Detailed photophysical investigations have been carried out using a probe dye, coumarin-153 (C153), to understand the microenvironments of micelles formed by the newly introduced Tetronic star block copolymers, T1304 and T1307, having the same poly(propylene oxide) (PPO) block size but different poly(ethylene oxide) (PEO) block sizes. Ground state absorption, steady-state fluorescence, and time-resolved fluorescence measurements have been used to estimate the micropolarity, microviscosity, and solvation dynamics within the two micelles. To the best of our knowledge, this is the first report on these important physicochemical parameters for this new class of the star block copolymer micelles. Our results indicate that T1307 micelle offers a relatively more polar and less viscous microenvironment in the corona region, compared to T1304. The effect of the two micellar systems has subsequently been investigated on the bimolecular photoinduced electron transfer (ET) reactions between coumarin dyes (electron acceptors) and aromatic amines (electron donors). On correlating the energetics and kinetics of the ET reactions, clear Marcus inversion (MI) behavior is observed in both of the micellar media. Interestingly, the ET rates for all of the donor-acceptor pairs are much higher in T1307 than in T1304, and the onset of MI also appears at a relatively higher exergenocity (-Δ G) in the former micelle (∼0.45 eV for T1307) than the latter (∼0.37 eV for T1304). The effect of added NaCl salt studied selectively in T1307 micelle shows that the ET rate decreases significantly along with a shift in the onset of MI toward lower exergenocity region, so that in the presence of 2 M NaCl the system becomes quite comparable to T1304. On the basis of the observed results, it is realized that the micropolarity and hence the dynamics of the ET process can be tuned very effectively either by changing the constitution of the star block copolymer or by using a suitable additive as a modifier of the micellar microenvironment.

This study investigates the role of varying alkyl chain lengths of a series of surface-active 1-alkyl-3-methylimidazolium tetrafluoroborate ([C n MIm]­[BF4], n = 4, 6, and 10) ionic liquids (ILs) as cosurfactants in modifying the micellar characteristics of a tetronic star-block copolymer, T1304, and the consequent effects on bimolecular photoinduced electron transfer (PET) reactions carried out in these T1304–IL mixed micellar systems. Using coumarin 153 as the probe dye and following ground-state absorption, steady-state fluorescence, and time-resolved emission measurements, the micropolarity, microviscosity, and solvent relaxation dynamics in the micellar palisade layer have been revealed both in pure T1304 and in T1304–IL systems. With increasing alkyl chain length of the ILs, the palisade layer of the micelles gradually becomes more polar and less viscous, suggesting better incorporation of the longer alkyl chain length ILs as cosurfactants into the T1304 micelles. The bimolecular PET reactions, involving 7-aminocoumarins as acceptors and N,N-dimethylaniline as the donor, are considerably modulated in T1304 micelles by the presence of the ILs, the effect being more prominent for ILs with longer alkyl chain lengths. In all of the micellar systems, correlations of the electron transfer (ET) kinetics with the reaction exergonicity (−ΔG 0) show clear Marcus inversion (MI) behavior where onsets of MI invariably appear at significantly lower exergonicities, suggesting the involvement of a two-dimensional ET mechanism. Interestingly, the Marcus correlations display significant variations, namely, enhanced reaction rates and gradual shift in the onset of MI toward higher exergonicity, as longer alkyl chain length ILs are sequentially introduced as cosurfactants. From the observed results, it is convincingly realized that 1-alkyl-3-methylimidazolium-based ILs can be used satisfactorily as cosurfactants in tetronic star-block copolymer solutions to modulate PET reactions very significantly for their better utilizations in suitable applied areas.

Understanding the kinetics and energetics of photoinduced electron transfer (PET) reactions in constrained media has attracted considerable research interest, as constrained media provide a handle to tune the microenvironments and consequently the mechanisms of PET reactions. In this study, PET reactions between excited 7-aminocoumarin acceptors and ground-state N,N-dimethylaniline (DMAN) donor have been investigated in mixed micellar media composed of triblock copolymer, P123, and anionic surfactant, sodium dodecyl sulfate (SDS), with varying SDS-to-P123 molar ratios (n values). The objective is to elucidate the role of the n values in the rates and energetics of PET reactions over the entire time range from the subpicosecond to the subnanosecond domain, especially in regard to the applicability of the two-dimensional ET (2DET) mechanism. It is observed that by changing the n values, there is a significant change in the hydration characteristics of the SDS-P123 mixed micelles, which in turn changes the kinetics to energetic correlations for the PET reactions. Fluorescence from the excited coumarin acceptors undergoes substantial quenching due to PET from DMAN donor in all of the studied micelles as evidenced from steady-state, subnanosecond time-resolved (TR) and ultrafast (subpicosecond/femtosecond) fluorescence up-conversion measurements. The quenching rate constants (k q), estimated from subnanosecond TR fluorescence studies, and the individual component-wise decay rates (τi –1), estimated from up-conversion measurements, increase gradually with increasing n value, corroborating well with the sequentially increased micropolarity of the mixed micelles. Interestingly, it is observed that the correlations of either k q (from subnanosecond studies) or τi –1 (from femtosecond studies) with the reaction exergonicity (−ΔG°) show the noteworthy Marcus inversion (MI) behavior in a very consistent and similar manner for the entire time window, from subpicoseconds to subnanoseconds. The onset of MI always appears at an exergonicity (−ΔG°MI) much lower than solvent reorganization energy (λs), suggesting the involvement of 2DET mechanism throughout the subpicosecond to subnanosecond time domains. The present results thus provide a comprehensive picture of the kinetics and energetics of the PET reactions in constrained media for the whole time span and unequivocally establish the applicability of 2DET mechanism for the PET reactions in constrained media, eliminating any apprehensions about the effect of time resolution of the subnanosecond setup on the observed Marcus inversion behavior. This is indeed an important finding, providing valuable insights for PET reactions in constrained media, which has not been explored explicitly in any of the previous studies. Observation of MI behavior and the modulations in the PET reactions by simply changing the composition of SDS in the SDS-P123 mixed micelles are noteworthy findings of the present study and are expected to find suitable applications for better utilization and outcome...