Photoinduced Magnetic Exchange-Jump Promotes Ground State Biradical Electron Spin Polarization.

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.