Intermolecular and intramolecular energy and electron transfer reactions between porphyrin and fluorescein

Abstract

A porphyrin (TTP)-fluorescein (FL) heterodimer covalently linked with a flexible polyatomic chain has been synthesized and characterized. The rate constants and the efficiencies of the inter- and intramolecular energy and electron transfer processes were determined. Their UV-Vis absorption, steady-state and time-resolved fluorescence spectra were investigated. The UV-Vis absorption and 1H NMR spectroscopy suggest that there is some exciton coupling between the two chromophores in this system, whilst fluorescence spectroscopy shows that the FL unit transfers singlet-state excitation energy to the TTP. From time-resolved fluorescence studies, it is concluded that the heterodimer exists in solution in different, nonequili-brating, conformations. The effects of the solvent polarity on the intramolecular energy and electron transfer efficiencies are discussed. The results showed that on selective excitation of the FL chromophore, only a very efficient singlet state energy transfer process from FL to TTP was observed in different solvents, but on selective excitation of the TTP chromophore, only in a polar solvent (DMF) could the intramolecular electron transfer reaction occur. The difference of intramolecular interaction in solvents of various polarity may be explained in terms of conformational change due to the nature of solvent interaction.