In addition, the evolutions of the C* and K* bands in the time-resolved fluorescence spectra were analyzed by fitting the spectra to the sum of two lognormal peaks. Time-dependent spectral shifts of species with different formal charges revealed characteristic hydration behavior at the interface of the AOT anionic groups and the bound water region, as well as rotational and translational diffusion of the probe within the RM. The red solid lines represent the sum of the individual fitting curves of the log-normal distribution of the component spectra (dashed lines) and are superimposed on the measured spectra (gray solid line).
Results of lognormal fitting of UV/vis absorption spectra and emission spectra of NM7HQ+ in AOT/n-heptane RM as a function of w0. Schematic showing the rotational motion and hydration of NM7HQ+ after diffusion by ESPT within the water/AOT/n-heptane RM for (a) C*- and (b) K*-forms of the probe. Biexponential fit parameters (single exponential decay parameters for w0 = 0.5) of fluorescence anisotropy of NM7HQ+ in water/AOT/n-heptane RM at different values of w0 monitored at 440 nm with λex = 375 nm.
Bi-exponential fitting parameters (single exponential decay parameters for w0 = 0.5) of the fluorescence anisotropy of NM7HQ+ in water/AOT/n-heptane RMs at different w0 values, followed at 550 nm with λex = 375 nm. Bi-exponential fitting parameters (single exponential decay parameters for w0 = 0.5) of the fluorescence anisotropy of NM7HQ+ in water/AOT/n-heptane RMs at different w0 values, followed at 550 nm with λex = 450 nm.
Introduction
In the ground state, two prototropic forms of NM7HQ+ are in equilibrium, namely the larger protonated cationic form (C) and the smaller deprotonated zwitterionic form (Z). Upon photoexcitation, the acidity of NM7HQ+ increases drastically; its pKa value decreases from 5.9 in the ground state to -2.3 in the excited state [23–26]. In the excited state, the equilibrium thus shifts towards the keto form (K*) as a result of the deprotonation of C* (Scheme 1).
The positively charged C form is expected to be localized in the vicinity of the interfacial region between the headgroups and the water pool due to its strong ion-ion interaction with the anionic headgroups. In contrast, the deprotonated Z form, which is in equilibrium with C in the ground state, is expected to reside further from the charged interface due to its neutral formal charge. Ka and kpt denote the equilibrium constant in the ground state and the forward rate constant of ESPT, respectively.
Material and methods
- Materials
- Steady-state and time-resolved spectral measurements
- Data analysis
- DFT calculations
TRES, which depicts the time evolution of the emission intensity, I(t), and the peak position, ν(t), of each of the constituent fluorescence bands, was fitted to the sum of lognormal peak functions [27–31] . Among these parameters, we focused on the ν(t) profiles, which reveal the hydration dynamics and diffusion of each prototropic species involved in the ESPT accompanying the positive-to-neutral charge evolution of NM7HQ+. To quantify the dipole moments of the ground state forms of NM7HQ+ (C and Z), DFT calculations were performed using the CAMB3LYP functional with the 6-311G+(d, p) basis set.
Time-dependent density functional theory (TDDFT) calculations were performed to optimize the geometry of C* and K*. Water solvent effects in S0 and S1 were included via the polarizable continuum model (PCM) using a version of the integral equation formalism (IEFPCM) for all calculations.
Results
- Steady-state absorption and emission spectra
- TRES
- Spectral deconvolution of TERS
- Time-resolved fluorescence anisotropy
Furthermore, increasing the size of the water pools seemed to induce the blue shift of the C and Z bands (Figure 4 (b)). It follows that the local environment of the probes became more polar with increasing w0 [33]. Figure 4(c) shows the relative intensity of the emission peaks of the two prototropic species as a function of w0.
The increase in solvent polarity, explaining the gradual hypsochromic shift of the absorption bands at w0 ≤ 4.5, cannot account for the opposite trend observed in the emission spectra. At w0 > 4.5, the peak position of the C* band remained relatively constant, while the K* band showed a small hypsochromic shift with increasing water pool size. Interestingly, the trend in the shift of the emission maximum of K* was identical regardless of its origin, i.e. if K* was produced via C*'s ESPT or Z's Franck-Condon excitation.
At w0 = 8, the initial intensities of the C* and K* bands were comparable, while at w0 = 20, the K* emission band was more intense than the C* band. These results excluded the effect of ESPT and thus can be compared with the dynamics of K* originating from C* via ESPT. A comparison of the ν(t) profiles of the C* and K* emission revealed an anomalously slow component decaying on a time scale of tens.
In addition, the wavenumber of the maximum emission at infinite time, v∞, showed a gradual decrease or redshift with increasing water pool size. As w0 increased, the spectral relaxations for the fast and slow component ν(t) of the C* band profiles accelerated. The rates of depolarization processes for C* and K* were higher at larger RMs.
For K*, as w0 increased, the amplitude of the slow component, Aslow, decreased, while the amplitude of the fast component, Afast, simultaneously increased. The main difference in the anisotropy decays of K* originating from C via ESPT and from Z via Franck–Condon excitation (λex = 375 and 450 nm, respectively) were the initial anisotropy values (r0). Our TDDFT calculations of the excited state C* and K* forms of NM7HQ+ show that deprotonation causes changes in the direction and strength of the dipole moments, as shown in Figure 5 .
Discussion
Location of NM7HQ +
18. of the AOT headgroup), depolarization via the rotational diffusion of the cationic probe is severely limited. The closer β approaches 1, the more tightly bound the probes, and therefore, the lower the contribution of the fast component to the overall depolarization process. It follows that for larger water pools, the curvature of the spherical interfacial layer of AOT became shallower, and the binding force restraining the NM7HQ+ molecules became weaker, leading to larger θc values.
Before ESPT, C* has a positive formal charge and is expected to bind strongly to the negatively charged AOT headgroups through electrostatic interactions. It also follows that K* was no longer confined to the interface region and was pushed towards the water pool, where the environment is more unstable. Thus, in terms of location, one can envisage a scenario in which C* is tightly bound to the interface layer and thus remains partially hydrated, while K* takes a full hydration shell (with bound water) in the vicinity hers.
This conclusion was cross-checked by comparing the lateral diffusion times, τL, and the corresponding lateral diffusion coefficients, DL, for the two forms of the probe. The trend in the DL values for K* formed from C* via ESPT with increasing w0 (Table 8) was similar. At the time of photoexcitation, C* is initially tightly bound to the sulfonate moieties of the AOT headgroups.
Due to the changed charge state, K* is no longer confined to the anionic interface and can diffuse towards the core of the water pool. This indicates that C* had a similar restriction in its movement near the sulfonate moiety in all cases. Although the τw values for C* were not significantly different from those for K*, the latter showed larger Dw values that generally increased with increasing water pool size.
A comparison of the wobble parameters of K* generated by the ESPT of C* and those of K* generated by the direct Franck–Condon excitation of Z (Tables 8 and 9) indicated that the θc and Dw -values tended to be slightly higher for K* generated from C*, implying slightly increased mobility due to the ESPT-assisted diffusion.
Hydration in the water pool
The ν1 component contributed more to the total spectral relaxation for all water pool sizes, implying that the relatively free water molecules towards the RM core made a larger contribution to the overall solvation dynamics of K* than the water molecules connected (Table 2) . This argument is coherent with the picture created for the location of the probes based on the anisotropy studies. The dependence of the early spectral shift component (ν1) of K* generated through the Franck-Condon excitation of Z on the water pool size showed a similar trend to that of C* (Tables 1 and 3).
This component initially increased with the water pool size (w0 ≤ 4.5) due to the abundance of more labile water. However, some of this ultrafast solution could not be detected within our IRF; The fraction of undetected solvent is expected to increase with the size of the water pool. Compared to that of C*, the reduction in the ν1 value for K* was much smaller, because the K*.
This implied that the contribution of ν2, i.e. the contribution of the headgroup bound slow water, became less significant as the water pool size was increased, and more labile water became available. The local pH around a photoacid immediately after photoexcitation is expected to be higher than bulk due to the residence of the ejected proton in its environment. Although the order of the reported values is the same as that of τ2 in Table 2, such values are also observed even when no ESPT is involved (Table 3).
A comparison of the ν(t) fit parameters for K* populated via the ESPT of C* and K* produced by the direct photoexcitation of Z indicated that the νꝏ values tended to be similar for a given pool size, especially with larger pool sizes (Tables 2 and 3). Because the local heterogeneity in such crowded environments varies greatly over short distances, the probe's location determined the initial, observable emission energies of K* and thus the magnitude of the total spectral shift. This extra stabilization of K* may have arisen from the effect of the large polarizable iodide counterion, which is expected to remain in the vicinity of the cationic C* as it transforms into K* via the ESPT.
Finally, it can be deduced that when C* loses a proton to the surrounding water molecules to generate K*, the resulting K* tends to diffuse slightly from the original bound location to the core of the water pool to increase the free energy minimize (Figure 11).
Conclusions
