Vibronic coupling in light-harvesting complex II revisited

A growing body of work has pointed to vibronic mixing as a crucial design principle for efficient energy and charge transfer in natural^1–3 and artificial systems.^4,5 Notable among these studies was the recent observation of vibronically promoted ultrafast energy flow in the major antenna complex of green plants and algae light-harvesting complex II (LHCII)—the most abundant membrane protein on the Earth⁶—via the emerging experimental technique two-dimensional electronic-vibrational (2DEV) spectroscopy.³ This spectroscopy, which correlates electronic and nuclear degrees-of-freedom, shows promise for providing mechanistic insight into vibronic coupling; however, explicit theoretical input is necessary to extract such a detail. In a separate paper,⁷ we have developed a heterodimer model that describes various forms of vibronic coupling—which were speculated to be present in LHCII—resulting from diagonal electron–phonon coupling giving Franck–Condon (FC) activity and the nuclear dependence of the electronic transition dipole moment giving Herzberg–Teller (HT) activity. Here, we draw connections between this theoretical work and recent experimental studies in order to demonstrate how HT activity is leveraged in the function of LHCII.