Coherent energy and charge transport in materials could revolutionize technologies in energy harvesting, catalysis, electronics, but remains difficult to directly characterize in molecular materials. Here we report ultrafast pump-probe measurements, ab initio calculations, and analytical models demonstrating non-trivial coupling of quantum coherences localized on separate sites of a functional molecular material. Following optical pumping of inter-site charge transfer in a quinhydrone single crystal, the dynamics of coherent features centered at 172 cm-1 and 216 cm-1 show anti-correlated behavior. Calculations of the vibrational spectrum of quinhydrone using density functional theory (DFT) find nearly degenerate lattice phonons of different irreducible representation near each experimental frequency, which localize on different molecular sites. An analytic density matrix model using a phenomenological vibrational interaction indicates the experimental results correspond to Rabi-like oscillatory coherence transfer between the involved vibrations. In combination with the atomic motion comprising each phonon derived from DFT calculations, the form and time dependence of the model interaction leads us to propose that nonlinear electron-phonon coupling driven by intermolecular charge exchange explains this coherence transfer process in quinhydrone. These results provide foundational physical knowledge of the capability of functional organic materials to support quantum coherent transport of charge and energy as well as the experimental signatures of these transport mechanisms.

Intermolecular vibronic coherence transfer in an organic electrode material

253rd National Meeting (2017)

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