Phosphorescence spectrum and mechanisms of benzaldehyde in methyl-cyclohexane at 4.2°K

Abstract

The Shpolskii-type quasi-linear phosphorescence emission spectrum of benzaldehyde in methyl cyclohexane has been observed and analyzed under high resolution in mixed crystals at 4.2°K. Most of the intensity of the emission is accounted for by a direct spin-orbit interaction. The ratio of the intensity appearing in bands involving the CO stretching vibration to those involving the in-plane (symmetric) ring modes is 4:1, indicating that the excitation involves primarily the CO electronic system. This, together with the emission lifetime and the similarity of the spectrum to the emission of other simple carbonyls having their high-lying T π,π ∗ states, strongly suggests that the emission is of the 3A″ (n, π ∗) → 1A′ type. The Herzberg-Teller-type emission (due to vibronic coupling with nontotally symmetric vibrations) accounts for only 7% of the total intensity. The importance of spin-vibronic interactions leading to the aldehyde ketone phosphorescence is pointed out. Comparison of the benzaldehyde ( n, π ∗ ) phosphorescence with the benzaldehyde ( n, π ∗ ) fluorescence and the ( π, π ∗ ) phosphorescence of toluene suggests that the π ∗- orbital of the ( n, π ∗ ) benzaldehyde triplet resembles the toluene π-system more than does the π ∗- orbital of the ( n, π ∗ ) singlet. The relative importance of the different out-of-plane vibrations in this coupling is found to be in serious disagreement with recently published predictions.