Fluorescence quenching of hydrogen-bonded coumarin 102-phenol complex: effect of excited-state hydrogen bonding strength.

The fate of intermolecular hydrogen bond (H-bond) upon electronic excitation of a H-bonded complex has been debated in literature. For a model H-bonded complex, coumarin 102 (C102)-phenol in a noninteracting solvent ethylene tetrachloride, time-resolved infrared spectroscopy experiment of Nibbering and coworkers suggests that the H-bond between the C102 and phenol ruptures upon electronic excitation (C. Chudoba et al. J. Phys. Chem. A1999, 103, 5625-5628). On the contrary, Zhao and Han have demonstrated for the first time that the intermolecular hydrogen bond is significantly strengthened, while not disrupted, in the electronically excited states of the hydrogen-bonded complexes upon electronic excitation using the time-dependent density functional theory method (G. J. Zhao and K. L. Han J. Phys. Chem. A2007, 111, 2469-2474). The two excited-state hydrogen bonding dynamics mechanisms have widely different predictions of the emission or electronic relaxation of the excited H-bonded complex. The excited-state hydrogen-bond strengthening mechanism proposed by Zhao and Han anticipates a stronger intermolecular interaction, while the H-bond breaking mechanism speculates no interaction between C102 and phenol. The speculation has been tested here on the same system (H-bonded C102-phenol complex) in another noninteracting solvent cyclohexane. We found a strong quenching of the C102 emission in the H-bonded complex. Selectively excited (λex = 405 nm) H-bonded complex relaxes on a fast time scale of 400-600 ps and may be attributed to the conversion of the locally excited (LE) state to a nonfluorescent charge transfer (CT) state assisted by the strong excited-state H-bond formation. A minor component (∼10%) of 2.5 to 1.8 ns is ascribed to the LE complex without a H-bond. The findings are in accordance with the new fluorescence quenching mechanism that the excited-state intermolecular hydrogen bond strengthening facilitates CT from phenol to coumarin in the excited state (G. J. Zhao et al. J. Phys. Chem. B2007, 111, 8940-8945). Fluorescence quenching was absent for anisole, where H-bond formation is not possible and was more pronounced for p-Cl-phenol, where even stronger H-bonding is expected.