The asymmetric nature of charge transfer states of the cyano-substituted proton sponge

The geometric and electronic structure of 1,8-bis(dimethylamino)-4-cyano-naphthalene (DMAN-CN) and its mono-protonated cation (H+DMAN-CN) have been investigated by density functional theory (DFT) and time dependent density functional theory (TD-DFT) methods. Ground state geometry optimization leads to a significant structural asymmetry and polarity of both molecules. Vertical excitation energies and oscillator strengths calculated for the lower-lying excited singlet states of DMAN-CN are in excellent agreement with experimental absorption maxima in n-hexane. Similarly, it is found that experimentally observed solvatochromic shifts are favourably reproduced by the calculations. The calculations reveal that the S1 and S2 states of the neutral molecule and the S3 state of the cation are characterised by an asymmetric charge transfer, i.e., the two dimethylamino groups contribute unequally to the transfer of charge to the cyanonaphthalene moiety.