L exhibited good fluorescence sensing ability towards Al³⁺ ion over a wide range of pH (Fig. 8). Based on the experimental results we can conclude that our chemosensor can be utilized as a selective fluorescent probe to recognize and distinguish Al³⁺ ion in presence of other metal ions in biological system under physiological condition.

Fig. 8 Fluorescence intensity of H₂L (black box; 10 μM) in the absence and presence of Al³⁺ ion (red circle, 10 μM) ion at various pH values in HEPES buffer.

DFT study

Geometry optimization of both H₂L and [Al(L)]⁺ has been performed using DFT/B3LYP method. Mass spectral data analysis has confirmed that the composition of the complex ion is [Al(L)]⁺. The energy minimized structure of both H₂L and [Al(L)]⁺ are shown in Fig. 9. Some selected bond distances and bond angles of the optimized geometries of [Al(L)]⁺ are listed in Table 3. DFT optimized structure reveals that Al³⁺ is tetra-coordinated and distorted tetrahedral in geometry (Fig. 9) where the chemosensor H₂L act as a N₂O₂ donor center and binds to Al³⁺ through phenolic-O atom and imine-N. Theoretically calculated Al–N (imine) and Al–O (phenolato) distances are around 1.906 Å and 1.326 Å, respectively and have been found comparable with similar structure.^14e It is interesting to mention that upon complex formation the C(15)–N(3), C(17)–N(4), C(1)–O(1) and C(20)–O(2) bond distances have been significantly elongated compared to free ligand (Table 3). Theoretical calculations show that electron density in both HOMO and LUMO of H₂L is mainly distributed on the one part of the aromatic moiety. In metal complex ion i.e. [Al(L)]⁺ electron density in both HOMO and LUMO is allocated over the entire ligand system. Energy of some selected M.O. of both H₂L and [Al(L)]⁺ are given in Table S3.† Mulliken charge distribution shows positive charge on aluminium ion i.e. 1.227343. Mulliken charge distribution of [Al(L)]⁺ is provided in Table S4.†

Fig. 9 DFT optimized structures of H₂L and [Al(L)]⁺. H-Atoms are omitted for clarity.

Table 3 Selected bond lengths (Å) and bond angles (°) of DFT optimized structure of [Al(L)]⁺. (B3LYP/6-31+G(d) basis set)

	Calculated		Calculated		Calculated
N1–N2	1.26019	C1–O1	1.32767	Al–O1	1.7584
N5–N6	1.26320	C2–O2	1.32668	Al–O2	1.7553
C15–N3	1.31355	Al–N3	1.90639
C17–N4	1.31661	Al–N4	1.92372
C15–C16–C17	115.79	C15–C16–C17	116.50435

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TDDFT study

For better understanding of electronic transition, TDDFT calculations were performed using B3LYP/CPCM method using same basis sets in acetonitrile. The calculated electronic transitions are given in Table 4. H₂L shows intense absorption band for ligand based n–π and π–π* transitions around 290 nm and 388 nm respectively. The band at 290 nm is due to the contribution of HOMO−4 → LUMO+1 and HOMO → LUMO+2 transitions, whereas the band at 388 nm is due to the contribution of HOMO−1 → LUMO, HOMO−1 → LUMO+1 and HOMO → LUMO transitions. For the L–Al species the intense absorption band around 375 nm corresponding to HOMO → LUMO+1, HOMO → LUMO+2, HOMO → LUMO+3 and HOMO−2 → LUMO transitions (Fig. 10). The spectra calculated for the ligand and L–Al species were found to be compatible with those obtained experimentally. It is important to mention that in case of [Al(L)]⁺ species all transitions are ligand based.

Table 4 Electronic transition calculated by TDDFT using B3LYP/CPCM method in acetonitrile solvent for ligand (H₂L) and [Al(L)]⁺

	Eexcitation (eV)	λexcitation (nm)	Osc. strength (f)	Key transition	Character
Ligand (H2L)	3.60	344.43	0.1608	HOMO−4 → LUMO+1 (42%)	π(L) → π*(L)
	3.57	346.63	0.0469	HOMO → LUMO+2 (29%)	π(L) → π*(L)
	3.19	388.28	0.902	HOMO−1 → LUMO (56%)	π(L) → π*(L)
	3.14	394.53	0.6101	HOMO−1 → LUMO+1 (48%)	π(L) → π*(L)
	3.019	410.64	0.0149	HOMO → LUMO (71%)	π(L) → π*(L)
[Al(L)]^+	3.17	390.79	0.2576	HOMO−1 → LUMO+1 (86%)	π(L) → π*(L)
	3.15	393.75	0.0254	HOMO → LUMO+2 (82%)	π(L) → π*(L)
	3.02	411.01	0.0011	HOMO−1 → LUMO (86%)	π(L) → π*(L)
	2.95	419.85	0.6153	HOMO → LUMO (89%)	π(L) → π*(L)

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Fig. 10 Pictorial representation of key transitions of H₂L and [Al(L)]⁺.

In order to have better understanding of the emission spectrum we have optimized the triplet state (T1) of both ligand (H₂L) and ([Al(L)]⁺) using same basis set. The emission wavelength obtained from the computation is very much comparable with that of experimental data. The data i.e. emission wavelength, emission energies and the nature of the transition obtained from the computation are given in Table 5. The emission band of the chemosensor (H₂L) at 510 nm was theoretically obtained at 509.84 nm with major key transitions of HOMO(α) → LUMO(α)+1 (58%), HOMO(α) → LUMO(α)+2 (10%) and HOMO(β)−1 → LUMO(β) (11%) respectively. Whereas for the [Al(L)]⁺ the emission band at 478 nm was theoretically obtained at 476.93 with major key transitions viz. HOMO(β) → LUMO(β)+2 (22%) and HOMO(α)−1 → LUMO(α) (66%) respectively.

Table 5 Emission spectrum calculated by TDDFT using B3LYP basis set for ligand (H₂L) and [Al(L)]⁺

	Eexcitation (eV)	λemission (nm)	Excited state	Osc. strength (f)	Key transition
Ligand (H2L)	2.43	509.84	5	0.0116	HOMO(α) → LUMO(α)+1 (58%)
					HOMO(β)−1 → LUMO(β) (11%)
					HOMO(α) → LUMO(α)+2 (10%)
[Al(L)]^+	2.17	476.93	8	0.2476	HOMO(α)−1 → LUMO(α) (66%)
					HOMO(β) → LUMO(β)+2 (22%)

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NMR studies

Firstly, the chemosensor was well characterized in CDCl₃ solvent. In CDCl₃, imine proton appeared at 8.43 ppm. Aromatic, aliphatic and methoxy protons appeared at 7.42–7.90 ppm, 3.82–3.84 ppm, 2.25 ppm and 4.01 ppm respectively. Then, we have performed ¹H NMR studies of both free H₂L and H₂L + Al³⁺ in 1 : 1 molar ratio in DMSO-d₆ solvent (Fig. 11). The free chemosensor (H₂L) exhibit clear peaks for different protons. Imine proton appears at 8.66 ppm. All aromatic protons appear as multiplate at around 7.74–7.30 ppm. Aliphatic protons appear as multiplate at 3.74 and 2.06 ppm respectively. –OCH₃ protons appear around 3.74 ppm. In NMR spectrum signal of aliphatic one –CH₂ got merged with that the signal of –OCH₃. Unfortunately we could not identify signal for phenolic OH proton due to extensive hydrogen bonding with solvent molecules. Upon addition of Al³⁺ to the chemosensor peak positions of different protons have changed significantly. Imine protons have shifted notably from 8.66 ppm to 9.24 ppm indicating coordination of imine nitrogen with aluminium ion. In case of aromatic and aliphatic protons broadening and overlapping of signals observed. Therefore, formation of complex 1 in the presence of Al³⁺ is strongly evidenced from the NMR spectral studies.

Fig. 11 ¹​H NMR titration of H₂L in presence of 1 eq. of Al³⁺ ion.

Molecular logic gates

The property of this chemosensor encourages us to construct a molecular logic gate with two binary inputs. The two inputs are Al³⁺ and Na₂EDTA where the output is monitored as change in fluorescence emission spectrum at 478 nm. In absence of both of the inputs there is no significant change in emission band which implies that the gate is ‘OFF’. On addition of Na₂EDTA to the chemosensor there is no significant changes, rather the emission band remained unchanged. So, in presence of Na₂EDTA the output was considered to be zero. On the other hand, upon addition of Al³⁺ ion solution alone to the chemosensor shows significant enhancement of the fluorescence intensity at 478 nm and now the output signal is 1 i.e. the gate is ‘ON’. However in presence of both of the inputs the fluorescence intensity is significantly quenched and the output is zero implying that the gate is ‘OFF’ again. These studies suggest that this molecular gate is acting as INHIBIT (INH) logic gate where the inputs and output are Al³⁺, Na₂EDTA and change in fluorescence intensity at 478 nm, respectively. It is basically an ‘AND’ gate where one of its inputs is negated. The input which is negated, acts to inhibit the gate. We can say that the gate will behave like an ‘AND’ gate only when the negated input is set at a logic level ‘0’. Here the negated input is Na₂EDTA. The respective truth table and pictorial diagram of the INH logic gate is depicted in Fig. 12 and Table 6.

Fig. 12 Pictorial representation of logic gate.

Table 6 Truth table

IN1 (Al^3+)	IN2 (EDTA)	Output (emission at 478 nm)
0	0	0
0	1	0
1	0	1
1	1	0

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Conclusion

In summary, an azo based low cost, simple, easy to prepare, chemosensor H₂L has been successfully prepared which is capable of recognizing Al³⁺ ion in presence of large number of other metal ions in HEPES buffer solution (1 : 100 v/v, HEPES buffer at pH 7.4) at 25 °C. The chemosensor has been structurally characterized. Fluorescence intensity of the probe has enhanced by ∼61 fold in presence of Al³⁺. The low detection limit of H₂L for Al³⁺ (6.93 nM) suggests that the chemosensor could be a good choice for efficient monitoring of the Al³⁺ in real samples. H₂L forms 1 : 1 complex with Al³⁺ which has been established by ¹H NMR, MS studies and further supported by DFT calculations. H₂L exhibited good fluorescence sensing ability towards Al³⁺ ion over a wide range of pH, therefore, H₂L could be successfully applied to living cells for detecting Al³⁺. We also establish molecular logic gates (INH) based on two inputs (Al³⁺ and EDTA) and one output. Thus, this probe could be considered as a potential candidate for sensing Al³⁺ in less organic solvents.

Acknowledgements

A. S. gratefully acknowledges the financial support of this work by the DST, India (Sanction No. SB/FT/CS-102/2014, dated – 18.07.2015). The authors are thankful to Prof. Mahammad Ali and Mr Pravat Ghorai, Department of Chemistry, Jadavpur University, India for their scientific suggestions. The authors also acknowledge the use of the DST-funded National Single Crystal X-ray Diffraction Facility at the Department of Chemistry, Jadavpur University, Kolkata-700032, India for X-ray crystallographic studies.

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Footnote

† Electronic supplementary information (ESI) available. CCDC 1478332 and 1478333 contain the supplementary crystallographic data for azoaldehyde and H₂L. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ra21217d

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