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

Abstract

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 (k_nr ∼ 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 T_n manifold of biacetyl occurs from the onset of ¹B_2u 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.