Theoretical study on the dual fluorescence of 2-(4-cyanophenyl)-N,N-dimethylaminoethane and its deactivation pathway.

Low-lying singlet states and photophysical deactivation pathways of the electronically decoupled 2-(4-cyanophenyl)-N,N-dimethylaminoethane (PCN2NM) have been investigated by the density functional theory and CASSCF/CASPT2 approaches. PCN2NM in the ground state has two dominant configurations of the alkyl-twisted and alky-stretched structures with an interconversion barrier of 2.5 kcal/mol. The predicted vibrationally resolved weak absorption and fluorescence emission of (1)(S(1)-L(b)) exhibits clearly well-resolved vibronic features, whereas (1)(S(1)-CT) state corresponding to the redshifted fluorescence has no fine structure. Due to the presence of low-energy (1)(S(1)/S(0))(CI), radiationless decay of the excited PCN2NM to the ground state is facile, once the excess energy beyond the barrier is available. Present results show reasonable agreement with experimental observations available and provide a basis for understanding of the dual fluorescence of the electronically decoupled species.