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

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.