Optical bandgap of Cd, Zn, and Ag nitroprussides. A combined experimental and computational study

The optical bandgap (E_g) of transition metal nitroprussides remains practically undocumented, probably because these materials are usually considered wide bandgap solids, characterized by a strong metal-ligand charge-transfer band below 500 nm. In this contribution, we are reporting experimental and DFT computed data for both, 3D and 2D phases of Zn and Cd nitroprussides to get physical insight into the metal and structure effect on the optical bandgap (E_g) of these materials. 3D Ag nitroprusside, an analog solid for which the E_g value has been reported, was included for comparison. E_g values were obtained from both, UV–vis–NIR spectra and DFT calculations. The solids under study show a direct bandgap with diverse Urbach tails, which was ascribed to the presence of structural defects. Metal to ligand charge transfer within nitroprusside ion dominates the electronic structure in the valence-gap-conduction region for the 3D systems. Both, the outer metal, and the incorporation of 1-Methyl-2-pyrrolidone as an organic pillar molecule between adjacent layers for the 2D analogs, influence the electronic structure and consequently, the E_g values obtained. In these materials, the polarizing power of outer metal has a relevant role in the observed E_g value.