Single-Photon-Induced Electron Spin Polarization of Two Exchange-Coupled Stable Radicals

Transient electron paramagnetic resonance spectroscopy has been used to probe photoinduced electron spin polarization of a stable exchange-coupled organic biradical in a Pt(II) complex comprising 4,4'-di-tert-butyl-2,2'-bipyridine (bpy) and 3,6-bis(ethynyl-para-phenyl-nitronyl nitroxide)-o-catecholate (CAT(o-C≡C-Ph-NN)₂). Photoexcitation results in four unpaired spins in excited states of this complex, with spins being localized on each of the two radicals, CAT^•+ and bpy^•-. The four spins are all exchange-coupled in these excited states, and an off-diagonal matrix element in the CAT^•+-NN exchange allows for exchange-enhanced intersystem crossing to the ³T_1a state, which possesses (bpy^•-)Pt(CAT^•+) chromophoric triplet character. Fast mixing between this ³T_1a state and thermally accessible excited LL'CT state(s) followed by fast relaxation provides spin polarization of the exchange-coupled NN radicals in the ³S₀ ground state of the complex. Our results demonstrate that well-defined quantum states of a ground-state biradical can be initialized with single-photon excitation and have the potential for further spin manipulation directed toward quantum information science applications.