Electron-vibration (vibronic) couplings are increasingly found to facilitate photoinduced electron dynamics in various biological and chemical systems. To observe vibronic coupling features there must be multiple electronic surfaces with similar energies that can cross along the reaction coordinate, and there must be non-adiabatic coupling (NAC) between electrons and high frequency vibrations. This scenario can be more common for photoinduced processes where upon visible excitation there are many accessible electronic states. When considering excitation of vibrational transitions with mid-IR pulses the lack of nearby lower-lying electronic states usually prohibits the observation of vibronic interactions. Here we demonstrate that vibronic coupling can be probed with mid-IR pulses for multi-configuration systems such as TCNQ (tetracyanoquinodimethane). TCNQ has contributions from both the quinone resonance structure (closed-shell singlet spin configuration, CS) and benzylic resonance structure (open shell singlet spin configuration, OSS) forming the ground electronic state. We apply ultrafast two-dimensional infrared spectroscopy (2DIR) to TCNQ, where we observe a 0 to 1 transition that lies along the diagonal at 2222 cm^-1. At early times we observe a transient spectral feature at 2196 cm^-1 that decays on the ps timescale in addition to an anharmonically shifted 1 to 2 transition of CN stretch peak at 2212 cm^-1 that decays on the same timescale as the diagonal peak recovery. Through comparison with DFT calculations, we assign the initial transient peak to the OSS, which relaxes on the ps timescale. We show that the high frequency nitrile vibration in TCNQ can be strongly coupled to the electron dynamics, resulting in ultrafast formation of purely OSS TCNQ by vibrational perturbation. More generally, this study shows that 2DIR can identify vibronic interaction driven dynamics in multiconfiguration chemical systems.

Eitan Geva

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