The cavity ringdown spectrum of the visible electronic system of thiophosgene: An estimation of the lifetime of the T1(ã 3A2) triplet state

Abstract

To obtain insights into the photophysical properties of collision-free T1(ã 3A2) thiophosgene, Cl2CS, the cavity ringdown (CRD) spectrum of the T1←S0 absorption system was recorded under supersonic jet conditions and compared with the corresponding excitation spectrum of the total emission. It was found that none of the T1←S0 bands in the CRD spectrum appears in the excitation spectrum, indicating that the T1 thiophosgene decays almost exclusively by the nonradiative T1→S0 intersystem crossing (ISC). An estimation of the T1 nonradiative lifetime was made using the T1–S0 spin-orbit coupling and the Franck–Condon factors for the T1→S0 ISC based on the ab initio equilibrium structures and vibrational frequencies for the T1 and S0 states, computed at the MP2/6-31G(d,p) and MP4/6-31G(d,p) level of theory. The nonradiative life, calculated as the 1/e of the survival probability, is approximately 20 ps for barrier heights of 770–845 cm−1 and out-of-plane angles of 32.07°–32.69°. (The thiophosgene adopts a pyramidal conformation with the C=S bond bent from the ClCCl plane by about 32°.) The computed lifetime is comparable to the lower-limit lifetime of ∼50 ps obtained from the quantum yields ΦP⩽10−3ΦF with ΦF≈1.0 and the measured S1 fluorescence lifetime of 4 μs. The short T1 nonradiative lifetime of thiophosgene can be attributed to the large matrix elements of the T1–S0 spin-orbit coupling (V=150 cm−1) and the strong out-of-plane deformation of the T1 state relative to the planar S0 state, that provides the large Franck–Condon factors for the T1→S0 ISC. It is proposed that a similar pyramidal deformation is also responsible for the absence of phosphorescence from T1 thiocyclobutanone. Consistent with this supposition, T1 cyclopentanone, which is nearly planar at the thiocarbonyl carbon center, exhibits strong phosphorescence under similar experimental conditions.