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 E gap, 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 Sn4 4+ 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.

Electron delocalization in aromatic materials significantly impacts their third-order nonlinear optics (NLO). Despite organometallic complexes with Craig-Möbius aromaticity attracting great attention for their unusual physicochemical properties, their third-order NLO have been little studied to date. Herein, 12 Craig-Möbius aromatic organometallics with a stable structure similar to osmapentalyne, namely, carbolong complexes, are screened by DFT. They exhibit high third-order NLO responses because of the d and p electron delocalization in the organometallic ring. Furthermore, electron–hole distribution analyses draw a conclusion that extending the conjugated plane will increase the π-conjugation system to enhance the local excitation in the plane, and the introduction of typical aromatic ligands can result in the organometallic ring-to-ligand charge transfer (RLCT), which are effective methods to improve the third-order NLO response. This study opens a new window in the application of Craig-Möbius aromatic complexes and provides a new approach for third-order NLO materials design.

Comprehensive Summary Carbolong compounds as a metal‐heteroaromatic compound with both organometallic properties and π‐conjugated systems exhibit great potential in organic catalysis and optoelectronic devices. In this work, for the first time, the “Bucket Effect” is revealed to promote the third‐order nonlinear optical (NLO) performance in metal‐heteroaromatic compounds. We have successfully constructed and investigated a series of novel metallapentalenes with higher third‐order NLO performance benefited from the “Bucket Effect”. Meanwhile, aromaticity and electron−hole analysis further confirm the internal homogeneity of organometallic rings, reduced bandgap, and enhanced low‐energy peak response resulted in the enhanced third‐order NLO effects. The success of this work is discovering an emerging material library of high third‐order NLO effects, and illustrating the feasibility of engineering the high response metal‐heteroaromatic optical devices at the electronic structure level.