INTRAMOLECULAR RADIATIONLESS TRANSITIONS IN SUBSTITUTED BENZALDEHYDES

Phosphorescence excitation spectra of benzaldehyde and its derivatives have been observed in supersonic jets. We have also observed the disp ersed phosphorescence spectrum of benzaldehyde and found the intramole cular vibrational redistribution (IVR) in the triplet manifold. The fa st radiationless transitions in aromatic carbonyl molecules are induce d by the interactions among four electronic states, that is, S-1(1)(n pi), 3(pi pi*), 3(n pi*), and the S-0 ground state. Particularly, the change in the vibronic coupling between the (3)(pi pi) and (3)(n pi* ) states is important for the substitution effect. In the halogen and methyl-substituted benzaldehydes, the vibronic bands were found to be remarkably weakened. It is considered to be due to the dilution of the (3)(n pi) levels by the nonphosphorescent (3)(n pi pi*) levels and t he coupling with the ground state. In 4-pyridinecarboxaldehyde, howeve r, the intersystem crossing (ISC) is relatively slow because the 3(pi pi) energy is higher and the coupling with the S-1(1)(n pi*) state is not strong compared with that of benzaldehyde.