Decay dynamics of aromatic thione triplet states in fluid solution

Abstract

This paper is dedicated to Pro/e.ssor Harry E. Gunrling on the occasiorl a/ his 65th birthdayA. SAFARZADEH-AMIRI, R. E. VERRALL, and R. P. STEER. Can. J . Chem. 61. 894 (1983). Phosphorescence emission and excitation spectra, quantum yields, and lifetimes of several aromatic thiones have been measured in fluid solution at room temperature. Excitation to any of the excited electronic states accessible in the near-uv and visible regions of the spectrum results in population of the lowest triplet state with near unit quantum yield. Triplets of relatively inflexible thiones decay non-radiatively with rate constants of the order of lo5 s-' whereas thiones capable of higher amplitude flexion or group rotation exhibit non-radiative decay constants 2 lo7 S-I at infinite dilution in aprotic solvents. Self-quenching occurs at diffusion-controlled rates.A. SAFARZADEH-AMIRI, R. E. VERRALL et R. P. STEER. Can. J. Chem. 61, 894 (1983). On a mesure 1'Cmission par phosphorescence, les spectres d'excitation, les rendements quantiques et les temps de vie de plusieurs thiones aromatiques dans une solution fluide ii la tempkrature ambiante. L'excitation de n'importe quel Ctat Clectronique excitt, accessible dans les rtgions du proche uv et visible des spectres, conduit B une population du plus faible &tat triplet avec un rendement quantique proche de !'unit&. Les triplets des thiones relativement inflexibles se dtsintkgrent d'une manikre non radioactive avec des constantes de vitesse de I'ordre de 105 s-' tandis que les thiones qui peuvent subir une flexion de plus grande amplitude ou une rotation de groupe ont des constantes de dCsintCgration non radioactive de 2 lo7 s-' a dilution infinie dans des solvants aprotiques. L'arrCt automatique se produit B des vitesses contrdltes de diffusion. [Traduit par le journal] IntroductionThiocarbonyl compounds exhibit a number of photochemical and photophysical properties which are unusual compared with those of compounds containing only first and second row elements (1-3). For example, as a consequence of their relatively large S 2 -S , electronic energy gaps, some thiones fluoresce efficiently and react chemically from excited singlet states other than S I (1). The tetraatomic thiocarbonyl, thiophosgene, has been shown to undergo a unique excited singlet -excited singlet energy pooling reaction in the gas phase which can populate S z with high efficiency (4). There is also substantial evidence of strong spin-orbit coupling in thiones which leads to unusual physical and dynamic properties of their triplet states (5-7). Thus, thiones are among the few organic molecules which phosphoresce in fluid solution at room temperature.The T I +-S o absorption in aromatic thiones in fluid solution is often spectrally distinct from other electronic absorption systems and has an oscillator strength which is only about one order of magnitude smaller than that of the corresponding S I +-So transition (1). Thus T I may be populated either by direct absorption in the T I + So system or by intersystem crossing ...

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