Engineering opposite electronic polarization of singlet and triplet states increases the yield of high-energy photoproducts

Abstract

Significance All known natural reaction centers employ high-energy singlet states as electron donors, but artificial photosystems struggle to overcome energy losses associated with lower-energy triplet-state formation, which critically limits solar energy conversion efficiency. Our results illustrate a design principle—seemingly at work in the photosynthetic reaction center of Rhodobacter sphaeroides—that employs oppositely polarized singlet and triplet excited states to avert intersystem crossing and dramatically increase the yield of high-energy photoproducts, critical for enhanced efficiency solar cells. Efficient photosynthetic energy conversion requires quantitative, light-driven formation of high-energy, charge-separated states. However, energies of high-lying excited states are rarely extracted, in part because the congested density of states in the excited-state manifold leads to rapid deactivation. Conventional photosystem designs promote electron transfer (ET) by polarizing excited donor electron density toward the acceptor (“one-way” ET), a form of positive design. Curiously, negative design strategies that explicitly avoid unwanted side reactions have been underexplored. We report here that electronic polarization of a molecular chromophore can be used as both a positive and negative design element in a light-driven reaction. Intriguingly, prudent engineering of polarized excited states can steer a “U-turn” ET—where the excited electron density of the donor is initially pushed away from the acceptor—to outcompete a conventional one-way ET scheme. We directly compare one-way vs. U-turn ET strategies via a linked donor–acceptor (DA) assembly in which selective optical excitation produces donor excited states polarized either toward or away from the acceptor. Ultrafast spectroscopy of DA pinpoints the importance of realizing donor singlet and triplet excited states that have opposite electronic polarizations to shut down intersystem crossing. These results demonstrate that oppositely polarized electronically excited states can be employed to steer photoexcited states toward useful, high-energy products by routing these excited states away from states that are photosynthetic dead ends.