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

The electron spin polarization (ESP) phenomenon in photoexcited chromophore–radical connected systems was analyzed by multireference electronic structure calculations. We focused on bpy-M-CATmPh-NN (bpy = 4, 4'-di-tert-butyl-2, 2'-bipyridine, M = Pt or Pd, CAT = 3-tert-butylcatecholate, mPh = meta-phenylene, and 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 multireference 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.