Indian Journal of Chemistry Vol. 42A , March 2003, pp. 484-492 Mixed ligand binuclear copper(l) complexes containing CU I2 NS chromophores. Observation of emissinn(' from MLCT excited states involving two diffc.LvIH ligands Derek A Tocher Department of Chemistry, University College Londo n, 20 Gordon Street, London WCIH OAJ , UIK a nd Michael G B Drew Department of Chemistry, University of Reading, Whiteknights, Reading RG6 6A D, UK and Shubhamoy Chowdhur y, Pradipta Purkayas th a &. Dipankar Datta* Department of In organic Chem istry, Indian Association for th e Cultiva ti on of Science, Calcutta 700 032. India Received 20 November 2002 Us in g the 1:2 condensate (L) of diethylenetriamine and benzaldehyde as the ma in ligand, binuclear copper(l) complexes [Cu2L2(4,4'-bipyridine)](CI04h.0.5H 20 (la) and [Cu2L2(1,2-bis(4-pyridyl)ethane)](CI04l2 ( lb ) are synthesised. The two metal ions in la are bridged by 4,4'-bipyridine and those in Ib by l,2 ·bis(4-pyridyl)ethane. From the X-ray crystal structure of la , each metal ion is found to be bound to three N atoms of L and one of the two N atoms of th e bridg in g li gand in a di storted tetrahe dral fashion. The Cu(l)-N bond lengths in la li e in th e range of 1.998(5)-2.229(6) A. Electrochemical studies in dichloromethane (DCM) show that th e CU ' 2NS moie ti es in la and Ib are composed of two non- interacting Cu'N 4 cores wi th Cu' u, potential of 0.44 V vs. SCE. While la di splays metal induced quenching of the inherent em iss io n of 4,4'-b ipyridine in DCM solution, Ib exhibits two weak emission bands in DCM solution at 425 and 477 nm (total quantum yield = 3.59 x 10. 5 ) originat in g from MLCT exc it ed states. With the help of Extended HUckel ca lcu lati on s it is estab li shed th at th e higher energy e mi ss io n in Ib is from Cu(l) bridg in g- li ga nd charge transfer exc it ed state and th e l owe r energy one in 1 b from Cu(l) L charge transfer exc it ed state. Photolumin esce nt homoleptic mononuclear copper(l) complexes are known mainly with the derivatives of 1,1O-phenanthroline t - 5 . Few other N-donor li gands are ca pable of rendering such copper(l) complexes 6 - 9 . However, photoluminescent binuclear copper(J) complexes of N-donor ligands are rare. So far, there are o nly three exa mpl es-C u2( Ph-NNN-Ph h containing a Cu t 2N4 core 10, a CU l 2 N 6 chromophore assembled with 2,2' -bis[( 4R)-phenyl] -1 ,3-oxazoline " and a CUt 2NS chromo phore iso lated using bi s(pyri- dinal)ethylenediimine I2 . The se three complexes are homo leptic. Herein we d escribe a binuclear copper(l) complex of mixed N-donor li gan ds co ntaining a photoluminescent CU l 2 N s c hromophore. To be specific, in the prese nt complex , a bidentate pyridyl ligand connects the two copper(I) centers, each of w hi ch is bound to a tridentate N-donor li ga nd. This allows us to observe emissions from MLCT (metal-to- ligand charge transfer) exc ited states involving two different li gand s. Materials and Methods Deuterated dimethylsulphoxide (DMSO-d 6 ) (99.9 atom % D) was purchased fro m Aldrich. [Cu(CH 3 CN)4] CI0 4 was prepared by a reponed procedure I3 . C, Hand N analyses were performed using a Perkin-Elmer 240011 analyse r. Copper was estimated gravimetrically as CuSC:-J. FTIR spectra (KBr disc; 4 000-400 cm- I ) were recorded on a Nicolet Magna-IR spect rophotometer (ser·es II ), UV -vis spec tra on a Shimadzu UV -160A spectrophotometer and NMR spectra (in DMSO-d 6 ; reference, TMS) by Brucker DPX300 spectromete r. Solution co nductivity was measured by a Systronics (India) direct read in g conductivity mete r (model 304). Cyclic vo lt ammetry was perf or med using EG&G PAR C electrochemical analysis sys tem (model 250/5/0) under dry nitrogen atmosphere in conventional three electrode configura ti ons in purified dichloromethane (DeM ) with tetrabutyla mmonium perchlorate (TBAP) as the suppo rting electrolyte. A planar EG&G PARe G0229