Organotellurium fluorescence probes for redox reactions: 9-aryl-3,6-diaminotelluroxanthylium dyes and their telluroxides

Several 9-aryl-3,6-diaminotelluroxanthylium dyes with phenyl, 2-methylphenyl, and 2,4,6-trimethylphenyl substituents at the 9-position were prepared. The characterization of these compounds included determination of ¹²⁵Te NMR spectra, fluorescence quantum yields (Φ_F), and quantum yields for the generation of singlet oxygen [Φ( ¹O₂)]. While these compounds were essentially nonfluorescent (Φ_F < 0.005), they produce ¹O ₂ with Φ(¹O₂) between 0.43 and 0.90. The tellurorosamines were oxidized with ¹O₂ via self-photosensitization to the corresponding telluroxides, which allowed their preparation free of excess oxidant. Telluroxides with a 9-(2-methylphenyl) or 9-(2,4,6-trimethylphenyl) substituent were fluorescent with quantum yields for fluorescence between 0.20 and 0.31. Steric bulk at the 9-position of the resulting telluroxides impacted rates of inter- and intramolecular attack of nucleophiles and stability of the telluroxide in aqueous media near physiological pH. The yield of reduction of the telluroxide with glutathione was also dependent on the steric bulk of the 9-aryl substituent. The structure of products from oxidation of the 9-(4-bromophenyl) tellurorosamine was determined by X-ray crystallography and indicated the addition of oxygen nucleophiles to the 9-position of the telluroxide oxidation state of the tellurorosamine.