Experimental and theoretical study of 2,5-diaryloxazoles whose aryl are para-substituted phenyl groups

Absorption and emission properties of some phenoxy derivatives of diphenyloxazole (PPO) are presented and discussed. The photophysical properties reflect a dependence on the substituted 2 or 5 position of the oxazole ring. The experimental data were correlated with molecular parameters such as molecular flexibility, electronic structure and the singlet-triplet gap. The substitution with heteroatomic-bridged phenyls maintains the same frontier orbitals (m, m+1) as in the parent compound, but introduces a new molecular orbital, m-1, located on the terminal phenyl. The presence of the so-called band B in the absorption spectra of some diaryloxazoles was attributed to the m-1-->m+1 transition participating to the second excited state wave function. The other main component of this state, the m-->m+2 transition, was found to have a forbidden character, explaining the lack or the low intensity of band B. The decrease of the fluorescence quantum yield subsequent to substitution of PPO with phenoxy fragments was found to be due to the enhanced molecular flexibility comparing to PPO. The differences between the 2- or 5-substituted derivatives were rationalized in terms of a smaller S(1)-T(2) gap for the former, thus increasing the rate of the overall nonradiative intersystem crossing processes.