Ligand Modification on Copper(I)-Sulfide Clusters for Enhanced Third-Order Nonlinear Optical Response

Two copper(I)-sulfide clusters were synergistically modified by alkylthiols and conjugated N,S-heterocyclic ligands, i.e., Cu6(tBuS)2(MBIZ)4(designated as Cu6) and Cu8(tBuS)4(MBIZ)4·CH3OH·2H2O (designated as Cu8) (MBIZ = 2-mercaptobenzimidazole; tBuS = tert-butyl mercaptan). Introducing more thiol ligands led to further growth of the cluster core, resulting in the transformation from an octahedral to a di-tetrahedral configuration, accompanied by a slight increase in the Cu···Cu distance. Notably, both clusters demonstrate high chemical stability and superhydrophobicity as a result of the modification with surface ligands, being stable at least 1 week in a wide range of pH (1–14 for Cu6and 3–11 for Cu8). Moreover, Cu8exhibited near-infrared (NIR) photoluminescence (PL) emission and better third-order nonlinear optical (NLO) performance, with nonlinear absorption coefficients (β) of 3.5 × 10–10m/W, 3.5 times than that of Cu6and superior to the majority of crystalline chalcogenide semiconductors. Theoretical calculations indicate that the organic ligands and cluster core jointly optimize the electronic configuration in a manner that regulates the optical properties. This work provides a new perspective into understanding the relationship between cluster core/surface compositions and optical properties at an atomically precise level, which is instrumental in the accurate design and synthesis of stable and efficient third-order NLO materials.