Optical Properties of Monolayer-Protected Aluminum Clusters: Time-Dependent Density Functional Theory Study

We examine the electronic and optical properties of experimentally known monolayer-protected aluminum clusters Al-4(C5H5)(4), Al-50(C5Me5)(12), and Al-69(N(SiMe3)(2))(18)(3-) using time-dependent density functional theory. By comparing Al-4(C5H5)(4) and the theoretical Al-4(N(SiMe3)(2))(4) cluster, we observe significant changes in the optical absorption spectra caused by different hybridization between metal core and ligands. Using these initial observations, we explain the calculated spectra of Al-50(C5Me5)(12) and Al-69(N(SiMe3)(2))(18)(3-). Al-50(C5Me5)(12) shows a structured spectrum with clear regions of low-intensity core-to-core transitions followed by high-intensity ligand-to-core transitions due to its high symmetry and pi-bonding to the Cp ligands. The spectrum of Al-69(N(SiMe3)(2))(18)(3-) is rather featureless as the core-to-core and ligand-to-core regions partially merge because of the lower symmetry found in the metal core and differences in the ligand-core hybridization. Though there are minimal features in the spectra, the most intense features are identified as excitations from ligand states to metal core states. We show that our calculated absorption spectrum for Al-69(N(SiMe3)(2))(18)(3-) agrees with recent experimental results for N(SiMe3)(2)-protected Al nanodusters with an average diameter of 1.5 nm.