AB-INITIO STUDIES OF THE COPPER(I) TETRAMERS CU(4)X(4)L(4) (X=I, BR, CL) - EFFECTS OF CLUSTER STRUCTURE AND OF HALIDE ON PHOTOPHYSICAL PROPERTIES

Ab initio calculations at the Hartree-Fock level are described for various compounds of the type Cu(4)X(4)L(4), in order to gain a better understanding of the rich luminescence behavior of these cuprous halide clusters. The calculations clearly demonstrate a relationship between the Cu-Cu distances (d(Cu-Cu)) in these ''cubane'' type clusters and the energies and distortions (from the ground state) expected for the ''cluster-centered'' (CC) excited states, which arise primarily from a redistribution of charge within the CU4I4 cluster core. Specifically, the lowest energy excited state in the pyridine complex Cu4I4(pY)(4) (d(Cu-Cu) = 2.69 Angstrom A) is such a CC state but the lowest excited state in the 2-(diphenylmethyl)pyridine complex Cu4I4(dpmp)(4) (d(Cu-Cu) = similar to 2.90 Angstrom A) is a halide-to-ligand charge-transfer state. The calculations furthermore indicate that as one varies the halide from I to Br to C1 for the structurally analogous Cu(4)X(4)(dpmp)(4) series, the nature of the lowest excited state progressively changes and the extent of charge transfer to the ligand diminishes while intraligand and Cu d --> s,p contributions increase. Altogether, the calculations indicate that the simplified excited-state models often employed for describing the photophysical properties of coordination compounds appear to be inadequate in these cases.