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

Abstract

Thiophosgene, C12CS, has been excited in the near-uv and the resulting quantum yields of fluorescence and of CI2CS consumption have been measured as a function of solvent composition (perfluoro-nhexane, n-hexane, CC14), temperature, and excitation wavelength. In agreement with previous work it is shown that (i) nonradiative processes dominate the decay of the B state in solution, and (ii) perfluoroalkane solvents act as inert "heat baths". The process by which the B state is apparently "quenched" by CC14 and rr-hexane has been discovered. The nonradiative process leading to B's decay involves activation and crossing a barrier, the height of which is a function of the nature and composition of the solvent. CC14 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 CC14 or 11-hexane form clusters around a C12CS solute molecule in mixed perfluoroalkane-CCI4 or rl-hexane solutions. Barrier crossing leads to photodecomposition via at least two parallel paths, one free radical (CI + ClCS) and the other likely molecular (C12 + CS), the relative contributions of which are a function of excitation wavelength. Key worc1.s: thiophosgene, photochemistry, solvent-mediated banier crossing.Resume : On a excitC le thiophosgkne, C12CS, dans le proche-uv et on a mesure les rendements quantiques de la fluorescenceB-2 et de la consommation du CI2CS qui en resultent en fonction de la composition du solvant (perfluoro-n-hexane, n-hexane et CC14), de la tempkrature et de la longueur d'onde de I'excitation. En accord avec les travaux antkrieurs, il est dCmontrC que (i) les processus non dissociatifs dominent la dCgtntrescence de 1'Ctat Ben solution et (ii) les solvants perfluoroalcanes agissent comme des "bains de chaleur" inertes. On a dtcouvert le processus par lequel I'Ctat Best apparemment dCsactivC par le CCl4 et par le n-hexane. Le processus non radiatif conduisant h la dCgCnCrescence de B implique une activation et le passage d'une barrikre dont la hauteur est une fonction de la nature et de la composition du solvant. Le CC14 et le n-hexane ne dtsactivent pas 1'Ctat excitC, mais ils accClkrent plut8t sa vitesse de relaxation en abaissant la barrikre entre la portion radiative like de la surface et la region noire non liCe. 11 existe des indications j . l'effet que le CC14 ou le n-hexane forment des agrCgats autour d'une molCcule de CI2CS en solution dans des mClanges de perfluoroalcane, CC14 ou n-hexane. Le passage d'une barrikre conduit h une photodCcomposition par le biais d'au moins deux voies paralleles, une impliquant un radical libre (C1+ CICS) et I'autre Ctant probablement molCculaire (CI2 + CS); les contributions relatives des deux sont fonction de la longueur d'onde d'excitation.Mots cles : thiophosgkne, photochimie, passage de barrikre dCterminC par solvent.[Traduit par la ridaction]

Photodissociation of CSCl2 at 235 nm: Kinetic energy distributions and branching ratios of Cl atoms and CSCl radicals

“…As the molecular channel ͑3.2͒ cannot be observed in our experiment and the three-body decay channel ͑3.3͒ is energetically not allowed at the wavelength of 235 nm, we concentrated on the three details of the two-body decay channel ͑3.1͒, the spin-orbit branching ratio of Cl*/Cl, and the relative importance of the different states of CSCl which are not yet established with certainty. 36…”