First photoresponsive liquid-crystalline materials with small layer shrinkage at the transition to the ferroelectric phase Vladimı ´ra Novotn a, * a Ve ˇra Hamplov a, a Alexej Bubnov, a Miroslav Ka  spar, a Milada Glogarov a, a Nadia Kapernaum, b Samuel Bezner b and Frank Giesselmann b Received 3rd December 2008, Accepted 26th March 2009 First published as an Advance Article on the web 5th May 2009 DOI: 10.1039/b821738f A homologous series of liquid-crystalline compounds with an azo-group in the molecular core have been synthesized. A tendency for the layer shrinkage at the SmA–SmC* phase transition to decrease with decreasing alkyl chain length (n) was observed, with one homologue exhibiting almost no change in layer spacing. All studied compounds exhibited the photoferroelectric effect, namely a lowering of the spontaneous polarization and tilt angle in the SmC* phase under illumination by visible light. This effect is stronger for homologues with lower n and the time constant of this process, connected with the trans–cis isomerization of the azo group, follows the Arrhenius law. In addition to these effects, a lowering of the phase-transition temperature to the ferroelectric phase by a few Kelvin also occurs. Under sufficiently high light intensity, the ferroelectric SmC* phase completely disappears. After the light is switched off, the spontaneous quantities are restored to their original values. Introduction It has been found that the ferroelectric properties of liquid- crystalline compounds containing an azobenzene group in the molecular core can be significantly changed under the action of incident light. 1 This effect is a result of disturbing the ferroelectric ordering in the smectic C* (SmC*) phase by the light-induced trans-to-cis isomerization of the azobenzene group. 2–6 Such light- induced isomerization can provide photoswitchable ferroelectric materials, which exhibit both a photoresponse and high-speed ferroelectric switching, and thus may offer numerous applica- tions in optoelectronics, photonics, data storage, etc. The optical homogeneity and quality of devices based on ferroelectric smectic liquid crystals filled in thin planar cells is typically damaged by the occurrence of zig-zag defects, which arise due to the decrease of the smectic layer thickness at the SmA–SmC* phase transition. These defects separate opposite chevrons formed in the planar cell due to layer shrinkage. 7 Thirty years ago de Vries proposed a model for the SmA–SmC* tran- sition that is not connected with layer shrinkage. 8,9 This model assumes an SmA phase with molecules tilted by a certain average angle, the direction of this tilt (defined as the azimuthal angle) being randomly disordered. The transition to the SmC* phase is then connected with the ordering of the azimuthal angle without changing the magnitude of the average tilt. Recently, several compounds have been found that exhibit just a slight layer shrinkage at the SmA–SmC* transition, which may be attributed to de Vries-type behaviour. 7,10–12 We have prepared two series of ferroelectric liquid-crystalline materials, 13 with a molecular core containing three phenyl rings connected by ester groups, which exhibit a tendency to de Vries- type behaviour. 10 Recently, we have established different degrees of layer shrinkage in this series depending on the length of non-chiral alkyl chain. 14 In this paper we now present the first materials which combine photoresponsive ferroelectric proper- ties and de Vries-type behaviour. Materials A new series of compounds was synthesized with a photosensi- tive azo-group in the molecule core. We have modified the core studied before 13 by replacing one connecting ester group located far from the chiral centre with an azo group. The new homolo- gous series, denoted as nAHL (n is the number of carbon atoms in the non-chiral chain), has the general formula where n ¼ 6–12. The synthesis of this type of compound was described in our previous work on the mesomorphic properties of different types of rod-like azo compounds. 15 Results Mesomorphic properties The phase-transition temperatures and enthalpies in the absence of light were evaluated by Differential Scanning Calorimetry (DSC). Phase types were assigned from the observation in the polarizing microscope of characteristic textures and their changes under cooling from the isotropic phase. For texture observations the samples were placed in planar glass cells with transparent ITO electrodes. The sample thickness was fixed by spacer sheets of 10 mm thickness. The results are summarized in a Institute of Physics, Academy of Science of the Czech Republic, Na Slovance 2, CZ-182 21 Prague 9, Czech Republic. E-mail: novotna@fzu.cz; Fax: +42 (0)286 890 527; Tel: +42 (0)266 053 111 b Institute of Physical Chemistry, University of Stuttgart, Stuttgart, Germany; Fax: +49 (0)711-685-62569; Tel: +49 (0)711-685-64460 3992 | J. Mater. Chem., 2009, 19, 3992–3997 This journal is ª The Royal Society of Chemistry 2009 PAPER www.rsc.org/materials | Journal of Materials Chemistry