Light-triggered reversible supramolecular self-assembly of azo groups-functionalized copper nanoclusters

Abstract

Metal nanoclusters (NCs) have attracted much attention due to their ultrasmall sizes and unique electronic structures. In recent years, the light-induced reversible assembly of Au NCs is already reported. However, to achieve light-triggered reversible self-assembly of Cu NCs is still challenging. Herein, we prepared photoresponsive Cu NCs by using an azo-containing ligand, i.e., 3,5-dimethyl-4-(phenyldiazenyl)-1H-pyrazole) (abbreviated to Hazopz). Each nanocluster contains three Cu(I) in the core and has atomic precision (denoted as Cu3-NCs hereafter). Self-assembly of Cu3-NCs into nanofibers occurred in binary solvent mixture with toluene as a good solvent and methanol (MeOH) as a poor one. When the suspension containing the nanofibers was irradiated by UV light, the originally straight nanofibers became bent. This morphological change is mainly due to the isomerization of the peripheral azo groups of the Cu3-NCs from trans- to cis-conformation, and to a small extent ascribed to the increased solubility of the cis-conformation of the azo groups in the binary solvent. The disassembled aggregates can reassemble and the bent nanofibers can become straight again when exposed to visible light. This process can be repeated for at least three times, indicating the high reversibility of this light-triggered morphological change of Cu3-NCs. The one-dimensional self-assemblies with light-triggered, reversible morphological changes achieved in this work can be potentially used as a medium material in an optical information storage device, and can write and erase information through the irradiation of UV light and visible light.