Excited-state intramolecular charge transfer of p-N,N-dimethylaminobenzoic acid in Y zeolites: hydrogen-bonding effects

The excited-state intramolecular charge transfer (ICT) properties of p-N,N-dimethylaminobenzoic acid (DMABA) have been investigated in NaY and HY zeolites by steady-state and time-resolved fluorescence. The ratio of the ICT emission intensity to the normal ICT/LE is greatly enhanced in the zeolites compared to the homogeneous polar solvent. In accordance with steady-state fluorescence properties, the decay times of the ICT emission are increased from 1.4 to 2.1 ns with a rise time of about 20 ps. The ICT/LE is reduced upon illumination of DMABA in NaY zeolites while no illumination effects are observed in HY or CaY zeolites. Observation of the marked differences between the diffusive reflectance spectrum and the fluorescence excitation spectrum of DMABA in the zeolite imply that hydrogen bonding between the zeolite surface and the carboxylic acid group is stimulated in the excited state, supporting the importance of hydrogen bonding in the ICT process.