High-resolution spectroscopy of methyl 4-hydroxycinnamate and its hydrogen-bonded water complex.

The electronic structure and spectroscopic properties of the lower excited singlet states of methyl 4-hydroxycinnamate, a model for the chromophore of the photoactive yellow protein in neutral form, have been investigated using various high-resolution gas-phase spectroscopic techniques and quantum-chemical calculations. The experiments show that under our experimental conditions the molecule can adopt four conformations with similar spectroscopic properties. From the detailed assignment of the vibrationally active modes in excitation and emission spectra, it is concluded that the S(1) and S(2) states should be assigned to the V' and V pipi* states that are characterized by, respectively, small and large contributions of the HOMO --> LUMO excitation. We find that complexation with a single water molecule affects the spectroscopic properties of methyl 4-hydroxycinnamate considerably in terms of stabilization of the lowest excited singlet state but in particular with respect to the transition intensities. The latter observation is tentatively interpreted as being caused by an increase in the oscillator strength of the respective electronic transition as well as by a rise/removal of conical intersections with the pisigma* state.