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 3 T 1a state, which possesses (bpy •- )Pt(CAT •+ ) chromophoric triplet character. Fast mixing between this 3 T 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 3 S 0 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.