Photoinduced electron and energy transfer processes in supported multilayers: Energy delocalization and antennae effects

Abstract

A number of surfactant trans-stilbene derivatives having the general structure: ▪ ( nS mA), have been synthesized and investigated in supported Langmuir-Blodgett assemblies. Layers of pure nS mA or mixtures with fatty acids show “H” aggregate characteristics including relatively low oscillator strengths for the lowest excited singlet state. Consequently these exhibit relatively low ability, individually, to donate excitation energy or transfer electrons over long ranges. Mixtures of these stilbenes, layered together to form mixed films, also show “H” aggregate characteristics but give evidence for delocalization or “supramolecular” behavior in which the chromophore can effectively span the width of an entire monolayer and result in efficient energy and electron transfer processes over distances spanning several monolayers. The extension to “H” aggregated and diluted (monomer) diphenylbutadiene and diphenylhexatriene layers results in assemblies in which vectorial energy transfer and redox processes occur. The stilbene-mixtures have also been used as a “medium” interposed between layers containing potential donors and acceptors for studies of both electron transfer and energy transfer. One of the molecules used as an energy transfer and electron transfer quencher is the cobalt(III) complex of 8S 1A. Although this molecule undergoes photochemical intramolecular-electron transfer-induced fragmentation in solution and microheterogeneous media, it shows net photostability in the films but serves as a highly effective trap. In the photoinduced electron transfer studies the fluorescence of a cyanine dye is examined with a remote ferrocene incorporated into an adjacent layer at a hydrophobic site. The cyanine dye fluorescence is weakly quenched by the ferrocene donor when separated by fatty acid “spacers”; the quenching is approximately doubled when the fatty acid is replaced by the stilbene mixture.