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. 2023 Feb 13;8(7):6349–6360. doi: 10.1021/acsomega.2c06559

Dual-Mode Multiple Ion Sensing via Analyte-Specific Modulation of Keto–Enol Tautomerization of an ESIPT Active Pyrene Derivative: Experimental Findings and Computational Rationalization

Suvendu Paul †, Abhijnan Ray Choudhury ‡, Nilanjan Dey †,*
PMCID: PMC9947992  PMID: 36844601

Abstract

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A pyrene-based excited-state intramolecular proton transfer (ESIPT) active probe PMHMP was synthesized, characterized, and employed for the ppb-level, dual-mode, and high-fidelity detection of Cu2+ (LOD: 7.8 ppb) and Zn2+ ions (LOD: 4.2 ppb) in acetonitrile medium. The colorless solution of PMHMP turned yellow upon the addition of Cu2+, suggesting its ratiometric, naked-eye sensing. On the contrary, Zn2+ ions displayed concentration-dependent fluorescence rise till a 0.5 mole fraction and subsequent quenching. Mechanistic investigations indicated the formation of a 1:2 exciplex (Zn2+:PMHMP) at a lower concentration of Zn2+, which eventually turned into a more stable 1:1 (Zn2+:PMHMP) complex with an additional amount of Zn2+ ions. However, in both cases, it was observed that the hydroxyl group and the nitrogen atom of the azomethine unit were involved in the metal ion coordination, which eventually altered the ESIPT emission. Furthermore, a green-fluorescent 2:1 PMHMP–Zn2+ complex was developed and additionally employed for the fluorimetric analysis of both Cu2+ and H2PO4– ions. The Cu2+ ion, owing to its higher binding affinity for PMHMP, could replace the Zn2+ ion from the preformed complex. On the other hand, H2PO4– formed a tertiary adduct with the Zn2+–complex, leading to a distinguishable optical signal. Furthermore, extensive and organized density functional theory calculations were performed to explore the ESIPT behavior of PMHMP and the geometrical and electronic properties of the metal complexes.

1. Introduction

Due to their significant role in physiochemical processes, selective recognition of transition metal ions particularly zinc and copper has become a focus of numerous studies in supramolecular chemistry. The Zn2+ ion, the second most abundant transition metal ion in the human body, plays essential roles in various cellular activities, such as regulation of gene expression, apoptosis, metalloenzyme catalysis, neurotransmission in biological systems, etc.1 However, it has also been reported that the Zn2+ ion is a potent killer of neurons via oxidative stress.2 Compared to other tissues, pancreatic islets contain relatively large concentrations of Zn2+, which play a critical role in insulin biosynthesis, storage, and secretion.3 A decrease in the concentration of Zn2+ can cause a reduction in the ability of the islet cells to produce and secrete insulin.4 The Zn2+ ion does not give a spectroscopic or magnetic signal originating from its 3d104s0 electronic configuration; thus, the fluorescence method is a practical choice for detecting Zn2+.5 On the other hand, the soft transition metal ion Cu2+ is the third most abundant essential trace element in the human body. Various redox processes, enzyme functions, and pigments involve copper ions as their cofactors.6 The World Health Organization (WHO) suggests that 10–12 mg per day must be the upper limit for copper consumption.7 Free solvated copper ions can catalyze the formation of reactive organic species, including radical and non-radical species that participate in the initiation and/or propagation of radical chain reactions that can damage biomolecules.8 Copper toxicity causes oxidative stress and related symptoms, leading to diabetes and neurodegenerative disorders such as Alzheimer’s, Parkinson’s, Menkes, and Wilson’s diseases.9 However, despite a considerable number of optical probes reported in the literature for simultaneous detection of both Zn2+ and Cu2+ ions,10−25 there is still a demand for new colorimetric probes that can make distinguishable “naked-eye” detection in the visible-wavelength region. This is particularly fascinating as it does not involve sophisticated instruments or skilled technicians for execution.

At the same time, phosphates also play a vital role in chemistry and biology. Phosphates are the essential building blocks for nucleic acids and thus play an important role in protein and enzyme synthesis. Also, phosphates are necessary for bone and teeth formation.26,27 Therefore, the development of selective receptors for phosphate anions and derivatives, such as H2PO4–, pyrophosphate (PPi, P2O74–), adenosine triphosphate (ATP), adenosine diphosphate (ADP), guanosine monophosphate, adenosine monophosphate, and phosphoproteins, has reached the peak interest because of the vital roles that these chemical species play in a range of life processes.28,29 In particular, fluorescent sensors are appealing as they allow for low-detection limit analysis and imaging.30,31

The excited-state intramolecular proton transfer (ESIPT) is one of the most popular photoprocesses in the field of colorimetric and fluorescent detection. The ESIPT behavior of the fluorophore PMHMP (Scheme 1) can be well presented in Figure 1.32−34 Naturally, monitoring of the “on–off” switching between the tautomers of an ESIPT active probe and the detection and imaging of important cations, vital anions, and biological molecules have been achieved during the last 2 decades.35−43 However, most surprisingly, there is almost no pyrene-based ESIPT active probe that has been utilized in the detection and imaging of analytes.44 Unsurprisingly, the uniqueness and novelty of the present report are still significant in this sense. Moreover, a comparison table is designed assembling the ESIPT active probes that are involved in colorimetric and/or fluorescent detection of Cu2+ and Zn2+ ions. Table 1 displays two major concerns. The first one is the unavailability of an ESIPT active probe that can recognize both Cu2+ and Zn2+ ions in pure aqueous medium. There is no such ESIPT active pyrene derivative that is successful in dual-mode detection of Cu2+ and Zn2+ ions. Naturally, such variable concentration-dependent fluorescence behavior of the probe in the presence of Zn2+ is rare, interesting, and of high significance.

Scheme 1. Molecular Structure of PMHMP.

Scheme 1

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Figure 1.

Figure 1

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(a) Keto–enol tautomers of PMHMP along with the plausible ESIPT process. (b) UV–visible and fluorescence spectrum of PMHMP (10 μM, λex = 350 nm) in acetonitrile medium.

Table 1. Comparison Table Containing the ESIPT Active Probes Involved in Colorimetric and/or Fluorescent Detection of Cu2+ and Zn2+ Including the Probe Structure, LOD of the Ions, Medium of Detection, and Fluorescence Response.

1.

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On the other hand, molecular logic gates are also one research focus of chemistry for further miniaturization in information technology. Various chemical systems have been developed to improvise different logic functions such as AND, OR, and NOT and their integrated operations.45−47 Among them, those exhibiting more than one output channel with single molecules are currently of particular interest because they are the basis for constructing molecular logic circuits capable of executing special arithmetic operations and future security devices. In addition, many functional integrated logic gates such as INHIBIT, the half-subtractor, the half-adder, the full adder, and the full subtractor with various single molecules have been exploited.48 Remembering these expediencies, PMHMP with a convenient logic function might be important in the field of the binary system and intelligent opto-chemical devices.

This report assembled the exploration of the ESIPT behavior of a pyrene-based amphiphilic probe (PMHMP) and dual-mode sensing of Cu2+ and Zn2+ ions along with H2PO4– and its opto-chemical application in the logic function. The addition of Cu2+ induced the formation of a yellow-colored solution and quenching of keto emission. On the other hand, at a lower concentration of Zn2+ ions, a bright-green fluorescence was observed due to exciplex formation. However, upon increasing the concentration further, we could observe the formation of a “thermodynamically stable” 1:1 complex with the Zn2+ ion with a relatively faint fluorescence signal. These results observed here can be used to construct an INHIBIT logic gate where both Cu2+ and Zn2+ ions will be involved as input signals. Furthermore, we prepared a highly fluorescent 1:2 Zn–PMHMP complex and employed the same for the fluorimetric detection of Cu2+ and H2PO4– ions. Furthermore, the ESIPT phenomenon and ion sensing mechanism of PMHMP were explored theoretically. To the best of our knowledge, there is practically no ESIPT active pyrene derivative probe in the existing literature which could be involved in the selective detection of Cu2+, Zn2+, and H2PO4– with such potential competency.

2. Experimental Section

2.1. Materials and Methods

All the chemicals, including the starting materials, cationic salts (Ag+, Co2+, Ca2+, Cd2+, Cu2+, Hg2+, Ni2+, Pb2+, Mg2+, and Zn2+), anionic tetrabutylammonium (TBA+X–; X– = F–, Cl–, Br–, I–, PF6–, ClO4–, AcO–, CN–, H2PO4–, NO3–, N3–, and CO32–) salts, solvents, and silica gel, were obtained from the reliable and reputed suppliers, like Sigma-Aldrich (Merck, Massachusetts, USA), Spectrochem (Spectrochem, Mumbai, India), etc., and used without further purification.

2.2. Methods

1H NMR and 13C NMR spectra were recorded in dimethyl sulfoxide-d6 (DMSO-d6) using a Bruker Advance DRX 400 spectrometer operating at 400 and 100 MHz, respectively. Fourier-transform infrared (FT-IR) spectra were recorded using KBr pellets on a PerkinElmer FT-IR spectrum BX (PerkinElmer, Massachusetts, USA). The mass spectrum was recorded using a Micromass ESI-TOF MS instrument (Bruker, Massachusetts, USA). Elemental analysis was recorded using Thermo Finnigan EA FLASH 1112 SERIES (Thermo Finnigan, California, United States). The UV–visible absorption spectra were obtained on a UV-2100 spectrophotometer (Shimadzu, Osaka, Japan). Fluorescence spectra were recorded on a Fluorolog Horiba Jobin Yvon spectrofluorometer (Horiba, Kyoto, Japan). The stock solutions of the probe molecules were prepared in DMSO and diluted with acetonitrile before the spectroscopic studies. The slit width for the experiment was kept at 5 nm. Sensing was carried out by adding requisite amounts of cations to the acetonitrile solution of PMHMP (1 × 10–5 M).

2.3. Computational Details

All the quantum mechanical calculations of the neutral and cationic species were executed using the Gaussian 09W software package ignoring symmetrical constraints.49 The ground (S0) and first singlet excited state (S1) calculations were performed with density functional theory (DFT) and time-dependent DFT (TD-DFT) methods, respectively, in combination with an M06-2X theoretical model and 6-311G basis set.50,51 In the beginning, all the species were optimized in vacuum and then in acetonitrile (ε = 35.688), combining the integral equation formalism polarizable continuum model (IEFPCM).52−55 For all the species, the global minima were confirmed by stability calculations and the IR spectral calculations with no imaginary frequency for S0 and one for S1. The potential energy scans of the ESIPT active probe were performed by altering one particular coordinate against the potential energy. The molecular electrostatic potential (MEP) 3D and highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) images were extracted from the check-point files. For more insights into computational methods, authors might follow somewhere else.56,57

3. Results and Discussion

3.1. Spectroscopic Studies

3.1.1. UV–Visible Studies

The fluorophore PMHMP was synthesized by following the procedure reported in the literature44 and characterized accordingly (Scheme 1, Supporting Information). A weak peak characterizes the UV–vis spectrum of PMHMP in the 300 nm region, presumably caused by the n−π* transition of the C=N group. Additionally, two prominent bands were observed in the 387 and 413 nm regions, which can be attributed to the π–π* transitions of phenyl and pyrene rings, respectively.58 A distinct color change from colorless to yellow was observed immediately upon the addition of Cu2+ ions into the CH3CN solution of PMHMP. No other metal ions, including Zn2+, displayed any detectable color change even after incubating for a longer time. With the addition of Cu2+, a red shift and broadening of the absorption band were observed. As expected, no spectral change in the UV–visible study was observed upon the addition of Zn2+ ions. The titration studies with the Cu2+ ion showed a hyperchromic shift at 290 nm with the accompanying hypsochromic response at 387 and 412 nm bands. At the same time, a new band centered at 445 nm was formed upon titration with Cu2+. Also, the two isosbestic points were observed at 360 and 424 nm during titration with Cu2+ ions, which indicated a one-to-one equilibrium between PMHMP and the corresponding Cu2+–complex (Figure 2). When the ratios of absorbance at 290 and 388 nm bands were plotted against the equivalent of Cu2+ added, they appeared to be a straight line that established ratiometric, naked-eye sensing of Cu2+ (Figure S1, Supporting Information). The titration studies also indicated that the present system can detect Cu2+ ions as low as 7.8 ppb. Furthermore, the stoichiometry of the complex formed with Cu2+ was calculated using the method of continuous variation or Job’s method.59−62 The total molar concentration of Cu2+ and PMHMP was held constant, but their mole fractions varied. The changes in absorbance were plotted against the mole fractions of these two components, and from the maximum, on the plot, we could predict 1:1 stoichiometry with Cu2+ (Figure S2, Supporting Information). The association constant (log K) for the complex was calculated at 5.27 ± 0.025 using the Benesi–Hildebrand method for 1:1 stoichiometry.63−65

Figure 2.

Figure 2

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(a) UV–visible spectra of PMHMP (10 μM) with different metal ions (1.0 equiv) in acetonitrile medium. (b) UV–visible titration of PMHMP (10 μM) with Cu2+ ions (0–1.2 equiv) in acetonitrile medium.

3.1.2. Fluorescence Studies

After a meaningful observation in the UV–vis study, we intended to perform a fluorescence study by exciting PMHMP at 350 nm in the CH3CN medium. The fluorescence spectrum of PMHMP appeared to be broad and red-shifted (≈432 nm) with no pyrene-specific vibronic features. Due to the extended conjugated structure, this indicated more assertive electronic communication between pyrene and the iminophenol unit.66 However, PMHMP exhibited faint-blue fluorescence (Φ = 0.04) under a long UV lamp, which might be due to the efficient photo-induced electron transfer (PET) attributed to the lone pair of electrons on the nitrogen atoms.67 In addition, a broadband with relatively low intensity was observed in the 567 nm region, which might be due to emission from the keto tautomer.44,68 To prove this, we recorded the fluorescence spectrum in the CH3CN–water (1:1) mixture medium, which displayed a significant increase in emission intensity at 575 nm (Figure S3, Supporting Information). Thus, we can comment that the fluorescence maxima of PMHMP at 432 and 565 nm in the CH3CN medium probably originated from the enol and keto tautomers, respectively, ascertaining the ESIPT phenomenon of PMHMP.

When Cu2+ was added to the acetonitrile solution of PMHMP, the emission intensity at 565 nm was quenched significantly, while the intensity at 432 nm remained primarily unaffected (Figure S4, Supporting Information). Thus, we could speculate that the prototropic equilibrium gets disrupted due to the involvement of the hydrazone and the adjacent hydroxyl group in coordination with Cu2+ ions. Also, the fluorescence turn-off response (Φ = 0.01) is partially attributed to the intramolecular charge transfer (ICT) process owing to Cu2+ coordination. Interestingly, the probe exhibited a selective fluorescence enhancement with Zn2+ with a maximum of 527 nm. Apart from Cu2+ and Zn2+ ions, no other metal ions induce any distinguishable change in the fluorescence signal.

It was observed that in the presence of Zn2+, the fluorescence intensity was enhanced ≈16-fold (Figure 3). The enhancement of fluorescence intensity (Φ = 0.14) with the addition of Zn2+ is probably due to the π–π stacking of pyrene moieties and the formation of a static exciplex.69 However, titration of the probe with Zn2+ in CH3CN showed an interesting result. A substantial increase in the fluorescence intensity at the 527 nm band was witnessed until the [Zn2+]/[PMHMP] mole ratio reached 0.5 (I/I0 ratio is ≈ 16), followed by fluorescence quenching. We speculated that this abrupt change in the fluorescence signal during titration with Zn2+ ions might be due to a concentration-dependent change in the stoichiometry. At low concentrations, PMHMP probably formed a (1:2) complex with Zn2+ ions, while at the higher concentration (>0.5 equiv), it produced a (1:1) complex. Furthermore, we plotted Fc/F0 as a function of varying [Zn2+], where Fc represent the fluorescence intensity for a specific concentration of added [Zn2+] and F0 is the intensity of the free receptor. A non-linear variation of Fc/F0 versus 1/[Zn2+] at a lower concentration of Zn2+ ions confirmed a 1:2 complex formation. On the other hand, the linear Fc/F0 versus 1/[Zn2+] plot at a higher Zn2+ concentration suggested a 1:1 complexation.70,71 The Job plot analysis with Zn2+ ions showed inflection points at mole fractions both 0.5 and 0.7 of PMHMP, indicating the presence of both 1:1 and 1:2 stoichiometric complexes with Zn2+ ions in the reaction medium (Figure S5, Supporting Information).

Figure 3.

Figure 3

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Fluorescence spectra of PMHMP (10 μM, λex = 350 nm) with Zn2+ ions. (a) 0 to 0.5 and (b) 0.5–1.5 equiv in acetonitrile medium. Change in fluorescence intensity of PMHMP at 527 nm upon addition of Zn2+. (c) 0 to 0.5 and (d) 0.5–1.5 equiv in acetonitrile medium.

To increase the practical applicability of the sensing system, we checked the metal ion sensing ability of PMHMP in different water–acetonitrile mixtures. Although we could not see any response in the pure aqueous medium, Cu2+-induced change in the absorption spectrum was witnessed in (9:1; v/v) acetonitrile–water mixture medium (very similar to what was observed in pure acetonitrile medium) (Figure S6, Supporting Information). Then, we performed UV–visible titration of PMHMP with Cu2+ under that condition, which similarly showed isosbestic points at 355 and 425 nm. Here also, we then plotted the ratio of absorbance at 288 and 388 nm with the equivalent of Cu2+ added; it appeared to be a straight line which established ratiometric sensing of Cu2+ (Figure S7, Supporting Information). However, no fluorescence enhancement was found after Zn2+ addition in the presence of the (9:1; v/v) acetonitrile–water mixture.

Furthermore, a synthetic reaction was carried out between ZnCl2 and PMHMP (1:2) in a MeOH medium (at room temperature). The resultant [Zn(PMHMP)2Cl2] complex was characterized by elemental analysis (Figure S8, Supporting Information). The mononuclear Zn complex exhibited a spectrum resembling the range obtained at a [Zn2+]/[L] mole ratio of 0.5 during titration studies (Figure 4). Titration of this complex with the Zn2+ ion showed a further quenching in fluorescence intensity, similar to what we observed during the titration studies (>0.5 equiv). The binding constant was calculated according to the Benassi–Hildebrand method and found to be (log K) 4.42 ± 0.031 (from the decreasing side of the titration curve, 1:1 stoichiometry) (Figure S9, Supporting Information). Fluorescence studies indicated that the present system can detect Zn2+ ions as low as 4.2 ppb in acetonitrile medium.

Figure 4.

Figure 4

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(a) Fluorescence spectra of PMHMP (10 μM, λex = 350 nm) with different metal ions (0.5 equiv) in acetonitrile medium. (b) Fluorescence spectra of PMHMP (10 μM, λex = 350 nm) with 0.5 equiv of Zn2+ and the preformed Zn–complex in acetonitrile medium.

3.1.3. Mechanistic Investigations

We conducted spectroscopic studies to comment on the mode of interaction with Cu2+ and Zn2+ ions. The reversible binding of Cu2+ (Figure 5a) and/or Zn2+ (Figure S10, Supporting Information) ions was also checked using ethylenediaminetetraacetic acid (EDTA). The first metal ion at the requisite amount was added to the CH3CN solution of PMHMP, and the same amount of EDTA was included. The addition of EDTA revived the original spectrum of PMHMP. This was repeated a few times, and we observed the revival of the original spectrum each time.

Figure 5.

Figure 5

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(a) Reversible interaction of PMHMP (10 μM) with Cu2+ ions (0.5 equiv) in acetonitrile medium. (b) FT-IR spectrum of PMHMP with Zn2+ ions (with 0.5 and 1 equiv). (c) 1H NMR spectrum of PMHMP (5 mM) with Cu2+ and Zn2+ (1 equiv) in DMSO-d6 medium.

To comment on the binding mode, we performed the FT-IR studies of PMHMP with both Cu2+ (Figure S11, Supporting Information) and Zn2+ (Figure 5b) ions. Upon interaction with both Cu2+ and Zn2+ ions, the stretching frequency of the C=N band shifted to the lower-energy region, while the band corresponding to N–N shifted to the higher-energy region. On the other hand, in the FT-IR spectrum of the Zn complex, we found that the band at 1594 cm–1, due to (νC=N), shifted to 1539 cm–1 in the complex, suggesting coordination of the nitrogen atom of the azomethine group with the metal atom.72 Due to (νN–N), the band shifts slightly from 1016 to 1021 cm–1 because it involves only one N atom of each azomethine unit in coordination. Furthermore, we performed 1H NMR titration of PMHMP with Cu2+ and Zn2+ ions in DMSO-d6 (Figure 5c). The NMR titration could not be performed in CD3CN due to limited solubility.

All aromatic protons (belonging to pyrene and salicylaldehyde groups) experience chemical shifts upon the addition of Cu2+. However, the extent of the shift was found to be more prominent for protons indicated as “a” and “b”. Similarly, protons “c” and “d” were also affected as they were nearer to the azomethine and phenolic groups. This indicates that the coordination of Cu2+ occurred through phenolic oxygen and both N-atoms of the azomethine group. However, at a higher concentration of Cu2+, there was a significant broadening of NMR signals. This must be due to the paramagnetic effect of Cu2+, owing to its open-shell electronic configuration.73 Interestingly, when titration was performed with Zn2+, despite prominent chemical shifts at both pyrene and salicylaldehyde protons, no such broadening was observed. Such upfield shifts with Zn2+ ions indicate that pyrene and phenyl units are placed on top of each other in the metal complex.74 To explain the concentration-dependent change in the stoichiometry of the in situ-formed Zn2+–complex, we have recorded electrospray ionization-mass spectrometry mass spectra of PMHMP at different concentrations of added Zn2+ ions. At a mole fraction <0.5, we could observe the major peak at m/z 434, indicating the formation of the [L2·Zn]2+·2H2O complex (L = PMHMP). On the other hand, beyond 0.5 equiv, the peak at m/z 257 became more intense, which suggests 1:1 complexation [L·Zn]2+·2H2O, with Zn2+ ions (Figure S12, Supporting Information).

3.1.4. Designing of the Logic Circuit

Molecular and supramolecular logic gates are candidates for computation at the nano-scale level. Logic circuits, capable of performing arithmetic operations, have often been implemented in semiconductor technology. The optical signal of PMHMP could be affected distinctly by adding both Cu2+ and Zn2+ ions. Now, we have constructed binary logic functions with Zn2+ and Cu2+ as dual stimulating inputs and absorbance and fluorescence as outputs (Figure 6).75,76

Figure 6.

Figure 6

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(a) Fluorescence spectra of the Zn complex (10 μM, λex = 350 nm) with anions (1 equiv) in acetonitrile medium. (b) Change in FL intensity of the Zn complex (10 μM, λex = 350 nm) with metal ions (1 equiv) in acetonitrile medium. (c) Logic gate application of the Zn complex (10 μM, λex = 350 nm) with Cu2+ and Zn2+ as inputs. (d) Truth table displayed the construction of the INHIBIT logic function.

Upon the addition of Zn2+ ions (Input-1), a new fluorescence band appeared at 527 nm, attributed to a typical exciplex formation as described above. On the contrary, in the presence of Cu2+ ions (Input-2), the emission band at 565 nm was quenched, probably due to the combined effect of PET and intramolecular charge transfer. However, when Cu2+ ions were added to the solution of PMHMP·Zn2+, the quenching of fluorescence intensity at the 527 nm band was observed. Thus, we can presume that Cu2+ replaced the Zn2+ ion in the coordinated complex due to the higher binding ability of Cu2+ than that of Zn2+. If we consider the relative fluorescence intensity (Fc/F0, F0 = fluorescence of PMHMP and Fc = PMHMP with metal ions) 14 as the threshold value, the output will be 0 when Fc/F0 is lower than 14; output = 1 when the intensity is higher. Now, the fluorescence intensity is high enough only in the presence of Zn2+, that is, only when IN1 = 1 and IN2 = 0; thus, under this condition, the output signal OUT = 1. Also, for all other cases when Cu2+ was present in the system, Fc/F0 was lower than 14, so the outputs were 0. Therefore, it behaves like an INHIBIT logic gate, where Cu2+ ions act as an inhibitor.77 This logic function demonstrates a non-communicative behavior in which one of the inputs can disable the whole system.

3.1.5. Sensing Applications with the Preformed Zn Complex

We have also checked the effects of different metal ions and anions upon the preformed Zn complex in the CH3CN medium. Most of the metal ions (Ag+, Co2+, Ca2+, Cd2+, Cu2+, Hg2+, Ni2+, Pb2+, Mg2+, and Zn2+) showed little blue shifts (Δλ ≈ 7–8 nm) in the emission maxima. However, with the addition of Cu2+ ions, along with a blue shift, we could also observe a large decrease (≈9-fold) in the fluorescence intensity. This observation indicated that Cu2+ ions, owing to their higher binding affinity for PMHMP, could replace Zn2+ ions from the complex.78,79 We also observed a similar result when PMHMP was treated sequentially with Zn2+ and Cu2+ ions (Figures S13 and S14a, Supporting Information).

Furthermore, the preformed Zn2+ complex was exposed to a large number of anions (F–, Cl–, Br–, I–, PF6–, ClO4–, AcO–, CN–, H2PO4–, SO42–, NO3–, N3–, and CO32–); only H2PO4– could selectively quench the intensity of the excimer band (≈18-fold). The other tested anions showed no interaction with the Zn complex. Then, we conducted fluorescence titration studies with H2PO4– ions under similar conditions, which showed a concentration-dependent linear quenching in emission intensity (Figure S14b, Supporting Information). The Job’s plot analysis indicated a 1:1 complexation between the H2PO4– ion and Zn complex (Figure S15, Supporting Information). From the fluorescence titration studies, the association constant for H2PO4– was determined to be (log K) 6.14 ± 0.07. Along with quenching, H2PO4– ions also induced a small red shift (Δλ ≈ 8 nm) in the emission maxima. This suggested that the coordination of the H2PO4– ion with the Zn2+ center probably perturbs the π–π stacking interaction. The fluorescence spectrum in the presence of H2PO4– ions was found to be significantly different than that of free PMHMP. When Zn2+ ions were added to the H2PO4–-treated solution of the Zn complex, the revival of the original fluorescence spectrum (of free PMHMP) was not observed (Figures 7 and S16, Supporting Information). Such observations indicated tertiary complex formation between the Zn complex and H2PO4– ions, where Zn2+ ions are simultaneously linked to PMHMP and H2PO4– ions.80,81 The 31P NMR spectrum of H2PO4– with the Zn complex showed a downfield shift in the signal (Figure S17a, Supporting Information). This further confirmed the binding of H2PO4– with the Zn2+ center. The ternary complex formation was also evident from mass spectral analysis, where we observed a new peak at m/z 855.1 (Figure S17b, Supporting Information). We also checked the interaction of the Zn complex with methyl phosphate, etc.82,83 In this case, the extent of quenching was found to be substantially low (∼2-fold) compared to H2PO4– ions (Figure S18, Supporting Information). This means that their extent of interaction was less effective as compared to H2PO4–.

Figure 7.

Figure 7

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The schematic diagram shows stoichiometry dependence of PMHMP differential complexation modes for Cu2+ and Zn2+ ions.

3.2. DFT Studies

3.2.1. Optimized Geometry

Since PMHMP contains two aromatic moieties (pyrene and benzene) which are connected by a rotatable azomethine moiety, we performed the potential energy scan study by keeping the pyrene moiety intact and rotating the benzene moiety along the dihedral axis, 27N–28N–29C–30C from −180 to +185° by varying 5° in each step to locate the minimum energy conformations (Figure S19, Supporting Information). From this figure, it was quite clear that PMHMP existed in three lower-energy conformations on its ground-state (S0) potential energy surface with dihedral angles −180, 0, and +180°, and the transition energies for interconversion among the minimum energy conformations were quite high to achieve. Moreover, the conformations at the dihedral angles −180 and +180° equivalently denoted the enol form of PMHMP. Among all the possible orientations, all three minimum energy geometries have the closest distance along the −N···H···O– axis. This result revealed that PMHMP exists in the enol form in S0 and might show ESIPT phenomena to switch over into the keto form in the first singlet excited state (S1) and is involved in intramolecular hydrogen bonding along the axis −N···H···O– in both keto and enol forms. Naturally, all the ground-state calculations were performed in the subsequent sections with the minimum energy conformation with the dihedral angle of +180°. Thereafter, PMHMP and its metal complexes (stoichiometry according to experimental observations) were optimized (Figure S20, Supporting Information). From the optimized geometry, it was observed that one N atom of PMHMP was in close proximity to −OH which might be involved in hydrogen bonding in S0, but the other N atom with a lone pair might be involved in PET to the pyrene moiety. On the other hand, upon coordination of Cu2+ and Zn2+, there was a flipping of the azomethine moiety of PMHMP. This might be a major reason behind the respective spectroscopic changes. On the flip side, in the (PMHMP)2–Zn2+ complex, the probe reverted to its original conformation and displayed reverse spectral changes from the 1:1 Zn2+ complex.

3.2.2. MEP Studies

Since in this particular study, we focused on the ESIPT phenomena of PMHMP and its binding behavior with Cu2+ and Zn2+ ions and expecting significant charge reorganization, respective 3D MEP plots (isodensity value = 0.0004 a.u.) (Figure 8) were developed. The red-colored portion of the enol form of the probe denoted the electrophilic nature of the phenolic −OH and susceptibility of formation of an intramolecular hydrogen bond with the azomethine nitrogen and might be involved in ESIPT to transform into the keto form in the S1 state. This type of conformation is a very common and natural behavior of the ESIPT active probes.84 In both 1:1 Cu2+ and Zn2+complexes, we observed a huge enhancement of electron density around the metal center including the benzene moiety. This indicated the possibility of ICT from the benzene to the pyrene moiety. However, for the Cu2+ complex, it occurred in the ground state, whereas for the Zn2+ complex, charge transfer occurred in the exciplex. Moreover, in the (PMHMP)2–Zn2+ complex, the electron density over the benzene moieties was very low, whereas over pyrene moieties, it was quite high, denoting the inhibition of the charge transfer process.

Figure 8.

Figure 8

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3D MEP plots of PMHMP and its metal complexes in CH3CN calculated using the DFT/M06-2X/6-311G theoretical model and IEFPCM solvent system.

3.2.3. Frontier Molecular Orbital Analysis

There is no need to mention that the change in electronic cloud distribution of frontier molecular orbitals (FMOs) particularly the HOMO and LUMO over a fluorophore evidently revealed its ESIPT probe.84 The HOMO–LUMO images of both the enol and keto forms of PMHMP are presented in Figure S21, Supporting Information, and the Cu2+ and two Zn2+ complexes of PMHMP are presented in Figure 9. The electron density over the N atom of the enol form sharply increased from the HOMO to the LUMO. This charge transfer phenomenon strengthened the −N···H–O– intramolecular hydrogen bond strength and endorsed the proton transfer from oxygen to the nitrogen atom in the S1 state. Moreover, there was a huge decrease in the HOMO–LUMO energy gap from the enol to the keto form which, in turn, supported the experimental observation (Figure 1a) and the ESIPT mechanism (Figure 1a). In addition, the computationally calculated UV–vis spectra of PMHMP displayed two absorption bands denoting the possibility of two tautomeric states of PMHMP (Figure S22, Supporting Information). In both the 1:1 Cu2+ and Zn2+ complexes of PMHMP, there was a decrease in the electron cloud over the benzene moiety from the HOMO to the LUMO, denoting the ICT process from the benzene to the pyrene moiety. However, in the 1:2 Zn2+–PMHMP complex, we did not observe such transfer of the electron cloud but rather a high electron density over the pyrene moiety in both the HOMO and LUMO which indicated the inhibition of the charge transfer behavior in the exciplex.

Figure 9.

Figure 9

Open in a new tab

HOMO–LUMO images of Cu2+ and two Zn2+ complexes of PMHMP along with corresponding transition energy values calculated involving the DFT/M06-2X/6-311G theoretical model and CH3CN/IEFPCM solvent system.

3.2.4. Potential Energy Scan Studies

The potential energy scan study is a fantastic tool to describe and establish any ESIPT process very effortlessly.85,86 Hence, to establish the ESIPT phenomenon of PMHMP conclusively, we performed the potential energy scan study along the ESIPT coordinate (N–H distance) of PMHMP for both S0 and S1 states as presented in Figure 10.

Figure 10.

Figure 10

Open in a new tab

Potential energy scan plots along the ESIPT axis of PMHMP in S0 (bottom) and S1 (top) states calculated involving the DFT/M06-2X/6-311G theoretical model and CH3CN/IEFPCM solvent system.

In S0, the energy difference between the enol–keto forms was very high (4.58 kcal mol–1) along with extremely high transition energy for the enol-to-keto conversion (5.92 kcal mol–1) but low for the keto to enol transition (1.34 kcal mol–1). Naturally, PMHMP exists majorly in the enol form in the ground state. On the reverse side, in S1, the energy difference between the enol–keto forms was extremely low (0.16 kcal mol–1) along with almost the same transition energy for the enol to keto conversion (3.05 kcal mol–1) and the keto to enol transition (2.89 kcal mol–1). Moreover, the keto form was slightly more stable than the enol form in S1. Naturally, no other process could be observed other than the ESIPT process of PMHMP.

Based on the above-mentioned discussion, we could propose this plausible binding mechanism (Figure 7), which has already been argued throughout the article. PMHMP undergoes keto–enol tautomerism. 1:1 Cu2+ displayed ICT, and Zn2+ formed an exciplex with PMHMP in different stoichiometric ratios and different charge transfer behavior. Moreover, PMHMP was recovered from the metal complexes upon complexation with EDTA. All the events and photoprocesses are argued soundly with experimental shreds of evidence and theoretical justifications.

4. Conclusions

In conclusion, we have synthesized and characterized a pyrene-based probe (PMHMP), established its ESIPT behavior experimentally and theoretically, and employed it in dual-mode, ppb-level optical sensing of Cu2+ and Zn2+ ions. On the other hand, the keto emission of the compound was quenched upon the addition of Cu2+ ions. This indicated the inhibition of ESIPT emission upon coordination with metal ions. The probe can also be used as a “turn on” fluorescent sensor for Zn2+ ions. The exciplex formation was observed upon the addition of 0.5 equiv of Zn2+ ions. However, at a higher concentration of Zn2+ ions, we noticed the formation of a 1:1 complex with a faint fluorescence signal. Thus, we can detect and differentiate both Cu2+ and Zn2+ ions using UV–visible and fluorescence modes. Furthermore, the preformed 1:2 Zn2+–PMHMP complex showed a “turn off” fluorescence response selectively in the presence of Cu2+ and H2PO4– ions. The Cu2+ ions, owing to higher binding affinity, could replace the Zn2+ ion, whereas H2PO4– ions could form a tertiary complex with the Zn complex. We have also constructed one INHIBIT logic function using Cu2+ and Zn2+ as the inputs in the emission mode. Therefore, pyrene derivative-based dual-mode, ppb-level detection of multiple ions with such unique behaviors is rare, interesting, and of high significance.

Acknowledgments

The authors acknowledge BITS Pilani, Hyderabad, for central analytical facilities. ND thanks the SERB-SRG grant (SRG/2022/000031) for funding and technical support.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.2c06559.

  • Synthesis and characterization of the probe molecule and additional spectral and computational data (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao2c06559_si_001.pdf (1.1MB, pdf)

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