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Nonradiative dynamics determined by charge transfer induced hydrogen bonding: A combined femtosecond time-resolved fluorescence and density functional theoretical study of methyl dimethylaminobenzoate in water

Abstract

As a case study of the interplay and consequence of the interplay between intramolecular charge transfer (ICT) and intermolecular hydrogen (H)-bonding, a femtosecond time-resolved fluorescence (fs-TRF) combined with density functional theoretical (DFT) and time-dependent DFT (TDDFT) study has been conducted on methyl dimethylaminobenzoate (MDMABA) in largely water solvent. Direct observation of the broadband spectra, the anisotropy, and kinetic decays of fs-TRF from photo-excited MDMABA revealed a rapid ICT reaction occurring with ~0.7 ps time constant from an initial locally excited (LE) state identified to have the Laππ* character; this produced a weakly emissive ICT state featuring radiative rate constant decreased by more than two orders of magnitude. The fluorescence of the ICT state is strongly quenched exhibiting decay time of ~49.7 ps, unusually faster than the nanosecond range lifetime in polar aprotic solvent when intersystem crossing (ISC) is the major deactivation channel. This, according to study of the solvent kinetic isotope effect, is identified to origiante from an instantly enhanced strong solute-solvent H-bonding induced by the ICT reaction which allows eliminating the ISC, enables the nonardiative decay to proceed almost solely through the otherwise inaccessible internal conversion from the ICT state. The enhancement of the H-bonding is verified by the calculation which presents theoretical evidence for not only the binding site and binding energy of the H-bonding configuration but also the electronic and structural characterization, lending support to the twisted ICT (TICT) description of the photo-excited MDMABA. This study contributes a prominent example for extraordinary ability of water and a decisive role of ICT promoted H-bonding in offering a highly effective molecular mechanism for rapid elimination of the electronic excitation energy. The results contain important insight for in-depth understanding of the excited state H-bonding dynamics, and also have significant implication for clarifying the “sunscreen controversy” of the DMABA type of UVB sunscreen molecules.

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Publication details

The article was received on 29 Jul 2017, accepted on 02 Dec 2017 and first published on 04 Dec 2017


Article type: Paper
DOI: 10.1039/C7CP05140A
Citation: Phys. Chem. Chem. Phys., 2017, Accepted Manuscript
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    Nonradiative dynamics determined by charge transfer induced hydrogen bonding: A combined femtosecond time-resolved fluorescence and density functional theoretical study of methyl dimethylaminobenzoate in water

    C. Ma, Y. Ou, C. T. Chan, A. K. Wong, R. C. Chan, P. Chung, C. Jiang, M. Wang and W. Kwok, Phys. Chem. Chem. Phys., 2017, Accepted Manuscript , DOI: 10.1039/C7CP05140A

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