Planar vs. twisted intramolecular charge transfer mechanism in Nile Red: new hints from theory

Using a time-dependent density functional theory approach and taking into account bulk solvent effects, we investigate the absorption and fluorescence spectra of Nile Red. In particular, we have assessed both the planar and twisted intramolecular charge transfer mechanism by using a panel of exchange correlation functionals including both global and range-separated hybrids, refined solvent models and the simulation of vibronic couplings. It turned out that the appropriate choice of the functional is of prime importance to obtain, not only quantitatively accurate values, but also qualitatively correct evolution of the spectral features with respect to the dihedral angles of the amino group. At the light of this study, the interpretation of the experimental data is critically re-examined and compared to typical dual-fluorescent molecules.