How Symmetry Governs the Dihedral Angle Dependence of Intermolecular Spin–Orbit Coupling

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 donor and acceptor moieties are oriented under an orthogonal dihedral angle. Here, we challenge this idea through a theoretical study unveiling an orthogonal scenario where spin-orbit couplings (SOCs) are minimized. This 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 associated SOCT-ISC pathways.