Efficient energy conversion requires quantitative, light-driven formation of high-energy, charge-separated states. Conventional artificial photosystem designs seek to promote electron transfer (ET) by polarizing excited donor electron density toward the acceptor (“one-way” ET). Enigmatically, the excited donor of the archetypal R. sphaeroides reaction center polarizes its electron density away from its electron acceptor: light absorption by the reaction center thus triggers a “U-turn” ET event. Whatever mechanistic importance lies behind this biological U-turn ET has been obscured by the inability to experimentally reverse donor excited-state polarization within the reaction center. We describe how U-turn ET produces a strikingly larger yield of high-energy photo-products compared to a conventional one-way ET scheme, by minimizing intersystem crossing to the donor triplet state. We directly compare one-way vs. U-turn ET strategies via linked donor-acceptor (DA) assemblies based on highly conjugated (porphinato)metal-(polypyridyl)metal constructs, in which selective optical excitation produces donor excited states polarized either toward or away from the acceptor. Ultrafast spectroscopic studies of these DA assemblies pinpoint the importance of realizing donor singlet and triplet excited states that have opposite electronic polarizations to shut down intersystem crossing, a scheme exploited by the reaction center of R. sphaeroides. These results offer an unexpected U-turn design principle, heralded by Nature, that averts intersystem crossing and dramatically increases the yield of high-energy photo-products critical for light-driven energy conversion reactions.

Oppositely polarized singlet and triplet states: A new strategy to control photo-triggered energy conversion reactions of coordination compounds

254th National Meeting (2017)

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