Luminescence Ranging from Red to Blue:  A Series of Copper(I)−Halide Complexes Having Rhombic {Cu₂(μ-X)₂} (X = Br and I) Units with N-Heteroaromatic Ligands

Abstract

A series of Cu(I) complexes formulated as [Cu(2)(mu-X)(2)(PPh(3))(L)(n)] were prepared with various mono- and bidentate N-heteroaromatic ligands (X = Br, I; L = 4,4'-bipyridine, pyrazine, pyrimidine, 1,5-naphthyridine, 1,6-naphthyridine, quinazoline, N,N-dimethyl-4-aminopyridine, 3-benzoylpyridine, 4-benzoylpyridine; n = 1, 2). Single-crystal structure analyses revealed that all the complexes have planar {Cu(2)X(2)} units. Whereas those with monodentate N-heteroaromatic ligands afforded discrete dinuclear complexes, bidentate ligands formed infinite chain complexes with the ligands bridging the dimeric units. The long Cu...Cu distances (2.872-3.303 A) observed in these complexes indicated no substantial interaction between the two Cu(I) ions. The complexes showed strong emission at room temperature as well as at 80 K in the solid state. The emission spectra and lifetimes in the microsecond range were measured at room temperature and at 80 K. The emissions of the complexes varied from red to blue by the systematic selection of the N-heteroaromatic ligands (lambda(em)(max): 450 nm (L = N,N-dimethyl-4-aminopyridine) to 707 nm (L = pyrazine)), and were assigned to metal-to-ligand charge-transfer (MLCT) excited states with some mixing of the halide-to-ligand (XL) CT characters. The emission energies were successfully correlated with the reduction potentials of the coordinated N-heteroaromatic ligands, which were estimated by applying a simple modification based on the calculated stabilization energies of the ligands by protonation.

Luminescent Complexes Containing Halogeno-bridged Dicopper(I) Unit {Cu2(µ-X)2} (X = Cl, Br, and I)

“…The geometries of the {Cu 2 X 2 P 2 N 2 } skeleton are almost the same among the complexes, and the bulkiness of the ligand is mainly reflected in the crystal packing. 11 The Cu£Cu distances in these complexes are in the range of 2.93.2 ¡, showing the flexibility of {Cu 2 X 2 } units and implying the minor contribution of Cu£Cu interaction which is usually observed for the complexes with a shorter Cu£Cu distance than the sum of van der Waals radii (<2.8 ¡). 3 The emission spectra of the complexes cover the entire visible region from blue to red by varying the N-heteroaromatic ligand L. 11,13 The emission maximum changes from 450 to 630 nm with pyridine and its derivative ligands (Figure 4).…”

“…The emission lifetimes of these complexes are in the¯s range showing the origins of emission as the triplet excited state. 11 The emission colors can be systematically tuned by the selection of N-heteroaromatic ligands, indicating that the emissive excited states of the complexes are ascribed to the triplet metal-to-ligand charge-transfer ( 3 MLCT) excited states. In fact, the linear correlation was observed between the reduction potential of the free ligand L and the emission maximum of the complexes with monodentate pyridine derivative ligands L, as shown in Figure 5.…”

“…11,12 Reflecting the closed shell configuration of d 10 copper(I) ion, Cu 3d orbitals mixed well with np orbitals of X ¹ exist as the HOMO and as occupied orbitals close to the HOMO, while the Cu 4s and 4p orbitals exist as unoccupied orbitals with other antibonding orbitals well separated from occupied orbitals in energy (Figure 6). Between these occupied and unoccupied orbitals, the ³* orbital of the N-heteroaromatic ligand occurs as the LUMO keeping an almost pure ligand nature, namely, without the contribution of metal orbitals.…”

4-(Dimethylamino)pyridinium trichlorido[4-(dimethylamino)pyridine-κN]cobaltate(II)

“…For applications and properties of DMAP, see: Araki et al (2005); Satgé et al (2004). For Co-N and Co-Cl bond lengths and angles in related compounds, see: Akbarzadeh Torbati et al (2010); Baker et al (1988).…”

“…DMAP is also known to form transition metal complexes which exhibit luminescence properties (Araki et al, 2005). We report here the synthesis and crystal structure of such a cobalt(II) complex with 4-(dimethylamino)pyridine.…”