Radiationless transitions of gas phase benzaldehyde have been investigated, by studying the excitation energy and isotope dependence of its phosphorescence. As in aromatic hydrocarbons, S1→T1 intersystem crossing (ISC) dominates over S1→S0 internal conversion (IC) for low to moderate excess energies (i.e., for excitation energies corresponding to S0→S1 and S0→S2 absorptions), but S1→S0 IC becomes important for large excess vibrational energies. The excitation energy dependence of phosphorescence in the region of S0→S1 and S0→S2 absorptions therefore reflects the excess vibrational energy (in T1) dependence of T1→S0 ISC. This dependence is exponential, indicating that efficient intramolecular vibrational energy redistribution precedes T1→S0 ISC. It is shown that the rates of T1→S0 ISC and S1→S0 IC are dramatically reduced by deuteration of ring hydrogens, but only moderately affected by deuteration of the aldehyde hydrogen. The isotope effects on T1→S0 ISC are discussed in light of recent spectroscopic findings of Goodman and co-workers.

Hirata, Y., & Lim, E. C. (1980). Nonradiative electronic relaxation of gas phase aromatic carbonyl compounds: Benzaldehyde. The Journal of Chemical Physics, 72(10), 5505–5510.