Tin Metal Cluster Compounds as New Third-Order Nonlinear Optical Materials by Computational Study.

It is quite appealing but challenging to predict and synthesize new nonlinear optical (NLO) materials with exceptional performance. Herein, the different Sn4 cluster core structures and third-order NLO properties are studied through electronic structure, excited hole-electron, bonding character, and aromaticity analysis. As a result, Sn4 clusters with ring core structure (Sn4-R) not only have the smallest Egap, the largest UV-vis response intensity, but also the strongest third-order NLO response in our work. As proved by natural bond orbitals' (NBO) analysis, electron localization function (ELF), and adaptive natural density partitioning (AdNDP), the Sn44+ has two in-plane four center-two electron (4c-2e) Sn-Sn σ-bonds, resulting in a good delocalization. For the first time, delocalization of metal cluster cores in tin clusters that is beneficial to the third-order NLO response is proposed, which provides a new guidance to design and prepare third-order NLO materials.