Persistent Electronic Coherence Despite Rapid Loss of Electron-Nuclear Correlation

Long-lived coherences of excited states are notable for their positive effect on energy conversion mechanisms and efficiencies in photosynthetic complexes. Rational engineering of such persistent coherences could open a new way to increase energy conversion rates in man-made photovoltaic and photocatalytic materials. Therefore, a comprehensive understanding of the fundamental principles behind the long-lived coherences is necessary. In this work we show that the main factor determining the decoherence rates is the magnitude of the nuclear-induced fluctuation of the energy gap between the electronic states of interest, rather than the electron nuclear correlation on its own. Utilizing combined atomistic and electronic structure calculations, we demonstrate an inverse relationship between decoherence times and magnitude of the energy gap fluctuation. We also show that the energy gap fluctuation can often correlate with the gap itself. For sufficiently small energy gaps, the coherence time can be nearly an order of magnitude larger than the electron nuclear correlation time.