Highly sensitive and selective chemosensors for Cu²⁺ and Al³⁺ based on photoinduced electron transfer (PET) mechanism

Abstract

Two novel acridine based chemosensors 7a and 7b were synthesized and configured as “fluorophore–spacer–receptor” systems based on photoinduced electron transfer. The probes 7a and 7b exhibited high selectivity and sensitivity for the detection of Cu²⁺ and Al³⁺ respectively over commonly coexistent metal ions in CH₃CN. The binding association constants (K_a) of 7a–Cu²⁺ and 7b–Al³⁺ were found to be 4.0 × 10⁴ M⁻¹ and 1.7 × 10⁴ M⁻¹ in CH₃CN, and the corresponding detection limits were calculated to be 2.8 × 10⁻⁷ M and 5.8 × 10⁻⁷ M, respectively. The fluorescence response of 7a–Cu²⁺ and 7b–Al³⁺ with respect to pH change was studied and the resulting demonstrated fluorescence enhancement was observed in the pH range of 7.0–9.0. The chromophores were characterized by FT-IR, ¹H-NMR, ¹³C-NMR and HR Mass spectral analysis.

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1. Introduction

The design and synthesis of fluorescent chemosensors for transition metal ions in various biological systems are of growing interest because of the fundamental role of metal ions in chemical and biological processes.^1–6 Copper is the third most abundant transition metal ion (after Fe²⁺ and Zn²⁺) in the human body and is involved in various metabolic processes, for instance copper combines with proteins to produce enzymes involved in oxygen processing and also acts as a catalyst in various functions of the body.^7–9 It is an essential element at very low concentration for all organisms but in high concentration it is hazardous to living organisms. Excess concentration of Cu²⁺ in the human body causes severe neurological diseases such as Alzheimer’s disease, Menkes, Wilson’s disease and Parkinson’s disease.^10–12 Recently copper ions in prolonged exposure to human body have been suspected to cause liver and kidney damage in children.¹² Therefore it is important to detect copper at very low concentrations in biological systems. It is also observed that Cu²⁺ deficiency leads to severe or fatal problems in animals,^13,14 thus a deficiency or excessive intake of Cu²⁺ is very harmful and this demands development of molecular probes for selective detection of Cu²⁺ ions.

Aluminum plays a very important role in human life and happens to be the third most abundant element and the most abundant metal in the earth’s crust (approx. 8% by mass). Because of the high chemical reactivity of aluminum metal it is easily converted to Al³⁺ for instance due to acid rain and human activities.^15,16 Aluminum compounds are also frequently utilized as pharmaceutical drugs in human and veterinary medicine.^17,18 Among them, buffered aspirin containing aluminum glycinate is commonly used as an analgesic¹⁹ as well as in antacids.^20,21 Accumulation of excessive amounts of aluminum metal damages the kidneys,^22,23 the central nervous system causing Alzheimer’s disease,^24,25 it reduces total bone and matrix causing osteoporosis or osteomalacia^26,27 and kills fish in acidic water.²⁸ The incremental increase of Al³⁺ concentrations in the environment disturbs growing plants.^29,30 Also the excess usage of aluminum-based antiperspirants causes breast cancer in women.¹⁷ Thus the development of fluorescent chemosensors for detection of hazardous metal ions is becoming of more importance for biological and environmental reasons.

Several methods are currently used for the detection of metal ions like atomic absorption spectrometry,³¹ liquid-phase chromatography,³² solid-phase extraction,³³ X-ray fluorescence,³⁴ inductively coupled plasma mass spectrometry³⁵ and voltammetry.³⁶ Most of these techniques are expensive and time consuming as well as being less suitable for biological systems and in environmental media. This demands the development of chemosensors that can work for all samples in physiological conditions.³⁷

A variety of fluorescent chemosensors for Cu²⁺ have been reported based on fluorescence resonance energy transfer (FRET),³⁸ intramolecular charge transfer (ICT),³⁹ photoinduced electron transfer (PET),⁴⁰ excimers⁴¹ and chelation enhanced fluorescence (CHEF).⁴² Compared to the transition metals, the detection of Al³⁺ has always been challenging and problematic due to its poor coordination ability⁴³ but a few sensors for Al³⁺ have been reported.^44–49 However, there is still scope for improvement in the design of such sensors as they often suffer from problems such as narrow pH range, lower selectivity, less response time, unfavourable absorption emission, lower stability, higher synthesis cost and low fluorescence quantum yield. Of the available sensors, most fluorescent sensors are based on the PET mechanism, in which enhancement or quenching of fluorescence intensity is observed. A typical fluorescent sensor contains a receptor (the recognition site) linked to a fluorophore (the signal source) which translates the recognition event into the fluorescence signal.⁵⁰ Thus the receptor unit must have strong binding affinity to the relevant target. Photoinduced electron transfer based sensors usually have three components such as fluorophore–spacer–receptor in which the ionic or molecular input at the receptor site modulates the emission such as fluorescence quantum yield or lifetime and leads to an On–Off or Off–On sensing mechanism.⁵¹ The choice of the receptor depends entirely on the target analytes. In the present paper, we have presented PET based sensors for Cu²⁺ and Al³⁺ in the presence of other metal ions in CH₃CN. The binding study of metals with the probe shows that the nature of the receptor plays a very important role in the selective detection of metal ions. The chromophores studied show a fluorescence turn on response to Cu²⁺ and Al³⁺, based on the classical photoinduced electron transfer principle developed by de Silva,^52,53 and are stable for the pH range 7.0–9.0.

2. Results and discussion

2.1. Chemistry

The acridine derivatives 7a and 7b were synthesized by substitution, formylation and addition reactions. The intermediate 1 was prepared from aniline and methyl bromoacetate in basic media by nucleophilic substitution reaction and was further converted to compound 2 by formylation reaction.⁵⁴ The intermediate 4 was prepared by formylation of the compound 3 in DMF/POCl₃ at 80 °C. The intermediates 2 and 4 were reacted with dimedone and underwent a classical Knoevenagel condensation leading to the formation of the tetraketone 5a–b, which on cyclization in the presence of ammonium acetate in methanol led to formation of the acridine esters 6a–b. Hydrolysis of the acridine esters in 10% NaOH led to formation of 7a–b. The structures of the dyes were confirmed by FT-IR, ¹H NMR, ¹³C NMR and mass spectral analysis. The synthetic details are presented in Scheme 1.

Scheme 1 Synthesis of acridine sensors 7a and 7b.

2.2. Absorption and emission properties

2.2.1. Metal sensing study. The binding activities of the sensors 7a and 7b with various metal ions such as Zn²⁺, Hg²⁺, Cd²⁺, Fe²⁺, Ba²⁺, Mn²⁺, Ni²⁺, Mg²⁺, Co²⁺, Cu²⁺, Cu⁺, Ag⁺, Na⁺, K⁺ and Al³⁺ were investigated by UV-visible absorption and fluorescence spectroscopy. The free probes 7a (20 μM) and 7b (20 μM) exhibited two absorption peaks at 305 and 365 nm in CH₃CN. With the addition of Cu²⁺ (5 equiv.) to the probe 7a (20 μM) in CH₃CN, the absorption peak at 305 nm disappeared. In contrast, no significant change was observed for the absorption band at 365 nm (Fig. S1, ESI†). The addition of Al³⁺ (5 equiv.) to the probe 7b (20 μM) led to a decrease in the absorbance at 365 nm while the absorption peak at 305 nm disappeared (Fig. S2, ESI†). However, no remarkable change in the absorption spectra was observed after the addition of other metal ions to probes 7a and 7b. Upon the addition of increasing concentrations of Cu²⁺ (0 to 5 equiv.) to the probe 7a (10 μM), a slight increase in the absorbance was observed at 365 nm while the absorption peak at 305 nm gradually disappeared (Fig. S3, ESI†). A similar trend was observed for the addition of Al³⁺ (0 to 5 equiv.) to 7b (10 μM) in CH₃CN (Fig. S4, ESI†). In addition, the fluorescence spectrum was obtained by excitation at 365 nm. In the absence of metal ions the probes 7a–b showed very weak fluorescence signals in the range from 420 to 440 nm in CH₃CN. Upon the addition of Cu²⁺ (5 equiv.) to 7a (20 μM) and Al³⁺ (5 equiv.) to 7b (10 μM) significant enhancement in the fluorescence intensity was observed at 420–440 nm (Fig. 1 and 2), whereas no considerable change in the fluorescence intensity was observed for other metal ions.

Fig. 1 Fluorescence emission spectra of 7a (20 μM) upon addition of various metal ions (5 equiv.) in CH₃CN. λ_ex = 365 nm (slit widths: 5 nm/5 nm).

Fig. 2 Fluorescent emission spectra of 7b (20 μM) upon addition of various metal ions (5 equiv.) in CH₃CN. λ_ex = 365 nm (slit widths: 5 nm/5 nm).

To investigate the interaction of Cu²⁺ with probe 7a and Al³⁺ with probe 7b, the emission spectra with varying Cu²⁺ and Al³⁺ concentrations in CH₃CN were recorded separately. The fluorescence intensity of the probe 7a (10 μM) reaches a maximum when 5 equivalents of Cu²⁺ were added (Fig. 3). After the addition of 5 equivalents of Cu²⁺ to probe 7a (10 μM), a blue shifted emission from 429 nm to 420 nm (∼9 nm) was observed. However, the fluorescence intensity of 7b (10 μM) increased gradually at 425 nm upon addition of increasing concentrations of Al³⁺ (0 to 5 equiv.) in CH₃CN (Fig. 4) while no shift in maximum emission was observed. It is, thus, expected that there is appreciable interaction between the probes 7a and 7b with Cu²⁺ and Al³⁺ respectively.

Fig. 3 Changes in the fluorescence emission spectra of probe 7a (10 μM) in CH₃CN upon titration with 0 to 5.0 equiv. of Cu²⁺. Inset: the relative fluorescence intensity (R.F.I) at 420 nm as a function of Cu²⁺ ion concentration. λ_ex = 365 nm (slit widths: 5 nm/5 nm).

Fig. 4 Changes in the fluorescence emission spectra of probe 7b (10 μM) in CH₃CN upon titration with 0 to 5.0 equiv. of Al³⁺. Inset: the relative fluorescence intensity (R.F.I) at 427 nm as a function of Al³⁺ ion concentration. λ_ex = 365 nm (slit widths: 5 nm/5 nm).

From the photophysical properties it is observed that the binding of the sensors 7a–b to Cu²⁺ and Al³⁺ ions takes place by electrostatic interactions with the –COOH and the nitrogen atom present in the receptor part of the molecule (Fig. 5). By conjugating a receptor to the metal ion, the binding would perturb the energy levels of the excited states, which results in a change of emission wavelength. The sensing ability depends on the size of the receptor, the spacer and the availability of binding units which form the coordination complex with the metal ions. It also depends upon the geometry of the coordination complex formed after ligand to metal binding. Cu²⁺ and Al³⁺ have different chemical coordination characteristics. Thus, the sensor 7a is connected to the receptor by a short spacer which shows selective fluorescence enhancement to Cu²⁺ ions while for sensor 7b, the receptor is connected to the fluorophore by a long spacer which shows strong selectivity and sensitivity to Al³⁺ ions.

Fig. 5 Proposed binding mechanism of Cu²⁺ and Al³⁺ with sensors 7a and 7b.

The selectivity of the probe 7a (10 μM) was studied in the presence of other competitive metal ions. The competition experiments revealed that the Cu²⁺ induced fluorescence enhancement was unaffected in 5 equivalents of environmentally relevant alkali or alkaline-earth metals, such as Na⁺, K⁺ and Mg²⁺ as well as the other transition metal ions Zn²⁺, Ni²⁺, Cd²⁺, Co²⁺, Mn²⁺, Fe²⁺ (Fig. 6). Obviously, all of these results confirmed that our proposed chemosensor 7a (10 μM) has remarkably high selectivity toward Cu²⁺ ions over the other competitive metal ions in CH₃CN. Similar observations are made for the sensor 7b in the presence of other metal ions (Fig. 7).

Fig. 6 Relative fluorescence intensity (R.F.I) of probe 7a (10.0 μM) in the presence of 2.0 equiv. of other metal ions (blue bars) in CH₃CN. The red bars represent the change of the emission that occurs upon the subsequent addition of 2.0 equiv. of Cu²⁺ to the above solution (25 °C), λ_ex = 365 nm (slit widths: 5 nm/5 nm).

Fig. 7 Relative fluorescence intensity (R.F.I) of probe 7b (10.0 μM) in the presence of 2.0 equiv. of other metal ions (blue bars) in CH₃CN. The red bars represent the change of the emission that occurs upon the subsequent addition of 2.0 equiv. of Al³⁺ to the above solution (25 °C). λ_ex = 365 nm (slit widths: 5 nm/5 nm).

The quantum yields of the free sensors as well as the metal–sensor complex were evaluated by relative methods.

The fluorescence quantum yields of 7a without and with Cu²⁺ were 0.083 and 0.85, respectively. Thus a 10 fold increase in fluorescence quantum yield of the sensor 7a was observed after binding with Cu²⁺ in CH₃CN. In the case of sensor 7b, a 15 fold increase in quantum yield was observed for the 7b–Al³⁺ complex (0.81) as compared to the free sensor (0.054). The change in fluorescence intensity of the probe 7a (10 μM) in 5 eq. of Cu²⁺ in a CH₃CN/H₂O mixture was studied at pH 9.0. Notably, we observed that the fluorescence intensity remains unaltered up to 60% CH₃CN and 40% H₂O mixture. However, further increase in H₂O concentration leads to a slight decrease in the fluorescence intensity (Fig. S5, ESI†).

The effect of pH on 7a, 7b and their complexes 7a–Cu²⁺, 7b–Al³⁺ were studied over a pH range of 3–12. The fluorescence intensity of the sensors 7a and 7b decreases from pH 3 to 7 and after that it remains constant to pH = 12. The fluorescence intensity of the 7a–Cu²⁺ complex increases slightly from pH 3 to pH 7 and after that remains constant up to pH 12. In case of the 7b–Al³⁺ complex, the fluorescence intensity slightly increases from pH 3 to pH 7 and it remains constant from pH 7 to pH 9. After pH 9 the fluorescence intensity of 7b–Al³⁺ complex decreases suddenly because of the hydrolysis of Al³⁺ under basic conditions (Fig. S6, ESI†). At lower pH both the probes 7a and 7b showed high fluorescence intensity. In acidic pH conditions the protonation of the aromatic amine nitrogen takes place which suppress the photoinduced electron transfer mechanism.

The binding association constants (K_a) for 7a–Cu²⁺ and 7b–Al³⁺ were determined from the Benesi–Hildebrand equation. The binding association constants for 7a with Cu²⁺ and 7b with Al³⁺ were observed to be 4.0 × 10⁴ M⁻¹ and 1.7 × 10⁴ M⁻¹, respectively. The estimated limit of detection (LOD) came out to be 2.8 × 10⁻⁷ M for 7a–Cu²⁺ and 5.8 × 10⁻⁷ M for the 7b–Al³⁺ complex in CH₃CN (Fig. S7, ESI†) and were compared with some recently reported Cu²⁺ and Al³⁺ sensors (Table 1).

Table 1 Performance comparison of various chemosensors reported for Cu²⁺ and Al³⁺ detection

Sr. no.	Receptor	Metal ion	Association constant (Ka) (M^−1)	Limit of detection (M)	Reference
1		Cu^2+	8.55 × 10^5	2.55 × 10^−6	57
2		Cu^2+	1.5 × 10^4	1.15 × 10^−6	58
3		Al^3+	8.84 × 10^3	5.0 × 10^−7	59
4		Cu^2+	NA	1.0 × 10^−4	60
5		Cu^2+	NA	1.0 × 10^−4	60
6		Al^3+	8.32 × 10^6	3.28 × 10^−6	61
7		Cu^2+	4.0 × 10^4	2.8 × 10^−7	This work
8		Al^3+	1.7 × 10^4	5.8 × 10^−7	This work

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The binding stoichiometry of the 7a–Cu²⁺ complex was determined using the continuous variation (Job’s) method (Fig. S8a, ESI†). When the molar fraction of the sensor was 0.5, the absorbance value approached a maximum, which demonstrated the formation of a 1 : 1 complex between the receptor sensor and Cu²⁺. A similar trend was observed for the 7b–Al³⁺ complex (Fig. S8b, ESI†). The reaction medium was investigated to obtain a suitable reaction system. The fluorescence response of 7a (20 μM) with Cu²⁺ (5 equiv.) was studied in different solvents such as CH₃CN, MeOH, EtOH and DMF. It was observed that in CH₃CN the fluorescence intensity of 7a–Cu²⁺ was slightly higher than in MeOH and EtOH. However, a noticeable decrease in the fluorescence intensity was observed for DMF (Fig. S9, ESI†). So, CH₃CN was chosen as the solvent for the analyses. The binding of Cu²⁺ with 7a and Al³⁺ with 7b was further supported by ¹H NMR study in DMSO-d₆ (Fig. S14 and S15, ESI†). The interaction of Cu²⁺ or Al³⁺ with the probes takes place through the carboxylic oxygen and nitrogen lone-pairs present in the receptor part of the molecule. Other parts of the molecule are not involved in the complex formation with Cu²⁺ and Al³⁺. Upon the addition of metal ions to 7a and 7b a shift in the signals for the receptor protons adjacent to the carboxylic acid group was not observed. In contrast, before the addition of metal ions the probes 7a and 7b showed signals at δ = 12.67 ppm and δ = 12.17 ppm respectively accounting for the two acid protons. But after addition of one equivalent of Cu²⁺ to probe 7a and of Al³⁺ to the probe 7b, the signal at δ = 12.67 ppm and δ = 12.17 ppm disappeared which supports the binding of Cu²⁺ and Al³⁺ to the probe 7a and 7b through the –COOH group.

3. Experimental

3.1. Methods and materials

All the reagents were purchased from S. D. Fine Chemical Limited (India) of commercial grade and used without further purification. All the common chemicals were of analytical grade. The solvents were purified by standard procedures. All the reactions were monitored by TLC (thin-layer chromatography) with detection by UV light. The absorption spectra of the chromophores were recorded on a Perkin-Elmer spectrophotometer, Lambda 25. The emission spectra were recorded on a Varian Cary Eclipse fluorescence spectrophotometer. Freshly prepared solutions in solvents of different polarities at a concentration of 1 × 10⁻⁶ mol L⁻¹ solution were used in 1 cm optical path length quartz cuvettes. The photophysical properties were investigated using solvatochromic and solvatofluoric behaviours of the chromophores. The excitation wavelength used for fluorescence measurements was the absorption maxima of the compounds in respective solvents. The FT-IR spectra were recorded on a Perkin-Elmer Spectrum 100 FT-IR Spectrometer. ¹H NMR spectra were recorded on a Varian 500 MHz instrument using TMS as an internal standard. Mass spectra were recorded on a Finnigan mass spectrometer.

3.2. Preparation of salt solution

Commercially available salts such as ZnCl₂·6H₂O, HgCl₂, CdCl₂, FeCl₂·4H₂O, BaCl₂·2H₂O, MnCl₂·4H₂O, NiCl₂·6H₂O, MgCl₂·6H₂O, CoCl₂·6H₂O, CuCl₂·2H₂O, CuCl, NaCl, KCl, AgNO₃ and Al(NO₃)₃·9H₂O were used to prepare stock solutions of metal ions in doubly distilled deionised water at room temperature.

Quantum yield determinations. The relative quantum yields of synthesized compounds in different solvents were calculated by using eqn (1). The refractive indices of the solvents have been taken from the literature.⁵⁵ The quantum yields of the sensors 7a–b were evaluated in free as well as metal bound form in acetonitrile at room temperature by using quinine sulfate in 0.1 N H₂SO₄ (Φ_f = 0.51) as a standard.^55,56where: ϕ_x = quantum yield of compound, Φ_st = quantum yield of standard sample. Grad_x = gradient of compound, Grad_st = gradient of standard sample η_x = refractive index of solvent used for synthesized compound, η_st = refractive index of solvent used for standard sample.

3.3. Synthesis

3.3.1. Synthesis of intermediate 4. Phosphorus oxychloride (POCl₃) (2.75 mL, 0.03 mol) was slowly added to dimethyl formamide (DMF) (3.65 mL, 0.05 mol) at 5–10 °C under constant stirring. To this cooled reagent N-substituted amino benzene (0.01 mmol) was added by dissolving it in DMF (6 mL) under constant stirring and the resulting mixture was heated at 75 °C for 4 h. The reaction mixture was cooled to room temperature and then poured into ice cold water (60 mL). The reaction mass was neutralized with sodium carbonate and extracted with ethyl acetate. The organic layer was dried with sodium sulphate and evaporated on a rotary evaporator to yield a brown liquid which was used for further reaction. Yield: 85%. FT-IR (cm⁻¹): 1730 (ester), 1705 (aldehyde), 1650, 1600 (–C=C–, aromatic). ¹H NMR (500 MHz, CDCl₃, ppm, Me₄Si): δ = 2.62 (t, 4H, –CH₂), 3.68 (s, 6H, –CH₃), 3.75 (t, 4H, –CH₂), 6.71 (d, J = 9 Hz, 2H), 7.72 (d, J = 9 Hz, 2H), 9.72 (s, 1H). ¹³C NMR (125.6 MHz, CDCl₃, ppm, Me₄Si): 32.0, 46.7, 51.9, 111.2, 126.0, 132.2, 151.3, 171.9, 190.2.

3.3.2. Synthesis of intermediates 5a–b. Dimedone (2.0 mmol) and the substituted aromatic aldehyde (1.0 mmol) were stirred in ethylene glycol at 80 °C for 6 h. The progress of the reaction was monitored by TLC. After completion of reaction, the reaction mixture was poured into water and the resulting solid was filtered and dried. The crude product was recrystallized from 95% ethanol.
5a. Yield = 95%, melting point: 190 °C. FT-IR (cm⁻¹): 2955 (–CH), 1770, 1740 (ester), 1665 (ketone), 1530 (–C=C–, aromatic). ¹H NMR (500 MHz, CDCl₃, ppm, Me₄Si): δ = 1.09 (s, 6H, –CH₃), 1.21 (s, 6H, –CH₃), 2.35 (m, 10H), 3.79 (s, 6H), 4.11 (s, 4H), 5.43 (s, 1H), 6.52 (d, J = 9 Hz, 2H), 6.92 (d, J = 9 Hz, 2H). ¹³C NMR (125.6 MHz, CDCl₃, ppm, Me₄Si): 27.3, 29.7, 31.3, 46.4, 47.1, 52.1, 53.3, 112.4, 115.7, 127.8, 145.8, 171.5, 190.2. Mass: m/z 527.4 [M]⁺.
5b. Yield = 93%, melting point: 195 °C. FT-IR (cm⁻¹): 2954 (–CH), 1766, 1741 (ester), 1662 (ketone), 1519 (–C=C–, aromatic). ¹H NMR (500 MHz, CDCl₃, ppm, Me₄Si): δ = 1.09 (s, 6H, –CH₃), 1.22 (s, 6H, –CH₃), 2.38 (broad, 10H), 2.57 (t, 4H), 3.61 (t, 4H), 3.66 (s, 6H, –CH₃), 5.44 (s, 1H), 6.61 (d, J = 9 Hz, 2H), 6.93 (d, J = 9 Hz, 2H). ¹³C NMR (125.6 MHz, CDCl₃, ppm, Me₄Si): 27.1, 29.7, 31.3, 31.8, 32.3, 46.4, 46.9, 51.7, 112.5, 115.8, 127.9, 144.7, 172.5, 190.3. Mass: m/z 556.9 [M + H]⁺.

3.3.3. Synthesis of intermediates 6a–b. The tetraketone intermediates 5a–b (1.0 mmol) were taken up in aq. ethanol, and ammonium acetate (3.0 mmol) was added and the reaction mixture heated for 6 h. The progress of the reaction was monitored by TLC. Ethanol was evaporated and the mixture diluted with water. The resulting solid was filtered and dried. The crude product was recrystallized from 95% ethanol.
6a. Yield = 91%, melting point: 210 °C. FT-IR (cm⁻¹): 2960 (–CH), 1727 (ester), 1661 (ketone), 1608, 1514 (–C=C–, aromatic). ¹H NMR (500 MHz, DMSO-d₆, ppm, Me₄Si): δ = 0.87 (s, 6H, –CH₃), 0.98 (s, 6H, –CH₃), 1.98–2.12 (2d, J = 16 Hz, 4H –CH₂), 2.35 (s, 4H), 3.79 (s, 6H), 3.98 (s, 4H), 4.77 (s, 1H), 6.26 (d, J = 9 Hz, 2H), 6.91 (d, J = 9 Hz, 2H). ¹³C NMR (125.6 MHz, DMSO-d₆, ppm, Me₄Si): 27.3, 29.4, 32.0, 32.6, 46.4, 50.8, 53.4, 111.1, 112.3, 128.6, 136.5, 145.9, 149.3, 173.1, 194.8. Mass: m/z 479.51 [M − H]⁺.
6b. Yield = 92%, melting point: 221 °C. FT-IR (cm⁻¹): 2963 (–CH), 1729 (ester), 1666 (ketone), 1618, 1519 (–C=C–, aromatic). ¹H NMR (500 MHz, DMSO-d₆, ppm, Me₄Si): δ = 0.99 (s, 6H,–CH₃), 1.07 (s, 6H,–CH₃), 1.98–2.12 (2d, J = 16 Hz, 4H, –CH₂), 2.32 (m, 4H), 2.52 (t, 4H), 3.55 (t, 4H), 3.64 (s, 6H), 4.97 (s, 1H), 6.62 (d, J = 9 Hz, 2H), 7.16 (d, J = 9 Hz, 2H). ¹³C NMR (125.6 MHz, DMSO-d₆, ppm, Me₄Si): 27.4, 29.3, 32.4, 31.7, 41.3, 47.0, 50.8, 51.6, 112.3, 114.1, 129.0, 135.7, 144.9, 146.9, 172.7, 195.3. Mass: m/z 537.5 [M + H]⁺.

3.3.4. Synthesis of sensors 7a–b. The ester intermediates 6a–b (1.0 mmol) were taken up in aq. ethanol, and 10% sodium hydroxide (3.0 mmol) was added and the reaction mixture refluxed for 2 h. The progress of the reaction was monitored by TLC. Ethanol was evaporated and water (10 mL) was added to the residue. The resultant reaction mass was acidified with dil. HCl to pH 6. The obtained solid was filtered, dried and recrystallized using by 95% ethanol.
7a. Yield = 91%, melting point: 227 °C. FT-IR (cm⁻¹): 2963 (–CH), 1729 (acid), 1666 (ketone), 1618, 1519 (–C=C–, aromatic). ¹H NMR (500 MHz, DMSO-d₆, ppm, Me₄Si): δ = 0.87 (s, 6H, –CH₃), 0.98 (s, 6H, –CH₃), 1.98–2.12 (2d, J = 16 Hz, 4H –CH₂), 2.35 (s, 4H), 3.98 (s, 4H), 4.67 (s, 1H), 6.26 (d, J = 9 Hz, 2H), 6.91 (d, J = 9 Hz, 2H), 9.15 (s, 1H, NH), 12.67 (s, 2H). ¹³C NMR (125.6 MHz, DMSO-d₆, ppm, Me₄Si): 27.3, 29.4, 32.0, 32.6, 50.8, 53.4, 111.1, 112.3, 128.6, 136.5, 145.9, 149.3, 173.1, 194.9. HRMS m/z [M + Na]⁺ Calcd for C₂₇H₃₂N₂O₆Na: 503.2158. Found: 503.2159. Mass: m/z 503.59 [M + Na]⁺.
7b. Yield = 92%, melting point: 221 °C. FT-IR (cm⁻¹): 2963 (–CH), 1732 (acid), 1636 (ketone), 1603, 1519 (–C=C–, aromatic). ¹H NMR (500 MHz, DMSO-d₆, ppm, Me₄Si): δ = 0.88 (s, 6H, –CH₃), 0.98 (s, 6H, –CH₃), 1.98–2.12 (2d, J = 16 Hz, 4H, –CH₂), 2.32 (s, 4H), 2.48 (t, 4H), 3.44 (t, 4H), 4.67 (s, 1H), 6.45 (d, J = 9 Hz, 2H), 6.93 (d, J = 9 Hz, 2H), 9.15 (s, 1H, NH), 12.17 (s, 2H). ¹³C NMR (125.6 MHz, DMSO-d₆, ppm, Me₄Si): 27.3, 29.4, 31.8, 32.5, 32.6, 46.7, 50.8, 111.8, 112.3, 128.8, 135.8, 145.1, 149.3, 173.7, 194.8. HRMS m/z [M + Na]⁺ Calcd for C₂₉H₃₆N₂O₆Na: 531.2471. Found: 531.2474. Mass: m/z 531.62 [M + Na]⁺.

4. Conclusion

In summary, we have demonstrated that the two chemosensors 7a–b showed ideal photoinduced electron transfer behavior for the detection of Cu²⁺ and Al³⁺ respectively. The absorption spectra do not change significantly upon binding with various metal ions. In contrast, the emission spectra change dramatically after addition of Cu²⁺ to probe 7a and Al³⁺ to probe 7b even in the presence of other metal ions. Upon the addition of Cu²⁺ to the probe 7a in CH₃CN, a 10 fold enhancement in the fluorescence intensity was observed. However, probe 7b showed a 15 fold enhancement in the fluorescence intensity with Al³⁺ in CH₃CN. The binding of Cu²⁺ and Al³⁺ to the sensors resulted in maximum fluorescence enhancement in the pH range of 7.0–9.0. The limit of detection (LOD) came out to be 2.8 × 10⁻⁷ M for 7a–Cu²⁺ and 5.8 × 10⁻⁷ M for the 7b–Al³⁺ complex in CH₃CN which makes 7a and 7b suitable candidates for the development of a potential probe for biological applications.

Acknowledgements

Santosh Chemate is thankful for JRF and SRF fellowships from the Principal Scientific Adviser (PSA), Government of India.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra00123d

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