Photoinduced Magnetic Exchange-Jump Promotes Ground State Biradical Electron Spin Polarization
Martin L. Kirk, David A. Shultz, Anil Reddy Marri, Art van der Est
Abstract
Photoinduced electron spin polarization (ESP) is reported in the electronic ground states of three Pt(II) complexes comprised of two S = 1/2 nitronyl nitroxide (NN) radicals attached through different length para -phenylethynyl bridges to the 3,6 positions of a catecholate (CAT, donor) and 4,4′-di- tert -butyl-2,2′-bipyridine (bpy, acceptor). Complexes 1 – 3 have from 17 to 41 bonds separating NN radicals and display cw-EPR spectra consistent with | J NN-NN | ≫ | a N |, | J NN-NN | ≥ | a N |, and | J NN-NN | < | a N |, respectively, where J NN-NN is the magnetic exchange coupling between NN radicals in the electronic ground state, and a N is the isotropic 14 N hyperfine coupling constant. Light-induced transient EPR spectra characterized as enhanced ground-state absorption were observed for all three complexes using 532 nm pulsed laser excitation into the ligand-to-ligand charge transfer (LL’CT) band of the (CAT)Pt(bpy) chromophore. The magnitude of the observed ESP increases in the order 1 < 2 < 3 and is inversely correlated with the magnitude of ground-state J NN-NN . In addition to the experimental observation of net absorptive polarization in 1 – 3, light excitation also produces multiplet polarization in 2 . Since the weak dipolar coupling leads to a strong spectral overlap of the absorptive and emissive components, the multiplet polarization is not observed in 1 and 3 and is very weak in 2 . The ability to spin-polarize multiple radical spins with a single photon is anticipated to advance new photoinduced multi qubit/qudit ESP protocols for quantum information science applications.