Tuning of Intermolecular Electron Transfer Reaction by Modulating the Microenvironment Inside Copolymer−Surfactant Supramolecular Assemblies

Abstract

Photoinduced intermolecular electron transfer (ET) dynamics between various 7-aminocoumarin acceptors and N,N-dimethylaniline (DMAN) donor has been studied in copolymer-surfactant supramolecular assemblies prepared in aqueous 1% P123 triblock copolymer micellar solution with varying concentration of surfactants (sodium dodecyl sulfate (SDS), cetyl trimethyl ammonium chloride (CTAC), and triton-X-100 (TX100)). The aim of the present study is to modulate the reaction environment, especially the degree of micellar hydration inside the P123 micelle by the addition of the surfactants, which can modulate the ET reaction through the changes in the ET rates and the reaction exergonicity. Within the limited surfactant to copolymer molar ratios (n) used in the present study, fluorescence spectroscopy, dynamic light scattering (DLS), and small-angle neutron scattering (SANS) investigations indicate that the copolymer-surfactant supramolecular assemblies retain their micellar structure, although the micellar size gradually decreases with n. The redox potentials of the electron donor and acceptors are also found to change with n, although the extent of the effect is different for SDS, CTAC, and TX100 cosurfactants. In the presence of CTAC, the estimated exergonicity (-ΔG(0)) of the ET reaction is found to increase with an increase in n compared with that in pure P123, whereas it decreases marginally with SDS and remains almost the same for TX100. Substantial quenching of coumarin fluorescence is observed in the presence of DMAN in all copolymer-surfactant micellar aggregates because of ET reaction. The ET rate is seen to increase gradually with an increase in SDS and CTAC concentration in the supramolecular assembly, although it remains unaffected on the addition of TX100. The increased ionic strength in the Corona region of the copolymer-surfactant supramolecular aggregates due to the addition of the ionic surfactants has been envisaged for the increase in the ET rates. A correlation of the quenching rate constants with the free-energy changes (ΔG(0)) of the ET reactions shows the typical bell-shaped curve as predicted by Marcus outersphere ET theory. A substantial shift along the exergonicity axis (~0.3 eV) for the appearance of the Marcus correlation is observed in some cases, although the extent of such shift depends on both the nature of the cosurfactant and the amount of cosurfactant used in the copolymer-surfactant supramolecular assembly. Therefore, these preliminary results suggest a possibility of not only modulating the ET rates but also tuning the appearance of Marcus inversion along the exergonicity scale by suitably tuning the reaction environment inside the copolymer-surfactant supramolecular assemblies with a relatively more hydrophilic cosurfactant.