AbstractThe role of molecular vibrations for the persistence of quantum coherences, recently observed in photoinduced charge transfer reactions in both biological and artificial energy conversion systems at room temperature, is currently being intensely discussed. Experiments using two-dimensional electronic spectroscopy (2DES) suggest that vibrational motion – and its coupling to electronic degrees of freedom – may play a key role for such coherent dynamics and potentially even for device function. In organic photovoltaics materials, strong coupling of electronic and vibrational motion is predicted, especially for ubiquitous C=C stretching vibrations. The signatures of such strong vibronic couplings in 2DES are, however, debated. Here we analyse the effect of strong vibronic coupling in model simulations of 2DES spectra and dynamics for an electronic dimer coupled to a single high-frequency vibrational mode. This system represents the simplest conceivable model for a prototypical donor–acceptor interface in the active layer of organic solar cells. The vibrational mode is chosen to mimic C=C stretching vibrations with typical large vibronic couplings predicted in organic photovoltaics materials. Our results show that the decisive signatures of strong vibronic coupling mediating coherent charge transfer between donor and acceptor are not only temporally oscillating cross-peaks, but also most importantly characteristic peak splittings in the 2DES spectra. The 2DES pattern thus directly reflects the new eigenstates of the system that are formed by strong mixing of electronic states and vibrational mode.
Ultrafast Charge Transfer Visualized by Two-Dimensional Electronic Spectroscopy