Tangled ← S spectra of isotopic benzaldehydes

Abstract

The T 1.2 ← S 0 phosphorescence excitation spectra of benzaldehyde and its deuterated derivatives have been obtained in methyl benzoate and acetophenone hosts at 4.2 K. The observed tangled vibronic bands have been analyzed in terms of Born—Oppenheimer vibronic wavefunctions of both the excited ππ* and nπ* triplet states. Two perturbation terms are employed in this calculations: one accounts for the vibronic interactions, the other for direct environmental coupling interactions between T 1(ππ*) and T 2(nπ*) vibronic levels. In these hosts the spectra show sparse tangledness indicating that the most important resonances are between the zero-order vibrational level of T 2(nπ*) and zero-order low-frequency out-of-plane vibronic levels of T 1(ππ*). Substitution of the aldehyde hydrogen by deuterium produces a dramatic effect on the spectral intensity distribution, but little effect on the band positions. We conclude that the activity of the T 1(ππ*) origin band is mainly controlled by direct environmental perturbations between T 1(ππ*) and T 2(nπ*) with a minor contribution from the first-order spin—orbit coupling interaction between T 1(ππ*) and S 1(ππ*). The lowest excited state energy level consists of an almost pure vibrationless level of T 1(ππ*). The vibrationless origin level of T 2(nπ*), however, is difficult to identify in the T ← S spectrum because this level interacts strongly with several out-of-plane vibronic levels of T 1(ππ*) in resonance with T 2 0(nπ a; 0-0). Diagonalization of the secular determinant in terms of a basis set made up of many 3ππ* and 3nπ* Born-Oppenheimer vibronic levels shows that the Condon and Born-Oppenheimer approximations are inappropriate for discussing the intensities and designating the assignments.