Vibronic coupling and energy transfer in bichromophoric molecules: The effect of symmetry

Abstract

The fluorescence spectra of a series of bichromophoric molecules consisting of covalently linked fluorene units were investigated in a supersonic jet. In three of the systems (spirobifluorene, d8h8-spirobifluorene and 1-methyl spirobifluorene) no electronic coupling and no corresponding exciton splitting were detected in the zero-point level of the S1 state. Only 9,9′-bifluorene exhibited an exciton splitting in the v=0 state. The lack of coupling was attributed to symmetry; in the spirobifluorenes the planes of the fluorene moieties and the S1←S0 transition moments are perpendicular. When low vibrational levels were excited, state mixing, and energy transfer between the chromophores was observed. This behavior is characteristic of the ‘‘small molecule’’ regime of radiationless transition theory. When higher vibrational 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 also investigated by comparing the bifluorenes with a series of reference compounds.