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

ABSTRACT

Both the sign and intensity of photoinduced electron spin polarization (ESP) in the electronic ground state doublet (2S0/D0) 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 m-phenylene bridge, (bpy)M(CAT-m-Ph-NN) (1) (bpy = 4,4'-di-tert-butyl-2,2'-bipyridine, M = PdII (1-Pd) or PtII (1-Pt), CAT = 3-tert-butylcatecholate, m-Ph = meta-phenylene). In both complexes, photoexcitation with visible light produces an initial exchange-coupled, three-spin (bpy•-, CAT•+ = semiquinone (SQ), and NN•), charge-separated doublet 2S1 (S = chromophore excited spin singlet configuration) excited state that rapidly decays to the ground state via a 2T1 (T = chromophore excited spin triplet configuration) state. This process is not expected to be spin selective, and only very weak emissive ESP is found for 1-Pd. In contrast, strong absorptive ESP is generated in 1-Pt. It is postulated that zero-field-splitting-induced transitions between the chromophoric 2T1 and 4T1 states (1-Pd and 1-Pt) and spin-orbit-induced transitions between 2T1 and NN-based quartet states (1-Pt) account for the differences in polarization.