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Issue 14, 2016
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Initial hydrogen-bonding dynamics of photoexcited coumarin in solution with femtosecond stimulated Raman spectroscopy

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Abstract

Hydrogen bond (H-bond) making and breaking dynamics in solute–solvent systems represent a fundamental class of intermolecular interactions that play a crucial role in numerous chemical reactions and biological processes. To reveal the initial H-bond dynamics following electronic excitation of a shape-responsive fluorophore in the condensed phase, we develop tunable femtosecond stimulated Raman spectroscopy (FSRS) with 660/670–760 nm Raman pump/probe pulses to pre-resonantly enhance transient coumarin 102 (C102) species in the singly excited state S1 following 400 nm photoexcitation. Within 400 fs (<140 fs time constant), prominent vibrational marker bands at ∼1700 and 1740 cm−1 of the photoexcited fluorophore exhibit an ultrafast decay and rise, respectively, revealing that the H-bond cleaves on that timescale. The subsequent dynamics of the 1740 cm−1 mode intensity show ∼13 ps rise, 37 ps and >1 ns decay, which are attributed to solvation of the nascent free C102, H-bond reformation of the solvated C102, and radiative emission from the relaxed excited state. The mechanistic understanding is corroborated by the time-resolved excited-state absorption of C102 in ethanol as a function of probe wavelength in the visible to near-IR range, as well as ground-state bleaching signal evolution in the UV region. The direct observation of H-bond breaking events followed by excited-state population bifurcation and solvation of both free and H-bonded C102 in ethanol provides an unambiguous, rich portrait of structural dynamics of a fluorophore in solution on crucial molecular timescales (fs to ps). These insights will enable the rational development of photosensitive molecules in general with prescribed functions for materials and biological applications.

Graphical abstract: Initial hydrogen-bonding dynamics of photoexcited coumarin in solution with femtosecond stimulated Raman spectroscopy

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Publication details

The article was received on 01 Nov 2015, accepted on 02 Dec 2015 and first published on 02 Dec 2015


Article type: Paper
DOI: 10.1039/C5TC03598H
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Citation: J. Mater. Chem. C, 2016,4, 2954-2963

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    Initial hydrogen-bonding dynamics of photoexcited coumarin in solution with femtosecond stimulated Raman spectroscopy

    F. Han, W. Liu, L. Zhu, Y. Wang and C. Fang, J. Mater. Chem. C, 2016, 4, 2954
    DOI: 10.1039/C5TC03598H

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