Optical properties of the hydrated charged silver tetramer and silver hexamer encapsulated inside the sodalite cavity of an LTA-type zeolite

Abstract

The optical spectra in the UV-VIS region of the hydrated doubly charged tetramer Ag₄²⁺ and hydrated multiply charged hexamer Ag₆^p+ silver clusters encapsulated inside the sodalite cavity of an LTA-type zeolite have been systematically predicted using DFT, TD-DFT and CASSCF/CASPT2 methods. The optical behaviour of the model hydrated clusters [Ag₆(H₂O)₈(Si₂₄H₂₄O₃₆)]^p+ is very sensitive to their charge. Among the cations [Ag₆(H₂O)₈(Si₂₄H₂₄O₃₆)]^p+, only the embedded hydrated quadruply charged silver hexamer [Ag₆(H₂O)₈(Si₂₄H₂₄O₃₆)]⁴⁺ shows a strong absorption band at ∼420 nm (blue light) and emits light in red color. The absorption spectrum of the hydrated doubly charged silver tetramer cluster [Ag₄(H₂O)_m(Si₂₄H₂₄O₃₆)]²⁺, which shifts slightly and steadily with the increasing amount of interacting water molecules to longer wavelengths, has a strong peak in the blue region. The water environment forces the silver tetramer to relocate into one side of the cavity instead of at its center as in the case of the non-hydrated [Ag₄(Si₂₄H₂₄O₃₆)]²⁺ cluster. Water molecules act as ligands significantly splitting the energy levels of excited states of the Ag₄²⁺ and Ag₆⁴⁺ clusters. This causes the absorption spectra of the clusters to broaden and the emission to shift to the green-yellow and red part of the visible region.