Tuning of electron transfer reactions in pluronic-surfactant supramolecular assemblies

Abstract

Photoinduced electron transfer (ET) reaction between an anionic acceptor, coumarin-343 (C343), and a neutral donor, N,N-dimethylaniline (DMAN), has been investigated in composite supramolecular assemblies (mixed micelles) comprised of a pluronic copolymer (P123: EO 20 -PO 70 -EO 20 or F88: EO 103 -PO 39 -EO 103 where EO: ethylene oxide and PO: propylene oxide) and a cationic surfactant (CTAC: cetyltrimethylammonium chloride), following fluorescence quenching studies. Systematic increase in the quenching rates for the studied donor-acceptor system with the increasing CTAC to pluronic molar ratio in the mixed micelles demonstrates a large modulation in the ET rates. The mixed micellar systems in the present cases are formed through the incorporation of the hydrocarbon chains of CTAC into the poly-PO core of the pluronic micelles whereby the cationic head groups of CTAC are placed at the periphery of the micellar core, protruded into the hydrated poly-EO corona region, leading to the formation of a positively charged layer deep inside these mixed micelles. Thus, the anionic C343 dye, initially dissolved at the micelle-water interface, experiences a gradually increasing electrostatic attraction and is therefore systematically dragged deeper inside the micellar corona, as the CTAC composition is increased in the mixed micellar systems. Consequently, the ET rate of the C343-DMAN pair undergoes a large enhancement in the studied mixed micellar systems with the increasing CTAC to pluronic molar ratio. The present strategy of modulating ET reactions using such composite supramolecular assemblies can find applications in areas where bimolecular ET is an integral reaction step.