Study of phase transitions in a bent-core liquid crystal probed by infrared spectroscopy

Vibrational Spectroscopy 86 (2016) 24–34

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Study of phase transitions in a bent-core liquid crystal probed by infrared spectroscopy Swapnil Singha , Harshita Singha , Anubha Srivastavaa , Poonam Tandona,* , Rahul Debb , Somen Debnathb , N.V.S. Raob , A.P. Ayalac a b c

Department of Physics, University of Lucknow, Lucknow 226007, India Chemistry Department, Assam University, Silchar 788011, Assam, India 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 (40 -n-octylphenylazo)-phenyl-4-yl-3-[N-(40 -n-dodecyloxy-2hydroxybenzylidene) amino]-2-methyl benzoate has been synthesized and investigated using differential scanning calorimetry (DSC), polarized optical microscopy (POM) and temperaturedependent 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

* Corresponding author E-mail addresses: [email protected], [email protected] (P. Tandon). http://dx.doi.org/10.1016/j.vibspec.2016.05.005 0924-2031/ã 2016 Elsevier B.V. All rights reserved.

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 [(40 -noctylphenylazo)-phenyl-4-yl-3-[N-(40 -n-dodecyloxy-2-hydroxybenzylidene) 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

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acceptor groups. This delocalization enhances the nonlinear optical properties of the liquid crystals that results in a secondharmonic generation [10]. The Schiff base exhibits E/Z isomerization that results into the conformational change during the phase transitions. The ester group also plays an important role and is involved in the conformational changes during the phase transition. Therefore, all these linking groups provide flexibility to the molecule during the phase transition while phenyl rings provide rigidness to the molecule. In the present communication, we focused on the design and synthesis of four-ring bent core compound exhibiting liquid crystalline phases followed by the identification of phases of 12O (OH)2MA8 by differential scanning calorimetry (DSC) and polarized optical microscopy (POM). The combination of both the techniques provides a clear picture about the mesophases of BCLC. TD-FTIR technique was used to analyze the mechanism of phase transitions and the associated changes at molecular level. Computational approaches following density functional theory (DFT) for conformational and vibrational analysis of the titled molecule. Herein, the focus is on the study of all the three phase transitions as they reflect clear and sharp signature in the TD-FTIR spectroscopy. This is the first time when both spectroscopy and DFT are coupled to study the dynamics of phase transitions in BCLC (such a huge system). 2. Experimental section 2.1. Synthesis The schematic representation of the synthesis of the titled molecule is shown in Scheme 1. The present four ring system consists of three different types of linkage units (azo  N¼N, ester  COO  and salicylidene  CH( C( OH)¼N)) bridging the phenyl rings to each other. The compound exhibits broad mesomorphic range with clearing temperatures below 140  C. (40 -n-octylphenylazo)-phenyl-4-yl-3-[N-(40 -n-dodecyloxy-2hydroxybenzylidene) amino]-2-methyl benzoate, 12O(OH)2M8: Appropriate quantity of 3-(N-4/-n-dodecyloxy-2-hydroxybenzylidene)-amino-2-methylbenzoic acid (0.44 g; 10 mmol) was dissolved in dichloromethane stirred with a teflon coated magnetic stirrer and the catalytic amount of N,N/-dimethylaminopyridine (DMAP 2 mg, 0.01 mmol) was added to the solution. Further a solution of 4-(40 -n-octylphenyl-azo) phenol (0.31 ml, 10 mmol) dissolved in dichloromethane was slowly added to the reaction

Scheme 1. Synthetic details of the compound, Reagents, and conditions: i). HCl, H2O, NaNO2, 0–5  C, Phenol, NaOH; ii). dry acetone, dry KHCO3, KI, C12H25Br, D, 24 h; iii). dry EtOAc, Pd/C 10%, iv) Abs EtOH, AcOH, D, 6 h; v). DCC, DMAP, dry DCM, Stirring 48 h, 25  C.

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mixture. To the resulting solution equimolar amount of N,N0 dicyclohexylcarbodiimide (DCC) (0.21 g, 10 mmol) was added and the mixture was stirred for 48 h under inert atmosphere at room temperature. After the completion of stirring, the N,N0 -dicyclohexylurea precipitate thus formed in the reaction mixture was filtered off. Evaporation of the solvent gave the crude product which was then recrystallized several times with ethanol to obtain the pure product as orange solid. Yield = 0.44 g, (72%). M. P. 131  C. FTIR nmax in cm1: 1608 (nCH¼N, imine); 1736 (nC¼O, ester), 3439 (nOH, Hbonded); 1H NMR (CDCl3, 500 MHz): d = 13.42 (s, 1H, OH); 8.37 (s, 1H, CH¼N ); 7.96 (d, 1H, J = 8.0 Hz, ArH); 7.93 (d, 2H, J = 8.8 Hz, ArH); 7.78 (d, 2H, J = 8.8 Hz, ArH); 7.33 (d, 2H, ArH); 7.31 (d, 2H, J = 8.8 Hz, ArH); 7.30 (t, 1H, J = 8.8 Hz, ArH); 7.24 (d, 2H, J = 7.8, ArH); 6.44(dd, 2H, J = 2.4 Hz, 7.6 Hz, ArH); 3.95 (t, 2H, J = 6.0 Hz, O CH2); 2.63 (t, 2H, J = 7.8, Ar-CH2); 2.61 (s, 3H, Ar-CH3); 1.59-1.60 (q, 4H,  CH2 CH2); 1.59-1.17 (m, 26H,  (CH2)2 ); 0.87 (t, 6H,  CH3). Elemental analysis calculated for C47H61N3O4: C = 77.12%; H = 8.40%; N = 5.74% Found C = 77.09%; H = 8.43%; N = 5.70%. 2.2. Measurements 2.2.1. Differential scanning calorimetry Differential scanning calorimetry (DSC) has been performed on a DSC 821e (Mettler Toledo, Switzerland) operating with version 5.1 of Stare software. The sample was heated from 25 to 149  C with the scanning rate of 5  C/min. 2.2.2. Polarizing optical microscope POM (Nikon Optiphot-2-pol, from INSTEC Inc. USA) and heating plate with covered slip were used to record the mesophases of the titled molecule. 2.2.3. Fourier transformed infrared spectroscopy Infrared spectrometer (Vertex 70, Bruker, Ettlingen, Germany) was used to record the spectra in different temperature range. A liquid nitrogen cryostat (Janis Research, Wilmington, USA) was employed to record FTIR spectra as a function of temperature. The pellets were prepared from the mixture of KBr and the sample in a 400:1 ratio using a hydraulic pressure machine. Attenuated total reflectance and diffuse reflectance sampling techniques provided similar spectra. 3. Computational methodology Conformational study and geometry optimization of the title molecule has been performed using DFT approach employing Becke-Lee-Yang-Parr hybrid functional (B3LYP) [11–13] and 6-31G (d) basis set. All the calculations were carried out using Gaussian 09 program [14] employing 6-31G(d) basis set augmented by ‘d’ diffuse functions on heavy atoms [15,16]. GaussView 05 [17] and ChemCraft [18] softwares were used for visualization of molecular structures and vibrations. The optimised energy and geometrical parameters (bond length, bond angle, and dihedral angle) of the most stable conformation were calculated at the same level of theory. The normal-mode calculations for each of the internal coordinates were done utilizing potential energy distribution (PED). For 115 atoms; a complete set of 339 internal coordinates was defined using Pulay’s recommendations [19,20]. The vibrational assignments of the normal modes were accomplished on the basis of the PED calculated using the program Gar2Ped [21]. Infrared spectra were simulated using a pure Lorentzian band profile (FWHM = 8 cm1) utilizing the indigenously developed software. PeakFitv4.1 software was used for analysis of the observed TD-FTIR spectra.

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Fig. 1. DSC thermograph of the 12O(OH)2MA8 recorded to determine the phase transition temperatures (  C) and liquid crystalline phase (confirmed by polarized optical microscopy); thermal range of the compound for heating (upward curve) and cooling (downward curve) cycles at 5  C/min from DSC and confirmed by polarized optical microscopy. The enthalpy (DH in kJ mol1) and entropy (DS in J mol1/K) are also presented.

4. Results and discussion 4.1. Thermal and mesophase analysis The compound 12O(OH)2MA8 exhibited three enantiotropic phase transitions and the DSC thermograph is displayed in Fig. 1. In heating cycle, the crystal transformed at 126.2  C (DH = 44.6 kJ mol1, DS = 111.7 JK1 mol1) to a viscous smectic phase followed by another transition at 128.1  C (DH = 0.13 kJ mol1, DS = 0.34 JK1 mol1) to fluid nematic phase and finally transformed at 136.7  C (DH = 0.35 kJ mol1, DS = 0.87 JK1 mol1) to isotropic phase. In cooling cycle, the isotropic phase transformed at 136.1  C (DH = 0.52 kJmol1, DS = 1.28 JK1 mol1) to fluid nematic phase followed by another transition at 114.6  C (DH = 0.58 kJmol1, DS = 1.52 JK1 mol1) to a viscous smectic phase and finally transformed at 101.5  C (DH = 37.1 kJmol1, DS = 99.1 JK1 mol1) to crystalline phase. Repeated DSC runs of consistent transition temperatures recorded in both the heating and cooling cycles confirmed the absence of thermal degradation. All the three phase transitions are confirmed by the POM studies. The optical characteristics of the sample are examined using POM with different assembled cells viz., (i) untreated glass plate with a coverslip (ii) aligned planar cell (HG) and (iii) homeotropic cell (HT). The last two are discussed in the supplementary section

(Figs. S1-S3). On cooling from isotropic phase, the optical textures of the sample sandwiched between an untreated glass plate with a coverslip observed under POM revealed 2- and 4-brush Schlieren texture (Fig. 2a and b) with threads as boundaries characteristic of the nematic phase below 135  C. Brownian motion accompanies the growth of Schlieren texture. On further cooling at 114.6  C the texture transformed to a characteristic texture of smectic C phase (Fig. 2c) and finally crystallised at 114.6  C. The lower temperature phase is found to be analogous to SmC phase, which is different from SmA phase. 4.2. Density functional theory, potential energy scan and conformational analysis The geometry optimisation and conformational analysis of the title molecule were performed using DFT/B3LYP/6-31G(d) method and the initial optimised molecular structure is shown in Fig. 3. In order to find out all the possible conformers, the one-dimensional potential energy scans (PES) along the dihedral angles w1 to w12 were performed and plotted (PES of w1-w4: Fig. 4; w5-w12: Fig. S4, Supplementry material) by varying the torsion angles at a step of 10 in the range of 0–360 rotation around the bond. As the side chains are in trans-form inferred from spectroscopic data PES is not performed on the side chains.

Fig. 2. Defect texture of the compound on a glass slide and coverslip during cooling from isotropic phase (a) majority 2-brush defect texture at 135  C; (b) 133  C at the same site; (c) during nematic- smectic phase transition (SmC) of the compound at 112.8  C.

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Fig. 3. Optimized structure of the most stable conformer I.

The PES of w1-w4 reflected the most stable five conformers with lowest energy (labelled as I–V in Fig. 4) that may exist at room temperature since their energy difference with the most stable conformer is found to be less than 0.5922 Kcal/mol. PES of w6 and w9-w12 gave lower energy conformers (labelled as VI–XI in Fig. S4, Supplementary material). In scans w5 and w8 no new conformer was found and hence the conformer with initial geometry was the most stable one. As expected, the scan along w7 did not provide any new conformation due to the symmetry of CH3 moiety. Thus, further calculations were performed on the most stable conformer I with minimum energy 2292.92288 hartree (Fig. 3). The geometrical parameters such as bond length, bond angle, and dihedral angle of the conformer I are presented in Table S1, Supplementary material. It is clear from Table S1 Supplementary material the azo group connecting the phenyl rings, R1 and R2, are all in the same plane (Fig. 3) that facilitates the maximum charge conjugation among the phenyl rings and azo groups. The alkoxy chains are nearly in the same plane and almost coplanar with neighboring phenyl ring (R4), while the alkyl chain and neighboring phenyl ring (R1) are not co-planar. In conformer I, strong

intramolecular hydrogen bonding formed between H52 atom of the hydroxyl group and N53 atom of imine group resulted in the formation of a planar six-membered ring. The interatomic distance between N53 and H52 is 1.740 Å that supports the strong intramolecular interaction of N    H which is in agreement with the previously reported work [22]. The molecular length and bent angle are measured as  44 Å and 143 , respectively and well supported by previous studies on bent-core molecules [23]. 4.3. Temperature-dependent FTIR In order to understand the changes in intra/intermolecular interactions and the resulting dynamics of the core as a function of temperature, TD-FTIR of 12O(OH)2MA8 has been recorded during heating cycle over the temperature range 25–149  C in the region 4000–400 cm1. In the FTIR spectra, changes during phase transitions Cr ! SmC, SmC ! N, and N ! Iso were observed at 127  C, 129  C, and 137  C and the associated thermal activation energies are 0.795, 0.799 and 0.815 kcal/mol, respectively. The FTIR spectra exhibited pronounced changes in

Fig. 4. One-dimensional potential energy scan along the dihedral angles, w1w4 plotted between potential energy and dihedral angle.

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Fig. 5. Temperature dependent FT-IR of compound recorded during the heating cycle from room crystal phase (25  C) to the isotropic phase (149  C) in the region 4000– 400 cm1.

spectral features (peak position, relative intensity, and peak width) with temperature (Fig. 5). The analysis of these spectral parameters had been performed to understand the dynamics of the molecules during the phase transitions. Voigt (Lorentz + Gauss) profile was used for the fitting of IR spectra. Subtle but progressive changes were observed during pre-melting (Cr ! SmC) stage while sharp spectral changes accompanied the SmC ! N and N ! Iso phase transitions. The peak assignment of observed room temperature spectrum was done using the theoretical IR spectrum (Fig. 6), and presented in Table S2, Supplementary material following the atomic labelling (Fig. 3). The molecule 12O(OH)2MA8 consists of two distinct parts: the four phenyl rings connected by azo-ester-imine linkage (N¼N COOC¼N) form one part called the core, while the other part consists of the long alkyl (C8H17) and alkoxy chains (OC12H25) attached at both ends of the core. Crystalline phase has three-dimensional periodic positional order, smectic phase exhibits both one-dimensional positional and orientational order and the nematic phase has only orientational order. The isotropic phase possesses random orientational order. SmC phase has layered structure that provide the lateral gap between the long flexible

chains facilitating free movement of these chains through diffusion between the layers, thereby producing strain in the core, especially on the peripheral phenyl rings. This results in slight changes in their individual wavenumber as well as broadening in the IR bands. Among the two mesophases, the molecular packing is stronger in the smectic phase compared to the nematic phase. These mesophases result in the relaxation of the molecular packing and sometimes induce intramolecular conformational changes. These conformations contribute to the changes in the environmental interaction that facilitates charge transfer within the molecule, and in turn reflects in spectral features. Smectic phase showed subtle spectral changes in some specific regions while nematic and isotropic phases showed distinct changes in spectral features throughout the spectra. TD-FTIR spectra revealed that some specific modes undergo prominent spectral changes while almost entire spectra showed slight variations (intensity and width) during the phase transitions and few of them are marker bands that respond strongly towards the phase transition. Herein, the entire spectra are separated in seven wavenumber regions and discussed separately one by one.

Fig. 6. Combine experimental and calculated (scaled) Infrared spectrum of 12O(OH)2MA8, at room temperature in the higher region (3700–2800 cm1) and a lower region (1800–400 cm1).

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Fig. 7. Fitted infrared spectra of 12O(OH)2MA8, over temperature range 25–149  C, for the higher region (780–920 cm1).

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4.3.1. The analysis of region 780–920 cm1 In this region six peaks related to the ring CH out-of-plane bending are observed at 788, 810, 832, 861, 881 and 909 cm1 and the corresponding calculated values are 782, 803, 841, 854, 891 and 901 cm1, respectively are presented in Table S2, Supplementary material. All these peaks are highly sensitive to the temperature and undergo spectral changes as shown in Fig. 5. To visualize clearly, the changes in spectral features of marker bands, IR spectra at various temperatures are fitted and shown in Fig. 7. The peak intensity decreases while peak width (FWHM) increases on heating. The marked changes are observed in spectral features especially in peak width at 127  C, 129  C, and 137  C. Initially very slight changes are seen in the spectra reflecting the premelting stage of the sample and on further heating, the marked changes are observed during Cr ! SmC, SmC ! N, and N ! Iso phase transitions. In addition to this, a new peak (915 cm1) originated at 137  C reflecting the signature of N ! Iso phase transition. Peaks at 810, 881 and 909 cm1 showed red shift while the peak at 832 cm1 showed blue shift during all the three phase transitions. To visualize clearly the changes in spectral features, the graphs are plotted (between spectral features and temperature) for one of the peaks at 810 cm1 that correspond to CH out-of-plane bending of ring R4 and presented in Fig. 8. To understand the intermolecular interactions and dynamics of the molecules at the molecular level and to visualize the environmental changes in both the phases, models are proposed (for both the mesophases) on the basis of observed spectroscopic signatures and available literature [24] (presented in Fig. 9(a and b). The environment of all the four phenyl rings is different in both the phases (see Fig. 9a) and b) indicating red shift with a broadening in the IR peak (Fig. 8). The observed changes in spectral features are reflecting the changes in chemical environment of molecular conformation (during the phase transitions) which results in the change of intra as well as intermolecular interactions with the neighboring moieties of the molecules. It can be clearly seen from both the models there are several interactions exhibited viz., inter-/intramolecular H-bonding, van der waal’s, electrostatic and Debye interactions in the mesophases and responsible for the packing of system. On heating, the crystalline phase switches to smectic phase further to nematic and finally to isotropic phase by losing these interactions and resulting change in molecular packing. The reasons for subtle spectral changes during Cr ! SmC phase transition might be due to the minor conformational changes (due to bulkiness of the molecules). Moreover, the nematic phase exhibits only orientational order and less ordered than smectic phase so the higher

freedom of molecules resulting abrupt changes in SmC ! N phase transition. 4.3.2. The analysis of region 940–1060 cm1 This region contains three peaks at 1019, 1006 and 976 cm1, which matched well with the calculated values at 1021, 1016, and 1000 cm1, respectively. The first two peaks correspond the C23– O56 stretching of end alkoxy chain and the last one corresponds to the ring CH3 rocking (Table S2, Supplementary material). The peaks at 1019 and 976 cm1 showed a small red shift at 127  C reflecting the signature of Cr ! SmC phase transition. The shoulder peak at 1006 cm1 disappeared at 129  C and a broad merged peak of 1017 and 1006 cm1 appeared at 1011 cm1. The peak at 975 cm1 showed broadening without shifting during SmC ! N phase transition. On further heating, these two peaks at 1011 and 975 cm1 showed a small red and blue shifts to 1010 cm1 and 977 cm1, respectively (Fig. S5, Supplementary material). The major change like blue/red shifts, appearance/disappearance of peaks, or drastic change in the intensity of some of the IR peaks arises due to a change in dipole moment, which is an indirect consequence of the local fields present in different liquid crystal phases. The changes in molecular geometry are mainly responsible for such changes, which modify the intrinsic dipole moment causing variation in the infrared intensity of the specific band. The spectral features of one of the marker band (C23–O56) at 1019 cm1 (presented in Fig. 8) showed remarkable changes in all the three parameters. On heating, the crystal melts with the loss in periodic positional ordered structure to form a smectic phase possessing new orientational order and layered structure. Further heating causes the loss of layered structure with a translational shift of molecules along the molecular axis result a change in molecular conformation to form a fluid nematic phase with diffusion of molecules that causes large changes in IR spectra. On heating, the charge distribution over the C23–O56 may shift towards the adjacent CH2 group resulted the increase in intensity in nematic phase and its intensity again higher in isotropic phase (Fig. 8). The molecular packing and intermolecular interactions are different in all the phases (crystalline, smectic, nematic and isotropic). The presence of strong molecular packing results in intermolecular interactions in smectic phase that are partially lost in nematic phase while completely lost in the isotropic phase that causes abrupt change in both the nematic and isotropic phase and might be responsible for the minor changes during Cr ! SmC phase transition. Thus, on the basis of the proposed model (Fig. 9) we can draw an idea about the dynamics of the molecules during these transitions.

Fig. 8. Variation of peak position*, IR intensity and peak width with a temperature of 12O(OH)2MA8 for most prominent bands as observed by temperature-dependent infrared spectroscopy. *Here, relative wavenumber are shown by taking tempearature induced wavenumber difference.

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Fig. 9. The proposed model for the molecules in (a) smectic C and (b) nematic phase.

4.3.3. The analysis of region 1060–1160 cm1 This region is extremely sensitive and provides a clear picture of the molecular structural transformation at the phase transitions [25] and [26]. The title molecule 12O(OH)2MA8 consists of four phenyl rings linked with azo-ester-imine linkages that favor the formation of inter or intramolecular hydrogen bonding [27]. The bands associated with the core such as CC stretching, CH stretching, CH in-plane bending of phenyl rings and CO stretching are informative and provide the knowledge about molecular packing as well the hydrogen bonding during the phase transitions. In this region, the spectral changes can be seen during all the three phase transitions at 127, 129 and 137  C (Fig. S6, Supplementary material). The five peaks at 1092, 1112, 1127, 1139 and 1150 cm1 in this region mainly corresponding to the CH inplane bending of the phenyl rings and are also in good agreement with calculated results (Table S2, Supplementary material). Peaks at 1127, 1112 and 1092 cm1 are red, blue, and blue shifted respectively at 127  C indicating the Cr ! SmC phase transition. However, rest of the peaks do not show any shifting during Cr ! SmC phase transition although peak intensity decreases with a broadening of the peak. On further heating, the peak intensity keeps on increasing with lowering in width (Fig. S6, Supplementary material) reflecting the change in inter/intramolecular interactions facilitating the charge transfer from donar to acceptor moiety. One of the peak at 1127 cm1 corresponding to the mixed mode of C8-O55 stretching and ring CH in-plane bending, transformed into a shoulder peak at 127  C and disappeared at 129  C during Cr ! SmC and SmC ! N phase transitions,

respectively. The abrupt change in the peak width as well in intensity is observed during the SmC ! N phase transition, which can be visualized clearly from Fig. S6, Supplementary material. All the four peaks at 1092, 1112, 1139 and 1150 cm1 showed consistent blue, blue, red and blue shifts, respectively during all the three phase transitions. One of the marker band at 1112 cm1 correspond to the ring R4 CH in-plane bending and the associated variations in spectral features are shown in Fig. 8. On analyzing both Fig. 8 and Fig. 9, the spectral changes originated from the change in molecular alignment plus molecular conformations (due to the relaxation in molecular packing) thereby affect the environment of phenyl rings (already discussed in 4.3.1). 4.3.4. The analysis of region 11601260 cm1 This region reflects the peaks of ring CC stretching, ring C6–O55 stretching and CH in-plane bending of the phenyl rings. Five peaks are observed at 1178, 1192, 1204, 1229 and 1248 cm1 and the corresponding calculated values are tabulated in Table S2, Supplementary material. The very strong peak in this region at 1204 cm1 is associated with coupled mode of ring C6-O55 stretching along with phenyl ring CC stretching, CH in-planebending and trigonal deformation of the ring. This region also showed the spectral changes during all the three phase transitions due to the change in relative orientation of four phenyl rings facilitating the intramolecular charge transfer [4]. Three peaks at 1204, 1229 and 1248 cm1 are red shifted to 1201, 1226 and 1246 cm1, respectively during Cr ! SmC phase transition and on further heating, the red shift is consistent with

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both the SmC ! N and N ! Iso transitions. The peak at 1192 cm1 slightly red shifted to 1191 cm1 during Cr ! SmC transition, however, this peak disappeared at 129  C reflecting the SmC ! N phase transition. A new peak (1261 cm1) originated at 137  C confirming the N ! Iso phase transition (Fig. S7, Supplementary material). The marked variation is observed in a peak at 1204 cm1 during SmC ! N transition that was associated with the phenyl C6-O55 stretching. To understand the dynamics of molecules, the scan graph w4 (Fig. 4) analyzed that suggested a possible molecular rotation (hindered) as evidenced from the highest potential barrier (maximum barrier height 0.756 kcal/mol) and the thermal activation energy at the Cr ! SmC phase transition (0.79 kcal/ mol). Thus, the possible molecular rotation along this bond facilitates the intramolecular charge transfer that leads a modification in the core conformation resulting changes in spectral features. 4.3.5. The analysis of region 1350–1500 cm1 In this region, there are nine peaks at 1341, 1355, 1381, 1395, 1409, 1458, 1474, 1492 and 1513 cm1. This region contains the peaks of ring CC stretching, CH rocking, CH2 wagging, CH2 scissoring and ring CH in-plane bending. Region 1486– 1454 cm1 particularly represents the CH2 scissoring and CH3 bending. Peaks at 1474 and 1458 cm1 are assigned to the CH2 scissoring. CH rocking of Schiff base is observed at 1354 cm1. All the nine peaks undergo spectral change during the Cr ! SmC phase transitions and few of them showed distinct changes during SmC ! N and N ! Iso phase transitions (1513, 1492, 1474, 1409, and 1395 cm1 Fig. S8, Supplementary material). Peaks at 1513 and 1492 cm1 are mainly associated with coupled mode of ring CC stretching and CH in-plane bending of rings R4 and R2, respectively. These modes showed distinct changes in peak intensity as well in width clearly reflecting that the core no longer remains rigid. It can be attributed to the free rotation of the associated linking groups that augment the charge transfer between the donor and acceptor moieties. Similarly, peaks at 1409 and 1395 cm1 showed pronounced changes in spectral features. These are mixed modes of CC stretching as well as CH inplane bending of ring R4 and R3 respectively. Peaks at 1474 and 1458 cm1 correspond to the CH2 scissoring mode of end chains and get intensified with the red shift on heating (Fig. S8, Supplementary material). The blue and red shifts in the peak position during the phase transitions are seen and justifiable as the change in molecular conformation as well alignment of the molecules is responsible for these spectral changes. 4.3.6. The analysis of region 1550–1800 cm1 This region contains five peaks at 1562, 1585, 1608, 1623 and 1736 cm1. A distinct sharp intense peak at 1736 cm1 is assigned to C¼O stretching mode. The C¼N stretching mode is assigned to intense peak at 1608 cm1. The medium intense peaks at 1562, 1585 cm1 and a shoulder peak at 1623 cm1, are associated with ring CC stretching. All the observed peaks are found to be in good agreement with the calculated values and presented in Table S2, Supplementary material. The modes corresponding to the polar moieties (C¼O and C¼N stretching) and ring CC stretching showed distinct changes during the phase transitions. Analysis of these modes depicts that the C¼O group is not involved in hydrogen bonding up to isotropic phase transition while, C¼N is involved in intra-molecular hydrogen bonding even in isotropic phase which is also in agreement with previous report [24]. Initially, the three peaks at 1608, 1623 and 1736 cm1 show a slight variation in the peak position as well as in the intensity on heating; it can be termed as the warm up or pre-melting period during the region (25–127  C). At 127  C we observed slight

changes in spectral features that were due to Cr ! SmC phase transition (Fig. S9, Supplementary material). On further heating, at 129  C, the peaks undergo distinct changes in peak position by a large magnitude of 4 cm1 (FWHM 4) in C¼O stretching, 8 cm1 (FWHM 4) in ring CC stretching and 4 cm1 (FWHM 3) in C¼N stretching. The pronounced blue shift in the peak position as well decrease in intensity reflects the phase transition from SmC ! N phase. The blue shift in C¼O peak (on heating) from 1736 to 1738 cm1 and further to 1744 cm1 in nematic phase with broadening suggests the breaking of H-bond between the carbonyl oxygen and H-atoms of the adjacent molecules. This also supports changes in conformation of the molecule around the C6-O55-C8 bond of the ester group between the phenyl rings is the characteristic of Cr ! SmC transition [25]. On release of hydrogen bonding, torsional rotation takes place to achieve stable configuration. The band at 1744 cm1 corresponds to a new rotational isomer of 12O(OH)2MA8 in which the phenyl rings must have rotated to make the tilted structure different from that of crystal phase. Similarly, the changes in rest of the peaks at 1562 and 1584 cm1 can be visualized from Fig. S9, Supplementary material. Generally, the rings are considered as the rigid core and not much affected by these transitions but the marked changes in the ring CC stretching simply suggest that the rotation of linking groups facilitates the involvement of phenyl rings in the intramolecular charge transfer that causes changes in spectral features. The increase in temperature results in a consistent increase in line width with a decrease in intensity, however, at SmC ! N transition, the peaks undergoes a distinct change in the line width that represents the major changes in the structural modification as well large change in conformation of the molecules during the phase transition. These spectral changes arise with the change in microenvironment due to the rotation of the associated bonds especially; the rotation along the bonds C8–O55, ring C6–O55 and ring C12–N53 would have a greater impact on the conformation of the molecule as well responsible for the bent shape. 4.3.7. The analysis of region 2800–3500 cm1 The high frequency region, 2800–3500 cm1 revealed the presence of CH3/CH2 stretching vibrational modes along with ring CH and OH stretching modes. Due to the presence of several CH2 groups in the molecule, it is difficult to assign this region accurately. Therefore, the spectroscopic analysis of these stretching bands for such liquid crystalline systems has not yet been reported in many cases. Asymmetric and symmetric CH3 stretching modes are assigned to the medium shoulder peaks at 2958 and 2876 cm1, respectively. The intense peaks at 2919 cm1 and additional shoulder peaks at 2934 and 2900 cm1 were assigned to the asymmetric CH2 stretching. The symmetric CH2 stretching modes were observed as intense peak at 2850 cm1. The variations in the observed and calculated results are due to overlapping of several CH2 peaks or inter-chain interaction as already explained. The ortho substituted hydroxyl group in ring R4 is involved in intramolecular interaction with imine linkage and stretching mode of this group is observed at 3439 cm1. Three small peaks at 3105, 3089 and 3057 cm1 correspond to the ring CH stretching vibration. The corresponding calculated values along with assignments are presented in Table S2, Supplementary material. On heating, all the peaks in this spectral region exhibited changes with temperature (Fig. S10, Supplementary material). The shoulder peak at 2934 cm1 merged at 137  C and a broad peak appeared at 2926 cm1. The signatures of both transitions at 129  C and 137  C are clearly visible in this region and shown in Fig. S10, Supplementary material. The broad peak observed at 3439 cm1 reflecting the intramolecular hydrogen bonding between the hydrogen of O-hydroxyl moiety and nitrogen of imine

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Fig. 10. Infrared spectra of the compound were recorded for the both heating and cooling cycles, blue color spectrum corresponds at the room temperature (25  C), black color spectrum recorded at 149  C, and the red color spectrum was recorded after cooling the sample at 30  C. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

linkage became broader and less intense when heated up to 127  C. On further heating to 137  C, the intensity of broad OH stretching band is slightly decreased with no change in its peak position. It is suggested that the different molecules vibrate differently resulting in this broadening while no change in peak position suggest that the hydrogen bond is neither weakened nor destroyed during the heating process. This feature of the hydroxyl group is justified since the energy required (18 Kcal/mol) to break this strong hydrogen bond is larger than the supplied thermal energy (see graph w10 in Fig. S4, Supplementary material) and hence supports the geometry of the most stable conformer. 4.4. Reversible nature of the bent core liquid crystal DSC and POM studies confirmed the enantiotropic phase transitions with super cooling of transition temperatures in the cooling cycle and complemented the FTIR investigations. FTIR spectra are recorded at three different temperatures; at room temperature in crystalline phase (25  C), in the isotropic phase (149  C) and after annealing the sample to ambient temperature (30  C) and displayed in Fig. 10. A close inspection of the FTIR spectra at these temperatures revealed the reversible nature of the characteristic peaks. All the sensitive peaks related to the most stable conformer that are under investigation, are indentified and labelled in Fig. 10. Peaks of asymmetric stretching of methyl group, CH3 rocking, ring CC stretching and ring CH in plane bending are found to diminish on heating, which reappeared with similar features after annealing. However, few of them did not show any change after heating or cooling the sample. The intensity variation (I30/I25)400–1500cm1  0.99 in the lower region is conserved while after annealing the compound, marked variation is observed in intensity (I30/I25)1700–3000cm1 1.28 in the higher region. The slight changes observed in the sample were due to the annealing that leads to small but nonvanishing changes and showed the strong ordering and packing in the sample. Thus, it can be concluded that the compound 12O(OH)2MA8 exhibits reversible characteristics with no thermal degradation. 5. Conclusion The key points from the above studies are 1. A novel four-ring bent-core compound exhibiting nematic and smectic phases had been designed, synthesized and

characterised. The differential scanning calorimetry and polarized optical microscopic studies revealed that the newly synthesized four-ring bend-core liquid crystal 12O(OH)2MA8 exhibits three phase transitions viz., Cr ! SmC, SmC ! N, and N ! Iso at 127  C, 129  C, and 137  C, respectively. 2. The conformational analysis performed using density functional theory (DFT) predicted the most stable conformer. The peak assignment of experimental spectra was done using the same approach. 3. The observed spectral features of C¼N and OH stretching peaks demonstrated that, ‘the intramolecular hydrogen bonding is intact even in isotropic phase’ and ‘observed changes in spectra are associated with the minor conformational change and molecular alignment in different phases without causing major changes to the intramolecular hydrogen bonding’. This also supports our calculated results. 4. The analysis of observed changes in C¼O stretching peak revealed the C¼O group is involved in H-bonding in smectic phase while H-bonding is released in nematic phase, resulting conformational changes in microenvironment around C6 O55 C8 bond. The present study revealed the potential of combined FTIR and DFT approaches to study the liquid crystalline materials and to understand the mechanism of phase transitions. Acknowledgements S.S. and A.S. are thankful to University Grant Commission, New Delhi, India for financial support under BSR meritorious fellowship and Woman Scientist Scheme, respectively. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. vibspec.2016.05.005. References [1] J.F. Bardeau, A.N. Parikh, J.D. Beers, B.I. Swanson, J. Phys. Chem. B 104 (2000) 627–635. [2] R. Ogawa, Y. Miwa, S. Kutsumizu, J. Phys. Chem. B 119 (2015) 10131–10137. [3] R. Nandi, H.K. Singh, S.K. Singh, B. Singh, R.K. Singh, Spectrochim Acta Part A 128 (2014) 248–256.

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