Photoisomerizable metallomesogens and soft crystals based on orthopalladated complexes

With the aim of obtaining light-responsive liquid-crystalline palladium complexes, six palladium complexes derived from an orthometalated imine, bearing one or two azocarboxylato bridges, have been synthesized: [Pd(2)(μ-SC(10)H(21))(μ-O(2)CAzo)(L(1,2))(2)] (7 and 8), [Pd(2)(μ-SC(10)H(21))(μ-O(2)CAzo3C(10))(L(1,2))(2)] (9 and 10), [Pd(μ-O(2)CAzo)L(1)](2) (11), and [Pd(μ-O(2)CAzo3C(10))L(1)](2) (12), in which L(1) = p-H(21)C(10)OC(6)H(3)CH═NC(6)H(4)OC(10)H(21)-p, L(2) = p-H(21)C(10)OC(6)H(3)CH═NC(14)H(29), AzoCO(2)(-) = p-(phenylazo)benzoate, and Azo3C(10)CO(2)(-) = p-(2',3',4'-tris-n-decyloxyphenylazo)benzoate. Three of them (7-9), as well as the precursor Azo3C(10)CO(2)H (3), are thermotropic liquid crystals displaying nematic and smectic A mesophases, while 10-12 have been identified by X-ray diffraction to give rise to "soft" crystal phases. Electronic spectroscopy and (1)H NMR show that all of them undergo a trans-cis isomerization of the azobenzene moiety at λ = 365 nm. The molecular structure determines the photoresponse in solution, which is faster and more stable when the trisubstituted azocarboxylate is present and the motion of the azo group is not hindered by the orthometalated imine. The photoresponse has also been observed in the condensed phases, which change from the ordered phase to the isotropic liquid upon irradiation, except for compound 10, a soft crystal in which a permanent photoalignment highly sensitive to light polarization is produced. The latter is a behavior with potential applications, rather unusual in low-molecular-weight compounds.