Fluorescence spectra and dynamics of structural relaxation of excited-state derivatives of 1,2-bis-(5-phenyloxazolyl-2)benzene

The concern of this study is to analyze the electronic absorption spectra, as well as the steady-state and time-resolved fluorescence spectra, of ster ically hindered derivatives of 1,2-bis-(5-phenyloxazolyl)-benzene (ortho-PO POP) with conformations such that their oxazolyl-phenyl (1,3,4-oxadiazolylp henyl) fragment is offset by 70 degrees-85 degrees from the plane of their quasiplanar 2,5-diphenyloxazol fragment. Conjugation between these fragment s is disrupted, and they show up in the spectra almost independently. In ex cited-state ortho-POPOP, conjugation is partly restored due to structural r elaxation (flattening) of the molecules. The activation parameters for flat tening of the excited-state compounds in question are determined. The analy sis reinforced the previous conclusion that the activation barrier mainly d epended on the nature of a heterocycle on either side of a single C-C bond, around which molecular fragments rotated in the course of flattening of or tho-POPOP molecules in the SI state.