Singlet-singlet intramolecular energy and electron transfer in covalently linked porphyrin-fluorescein/porphyrin-eosin heterodimers

Abstract

Two porphyrin-xanthene (including FL-TTP and Eo-TTP) heterodimers covalently linked with a flexible polyatomic chain have been synthesized and characterized. Their Uv-vis absorption, steady-state and time-resolved fluorescence spectra were investigated. The Uv-vis absorption spectroscopy suggests that there is some weak exciton coupling between the two chromophores in these systems, whilst fluorescence spectroscopy shows that the FL or Eo unit transfer singlet-state excitation energy to the TTP. From time-resolved fluorescence studies, it is concluded that the heterodimers exist in solution in different, non-equilibrating, conformations. The effects of the solvent polarity on the intramolecular energy and electron transfer efficiencies are discussed. The results show, in apolar solvent, selective excitation of the FL/Eo chromophore, a very efficient singlet state energy transfer process from FL/Eo to TTP was observed, but in polar solvent (DMF) intramolecular electron transfer reaction also occurs in addition to the singlet state energy transfer. The difference of the intramolecular interaction in solvents of various polarity may be explained in terms of conformational change due to the nature of solvent interaction. The rate constants and efficiencies of the intramolecular energy and electron transfer reactions in different polar solvents were determined.