Excited State Magneto-Structural Correlations Related to Photoinduced Electron Spin Polarization

Photoinduced electron spin polarization (ESP) is reported in the ground state of a series of complexes consisting of an organic radical (nitronylnitroxide, NN) covalently attached to a donor-acceptor chromophore either directly or via para-phenylene bridges substituted with 0-4 methyl groups. These molecules represent a class of chromophores that undergo visible light excitation to produce an initial exchange-coupled, three-spin [bpy^•-, CAT^•+ (= semiquinone, SQ) and NN^•], charge-separated doublet ²S₁ (S = chromophore spin singlet configuration) excited state that rapidly decays by magnetic exchange-enhanced internal conversion to a ²T₁ (T = chromophore excited spin triplet configuration) state. The ²T₁ state equilibrates with chromophoric and NN radical-derived excited states, resulting in absorptive ESP of the recovered ground state, which persists for greater than a millisecond and can be measured by low-temperature time-resolved electron paramagnetic resonance spectroscopy. The magnitude of the ground state ESP is found to correlate with the excited state magnetic exchange interaction between the CAT^+• and NN^• radicals, which in turn is controlled by the structure of the bridge fragment.