Direct and Energy-Transfer-Mediated Charge-Transfer State Formation and Recombination in Triangulene-Spacer-Perylenediimide Multichromophores: Lessons for Photovoltaic Applications

We study the dynamics of primary photoexcitations in three symmetric donor-spacer-acceptor-spacer-donor multichromophores with increasing oligophenylene spacer length, following selective donor or acceptor excitation. Energy levels of the donor and acceptor moieties are tailored to facilitate splitting of the excited state into a lower-lying charge-transfer (CT) state, mimicking the functionality of a donor-acceptor interface for charge generation, thus resulting in long-lived charge separation. Ultrafast electronic energy transfer (ET) from the donor followed by fast hole (back)transfer from the acceptor populates the molecules' CT states. However, the CT efficiency is found to be close to unity, independent of the donor or acceptor photoexcitation. The ratio of CT and recombination rates, which reflects the population of CT states, increases with the oligophenylene spacer length for both direct hole transfer and hole transfer following ET, boosting the population of CT states under continuous excitation. We observe the population of high-lying "dark" excited states following ET from the donor to the acceptor. The "dark" states successively undergo CT and form CT states of higher energy, with decreased recombination rates, while maintaining the high charge generation efficiency. Changes in CT reaction rates are rationalized within the Marcus theory, with driving forces and reorganization energies evaluated by density functional theory and polarizable continuum models. The present study demonstrates the importance of energetically higher-lying states, which cannot be directly photoexcited yet are accessible through ET from local excited states. Similar processes are anticipated in other donor-acceptor systems, which allow for both energy and CT processes, such as bulk heterojunctions of the polymer and small-molecule donor/nonfullerene acceptor typically used in photovoltaic systems.