Reveal the interplay between high frequency and low frequency intramolecular vibrational mode in ultrafast electron transfer reaction*

We are presenting a theoretical study of an ultrafast electron transfer reaction by employing the Redfield theory of quantum dissipation dynamics and a model Hamiltonian involving three electronic states and two vibrational modes. Accompanying a pump-probe spectroscopy experiment of N, N'-Bis(2,6-methylphenyl)-3,4,9,10-perylenetetracarboxylic Diimide (PDI) emerging in an electron-donating solvent, we are able to identify the role of vibronic coherence and the timescale separation due to the interplay between the high-frequency mode and the low-frequency mode. We found that vibronic coherences provide a complete blueprint of a three-stage process: an ultrafast ET event, an impulsive response of nuclear coherences (truest manifestation of Born-Oppenheimer approximation), and the relaxation of coherently prepared hot vibrational states. The outcome of this work also provides a potential design principle for preventing detrimental charge recombination in organic photovoltaics.