PHOTOPHYSICS OF PBD DERIVATIVES .1. THE FLUORESCENCE OF PARA-BIPHENYLYL-SUBSTITUTED 2-(BIPHENYL-4'-YL)-5-PHENYL-1,3,4-OXADIAZOLES

The room and low temperature absorption and fluorescence spectra and the fluorescence quantum yields and lifetimes of ten derivatives of 2-(biphenyl-4'-yl)-5-phenyl-1,3,4-oxadiazole with a substituent in the para-biphenylyl position are reported. The theoretical spectral characteristics of the electronic transitions of the derivatives with strong electro-releasing (-NH2) and strong electron-withdrawing (-NO2) substituents have been calculated by the PPP method. Differences between the spectral characteristics of the phenyl- and biphenyl-substituted PBDs are explained on the basis of the charge transfer character of the first electronic transition. No fluorescence has been observed for the carbonyl, thiocarbonylamide and nitro derivatives. An efficient non-radiative deactivation mechanism involving low energy npi* states localized on the substituent is assumed for these cases. The planarity of the compounds under study both in their ground and the lowest excited singlet pipi* state is discussed on the basis of the vibrational structure of the absorption and fluorescence spectra and on the basis of quantum chemical calculations.