Light-controlled qubit coupling in organic diradicals linked by an MR-TADF emitter

Photoexcited high-spin states in organic systems represent an attractive platform for the optical control of magnetic interactions in molecular spin-qubit architectures. Here, we theoretically study two multiresonant thermally activated delayed fluorescence (MR-TADF)-bridged organic diradicals incorporating stable 1,2,3,5-dithiadiazolyl (DTDA) radical units as molecular candidates for generating long-lived quintet excited states. Using high-level multireference ab initio electronic structure methods, namely CASSCF/QD-NEVPT2 and CASSCF/XMS-PDFT, we show that both systems preserve a disjoint ground state electronic structure with nearly degenerate singlet and triplet manifolds arising from weakly interacting radical spins. Upon photoexcitation of the MR-TADF bridge, population of the bridge-centered LUMO activates exchange interactions between the two DTDA radicals, stabilizing a low-lying metastable quintet state through coupling between the bridge-centered triplet excitation and the radical spins. The calculated spin–orbit couplings reveal that thermally activated torsional distortions around the bonds connecting the MR-TADF bridge to the radical units efficiently enable intersystem crossing between singlet, triplet, and quintet excited state manifolds. In particular, intersystem crossing from the low-lying triplet states to the quintet manifold reaches rates on the order of 10^7 s−1. Overall, these results establish MR-TADF emitter-linked organic diradicals as promising molecular platforms for accessing long-lived photoexcited quintet states and suggest a viable route toward the light-controlled activation of magnetic interactions between molecular qubits.