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)], cyclopall adated complexes of general formula [Azo-6)Pd(mu-X)](2), (X = Cl, Br, I, N- 3, SCN, OAc) and [Azo-6)(2)Pd-2(mu-Ox)] (Ox = oxalate) have been synthesize d and investigated for mesomorphism and spectroscopic properties. Single-cr ystal X-ray analysis of the dinuclear bromo- and iodo-bridged complexes has been performed. The structural data, compared with those of the known homo logous chloro compound, show that all the [Azo(6))Pd(mu-X)](2))] (X = Cl, B r, I) molecules crystallize in the monoclinic space group P2(1)/c and are i somorphous. They are arranged in slipped pairs with intermolecular non-bond ing Pd-Pd contacts ranging from 3.668(1) Angstrom(X = CI) to 3.758(3) Angst rom (X = I). The different nature of the bridging group allows variation of the distance between the palladium atoms and the bond environment experien ced by the metal centers. Thus, this comparative study reveals that the eff ectiveness of the bridging group in promoting thermotropic mesophases is gr eater for chloride, bromide, azide or oxalate than for iodide, thiocyanate. or acetate. The greatest range of liquid-crystal behavior was displayed by [Azo-6)(2)Pd-2(mu-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 tw o cyclometalated [H(Azo-6)] moieties, which can be varied so as tot tune th e optical properties. Blocking the azo group in the trans position results in several cases in weakly luminescent complexes, with luminescence efficie ncies phi approximate to 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(mu-Cl)](2), from the mononuclear acetylacetonate (acac) complexes [(Azo-6)Pd(acac)] and [(Azoxy-6)Pd(acac)], and from the un complexed [H(Azo-6)1 and [H(Azoxy-6)] ligands, the nature of the excited st ates relevant to the photophysical behavior are discussed. Copyright (C) 19 99 John Wiley & Sons, Ltd.