Metal Ion Control of Photoinduced Electron Spin Polarization in Electronic Ground States.

Both the sign and intensity of photoinduced electron spin polarization (ESP) in the electronic ground state doublet (S/D) of chromophore-radical complexes can be controlled by changing the nature of the metal ion. The complexes consist of an organic radical (nitronyl nitroxide, NN) covalently attached to a donor-acceptor chromophore via a -phenylene bridge, (bpy)M(CAT--Ph-NN) () (bpy = 4,4'-di--butyl-2,2'-bipyridine, M = Pd () or Pt (), CAT = 3--butylcatecholate, -Ph = -phenylene). In both complexes, photoexcitation with visible light produces an initial exchange-coupled, three-spin (bpy, CAT = semiquinone (SQ), and NN), charge-separated doublet S (S = chromophore excited spin singlet configuration) excited state that rapidly decays to the ground state via a T (T = chromophore excited spin triplet configuration) state. This process is not expected to be spin selective, and only very weak ESP is found for . In contrast, strong ESP is generated in . It is postulated that zero-field-splitting-induced transitions between the chromophoric T and T states ( and ) and spin-orbit-induced transitions between T and NN-based quartet states () account for the differences in polarization.