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--butyl-2,2'-bipyridine (bpy) and 3,6-bis(ethynyl--phenyl-nitronyl nitroxide)--catecholate (CAT(-C≡C-Ph-NN)). 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 T state, which possesses (bpy)Pt(CAT) chromophoric triplet character. Fast mixing between this T state and thermally accessible excited LL'CT state(s) followed by fast relaxation provides spin polarization of the exchange-coupled NN radicals in the S 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.