Abstract
Ultrafast photo-induced charge-transfer reactions are fundamental to a number of photovoltaic and photocatalytic devices, yet the multidimensional nature of the reaction coordinate makes these processes difficult to model theoretically. Here we use femtosecond stimulated Raman spectroscopy to probe experimentally the structural changes occurring following photoexcitation in betaine-30, a canonical intramolecular charge-transfer complex. We observe changes in vibrational mode frequencies and amplitudes on the femtosecond timescale, which for some modes results in frequency shifts of over 20 cm(-1) during the first 200 fs following photoexcitation. These rapid mode-specific frequency changes track the planarization of the molecule on the 400 ± 100 fs timescale. Oscillatory amplitude modulations of the observed high frequency Raman modes indicate coupling between specific high frequency and low frequency vibrational motions, which we quantify for 6 low frequency modes and 4 high frequency modes. Analysis of the mode-specific kinetics is suggestive of the existence of a newly discovered electronic state involved in a relaxation pathway, which may be a low-lying triplet state. These results directly track the multiple nuclear coordinates involved in betaine-30's reactive pathway, and should be of use in rationally designing molecular systems with rapid electron transfer processes.
Highlights
Many of these devices rely on ultrafast electron transfer reactions, in which the system does not have time to thermalize and intramolecular vibrational motions are thought to play a critical role in driving the reaction.[3]
The calculated ground state Raman spectrum of betaine-30 is in agreement with experimental values after scaling by 0.967.33 To probe mode specific anharmonic couplings, we performed a scan of frequency calculations for optimized geometries at several points along the central dihedral torsional coordinate
We will focus on the four vibrational features in the 1350 to 1680 cmÀ1 spectral region, which are highlighted in grey (Fig. 2a)
Summary
Electron transfer reactions are of paramount importance in a variety of photo-driven processes, including photovoltaic and photocatalytic systems.[1,2] Many of these devices rely on ultrafast electron transfer reactions, in which the system does not have time to thermalize and intramolecular vibrational motions are thought to play a critical role in driving the reaction.[3]. Based on the solid foundation of results obtained through studies on solvation, transient absorption, and resonance Raman intensity analysis, we set out to probe the rapid high frequency vibrational response on betaine-30’s excited electronic state. The calculated ground state Raman spectrum of betaine-30 is in agreement with experimental values after scaling by 0.967.33 To probe mode specific anharmonic couplings, we performed a scan of frequency calculations for optimized geometries at several points along the central dihedral torsional coordinate.
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