Vibronic Interactions between nπ* and ππ* States of Polycyclic Azines and Aromatic Aldehydes: A Study of Perturbing Vibrations from Phosphorescence Spectra

It has been shown that out-of-plane hydrogen wagging vibrations of CH groups, adjacent to the heteroatoms, are responsible for the appearance of relatively strong, in-plane polarized bands in π*→π phosphorescence of p-methoxybenzaldehyde and dibenzo[f, h]quinoxaline. In dibenzo[f, h]quinoxaline, these hydrogen vibrations are more effective than the out-of-plane ring vibrations in inducing the in-plane polarized phosphorescence intensity through vibronic spin–orbit coupling. In dibenzo[a, c]phenazine, where there is no CH group adjacent to the nitrogen atoms, the vibrational modes which give rise to the more intense in-plane polarized bands are out-of-plane ring modes. These results indicate that the nπ* state(s) is involved as an intermediate state(s) in vibronic spin–orbit coupling.