Nonradiative relaxation of pyridine vapor: Transient absorption studies of triplet state formation, decay, quenching, and structure

Abstract

Relaxation processes in pyridine vapor have been studied using high-performance transient absorption spectrometry following excitation low in the S1 manifold. The ultraviolet spectrum of the lowest triplet state has been identified and measured, and its dependence on delay time and background pressure has been investigated for a variety of collision partners. It is concluded that formation of the triplet through S1 → T1 intersystem crossing proceeds in the statistical limit. Subsequent radiationless decay of the triplet population was found to show unusual pressure-dependent kinetics which apparently reflects collisional interconversion between two forms having very different intrinsic lifetimes. A simple model is proposed to explain the nonradiative behavior, collisional quenching, and spectra of the lowest triplet in terms of strong pseudo-Jahn–Teller vibronic coupling between nearly degenerate 3ππ* and 3nπ* states that leads to a double minimum in the T1 potential surface along the out-of-plane coupling coordinate. It is suggested that vibrationally relaxed T1 pyridine is nonplanar in structure whereas the vibrationally activated form is quasiplanar, and that the nonradiative T1 → S0 decay rates of these two forms are <105 s−1 and ∼5×106 s−1, respectively. Quenching of the triplet by ground state molecular oxygen was found to follow a sequential kinetic mechanism in which a transient intermediate was spectroscopically intercepted. This species is thought to be a weak complex formed between triplet pyridine and oxygen.