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

Optically addressable molecular spins are promising candidates for quantum sensing and information processing.[1,2] While metal-free organic diradicals offer high chemical tunability, realizing robust optically detected magnetic resonance (ODMR) functionality requires precise, light- driven control over spin-spin interactions.[1,2] In this work, we present a comprehensive theoretical investigation of symmetric and asymmetric disjoint diradicals bridged by an inverted singlet-triplet (InveST) core. Specifically, we focus on a 1,3- diazete bridge coupled to allyl and phenalenyl radical units.[3] Employing the Pariser-Parr-Pople (PPP) Hamiltonian within a Restricted Active Space (RASCI) scheme, validated by multireference ab initio CASSCF/QD-NEVPT2 calculations, we demonstrate that these systems possess disjoint ground states with effectively degenerate singlet and triplet levels, indicating fully decoupled spins. However, the population of the bridge LUMO trough optical excitation triggers a finite exchange interaction that stabilizes the triplet excited state below the corresponding singlet. Crucially, we demonstrate that thermally accessible torsional fluctuations at the bridge-radical junctions break molecular planarity, activating the spin-orbit coupling (SOC) that mediates intersystem crossing (ISC), thereby driving the robust population of the T1 state from the excited singlet manifold.[3] These findings highlight InveST-bridged diradicals as a robust, metal-free paradigm for room- temperature molecular qubits and spin-optical interfaces.[2,3] References [1] S. M. Kopp et al., J. Am. Chem. Soc. 146, 27935 (2024). [2] L. Savi et al., J. Chem. Theory Comput. 22, 1465 (2026). [3] M. T. Barreca et al., ChemRxiv (2026).