Controlled self-assembly of chiral gelator molecules into aligned fibers induced by nematic to smectic B phase transitions Nataša Šijakovic ´ Vujic ˇic ´ a , Maja Šepelj b , Andreja Lesac b , Mladen Z ˇ inic ´ a, * a Laboratory of Supramolecular and Nucleoside Chemistry, Rudjer Boskovic Institute, Bijenicka 54, Zagreb 10000, Croatia b Laboratory for Stereoselective Catalysis and Biocatalysis, Rudjer Boskovic Institute, Bijenicka 54, Zagreb 10000, Croatia article info Article history: Received 8 April 2009 Revised 30 April 2009 Accepted 15 May 2009 Available online 23 May 2009 Keywords: Self-assembly Gelator Liquid crystalline gel Aligned fibers Controlled self-assembly abstract Chiral bisoxalamide 1 shows remarkable gelling capacity of the nematic and smectic B liquid-crystalline phases of heptylcyclohexanecarboxylic acid (HCCA). Chiral nematic-containing left-handed helical fiber bundles are formed if the gelator is present in amounts higher than 0.55 wt %. With lower amounts of 1, no nematic gel forms, however, a nematic to smectic B phase transition triggers instantaneous self- assembly of gelator molecules into aligned fibers. The latter liquid crystalline gel system represents an example of controlled self-assembly induced by a liquid crystalline phase transition. Ó 2009 Elsevier Ltd. All rights reserved. Research has shown that a small quantity of low molecular weight organic gelators can gel large volumes of various organic solvents or water, efficiently, giving organogels and hydrogels, respectively. 1 In addition, the group of Kato has reported on the formation of liquid crystalline gels (LC gels) where a liquid crystal- line phase was gelled playing the role of an ordinary solvent. 2 The latter systems represent a new type of soft materials which may possess induced or enhanced electro-optical, 3 photochemical, 4 and electronic 5 properties which are useful for the construction of supramolecular devices for advanced applications. LC gels repre- sent more complex systems consisting of a mesogen and a gelator each capable of giving different types of supramolecular organiza- tion at a specific temperature. Usually, the LC gels exhibit random or ordered phase-separated structures depending on the relation between sol–gel (T sol–gel ) and isotropic-LC (T iso-lc ) transition tem- peratures. When T sol–gel is higher than T iso-lc (Type I), a randomly dispersed network of gelator fibers is formed in the isotropic state of the liquid crystal. 6 In contrast, when T iso-lc is higher than T sol–gel (Type II), the anisotropic LC media serves as a template for aniso- tropic growth of gel fibers. 7,3a Until now, LC gels of nematic, smec- tic A and C, and discotic columnar phases have been reported which in some cases formed at a gelator concentration as low as 0.25 wt %. However, the behavior of gelator molecules in an orga- nized smectic B phase and its unexpected influence on the self- assembly of gelator molecules resulting in formation of highly aligned fibers have not been reported to date. Here, we report on the gelation of nematic and smectic B LC phases formed by mesogen heptylcyclohexanecarboxylic acid (HCCA) and provide evidence that gelation of each phase depends substantially on the concentration of chiral gelator 1. When 1 is present in concentrations higher than 0.55 wt % it induces initially the transformation of the nematic phase into a chiral nematic (cho- lesteric) phase which upon further cooling is gelled by 1. In con- trast, with 1 present in the concentration range 0.025–0.5 wt %, neither chiral induction nor gelation of the nematic phase was ob- served. However, the subsequent transition, at lower tempera- tures, of the nematic phase containing 1 into a homeotropically oriented smectic B phase triggers instantaneous self-assembly of the gelator molecules into aligned fibrous aggregates. We provide differential scanning calorimetry (DSC), FTIR, and electron micro- scopic evidence that the LC phase transition from nematic to orga- nized and viscous smectic B phase is able to trigger instantaneously the self-assembly of gelator molecules into fibrous aggregates and also serves as a template for their alignment. In continuation of our studies on oxalamide gelators, 8 a new type of chiral bisoxalamide gelator has been designed and their gelation ability and sol–gel transcription into various silica fibers examined 9 (Scheme 1). We also found that chiral gelator 1 shows remarkable gelation capacity for the liquid crystalline (LC) phases of HCCA. It is known that HCCA displays two mesophases, nematic and smectic B. 10 Addition of various amounts of gelator 1 resulted in the forma- tion of type II LC gels (T iso-lc > T sol–gel ). When 1 was used in the con- centration range 0.55–5.5 wt %, the mixture, upon cooling from an isotropic liquid displays a chiral (cholesteric) nematic phase 0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.tetlet.2009.05.058 * Corresponding author. Tel.: +385 1 456 1061; fax: +385 1 468 0195. E-mail address: zinic@irb.hr (M. Z ˇ inic ´). Tetrahedron Letters 50 (2009) 4430–4434 Contents lists available at ScienceDirect Tetrahedron Letters journal homepage: www.elsevier.com/locate/tetlet