Sulfur aromatic heterocycles: A new kind of solar ultraviolet-B radiation actinometers
Indoline-2-thione (BC) and benzimidazole-2-thione (BN) are two aromatic compounds containing a NCS synthonic unit in a heterocyclic ring of five centres, where the main electronic absorption bands are localized in the spectral range of the ultraviolet A and B. After ultraviolet irradiation by artificial sources or sunlight. these molecular systems in aqueous solution, under controlled pH, present a simple pattern of photochemical reactions defined by a typical isosbestic point in the electronic absorption spectra. The photolysis process observed follows a good linear behavior as function of the irradiation time, being a particular actinometer of short time exposition. On the other hand, the final photochemical products of indoline-2-thione and benzimidazole-2-thione, after irradiation, were identified as indole and benzimidazole, respectively. Other properties such as molecular stability in acid solution at pH < 2, photochemical conversion in a linear response in the time range from minutes to hours, and photochemical quantum yields are reported. Due to the use of water as solvent, without a health hazard by direct manipulation, from an environmental point of view we propose these two thione compounds as new potential photochemical actinometers of solar ultraviolet-B radiation.