Study of phase transitions in a bent-core liquid crystal probed by infrared spectroscopy Swapnil Singh a , Harshita Singh a , Anubha Srivastava a , Poonam Tandon a, *, Rahul Deb b , Somen Debnath b , N.V.S. Rao b , A.P. Ayala c a Department of Physics, University of Lucknow, Lucknow 226007, India b Chemistry Department, Assam University, Silchar 788011, Assam, India c Departamento de Física, Universidade Federal do Ceará, C.P. 6030, 60.455-900 Fortaleza, CE, Brazil A R T I C L E I N F O Article history: Received 29 December 2015 Received in revised form 28 April 2016 Accepted 26 May 2016 Available online 27 May 2016 Keywords: Bent-core liquid crystal Smectic C and nematic phase DSC POM TD-FTIR Conformational analysis A B S T R A C T A new four ring bent-core liquid crystal (4 0 -n-octylphenylazo)-phenyl-4-yl-3-[N-(4 0 -n-dodecyloxy-2- hydroxybenzylidene) amino]-2-methyl benzoate has been synthesized and investigated using differential scanning calorimetry (DSC), polarized optical microscopy (POM) and temperature- dependent Fourier transform infrared (TD-FTIR) spectroscopy. DSC and POM were used to detect transition temperatures and identify mesophases. TD-FTIR spectroscopic technique was utilized as a probe to analyze the dynamics of the liquid crystal molecules during the phase transition. The quantum chemical density functional theory (DFT) was used to predict the most stable conformation and the correct vibrational assignment of different modes. Combined theoretical and experimental techniques provided a clear picture of the structural modifications and dynamics of mesophases during the phase transitions. ã 2016 Elsevier B.V. All rights reserved. 1. Introduction Temperature dependent Fourier transform infrared (TD-FTIR) spectroscopy is a precise and sensitive tool for monitoring the structural, rotational and conformational (torsional) changes in molecules during phase transitions [1,2]. The combination of vibrational spectroscopy and quantum chemical approach has been widely used to understand the mechanism of phase transitions in liquid crystals at molecular level [3–5]. The crystal ! mesophase ! isotropic transitions and vice-versa result in a change in molecular orientation (that causes a change in inter/ intramolecular interactions) and thereby a change in dipole moment of the molecule. Therefore, infrared spectroscopy became a sensitive detection technique for the investigation of phase transitions. The fingerprint region is very informative regarding the structural changes and provides the specific information about the core (phenyl rings) and linking groups (azo, ester, imine) or functional groups (hydroxyl or H-bonding) of liquid crystals. The peak position, peak height and peak width are temperature sensitive therefore, very useful (especially for the liquid crystals) to investigate the dynamics of the molecule during the phase transition. The bent-core liquid crystals (BCLC) exhibits smectic phase [6] as well as nematic phase [7,8]. In the present work, a four ring bent-core liquid crystal [(4 0 -n- octylphenylazo)-phenyl-4-yl-3-[N-(4 0 -n-dodecyloxy-2-hydroxy- benzylidene) amino]-2-methyl benzoate] abbreviated as 12O(OH) 2MA8 possessing four phenyl rings linked through azo-ester-imine linkages was designed and synthesized. The central core of two phenyl rings are connected by an ester linkage followed by the extension with an azo linkage on one side and an imine linkage on the other side to connect the phenyl rings on either side of the central core. The imine linkage is stabilized by hydrogen bonding due to the presence of o-hydroxyl group. The presence of ortho hydroxyl group in benzylidene moiety enhances the stability of the imines through intramolecular hydrogen bonding to overcome the hydrolytic instability of the molecules towards moisture and also enhances the transverse dipole-moment. The resorcylidene core is more stable towards to atmospheric hydrolysis. The liquid crystalline phases (smectic to nematic or nematic to isotropic) exhibited by these materials possessing a photochromic azo group can be induced by the trans-to-cis isomerization under the influence of UV irradiation [9]. The azo group in the molecule also provides delocalization of p electrons between donor and * Corresponding author E-mail addresses: poonam_tandon@yahoo.co.uk, poonam_tandon@hotmail.com (P. Tandon). http://dx.doi.org/10.1016/j.vibspec.2016.05.005 0924-2031/ã 2016 Elsevier B.V. All rights reserved. Vibrational Spectroscopy 86 (2016) 24–34 Contents lists available at ScienceDirect Vibrational Spectroscopy journal homepage: www.else vie r.com/locate /vibspec