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-CC-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 center dot+ and bpy center dot-. The four spins are all exchange-coupled in these excited states, and an off-diagonal matrix element in the CAT center dot+-NN exchange allows for exchange-enhanced intersystem crossing to the 3T1a state, which possesses (bpy center dot-)Pt(CAT center dot+) chromophoric triplet character. Fast mixing between this 3T1a 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.