(19, 22-24) Intermediates A can be prepared via the addition of a diaryl dichalcogenide B bearing a m-amino substituent to an anion C prepared by amide-directed metalation. Structures related to A have shown a propensity to cyclize para to the amino substituent under these conditions. Chalcogenoxanthones 2– 4 can be prepared via electrophilic cyclization of the diaryl chalcogenide intermediates A (Scheme 1) under modified Friedel–Crafts conditions.
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Texas red analogues such as 5– 7 can be prepared via the addition of PhMgBr (or other Grignard or organolithium reagents) to chalcogenoxanthones 2– 4 followed by treatment with acid. (18, 19, 22-24) The synthesis of chalcogenoxanthones is similarly key to this work. The key to the successful synthesis of TMR-related molecules or derivatives incorporating a julolidine or half-julolidine was access to the corresponding chalcogenoxanthones as precursors to the rhodamine analogues. Our synthetic approach is analyzed in Scheme 1. (15-18) The synthesis of Texas red derivatives incorporating the heavier chalcogen atoms S, Se, and Te should give rhodamine analogues with values of λ max > 600 nm with control of relative yields for fluorescence and triplet production for different applications. As the chalcogen atoms increase in size, the resulting rhodamines have decreasing quantum yields of fluorescence (Φ FL) and increasing quantum yields for the generation of triplets and singlet oxygen. (19) The introduction of a single fused julolidine or half-julolidine group gives a small increase in absorption maxima (λ max), but larger shifts in λ max are realized by replacing the oxygen atom of the xanthylium core with the heavier chalcogen atoms S, Se, and Te. We have prepared heavy-atom analogues of the tetramethylrosamines/rhodamines ( TMR-E, Chart 1) (15-17) and analogues incorporating one julolidine group (18) and, more recently, one “half-julolidine” group (a trimethyltetrahydroquinoline, Chart 1) in the xanthylium core.
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A mesityl-substituted tellurorhodamine derivative localized in the mitochondria of Colo-26 cells (a murine colon carcinoma cell line) and was oxidized in vitro to the fluorescent telluroxide. The tellurorhodamine telluroxides absorb at wavelengths ≥690 nm and emit with fluorescence maxima >720 nm. The S-analogues were highly fluorescent, the Se-analogues generated single oxygen ( 1O 2) efficiently upon irradiation, and the Te-analogues were easily oxidized to rhodamines with the telluroxide oxidation state.
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The photophysics of the Texas red analogues were examined. Analogues containing two “half-julolidine” groups (a trimethyltetrahydroquinoline) and one julolidine and one “half-julolidine” were also prepared. The Texas red analogues incorporate two fused julolidine rings containing the rhodamine nitrogen atoms. The xanthones were prepared via directed metalation of amide precursors, addition of dichalcogenide electrophiles, and electrophilic cyclization of the resulting chalcogenides with phosphorus oxychloride and triethylamine. Analogues of Texas red incorporating the heavy chalcogens S, Se, and Te atoms in the xanthylium core were prepared from the addition of aryl Grignard reagents to appropriate chalcogenoxanthone precursors.