Single-Photon-Induced Electron Spin Polarization of Two Exchange-Coupled Stable Radicals
Martin L. Kirk, David A. Shultz, Anil Reddy Marri, Patrick Hewitt, Art van der Est
Abstract
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 3T1a state, which possesses (bpy•–)Pt(CAT•+) 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.