Flipping the Rules: From Inverted Singlet-Triplet Gaps to Diradical Switches

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.[1,2] In this talk, 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 (see Figure 1).[3] 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 electronically decoupled in the ground state, yielding degenerate singlet and triplet configurations. Upon photoexcitation, 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 emerge as promising scaffolds for molecular quantum technologies.