Viscosity-dependent fluorescence and low-temperature photochemistry of mesoionic 4-phenyl-1,3,2-oxathiazolylium-5-olate

Abstract

Irradiation of 4-phenyl-1,3,2-oxathiazolylium-5-olate (1) at cryogenic temperatures leads to formation of benzonitrile sulphide (4) and phenyl(nitrosothio)ketene (6) as primary products. They have been characterized by u.v. and i.r. spectroscopy. The relative yields of (4) and (6) depend strongly on the local viscosity of the medium. For example, poly(vinyl chloride) favours the formation of (4), whereas solid nitrogen favours (6). The observation of a strong fluorescence is also conditioned by a rigid environment. A vibrational fine structure extending half way through the main absorption band of (1) indicates the existence of a dissociative pathway in the excited singlet-state potential energy surface. This pathway is identified with the formation of (6). Isotopic labelling has been used to characterize (6) and the i.r. absorptions of its cis–trans-isomers have been located. The cis-isomer of (6) can regenerate (1) upon irradiation; similar treatment of the trans-isomer leads to the resonance-stabilized radical phenyl(oxomethylene)thiyl (7).