Triplet states of biacetyl and energy transfer as revealed by opto-acoustic spectroscopy

Using the technique of opto-acoustic spectroscopy, it is first demonstrated that radiationless decay from S1 of biacetyl when excited at wavelengths longer than 4430 Å involves a very slow (knr ∼ 103 sec−1) T1→S0 step, whereas beyond this wavelength a second fast channel opens: T2→S0. Addition of oxygen makes the apparent T1→S0 relaxation very fast. In mixtures of biacetyl and benzene, monitoring the slow component of the heat released from the biacetyl T1 state while irradiating at wavelengths absorbed by benzene shows that energy transfer from benzene directly to the Tn manifold of biacetyl occurs from the onset of 1B2u absorption to 2440 Å rather than throughout the band. In pyridine-biacetyl mixtures, the lowest n→π* state of pyridine transfers to the triplet manifold of biacetyl, but the lowest π→π* state does not.