AZD-5153 6-hydroxy-2-naphthoic – Verteporfin chemical

Prototropic forms of hydroxy derivatives of naphthoic acid within deep eutectic solvents†

Vaishali Khokhar and Siddharth Pandey *

Deep eutectic solvents (DESs) are not only recognized as benign and inexpensive alternatives to ionic liquids, they offer a unique solvation milieu due to the varying H-bonding capabilities of their constituents. Proton-transfer involving a probe and its prototropic forms strongly depend on the H-bonding nature of the solubilizing media. The presence of prototropic forms of three probes, 1-hydroxy-2-naphthoic acid (1,2-HNA), 3-hydroxy-2-naphthoic acid (3,2-HNA), and 6-hydroxy-2-naphthoic acid (6,2-HNA) is investigated in two DESs, named ChCl:urea and ChCl:glycerol, constituted of H-bond acceptor choline chloride and different H-bond donors, urea and glycerol, respectively, in a 1 : 2 mole ratio under ambient conditions. While 1,2-HNA and 3,2-HNA exhibit an intramolecular H-bonding ability, 6,2-HNA does not. In contrast to common polar solvents, where the monoanionic emitting form of 1,2-HNA is also supported along with the neutral one, in both the DESs only the neutral emitting form exists. Addition of acid to the two DESs, respectively, fail to generate the monocationic form of the probe. Addition of a base to ChCl:urea results in the generation of the monoanionic form; even a very high strength of the base fails to generate the monoanionic emitting form in ChCl:glycerol. Relatively higher H-bond donating acidity of ChCl:glycerol results in added hydroxyl getting involved in H-bonding with alcohol functionalities of ChCl:glycerol leading to the absence of proton extraction to create the monoanionic form of the probe. Only the monoanionic emitting form of 3,2-HNA is present in ChCl:urea; in ChCl:glycerol, due to its higher H-bond donor acidity, the neutral emitting form is also detected. Addition of high strength of acid to ChCl:urea does result in formation of the neutral emitting form. Addition of an aqueous base results in the formation of the dianionic form of 3,2-HNA in ChCl:urea; however, in ChCl:glycerol, the added base fails to convert the neutral form of this probe to the monoanionic form as eﬃciently as that in ChCl:urea. The monoanionic (carboxylate) form of 6,2-HNA exits in ChCl:urea, whereas the neutral form is present in ChCl:glycerol due to its higher H-bond donating acidity. Addition of an acid can induce a shift in prototropic equilibrium towards the neutral form of 6,2-HNA in ChCl:urea; no change is observed in the behavior of this probe in ChCl:glycerol as the acid is added. Both the DESs support the dianionic form of 6,2-HNA in the presence of the base; the added base helps extract both –OH and –COOH protons of this probe. The H-bond donor component of the DES is clearly established to play a critical role in the prototropic behavior of the probe.

1. Introduction

Deep eutectic solvents (DESs) have emerged as a neoteric class of alternate media and have found numerous applications in various fields of science and technology.1–6 These new- generation solvents are formed through the complexation of two or more components.7–9 The resulting mixture has high entropy and remains in liquid form at appreciably lower temperatures. The formation and depression in the freezing point of these solvents are a result of the extensive H-bonding interactions present between the components.10–13 These novel solvents offer several advantages, such as, ease of preparation, low cost, non-toxicity, and biodegradability, among others. The easy availability of a wide variety of components forming DESs extends their utility in various applications.1,9,14,15 Further- more, the physical and chemical properties of these solvents can be tailor-made through substitution of components, thus allowing preparation of task-specific solvents for desired applications.16–19 DESs, in general, are known to be good solvents for a wide range of organic and metallic solutes promoting their use in versatile applications, such as, organic material synthesis,17 biocatalytic reactions,20,21 drug solubilization,22 and electroplating.23–26 Hence, the environmentally-benign nature, adaptable properties and numerous potential applications of DESs have evoked the interest of both the scientific community and industrialists.

The proton transfer process has been the focus of intensive research in many interdisciplinary fields of science and technology.27–35 Excited-state proton transfer can be used in a variety of applications, such as, laser dyes, polymer photo- stabilizers, photodynamic therapy, and fluorescence sensing for biological purposes.36–40 It has been well established that the acidic and basic traits of a molecule are enhanced by optical excitation. Photoexcitation of a molecule induces charge redis- tribution, which in turn instigates structural changes thereby significantly altering the acidic and basic properties of the excited molecule.41 Also, the occurrence of excited-state intra- molecular proton transfer (ESIPT) in bifunctional molecules, possessing both acidic and basic moieties, is of particular interest. In a study on flavanols, Kasha has shown that the intramolecular proton transfer is facilitated by the formation of an intramolecular H-bonded (IMHB) ring.42 Following the ground-breaking work of Weller on salicylates and methyl salicylates, there has been an upsurge in studies on the ESIPT process in salicylate-related molecules.43–46 Woolfe and This- tlethwaite conducted a comparative photophysical study on o-hydroxy naphthoic acid derivatives.47 They found that methoxy- 3-hydroxy-2-naphthoate exhibits a blue-shifted band, while phenyl- 1-hydroxy-2-naphthoate only shows normal emission. However, Mishra et al. reported that even though 1-hydroxy-2-naphthoic acid (1,2-HNA) does not exhibit ESIPT under neutral conditions, in highly alkaline (pH 4 12) medium, a red-shifted band is observed, which is attributed to the ESIPT in deprotonated form.48 Catal´an and group observed that the photophysics of these compounds was influenced by the intramolecular H-bonding, and the presence of both the hydroxy and carboxyl groups as substituents makes their photochemistry very intriguing.49 These bifunctional mole- cules exist in different prototropic forms – neutral, cationic, monoanionic (phenolate/carboxylate), dianionic, or zwitterion – depending on the solvation media. Proton relaxation and pro- totropism in HNAs have been studied in various polar–protic, polar–aprotic and non-polar solvents as a function of temperature, concentration, and pH.50–57
The photophysical behavior of prototropic molecules is known to be controlled by their surrounding milieu. Aqueous media provides an extensive H-bonding network and high dielectric constant, making them highly suitable for proton-transfer studies. This has inspired us to investigate hydroxy derivatives of naphthoic acid in DESs as non-aqueous solubilizing media under ambient conditions. In the present study, three structurally-distinct hydroxy naphthoic acid derivatives are investigated in two diﬀerent DESs named ChCl:urea and ChCl:glycerol, constituted of H-bond acceptor (HBA) choline chloride (ChCl) and two diﬀerent H- bond donors (HBDs), urea and glycerol, in a 1 : 2 mole ratio, respectively. The structures of the HBA and HBDs are presented in Fig. 1. In this investigation, HBA is kept constant and the eﬀect of two diﬀerent HBDs on the prototropic equilibria is explored in detail. While both the constituents of ChCl:urea are solids under ambient conditions, ChCl:glycerol was favored as one of the DESs since the precursor HBD is present in liquid form under ambient conditions aﬀording insights to its eﬀect as solvent media in relation to the DESs. Among the three proto- tropic probes, the hydroxy group is present at the ortho position in 1,2-HNA and in 3-hydroxy-2-naphthoic acid (3,2-HNA), while the hydroxy group is at the para position in 6-hydroxy-2- naphthoic acid (6,2-HNA). The structural dissimilarity in the prototropic probes allows us to study intramolecular proton transfer facilitated by the IMHB ring as well as the solvent- assisted intramolecular proton transfer. Furthermore, we have compared the outcomes observed in neat DESs and acid/base- added DESs with those obtained in water at diﬀerent pHs. Our work helps in establishing the role of DESs in supporting the process of proton transfer.

Fig. 1 Structures of the components of DESs used.

2. Experimental
2.1. Materials

ChCl:urea and ChCl:glycerol were prepared by mixing choline chloride (499% from Sigma–Aldrich) with urea (499% from Sigma–Aldrich) and glycerol (499% from Sigma–Aldrich), respectively, in a 1 : 2 molar ratio followed by stirring under gentle heating (B80 1C) until a homogeneous, colorless liquid was formed. DESs thus prepared were rigorously dried under vacuum for at least 24 hours. A Karl Fisher titrator was subsequently used to measure the water content of the DESs prior to their use. DESs were dried until water content became o100 ppm. 1,2-HNA, 3,2-HNA, and 6,2-HNA were used as received from Sigma-Aldrich. HCl and NaOH were used as received from Merck and Sigma- Aldrich, respectively.

2.2. Methods

The required amounts of the fluorophores 1,2-HNA 3,2-HNA and 6,2-HNA were weighed using an analytical balance with a precision of 0.1 mg. Stock solutions of fluorophores 1,2-HNA, 3,2-HNA and 6,2-HNA were prepared by dissolving in ethanol in pre-cleaned amber glass vials and stored at 4 1 1C to retard any photochemical reactions. An appropriate amount of the probe solution from the stock was transferred to a 1 cm path length quartz cuvette. Ethanol was evaporated using a gentle stream of high purity nitrogen gas and DES was added in a pre-calculated amount to achieve the desired final concentration of the probe. Pre-calculated amounts of aqueous HCl and aqueous NaOH were directly added to the cuvette and mixed thoroughly to form a homogeneous solution.

A PerkinElmer Lambda 35 double beam spectrophotometer with variable bandwidth was used for the acquisition of the UV-vis molecular absorbance spectra. Steady-state fluorescence spectra were acquired on a Jobin-Yvon Fluorolog-3 (model FL-3-11) modular spectrofluorometer equipped with a 450 W Xe arc lamp as the excitation source and single-grating monochromators as wavelength selection devices with a photomultiplier tube as the detector. All spectra were duly corrected by subtracting the spectral responses from suitable blanks prior to data analysis. Data analysis was performed using the Sigma Plot v10 software.

In order to obtain the time-resolved fluorescence emission of 1,2-HNA, 3,2-HNA, and 6,2-HNA, excited-state intensity decay data were acquired in the time-domain using a Horiba Jobin Yvon, Inc. fluorocube time-correlated single photon counting (TCSPC) fluorometer. 1,2-HNA and 3,2-HNA were excited using a 340 nm UV-pulsed NanoLED-340 source and 6,2-HNA was excited using a 295 nm UV-pulsed NanoLED-295 source having pulse width o1.0 ns. The emission was collected using a Peltier-cooled red-sensitive TBX-04 PMT detection module at the respective emission maxima wavelength. The data were collected using a DAQ-MCA-3 Series (P7882) multichannel analyzer. The instrument response function (IRF) was obtained using a scattering solution of glycogen in water (glycogen from bovine liver, Type IX, Aldrich). The excited- state intensity decays were analyzed using DAS6 analysis software and were fitted to the desired decay models.

Fig. 2 Normalized UV-vis absorbance and fluorescence emission spectra (lex = 340 nm; excitation and emission slits are 3 and 3 nm, respectively) of 1,2-HNA (25 mM) dissolved in different pH aqueous solutions (panel A) and in ChCl:urea, ChCl:glycerol and glycerol (panel B) under ambient conditions.

3. Results and discussion
3.1. Prototropism of 1,2-HNA
3.1.1. In neat DESs. The UV-vis absorbance and steady-state fluorescence emission spectra of 1,2-HNA dissolved in water at diﬀerent pHs are shown in Fig. 2A (band maxima are depicted in Table 1). At pH 3, a low energy transition band (Lb) appears at 342 nm and a high energy transition (La band) is observed at 287 nm. The emission maxima observed at 415 nm is assigned to the neutral form of 1,2-HNA. The absorbance spectra in the water at pH 7 is characterized by Lb band at 343 nm and La band at 287 nm while the emission spectrum shows a maximum at 413 nm corresponding to the neutral form. In highly basic conditions (at pH 13), La and Lb bands merge and a single maximum is observed at 340 nm in the UV-vis absorbance spectrum while the emission maxima occurs at 458 nm. The band observed at 458 nm is assigned to the monoanionic form of 1,2-HNA and the large bathochromic shift in emission spectra is attributed to the ESIPT in monoanionic form resulting in the formation of the naphtholate form of 1,2-HNA.

The structures of diﬀerent prototropic forms of 1,2-HNA are presented in Scheme 1(a). The spectral features and excited- state emission intensity decay of distinct forms of 1,2-HNA were reported in diﬀerent solvents including water at varied pH values, cyclohexane, ether, dioxane, and acetonitrile, among others.48 The results obtained by us in water are in good agreement with those reported in the literature.

The UV-vis absorbance/fluorescence emission spectra and the corresponding band maxima of 1,2-HNA dissolved in DESs ChCl:urea and ChCl:glycerol, and in glycerol are presented in Fig. 2B and Table 1, respectively. The spectral features in the absorbance spectra of 1,2-HNA dissolved in ChCl:urea are somewhat in contrast with those observed in water. The Lb band appears at 342 nm; however, in comparison to water, the La band in ChCl:urea shifts to 308 nm exhibiting a B21 nm bathochromic shift. An emission maximum centered at 415 nm is assigned to the neutral form of 1,2-HNA. The excited-state intensity decay of this probe was collected at 415 nm and the data fit best to a single-exponential decay with a recovered decay time of 1.2 ns further characterizing the neutral form. This lifetime was found similar to the lifetime of 1.8 ns reported for the neutral form in water and in acidic–ethanol solution.48 The absorbance spectrum of 1,2-HNA in ChCl:glycerol is fairly similar to those obtained in water at pH3 and pH 7. The Lb band is located at 343 nm and the La band is observed at 291 nm, while an emission maximum appears at 414 nm. To assess the role of HBD in a DES, the absorbance and emission spectra of 1,2-HNA were also acquired in glycerol. In glycerol, the Lb and La bands appear at 344 nm and 290 nm, respectively, whereas, the emission spectrum is char- acterized by a maximum at 411 nm corresponding to the neutral form. Furthermore, the excited-state intensity decay times in ChCl:glycerol and glycerol also fit best to a single-exponential decay with recovered lifetimes of 1.4 ns and 1.8 ns, respectively. These results reveal that the IMHB in both the DESs is such that it allows minimal intramolecular proton transfer in 1,2-HNA. The DESs clearly do not support the monoanionic form of the probe in contrast with that observed in water, ethanol, dioxane, acetonitrile, (cyclohexane + ether), and (TEA + ether), respectively, where monoanionic emitting forms are identified along with the neutral one. It is clear that the behavior of the DESs is more similar to that of cyclohexane and ether in this respect. Based on the reported pKa of B2.7 in
aqueous media for neutral-to-monoanionic transition, it is clear that DESs, in contrast, do not favor intramolecular proton transfer in 1,2-HNA. The subtle diﬀerences in the photophysi- cal behavior of the probe in ChCl:urea versus that in ChCl:gly- cerol (and glycerol) may be attributed to the relatively higher HBD acidity of the latter (Table 1).58,59 Also, the carboxyl group adjacent to the amine groups in urea further decreases the acidity of ChCl:urea. Increased HBD acidity from ChCl:urea to ChCl:glycerol to glycerol results in subsequent corresponding emission band broadening (FWHM values in ChCl:urea, ChCl: glycerol, and glycerol are 41, 49, and 54 nm, respectively). This diﬀerence in the HBD acidity values of ChCl:urea, ChCl:glycerol and its HBD component glycerol is further manifested in the excited-state intensity decay where no monanionic form is detected in the three solvent media; however, the recovered decay times increase in the order ChCl:urea (1.2 ns) o ChCl:glycerol (1.4 ns) o glycerol (1.8 ns). While the decay time of 1,2-HNA in glycerol is the same as those in water and in ethanol, respectively, the decreased decay times in the DESs may be attributed to the presence of HBA component choline chloride, which results in the reduction in HBD acidity values of the two DESs as compared to those of their HBD constituents.

3.1.2. In acid/base-added DESs. To get further insight into the existence of various prototropic forms of 1,2-HNA within DESs, aqueous HCl and aqueous NaOH, respectively, of varying acid/base strengths were added to the DESs. We acquired data for a 10 wt% aqueous solution of HCl (0 to 1.0 M) and NaOH (0 to 5.0 M) in DESs ChCl:urea and ChCl:glycerol, and in glycerol, respectively (Fig. 3 and S1 (ESI†) present the resulting UV-vis absorbance and fluorescence emission spectra, respectively). A careful examination of data reveals that while no change in the UV-vis absorbance spectra of 1,2-HNA in both the DESs is observed on addition of 10 wt% water, the presence of HCl results in the appearance of a well-structured La band akin to that in aqueous media at pH 3; the shape and the structure of the Lb band also start to resemble that in pH 3 water solution (vide supra).

The addition of aqueous HCl, however, leads to an increase in the acidity, which subsequently results in considerable broadening of the emission spectra in both DESs and also in glycerol (increase in FWHM of 17, 12, and 7 nm, respectively, in going from neat ChCl:urea, ChCl:glycerol, and glycerol to 10 wt% 1.0 M aqueous HCl solutions of the three). Recovered parameters from excited-state intensity decay of 1,2-HNA clearly attribute these observations to the change in the acidity of the solubilizing milieu rather than the possible presence of the monocationic form. The decay in both the DESs continued to fit best to a single- exponential decay function; however, decay time is observed to increase towards its value in water with increase in the strength of the acid (Fig. 4 and Table 2).

As the monocationic form is assigned a relatively short decay time of 0.85 ns,48 the increased decay times within DESs upon addition of aqueous HCl hint towards medium eﬀect and exclude the possibility of the presence of the monocationic form. Thus, even in the presence of 10 wt% 1.0 M aqueous HCl in ChCl:urea and in ChCl:glycerol, respectively, the monocationic forms are not supported in the solutions. It is important to mention here that the outcomes are fairly similar even for the addition of a higher amount of aqueous HCl in the two DESs (data not shown). The behavior of 1,2-HNA dissolved in 10 wt% aqueous NaOH solutions of the two DESs with varying NaOH strength, on the other hand, highlights diﬀerences that are due to the HBD component of the DES. For the 10 wt% aqueous NaOH solution with up to 5.0 M NaOH, the UV-vis absorbance and emission spectra of 1,2-HNA in ChCl:urea are distinctly diﬀerent from those in ChCl:glycerol; the spectral characteristics of the probe in ChCl:glycerol resemble closely to those recorded in glycerol (Fig. 3 and S1, ESI†). While for ChCl:urea, the addition of the base results in a change in the UV-vis absorbance spectral shape of 1,2-HNA towards that observed in pH 13 aqueous solution (vide supra) with additional broadening, ChCl:glycerol is clearly observed to resist this change, which is also mani- fested in the behavior of this probe in 10 wt% aqueous NaOH solutions of glycerol. The emission maximum of the probe within aqueous NaOH-added ChCl:urea gradually shifts from 415 nm to 450 nm implying the presence of both neutral and monoanionic forms in equilibrium at intermediate base con- centrations; the monoanionic form starts to dominate at higher base concentrations (43.0 M). The transition from neutral-to- monoanionic form is mirrored in the parameters recovered from excited-state intensity decay data as well (Fig. 4 and Table 2). While the best fit of the decay is to the single- exponential function in neat and 10 wt% water-added ChCl:urea, the decay starts to exhibit a satisfactory fit to only a double- exponential decay function in 10 wt% 1.0–5.0 M aqueous NaOH solutions of ChCl:urea. While one of the two recovered decay times (1.1–1.4 ns) is easily assignable to the neutral form of 1,2-HNA, the other one (4.3–5.5 ns) is similar to that reported for the monoanionic form (the lifetimes in water were reported to be 1.78 ns and 4.5 ns for neutral and monoanionic 1,2-HNA, respectively).48 Thus, it is easy to infer that the presence of base within ChCl:urea supports intramolecular proton-transfer in 1,2-HNA, which results in the formation of the monoanionic form. As mentioned earlier, in 10 wt% aqueous NaOH solution of ChCl:glycerol, almost no presence of the monoanionic form of the probe is observed even at 5.0 M NaOH concentration; this behavior is similar to that observed in its HBD constituent glycerol (Fig. 3, 4 and Table 2). While certain changes in band shape may be seen in UV-vis absorbance spectral profiles upon addition of 10 wt% 1.0–5.0 M aqueous NaOH, La and Lb bands clearly do not appear to merge. Furthermore, upon addition of aqueous base the emission spectra of 1,2-HNA exhibits minimal shift accompanied by minimal band broadening with no indi- cation of the presence of the monoanionic form. To corroborate this further, the excited-state intensity decay fits best to the single exponential decay irrespective of the strength of the added aqueous NaOH (the recovered decay times are character- istics of that assigned to the neutral form). It is interesting to note that the photophysical behavior of 1,2-HNA in 10 wt% 1.0– 5.0 M aqueous NaOH solutions of glycerol is similar to that for ChCl:glycerol – almost no monoanionic form is observed (Fig. 3, 4 and Table 2). This drastic difference in the behavior of ChCl:urea versus ChCl:glycerol is proposed to be due to the vastly differing HBD acidity values of the two DESs (Table 1). Added hydroxyl gets involved in rather strong H-bonding with –OH functionalities present in ChCl:glycerol due to relatively higher HBD acidity of this DES, and thus is not able to involve and assist in proton extraction to create the monoanionic form of the probe. On the other hand, due to relatively lower HBD acidity of ChCl:urea (urea as the HBD), the added hydroxyl is able to extract proton from 1,2-HNA giving rise to the monoanionic form within the solution. The HBD constituent is clearly controlling the proto- tropic behavior of this probe within the DESs.

Fig. 3 Normalized fluorescence emission spectra (lex = 340 nm; excitation and emission slits are 3 and 3 nm, respectively) of 1,2-HNA (25 mM) dissolved in ChCl:urea, ChCl:glycerol and glycerol, respectively, in the presence of aqueous HCl and aqueous NaOH with varying acid/base strength under ambient conditions.

Fig. 4 Excited-state intensity decay times of 1,2-HNA (25 mM; excitation with 340 nm NanoLED) dissolved in neat and aqueous acid/base-added ChCl:urea, ChCl:glycerol and glycerol, respectively, under ambient conditions. Residuals are provided below each panel.

3.2. Prototropism of 3,2-HNA
3.2.1. In neat DESs. The prototropic forms of 3,2-HNA, presented in Scheme 1(b), are somewhat similar to those of decay in all the investigated systems with recovered decay times of 7.7 ns (in ChCl:urea), 5.4 ns (in ChCl:glycerol) and 3.1 ns (in glycerol) that are considerably higher than the lifetime of 1.5 ns reported in water.53 The presence of neutral emitting species along with the monoanionic one is not manifested in the excited-state intensity decay of this probe in DES ChCl:glycerol and its HBD component glycerol. It is clear, however, that increasing HBD acidity results in decreased lifetimes for 3,2- HNA; H-bonding interaction is known to result in increased rate of nonradiative decay leading to a decrease in singlet-state lifetime.60,61 These outcomes suggest the importance of HBD acidity of a DES in controlling probe prototropism nonetheless.

1,2-HNA (i.e., possibility of intramolecular six-membered H-bonded state in monoanionic form) except for the position of the –OH group. The UV-vis absorbance and fluorescence emission spectra of 3,2-HNA in water at different pH values are presented in Fig. 5A (band maxima are reported in Table 1). The absorbance spectra in aqueous solutions of pH 3, 7, and 13, respectively, are fairly similar, and are characterized by an Lb band in the region 351–355 nm and a structured La band at higher energy. The emission spectrum in aqueous pH 7 exhibits a low energy peak at 523 nm, which is assigned to the monoanionic emitting form along with a high energy peak at 400 nm characterizing the neutral emitting form of 3,2-HNA. In basic aqueous solution (pH 13), the band centered at 400 nm is absent; the band characterizing the presence of both monoanionic and dianionic forms in equilibrium is hypsochromically-shifted to 498 nm. In a pH 3 aqueous solution, apart from a shoulder appearing around 410 nm (neutral form), the emission maximum at 527 nm is assigned to the monocationic form; the emission band has broadened considerably in com- parison. It has been reported in the literature that the monoanionic form undergoes ESIPT in aqueous solution.51,53 The results in water presented here are in line with those reported previously.51

Fig. 5 Normalized UV-vis absorbance and fluorescence emission spectra (lex = 340 nm; excitation and emission slits are 3 and 3 nm, respectively) of 3,2-HNA (25 mM) dissolved in different pH aqueous solutions (panel A) and in ChCl:urea, ChCl:glycerol and glycerol (panel B) under ambient conditions.

3.2.2. In acid/base-added DESs. We next explored whether prototropic forms of 3,2-HNA within DESs can be modulated by addition of external acid/base. Fig. S2 and S6 (ESI†) present UV-vis absorbance and fluorescence emission spectra of 3,2-HNA in 10 wt% aqueous HCl solutions of ChCl:urea, ChCl:glycerol, and glycerol, respectively, with varying strengths of acid (0 to 1.0 M). While not much change is observed in the UV-vis absorbance behavior of 3,2-HNA in ChCl:urea as the strength of HCl is increased to 1.0 M, the emission spectra clearly exhibit emergence of a shoulder at B420 nm indicating the formation of the neutral emitting form.

Added acid protonates the monoanionic emitting form to some extent and subsequently a neutral emitting form is generated. It is further corroborated by the intensity decay data (Fig. 7 and Table 3). While fits to single-exponential decay are satisfactory at low acid strengths, at higher HCl concentrations, double-exponential fits are required with the longer decay time (44.5 ns) characterizing the monoanionic emission, whereas the shorter decay time (o1.2 ns) may be assigned to the neutral emitting species. Addition of acid is able to produce the neutral emitting form to some extent along with the monoanionic form of 3,2-HNA within ChCl:urea.

The UV-vis absorbance and fluorescence emission spectra of 3,2-HNA in DESs ChCl:urea and ChCl:glycerol, and in glycerol, respectively, are presented in Fig. 5B (Table 1 depicts the corresponding band maxima). The absorbance spectra in ChCl:urea, ChCl:glycerol, and glycerol are fairly similar to each other and also have distinct similarity with the corresponding spectra of 3,2-HNA in water. Emission spectral characteristics of 3,2-HNA in ChCl:urea show appearance of just one band centered at 521 nm indicating the presence of only the mono- anionic emitting form. In ChCl:glycerol, however, along with the emission band centered at 518 nm characterizing emission from the monoanion, a prominent shoulder appears at 420 nm, which is assigned to the neutral emitting form. From the emission band shape and intensity, it may be inferred that the monoanionic form is the dominating emitting form in ChCl: glycerol as well. The presence of the neutral emitting form in ChCl:glycerol is easy to comprehend as the emission behavior of 3,2-HNA in glycerol, the HBD constituent of this DES, also reflects a high energy prominent shoulder along with a dominating band centered at 521 nm. Thus, the presence of both neutral and monoanionic emitting states is controlled by the HBD constituent of the DES – stronger H-bond donating acidity of glycerol, and thus ChCl:glycerol, supports the neutral emitting form along with the monoanionic form, whereas ChCl:urea with HBD component urea having lower HBD acidity does not. The excited-state intensity decay best fits to single-exponential.

The behavior of 3,2-HNA in 10 wt% aqueous HCl solutions of DES ChCl:glycerol is diﬀerent from that in corresponding solutions of ChCl:urea (Fig. 6 and S2, ESI†). As 10 wt% water is added to ChCl:glycerol, the neutral emitting form disappears. It is easy to comprehend as water is known to support only the monanionic emitting form of 3,2-HNA.51 However, surprisingly, in 10 wt% 0.2 M aqueous HCl solution of ChCl:glycerol, the monoanionic emitting form nearly disappears and converts into the neutral emitting form, which is clearly evident from the strong emergence of a band centered at 428 nm. A relatively small amount of acid dramatically shifts the equilibrium towards the neutral emitting species in ChCl:glycerol. To our convenience, the behavior of 3,2-HNA in acid-added glycerol, the HBD constituent of ChCl:glycerol, is the same (Fig. 6). These observations are further supported by the intensity decay times of 3,2-HNA in 10 wt% aqueous HCl solutions of ChCl: glycerol and glycerol, respectively (Fig. 7 and Table 3). In the presence of acid (i.e., 0.2–1.0 M HCl), the fits to the decay require double-exponential function with emergence of a shorter decay time along with a longer one clearly highlighting the presence of the neutral emitting form. It is clear that in ChCl:glycerol, in contrast to that in ChCl:urea, even a small amount of acid helps to shift the excited-state prototropic equilibria almost completely towards the neutral form. It appears that the added acid efficiently protonates the monoanionic emitting form of 3,2-HNA in ChCl:glycerol (and in glycerol) resulting in the emergence of the neutral emitting form as the dominating form.

Fig. 6 Normalized fluorescence emission spectra (lex = 340 nm; excitation and emission slits are 3 and 3 nm, respectively) of 3,2-HNA (25 mM) dissolved in ChCl:urea, ChCl:glycerol and glycerol, respectively, in the presence of aqueous HCl and aqueous NaOH with varying acid/base strengths under ambient conditions.

As the monoanionic emitting form of 3,2-HNA dominates in neat ChCl:urea with the neutral emitting form also showing its presence in neat ChCl:glycerol (vide supra), behavior of the probe in 5 wt% aqueous NaOH (0–5.0 M) solutions of the DESs is easy to comprehend (Fig. 6, 7, and S2, ESI† and Table 3). Addition of aqueous base results in the formation of the dianionic form of 3,2-HNA in equilibrium with the ESIPT monoanionic form characterized by a substantial hypsochro- mic shift in the emission maximum from 521 nm to 497 nm. The dianionic emitting form of 3,2-HNA is reported to exhibit emission that is hypsochromically-shifted as compared to the emission from the monoanionic form.51 It is noteworthy that in the presence of base, the photophysical behavior of 3,2-HNA in ChCl:urea resembles that observed at pH 13 (vide supra). The excited-state intensity decay of 3,2-HNA in base-added ChCl:urea exhibits a better fit to the double-exponential decay model (Fig. 7 and Table 3). The shorter of the two recovered decay times (in the vicinity of 6 ns, in general) corresponds to the monoanionic emitting form, whereas the longer one (14.6 to 15.8 ns) is assigned to the dianionic emitting form of 3,2-HNA.

Increased addition of base to the 3,2-HNA solution of ChCl:glycerol results in gradual decrease in the shoulder at ca. 428 nm indicating conversion of the neutral form to the mono- anionic one. However, a small albeit distinct hypsochromic- shift with increase in the concentration of NaOH in the solution may hint at the possible formation of a small amount of dianionic emitting species as well. The excited-state intensity decay aﬀords support to this proposition as it starts to resemble better to double-exponential decay at higher base concentrations with the appearance of longer decay times in the vicinity of 15 ns (Table 3). It is clear, however, that the conversion of neutral-to- monoanionic emitting form is not very eﬃcient in ChCl:glycerol, and the extent of conversion of monoanionic-to-dianionic emitting form is also not nearly as eﬃcient as that observed in ChCl:urea. The reason for this contrast in behavior within the two DESs is revealed by the response of 3,2-HNA in glycerol, the HBD constituent of ChCl:glycerol. In glycerol, the addition of 5 wt% aqueous NaOH of even 5.0 M strength fails to convert the neutral emitting form to the monoanionic one; the absence of hypsochromic shift in band characterizing the monoanionic form implies no significant formation of dianionic emitting species (Fig. 6) (intensity decay fits do show some formation of dianionic form for Z4.0 M NaOH, Table 3). It is easily inferred that the higher HBD acidity of the milieu resists base-induced changes in the behavior of the probe 3,2-HNA. These outcomes for 3,2-HNA solution of base-added DESs are similar to those observed for 1,2-HNA (vide supra). The added base prefers to interact with the –OH functionalities of the ChCl:glycerol, and thus fails to convert the neutral form of the probe to the monoanionic form as efficiently as in ChCl:urea, which has considerably lower HBD acidity (Table 1) – the added base reacts with the probe thus converting the monoanionic form to the dianionic one in a rather efficient manner. These outcomes further validate the critical role of the HBD acidity of a DES in controlling the probe behavior.

3.3. Prototropism of 6,2-HNA
3.3.1. In neat DESs. Probe 6,2-HNA represents a class of interesting and useful aromatic photoacids substituted with both –OH and –COOH in such a manner so that the acidity of the –OH is increased upon optical excitation (for the –OH group in 6,2-HNA, pKa E 8.9 and pKa* E 1.4), whereas the –COO— substituent becomes more basic in the excited-state relative to its basicity in the ground-state (for the –COOH group in 6,2-HNA, pKa E 4.3 and pKa* E 7.8).52 While within 1,2-HNA and 3,2-HNA, intramolecular proton transfer occurs readily, in 6,2-HNA, on the other hand, the proton transfer is usually with the solvent, which may or may not couple the protonation/ deprotonation of the two functionalities depending on the solubilizing milieu.52 The probe 6,2-HNA, in this respect, is clearly distinct from 1,2-HNA and 3,2-HNA.

The UV-vis absorbance and fluorescence emission spectra of 6,2-HNA in water at diﬀerent pH values obtained by us are similar to those reported earlier (Fig. 8A; the band maxima are listed in Table 1).52 The absorbance maximum at 305 nm (at pH 3) is assigned to the neutral form of 6,2-HNA, that at 292 nm exponential decay model with similar recovered decay times of 3.8 ns, 3.5 ns, and 3.8 ns, respectively (Fig. 9 and Table 1). Based on these observations, it may be deduced that excited-state proton transfer is not facilitated in either of the DESs. However, the dissimilarities in the spectral features leading to the presence of diﬀerent forms in ChCl:urea and ChCl:glycerol are attributed to the diﬀerence in the HBD acidities of the two DESs.

Fig. 7 Excited-state intensity decay times of 3,2-HNA (25 mM; excitation with 340 nm NanoLED) dissolved in neat and aqueous acid/base-added ChCl:urea, ChCl:glycerol and glycerol, respectively, under ambient conditions. Residuals are provided below each panel.

Fig. 8 Normalized UV-vis absorbance and fluorescence emission spectra (lex = 295 nm; excitation and emission slits are 1 and 1 nm, respectively) of 6,2-HNA (25 mM) dissolved in different pH aqueous solutions (panel A) and in ChCl:urea, ChCl:glycerol and glycerol (panel B) under ambient conditions.

3.3.2. In acid/base-added DESs. Further pursuing the diﬀer- ence in the prototropic behavior of 6,2-HNA in ChCl:urea versus ChCl:glycerol, we explored the eﬀect of increasing acidity of solubilizing DES on the photophysical properties of the probe. The UV-vis absorbance and fluorescence spectra of 6,2-HNA in ChCl:urea, ChCl:glycerol, and glycerol, respectively, are presented in Fig. 10 and S3 (ESI†).
In 2 wt% aqueous HCl (0 to 1.0 M) solution of ChCl:urea, no substantial change is observed up to the HCl strength of 0.8 M (slight bathochromic shift is accompanied by emission band broadening at 0.8 M HCl), and at 1.0 M HCl, both the absorbance and emission band exhibit abrupt substantial bathochromic shift (to 307 nm and 392 nm, respectively) suggesting a shift of equilibrium from carboxylate monanionic form to neutral form in both ground and excited states. This is accompanied by a decrease in the decay time upon the addition of 2 wt% 1.0 M aqueous HCl (Table 4).

It is clear that the addition of acid can induce a shift in prototropic equilibria towards the neutral form of 6,2-HNA within
ChCl:urea. As there is no mention of any cationic form either in (at pH 7) to the anionic (carboxylate) and that at 320 nm (at pH 13) to the dianionic forms, respectively. At pH 3, the emission maximum at 463 nm is assigned to the monoanionic (pheno- late) emitting form, whereas the one at 431 nm at pH 13 is assigned to the dianionic emitting form. Both the monoanionic emitting forms [phenolate at 457 nm as well as carboxylate at 358 nm (strong shoulder)] are present at pH 7. The results indicate that 6,2-HNA at pH 3 deprotonates in the excited state and a solvent-assisted excited proton transfer occurs at pH 3 and pH 7. The structures of these diﬀerent prototropic forms of 6,2-HNA are depicted in Scheme 1(c).

While Fig. 8B presents UV-vis absorbance and fluorescence emission spectra of 6,2-HNA in neat ChCl:urea, ChCl:glycerol, and glycerol, respectively, the corresponding band maxima are listed in Table 1. The absorbance and emission maxima at 292 nm and 359 nm, respectively, in ChCl:urea indicate the presence of the monoanionic (carboxylate) form in both ground and excited states. In contrast, the appearance of absorbance and emission maxima at 308 nm and 385 nm, respectively, in ChCl:glycerol implies that 6,2-HNA exists in its neutral form in both the states. The observations in glycerol are similar to those in ChCl:glycerol (small bathochromic shifts are due to the diﬀerences in the structure of the solubilizing milieu, as choline chloride is also present in ChCl:glycerol).

It is clear that, unlike ChCl:urea, strong HBD acidity of ChCl:glycerol (due primarily to the glycerol) leads to the neutral form of 6,2-HNA in both the ground and excited states in this DES. The excited-state intensity decay times in ChCl:urea, ChCl:glycerol, and glycerol, respectively, fit best to a single the ground or excited states of 6,2-HNA in aqueous or any other solvents, addition of acid does not result in any significant change in optical spectral behavior (Fig. 10) or excited-state intensity decay (Table 4) of the probe within ChCl:glycerol and glycerol as in both these media the neutral form of the probe exists in both ground and excited states. Thus, 6,2-HNA remains in its neutral form within 2 wt% aqueous acidic solutions of ChCl:glycerol and glycerol.

Addition of base to the 6,2-HNA solutions of ChCl:urea, ChCl:glycerol, and glycerol, respectively, results in the eﬃcient formation of the dianionic prototropic form in both ground and excited states (UV-vis absorbance and fluorescence emission spectra of 6,2-HNA in 2 wt% 0–5.0 M aqueous NaOH solutions of ChCl:urea, ChCl:glycerol, and glycerol, respectively, are presented in Fig. 10 and S3 (ESI†); excited-state intensity decay times are presented in Fig. 9 with recovered decay times listed in Table 4). In ChCl:urea, the addition of aqueous NaOH results in a shift in absorbance maximum from 292 nm to 321 nm along with appearance of a shoulder around 355 nm clearly implying the formation of a dianionic form from the predominant monoanio- nic form. The emission spectrum also reflects a similar change in maximum which shifts from 359 nm to 441 nm in the presence of the base. The presence of a small amount of base facilitates the proton transfer from the probe to the ChCl:urea resulting in the formation of a dianion. There is a significant bathochromic shift (B10 nm) for the dianion peak as compared to the corresponding peak in water (431 nm), which suggests that ChCl:urea may stabilize electronic transition for the dianion. The excited-state intensity decay data further corroborates the formation of dianion as the recovered decay time increases drastically from 3.8 ns in neat and 2 wt% water-added ChCl:urea to ~7.6 ns (the lifetime of dianion in water is reported to be 6.36 ns).52 Bathochromic shifts in absorbance and emission bands are also observed for 6,2-HNA dissolved in base-added ChCl:glycerol (and glycerol,Fig. 10). Specifically, the emission maximum shifts from 385 nm to 431 nm in ChCl:glycerol (and from 392 nm to 432 nm in glycerol). The neutral form of 6,2-HNA in ChCl: glycerol is readily converted to the dianionic form upon addition of a small amount of base; clearly, the added base helps in extracting both the protons (phenol and carboxylic acid) from the probe in a highly efficient manner. The decay times of 6,2-HNA in base-added ChCl:glycerol increases to 6.7 ns from its value of 3.5 ns in water-added ChCl:glycerol. This further implies the formation of the dianionic form of the probe on base addition (similar outcomes are observed in glycerol). It is established that the prototropism of 6,2-HNA within DESs is dependent on the identity of the HBD compo- nent of the DES; the HBD acidity imparted to the DES by the HBD component plays a critical role in controlling the proto- tropic forms of the probe within the DES.

Fig. 9 Excited-state intensity decay times of 6,2-HNA (25 mM; excitation with 295 nm NanoLED) dissolved in neat and aqueous acid/base-added ChCl:urea, ChCl:glycerol and glycerol, respectively, under ambient conditions. Residuals are provided below each panel.

Fig. 10 Normalized fluorescence emission spectra (lex = 295 nm; excita- tion and emission slits are 1 and 1 nm, respectively) of 6,2-HNA (25 mM) dissolved in ChCl:urea, ChCl:glycerol and glycerol, respectively, in the presence of aqueous HCl and aqueous NaOH with varying acid/base strengths under ambient conditions.

4. Conclusions

The presence of prototropic forms of a probe within a DES not only depends on the chemical architecture of the probe (whether capable of intramolecular H-bonding), but also on the HBD acidity of the DES. Unlike polar solvents, both the DESs fail to support the intramolecular H-bonded monoanionic emitting form of 1,2-HNA. Addition of a base to ChCl:urea is able to form monoanionic species; it is not formed in ChCl:glycerol even at very high base strength. The hydroxyl of the base gets involved in H-bonding with alcohol functionalities of ChCl:glycerol and fails to extract proton to create the monoanionic form of the probe. For 3,2-HNA, only the monoanionic emitting form is present in ChCl:urea; in ChCl:glycerol, the neutral emitting form is also detected due to H-bond donation from the solvent. The external base helps to form the dianionic form of 3,2-HNA in ChCl:urea; however, formation of anionic forms in ChCl:glycerol is not as eﬃcient. In 6,2-HNA, a probe with no intramolecular H-bonding, the involvement of DES as solvent is more prominent. Carboxylate form of 6,2-HNA, which exits in ChCl:urea, is not supported in ChCl:glycerol due to its high HBD acidity. The added base extracts both –OH and –COOH protons to form the dianionic form of 6,2-HNA in ChCl:urea as well as in ChCl:glycerol. The prototropic equilibrium within a DES is a complex interplay of the HBD acidity of the DES and the structure of the probe itself. The tailorability of DESs in terms of their HBD–HBA constituents thus offers widespread possibilities of proton-transfer processes within these neoteric media. The investigation presented here clearly establishes DESs as highly suitable solvents to modulate prototropic forms, which thus enhances the overall application potential of these upcoming versatile solvent systems.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

V. K. would like to thank the University Grants Commission (UGC), Government of India for her fellowship.

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