Tandem photoarylation-photoisomerization of halothiazoles: synthesis, photophysical and singlet oxygen activation properties of ethyl 2-arylthizole-5-carboxylates.

The photochemical reaction between ethyl 2-chlorothiazole5-carboxylate and benzene gave no product. In contrast, the reaction with ethyl 2-iodothiazole-5-carboxylate gave ethyl 3-phenylisothiazole-4-carboxylate. The same behaviour was observed if the reaction was performed in the presence of furan, thiophene and 2-bromothiophene. The products thus obtained were studied for their photophysical properties and it was found that the observed absorptions [λmax=257 nm (ε = 7000 M-1 cm -1) for ethyl 3-phenylisothiazole-4-carboxylate, 278 nm (ε = 7000 M-1 cm-1) for ethyl 3-(2-furyl)isothiazole- 4carboxylate, 269 and 285 nm (ε = 7500M-1Cm-1) for ethyl 3(2-thienyl)isothiazole-4-carboxylate] are mainly due to π → π* transitions from the HOMO to the LUMO+1 orbital. The tested compounds showed fluorescence [λem = 350-400 (λexc = 300 nm), 360 and 410 nm (λexc = 320 nm), 412 nm (λexc = 340 nm) for ethyl 3-phenylisothiazole-4- carboxylate, 397 and 460 nm (λexc = 300 nm), 381, 394 and 460 nm (λexc = 320 nm), 381, 398 and 466 nm (λ exc, = 340 nm) for ethyl 3-(2-furyl)isothiazole-4-carboxylate, 372, 377 and 414 nm (λexc = 300, 320 and 340 nm respectively) for ethyl 3-(2-thienyl)isothiazole-4carboxylate], possibly due to dual emission from different excited states. The use of the compound obtained as a sensitizer in the photo-oxidation of trans-α,α'-dimethylstilbene showed that all the new compounds are singlet-oxygen sensitizers.