Exploring the origin of electron spin polarization in the metal-containing chromophore-radical system via multireference calculations

The electron spin polarization (ESP) phenomenon in photoexcited chromophore-radical connected systems was analyzed by multi-reference electronic structure calculations. We focused on the bpy-M-CAT-mPh-NN (bpy = 4,4-di-tert-butyl-2,2-bipyridine, M = Pt or Pd, CAT = 3-tert-butylcatecholate, mPh = meta-phenylene, NN = nitronyl nitroxide) reported by Kirk et al., which is a connected system consisting of a donor-acceptor complex and a radical, and elucidated the mechanism behind the reversal of the sign of photoinduced ESP depending on the metal species. The low-lying electronic states of these molecules were revealed through the multi-reference theory, suggesting that the ligand-to-ligand charge-transfer states play a significant role. Additionally, several structural factors that influence the energies of the excited states were identified. To enhance our understanding of the ESP, we incorporated spin-orbit coupling as a direct transition term between excited states and explicitly considered its effects on the ESP. The results of evaluating transition rates through a transition simulation indicate that when the influence of spin-orbit coupling is significant, the sign of the ESP in the ground state can reverse. This novel ESP mechanism mediated by spin-orbit coupling may offer fundamental insights for designing molecules to precisely control electron distribution across multiple spin states.