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

Abstract

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 S 1 and S 2 states of the neutral molecule and the S 3 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.