Dinuclear Cyclopalladated Azobenzene Complexes: a Comparative Study on Model Compounds for Organometallic Liquid-Crystalline Materials

The series of dinuclear 4,4′-bis(hexyloxy)azobenzene, [H(Azo-6)], cyclopalladated complexes of general formula [Azo-6)Pd(μ-X)]2, (X = Cl, Br, I, N3, SCN, OAc) and [Azo-6)2Pd2(μ-Ox)] (Ox = oxalate) have been synthesized and investigated for mesomorphism and spectroscopic properties. Single-crystal X-ray analysis of the dinuclear bromo- and iodo-bridged complexes has been performed. The structural data, compared with those of the known homologous chloro compound, show that all the [Azo-6)Pd(μ-X)]2)] (X = Cl, Br, I) molecules crystallize in the monoclinic space group P21/c and are isomorphous. They are arranged in slipped pairs with intermolecular non-bonding, Pd-Pd contacts ranging from 3.668(1) Å(X = Cl) to 3.758(3) Å(X = I). The different nature of the bridging group allows variation of the distance between the palladium atoms and the bond environment experienced by the metal centers. Thus, this comparative study reveals that the effectiveness of the bridging group in promoting thermotropic mesophases is greater for chloride, bromide, azide or oxalate than for iodide, thiocyanate or acetate. The greatest range of liquid-crystal behavior was displayed by [Azo-6)2Pd2(μ-Ox)]. Remarkably, this compound is the first example of a metallomesogen containing the bridging oxalate group. The bimetallic complexes exhibit different absorption spectra (i.e. colors) depending, in general terms, on the nature of the bridge connecting the two cyclometalated [H(Azo-6)] moieties, which can be varied so as to tune the optical properties. Blocking the azo group in the trans position results in several cases in weakly luminescent complexes, with luminescence efficiencies φ ≈10-4 and luminescence lifetimes of the order of nanoseconds. Using the data obtained from the 4,4′-bis(hexyloxy)azoxybenzene [H(Azoxy-6)] derivative, [Azoxy-6)Pd(μ-Cl)]2, from the mononuclear acetylacetonate (acac) complexes [(Azo-6)Pd(acac)] and [(Azoxy-6)Pd(acac)], and from the uncomplexed [H(Azo-6)] and [H(Azoxy-6)] ligands, the nature of the excited states relevant to the photophysical behavior are discussed.