Radiationless transition T1→ S0 in aromatic ketones

Abstract

Measurements of quantum yields of phosphorescence are reported for a series of aromatic ketones and aldehydes in a glass at 77 K. Values for the radiative and radiationless rate-constants are calculated from the associated lifetimes. For molecules with ³nπ^* as the lowest excited state a correlation between the spin-orbit electronic matrix elements for the radiative and radiationless processes is observed. The radiative and radiationless matrix elements are due to mixing with an excited ππ^* state and the ground state respectively, and these are discussed in terms of the one-centre integrals on the oxygen atom. Quantitative estimates of the radiationless rate-constant indicate that the dominating vibration in the Franck-Condon factor is the carbonyl stretch. For molecules with ³ππ^* as the lowest excited state, both radiative and radiationless processes occur through spin-orbit mixing with the ¹nπ^* state. As the ³ππ^*, ¹nπ^* energy gap increases the phosphorescence characteristics approach those of aromatic hydrocarbons. Radiationless processes in fluorenone, showing ππ^* fluorescence but little phosphorescence are also discussed.