Optical and vibrational properties of nickel integrated germanium Zintl ion clusters

Abstract

Vibrational and optical properties of Ni-Ge clusters ([Ni2Ge9(PPh3)]2−, [Ni6Ge13(CO)5]4−) have been investigated via UV–Vis, FTIR, and Fluorescence spectroscopies. Moreover, frequency and time-dependent (TD) electronic transition calculations have been performed to better characterize experimental findings, due to the scarcity of vibrational and optical spectroscopic studies on these type of clusters in the literature. UV–Vis spectroscopic analyses demonstrate a broad absorption band, with a maximum at 270 nm for both Ni-Ge clusters. [Ni6Ge13(CO)5]4− cluster band is broader and has a shoulder around 320 nm. Vibrational modes associated with the clusters appear at low-frequency region of FTIR spectra. [Ni6Ge13(CO)5]4− cluster spectrum is very rich as expected due to higher number of atoms in the cluster structure. Main vibrational modes of [Ni2Ge9(PPh3)]2− are observed at 318(m) cm−1 and 130(w) cm−1, and those of [Ni6Ge13(CO)5]4− appear at 232(s) cm−1 and 155(s) cm−1. Similarities observed between simulated and experimental spectra strongly support a stable, isolated cluster behavior in the crystal structure. Fluorescence spectroscopic analysis demonstrates that excitation of [Ni2Ge9(PPh3)]2− and [Ni6Ge13(CO)5]4− clusters at 260 nm has resulted relatively strong and broad emissions at 400 nm and 410 nm, respectively. [Ni2Ge9(PPh3)]2− and [Ni6Ge13(CO)5]4− clusters are two of the very few examples of Zintl ion clusters that have been characterized with UV–Vis and FTIR spectroscopic analyses, and the only ones that have been studied with fluorescence spectroscopy.