The UV-Vis absorption spectra of ensulizole in different solvents such as carbon tetrachloride, acetone, water, methanol and ethanol is displayed in Fig. 1(a). In polar neutral medium, ensulizole exists in monoanionic form and its sulfonic acid group donates proton that is accepted by the polar solvents like water, methanol and ethanol and subsequently stabilize the conjugate base of ensulizole. Removal of one or both protons from ensulizole results in an increase of delocalization of electrons, which tends to cause a red-shift in the UV absorption spectrum. The imidazole ring proton does not detach due to the neutral medium. For the removal of the imidazole ring proton, highly basic medium (pH =12 or above) is required. This is evidenced by Fig. 1(b), where a slight red-shift in the UV spectrum is observed in case of methanol and ethanol as compared to water. This is attributed to the electron releasing inductive effect [42] of methyl (CH3-) moiety in methanol and ethyl (C2H5-) moiety in ethanol, which stabilizes both the protonated methanol and protonated ethanol.
As the ethyl- group has greater electron donating inductive effect, therefore, the absorption peak of ensulizole in ethanol is more red-shifted than in methanol, which in turn is more red-shifted than water, as it does not offer any such inductive stabilization effect. Acetone is a weakly polar solvent. It does not facilitate the ionization process as polar solvents demonstrate, for instance water, methanol and ethanol. This is manifested by the less solubility of ensulizole in acetone. Therefore, ensulizole shows a blue-shift in UV absorption spectra as compare to water, Fig. 1(a). The ensulizole is more soluble in carbon tetrachloride (CCl4) due to non-polar interactions and these interactions do not favour the ionization of sulfonic acid group proton of ensulizole. Therefore, ensulizole remains undissociated in CCl4 and its UV-Vis absorption spectrum exhibits a blue-shift as displayed by Fig. 1(a). The reaction scheme 1 displays the proposed mechanism of proton transfer in various solvents.
Some of the polar solvent molecules are converted into their respective conjugate acids form and remaining free solvent molecules develop hydrogen bonding with ensulizole at specific sites. The hydrogen bonding interaction affects the electronic excitations, in general an increase in solvent polarity causes a blue-shift in n→π* transitions and a red-shift in π→π* transitions [8].
The UV-Vis absorption spectra of ensulizole in water at pH ≤ 2 (Fig. 1(b)) demonstrate in strongly acidic conditions the ensulizole exists as undissociated acid. It exhibits an absorption peak at ~300 nm because of π→π* transition and a shoulder at 314 nm due to n→π* transition. As ensulizole is a weak acid (pKa1 = 4 and pKa2 = 11.9), so its solution is mildly acidic (pH = 6) when dissolved in water. It is the sulfonic acid group proton, which dissociates under these conditions and the absorption spectrum at this pH is slightly red-shifted. Reaction scheme 2 presents the proposed mechanism of proton transfer in different pH environments.
In highly alkaline medium (i.e., at pH >12), both the sulfonic acid and imine moieties protons of ensulizole are removed and the absorption peak even further red-shifts at this pH. The red-shift occurs due to increase in the delocalization of the π-electrons. The pronounced red-shift observation following removal of imine proton confirms the N-H group in imidazole ring is the chromophore and controls the photochemistry of ensulizole following excitation at 306 nm, as substantiated by laser flash photolysis experiments [43].
Since it has been justified above with the help of UV-Vis measurements that ππ* state along the N-H coordinate of imidazole ring will be excited at 306 nm with pulsed PLS excitation source. Henceforth, all the results of SSPL and TRPL measurements will be discussed after pulsed excitation at 306 nm. The SSPL spectra of ensulizole in solid form as well as in solution form in different solvents is displayed by Fig. 2(a). Following excitation at 306 nm the PL spectrum of ensulizole exhibits a peak at ⁓445 nm in solid form as well as in slightly polar or non-polar solvents, like acetone and CCl4, where there is no or very weak solute-solvent interactions exist. In polar solvents like methanol, ethanol and water the PL spectra of ensulizole exhibits a pronounced blue-shift and the PL peaks maxima moves to 412 nm as compare to slightly polar or non-polar solvents (λemission = 445 nm) like acetone and CCl4, Fig.2 (a). This trend can be attributed to the occurrence of hydrogen bonding between the aforementioned polar solvents and the ensulizole solute molecules.
Polar solvents like water, methanol and ethanol develop hydrogen bonding with ensulizole and due to this effect, non-bonding electrons on the nitrogen and oxygen atoms are engaged with the solvent molecules. This results in decrease of the delocalization of non-bonding electrons and consequently a blue-shift in PL spectrum is observed. Slightly polar or non-polar solvents like acetone and CCl4 are unable to develop hydrogen bonding with the hetero-atoms of ensulizole, so the lone pairs of electrons are unrestricted and enhance the delocalization of π-electrons and causes a red-shift in the PL spectrum. A major evidence in this regard is the SSPL spectrum of ensulizole in the solid-state sample where there no solute-solvent interactions are possible. Amongst the polar solvents, SSPL spectra exhibit a slight red-shifted tail, which is more pronounced in water as compared to the ethanol and methanol. This is probably due to the polarity of the solvent and the delocalization effect, as justified above. The SSPL spectra of ensulizole in non-polar solvents are much broader as compared to the polar solvents. This is attributed to the enhanced delocalization effect in non-polar solvents. A slight red-shift in the SSPL spectra of ensulizole is observed in the acidic medium Fig. 2 (b). This again can be attributed to the fact that the increase in pH results in enhancement of delocalization and these observations are in consistent with the UV-Vis absorption measurements.
In order to assess the lifetime of ππ* state the PL kinetics is also measured. The ensulizole samples were excited at 306 nm and the PL decay kinetics was monitored at room temperature. Fig. 3 (a) depicts the PL decay kinetics of ensulizole in different solvents. The PLS-306 LED excitation source along with the electronics of time-correlated single photon-counting (TCSPC) setup allow conducting the measurements with a time-resolution of 500 ps. It is a well-known fact that an increase of solute-solvent interaction facilitates the internal conversion process. This leads to a decrease in fluorescence efficiency [44]. Reduction in the fluorescence efficiency is directly related to shortening of fluorescence lifetime. In aforementioned polar solvents, the ensulizole exhibits short PL lifetime and the PL decay kinetics is found to be monoexponential (Fig.3 (a)), in comparison to the non-polar solvents due to solute-solvent interactions, which are present in former and are absent in the latter. Thus, the polar solvents due to existence of solute-solvent interaction increase the internal conversion (IC) rate and decrease the fluorescence lifetime, confirming the ensulizole can be the most effective sunscreen in polar solvents particularly in water, where the ππ* state exhibits the shortest lifetime. To further corroborate this finding, the solvent polarity was gradually decreased that resulted in enhancement of PL lifetime. In order to eliminate the solvent-solute interactions the measurements were also conducted in the solid film of the ensulizole (Fig. 3(b)). The measured PL kinetics of all the solution-phase samples is fitted by a suitable exponential decay model (eq. 1) and the fitting parameters are displayed in table 1. The solid sample of ensulizole exhibited the longest PL lifetime (τav = 6.879 ns) and this can be attributed to the population transfer from the single to triplet state by intersystem crossing (ISC) process. The average PL lifetimes were estimated by eq. (2) using the time constants and the coefficients extracted from eq. (1). The τav and the extracted fitting parameters from the aforementioned equations (1) & (2) are presented in table 1.
Here Ai, x0 and ti are the associated coefficients, time zero and the time constants, respectively.
Table 1: The fitting parameters extracted from a unimolecular (monoexponential), biexponential, triexponential and tetraexponentail decay models for ensulizole in different solvents.
Sample ID.
|
A1
|
τ1 (ns)
|
A2
|
τ2 (ns)
|
A3
|
τ3 (ns)
|
A4
|
τ4 (ns)
|
τ(Average)
(ns)
|
Ensulizole film
|
1849.5
|
7.229
|
10958
|
1.757
|
1302.5
|
34.042
|
131.20
|
143.40
|
6.879
|
CCl4
|
1808.1
|
9.957
|
10915
|
1.869
|
874.6
|
43.363
|
---
|
---
|
5.918
|
Acetone
|
2124.1
|
28.762
|
15951
|
1.567
|
---
|
---
|
---
|
---
|
4.763
|
Ethanol
|
15557.6
|
2.847
|
---
|
---
|
---
|
---
|
---
|
---
|
2.847
|
Methanol
|
16167.9
|
2.505
|
---
|
---
|
---
|
---
|
---
|
---
|
2.505
|
Water
|
16382
|
1.688
|
---
|
---
|
---
|
---
|
---
|
---
|
1.688
|
In order to study the effect of pH on the excited ππ* state lifetime, the PL kinetics was also conducted in various pH environments in water as solvent, Fig. 3(c). It displays different PL decay kinetics for ensulizole in mono-anionic form i.e., at pH = 6, in molecular form i.e., at pH= 2 and in highly basic medium i.e., at pH = 14. The measured PL kinetics is best fitted by biexponential decay model for pH 2 – 6 and a unimolecular (monoexponential) decay model for pH 8 - 14, table 2. The increase in PL lifetime by increasing the pH from acidic to highly basic medium is due to enhanced stabilization of π-electronic system after removal of the second proton from the imino group. These findings again confirm that imidazole ring is the chromophore of the ensulizole and are in consistent with the steady-state absorption analysis. This inference is also in accordance with previously reported laser flash photolysis experiments, where following excitation at 266 nm the dissociation of imino group and the formation of dianion of ensulizole through double deprotonation has been observed [43]. Altough these experiments do not exactly mimic the TRPL measuremnets performed here, but as justified above the high pH extracts the second H+ from the imino group that further stablizes the π-electron system and enhnaces the PL lifetime.
Table 2: The fitting parameters extracted from a unimolecular (monexponential), biexponentail decay kinetics models for ensulizole at different pH.
Sr. no.
|
pH
|
A1
|
τ1 (ns)
|
A2
|
τ2 (ns)
|
τ(Average) (ns)
|
τo/τ
|
1.
|
2
|
17283
|
1.367
|
187.5
|
3.481
|
1.389
|
1.709
|
2.
|
4
|
15857
|
1.485
|
27.51
|
16.38
|
1.511
|
1.572
|
3.
|
6
|
16262
|
1.678
|
17.42
|
14.84
|
1.692
|
1.404
|
4.
|
8
|
15009.5
|
1.910
|
---
|
---
|
1.911
|
1.243
|
5.
|
10
|
15551
|
2.056
|
---
|
---
|
2.056
|
1.156
|
6.
|
12
|
15522.4
|
2.347
|
---
|
---
|
2.347
|
1.012
|
7.
|
14
|
15190
|
2.376
|
---
|
---
|
2.375
|
1
|
The removal of the second H+ and enhancement of the PL lifetime confirms that the photoexcited ππ* state along the N-H coordinate of imidazole ring controls the photochemistry of ensulizole following excitation at 306 nm.
A gradual drop in PL intensity of ensulizole is observed with a decrease in pH value from 14 to 2 as shown in Fig. 2 (b). This reflects a quenching role of hydrogen ions (H+) in this process. With increase in H+ concentration (decrease in pH value), a decrease in PL intensity and average PL lifetime has been observed. By applying Stern-Volmer equation [45,46] (eqs. 3 & 4) to the measured PL data, a straight line with intercept equal to one is obtained (Fig.3 (d)), thus confirming the dynamic and diffusion controlled PL quenching of the ensulizole. The linear fit to the data enabled to evaluate the Stern-Volmer quenching constant KSV, 0.06995 M-1, where KSV = kq×τo.
Where [Q] is concentration of quencher, τo is average lifetime in the absence of quencher, τ is average lifetime in the presence of quencher and Kq is bimolecular quenching rate constant (eq. 4). The estimated value of bimolecular quenching constant is 2.945 × 107 M-1 s-1. In this study, we quantitatively demonstrate the lifetime of ππ* state locating on N-H coordinate of imidazole ring of ensulizole that strongly depends on the solvent polarity and the pH of the medium. Our future studies will focus on the photochemistry of ensulizole attached semiconductor quantum dots (QDs) to address how the QDs alter the photochemistry of the chromophore, in particular the N-H coordinate.