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 (S1) 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 S1 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 S1 potential surface. The S1 state is a precursor to the formation of the ICT emissive state and photoinduced trans–cis isomerization. The S1/S0 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 S1/S0 crossing in the strong polar solvent reduces the isomerization efficiency.
