Intensity distribution in the fluorescence low-frequency mode progressions of the “slightly” asymmetric hydrogen bonded AHB fragment Original Research Article

The model two-dimensional potential energy surface (PES) of the “slightly” asymmetric hydrogen bonded OHO fragment is used for treatment of the intensity distribution in the fluorescence low-frequency mode progressions (LFMPs) of systems governing excited state proton transfer. The PES resembles those suggested earlier (Chem. Phys. 168 ( 1992) 29) and allowing for a strong coupling of the proton movement and the O⋯O stretching vibrations. The respective wave equation was solved numerically with adiabatic separation of the “fast” (proton) and “slow” (O⋯O vibrations) subsystems. The existence of two closely spaced proton vibrational levels in the lowest excited singlet ππ∗ state is shown to cause the appearance of two different LFMPs for each excited tautomer. It was found that the spectral distribution in one LFMP does not depend on the barrier height of the excited state potential for the “fast” subsystem. The results are interpreted in terms of the overlap of the respective matrix elements. Theoretical values of the relative intensities of bands in the LFMPs are compared with the available experimental data.