Serendipitous Triplet Harvesting in Donor-acceptor Dyads with Low Spin-orbit Coupling

Here, we present a heuristic mechanism for efficient intersystem crossing (ISC) between singlet and triplet states with low spin-orbit coupling (SOC) in molecules having donor-acceptor (D-A) moieties separated by a sigma bond. We show that SOC between the first singlet and the higher-lying triplet states, together with nuclear motion-driven coupling of this triplet state with lower-lying triplet state during the free rotation about a sigma bond is the only plausible way to achieve the experimentally observed ISC rate for a class of D-A type photoredox catalysts. We found this mechanism to be the dominant contributor to the ISC process with the corresponding rate reaching maximum at an optimal dihedral angle between D-A moieties. An extension to this is provided by discovering the same mechanism to be operative in a specific spirobis[anthracene]diones molecule where the D and A moieties are interlocked near to the optimal dihedral angle, indicating the plausible universality of the proposed mechanism. The present finding is expected to have implications in strategies for the synthesis of a new generation of triplet-harvesting organic molecules.