J O U R N A L O F C H E M I S T R Y Materials Communication Calamitic smectic liquid crystalline supramolecular architecture from octaalkoxy-substituted PdII–g1-benzylideneaniline complexes Myongsoo Lee,* Yong-Sik Yoo, Moon-Gun Choi and Hong-Young Chang Department of Chemistry, Yonsei University, Sinchon 134, Seoul 120–749, Korea compounds.† Also, an X-ray crystallographic analysis performed for a model compound10 has confirmed that the Octaalkoxy substituted PdII–g1-benzylideneaniline complexes with a more disc-like shape can give rise to a calamitic smectic molecular structure appeared to be the PdII complex with g1- benylideneaniline.The mesomorphic properties of imine phase and low melting transition temperatures comparable to those of the corresponding free ligands. ligands 1, 2 and the complexes 3, 4 were studied using diVerential scanning calorimetry (DSC) and thermal optical polarized microscopy.Fig. 1 presents the DSC traces for the complexes obtained from the cooling scans. The thermal properties of the complexes are summarized and compared It is well documented that in general the number of peripheral with those of the corresponding ligands in Table 1.aliphatic chains connected to an aromatic core and the aniso- Both imine ligands exhibit only a crystalline melting and do tropic shape in metallomesogens are main factors in determin- not show liquid crystalline phase behavior. In contrast to the ing their supramolecular structure in the liquid crystalline thermal behavior of the ligands, the palladium complexes phase.1–7 Lamellar arrangements of liquid crystalline phases exhibit a liquid crystalline phase.Complex 3 with hexyloxy are formed for molecules with two to four peripheral chains, peripheral chains melts into an isotropic liquid at 91.8 °C. On whereas columnar arrangements are exhibited by metallomeso- cooling from the isotropic liquid, first bato�nnet-like growth of gens with six or more flexible aliphatic chains peripherally texture can be observed with a final development of focal conic connected to a central aromatic core.For example, dinuclear domains which are characteristic of a smectic A mesophase orthopalladated imine complexes with tetrasubstituted ali- exhibited by conventional calamitic mesogens. Thus, the phatic chains are reported to show calamitic smectic phases.8 On the other hand, the presence of four additional chains in † Supplementary material (SUP. 57325, 4 pp.) available from the the aromatic core of dinuclear orthopalladated complexes British Library. Details are available from the editorial oYce. produces a more disc-like molecular shape, thus leading to the formation of the discotic mesophase.9 This can be explained by the fact that the main factor governing the geometry of supramolecular structure in the liquid crystalline phase is the ratio of the volume occupied by the rigid core to the volume filled by the flexible peripheral substituents of the molecules.Here we report on the first examples of more disc-like octaalkoxy-substituted mononuclear PdII metallomesogens with g1-benzylideneaniline giving rise to a calamitic smectic A mesophase and low melting transition temperatures comparable to those of the corresponding free ligands in contrast to those of palladium(II ) orthometallated complexes.8 The mononuclear palladium complexes with monodentate imine ligands without orthocyclopalladation were synthesized by a ligand exchange reaction with [Pd(PhCN)2Cl2] and the corresponding imine ligands as shown in Scheme 1.The complexes were purified by column chromatography (silica gel ) using methylene chloride as eluent, and were obtained in 44–53% yields. Satisfactory analytical data were obtained for all the Fig. 1 DSC traces (10 °C min-1) recorded during the first cooling scans of 3 and 4 Table 1 Thermal transitions of the imine ligands and the Pd complexes (K, crystalline phase, SA, smectic A phase, I, isotropic phase) phase transitions/°C and corresponding enthalpy changes/kJ mol-1 compound heating cooling 1 K 84.2 (45.2) I I 71.0 (44.6) K 2 K 95.5 (90.1) I I 74.4 (90.7) K 3 K 91.8 (24.5) I I 61.2 (1.5) SA 50.3 (0.9) K RO RO N RO RO RO RO N RO RO Pd N OR OR OR OR Cl Cl Pd(PhCN)2Cl2 n-hexane R = CH3(CH2)5 1 CH3(CH2)9 2 R = CH3(CH2)5 3 CH3(CH2)9 4 4 K 79.7 (19.3) I I 64.5 (4.3) SA 55.0 (13.2) K Scheme 1 Synthesis of PdII complexes with g1-benzylideneaniline J.Mater. Chem., 1998, 8(2), 277–278 277performed on the mesophase of complex 4. The X-ray pattern of 4 exhibits two Bragg reflections in the small angle region with the ratio of positions of 152 and a diVuse scattering in the wide angle region as shown in Fig. 3. This result supports the existence of a disordered smectic mesophase. The existence of a layered smectic phase is in contrast with the normal behavior of disc-like octasubstituted metallomesogens which show a columnar type of supramolecular structure. The aromatic moieties of the imine ligands attached to a central metal will be twisted out of coplanar geometry to reduce steric repulsion.This distortion may provide a larger space for chains, and thus the peripheral chains are still not able to fill the space around the core eYciently. Consequently, the flexible chains tend to align in a preferred direction, leading to the lamellar self organization. This explains qualitatively the phase behavior of this palladium complex system.Fig. 2 Representative optical polarized micrograph (100×) of the texture exhibited by the smectic A mesophase of 4 at 64 °C on the Financial support of this work by the Non Directed Research cooling scan Fund, the Korea Research Foundation (1996) is gratefully acknowledged. We also acknowledge Mr. J. H. Im for the synthesis of a model compound and Prof. W.-C. Zin for X-ray powder diVraction experiments.References 1 Metallomesogens, ed. J. L. Serrano, VCH Publishers, Weinheim, 1996. 2 Q. M. Wang and D. W. Bruce, Angew. Chem., Int. Ed. Engl., 1997, 36, 150. 3 K. Outa, H. Akimito, T. Fujimoto and I. Yamamoto, J. Mater. Chem., 1994, 4, 61. 4 M. J. Baena, P. Espinet, M. B. Ros and J. L. Serrano, J. Mater. Chem., 1996, 6, 1291. 5 M. Ghedini, S. Morrone, O.Francescangeli and R. Bartolino, Chem. Mater., 1994, 6, 1971. 6 H. Zheng and T. M. Swager, J. Am. Chem. Soc., 1994, 116, 761. 7 S. N. Poelsma, A. H. Servante, F. P. Fanizzi and P. M. Maitlis, L iq. Cryst., 1994, 16, 675. 8 M. Ghedini, S. Licoccia, S. Armentano and R. Bartolino, Mol. Cryst. L iq. Cryst., 1984, 108, 269; M. Ghedini, S. Armentano, Fig. 3 Wide angle X-ray diVraction pattern for compound 4 at 60 °C F.Neve and S. Licoccia, J. Chem. Soc., Dalton T rans., 1988, 1565; on cooling M. Hoshino, H. Hasegawa and Y. Matsnaga, L iq. Cryst., 1991, 9, 267; J. Barbera, P. Espinet, E. Lalinde, M. Marcos and smectic A liquid crystalline phase of 3 is monotropic. Complex J. L. Serrano, L iq. Cryst., 1987, 2, 833. 4 with decyloxy peripheral chains shows similar thermal 9 K.Praefcke, D. Singer and B. Gundogan, Mol. Cryst. L iq. Cryst., 1992, 223, 181; D. Singer, A. Liebmann, K. Praefcke and behavior to 3 which exhibits a monotropic smectic A meso- J. H. WendorV, L iq. Cryst., 1993, 14, 785. phase. On heating, 4 melts into an isotropic liquid at 79.7 °C 10 We have synthesized bis[g1(N)-4-ethyloxybenzylidene-4¾-ethylox- which is an even lower melting transition temperature than yaniline]dichloropalladium(II) as a model compound in order to that of the corresponding ligand.The transition from the confirm the molecular structure by a single crystal X-ray structural isotropic liquid can be seen by the formation of a focal conic analysis. Full crystallographic details, excluding structure factors fan-like texture indicating a smectic A mesophase as shown in have been deposited at the Cambridge Crystallographic Data Centre (CCDC). See Information for Authors, Issue 1.Any request Fig. 2.11 Induction of comparable or even lower melting transto the CCDC for this material should quote the full literature ition temperatures in the complexes compared to those of the citation and the reference number 1145/75. ligands is very rare in calamitic metallosogenic systems, and 11 D. Demus and L. Richter, T extures of L iquid Crystals, Verlag is rewarding with respect to their potential processability. This Chemie, Weinheim, 1978; G. W. Gray and J. W. Goodby, Smectic is mainly due to disturbance of the crystal packing of the L iquid Crystals. T extures and Structures, Leonard Hill, Glasgow, molecules caused by the chlorine atoms which are out of the 1984. plane of the aromatic core. Communication 7/08128F; Received 11th November, 1997 Preliminary X-ray diVraction measurements have been 278 J. Mater. Chem., 1998, 8(2), 277–278