Av. Kirichenko et al., Fluorescence spectra and dynamics of structural relaxation of excited-state derivatives of 1,2-bis-(5-phenyloxazolyl-2)benzene, CHEM PHYS R, 17(9), 1998, pp. 1643-1651
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.