Triplet Harvesting in Single-bond Connected Donor-acceptor Dyad: A Universal Mechanism

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 finding 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 universality of the proposed mechanism. The present findings pave the routes for the synthesis of a new generation of triplet-harvesting organic molecules.