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

Abstract

The photophysical properties and a detailed mechanism of the photoinduced ring-opening reactions of benzothiazolinic spiropyran derivatives BTSP–X_1-3 (X_1–3 = 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–X_1-3 → BTMC-X_1-3 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 C_btspiro–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-X_2,3. The redshift in fluorescence wavelength of the trans BTMC-X_1-3 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 X_2,3 is greater than X₁.