Spin Coupling in Symmetric and Asymmetric Allyl and Phenalenyl Diradicals Bridged by an Inverted Singlet–Triplet System

Organic diradicals bridged by inverted singlet-triplet (InveST) units have recently emerged as promising molecular platforms for spin-optical functionality, enabling optical control of spin-spin interactions within a fully organic framework. Here, we study a series of symmetric and asymmetric InveST-bridged diradicals based on the smallest InveST motif, 1,3-diazete, functionalized with allyl and phenalenyl radical units. Within the Pariser-Parr-Pople (PPP) framework, these systems possess disjoint diradical ground states with degenerate singlet and triplet levels, while population of the bridge LUMO triggers finite exchange interactions that stabilize the triplet excited state. Torsional flexibility at the bridge-radical junctions plays a key role in triggering spin-orbit coupling, thereby promoting intersystem crossing between the lowest excited singlet and triplet states. Through evaluation of intersystem crossing and reverse intersystem crossing rates using a vibronic model, we find that all three diradicals support net population transfer from the singlet to the triplet manifold under ambient conditions.