Liquid Crystals Synthesis, characterisation

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Synthesis, characterisation, mesomorphic investigation and temperature-dependent Raman study of a novel calamitic liquid crystal: methyl 4-(4′-nalkoxybenzylideneamino)benzoate a

b

b

b

Rajib Nandi , Hemant Kumar Singh , Sachin Kumar Singh , Bachcha Singh & Ranjan K. Singh a

a

Department of Physics , Banaras Hindu University , Varanasi , 221005 , India

b

Department of Chemistry , Banaras Hindu University , Varanasi , 221005 , India Published online: 30 Apr 2013.

To cite this article: Rajib Nandi , Hemant Kumar Singh , Sachin Kumar Singh , Bachcha Singh & Ranjan K. Singh (2013) Synthesis, characterisation, mesomorphic investigation and temperature-dependent Raman study of a novel calamitic liquid crystal: methyl 4-(4′-n-alkoxybenzylideneamino)benzoate, Liquid Crystals, 40:7, 884-899, DOI: 10.1080/02678292.2013.791380 To link to this article: http://dx.doi.org/10.1080/02678292.2013.791380

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Liquid Crystals, 2013 Vol. 40, No. 7, 884–899, http://dx.doi.org/10.1080/02678292.2013.791380

Synthesis, characterisation, mesomorphic investigation and temperature-dependent Raman study of a novel calamitic liquid crystal: methyl 4-(4 -n-alkoxybenzylideneamino)benzoate Rajib Nandia, Hemant Kumar Singhb, Sachin Kumar Singhb, Bachcha Singhb* and Ranjan K. Singha* Department of Physics, Banaras Hindu University, Varanasi 221005, India; b Department of Chemistry, Banaras Hindu University, Varanasi 221005, India

Downloaded by [Banaras Hindu University BHU] at 01:10 09 September 2013

a

A new series of Schiff base calamitic liquid crystal; methyl 4-(4 -n-alkoxybenzylideneamino)benzoate (MABAB), H2n+1 Cn OC6 H4 C(H)=NC6 H4 COOCH3 (n = 6, 8, 10, 12, 14, 16) has been synthesised and characterised by elemental analyses, Fourier transform infrared spectroscopy (FT-IR), 1 H and 13 C Nuclear Magnetic Resonance (NMR) spectroscopy. The mesomorphic properties of these compounds were studied by differential scanning calorimetry (DSC) and polarising optical microscopy (POM). All members of the series exhibit enantiotropic smectic A (SmA) mesophase. Temperature-dependent micro-Raman study of one of the members, MABAB-10 has been employed to identify phase transitions and the molecular rearrangement therein. Analysis of Raman marker bands; C–H in-plane bending, C–C stretching of phenyl rings and –C(H)=N– linking group of core confirms the transitions clearly as observed through DSC and POM. An in situ Raman measurement of C–H in-plane bending mode has also been performed to visualise the molecular changes more clearly. The Raman study gives an evidence of induced co-planarity of rings at Cr→SmA phase transition. The density functional theoretical (DFT) optimisation of monomer, dimer and rotational conformer of MABAB-10 also support the induced co-planarity at Cr→SmA phase transition. Keywords: liquid crystals; smectic A mesophase; polarising optical microscopy; differential scanning calorimetry; temperature-dependent micro-Raman study

1. Introduction Liquid crystals (LCs) have been referred to as the curious state of matter and their impact on modern technology has been profound because of their unique properties, including the selective reflection of light and ferroelectricity. They have potential applications in numerous areas, especially in the field of optics, electro-optics, thermoconducting materials and fast switching [1–8]. Therefore, LCs are known as an extraordinary system, in that they continue to have a revolutionary technological impact and also consistently pose new challenges of fundamental interest. The characterisation of the liquid crystalline behaviour is usually achieved by three techniques: hot-stage polarising optical microscopy (POM), differential scanning calorimetry (DSC) and X-ray diffraction (XRD). POM is based on the birefringent property of LC materials and, therefore, one can get a unique textural pattern of each type of mesophase due to the interference phenomenon of light. DSC is based on fact that all the phase transitions (solid–mesophase, mesophase–mesophase and mesophase–isotropic phase) are accompanied by an enthalpy change (and consequently by an entropy change S = H/T). The mesophase–mesophase transition can also be a second-order transition, which ∗

results in a change of heat capacity (Cp). The magnitude of the enthalpy change is related to the amount of order that is lost or gained during the transition. In a DSC experiment, the enthalpy change related to a phase transition is measured. Another method for the characterisation of mesophases is XRD. X-rays interact with the clouds of electrons present in the mesophase and are scattered accordingly. The various scattered wavelets combine, undergoing constructive or destructive interference. The position, intensity and sharpness of diffraction peaks give specific information about mesophase pattern and, therefore, the structure of the mesophase can be revealed. Therefore, each characterisation technique is based on the unique properties of liquid crystalline materials and provides specific information about the mesophase [9,10]. Some of the unique properties of the LC material are its polarisability anisotropy and its anisometric shape. The anisometric shape induces the anisotropic intermolecular interactions responsible for the origin of various LC phases [10,11]. During the crystal to mesophase transition, the molecules gain translational as well as rotational freedom, and as a result, changes in molecular arrangement, intra/intermolecular interaction and molecular symmetry take place. Due to polarisability anisotropy and its anisometric shape,

Corresponding authors. Email: [email protected] (Bachcha Singh); [email protected] (Ranjan K. Singh) Rajib Nandi and Hemant Kumar Singh contributed equally to this study.

© 2013 Taylor & Francis


Liquid Crystals the Raman spectroscopy also has a great potential to reflect the molecular changes during phase transitions [12–19] in terms of some measurable parameters of marker. The phase transitions are more often associated with a peak shift, change in relative intensity and linewidth of marker bands. The precise study of linewidth changes may give information about the dynamical processes causing transition and the peak shifts together with intensity variation may give information about the static processes [20]. The DFT study, on the other hand, gives information about the chemical properties such as shape, size and conformational changes of the molecule that may be induced in the mesophases. In this article, we report the synthesis of a novel homologous series of Schiff base liquid crystalline system, methyl 4-(4 -n-alkoxybenzylideneamino)benzoate (MABAB); Cn H2n+1 C6 H4 –CH=N–C6 H4 C(O)OCH3 (n = 6, 8, 10, 12, 14, 16) in which a long alkyl chain is incorporated at the aldehydic moiety and a methyl ester derivative at the amine moiety to provide better polarity (and therefore, polarisability anisotropy) to the whole mesogenic molecule. Their mesogenic nature is investigated by DSC and POM. The optimised structures of all members, role of rotations of the linking group about a specific bond in the core part and presence of possible optimised dimers were investigated by employing density functional theory (DFT). The vibrational assignment and potential energy distribution of methyl 4(4 -n-decyloxybenzylideneamino)benzoate (MABAB10) were done for all experimentally observed Raman bands. In order to know the structural changes as well as changes in the intermolecular interaction at phase transitions, we have performed temperaturedependent Raman study of the MABAB-10 system. A detailed discussion of the core containing vibrations O (i)

HO

(ii)

RO

885

such as C–H in-plane bending of aromatic rings, C–N and C=N stretching of linking group and C– C stretching of rings gives an evidence of induced co-planarity of rings of MABAB-10 molecule at Cr→SmA phase transition. Also, the in situ Raman measurement of C–H in-plane bending mode supports the result. 2.

Methodology

2.1 Synthesis The synthetic route for the preparation of Schiff’ base ligands – methyl 4-(4 -n-alkoxybenzylideneamino)benzoate is outlined in Scheme 1. Alkylation of 4-hydroxybenzaldehyde with appropriate alkyl bromide in the presence of a mild base in 2butanone allowed us to obtain a reasonable yield of 4-alkoxybenzaldehydes. The Schiff bases MABAB 6–16 were obtained by the condensation reaction of methyl-4-aminobenzoate with appropriate alkylated aldehyde in refluxing ethanol. All the ligands were characterised by elemental analyses and standard spectroscopic techniques. The elemental data are in agreement with the stoichiometry of the compounds. The IR and NMR spectral data are fully consistent with the structure. The IR spectrum of methyl 4-(4 -decyloxybenzylideneamino)benzoate shows absorption bands at (2906–2857), 1712, 1630, (1590,1511), (1275,1250) cm−1 which are assigned to (aliphatic C–H), (ester, C=O), ν(–C=N), ν(Ph) and ν(OPh), respectively. The disappearance of the band at 1689 cm−1 due to ν(CH=O) and appearance of a new band at 1630 cm−1 due to ν(C=N) indicates the condensation of the aldehyde with the amine, forming a Schiff base. The proton NMR spectrum of methyl exhibits 4-(4 -decyloxybenzylideneamino)benzoate peaks at 8.35(s), 8.07–6.99(m), 4.05–4.1(t), 3.92(s),

O

a RO

O

O +

H2N OCH3

b O N RO

OCH3 R = C6H13, C8H17, C10H21, C12H25, C14H29, C16H33

Scheme 1. Reactions and reagents: (a) RBr (1.0 equiv.), K2 CO3 (3.0 equiv.), refluxing in 2-butanone, 24 h, 60–67%; (b) acetic acid (three drops), refluxing in EtOH, 4 h, 85–92%.

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1.84–1.36(m), 0.91(t) ppm, which are attributed to (–CH=N), ring, –COOCH3 , –OCH2, –[CH2 ]n and –CH3 protons, respectively. The signals observed at 8.35 ppm due to –CH=N indicate the condensation of the aldehyde with the amine, forming a Schiff base. The length of the alkyl chain has no significant effect on the position of the signals of –CH=N, ring, –OCH2 , –[CH2 ]n and –CH3 protons in the 1 H NMR spectra of the series.


2.2 Characterisation of mesophase 2.2.1 Differential Scanning Calorimetric (DSC) Measurement Phase transition and thermodynamic parameters of methyl 4-(4 -n-alkoxybenzylideneamino)benzoate (MABAB) series were determined from DSC thermograms in second heating and cooling cycle scans by a Mettler Toledo TC 15 TA differential scanning calorimeter at the rate of 5.0◦ C min−1 under nitrogen atmosphere using spec pure grade indium as standard by taking samples in close-lid aluminium pans. The transition temperatures from DSC thermograms have been determined with an accuracy of ±0.1◦ C.

2.2.2 Polarising Microscopic (POM) Study The mesophase type was identified by visual comparison with known phase standards using an HT 30.01 NTT 268 Lomo polarising optical microscope (POM) fitted with a hot stage with temperature controlling accuracy of 0.1◦ C. 2.2.3 Temperature-dependent Raman spectral Measurement Raman spectra of MABAB-10 were recorded on a micro-Raman setup from Renishaw, UK equipped with a grating of 2400 lines/mm and a Peltier-cooled charge-coupled device (CCD). GRAM-32 software was used for data collection. Accumulation time for one window was selected as 60 seconds and five spectra were accumulated in each window. The spectral resolution of the spectrometer with 50 μm slit opening was ∼1 cm−1 . An Ar+ laser line at 514.5 nm wavelength was used as the excitation source. A microscope from Olympus (Model: MX50 A/T) attached to the spectrometer focused the laser light on to the sample and collected the scattered light at 180◦ scattering geometry. The laser power was kept very low (5–10 mW) to avoid laser heating. The sample was kept in a quartz sample holder which was put on a temperature-controlled THM 600 hot stage (Linkam Scientific Instruments, Tadworth, Surrey, UK). The stage containing the sample was placed

on an automated X–Y stage below the Olympus long-distance 50× microscope objective. After setting the temperature to the desired value, sufficient time was given to stabilise the temperature before collecting the spectra. The reported accuracy in the measurement of temperature is ± 0.1◦ C. 2.2.4 Theoretical Approach All quantum chemical calculations were performed using DFT with Gaussian-03 program package [21]. The method adopted for this calculation was hybrid density functionals, that mixes the Becke three hybrid functionals for the exchange part and the Lee, Yang and Parr hybrid functional for the correlation part (B3LYP) [22,23] with appropriate level basis set 6–31G(d). The input structure was drawn and the output geometry as well as vibrational animations were observed in Gauss View 4.1 [24]. Potential energy distribution (PED) for accurate vibrational assignment was done using GAR2PED software [25]. The Raman activity for each band has been converted into the relative Raman intensity using a standard procedure [26,27]. 3.

Results and Discussion

3.1 Mesomorphic property 3.1.1 Differential scanning calorimetry (DSC) and polarised optical microscopic (POM) studies All the synthesised compounds are crystalline at room temperature but show liquid crystallinity at high temperatures. The thermotropic mesomorphism of the compounds was investigated by a combination of DSC and POM. The DSC thermograms of the compounds are shown in Figure 1, while few examples of their POM textures are given in Figure 2. In the first heating scan of MABAB-10, two sharp peaks appear at 99.2 and 119.9◦ C (Figure 1). The DSC thermogram recorded in the cooling scan exhibits two distinct peaks at 118.8 and 81.7 plus three overlapping peaks at ∼67.0◦ C. POM observations reveal that upon heating from the crystal state to 99.2◦ C, focal conic textures corresponding to the SmA mesophase emerge. Further heating of the compound leads to formation of fully grown focal conic SmA mesophase, having fluid property with higher viscosity and stratified structures with well-defined interlayer spacing. The interlayer attractions are weak and the layers are able to slide over one another relatively easily. The flexibility of the layers leads to distortions, which give rise to beautiful optical patterns known as the focal–conic texture. The molecules are aligned parallel to the layer normal, maintaining long-range orientational ordering and short-range positional ordering, which lead

Liquid Crystals

887

108.25

< exo Heat flow endo>

MABAB-16

SmA 112.37

k

115.15 MABAB-14

i

SmA

k

100.96 MABAB-12

k

MABAB-10

k

108.88

SmA 99.20

MABAB-8

119.92

119.14

88.82

60

i 117.62

k

40

i

SmA

k

MABAB-6

i

SmA 93.50

i

SmA

80 100 Temperature (°C) (a)

140

120

MABAB-16 91.12 SmA E 111.58 88.69

k MABAB-14

i

k′ k

< exo Heat flow endo>


i

106.07

i

SmA 86.49

k′

114.98

MABAB-12 k

k′′′ k′′

SmA E 74.10

i

102.78

k′ 73.21

MABAB-10 k

E k′′

MABAB-8 k

k″ k′

k′ 67.7

E 59.62

81.75

81.40

SmA

i 118.88

SmA

118.13

i

MABAB-6 k

40

k″

k′

E 51.82

60

84.01

SmA 116.40

80 100 Temperature (°C)

120

i

140

(b)

Figure 1. DSC thermogram of methyl 4-(4 -alkoxyenzylideneamino)benzoates: (a) heating cycle; (b) cooling cycle.

to the SmA mesophase. The molecular packings are found to be random for the SmA phase. A hexagonal pattern with a focal–conic texture is seen for this compound through optical microscopy. In the cooling cycle, the POM observation reveals the same textural observations which emerge from 118.8◦ C and solidify at 81.7◦ C. It thus becomes clear that the two overlapping peaks observed in DSC cooling scan are due to crystal transformation from one state to another (k–k transition); such polymorphism has often been observed in Schiff base-containing compounds [28]. Reheating regenerates the focal conic

SmA mesophase with similar pattern; that is, the mesomorphism is enatiotropic. The lower homologues MABAB-6 and MABAB-8 show similar phase and transition behaviour. The compound MABAB-12 with dodecyloxy chain length shows two exothermic peaks at 100.9 and 108.8◦ C in the second heating scan. The DSC thermogram recorded in the cooling scan exhibits two distinct peaks at 102.7◦ C and ∼73.0◦ C. The peak at ∼73.0◦ C is an overlap of five peaks at 74.1◦ C, 73.2◦ C, 72.06◦ C, 70.8◦ C and 69.3◦ C. The DSC thermogram of MABAB-14 with tetradecyloxy chain length shows


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(a)

(b)

(c)

(d)

Figure 2. (colour online) Optical textures of methyl 4-(4 decyloxybenzylideneamino)benzoate (MABAB-10) under a polarising optical microscopy. Heating cycle: (a) SmA mesophase at 107.3◦ C (b) SmA mesophase to isotropic transition at 117.2◦ C. Cooling cycle: (c) Isotropic to SmA mesophase transition 117.1◦ C (d) SmA mesophase at 110.7◦ C.

two transition peaks at 106.0◦ C and 115.1◦ C in the second heating cycle. Two exothermic peaks are observed at ∼114.9◦ C and 86.0◦ C in the second cooling cycle. Similar transition behaviour is also observed for hexadecyloxy derivative (MABAB-16) with two endothermic peaks at 108.2◦ C and 112.3◦ C and two exothermic peaks at 111.5◦ C and 91.1◦ C, respectively. The peaks at 86.4◦ C in MABAB-14 and at 88.6◦ C in MABAB16 result from two overlapping peaks. Therefore, in the MABAB homologous series, the degree of polymorphism (k–k transition) is three for the hexyl- to decylderivatives, MABAB-6→10, and then it increases to four for the dodecyloxy derivative MABAB-12, but on increasing the chain length further, it decreases to two for the higher homologues; MABAB-14 and MABAB-16. Table 1.

Table 1 summarises the thermodynamic parameters associated with the mesomorphic transitions of all the members of the MABAB series. Except at the clearing points of MABAB-12, all the SmA and crystal transitions are characterised by large enthalpy (H) and entropy changes (S), confirming that these transitions are first order in nature [29] and suggesting that liquid crystal dimers are acting as virtual mesogenic units [30,31]. Two mesogenic units may form a dimer through dipole interactions and weak hydrogen bonding (O.....H–C, N.....H–C) in a headto-tail antiparallel interdigitating fashion due to the polar nature of the methyl benzoate end group; this aspect is discussed again in the DFT section. As such dimer species are bulky in volume and anisotropic in shape, their packing/disassembling would involve a large change in the order of the system, thus giving the observed large S values [30,32]. Since the phase behaviour of all members of MABAB series is same, the reason for difference in H and S seems to be due to two stabilising factors, viz. dipole moment and hydrogen bonding. The dipole moment consistently increases but hydrogen bonding decreases with increasing chain length because of larger thermal agitation and bulkiness of the molecule. This will lead to different interaction of different magnitude in different members. It so happens that for MABAB-12, the intermolecular interaction is the least. The resultant effect is lowest for a moderate chain-length compound (n = 12) and therefore, it would have very weak interaction with the neighbouring layers and consequently, H and S are lowest at the clearing point. In the series of methyl 4-(4 -n-alkoxybenzylideneamino)benzoates (MABAB 6→16), the liquid crystal transition temperature decreases from the hexyloxy (MABAB-6) to decyloxy derivatives (MABAB-10) and remains almost same for the dodecyloxy (MABAB-12) to hexadecyloxy derivatives (MABAB-16) in the heating cycle. In the cooling cycle, the story is quite different as the

Thermal transitions and corresponding thermodynamic parameters of MABAB 6–16. T, ◦ C [H, kJ/mol; S, J/(mol K)]

No.

Compounds

Heating

1

MABAB-6

k 88.8 (39.3; 108.5) SmA 117.6(6.9; 17.8) i

2

MABAB-8

k 93.5 (47.6; 129.8) SmA 119.1(7.4; 19.0) i

3

MABAB-10

k 99.2 (52.1; 140.2) SmA 119.9(7.2; 18.5) i

4

MABAB-12

k 100.9 (46.6; 125.0) SmA 108.8(1.9; 5.0) i

5 6

MABAB-14 MABAB-16

k 106.0 (88.8; 234.5) SmA 115.5 (10.2; 26.3) i k 108.2 (82.3; 215.9) SmA 112.3(8.0; 20.9) i

Cooling i 116.4 (–7.0; –18.0) SmA 84.0 (–2.9; –8.3)E 51.8 (–28.1; –86.6) k 44.6 (-) k 44.8 (-) k i 118.1 (–7.4; –19.1) SmA 81.4 (–2.4; –6.8)E 59.6 (–35.8; –108.3) k 58.2 (-) k 55.5 (-) k i 118.8 (–7.3; –18.7) SmA 81.7 (–2.1; –5.9)E 67.7 (–42.1; –123.6) k 67.5 (-) k 66.2 (-) k i 102.7 (–5.0; –13.3) SmA 74.1 (-)E 73.2 (–46.1; –133.1) k 72.0 (-) k 70.8 (-) k 69.3 (-) k i 114.9 (–10.7; –27.6) SmA 86.4 (85.5; 238.0) k 83.0 (-) k i 111.5 (–9.0; –23.6) SmA 91.1 (–3.2; –8.8)E 88.6 (–81.5; –225.5) k 86.6 (-) k

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120

100

T (°C)

80

60


40

20

0

MABAB-6

MABAB-8

MABAB-10 MABAB-12 MABAB-14 MABAB-16

Figure 3. Comparison of mesomorphic behaviour of the compounds MABAB 6–16 (left, heating cycle; right, cooling cycle; grey part, crystal phase; patterned part, mesophase).

Figure 4. The optimised structure of MABAB-10 molecule using DFT method employing B3LYP/6–31G(d) functional.

transition temperature remains almost similar for the hexyloxy to decyloxy derivatives (MABAB 6→10) decreases for the dodecyloxy (MABAB-12) and tetradecyloxy (MABAB-14) derivatives and is the lowest for the hexadecyloxy derivative (MABAB-16). Thus, in the series of methyl 4-(4 -nalkoxybenzylideneamino)benzoates (MABAB 6→16), a random order of melting (T m ) and isotropisation temperatures (T i ) is found as shown in Figure 3. Although, the melting and isotropisation temperatures generally decrease with increase in the spacer length due to the plasticisation effect of the long alkyl spacers in mesogenic compounds. Therefore, it can be concluded that along with packing/disassembling, some intramolecular phenomenon is also playing a role during melting and isotropisation, which is confirmed from detail Raman Study.

3.2 DFT Study The various intermolecular interactions and induced intramolecular changes decide the symmetry and packing of the molecules which results in the LC phase. The single molecular optimisation, simulation

of vibrational frequencies, investigation of rotamers and possible dimer optimisation were done to investigate the single-molecule physical properties as well as possible interaction environment using the DFT method. We optimised all the members of the MABAB series. It helps to observe the effect of chain length on different dihedral angles as well as dipole moment. The optimised structure of MABAB10 molecule is shown in Figure 4. The optimised energy, dihedral C=N–C–C and dipole moment of all compounds of the series is given in Table 2. Dipole Table 2. Comparison of geometrical parameters of methyl 4-(4 -alkoxybenzylideneamino)benzoate (MABAB) series calculated from DFT calculation.

System

Optimised energy Dipole moment Dihedral angle (◦ ) (Hartree) (Debye) –C10 –N12 –C16 –C18 –

MABAB-6 MABAB-8 MABAB-10 MABAB-12 MABAB-14 MABAB-16

−1095.731 −1174.359 −1252.986 −1331.614 −1410.241 −1488.862

4.819 4.852 4.861 4.863 4.869 4.878

141.77 141.41 141.13 141.17 141.29 141.44

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Potential energy (kJ/mol)

–3,289,690 –3,289,695 –3,289,700 –3,289,705


–3,289,710 4.64 kJ/mol

–3,289,715 –3,289,720 –20

0

20

40

60 80 100 120 140 160 180 200 Dihedral angle (°)

Figure 5. Variation of energy with dihedral angle C10 –N12 –C16 –C18 of MABAB-10 molecule.

moment increases with increasing chain length at the single molecule level. The dihedral angle (–C10 –N12 – C16 –C18 –) for all members of the series is nearly the same (Table 2). This is because the dihedral angle between the two phenyl rings is not affected by variation of chain length in the gaseous phase. So changes in relative orientation between the phenyl rings in core is due to the change in intermolecular interaction in different phases. The dihedral angle of linking group C10 –N12 –C16 – C18 between two phenyl rings is an important parameter because it can change during phase transitions in LC systems [33]. To investigate the different stable rotational conformers, we have calculated energy by variation of the dihedral angle C10 –N12 –C16 –C18 in between 0–180◦ at 10◦ interval. The relative energy with respect to the optimised structure vs. dihedral angle is shown in Figure 5. From this, we see that the most stable conformer for MABAB-10 at the potential energy surface is at a dihedral angle of 141.13◦ and there is another local minimum at a dihedral angle of 60◦ with an energy difference of 7 kJ/mol and a barrier height of 10 kJ/mol with respect to the global minima. From this, we conclude that most of the molecules prefer to be present in the global minima in solid phase. The experimental room temperature and theoretical spectra of MABAB-10 LC are shown in Figure 6. The calculated harmonic wavenumbers were scaled [34] by a factor of 0.9612 in the region 500–2000 cm−1 in order to compare with the experimental spectra. A slight difference between the experimental and theoretical spectra is due to large intermolecular interaction in the crystalline phase and no interaction in single

molecular level calculation. Since this is a new LC system, the vibrational assignment has been done for further meaningful discussion in temperature-dependent Raman study in the next section. The potential energy distribution (PED) of experimentally observed Raman bands has been shown in Table 3. Due to the presence of ester as the terminal group and nitrogen atom in linking group, dimer formation as discussed in previous section is possible through hydrogen bonding (O.....H–C, N.....H–C) and van der Waals interaction. The typical strength of such hydrogen bonds is 2–10 kJ/mol and that for van der Waal forces is 1–3 kJ/mol, but these relatively small forces have contribute greatly to determine the mesophases in liquid crystalline systems [10,11]. We have optimised the most possible dimer structure as shown in Figure 7 in which weak H bonds (O.....H–C, N.....H–C) are also responsible for dimer formation. 3.3 Temperature-Dependent Raman spectroscopic study In order to know the change in the intra- and intermolecular interactions as well as molecular realignment with temperature of the above-synthesised LC series, we have recorded temperature-dependent Raman spectra of MABAB-10 system in both heating and cooling cycles from room temperature to 122◦ C. The spectra in heating and cooling cycles are the same. The DSC and POM study of MABAB10 have revealed Cr→SmA and SmA→isotropic phase transitions at 99.2◦ C and 119.9◦ C, respectively, in the heating cycle, and in the cooling cycle,

891

1619

1605

1572

800

1000

1639

1600

1727

1400

1604

1200 Raman Shift (cm–1)

1487 1507 1557

1298 1370 1416

600

1254 1261 1309

B3LYP/6-31(d) Scaled

1146 1155 1185

1582

1727

1494 1507

1417

1365

1287 1303

1242

1252

1163 1169 1189 1107

Experimental

1100


Raman Intensity

1591

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1800

Figure 6. (colour online) The experimental and DFT/B3LYP 6–31(d)-derived Raman spectra at room temperature of MABAB-10 molecule.

the isotropic→SmA and Cr→SmA transitions are observed at 118.8 and 81.7◦ C, respectively. The Raman signatures for Cr→SmA transition is observed between 98–100◦ C in the heating cycle and between 80–90◦ C in the cooling cycle. The Raman signatures for the Cr→SmA transition are prominent. A systematic study of peak position and linewidth variation of some Raman marker bands with temperature may give information on structural transformation. It may also reveal change in the interaction environment during the Cr→SmA transition. In next sections, we shall focus on temperature dependence of Raman bands in two regions, 1120–1240 cm−1 and 1520–1750 cm−1 in the heating cycle only. 3.3.1 Temperature-dependent region 1120–1240 cm−1 This region contains C–H in-plane bending and C– N stretching vibrations which are very sensitive for studying the molecular dynamics at phase transitions as reported in other LC systems [33,35]. At room temperature, the C–H in-plane bending mode has two components which are at 1163 cm−1 and 1169 cm−1 as shown in Figure 8. They correspond to calculated bands at 1146 cm−1 and 1155 cm−1 as given in Table 3. These two components of C–H in-plane bending mode are due to two phenyl rings having different neighbouring interactions. As it can be seen from the optimised structure of MABAB-10 molecule shown in Figure 4 the two phenyl rings are at an angle of 39.8◦ . It is

obvious that the interaction environments for C–H bonds belonging to two phenyl rings are different in the crystal phase. It is, therefore, likely that the C– H in-plane bending vibration of the two rings gives two separate bands till 98◦ C (Figure 8). On increasing the temperature to 100◦ C, the MABAB-10 molecules undergo Cr→SmA transition and a new band appears at 1157 cm−1 . There sometimes seems to be a sudden shift of 1163 cm−1 to 1157 cm−1 . In order to confirm whether it is a sudden shift or a new band appears at 1157 cm−1 and to understand the changes in C–H in-plane bending mode, we have performed in situ Raman measurement during the Cr→SmA transition. The results of the in situ measured spectra are presented in Figure 9 that contains five spectra during transition and one spectrum each just before and just after the transition. The spectra observed during in situ measurement were recorded at 1-s intervals, allowing the temperature to increase by 0.2 K in 1 minute. Figure 9 clearly shows the appearance of the new band at 1157 cm−1 and simultaneously the decrease in intensity of 1163 cm−1 and 1169 cm−1 during the Cr→SmA transition. In an earlier in situ measurement Raman study of TB7A [35] on the same C–H in-plane bending mode, it was also observed that the band at 1157 cm−1 is a new band. The 1157 cm−1 band also belongs to C–H in-plane bending mode of the MABAB-10 molecules that are arranged in the molecular structure typical of the SmA phase.

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Table 3. Vibrational assignments of some specific modes of methyl 4-(4 -decyloxybenzylideneamino)benzoate MABAB-10 by comparing the DFT-derived wavenumber and potential energy distribution (PED) obtained from DFT calculations..


Harmonic wavenumber scaled (cm−1 )

Experimental wavenumber Raman (cm−1 )

1727 1639 1604

1727 1619 1605

1582

1591

1557 1507

1572 1507

1487

1494

1416

1417

1370 1309

1365 1303

1298

1287

1261 1254 1185

1252 1242 1189

1155

1169

1146

1163

1108

1126

1104

1119

1100

1107

Assignmenta and PED ν(C26 O27 ) (84) – ν(C26 C23 ) (7) – ν(C26 O31 ) (5) ν(C10 N12 ) (54) + βIN (C10 H11 ) (11) − ν(C8 C10 ) (5) – δasym (C8 C10 N12 C15 ) (5) ν(C2 C4 ) (15) + ν(C3 C6 ) (13) + δasym (Ring I) (8) − ν(C17 C19 )(7) − ν(C18 C21 ) (6) − ν(C4 C8 ) (5) − ν(C6 C8 ) (5) ν(C17 C19 ) (13) + ν(C18 C21 ) (12) + ν(C18 C21 ) (11) – ν(C10 N12 ) (10) + δasym (Ring II) (7) − ν(C21 C23 ) (5) + ν(C2 C4 ) (5) + ν(C3 C6 ) (5) ν(C1 C2 ) (17) + ν(C6 C8 ) (17) − ν(C1 C3 ) (15) − ν(C4 C8 ) (15) + δasym (Ring I) (10) + ν(C10 N12 ) (5) S(C13 H14 H15 ) (16) – βIN (C3 H7 ) (11) + βIN (C4 H9 ) (9) + ν(C1 O60 ) (8) – βIN (C6 H62 ) (7) + ν(C6 C8 ) (7) + βIN (C2 H5 ) (7) − ν(C1 C3 ) (6) − ν(C1 C2 ) (6) − ν(C8 C10 ) (6) + ν(C4 C8 ) (6) βIN (C18 H22 ) (8) – βIN (C17 H20 ) (7) + βIN (C21 H25 ) (7) – βIN (C19 H24 ) (6) − ν(C17 C19 ) (5) + S(C33 H37 H38 ) (5) – S(C32 H34 H36 ) (5) − ν(C16 C17 ) (5) + ν(C21 C23 ) (5) ν(C2 C4 ) (26) − ν(C3 C6 ) (24) + βIN (C2 H5 ) (11) + βIN (C3 H7 ) (9) – βIN (C6 H62 ) (8) – βIN (C4 H9 ) (6) + βIN (C10 H11 ) (5) βIN (C10 H11 ) (46) + ν(C3 C6 ) (6) − ν(C10 N12 ) (5) − 151(4) – ν(C8 C10 ) (4) − ν(C18 C21 ) (4) ν(C6 C8 ) (11) + ν(C1 C3 ) (9) − ν(C4 C8 ) (7) − ν(C1 C2 ) (6) + ν(C2 C4 ) (6) − ν(C16 C17 ) (5) − ν(C19 C23 ) (5) + ν(C16 C18 ) (5) ν(C21 C23 ) (12) − ν(C19 C23 ) (12) − ν(C16 C17 ) (11) + ν(C17 C19 ) (10) − ν(C1 C3 ) (7) − ν(C6 C8 ) (5) + ν(C17 C19 ) (5) + ν(C1 C2 ) (5) ν(C23 C26 ) (27) − ν(C26 O31 ) (19) − ν(C1 O60 ) (8) ν(C1 O60 ) (35) + ν(C23 C26 ) (7) − ν(C3 C6 ) (6) − ν(C13 O60 ) (5) – τ(C13 H14 H15 ) (5) − ν(C26 O31 ) (5) ν(C16 N12 ) (22) + ν(C8 C10 ) (11) − ν(C3 C6 ) (7) + βIN (C19 H24 ) (6) − ν(C18 C21 ) (6) − βIN (C3 H7 ) (6) + δtri (Ring II) (5) βIN (C18 H22 ) (18) − βIN (C21 H25 ) (13) -βIN (C17 H20 ) (11) – βIN (C6 H62 ) (8) + βIN (C19 H24 ) (7) + βIN (C3 H7 ) (5) – ν(C18 C21 ) (5) − βIN (C2 H5 ) (5) βIN (C4 H9 ) (13) − βIN (C6 H62 ) (11) − βIN (C2 H5 ) (9) + ν(C16 N12 ) (7) − ν(C4 C8 ) (6) + ν(C8 C10 ) (6) − βIN (C18 H22 ) (6) + βIN (C3 H7 ) (5) + βIN (C17 H20 ) (5) δasym (C54 H57 H59 ) (9) –S(C32 H34 H35 ) (8) – S(C33 H37 H38 ) (7) – S(C36 H39 H40 ) (7) + ν(C50 C51 ) (7) – S(C41 H43 H44 ) (6) – S(C42 H46 H47 ) (6) − S(C45 H48 H49 ) (5) − S(C50 H52 H53 ) (5) – S(C13 H14 H15 ) (5) ν(O31 C28 ) (21) − ν(O31 C26 ) (13) + βIN (C19 H24 ) (13) + ν(C21 C23 ) (7) + ν(C18 C21 ) (6) − ν(C23 C26 ) (5) + ν(C19 C23 ) (5) βIN (C2 H5 ) (22) − βIN (C6 H62 ) (14) − ν(C16 C17 ) (14) – βIN (C4 H9 ) (13) + βIN (C3 H7 ) (11) + ν(C3 C6 ) (9)

Notes: a ν: stretching, μ: ring puckering, β: bending, γ : twisting, δ: deformation, ρ: rocking, τ: torsion, S: scissoring, W: wagging, IN: in plane, OUT: out of plane, sym: symmetric, asym: asymmetric, LIN: linear bending, RI: ring(1 3 6 8 4 2), RII: ring(16 17 19 23 21 18). All assignments have been done with respect MABAB-10 molecule shown in Figure 4.

Figure 7. Optimised dimer structure with weak hydrogen bonds.

The two components in crystal phase can be associated with two close potential minima. During the Cr→SmA transition, the band at 1169 cm−1 vanishes completely and an extremely small contribution from the 1163 cm−1 band remains. Thus, the double minima potential well can be assumed to converge to a single minima potential which leads

to a single band at 1157 cm−1 . In the SmA phase, the MABAB-10 molecules are loosely packed with one-dimensional positional order and orientational order compared to the crystal phase that is characterised by larger interaction and close molecular packing due to three-dimensional positional order as well as orientational order. The MABAB-10 molecules

Liquid Crystals ∼1157 cm–1

893

∼1189 cm–1

30.0°C

1150 1155 1160 1165 1170 1175 –1 Raman shift (cm )

∼1163 cm–1

1120

1140

1160

98.0°C 90.0°C 80.0°C 70.0°C 60.0°C 50.0°C 40.0°C 30.0°C

∼1169 cm–1

1180 1200 Raman shift (cm–1)

1220

1240

Figure 8. Temperature-dependent Raman spectrum of MABAB-10 at different temperatures in the range 1120–1240 cm−1 .

1157 cm–1

Raman intensity

99.4°C During transition


Raman intensity

122.0°C 115.0°C 110.0°C 100.0°C

98.4°C –1

1163 cm

1120

1140

1160

–1

1169 cm

1180

1200

1220

1240

1260

Raman shift (cm–1)

Figure 9. In situ observed Raman spectra at Cr→SmA transition of MABAB-10 molecule in the region 1120–1240 cm−1 .

acquire enough energy to cross over the barrier height between the two minima (giving 1163 cm−1 and 1169 cm−1 bands) as the transition temperature is approached and the MABAB-10 molecules with activation energy higher than the barrier height effectively experience a single potential leading to a single band at 1157 cm−1 . In terms of molecular geometry, below the transition temperature, the two phenyl rings being at angle of 39.8◦ , experience different intermolecular interaction, and above the transition temperature,

the molecules become more co-planar. The induced co-planarity is further estimated by drawing the potential energy vs. dihedral angle C10 –N12 –C16 –C18 of linking group curve shown in Figure 5. Thermal activation energy corresponding to Cr→SmA transition temperature at 99.4◦ C is 4.64 kJ/mol. It crosses the potential energy curve at 162.5◦ , which is close to co-planarity (180◦ ). The small discrepancy lies in the single-molecule approach in DFT calculation. Almost complete absence of 1163 cm−1 and 1169 cm−1 bands

R. Nandi et al.

in the SmA phase shows that most of the molecules are arranged in SmA phase structure, leaving almost no residue of crystalline structure. Another important point to be noted in Figure 8 is the higher shift of 1189 cm−1 band (C–N stretching) at Cr→SmA transition. During the Cr→SmA transition, the C–H in-plane bending mode is lower shifted by 6 cm−1 and C–N stretching mode is higher shifted by ∼1.5 cm−1 . Considering the fact that the wavenumbers of C–N stretching or C=N stretching bands shift higher and those of the bands associated with phenyl rings such as C–H in-plane bending and C–C stretching shifts lower, charge transfer takes place from phenyl rings to the linking group of MABAB10 molecules. The detailed discussion of C–C and C=N stretching bands will be given in the next section.

3.3.2 Temperature-dependent region 1520–1750 cm−1 We observed four intense bands in the region 1510–1750 cm−1 as shown in Figure 10. They appear at ∼1572 cm−1 , 1591 cm−1 , 1619 cm−1 and 1720 cm−1 . The two bands at ∼1572 cm−1 and 1591 cm−1 are due to C–C stretching vibration of the phenyl rings. In Wilson notation, they are named as 8a and 8b, respectively [36]. For pure benzene, these are two degenerate modes but on substitution, the degeneracy may be removed. They are also known as quadrant stretching vibration of aromatic ring [37]. The band towards the higher wavenumber side at 1619 cm−1 contains mainly C=N stretching motion of the linking group –C(H)=N–. There is another band at 1720 cm−1 , which is C=O stretching band of the MABAB-10 molecule.

∼1572 cm–1

∼1591 cm–1 ∼1720 cm–1 ∼1619 cm–1

Raman intensity


894

122.0°C 115.0°C 110.0°C 100.0°C 98.0°C 90.0°C 80.0°C 70.0°C 60.0°C 50.0°C 40.0°C 30.0°C

1530 1560 1590 1620 1650 1680 1710 1740 1770 Raman shift (cm–1)

Figure 10. Temperature-dependent Raman spectrum of MABAB-10 at different temperatures in the range 1510–1760 cm−1 .

All bands in this region give convincing signatures of Cr→SmA transition (Figure 10). In order to demonstrate the transition more clearly and to understand structure modification precisely, we fitted these bands as mixtures of Lorentzian and Gaussian profiles with Spectra calc Software and obtained precise values of peak position and linewidth. It is interesting to see that the two bands belonging to the phenyl rings; 1572 and 1591 cm−1 shift lower whereas the C=N band belonging to the linking group shifts higher at Cr→SmA transition as shown in Figure 11 (a) and (b). The C=O stretching band does not shift with temperature (Figure 11b). The higher shift of the linking group C=N and lower shift of bands associated with the phenyl rings gives evidence of charge transfer from the phenyl rings to the linking group region. Such charge transfer is usually associated with intramolecular rotation in liquid crystalline systems. The intramolecular strain on different bonds of the molecule changes, resulting in charge density shift from one part to the other. In the MABAB10 molecule, the bond through which intramolecular rotation is possible is C16 –N12 . If the rotation takes place through C16 –N12 bonds, the relative orientation of two phenyl rings changes, leading to identical intermolecular interaction in the SmA phase, which is evidenced from the symmetric single component of C–H in-plane bending mode. However, C–N stretching frequency increases slightly. If we assume rotation about the C–N bond, the frequency of the C–N bond should decrease. It qualitatively suggests that a large amount of charge from the two phenyl rings shifts to the linking group (C–C=N–C) and in this process, some amount of charge is given to the C–N region causing its frequency to increase. The C–N bond experiences two competing mechanisms; red shift by rotation and blue shift by charge transfer simultaneously at Cr→SmA transition. The net effect is blue shift of the C–N band. Another plausible reason for lower shift of C–C stretching of rings may be due to the fact that the rotation about the C–N bond causes the phenyl rings of different MABAB-10 molecules to come closer, resulting in a π − π stacking between neighbouring mesogen molecules. The π − π stacking induces the frequency to shift lower since the π electrons of the phenyl ring participating in the π − π stacking reduce the force constant of the C–C bond. The variation of linewidth with temperature of 1572, 1591 1619 and 1720 cm−1 band shown in Figure 12 increases suddenly at Cr→SmA transition. It is important to note that all the Raman bands associated with the core show a sudden increase in linewidth, indicating that the core does not remain rigid during the Cr→SmA transition that is already clear from rotation about the C–N bond. The

Liquid Crystals

895

1592 1572 cm–1 1591 cm–1 1590

Peak Position (cm–1)

1588

1586 1572

1568

1566 30

40

50

60


90

100

110

120

(a) 1724 1619 cm–1 1720 cm–1

1722 1720 Peak Position (cm–1)


1570

1718 1716 1624 1622 1620 1618 1616 30

40

50

60


90

100

110

120

(b)

Figure 11. Peak position variation with temperature of the bands: (a) ∼1591, 1572 cm−1 and (b) 1619, 1720 cm−1 .

experimentally observed Raman linewidth has contributions from both the intrinsic linewidth and the temperature-dependent reorientation part. Additional broadening may be introduced by the instrumental slit also. Since the linewidth of the bands studied is more than 6 cm−1 , the effect of slit function (∼1 cm−1 ) can be considered as negligible. The experimentally observed linewidth can then be expressed as the sum of

two contributions: G = Gi + Gt , where Gi is the intrinsic linewidth and Gt is the temperature-dependent contribution to the linewidth. Due to increase in temperature, there is a consistent increase in linewidth. At the Cr→SmA transition point, the sudden increase in the linewidth is due to the structural change. The C=O band is important because hydrogen bonding takes place through the O atom. The discontinuous

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R. Nandi et al. 18 1720 cm–1 16

1572 cm–1 1591 cm–1


Linewidth (cm–1)

14

1691 cm–1

12

10

8

6

4 30

40

50

60

70

80

90

100

110

120

Temperature (°C) Figure 12. (colour online) Variation of linewidth of 1572, 1591, 1619 and 1720 cm−1 bands with temperature.

broadening in C=O bond at the crystal →SmA transition is due to structural change as a result of breaking of hydrogen bonds. 4. Conclusions A Schiff base liquid crystalline homologous series methyl 4-(4 -n-alkoxybenzylideneamino)benzoates (MABAB) with n = 6, 8, 10, 12, 14, 16 has been synthesised and the mesomorphic behaviour has been analysed using DSC, POM, Raman and DFT techniques. All members show SmA mesophase with large values of enthalpy and entropy change during the transitions except for MABAB-12 at clearing point. The high values of H and S indicate the formation of dimer through weak intermolecular hydrogen bonding and dipole interactions. This fact has been confirmed by temperature-dependent Raman and DFT studies. An unusually low value of H and S during SmA→isotropic and isotropic→SmA transitions for a moderate chain length compound MABAB-12 might be caused due to decreased intermolecular interaction in the course of achieving stability by creating a balance between the increasing dipole moment and decreasing hydrogen bonding with increase in chain length. The vibrational study of the compound MABAB-10 was performed for the first time. Therefore, optimised structures and the vibrational assignment and PED of experimentally

observed Raman bands have been done. The most possible dimer structure was also optimised. The rotational isomers help understand the intramolecular rotation about the C–N bond. Temperature-dependent Raman study gave clear signature of Cr→SmA transition in both the heating and cooling cycles. The transition temperatures are also consistent with those observed in DSC. As a result of intramolecular rotation around the C–N bond of core, the wavenumbers of the C–H in-plane bending, C–C stretching of rings shifted lower and the bands corresponding to C–N and C=N vibration shifted higher at the Cr→SmA transition. The C–H in-plane bending mode has two components at room temperature and at Cr→SmA transition, a new band appears at 1157 cm−1 . The DFT calculation and in situ Raman measurement of C–H in-plane bending mode leads to the conclusion that in the crystalline phase, the two phenyl rings being at angle of ∼39.8◦ experience different intermolecular interaction and in the SmA phase, molecules of MABAB-10 become more co-planar getting almost identical interaction environment. The wavenumber shift of bands belonging to the core further gives the evidence of charge transfer due to intramolecular rotation about the C–N bond. This study also shows that Raman and DFT techniques combined with standard techniques to study the liquid crystalline behaviour are extremely useful to know the details at transitions.

Liquid Crystals 5.

Experimental Section


5.1 Materials Methyl-4-aminobenzoate, 4-hydroxybenzaldehyde, bromoalkanes from Aldrich Chemicals, USA were used as received. All other solvents and reagents were purchased from Merck. The solvents were dried using standard methods [38] when required.

5.2 Techniques Elemental analyses were performed on a CE440 Exeter Analytical CHN analyser. IR spectra (4000–400 cm−1 ) were recorded on a Varian 3100 FT-IR Excalibur spectrophotometer using KBr pellets. 1 H and 13 C NMR spectra were obtained on a JEOL FT-NMR AL 300 MHz spectrometer using tetramethylsilane as the internal standard and deuterated chloroform (CDCl3 ) as the solvent.

5.3 Synthesis 5.3.1 Synthesis of 4-decyloxybenzaldehyde To a solution (30ml) of 4-hydroxybenzaldehyde (0.61g, 5 mmol) in acetone, 1-bromodecane (1.03ml, 5mmol) K2 CO3 (1.037g, 7.5 mmol) and KI (catalytic amount) were added. The reaction mixture was refluxed for 24h.The residue was filtered off and the solvent was removed from the filtrate under reduced pressure. Yield: 84 %. IR (KBr, cm−1 ): 2937, 2858 (aliphatic C–H), 1689 (C=O), 1602, 1510 (Ph), 1311, 1259 (–OPh); 1 H NMR (CDCl3 , TMS): δH (ppm):9.88 (s, 1H, –CHO), 7.84-7.78 (d, 2H, ArH), 7.00–6.95 (d, 2H, ArH), 4.06–4.02 (t, 2H, –OCH2 ), 1.84–1.27 (m, 16H, –[CH2 ]8 ), 0.90–0.86 (t, 3H, –CH3 ). All other members of the homologous series 4hexyloxy-, octyloxy-, dodecyloxy-, tetradecyloxy- and hexadecyloxy benzaldehydes were prepared using the above procedures. 5.3.2 Synthesis of methyl 4-(4 -decyloxybenzylideneamino)benzoate (MABAB-10) 4-Decyloxybenzaldehyde (1.31 g, 5 mmol) and methyl4-aminobenzoate (0.755 g, 5 mmol) were mixed together in ethanol (10 ml). To this, 7–8 drops of glacial acetic acid was added as a catalyst. The reaction mixture was refluxed for 4h. The colour of solution became yellow. The yellow precipitate that formed on cooling was filtered off, washed with cold ethanol and recrystallised from ethanol–chloroform mixture (1/1,v/v). Yield: 72%. IR (KBr, cm−1 ): 2906–2857 (aliphatic C–H), 1712 (ester, C=O), 1630 (–C=N), 1590–1511 (Ph), 1275, 1250 (OPh); 1 H NMR

897

(CDCl3 , TMS) δH (ppm): 8.35 (s, 1H, –CH=N), 8.07 (d, J1 (H,H) = 8.4 Hz, 2H, –C6 H4 ), 7.85 (d, J1 (H,H) = 8.4 Hz, 2H, –C6 H4 ), 7.25 (d, J1 (H,H) = 8.4 Hz, 2H, –C6 H4 ), 6.99 (d, J1 (H,H) = 8.4 Hz, 2H, –C6 H4 ), 4.05–4.00 (t, 2H, –OCH2 ), 3.92 (s, 3H, COOCH3 ), 1.84–1.36 (m, 16H, –[CH2 ]8 ), 0.91 (t, 3H, –CH3 ). 13 C NMR (75 MHz, CDCl3 , 25◦ C): δc 166.84, 162.24, 160.85, 156.55, 130.77, 128.53, 126.88, 120.69, 114.69, 77.42, 76.57, 68.19, 51.95, 31.85, 29.51, 29.10, 25.95, 22.64, 14.08; Elemental analyses: calculated for C25 H33 NO3 (%), C, 75.91; H, 8.41; N, 3.54; Found, C, 75.80; H, 8.67; N, 3.39. 5.3.3 Synthesis of methyl 4-(4 -hexyloxybenzylideneamino)benzoate (MABAB-6) Yield: 74%. IR (KBr, cm−1 ): 2906–2857 (aliphatic C– H), 1713 (ester, C=O), 1628 (–C=N), 1590–1513 (Ph), 1275, 1250 (OPh); 1 H NMR (CDCl3 , TMS) δH (ppm): 8.35 (s, 1H, –CH=N), 8.06 (d, J1 (H,H) = 7.2 Hz, 2H, –C6 H4 ), 7.84 (d, J1 (H,H) = 7.5 Hz, 2H, –C6 H4 ), 7.25 (d, J1 (H,H) = 7.5 Hz, 2H, –C6 H4 ), 6.98 (d, J1 (H,H) = 8.1 Hz, 2H, –C6 H4 ), 4.05–4.01 (t, 2H, –OCH2 ), 3.92 (s, 3H, COOCH3 ), 1.82–1.52 (m, 8H, –[CH2 ]4 ), 0.94 (t, 3H, –CH3 ). 13 C NMR (75 MHz, CDCl3 , 25◦ C): δc 166.83, 162.21, 160.86, 156.55, 130.76, 128.53, 126.88, 120.68, 114.69, 68.19, 51.95, 31.85, 29.513, 25.95, 22.63, 14.08; Elemental analyses: calculated for C21 H25 NO3 (%), C, 74.31; H, 7.42; N, 4.12; Found, C, 75.10; H, 7.67; N, 4.29. 5.3.4 Synthesis of methyl 4-(4 -octyloxybenzylideneamino)benzoate (MABAB-8) Yield: 72 %. IR (KBr, cm−1 ): 2908–2857 (aliphatic C–H), 1715 (ester, C=O), 1628 (–C=N), 1591–1513 (Ph), 1275,1250 (OPh); 1 H NMR (CDCl3 , TMS) δH (ppm):8.35 (s, 1H, –CH=N), 8.07 (d, J1 (H,H) = 8.4 Hz, 2H, –C6 H4 ), 7.85 (d, J1 (H,H) = 8.4 Hz, 2H, –C6 H4 ),7.26 (d, J1 (H,H) = 8.4 Hz, 2H, –C6 H4 ), 6.99 (d, J1 (H,H) = 8.4 Hz, 2H, –C6 H4 ), 4.05–4.01 (t, 2H, –OCH2 ), 3.92 (s, 3H, COOCH3 ), 1.84–1.30 (m, 12H, –[CH2 ]6 ), 0.89 (t, 3H, –CH3 ). 13 C NMR (75 MHz, CDCl3 , 25◦ C): δc 166.84, 162.26, 160.85, 156.52, 130.77, 128.53, 126.88, 120.69, 114.69, 76.57, 68.19, 51.93, 31.85, 29.51, 25.95, 22.64, 14.08; Elemental analyses: calculated for C23 H29 NO3 (%), C, 75.17; H, 7.95; N, 3.81; Found, C, 74.90; H, 7.71; N, 3.70. 5.3.5 Synthesis of methyl 4-(4 -dodecyloxybenzylideneamino)benzoate (MABAB-12) Yield: 70 %. IR (KBr, cm−1 ): 2908–2854 (aliphatic C–H), 1717 (ester, C=O), 1629 (–C=N), 1591–1513 (Ph), 1275,1250 (OPh); 1 H NMR (CDCl3 , TMS) δH

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(ppm):8.35 (s, 1H, –CH=N), 8.06 (d, J1 (H,H) = 8.4 Hz, 2H, –C6 H4 ), 7.84 (d, J1 (H,H) = 8.4 Hz, 2H, –C6 H4 ), 7.20 (d, J1 (H,H) = 8.1 Hz, 2H, –C6 H4 ), 6.99 (d, J1 (H,H) = 8.7 Hz, 2H, –C6 H4 ), 4.02–3.92 (t, 2H, –OCH2 ), 3.84 (s, 3H, COOCH3 ), 1.83–1.27 (m, 20H, –[CH2 ]10 ), 0.882(t, 3H, –CH3 ). 13 C NMR (75 MHz, CDCl3 , 25◦ C): δc 166.81, 162.22, 160.86, 156.55, 130.78, 128.55, 126.89, 120.69, 114.69, 77.42, 76.57, 68.19, 51.95, 31.85, 29.53, 29.28, 29.10, 25.94, 22.64, 14.06; Elemental analyses: calculated for C27 H37 NO3 (%), C, 76.558; H, 8.803; N, 3.306; Found, C, 76.70; H, 8.67; N, 3.19. 5.3.6 Synthesis of methyl 4-(4 -tetradecyloxybenzylideneamino)benzoate (MABAB-14) Yield: 67 %. IR (KBr, cm−1 ): 2908–2854 (aliphatic C–H), 1717 (ester, C=O), 1629 (–C=N), 1590–1513 (Ph), 1275,1250 (OPh); 1 H NMR (CDCl3 , TMS) δH (ppm):8.38 (s, 1H, –CH=N), 8.07 (d, J1 (H,H) = 8.1 Hz, 2H, –C6 H4 ), 7.85 (d, J1 (H,H) = 8.4 Hz, 2H, –C6 H4 ),7.26 (d, J1 (H,H) = 8.4 Hz, 2H, –C6 H4 ), 6.99 (d, J1 (H,H) = 8.7 Hz, 2H, –C6 H4 ), 4.05–4.00 (t, 2H, –OCH2 ), 3.92 (s, 3H, COOCH3 ), 1.81–1.26 (m, 24H, –[CH2 ]12 ), 0.88 (t, 3H, –CH3 ). 13 C NMR (75 MHz, CDCl3 , 25◦ C): δc 166.82, 162.23, 160.86, 156.56, 130.77, 128.53, 126.88, 120.69, 114.69, 77.42, 76.58, 68.19, 51.95, 31.86, 29.51, 29.29, 29.11, 25.95, 22.64, 14.09. Elemental analyses: calculated for C29 H41 NO3 (%), C, 77.12; H, 9.15; N, 3.10; Found, C, 77.30; H, 8.97; N, 3.21. 5.3.7 Synthesis of methyl 4-(4 -hexadecyloxybenzylideneamino)benzoate (MABAB-16) Yield: 65%. IR (KBr, cm−1 ): 2908–2854 (aliphatic C– H), 1716 (ester, C=O), 1630 (–C=N), 1590–1513 (Ph), 1274,1250 (OPh); 1 H NMR (CDCl3 , TMS) δH (ppm):8.35 (s, 1H, –CH=N), 8.07 (d, J1 (H,H) = 8.4 Hz, 2H, –C6 H4 ), 7.85 (d, J1 (H,H)=8.7 Hz, 2H, –C6 H4 ), 7.26 (d, J1 (H,H) = 8.4 Hz, 2H, –C6 H4 ), 6.99 (d, J1 (H,H) = 8.7 Hz, 2H, –C6 H4 ), 4.05–4.00 (t, 2H, –OCH2 ), 3.92 (s, 3H, COOCH3 ), 1.85–1.26 (m, 28H, –[CH2 ]14 ), 0.88 (t, 3H, –CH3 ). 13 C NMR (75 MHz, CDCl3 , 25◦ C): δc 166.81, 162.22, 160.86, 156.55, 130.79, 128.54, 126.88, 120.68, 114.70, 77.42, 76.57, 68.20, 51.96, 31.84, 29.52, 29.28, 29.10, 25.95, 22.64, 14.09; Elemental analyses: calculated for C31 H45 NO3 (%), C, 77.62; H, 9.45; N, 2.92; Found, C, 77.50; H, 9.67; N, 2.79. Acknowledgements RN is grateful to the University Grants Commission, India for providing fellowship under Research Fellowship Scheme

for Meritorious Students. HKS (Junior Research Fellow), SKS (Senior Research Fellow) and RKS are grateful to the Council of Scientific and Industrial Research (CSIR), New Delhi, India, for financial support. BS is grateful to CSIR (Project No. 01(2550)12/EMR-II) for providing financial assistance.

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