Nile blue and Nile red optical properties predicted by TD-DFT and CASPT2 methods: static and dynamic solvent effects

The absorption and emission spectra of the fluorescent dyes Nile blue (NB) and Nile red (NR), widely used in biology and histology, were simulated with different methods, considering the effect of water as solvent. The aforementioned dyes are extremely significant because they also act as DNA sensitizers and hence can be used in photodynamic therapy. Especially, time dependent-density functional theory (TD-DFT) including different functionals, and ab initio single-state-and multi-state-complete active space perturbation theory (SS-and MS-CASPT2) including the effect of the basis set, were considered. The solvent environment was taken into account statically and dynamically: static optical properties were calculated with the polarizable continuum model as vertical transitions from the ground state equilibrium geometry, while dynamic properties were obtained by performing ground state molecular dynamics of NB and NR in explicit water, followed by hybrid quantum mechanics/molecular mechanics calculations of a statistical number of geometries along the trajectory. The results show that a dynamic treatment is required in order to reproduce the experimental absorption spectra, since the static approach gives rise to a hypsochromic shift of ca. 0.3 eV for NB and 0.2 eV for NR, at the TD-DFT and CASPT2 level of theory. This can be explained in terms of out-of-plane vibrational normal modes, which are properly taken into account only in the dynamic approach. Moreover, an exhaustive description of the charge transfer character in the excited state is given, at both TD-DFT and CASPT2 levels of theory.