Interplay of conformational relaxation and hydrogen bond dynamics in the excited states of fluorescent Schiff base anions

Abstract

Time resolved fluorescence spectroscopic investigation of four Schiff base anions has established that their excited state dynamics is governed by several solvent properties: polarity, viscosity and hydrogen bond donating ability. With viscous protic solvents like glycerol, fluorescence lifetimes of anions have been found to be markedly longer than those in ethanol, implying that conformational relaxation of molecules plays a key role in their nonradiative relaxation. Surprisingly, the lifetimes in less viscous aprotic solvents, like acetonitrile, are found to be even longer. The only plausible rationalization of this observation is in the light of hydrogen bond-assisted nonradiative phenomena that are operative in protic solvents. This contention draws support from a time evolution of the emission in the red end of the spectrum in low to moderately hydrogen bond donating protic solvents, with regard to an absence of such a rise time in aprotic solvents and strongly hydrogen bond donating solvents, viz., 2,2,2-trifluoroethanol. Rudimentary quantum chemical calculations provide a preliminary idea about the nature of excited state hydrogen bond redistribution involved in the process.

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1. Introduction

Fluorogenic molecules have attracted significant attention in recent times because of their importance in the fields of organic electronics and sensing of analytes.^1–11 Excited state dynamics of such molecules are governed by highly efficient nonradiative processes, cessation of which by specific chemical or physical inputs renders the molecules strongly fluorescent.¹² Schiff bases constitute a class of fluorogenic molecules that are attractive from several different perspectives. They are easy to synthesize and derivatise,¹³ making them excellent candidates as building blocks for novel functional materials.^14–16 Generally, their fluorescence is very weak, as their flexible structures sustain efficient nonradiative pathways associated with the rotation of carbon–carbon and carbon–nitrogen bonds.^17–23 On the other hand, they are excellent ligands that can bind strongly and selectively with metal ions. The consequent rigidification can lead to efficient suppression of the non-radiative pathways in principle, provided it is not overwhelmed by factors like the heavy atom effect. This forms the basis of the much explored applications of Schiff bases as fluorogenic “turn on” sensors of metal ions.^24–26 The practical utility of Schiff bases as sensors in real biological media is limited by their well-known propensity for hydrolysis, the mechanism of which is an interesting problem in itself.²⁷ The pursuit for Schiff bases that are immune to hydrolysis continues to be one of the Holy Grails of this field.²⁸

Over the last few years, our group has explored a family of Schiff bases (Scheme 1), with the mandate of understanding and optimizing the conditions for obtaining enhanced fluorescence from them, by complexation and aggregation.^29–34 It has been established that these compounds exhibit selective enhancement of fluorescence with Zn(II) and Al(III) and that the Al(III) complexes emit more strongly than the Zn(II) complexes in solution. For salophen ((2,2′-((1,2-phenylenebis(azaneylylidene))bis(methaneylylidene))diphenol), Scheme 1), this has been explained in the light of opening of additional non-radiative channels in the dimeric Zn(II) complex, in which the ligands are π-stacked.³⁰ This contention draws support from the marked decrease in the fluorescence of the Al(III) complex in its crystals, due to the π-stacking interactions induced between the ligand molecules in these complexes.²⁹ Interestingly, fluorescence is restored in the solid state, upon frustrating the stacking by small chemical substitutions.³¹ Moreover, Zn(II) complexes with significantly stronger fluorescence are obtained with salampy ((2-(((pyridin-2-ylmethyl)imino)methyl)phenol), Scheme 1), a Schiff base, in which the ligands are not π-stacked.³² However, the Al(III) complex of this Schiff base has a larger fluorescence quantum yield (ϕ_f) and a longer fluorescence lifetime (τ_f), possibly because they are more rigid.³⁵ Hence, a systematic understanding of the factors that determine the strength of fluorescence complexes has begun to emerge from this series of studies.

Scheme 1 Chemical structures of the Schiff bases.

An interesting aspect of the photophysics of Schiff bases has come to light in the course of these investigations. The anions of all the four Schiff bases (Scheme 1), namely salophen, salampy, HBAP (2-((2-hydroxybenzylidene)amino)phenol) and salbn (2,2′-((butane-1,4-diylbis(azaneylylidene))bis(methaneylylidene))diphenol) have been found to be emissive to a significantly greater extent compared to the neutral molecules, but to a lesser extent than the metal complexes. The neutral Schiff bases typically have τ_f values of tens of picoseconds and the complexes have lifetimes in the nanosecond regime, unless π-stacking is at play. The anions typically have fluorescence lifetimes of 800–900 ps in methanolic solutions^32–34 and their emission spectra exhibit a time dependent fluorescence Stokes shift (TDFSS) over timescales comparable to the solvation times reported earlier.³⁶ This observation is in line with an earlier report on a Schiff base anion.¹⁹ The present work takes a closer look at the excited state dynamics involved. Time evolution of fluorescence of the anions of four Schiff bases (Scheme 1) have been investigated in protic and aprotic solvents, with a range of polarity and viscosity. The possible contributions of solvation and hydrogen bond dynamics have been weighed against each other.^37–41 A clearer picture of the excited state dynamics of these anions has hence emerged.

2. Materials and methods

2.1. Materials

All the chemicals and spectroscopy grade solvents (Spectrochem, Mumbai, India) are used as received without further purification unless mentioned. Deuterated ethanol (EtOD) and Coumarin-153 (C153) are procured from Sigma Aldrich and used for the experiments.

2.2. Synthesis of Schiff bases and their anions

Salophen, salampy and HBAP are prepared and purified following our previous reports, using salicylaldehyde (SD Fine Chemicals, Mumbai, India) and respective aryl amines.^30,32,34,42 Salbn is prepared by dropwise addition of salicylaldehyde (2 equiv.) to a methanolic solution of 1,4-butanediamine (1 equiv., Sigma Aldrich) at room temperature.⁴³ The yellow precipitate obtained is recrystallized from methanol. The structures of the products are determined by ¹H NMR spectra on a Bruker 400 MHz NMR spectrometer and by single X-ray diffraction. ¹H NMR spectra and crystal structure (CCDC 176004) of salbn are presented subsequently (Fig. S1, ESI†). Detailed characterization of the other Schiff bases have been reported in our earlier reports.^30,32,34 The Schiff base anions are prepared by adding the Schiff bases to a saturated alcoholic solution of potassium hydroxide (Merck, Mumbai). For the studies carried out in aprotic solvents, preparation of the anions in methanol is followed by vacuum evaporation of the solvent and addition of the corresponding aprotic solvent, such as acetonitrile (ACN) and N,N-dimethylformamide (DMF). Filtration of the same solution is performed to get rid of the KOH residue.

2.3. Steady state fluorescence spectroscopy

Steady state spectra are recorded on a Shimadzu UV-NIR-3600 spectrophotometer and a Horiba Fluoromax 4 spectrofluorimeter. Quinine sulphate in 0.1 M H₂SO₄ (ϕ_f = 0.54) is used as a standard for measurement of fluorescence quantum yields (ϕ_f).⁴⁴

2.4. Time-resolved fluorescence spectroscopy

Fluorescence lifetimes in the pico–nanosecond time regime are recorded on a DeltaFlex Time Correlated Single Photon Counting (TCSPC) spectrometer. Vertically polarized second harmonic output of a Mai-Tai Laser, with a repetition rate of 8 MHz, is used as the excitation source. Fluorescence decays are collected at magic angle (54.7°) polarization with respect to the excitation light. The full width at half maxima (FWHM) of the instrument response function (IRF), using a hybrid photomultipler detector (HPPD), is 74 ps. Decays are fitted to the sum of exponential functions (eqn (1)), with iterative reconvolution by a commercially available EzTime software package.A_i, τ_i are the amplitude and lifetime of the ith component, respectively, with . B is the background signal. Fluorescence decays in the femto–picosecond domain are recorded using Femtosecond Optical Gating (FOG) technique with λ_ex = 400 nm. The second harmonic, 400 nm of a modelocked 100 fs Ti:Sapphire (Tsunami, Spectra Physics, USA, 1 W, 80 MHz repetition rate) was used for the excitation. Details of the setup are discussed elsewhere.⁴⁵

Time Resolved Emission Spectra (TRES) are constructed using steady state fluorescence intensities and temporal parameters from fluorescence transients (FOG decays for all the solvents except glycerol) at regular intervals across the range of fluorescence wavelengths of the Schiff base anions. Jacobian correction (i.e., I(v̄) = λ²I(λ)) is incorporated into the steady state spectral intensity.⁴⁶ Each of the TRES is fitted to a modified log-normal function:

where y₀ = peak height, b = asymmetry parameter, [small nu, Greek, tilde]₀ = spectral peak position, and Δ = bandwidth of the spectral band.

The apparent solvent correlation C(t) as obtained from the TRES function is defined as:

All of the experiments in the preceding sections have been performed at 298 K.

2.5. Quantum chemical calculations

The electronic structure calculations for the ground and excited states are performed with the Gaussian 09 program suite.⁴⁷ The ground state geometry optimization of the salampy anion and its hydrogen bonded complexes with ethanol molecules are carried out using a density functional theory (DFT) method with the CAM-B3LYP/6-31G(d,p) level of theory. The final ground state optimized structure of the salampy⁻-ethanol hydrogen bonded complex obtained is used to carry out the excited state electronic structure calculations using the time-dependent density functional theory (TDDFT) method with the same CAM-B3LYP/6-31G(d,p) level of theory.³⁷ Visualization of all the electronic structures and the measurements of the hydrogen bond lengths are done by Chemcraft software.

3. Results and discussion

The emission spectra of all the anions are significantly Stokes shifted compared to their absorption spectra (Fig. 1, Table 1 and Fig. S2, Table ST1, ESI†). For example, salampy⁻ exhibits absorption (λ^max_abs) and emission (λ^max_em) maxima at 383 and 492 nm, respectively in ethanol (EtOH). The large Stokes shift (109 nm for salampy⁻) indicates that the excited state processes are operational in the anions. For neutral Schiff bases, a similar observation is ascribed to the excited state intramolecular proton transfer (ESIPT) and the rapid non radiative depopulation of the cis-keto excited state associated with segmental motions of C–N bonds.^{17,20,21,48–54} ESIPT is precluded for anions, as they lack the proton that is essential for its occurrence. In polar aprotic solvents like N,N-dimethyl formamide (DMF) and acetonitrile (ACN), the emission spectra are slightly blue shifted (5–10 nm) compared to those in polar protic solvents (the Stokes shift for salampy⁻ is 67 nm in DMF, but 84 nm in ACN, Table 1). However, in strongly hydrogen bonding solvents like 2,2,2-trifluoroethanol (TFE) the emission spectrum is further blue shifted (∼40 nm) as compared to methanol (MeOH). Similar observations are made for the other anions as well (Table ST1, ESI†). Hence, the emissive excited state is found to be stabilized to a greater extent in protic solvents with low to moderate hydrogen bond donating ability, than in aprotic solvents. This drastic difference in nature of the emissive excited state in TFE as compared to other protic solvents indicates the critical role of hydrogen bonding in the excited state processes of this class of anions. Viscosity also has a major role to play, as is indicated by the largest values of fluorescence quantum yields (ϕ_f) of all of the four anions in glycerol (Table 1 and Table ST1, Fig. S3, ESI†). This is likely to be due to the hindrance to the torsional motions associated with non-radiative channels, in solvents with high viscosity.⁵⁵ The steady state spectra of salampy⁻, HBAP²⁻ and salophen²⁻ in methanol (MeOH) has been recorded afresh to reproduce our recent reports.^33–35 Steady state spectra thus indicate the dependence of the excited state phenomena in the Schiff base anions on multiple parameters of the fluid medium in which it resides.

Fig. 1 Absorption (black), fluorescence emission (blue) and excitation (red) of salampy⁻ in (a) TFE (b) ACN (c) DMF (d) glycerol (e) BuOH (f) EtOH (g) MeOH. The emission spectra are recorded by exciting the samples at their corresponding λ^max_abs. Excitation spectra are monitored at their respective λ^max_em.

Table 1 Fluorescence spectral and temporal parameters of salampy⁻ in different solvents

Solvent	Δf	[small nu, Greek, tilde] abs (cm^−1)	[small nu, Greek, tilde] em (cm^−1)	Δλ (nm)	Δ[small nu, Greek, tilde] (cm^−1)	ϕ f	τ f (ns)
ACN	0.31	24 752	20 492	84	4260	0.07 ± 0.01	3.3
DMF	0.27	24 570	21 097	67	3473	0.05 ± 0.01	4.4
Glycerol	0.27	26 525	19 920	125	6605	0.10 ± 0.05	1.3
BuOH	0.24	25 974	19 960	116	6014	0.03 ± 0.008	1.7
EtOH	0.29	26 110	20 325	109	5785	0.01 ± 0.006	1.0
MeOH	0.31	26 596	19 960	125	6636	0.03 ± 0.003	0.8
TFE	0.57	27 269	23 049	67	4220	0.01 ± 0.001	—

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The plot thickens upon closer inspection of the time resolved fluorescence data in the femtosecond–picosecond and picosecond–nanosecond range when using femtosecond optical gating (FOG) and time correlated single photon count (TCSPC), respectively. The fluorescence decays, recorded at λ^max_em, are slower in glycerol than in ethanol (Fig. 2a). This observation can be explained in light of hindrance to the segmental motion and consequently, conformational relaxation at higher viscosity^32–34 and is in line with the larger ϕ_f values observed in glycerol. However, viscosity is not the sole factor that determines the excited state dynamics, as is borne out by a distinctly slower fluorescence decay of salampy⁻ in ACN (Fig. 2a and Tables ST1, ST2, ESI†). Experiments with a library of solvents reveal that the slowest fluorescence decays are observed for solvents that are polar, but aprotic (Fig. S4, ESI†). Hence, it is inferred that hydrogen bonding interactions provide additional nonradiative decay channels for the anions in protic solvents. The dynamics of their excited states are thus experimentally indicated to be governed by potential energy surfaces that involve multidimensional reaction co-ordinates.^56,57 This contention is fortified by the strong dependence of the fluorescence dynamics on the emission wavelength, observed in the nanosecond timescale for glycerol (Fig. 2b and Fig. S5, ESI†) and in the picosecond timescale for EtOH and butanol (BuOH) (Fig. 2c and Fig. S5, ESI†). Rise times at the red end of the emission spectra and corresponding fast decays at the blue end confirm the occurrence of an excited state relaxation (Table ST2, ESI†). The lack of rise times at the red of spectra in TFE is reminiscent of the fact that TFE is a strong hydrogen bonding solvent and it forms very strong hydrogen bond networks with the anions in the ground state itself, thus vindicating the role of hydrogen bond dynamics in the electronic excited state (Fig. 2e and Fig. S6, ESI†). The markedly slower dynamics in glycerol reaffirms the viscosity dependence and indicates the involvement of conformational relaxation in the process.

Fig. 2 Fluorescence decays of salampy⁻ using (a and b) TCSPC (nanosecond regime) and (c–e) FOG (femtosecond–picosecond regime) in (a) EtOH, glycerol and ACN at their corresponding emission maxima, (b) glycerol with λ_ex = 400 nm, λ_em = 450 nm (blue end) and λ_em = 600 nm (red end) (c) in EtOH at λ_ex = 400 nm, λ_em = 450 nm (blue end) and 600 nm (red end), (d) ACN with λ_ex = 400 nm, λ_em = 450 nm (blue end) and 600 nm (red end), (e) in TFE at λ_ex = 400 nm, λ_em = 450 nm (blue end), 550 nm (intermediate wavelength), 600 nm (red end).

Fast decays at the blue end and corresponding rise times in the red end are commonly associated with solvation dynamics.^{46,56,58–62} Indeed, the time resolved emission spectra (TRES) of salampy⁻ in EtOH (Fig. 3c) exhibit a TDFSS up to 50 ps. For BuOH and glycerol this TDFSS is slow and it saturates near the emission maxima in around 500 ps which is unlike that for EtOH (Fig. 3a, b and Fig. S7, ESI†). This can be attributed to the higher viscosity of BuOH and glycerol that tends to arrest different conformations of the probe in the excited state making their relaxation pathways slower. The decay of the apparent solvent correlation function, C(t) is fastest in ethanol followed by BuOH and glycerol (Fig. 3i and Fig. S8, Table ST3, ESI†), following the trend of increasing viscosity of the solvents. In deuterated ethanol (EtOD) the faster component is slightly slower as compared to EtOH because of the inertial effect on the rotation of the solvent molecules. However because of inhomogeneous broadening in liquids the slower component slows down only slightly on deuteration (Fig. 3i).⁶³ Similar observations are obtained for the other Schiff base anions (Fig. S8, ESI†). Hence, a unified picture of viscosity dependent solvent relaxation, associated with TDFSS, is apparently obtained for the protic solvents other than TFE.

Fig. 3 Time resolved emission spectra of salampy⁻ in (a) glycerol (b) BuOH (c) EtOH (d) EtOD; time resolved area normalized emission spectra (TRANES) of salampy⁻ in (e) glycerol (f) BuOH and (g) EtOH (h) EtOD; (i) apparent solvent correlation function [small nu, Greek, tilde](t) − [small nu, Greek, tilde](∞) of salampy⁻ in glycerol, BuOH and EtOH.

A twist in the tale is encountered in the form of an apparent absence of the emission wavelength dependence of the fluorescence decays of salampy⁻ in polar aprotic solvents, namely ACN and DMF (Fig. 2d and Fig. S9, Table ST2, ESI†). There are previous reports where sub picosecond solvation is said to have been operative for C153 in ACN.⁶⁴ Although that resolution is not achievable in the present set of experiments, it is evident that the magnitude of the rise at the red end obtained in solvents, viz., EtOH, BuOH and glycerol is perceivably much higher than that in polar aprotic solvents like DMF and ACN which prepares the basic foundation for us to believe in the difference of the nature of the operational excited state relaxation pathways. However, the absence of such a dependence in another polar protic solvent, viz., TFE further raises our interest. Experiments with C153, a well-known solvation probe reveal the presence of a 10 ps rise at the red end of the spectra owing to solvent relaxation in the excited state (Fig. S10 and Table ST4, ESI†). The decays of HBAP²⁻ and salampy⁻ in DMF (Fig. S9, ESI†) exhibit a weak, but perceivable wavelength dependence with the decays at the red end becoming faster. No rise time is observed at the red end of the spectra. This small wavelength dependence may be attributed to the conformational heterogeneity of the probes in these specific solvents. The absence of an apparent rise time indicates that dynamic solvation may be precluded in these liquids, the polarities of which are comparable to those of the protic solvents in which the rise times are observed (Table ST5, ESI†). In fact, it poses a serious question about the assignment of the observed relaxation to dynamic solvation, which is considered to arise out of non-specific, electrostatic interactions between solutes and solvent dipoles.⁴⁶ In this situation, it is prudent to examine the possibility of a relaxation involving two states, rather than a continuous evolution by dynamic solvation. A convenient modality in this context is available in the form of time resolved area normalized emission spectra (TRANES), in which the presence of one or more isoemissive points is indicative of the involvement of discrete electronic states.⁶⁵ Indeed, an isoemissive point, which is sometimes diffuse, is seen for each of the anions in hydrogen bonding polar protic solvents, viz., EtOH, BuOH, glycerol and EtOD (Fig. 3e–h and Fig. S11, ESI†), hence indicating relaxations involving two discrete states. The diffused nature of the isoemissive “points” may have two origins. The first one is trivial. TRES in EtOH and BuOH have been constructed from FOG data, for which the signal to noise ratio is not as good as TCSPC. The second possible issue is perhaps more important. Conformational heterogeneity of the excited state has been indicated for these anions. This would lead to a deviation from an ideal two state scenario.

At this point, it is imperative to ask if the Schiff base anions under study are good solvation probes at all. This question is best answered by constructing Lippert–Mataga plots for the anions, with a view to estimate the values of Δμ, the change in magnitude of the dipole moment brought about by photoexcitation (Fig. 4, Table 1 and ST1, ESI†).⁴⁴ This simple exercise turns out to be rather rewarding. For each of the anions, two distinct data sets with linear variation are obtained, one for polar aprotic solvents and the other for polar protic solvents (Fig. 4). Moreover, both the lines are practically parallel. In fact, some of them exhibit a slightly negative slope. Considering the slight deviation on either side to be within the experimental error limits the slopes are almost zero, implying Δμ = 0. In other words, photoexcitation does not lead to the creation of a dipole and hence, the anions are not good solvation probes. This realization necessitates a second look at the excited state relaxation observed in protic solvents (except TFE), as it can no longer be ascribed to solvation. However, the rise times at the red ends for solvents, viz., EtOH, glycerol, butanol necessarily means that a relaxation pathway is operative and more often than not these pathways are associated with a photoinduced electron transfer and consequent change in dipole moment in the electronically excited state. It is better to mention here that it is not imperative to assume that a change in magnitude of the dipole moment can only bring about relaxation, the direction of dipole moment in the photoexcited state also has an equal role to play and the Lippert–Mataga model does not account for that change (Fig. 5a).^66,67 At the same time, it is important to recognize an additional degree of stabilization of the emissive excited states in protic solvents, which takes place in tens of picoseconds to a few nanoseconds, depending on the viscosity of the solvent. This extra degree of stabilization leaves its mark in the Lippert–Mataga plots as well. For solvents of similar polarity like MeOH and ACN, the difference in Δ[small nu, Greek, tilde] is 1850 ± 500 cm⁻¹ (Fig. 4) which is comparable to the magnitude of TDFSS of the probes in polar protic solvents (Table ST6, ESI†). This agreement between the two values is in line with the fact that the total amount of TDFSS essentially amounts to the energy stabilization brought about by excited state relaxation. In the present experiment, the extra stabilization is observed only in the protic solvents that have moderate hydrogen bond donating abilities and dynamic solvation is ruled out. The most likely candidate for the process that brings about the relaxation would be the reorganization of the solute–solvent hydrogen bonds post excitation. It is prudent to discuss the curious case of TFE at this point (Fig. 2 and Fig. S6, ESI†). Owing to its very strong hydrogen bond donating ability TFE forms very strong hydrogen bond networks with the anions in the ground state itself. So, the possibility of relaxation of the locally excited state via repositioning of hydrogen bonds is impeded and hence there is no rise time at the red end of the emission spectrum. However, the presence of a 10 ps rise at the red end of C153, a well-known solvation probe, is observed in TFE. Absence of such a rise in the Schiff base anions (Fig. 2e and Fig. S6, ESI†) indicates the absence of dynamic solvation of the excited state of the anions in this solvent and bolsters the contention about the predominant role of hydrogen bond dynamics. The slight wavelength dependence arises mostly because of conformational reorganization which led to a slight wavelength dependence in solvents like ACN, DMF as well. Dynamics of such hydrogen bond reorganization has been elucidated for several other molecular systems like fluorenone and coumarin 102, using ultrafast spectroscopic techniques and quantum chemical calculations.^68,69

Fig. 4 Lippert–Mataga plots of (a) salampy⁻ (b) HBAP²⁻ (c) salophen²⁻ (d) salbn²⁻.

Fig. 5 (a) Energy optimized structure in the ground and electronic excited state depicting the direction of dipole moment. (b) Quantum chemical calculations depicting the energy of the ground state (GS), locally excited state (LE) and relaxed state (RS).

Taking a leaf out of these literature reports, elementary quantum chemical calculations have been performed to obtain a preliminary idea about the energetics of the ground and excited states of salampy⁻ in EtOH (Fig. 5 and Table ST7, ESI†). These calculations indicate that the excited state relaxation involves the lengthening of the hydrogen bond between the phenyl O atom in the anion and the hydroxyl H atom of a solvent EtOH molecule, along with a shortening of the hydrogen bond between the pyridinium N atom of the anion and a solvent molecule. This redistribution of hydrogen bonds is estimated to be associated with a stabilization of the excited state by 1229 cm⁻¹. This estimate is in excellent agreement with the experimentally observed TDFSS of 1000 cm⁻¹ in EtOH (Table ST6, ESI†) and also with the amount of stabilization indicated by the difference in the Δv values between protic and aprotic solvents of the same polarity, in the Lippert–Mataga plots (Fig. 4). It should be reiterated that the results obtained here might not be quantitative. A more detailed study considering the solvent molecules explicitly is likely to portray a more holistic picture. However, the agreement between the experimental and computational results is indeed encouraging. The extra stabilization by hydrogen bond reorganization in the excited states decreases the energy gap between the excited and ground states. It appears that this is the driving force, in keeping with the energy gap law,⁷⁰ for an increase in the rate of radiationless transition and consequently, smaller fluorescence lifetimes for the anions in protic solvents. The lifetimes are longer in aprotic solvents of the same polarities, due to the absence of such an extra amount of stabilization.

4. Conclusion

To conclude, the combination of conformational relaxation and hydrogen bond redistribution is found to play a significant role in determining the course of evolution of the excited state of Schiff base anions in protic solvents, viz., EtOH, BuOH and glycerol. The critical role of hydrogen bond dynamics is further evident from the absence of a rise time in strongly hydrogen bond donating solvents like TFE. The observed time evolution of the emission spectra is not due to solvation, as photoexcitation does not lead to a significant amount of dipole creation in these anions. Besides, such time evolution is not observed in polar aprotic solvents. The nature of the hydrogen bond redistribution in the excited state has been further bolstered by quantum chemical calculations, which indicate the shortening of the hydrogen bond between one of the N atoms of the anion and a solvent molecule, along with the lengthening of that between the O atom of the anion and a solvent molecule, in the excited state. The profound effect of these subtle changes in the state of solute–solvent hydrogen bonding, on the excited state dynamics of the anions, is illuminating. This could possibly open up future applications of this class of fluorophores in sensing minute changes in complex microenvironments of biological relevance.

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

S. D. acknowledges DST-INSPIRE and IRCC, IIT Bombay for a Senior Research Fellowship. A. C. acknowledges the Ministry of Education, India for a Prime Minister's Research Fellowship. D. K. S. acknowledges IIT Bombay for an institute postdoctoral fellowship. A. D. thanks CSIR for a generous research grant and Prof. Sukhendu Nath for a fruitful discussion. The Central Instrumentation Facility, Department of Chemistry is thankfully acknowledged. The TCSPC experiments were performed in the central facility for Time Resolved Fluorescence Spectroscopy and Microscopy, IIT Bombay. The FOG experiments have been done on a Fund for Improvement of S&T Infrastructure (FIST)-funded facility of the Department of Chemistry.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2cp05007b

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