Size evolution study on the electronic and optical properties of gold-cluster complexes Au 4 -S- C n H 2 n - S ′ - Au 4 ′ (n = 2–5)

Highlights: • Theoretical study on the optical properties of gold cluster complexes. • Structure and bonding at the metal-molecule interface affets the UV–vis spectra. • HOMO-LUMO gap of the cluster complexes shows a zig-zag behavior with size. -- Abstract: Relativistic unrestricted time-dependent density functional theory calculations were performed for obtaining the absorption spectra of gold-cluster complexes ${\mathrm{Au}}_4-S-C_n{H}_{2n}-S^{\prime }-{\mathrm{Au}}_4^{\prime }$ (n = 2–5). The range-separated exchange-correlation functional CAMYB3LYP was used to properly predict charge transfer excitations. The absorption UV–vis spectrum is not affected by size if structure and metal-molecule bonding remains similar. Changes in the 'local' structure and bonding have drastic effects on these systems' electronic transitions. The HOMO-LUMO gap of the cluster complexes shows a zig-zag behavior typical of gold nanoclusters with respect to the size of the alkanedithiol chain (n).