Theoretical investigations on forward-backward ESIPT processes of three fluorophores deriving from 2-(2'-hydroxyphenyl)thiazole.

The photophysical properties and excited-state intramolecular proton transfer (ESIPT) processes for 2-(2'-hydroxyphenyl)-4-chloromethylthiazole (1), 2-(2'-hydroxyphenyl)-4-phenylthiazole (2), 2-(2'-hydroxyphenyl)-4-hydroxymethyl-thiazole (3) were studied at the TD-B3PW91/6-31 + G(d, p)/IEFPCM level. The structures of 1-3 were fully optimized and the corresponding structural parameters, infrared spectra and electron densities in the ground (S0) and the first excited (S1) states were analyzed. The calculated absorption and fluorescence wavelengths of 1-3 reproduced the experimental data. The potential energy curves of the S0 and S1 states were built and the ESIPT processes were clarified. Our results showed that the intramolecular H-bonds of 3 and 2 in the S1 state were the strongest and the weakest, respectively, and then the ESIPT potential barriers of 3 and 2 were the lowest and highest, respectively. Among the three phenol-thiazole type probes, the compound 2 with phenyl ring group at the 4 position of the thiazole ring had the larger π-conjugation, and had the higher ESIPT potential barrier at the same time. The corresponding compound 1 and 3 with CH2Cl and CH2OH had the lower ESIPT barrier.