Computational Comparative Analysis of Small Atomically Precise Copper Clusters.

Atomically precise copper clusters (APC) have attracted attention for their promise in sensing, water remediation, and electrochemical technologies. However, smaller-sized APCs and the evolution of their properties as a function of size and composition are not clearly understood. Here, we have performed an investigation into the electronic structure, geometry, and optical properties of small atomically precise copper clusters using density functional theory (DFT) and time-dependent DFT. Through comparative analysis, we show that the electronic structures of the experimentally characterized clusters, Cu4(PN(C6H5)2CH)4 and Cu4(SN2C7H11)4, are similar with the closed-shell superatom character 1S21P2. By changing the ligand on Cu4(PN(C6H5)2CH)4 and Cu4(SN2C7H11)4, there were no major changes observed in the tetrahedral Cu4 core geometry, electronic structure, or optical spectra. However, a change in the anchor atom causes an increase in the electronic gap and induces a hypochromic shift in the onset peak in the optical spectrum of the small clusters. Increasing the copper core size showed small changes Cu-Cu bond lengths, lower electronic gap values, and a bathochromic shift in the optical spectra. Computational results not only provide detailed physical insight into APCs but also aid in identifying compound compositions of small atomically precise nanoclusters from data collected in the experiment.