Dalton Transactions Dynamic Article Links Cite this: Dalton Trans., 2011, 40, 8320 www.rsc.org/dalton PAPER A dinuclear extension to constrained heteroleptic Cu(I) systems† Bel´ en Gil, Gareth A. Cooke, Deanne Nolan, Gear´ oid M. ´ OM´ aille, Sunil Varughese, Longsheng Wang and Sylvia M. Draper* Received 18th February 2011, Accepted 13th May 2011 DOI: 10.1039/c1dt10275c This article reports the synthesis and optical properties of three dinuclear, cationic copper complexes [Cu 2 (m-dppm) 2 (m-L)](NO 3 ) 2 (dppm diphenyldiphosphinomethane, L: L A 3,6-bis(2-pyridyl)-4,5- diphenyl-pyridazine, L B 3,6-bis(2-pyridyl)-4,5-di(4-pyridyl)-pyridazine and L C 3,6-bis(2-pyridyl)-8,9- diazafluoranthene). These were formed on the reaction of [Cu(m-dppm)(NO 3 )] 2 with a series of N-donor (bppn) ligands L. The single crystal X-ray structures of [Cu 2 (m-dppm) 2 (m-L)](NO 3 ) 2 ·CH 2 Cl 2 were determined and revealed that in both, the two copper atoms are held by three bridging ligands, two dppm ligands and one bppn ligand acting as a tetradentate bridge. The absorption spectra of the complexes present a MLCT [Cu →p*(N Ÿ N)] band in the l 370–425 nm region. These new complexes exhibit red-orange MLCT-based emission in the solid-state with lifetimes in the microsecond range. In oxygen-free dichloromethane solution, the complex [Cu 2 (m-dppm) 2 (m-L C )] 2+ has a long lifetime of 22.8 ms. The long emission lifetimes are attributed to a rigid conformation that precludes the possible distortion of the copper in the excited state. Introduction Although few in number, recent publications have begun to expose the potential of Cu(I) complexes as an alternative to Ru(II) in low-cost solar-conversion devices. 1–3 The rich photochemistry of Ru(II), 4 Pt(II), 5 and Ir(III) 6 complexes is undermined by the reliance on expensive, rare metals, which in some cases exhibit an undesirable level of toxicity. The ease of preparation of Cu(I) complexes, their ability to absorb light in the visible region, intense luminescence and low cost has captured the interest of researchers in the field. Cu(I), however, undergoes a conformational change on oxidation to Cu(II), thus decreasing device efficiencies. Strate- gies to overcome the resultant low quantum yields and short luminescence lifetimes of [Cu(N Ÿ N) 2 ] + complexes require further development. 1,3 The electronic nature, bulk and rigidity of the diimine ligand all play an important role in determining the photophysical properties of Cu(I) complexes. 3 Traditionally bipyridyl or phenanthroline- based systems have been used but there is scope to rethink and redesign ligand systems. A synthetic protocol which can be extended to multiple systems is very attractive and new aromatic School of Chemistry, Trinity College Dublin, Dublin, 2, Ireland. E-mail: smdraper@tcd.ie; Fax: +353 1 671 2826; Tel: +353 1 896 2026 †Electronic supplementary information (ESI) available: Further crystal structures of [Cu(m-dppm) 2 L B ] 2+ and [Cu(m-dppm) 2 L A ] 2+ (Fig. S1, S2), 1 H NMR spectrum (Fig. S3) and 1 H– 1 H correlation NMR spectrum (Fig. S4) of [Cu(m-dppm) 2 L C ] 2+ , cyclic voltammogram of [Cu(m-dppm) 2 L A ] 2+ (Fig. S5), Comparative photophysical data of the ligands bppn L (Table S1). CCDC reference numbers 768169, 768170 and 824325. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c1dt10275c ligands generated by Diels–Alder cycloaddition have come to the fore in Ru(II) chemistry. 7–10 One development in copper chemistry has been the inclusion of stabilizing phosphine ligands, offering a significant improvement on the photophysical properties. McMillin and co-workers have demonstrated that bulky bidentate phosphines (such as POP = bis[2-(biphenylphosphino)phenyl]ether) inhibit the formation of quenching exciplexes, providing unusually long emission lifetimes and rather good quantum yields, typically in the green spectral window. 11,12 Another possibility is to increase the number of metal centers or to seek cooperative interactions between metal centers in a diimine-based system. 13 Only a few examples of binuclear systems exist and these use ligands such as bipyrimidine 14,15 or 2,5-bis(2-pyridyl)pyrazine. 16 One downside is that the overall rigidity is reduced, e.g. in a transoid disposition. Rigid systems are needed to prevent distortion and/or exciplet-quenching of the excited state and thus enhance the optical properties. 2,5- Bis(2-pyridyl)tetrazines, can favour cisoid conformation and as precursors via inverse electron demand Diels–Alder to pyri- dazines offer a synthetic procedure for the inclusion of new chromophores. 17,18 Results and discussion Synthesis Trans-stilbene and 1,4-bis(4-pyridyl)ethylene were each reacted with 3,4-bis(2-pyridyl)-1,2,4,5-tetrazine (bptz) in toluene by heating to reflux overnight in a sealed tube. On the loss of the characteristic pink colour of bptz, the products were 8320 | Dalton Trans., 2011, 40, 8320–8327 This journal is © The Royal Society of Chemistry 2011 Published on 12 July 2011. Downloaded by Trinity College Dublin on 24/01/2014 09:43:35. View Article Online / Journal Homepage / Table of Contents for this issue