Ultrafast solvent-assisted electronic level crossing in 1-naphthol.

Nonadiabatic electronic transitions between potential energy surfaces play a dominant role in many photoinduced chemical reactions, such as charge transfer, branching pathways in dissociation, and cis/trans isomerizations. Furthermore, nonadiabatic internal conversion (IC) to the ground state (S0) is believed to protect photoexcited natural chromophores (e.g. nucleic acid bases of DNA.) against UV radiation damage. The theoretical description of nonadiabatic level crossing (LC) dynamics is a highly active field of research for both gas phase and condensed phase molecular systems. Importantly, in the condensed phase the solvent often provides more than just a heat bath for the reactive system, as it can strongly affect the outcome of different reaction products. Herein, we report the experimental observation of LC occurring in 60 fs between the two lowest electronic excited singlet states (La and Lb) of 1-naphthol (1N). Like other aromatic molecules such as benzene, naphthalene, or indole, the La and Lb electronic states of 1N have transition dipole moments (TDM) in the plane of the aromatic ring, pointing almost along the short (La) and long (Lb) molecular axes (Figure 1). [6] The energies of the two states depend on the nature of the aromatic structure and on the position of functional side groups, while their lifetimes depend on their energetic order. In line with Kasha s rule (i.e., fluorescence emission occurs only from the lowest-lying excited S1 state), IC from the higher lying S2 state must occur on ultrafast time scales (e.g., < 40 fs as shown for La-toLb IC in 5-methoxyindole). However, for some aromatic systems, a solvent-dependent fluorescence emission has been interpreted to arise from an inversion of the energetic order of the La and Lb states with increasing solvent polarity. Here, we address the IC and LC mechanisms in 1N with ultrafast time resolution. 1N is a prototypical case for which the La state is more polar than the Lb state (Figure 1). In the gas phase, or in