Reversible Switching of Organic Diradical Character via Iron-Based Spin-crossover

Organic diradicals are uncommon species that have been intensely studied for their unique properties and potential applicability in a diverse range of innovative fields. While there is a growing class of stable and well characterized organic diradicals, there has been recent focus on how diradical character can be controlled or modulated with external stimuli. Here we demonstrate that a diiron complex bridged by the doubly oxidized ligand tetrathiafulvalene-2,3,6,7-tetrathiolate (TTFtt²⁻) undergoes a thermally induced Fe-centered spin-crossover which yields significant diradical character on TTFtt²⁻. UV-vis-Near-IR, Mössbauer, NMR, and EPR spectroscopies with magnetometry, crystallography, and advanced theoretical treatments suggest that this diradical character arises from a shrinking TTFtt²⁻ π-manifold from the Fe(II)-centered spin-crossover. The TTFtt²⁻ centered diradical is predicted to have a singlet ground state by theory and variable temperature EPR. This unusual phenomenon demonstrates that inorganic spin transitions can be used to modulate organic diradical character.