Ground-state
electron spin polarization (ESP) is generated in radical
elaborated (bpy)Pt(CAT-NN) and (bpy)Pt(CAT-p-Me2PhMe2-NN) (bpy = 5,5′-di-tert-butyl-2,2′-bipyridine, CAT = 3-tert-butylcatecholate, p-Ph = para-phenylene, NN = nitronylnitroxide).
Photoexcitation produces an exchange-coupled, three-spin, charge-separated
doublet 2S1 (S = chromophore excited spin singlet
configuration) excited state that rapidly decays to a 2T1 (T = chromophore excited spin triplet configuration)
excited state. The SQ-bridge-NN bond torsions affect the magnitude
of the excited state exchange interaction (
J
SQ‑NN
), which determines the 2T1–4T1 energy gap.
Ground state ESP is dependent on the magnitude of
J
SQ‑NN
, and we postulate that
this results from differences in 2T1 and 4T1 state mixing. Mechanisms that lead to the rapid
transfer of the excited state ESP to the ground state are discussed.
Although subnanosecond 2T1 state lifetimes are
measured optically in solution, the ground state ESP decays very slowly
at 20 K and is observable for more than a millisecond.