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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 <sup>125</sup>Te NMR spectra, fluorescence
quantum yields (Φ<sub>F</sub>), and quantum yields for the generation
of singlet oxygen [Φ(<sup>1</sup>O<sub>2</sub>)]. While these
compounds were essentially nonfluorescent (Φ<sub>F</sub> <
0.005), they produce <sup>1</sup>O<sub>2</sub> with Φ(<sup>1</sup>O<sub>2</sub>) between 0.43 and 0.90. The tellurorosamines were oxidized
with <sup>1</sup>O<sub>2</sub> 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