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

TL;DR

This study reveals that in donor-acceptor dyads, optimal spin-orbit coupling for intersystem crossing occurs at oblique angles due to symmetry constraints, highlighting the role of chirality.

Contribution

It challenges the common belief that orthogonal dihedral angles maximize spin-orbit coupling, showing instead that oblique angles can be more effective due to symmetry considerations.

Findings

Orthogonal angles do not always maximize spin-orbit coupling.

Finite SOCs require oblique orientations, often involving molecular chirality.

Chirality may be necessary to activate SOC pathways in these systems.

Abstract

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.