THE LOWEST EXCITED SINGLET-STATE OF 1,4-DIPHENYL-1,3-CYCLOPENTADIENE

We report a high-resolution excitation spectrum for 1,4-diphenyl-1,3-cyclopentadiene cooled in a supersonic expansion. The spectrum, which was obtained by the use of two-color resonance-enhanced multiphoton ionization techniques, has an origin at 27 008 cm-1. The dependence of absorption and fluorescence spectra on solvent and the vapor phase absorption spectrum shows that the lowest energy excited singlet state in the isolated molecule is the 2A state. The order of excited singlet states reverses in the condensed phase: the 1B state is found to be S1 in solvents with refractive indices ranging from 1.25 to 1.56. The lifetime of the 2A state in the isolated molecule could not be accurately determined, though an upper bound of 10 ns could be placed on it. These results give insight into the effect that an s-cis conformation in a polyene has on its electronic structure.