Hydrogen-Bond Dynamics in the Excited State of Coumarin 102−Aniline Hydrogen-Bonded Complex

Comparison of the steady-state FTIR absorption spectra of coumarin-102 (C-102) in tetrachloroethylene with added aniline of various concentrations, in neat aniline and in neat N, N -dimethylaniline (DMA), indicates formation of a hydrogen-bonded complex between C-102 and aniline in solution. Subpicosecond time-resolved infrared absorption spectroscopy has been applied to study the dynamics of the hydrogen-bond following photoexcitation of C-102 chromophore in a C-102−aniline hydrogen-bonded complex. Upon photoexcitation at 400 nm, the hydrogen bond between C-102 and aniline breaks within 250 fs. Reformation of hydrogen-bond between the excited C-102 molecule and aniline takes place within about 30 ps. Biexponential temporal dynamics monitored at C O stretching vibration (1736−1742 cm -1 ) in neat aniline, which is a strongly structured solvent due to formation of intermolecular hydrogen bonds, reveals the biphasic solvation dynamics of aniline with solvation times 0.6 and 7.2 ps. These time constants have been assigned to nondiffusive and diffusive structural reorganization of the solvent.