Tetronic Star Block Copolymer Micelles: Photophysical Characterization of Microenvironments and Applicability for Tuning Electron Transfer Reactions.

Abstract

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 G0) 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.