Computational Study on the Relationship between Charge Localization and Second Hyperpolarizability in Mixed‐Valence Systems

Molecular systems with symmetric resonance structures with invertible polarization (SRIP) have been reported to exhibit negative second hyperpolarizability, which is unusual for ordinal molecular systems. We herein report that mixed-valence systems can also be recognized as SRIP systems showing negative second hyperpolarizability. Since electronic structures of MV systems are often represented by the Robin-Day classification, we theoretically investigated the correlation between the Robin-Day class and second hyperpolarizability γ in simple MV systems such as He₂ ^•+ and biaryl derivatives. We found that the absolute value of γ increases and its sign reverses near the boundary of the Robin-Day class-III and II, where the charge begins to localize. The significant variation of γ was rationalized by the context of the intervalence charge transfer (IVCT) transitions, which are unique to MV systems. The calculated γ for MV systems is more than 100 times larger in absolute value than in the neutral state near the charge localization boundary, and the sign of γ switches at the class-II/III boundary. These results suggest that the nonlinear optical response of materials can be significantly switched by redox stimuli.