CHARACTERIZATION OF THE LUMINESCENT CU-ASTERISK(CN)(2)BR2- EXCIPLEX

The efficient luminescence from certain Cu(I) complexes has been the s ubject of intense studies, particularly within the context of assignin g the optical transitions involved.(2,3) On the one hand, the copper(I ) transitions resulting in luminescence have been ascribed to metal-to -ligand charge transfer (MLCT), where the electron is transferred from the copper(I) center to the unoccupied pi orbital of the ligand,4-10 or to charge transfer to solvent (CTTS), where the electronic charge moves to the solvent(11-13) molecule from the central atom. On the oth er hand, a metal-centered transitions of the types 3d(10) --> 3d(9)4s and 3d(10) --> 3d(9)4p on Cu(I) are considered(14-17) as another sourc e of light emission. The common feature of all of the emissions is the pronounced Stokes shift, which especially in the case of the halocupr ate(I) complexes in aqueous solutions is rather large.(3) Recently we demonstrated that coordinatively-unsaturated Cu(CN)(2)(-), formed upo n UV irradiation in aqueous solutions of dicyanocuprate(I), associates with halide ions to create a longer-lived, more highly luminescent sp ecies assigned as an exciplex.(18) However, since halo ligands also co ordinate to the metal center in the ground state, with a stepwise form ation constant that is about an order of magnitude smaller than that f or the excited-state reaction,(18,19) the luminescence characteristics were explained by a mechanism involving both ground-and excited-state equilibrium reactions. We have observed that there is a dependence of lifetime on both the concentration and the nature of halo ion. At amb ient temperature, the longest luminescence lifetime, similar to 200 ns , was observed for the Cu-(CN)(2)(-)-Br- system in aqueous solutions a t 5 M ionic strength, providing favorable conditions for characterizin g the mixed-ligand inorganic exciplex. In this work, the absorption an d emission spectra of the equilibrated, aqueous Cu(CN)(2)(-)-Br- syste m have been investigated, as well as the luminescence quenching dynami cs, using steady-state and time-resolved photochemical techniques, in order to elucidate the nature of the luminescent exciplex, Cu(CNBr2-, formed following absorption of the photon.