Theoretical study on photochemical behavior of trans-2-[4′-(dimethylamino)styryl]benzothiazole

Photochemistry of trans-2-[4′-(dimethylamino)styryl]benzothiazole (t-DMASBT) has been examined theoretically. Ground state calculations are performed by density functional theory (DFT) method. For excited-state calculations restricted configuration interaction singles (RCIS) combined with time dependent DFT (TDDFT) approach was used. Theoretical calculations predicted the existence of two ground state conformers and the photophysics of both the conformers are nearly identical. With torsional rotation of dimethylamino group, the energies of S1 and S2 states increase, but that of S3 state decreases. In contrary to an earlier report [J. Photochem. Photobiol. A: Chem. 195 (2008) 368–377], the calculation predicted at the perpendicular geometry the HOMO is localized on dimethylamino and, HOMO and LUMO are decoupled. Excitation from HOMO to LUMO at perpendicular geometry results in twisted intramolecular charge transfer (TICT) state. Avoided crossing of S3 and S1 states causes a barrier for formation of TICT state from locally excited state. Potential energy surface of isomerization constructed by TDDFT method suggests that the photoisomers are formed through phantom state in a nonradiative way, not by a radiative way from cis and trans isomers as proposed earlier by Saha et al. [J. Photochem. Photobiol. A: Chem. 199 (2008) 179–187].