Evidence of a solvent-mediated barrier to radiationless decay in the state of thiophosgene in solution

Abstract

Thiophosgene, Cl2CS, has been excited in the near-uv and the resulting quantum yields of [Formula: see text] fluorescence and of Cl2CS consumption have been measured as a function of solvent composition (perfluoro-n-hexane, n-hexane, CCl4), temperature, and excitation wavelength. In agreement with previous work it is shown that (i) nonradiative processes dominate the decay of the [Formula: see text] state in solution, and (ii) perfluoroalkane solvents act as inert "heat baths". The process by which the [Formula: see text] state is apparently "quenched" by CCl4 and n-hexane has been discovered. The nonradiative process leading to [Formula: see text] decay involves activation and crossing a barrier, the height of which is a function of the nature and composition of the solvent. CCl4 and n-hexane do not quench the excited state, but instead accelerate its rate of relaxation by lowering the barrier between the bound, radiative portion of the surface and a dark, unbound region. There is evidence that CCl4 or n-hexane form clusters around a Cl2CS solute molecule in mixed perfluoroalkane–CCl4 or n-hexane solutions. Barrier crossing leads to photodecomposition via at least two parallel paths, one free radical (Cl + ClCS) and the other likely molecular (Cl2 + CS), the relative contributions of which are a function of excitation wavelength. Keywords: thiophosgene, photochemistry, solvent-mediated barrier crossing.