A computational study on the excited state properties of a cationic cyanine dye: TTBC

The ground and excited state behaviors of a biologically important mitochondria selective dye, 5,5′,6,6′-tetrachloro-1,1′,3,3′-tetraethyl-benzimidazolo carbocyanine (TTBC), were investigated in order to give useful information related to the enhancement of its electronic and optical properties. DFT and TDDFT levels of theory with different functionals and basis sets were used. The results obtained by using PBEPBE/6-31+G(d,p)/C-PCM are in good agreement with the experiments. The potential energy surface sections of TTBC were explored and the geometry optimized in the first excited state, S1, to analyze the fluorescence spectrum of the dye. There are two distinct minima on the S1 surface, one located near the ground state geometry and an energetically favorable twisted geometry, having a charge transfer character. The observation of similar ground and excited state dipoles as well as solvent-independent fluorescence and absorption wavelengths reveals that there is no competition between the two minima; the emission is dominantly coming from the more planar configuration.