VIBRONIC COUPLING AND ENERGY-TRANSFER IN BICHROMOPHORIC MOLECULES - THE EFFECT OF SYMMETRY

The fluorescence spectra of a series of bichromophoric molecules consi sting of covalently linked fluorene units were investigated in a super sonic jet. In three of the systems (spirobifluorene, d(8)h(8)-spirobif luorene and 1-methyl spirobifluorene) no electronic coupling and no co rresponding exciton splitting were detected in the zero-point level of the S-1 state. Only 9,9'-bifluorene exhibited an exciton splitting in the nu=0 state. The lack of coupling was attributed to symmetry; in t he spirobifluorenes the planes of the fluorene moieties and the S-1<-- S-0 transition moments are perpendicular. When low vibrational levels were excited, state mixing, and energy transfer between the chromophor es was observed. This behavior is characteristic of the ''small molecu le'' regime of radiationless transition theory. When higher vibrationa l levels were excited, the systems exhibited typical ''large molecule' ' behavior. In this limit, both electronic energy transfer, as well as intramolecular vibrational relaxation contribute to the decay of the initially excited state. Intramolecular dispersive interactions were a lso investigated by comparing the bifluorenes with a series of referen ce compounds. (C) 1995 American Institute of Physics.