Organic Diradicals Bridged by Inverted Singlet–Triplet Units for Optical–Spin Interfaces

Molecular platforms for optically addressable spin states are emerging as fascinating alternatives to solid-state spin centers, offering scalable synthesis, structural tunability, and chemical versatility. Here, we present a molecular design strategy for achieving photoinduced spin polarization in organic diradicals bridged by systems featuring an inverted singlet−triplet (InveST) energy gap. These InveST units possess HOMO and LUMO orbitals localized on complementary atomic sites. By covalently linking the non-SOMO-bearing positions of alternant hydrocarbon radicals to the LUMO-localized atoms of the InveST bridge, we construct diradicals in which the radical centers remain electroni- cally decoupled in the ground state, yielding degenerate singlet and triplet configurations. Upon photoexcitation, the population of the InveST LUMO activates an excited-state exchange interaction between the radicals, generating a finite singlet−triplet gap and enabling spin-selective intersystem crossing to polarized triplet states. Using a combination of model Hamiltonians and multireference ab initio calculations, we establish design principles for tuning exchange interactions and spin−orbit coupling to achieve molecular-level control over optical−spin interfaces. The resulting InveST- bridged diradicals have emerged as promising scaffolds for molecular quantum technologies.