Photophysical studies on the benzophenones. Prompt and delayed fluorescences and self-quenching (original) (raw)

1975, Journal of the American Chemical Society

The photophysical pathways of benzophenone and several 4,4'-disubstituted benzophenones (fluoro, chloro, methyl, methoxy, dimethylamino) in benzene a t room temperature have been investigated using time-resolved laser spectroscopy. Prompt fluorescence spectra are reported for benzophenone and 4.4'-difluoro-, 4,4'-dichloro-, 4.4'-dimethyl-and 4,4'-dimethoxybenzophenone. The relative prompt fluorescence and absolute phosphorescence quantum yields at room temperature in benzene are reported. Both thermal ( T p -4 1 ) and P type (TI + T I -SI + SO) delayed fluorescence are documented for these ketones. From variable temperature data on the thermal delayed fluorescence, the singlet-triplet splittings are estimated to be: benzophenone, 4.9 f 0.5 kcal/mol; 4,4'-difluoro-, 3.9 f 0.4 kcal/mol; 4,4'-dichloro-, 4.1 f 0.4 kcal/mol; 4,4'-dimethyl-, 4.5 i 0.5 kcal/mol; 4,4'-dimethoxy-, 5.1 f 0.5 kcal/mol. All of these ketones and also 4,4'-bis(dimethy1amino)benzophenone are shown to undergo self-quenching (TI + SO -2So(ksQ)). The ~S Q ' S vary from ca. lo5 to IO9 M-I sec-' and correlate well with up+, p = -1.7. It is proposed that self-quenching involves formation of an exciplex wherein the half-filled n orbital of T I is directed toward the a electron density of an aromatic ring of So.