Theoretical Exploration of the Layered Sandwich Cobaltacarborane as a Multi-State NLO Molecular Switch Triggered by Redox

Layered sandwich metallacarboranes have various advantages including structural diversity, tailoring ability, rich electrochemical properties, and high thermal stability and thus could be potential multifunctional nonlinear optical (NLO) materials. In this study, second-order NLO properties and the multi-state switching effect of the layered cobalta-carborane, induced by electrochemical behavior, were explored using density functional theory (DFT). The calculated first hyperpolarizabilities (beta) illustrated that the NLO responses are enhanced with the increasing of layers, which was mainly attributed to the C -> B intraligand charge transfer (C2B3) (ILCT) and metal-centered d -> d transition (MCT) as indicated by the time-dependent (TD)DFT results. Remarkable beta contrasts were observed after undergoing two sequential one-electron redox processes. Therefore, the pronounced multi-state NLO switch can be achieved. In mixed-valence systems, the metal metal interaction plays a crucial role in the NLO responses through intervalence charge transfer (IVCT) and the pi -> pi* transition of the whole metal bridge metal system.