Structural background of fast nonradiative deexcitation of benzo[cd]indole polymethine dyes

Abstract

The electronic structure of a series of benzo[cd]indole-based merocyanines and cationic polymethine dyes has been analyzed in both the ground and fluorescent states using the DFT and TD-DFT computations. The obtained data has shown the marked difference between the benzo[cd]indole dyes and those based on such typical heterocyclic cores as indole and benzimidazole. Namely, in the former, the frontier MOs have much greater coefficients at the end group(s), which leads to an enhancement of vibronic interactions in the long-wavelength absorption and fluorescence transitions. This effect seems to be the major cause of the low fluorescence lifetimes and fluorescence quantum yields of benzo[cd]indole dyes.