ORIGINS OF THE DOUBLE EMISSION OF THE TETRANUCLEAR COPPER(I) CLUSTER CU4I4(PYRIDINE)4 - AN ABINITIO STUDY

The Cu(I) tetranuclear cluster Cu4I4py4 (I, py = Pyridine), together with several analogues of general formula Cu4I4L4, presents remarkable photophysical properties, which have been attributed to two poorly coupled emissive excited states of different orbital parentages.2 Ab initio calculations at the restricted Hartree-Fock self-consistent field level on this and related systems have been used to complement the results of the experimental spectroscopic studies. The outcome indicates an iodine-to-pyridine charge transfer nature for the state(s) responsible for the high-energy emission band of 1. In contrast, the low-energy emission band is assigned to a transition with both iodine-to-copper charge transfer and metal centered 3d --> 4s promotion within the Cu4I4 cluster core; thus this band is labelled a cluster-centered (CC) transition. The lack of communication between the two excited states is explained by different respective distortions relative to the ground state, as indicated by excitation-induced changes in the calculated bond orders. Calculations are also described for the hypothetical cluster Cu4I4(NH3)4, as a model for compounds like Cu4I4pip4 (pip = the saturated amine piperidine) for which only CC emission has been observed.