Dithiol-Induced Contraction in Ag14 Clusters and Its Manifestation in Electronic Structures

We report a dithiol-protected silver cluster, Ag14(BDT)6(PPh3)8 (BDT = 1,2-benzene dithiol), abbreviated as Ag14DT, which exhibits distinctly different optical properties than the analogous monothiol-protected Ag14(SC6H3F2)12(PPh3)8, abbreviated as Ag14MT. Replacement of monothiol by dithiol, keeping the composition constant, has not been possible so far. The inner cores of both Ag14DT and Ag14MT are composed of octahedral Ag6, but because of the presence of dithiol, the outer cubic Ag8 shell became distorted in the former. Consequently, Ag14DT showed a unique absorption in the near-infrared (NIR) region, which is mainly due to transitions derived from ligands. It exhibits dual visible/NIR emission, at around 680 and 997 nm. Ag14DT exhibited greater thermal stability because of the rigidity provided by dithiol ligands. The clusters with NIR absorption and emission open up a possibility for their application in solar thermal conversion and medical imaging. NIR luminescence in the range of 1000 nm in ultrasmall clusters is very new.