INTRAMOLECULAR CHARGE TRANSFER AND PHOTOISOMERIZATION OF THE DCM STYRENE DYE: A THEORETICAL STUDY

Abstract

Structures and properties of the low-lying states in 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM) have been investigated theoretically. Calculations show that the dimethylamino and dimethylanilino twisted conformations of DCM on the potential energy surface of the first excited state ( S 1) have relatively high stabilities and remarkable intramolecular charge transfers (ICT). Both structures can serve as candidates for the red-shifted emissive state in polar solvent. In particular, the dimethylanilino twisted ICT state has been predicted to have a dipole moment increment of 20 Debye with respect to the ground state by CASSCF calculations, in good agreement with the suggested experimental values. The optimized geometry of the S 1 state exhibits a long central CC bond of 1.458 Å, which makes the trans–cis isomerization quite facile through intramolecular rotation around the central CC bond on the S 1 potential surface. The S 1 state is a precursor to the formation of the ICT emissive state and photoinduced trans–cis isomerization. The S 1/ S 0 crossing in polar solvent and avoid-crossing in the gas phase as well as in non-polar solvent are involved in the trans–cis isomerization process. The presence of an early S 1/ S 0 crossing in the strong polar solvent reduces the isomerization efficiency.