Self-quenching and electronic energy transfer of triplet molecules. The benzaldehyde-biacetyl system in the vapor phase

The triplet-triplet energy transfer from benzaldehyde to biacetyl and the competing self-quenching between triplets and ground state molecules of benzaldehyde were investigated in the dilute vapor phase by monitoring the phosphorescence (T₁(nπ^*)S_o) decay of benzaldehyde. Following excitation into the S₁(nπ^*)S₀ absorption band, a triplet self-quenching rate constant of k_SQ=(2.4±0.1) × 10⁴ s^?1 Torr^?1, corresponding to a gas-kinetic cross section of σ_SQ=0.22 A², was measured. The collision-free lifetime of the benzaldehyde triplet was found to be 2.3 ± 0.4 ms. Substitution of the aldehydic proton by deuterium reduces k_SQ by a factor of two: complete deuteration of the molecule has no further effect. Under the same excitation conditions, the energy transfer rate to biacetyl is k_ET=(2.8 ± 0.1) × 10⁶ s^?1 Torr^?1, with σ_ET = 24 A². This process is not influenced by deuteration.