Liquid Crystals Hydrogen-Bonded Banana Liquid Crystals** NØlidaGimeno,MariaBlancaRos,*JosØLuisSerrano,* andMariaRosariodelaFuente There are numerous examples in nature that demonstrate the crucial role of hydrogen bonds. Through imitation of this phenomenon, hydrogen bonds have been used to good effect by researchers to achieve specific synthetic targets. [1] Fields such as supramolecular chemistry and materials science are indebted to this strategy, [2] and important breakthroughs have been achieved by exploiting these inter- or intramolecular “strong and directional, and… lovely” [3] interactions. Liquid crystals, which are related to both of the aforementioned fields, are another example of the attractive possibilities of this interaction. [4] Mesomorphic properties result from a suitable combination of the shape of a molecule and the magnitude and direction of interactions between the mole- cules. Hydrogen bonding, through self-assembly, has been used to order thermotropic and lyotropic liquid crystals in which the magnitude and the direction of the interactions have been appropriate to maintain order within the fluid state. Calamitic and columnar mesophases of low or high molecular-weight materials have been stabilized with this approach. Such systems have also been obtained by the use of either mesogenic or non-liquid-crystalline moieties. In 1996, a new type of mesogenic material appeared in the field of liquid crystals: the so-called “banana-compounds”. [5] These mesomorphic materials are of interest from both an academic and a practical point of view. [6] These systems form a distinct class of liquid-crystalline compounds as they give rise to new types of mesophases that do not have analogues among classical calamitic phases. Interestingly, some of these compounds exhibit the unique feature of forming polar ordered mesophases with achiral molecules to provide anti- ferroelectric, ferroelectric or nonlinear optical responses— often with exceptional values for the relevant parameters. [7] Herein, we report a study that addresses a pertinent question that has not successfully been answered to date: is it possible to stabilize this type of mesophase through hydrogen bonding interactions? On the basis of the following results, the answer to this question is a resounding yes! Having taken into account the structural molecular requirements that 1) govern these new macroscopic liquid- crystal phase arrangements [5] and 2) allow appropriate H bonded complexation, we focused our interests on the V- shaped H acceptor structures 1 and 2 shown in Scheme 1 and two benzoic acid derivatives of varying lengths as the H donor moieties 3 and 4. The V-shaped 4’-stilbazoles (1 and 2) were prepared according to the synthetic route outlined in Scheme 2 (see also Supporting Information), and the acids (3 and 4) were prepared according to literature methods. [8] The synthesis of the desired bent complexes required the two components (H donor and H acceptor) to be mixed in precise equimolecular proportions in a common solvent (THF) followed by removal of the solvent. The formation of the complex was easily confirmed through polarizing optical microscopy measurements; the solid samples melted cleanly without the appearance of biphasic regions, which would otherwise have indicated the presence of nonstoichiometric complexes. Despite the nonmesomorphic nature of both of the H acceptor compounds studied, all of the complexes were liquid crystalline over temperature ranges that were different to the ranges at which the carboxylic acids displayed calamitic phases (Table1). More interestingly, the complexes exhibit textural features which are identical to those reported [8,9] and observed by us for SmCP (smectic C polar) mesophases. A schlieren texture and highly birefringent domains were observed upon cooling the sample from the isotropic liquid (Figure 1a). From a structure–activity point of view, the larger the number of aromatic rings, the broader and more stable the mesophases are. Furthermore, hysteresis of the solidification processes of around 20 degrees was also observed. The stability of the hydrogen bonding that leads to these banana complexes was also examined. The IR spectra of these materials (KBr pellets) show features that are characteristic of pyridine–carboxylic acid complexes. [4b,10] Thus, the forma- Scheme 1. General structures of the H acceptors (1 and 2) and H do- nors (3 and 4). [*] N. Gimeno, Dr. M. B. Ros, Prof. J. L. Serrano Dpto. Química Orgµnica, Facultad de Ciencias–ICMA Universidad de Zaragoza–CSIC, 50009-Zaragoza (Spain) Fax: (+ 34)976-761209 E-mail: bros@unizar.es joseluis@unizar.es Prof. M. R. de la Fuente Dpto. Física Aplicada II, Facultad de Ciencias Universidad del País Vasco, 48080-Bilbao (Spain) Fax: (+ 34)94-664-8500 [**] This work was supported by the Spanish Government (project CICYTMAT2003-07806-C02),theEuropeanUnion(FEDER),andthe Government of Aragón. Supporting information for this article is available on the WWW under http://www.angewandte.org or from the author. Angewandte Chemie 5235 Angew. Chem. Int. Ed. 2004, 43, 5235 –5238 DOI: 10.1002/anie.200460549 # 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim