The photoluminescence properties of the copper(I) clusters Cu4I4A4 (A = aromatic amine) in solution

Solutions of the copper(I) tetranuclear clusters Cu4I4A4 (A = pyridine or substituted pyridine) in room temperature toluene display two luminescence bands in the visible region of the spectrum. The higher energy emission has been assigned as a transition from a metal-to-pyridine charge transfer state on the basis of pyridine substituent effects, while the much more intense lower energy emission has been assigned as a cluster-centered transition owing to its presence even when A is a saturated amine. The two emission display relatively long, but different, lifetime and appear to originate from states which are essentially uncoupled. Calculations using effective relativistic core potential methods suggest that the cluster-centered emission is from an excited state which can be visualized as the result of electronic promotion from a HOMO which is largely iodide in character to a LUMO which is largely copper (4s and 4p) in character.