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This theoretical study challenges the conventional wisdom that orthogonal dihedral angles between donor and acceptor moieties in donor-acceptor dyads always maximize spin-orbit couplings (SOCs) for efficient triplet excited state production. The authors demonstrate a scenario where SOCs are minimized at orthogonal geometries due to symmetry-imposed constraints on the involved singlet and triplet states. They further show that oblique orientation angles and molecular chirality are necessary to activate SOC pathways in this scenario.
Orthogonal geometries, long thought optimal for spin-orbit coupling in donor-acceptor dyads, can actually *minimize* it, flipping our understanding of triplet excited state production.
Spin-orbit, charge-transfer intersystem crossing (SOCT-ISC) allows for the efficient production of triplet excited states in donor-acceptor (DA) dyads without the involvement of heavy atoms, for use in a myriad of technologies. This process is commonly believed to proceed optimally when the dihedral angle between donor and acceptor moieties is orthogonal. Here, we challenge this idea through a theoretical study unveiling a scenario where spin-orbit couplings (SOCs) are minimized under orthogonal conditions. This scenario is rationalized based on an analysis of the structure-imposed symmetry properties of the involved singlet and triplet states. Notably, in this scenario, finite SOCs demand oblique orientation angles, which in turn requires molecular chirality, suggesting chirality to be a prerequisite for activating the involved SOC pathways.
