TUNING OF PHOTOINDUCED ENERGY-TRANSFER IN A BICHROMOPHORIC COUMARIN SUPERMOLECULE BY CATION BINDING

A bichromophoric molecule consisting of two coumarins linked by a pentakis(ethylene oxide) spacer can efficiently bind Pb2+ ions in acetonitrile and in propylene carbonate. The resulting changes in photophysical properties are reported with special attention to photoinduced electronic energy transfer. Steady-state data allow one to determine the transfer efficiency and stoichiometry of the complex, whereas subpicosecond time-resolved experiments by the excite-and-probe technique provide information on the transfer kinetics. The stoichiometry of the complex is 1:1 in acetonitrile and 1:3 (ligand:metal) in propylene carbonate. A significant increase in efficiency and rate of energy transfer for the complex is observed in acetonitrile, whereas there is almost no change in propylene carbonate. The results are discussed in terms of a possible structure of the complex which is different in the two solvents.