Exploring the origin of electron spin polarization in metal-containing chromophore–radical systems <i>via</i> 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-CAT-<i>m</i>Ph-NN (bpy = 4,4'-di-<i>tert</i>-butyl-2,2'-bipyridine, M = Pt or Pd, CAT = 3-<i>tert</i>-butylcatecholate, <i>m</i>Ph = <i>meta</i>-phenylene, and NN = nitronyl nitroxide) reported by Kirk <i>et al.</i>, 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.