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: [Pd2(μ-SC10H21)(μ-O2CAzo)(L1,2)2] (7 and 8), [Pd2(μ-SC10H21)(μ-O2CAzo3C10)(L1,2)2] (9 and 10), [Pd(μ-O2CAzo)L1]2 (11), and [Pd(μ-O2CAzo3C10)L1]2 (12), in which L1 = p-H21C10OC6H3CHNC6H4OC10H21-p, L2 = p-H21C10OC6H3CHNC14H29, AzoCO2− = p-(phenylazo)benzoate, and Azo3C10CO2− = p-(2′,3′,4′-tris-n-decyloxyphenylazo)benzoate. Three of them (7−9), as well as the precursor Azo3C10CO2H (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 1H 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.

为获取光响应性液晶钯配合物，本研究合成了6种由环金属化亚胺（orthometalated imine）衍生得到、带有单或双偶氮羧酸桥基的钯配合物：[Pd2(μ-SC10H21)(μ-O2CAzo)(L1,2)2]（7、8）、[Pd2(μ-SC10H21)(μ-O2CAzo3C10)(L1,2)2]（9、10）、[Pd(μ-O2CAzo)L1]2（11）与[Pd(μ-O2CAzo3C10)L1]2（12）。其中，L1 = p-H21C10OC6H3CHNC6H4OC10H21-p，L2 = p-H21C10OC6H3CHNC14H29，AzoCO2− = 对（苯基偶氮）苯甲酸根，Azo3C10CO2− = 对（2′,3′,4′-三-正癸烷氧基苯基偶氮）苯甲酸根。其中3种配合物（7~9）及前驱体Azo3C10CO2H（3）为热致液晶，可呈现向列相与近晶A介晶相；而配合物10~12经X射线衍射（X-ray diffraction）表征，被证实可形成软晶相（soft crystal phases）。电子光谱学与氢核磁共振谱（1H NMR）结果显示，所有配合物在波长365 nm的光照下均可发生偶氮苯基团的反式-顺式异构化反应。分子结构决定了溶液中的光响应特性：当配体为三取代偶氮羧酸酯且偶氮基团的运动未受环金属化亚胺阻碍时，光响应更快且稳定性更强。在凝聚态相中同样可观测到光响应现象：除配合物10外，其余样品在光照下均会从有序相转变为各向同性液体；配合物10作为软晶相，可产生对光偏振高度敏感的永久性光取向（photoalignment）。该特性具有潜在应用价值，在小分子化合物中较为罕见。