Mechanism of a photo-induced ring-opening reaction in benzothiazolinic spiropyran BTSP–X1-3 (X1–3 = NO2, OMe, and NH2) derivatives in gas and solvent media: a TD-CAM-B3LYP study.

The photophysical properties and a detailed mechanism of the photoinduced ring-opening reactions of benzothiazolinic spiropyran derivatives BTSP–X1-3 (X1–3 = NO2, OMe, and NH2) with the X substituent in the para position as well as an OMe group in the ortho position of the phenolic oxygen atom were investigated at the TD-CAM-B3LYP/6-31G(d) level of theory in gas and solvent media. The environmental effects of the solvent on the transition energies were accounted for by the corrected linear-response (CLR-IEFPCM) formalism in seven steps. Based on the calculated energy barriers, the cis-BTMC → trans-BTMC step is predicted to be the rate-determining step of BTSP to trans-BTMC switching in the excited state. The process of BTSP–X1-3 → BTMC-X1-3 photoswitching in the excited state is predicted to be much faster than in the ground state. From the obtained results, it is predicted that the process of the Cbtspiro–O1 bond dissociation occurs more easily in the more polar solvents and for BTSP-X1 it is easier than the others. In the gas and solvent media, the more populated (bright) state is predicted to be the S2 state for BTSP-X1 and the S1 state for BTSP-X2,3. The redshift in fluorescence wavelength of the trans BTMC-X1-3 on going from polar solvent to nonpolar one is in the range of 31.6 to 66.8 nm for X1 = NO2, 196.2 to 212.3 nm for X2 = OMe and 179.2 to 199.7 nm for X3 = NH2, indicating that its value for X2,3 is greater than X1. [ABSTRACT FROM AUTHOR]