Supracrystals of N-Heterocyclic Carbene-Coated Au Nanocrystals

Controlling the generation of organized 3D assemblies of individual nanocrystals, called supracrystals, as well as their properties, is an important challenge for the design of new materials in which the coating agent plays a major role. We present herein a new generation of structured fcc Au supracrystals made of N-heterocyclic carbene (NHC)-coated Au nanocrystals. The 3D assemblies were achieved by using benzimidazole-derived NHCs tailored with long alkyl chains at different positions. The average size of the nanocrystal precursors (4, 5, or 6 nm) and their ability to self-assemble were found to be dependent on the length, orientation, and number of alkyl chains on the NHC. Thick and large supracrystal domains were obtained from 5 nm Au nanocrystals coated with NHCs substituted by C14 alkyl chains on the nitrogen atoms. Here, the geometry of both the C-carbene-Au and N-C-alkyl bonds induces a specific orientation of the alkyl chains, different from that of alkylthiols, resulting in Au surface covering by the chains. However, the edge-to-edge distances in the supracrystals suggest that the supracrystals are stabilized by interdigitation of neighboring nanocrystals alkyl chains, whose terminal part must point outward with the appropriate geometry.