Evidence for occurrence of the intramolecular CHO group rotation in the excited state for 3-pyridinecarboxaldehyde vapor

When only one of the two rotamers of 3-pyridinecarboxaldehyde (3-PCA) vapor was excited into the S1(n, π∗) state, the T1(n, π∗) phosphorescence bands of both of the two rotamers were observed with almost the same band intensity ratio as that observed under thermal equilibrium condition. When the S0 → S1 excitation spectrum was measured by monitoring the phosphorescence band of only one of the two rotamers, the excitation bands of both of the two rotamers were observed with almost the same band intensity ratio as that observed under equilibrium condition. These observations show occurrence of the intramolecular CHO group rotation in the excited state.