Morphology, Mesophase, and Molecular Order of 3-Hexyl Thiophene

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Morphology, Mesophase and Molecular Order of 3Hexyl Thiophene Based #-Conjugated Mesogens Santhosh Kumar Reddy Yanati, Nitin Prakash Lobo, Srinivasan Sampath, and Tanneru Narasimhaswamy J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b06169 • Publication Date (Web): 22 Jul 2016 Downloaded from http://pubs.acs.org on July 23, 2016

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

Morphology, Mesophase and Molecular Order of 3-Hexyl Thiophene Based

π-Conjugated Mesogens Y. Santhosh Kumar Reddy, † Nitin P. Lobo, ‡ Srinivasan Sampath,† T. Narasimhaswamy*† †

Polymer Laboratory, ‡Chemical Physics Laboratory, CSIR-Central Leather Research Institute, Adyar, Chennai 600020, India.

Abstract Molecular materials that are built with π-conjugated moieties which self-assemble to form organic semiconductors have gained greater recognition for their applications in organic electronics. By judicious molecular designing, the liquid crystallinity can be incorporated into them to realize π-conjugated liquid crystals. In this work, we report 3-hexyl thiophene based π-conjugated liquid crystals in which alkoxy biphenyls are introduced in to the core by palladium acetate catalyzed direct arylation. The π-conjugated molecules exhibit selfassembly in solution and enantiotropic nematic phase upon melting. Therefore, the morphological studies of molecules in solution by AFM and SEM, mesophase properties by HOPM and DSC as well as molecular order by solid state

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C NMR is performed. The

morphological studies indicate the formation of supramolecular fibers due to the selfassembly of molecules through assisted π-π stacking and van der Waals interactions. The static 1D and

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C NMR studies reveal the molecular organization and orientational order in

the nematic phase. Due to the asymmetry of the thiophene ring because of 3-hexyl chain, the orientational order in nematic phase is characterized by three values i.e. Szz,, Sxx-Syy and Sxz. The important feature of the

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C NMR investigations is the contrasting orientational

constraints for terminal octyloxy and lateral hexyl chains. The large difference in

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C-1H

dipolar couplings of the individual carbons of the terminal and lateral chains reveal that the mean orientation of former is in fully trans conformation while the lateral hexyl chain carbons prefer cis conformation in order to align with the core unit. A plot of alignment 1 ACS Paragon Plus Environment


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induced chemical shifts versus temperature follow the decreasing trend with rise in temperature. The optical absorption and emission in solution display that the absorption band is structure less while the fluorescence spectrum is structured with two maxima located at 422 nm and 445 nm which are independent of terminal chain length. The investigations point out that the 3-hexylthiophene in combination with alkoxy biphenyls can result in π-conjugated low melting mesogens which undergo self-assembly in solution besides displaying fluorescence emission. Introduction The functionalized thiophenes are one of the most often utilized families of molecules for application in opto-electronic as well as opto-photonic materials.1-3 The 3-hexyl thiophene has particularly attracted the attention and the oligomeric as well as polymeric materials of it showed great promise for OLED and OPV applications.4-6 Quite interestingly, 3-hexyl thiophene as part of the mesogenic core is not well exploited despite the extensive use of thiophene moiety for π-conjugated molecules.7,8 The insertion of hexyl chain at 3- position in thiophene renders the molecule to be suitable for polymerization at 2, 5- sites and also for further functionalization. There is also growing interest on thiophene based π-functionalized mesogens since they exhibit the fluorescence emission in addition to mesomorphism and accordingly are classified as light emitting liquid crystals.9-11 In contrast to π-conjugated solid state materials, the liquid crystals built with π-conjugated cores provide better organization and show excellent charge carrier mobility.12,13 Further, the mono domain samples of them offer improved alignment of the molecules and avoid charge trapping characteristic of grains in solid state.14,15 The underlying principle of constructing the thiophene based π-conjugated molecules is establishing the direct link between thiophene and other moieties of the core unit. However, the absence of linking units in the core unit enhances the phase transition temperature owing to the rigidity besides suppressing the solubility in common solvents.16,17

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Hence, a right balance of core length, location as well as span of the alkyl/alkoxy chains is crucial for realizing the π-functionalized mesogens. In this work, we report the results obtained from π-conjugated molecular mesogens in which 3-hexyl thiophene is central part of the core. By linking 4-alkoxy bromo biphenyls to both sides of thiophene ring through direct arylation, non-centro symmetric mesogens are accomplished. These molecules exhibit nano– segregation in solution as revealed by Atomic Force Microscope (AFM) and Field Emission Scanning Electron Microscope (FESEM) studies. Up on melting the molecular crystals, an enantiotropic nematic phase with low melting transition values is noticed. The synthesized mesogens are further characterized by UV–Visible and Fluorescence spectroscopy and high resolution solid state 13C NMR spectroscopy. To the best of our knowledge this work is the first attempt in understanding the molecular orientation of π- conjugated molecules by

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C

NMR in liquid crystalline phase. It is expected that the investigation of self-assembly in solution state and molecular organization in liquid crystalline phase provides unique opportunity to find the role of molecular design in realizing the end properties. Experimental Materials 3-Hexylthiophene, n -bromo alkanes (C6, C8, C10, C12), 4’-bromo biphenyl -4-ol, K2CO3, Pd(OAc)2, PCY3HBF4, pivalic acid were purchased from Sigma-Aldrich USA and used without further purification. Dimethyl formamide, chloroform, sodium hydroxide, anhydrous sodium sulfate, acetonitrile, dichloromethane, hexane, ethyl acetate, dimethyl acetamide and silica gel (100 - 200 mesh) obtained from Merck, India, and used as received. Instrumental details FT-IR spectra of the compounds were recorded on an ABB BOMEM MB 3000 spectrometer using a KBr pellet. Both 1H and

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C NMR spectra of the samples were run on a 400 MHz

Bruker Avance–III HD spectrometer at room temperature using CDCl3 as a solvent and 3 ACS Paragon Plus Environment


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tetramethylsilane (TMS) as an internal reference. The resonance frequencies of 1H and

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C

were 400.23 and 100.64 MHz respectively. Optical polarizing micrographs were taken using a Carl Zeiss axiocam MRC5 Polarizing microscope equipped with a Linkam THMS heating stage with TMS 94 temperature programmer. The samples placed between 12 mm glass coverslips and transferred to the heating stage and heated at a programmed heating rate. The photomicrographs were taken using an imager A2M digital camera. Differential scanning colorimetry traces were recorded using a DSC Q 200 instrument at a heating rate of 10°C for minute in a nitrogen atmosphere. The samples were subjected to two heating and two cooling cycles. The data obtained from the second heating and cooling cycle used for discussion. UV visible absorption spectra recorded on a Varian Cary 50 Bio UV–visible spectrophotometer in chloroform solution and the photoluminescence spectra were measured in chloroform on a Varian Cary Eclipse fluorescence spectrophotometer. Atomic force microscopy (AFM) imaging was performed using the NT-MDT RUSSIA Model: NTEGRA PRIMA instrument under ambient conditions while, Field emission scanning electron microscopy (FESEM) imaging was carried out on a HITACHI SU-6600 at an accelerating electron voltage of 15 kV. Solid state NMR measurements The solid-state NMR experiments were performed on a Bruker Avance III HD 400 WB NMR spectrometer (9.4 T) operating at 400.07 and 100.61 MHz frequencies for 1H and 13C nuclei respectively. A Bruker double resonance static probe with a 5 mm horizontal solenoid coil was used for the experiments in the nematic phase. The sample was packed in a 4 mm Zirconia rotor with a Zirconia cap which was then inserted into a 5 mm glass tube. The sample was heated to isotropic temperature and allowed to cool slowly to nematic phase to achieve the alignment in the magnetic field. The 1D

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C NMR spectra in nematic phase at

100 °C and 88 °C were obtained by cross-polarization (CP) scheme, typically with 50 kHz of 4 ACS Paragon Plus Environment

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radio frequency (RF) field strength on both the 1H and 13C channels for a contact time of 3ms, number of scans 196, and a recycle delay 9 s. The 2D high resolution separated local field (SLF) spectra were acquired using the SAMPI-4 pulse sequence18 that correlates the chemical shift with the corresponding

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C

C-1H dipolar frequencies under static condition

(Figure S1 of Supporting Information). The applications of SAMPI-4 for liquid crystalline samples were described in our recent work.19,20 The experimental parameters utilized for HT8 in nematic phase were: CP contact time τ =3 ms, t1 increments =100, number of scans=36 and recycle delay= 14 s (to minimize the RF heating effect). For both 1D and 2D experiments, the typical 1H 90º pulse length was 5 µs and heteronuclear decoupling during the acquisition period was achieved by SPINAL-6421 irradiation with a 30 kHz RF strength. Synthesis of Mesogens Synthesis of 4 – bromo – 4’- (octyloxy) biphenyl (2b) In a 250 mL round bottom flask, (10 g, 28 mmol) 4’-bromo biphenyl -4-ol and (3.86g, 28 m mol) K2CO3 were taken and 100 mL DMF was added. The mixture was stirred while heating at 90ᵒC followed by dropwise addition of (5.4g, 28 mmol) 1-bromo octane. The reaction was continued for 5h. Then the reaction mixture was poured into 1L distilled water and transferred to a separating funnel and extracted with chloroform. The chloroform layer was washed using 5% NaOH solution and distilled water, and the organic layer was dried over anhydrous sodium sulfate. Upon removal of solvent using rotary evaporator under reduced pressure, white solid was obtained. It was further purified by recrystallization from acetonitrile. The similar procedure was followed for the synthesis of hexyloxy (2a), decyloxy (2b), dodecyloxy (2d) and tetradecyloxy (2e) homologs. Yield: 89%. 1H NMR (400 MHz, CDCl3) δ 7.57 – 7.32 (m, 6H), 7.00 – 6.90 (m, 2H), 3.99 (t, J = 6.6 Hz, 2H), 1.87 – 1.73 (m, 2H), 1.48 (m, 2H), 1.43 – 1.17 (m, 8H), 0.89 (t, J = 6.7 Hz,



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3H). 13C NMR (100 MHz, CDCl3) δ 159.02, 139.83, 132.25, 131.78, 128.28, 127.94, 120.72, 114.91, 68.15, 31.84, 29.39, 29.29, 29.27, 26.08, 22.68, 14.13. Synthesis of 3 – hexyl – 2, 5 –bis (4’ – (octyloxy) biphenyl- 4 –yl) thiophene (HT8) A 100 mL round bottom flask equipped with a magnetic stirrer, (1.24g, 9 mmol) K2CO3, (40.4 mg, 0.18m mol) Pd (OAc)2, (105.1 mg, 0.36 m mol) PCy3HBF4 and (91.9 mg) pivalic acid were placed. The reaction flask was purged with nitrogen gas and (0.5g, 3 mmol) 3hexylthiophene and (3.24g, 9 m mol) 4 -bromo - 4’ - (octyloxy) biphenyl in 50 mL dimethyl acetamide were added. The reaction mixture was then vigorously stirred at 110ᵒC for 10h. After that the solution was cooled to room temperature, diluted with dichloromethane and distilled water. The aqueous phase was extracted with dichloromethane and the combined organic layer was dried over anhydrous MgSO4 and removed the solvent under vacuum. The light green color crude solid obtained was purified by column chromatography using chloroform and hexane as eluents (v/v 8:2). The same work up was followed for the synthesis of other homologs i.e. hexyloxy (HT6), decyloxy (HT10), dodecyloxy (HT12) and tetradecyloxy (HT14) mesogens. Yield: 75%. FT-IR (KBr, cm-1): 2926, 2858, 1606, 1494, 1468, 1392, 1286, 1249, 1177, 1122, 1029, 997, 819, 721, 628. 1H NMR (400 MHz, CDCl3) δ 7.65 – 7.38 (m, 12H), 7.17 (s, 1H), 6.99 – 6.85 (m, 4H), 3.93 (m, 4H), 2.71 – 2.56 (t, J = 6.8 Hz, 2H), 1.78 – 1.57 (m, 4H), 1.51 – 1.40 (m, 4H), 1.40 – 0.99 (m, 24H), 0.90(m, 9H).

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C NMR (100 MHz, CDCl3) δ

158.89, 158.86, 141.81, 139.82, 139.75, 137.10, 133.01, 132.85, 132.78, 129.47, 128.00, 127.87, 126.99, 126.71, 125.86, 125.63, 114.87, 68.15, 31.85, 31.68, 31.01, 29.40, 29.33, 29.28, 29.05, 26.10, 22.69, 22.64, 14.13. 3 – hexyl – 2, 5 –bis (4’ – (hexyloxy)biphenyl- 4 –yl) thiophene (HT6) Yield: 79%. FT-IR (KBr, cm-1): 2926, 2858, 1606, 1494, 1468, 1392, 1286, 1249, 1177, 1122, 1029, 997, 819, 721, 628. 1H NMR (400 MHz, CDCl3) δ 7.64 – 7.37 (m, 12H), 7.17 (s,


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1H), 6.96 – 6.85 (m, 4H), 3.92 (m, 4H), 2.70 – 2.54 (t, J = 6.8, 2H), 1.80 – 1.64 (m, 4H), 1.65 – 1.36 (m, 6H), 1.36 – 1.07 (m, 14H), 0.82 (m, 9H). 13C NMR (100 MHz, CDCl3) δ 158.90, 158.86, 141.81, 139.82, 139.74, 137.11, 133.02, 132.85, 132.79, 129.47, 128.00, 127.87, 127.00, 126.72, 125.86, 125.64, 114.88, 68.14, 31.70, 31.65, 31.02, 29.31, 29.28, 29.07, 25.79, 22.66, 14.13, 14.09. 3 – hexyl – 2, 5 –bis (4’ – (decyloxy)biphenyl- 4 –yl) thiophene (HT10) Yield: 74%. FT-IR (KBr, cm-1): 2926, 2859, 1606, 1497, 1467, 1392, 1289, 1251, 1174, 1121, 1028, 996, 816, 721, 628. 1H NMR (400 MHz, CDCl3) δ 7.65 – 7.36 (m, 12H), 7.17 (s, 1H), 6.91 (m, 4H), 3.93 (m, 4H), 2.73 – 2.53 (t, J = 6.8 Hz, 2H), 1.94 – 1.58 (m, 4H), 1.62 – 1.38 (m, 6H), 1.38 – 1.05 (m, 30H), 0.81 (m, 9H).

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C NMR (100 MHz, CDCl3) δ 158.88,

158.85, 141.80, 139.82, 139.74, 137.10, 133.00, 132.84, 132.77, 129.47, 128.00, 127.87, 127.00, 126.72, 125.86, 125.63, 114.86, 68.14, 31.93, 31.69, 31.02, 29.63, 29.60, 29.45, 29.36, 29.33, 29.27, 29.05, 26.10, 22.72, 22.65, 14.16, 14.13. 3 – hexyl – 2, 5 –bis (4’ – (dodecyloxy)biphenyl- 4 –yl) thiophene (HT12) Yield: 72%. FT-IR (KBr, cm-1): 2926, 2858, 1606, 1493, 1468, 1391, 1286, 1249, 1175, 1121, 1029, 997, 819, 721, 628. 1H NMR (400 MHz, CDCl3) δ 7.64 – 7.36 (m, 12H), 7.25 (s, 1H), 6.98 (m, 4H), 4.00 (m, 4H), 2.80 – 2.57 (t, J = 6.8 Hz, 2H), 2.01 – 1.59 (m, 6H), 1.49 – 1.14 (m, 42H), 0.88 (m, 9H). 13C NMR (101 MHz, CDCl3) δ 158.90, 158.86, 141.81, 139.81, 139.75, 137.11, 133.01, 132.86, 132.79, 129.46, 127.99, 127.86, 126.99, 126.70, 125.86, 125.62, 114.88, 77.34, 77.02, 76.70, 68.15, 31.94, 31.68, 30.99, 29.68, 29.66, 29.62, 29.61, 29.43, 29.37, 29.32, 29.25, 29.04, 26.08, 22.71, 22.63, 14.13, 14.09. 3 – hexyl – 2, 5 –bis (4’ – (tetradecyloxy)biphenyl- 4 –yl) thiophene (HT14) Yield: 68%. FT-IR (KBr, cm-1): 2926, 2858, 1605, 1493, 1468, 1394, 1286, 1249, 1176, 1123, 1029, 998, 819, 723, 628. 1H NMR (400 MHz, CDCl3) δ 7.65 – 7.38 (m, 12H), 7.22 (s, 7 ACS Paragon Plus Environment


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1H), 7.04 – 6.86 (m, 4H), 3.96 (m, 4H), 2.77 – 2.60 (m, J = 6.8 Hz, 2H), 1.73 (m, 6H), 1.45 – 1.05 (m, 50H), 0.97 – 0.73 (m, 9H). Results and Discussion The molecular mesogens are designed to have thiophene flanked by 4-alkoxy biphenyls at 2 and 5 positions and n-hexyl chain at 3-location (Scheme 1). The molecules do not possess center symmetry due to the presence n-hexyl chain in thiophene. The presence of five rings i.e. two biphenyl units and one thiophene ring in the core unit facilitates the π-conjugation while the alkoxy chains at terminal position enable the molecules to undergo self-assembly in solution. By varying the length of terminal alkoxy chain from C6-C14 (even carbons), five homologs are realized. Further, due to the π-conjugated phenyl rings as well as thiophene, the fluorescence emission in solution is noticed. Up on melting of the crystals of mesogens, an enantiotropic nematic mesophase is observed. Despite the presence of five rings in the mesogenic core, the melting transition temperatures are found to be lower than 100 oC. In other words, the molecules exhibit self-assembly and fluorescence emission in solution and show liquid crystallinity with low melting transition. As a consequence, the investigation is deigned to cover the morphological studies of mesogen in solution by AFM and FESEM, mesophase properties by Hot-stage Optical Polarizing Microsocpe (HOPM) and Differential Scanning Calorimetry (DSC) and the molecular order by solid state

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synthesized mesogens are initially characterized by FT–IR, 1H and solution

C NMR. The 13

C NMR for

establishing the structural integrity. Morphological studies Generally the π-conjugated molecules are composed of aromatic rings (sp2 hybridized carbons)

along with alkyl chains (sp3 hybridized carbons).22,23 As a consequence, the

structural difference causes diverse polarity and interactions between the rigid π-conjugated core and the flexible alkyl chains leading to micro/nano segregation.24,25 Owing to the 8 ACS Paragon Plus Environment

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hydrophobic nature of the alkylated π-conjugated molecules, they are classified as hydrophobic amphiphiles which on processing from solution induce remarkable selfassembly.26 Hence, the synthesized mesogens are subjected to AFM and FESEM investigations to understand their self-assembly process in solution.27-29 The AFM imaging is performed on samples drop casted from dilute solutions (5 x 10-5 M) of series of mesogens (HT6 to HT12) on a freshly cleaved mica surface. AFM images of HT6, HT8, and HT10 in decane, HT12 in heptane are shown in the Figure 1. It is observed that the terminal chain length had a significant influence on the morphology of these mesogens. It is found that the HT6 formed tape-like strand with 4 µm width and nearly 30 nm height while HT8 showed entangled network of supramolecular fibers consisting of fiber bundles in the range of 2-5 µm width and several micrometers in length with height in the range of 1-2 nm. Notably, the large width and thickness of the fibers is an indication of the three-dimensional growth of the self-assembly. HT10 forms spherical interconnected grains with high nucleation density. HT12, on the other hand, shows island growth with low nucleation density. Usually, the island growth is observed where intermolecular interactions dominate over the substratemolecule interactions. The morphological studies conclusively indicate that the formation of supramolecular fibers is due to the self-assembly of mesogenic units through assisted π-π stacking and van der waals interactions. In order to understand the formation of supramolecular 3-D network structure by the selfassembled mesogens, the gelation studies are undertaken.29,30 The gel forming ability of these mesogens are examined in solvents such as heptane, toluene, decane, cyclohexane and acetonitrile. Accordingly, the saturated solutions of the mesogens are subjected to heating and cooling cycles and no gelation is noticed. This observation indicates that the stronger intermolecular interactions between the mesogens dominate over the solvent-mesogen



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interactions. As a consequence, the gelation involving 3-D supramolecular entangled fiber network that can hold the solvent molecules is not noticed.29,30 Further evidence for the formation of fibrous assemblies is esatblsihed from the FESEM investigation. The FESEM images of mesogens in heptane at a concentration of 3 x 10-3 M are shown in Figure 2. The morphology of HT6, HT8, HT10, HT12 mesogens is significantly different from one another. The HT6 showed thin fiber rod-like networks, HT8 exhibited the formation of fiber bundles (flower-like structure), HT10 forms fibrous aggregates, and HT12 displayed linear supramolecular tape-like structures. The AFM and SEM analyses indicate that the terminal chain length plays a crucial role in the morphology of the self-assembled structures in dilute (5 x 10-5 M) as well as in concentrated solutions (3 x 10-3 M). Optical absorption and emission in solution The Figure 3 shows the normalized UV–visible absorption spectra of all the mesogens (HT6 – HT14) at 1 x 10-6 M solution in chloroform. The absorption maxima (λmax) of all of them is centered at ~ 344 nm which indicates π – π* transition of the mesogenic core unit. The absorption band does not show any resolvable fine structure. The normalized fluorescence spectra of all mesogens at 1 x 10-6 M solution in chloroform shows negligible change with variation in the terminal alkoxy chain length. The emission spectra of mesogens at 1 x 10-6 M solution in chloroform are obtained with an excitation wavelength (λex) of 344 nm and emission collected in the range of 350 to 650 nm. It is clear from the fluorescence spectra that the emission spectra have two vibronic bands positioned at around 422 nm and 445 nm. Belletete et al.31,32 observed such vibronic bands in fluorescence emission spectra of thiophene oligomers and polymers. Similar to our observation, the absorption band is structure less while the fluorescence spectrum is structured and sharper. The two maxima located at 422 nm and 445 nm are indication of fine structure of emission and is attributed to vibrational coupling with electronic transition involving C=C ring stretching. The difference 10 ACS Paragon Plus Environment

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amongst the structure less absorption and structured fluorescence bands indicates that molecule is more rigid in its first excited singlet state. Further, the influence of terminal alkoxy chain length on fluorescence spectra is similar to the absorption spectra. In other words, no appreciable change in either absorption or fluorescence spectra is noticed with varying terminal chain length Mesophase Transitions The mesophase properties of synthesized mesogens are evaluated by HOPM and DSC investigations. In HOPM, the samples, upon cooling the isotropic phase, formation of birefringent spherical droplets are noticed which on further cooling coalesced to form threaded texture indicating the nematic mesophase (Figure 4).33 On continued cooling, the nematic phase transformed to solid phase at much lower temperatures due to crystallization by super cooling. Table 1 lists the crystal-nematic and nematic-isotropic temperatures for all the mesogens and with increase in terminal chain length, the TN-I values showed decreasing trend.34 For HT14, the nematic mesophase is observed on cooling the isotropic phase only indicating the monotropic nature of the transition (Table S1 of Supporting Information). These observations are further confirmed by DSC measurements (Figure 5) where crystalnematic and nematic-isotropic transitions are clearly noticed. The typical enthalpies of N-I transition are found to be 0.10-0.16 kcal/mol.35 The important observation of the DSC measurements is enantiotropic nature of the mesophase transitions for HT6-HT12 and monotropic nature for HT14. It is remarkable to note that the morphological studies of the mesogens by AFM and FESEM revealed self-assembly of the molecules in solution state due to segregation of the aromatic rings and flexible alkoxy chains. However, in liquid crystalline phase the observation of nematic phase for all the mesogens indicate that such effects are not prevalent. Usually, smectic mesophase is seen when segregation of incompatible parts i.e. separation of flexible aliphatic chains from rigid aromatic units occurs.36-38 In the present 11 ACS Paragon Plus Environment


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case, the absence of smectic mesophase(s) in the mesogens could be attributed to the presence of n-hexyl chain on thiophene ring. It is apparent that the hexyl chain interrupts the lateral interactions between the π-conjugated molecules there by suppressing the layer ordering characteristic of smectic mesophase. Solid state 13C NMR and orientational order in nematic phase One of the synthesized mesogens i.e. HT8 (Figure 6) is characterized by solid state 13C NMR to find the orientational constraints and the molecular order in nematic phase. To accomplish the task, both static 1D and 2D NMR experiments are performed. To determine the alignment induced chemical shifts of resolved carbons of the molecule, the chemical shift values of the mesogen in solution is essential. Figure7a shows the

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C NMR spectrum of HT8 run in

chloroform solution. The spectrum in the range shows 14-159 ppm well resolved lines accounting for core unit as well as lateral n-hexyl and terminal octyloxy chains. The assignment of the lines is attempted by comparing the spectrum generated from chemsketch software and further refined by 2D 1H-1H DQF COSY,

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C-1H HSQC and HMBC

experiments (Figure S2-S11 of Supporting Information). The assigned chemical shift values of all the carbons are listed in Table 2. It is clear from the spectrum that the core unit carbons showed 18 lines accounting for 20 carbons. Among them, the line appeared at 114.87 ppm is more intense than other core unit carbons and is accounted for two carbons (C2 and C19). For the terminal octyloxy chain and the hexyl chain of the thiophene ring, lines in the range 14.0-68.5 ppm are noted. Of them, the lines arising from the OCH2 carbon and methyl carbon are noticed at 68.15 ppm and 14.11 ppm. The rest of methylene carbons of octyloxy as well as hexyl chain are seen in the range 22.00 to 32.00 ppm. The interesting feature of the aliphatic segment of the spectrum is low intensity of carbons arising from hexyl chain of thiophene ring in contrast to terminal octyloxy chain carbons. As the terminal chains are located at both ends of the core unit whereas one hexyl chain is positioned at thiophene ring,


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the variation in intensities is evidently seen. This trend is also observed for methyl carbon of hexyl chain which is seen as low intense shoulder to the methyl carbon of octyloxy chain (14.12 ppm). Figure 7b shows the 1D static 13C NMR spectrum of HT8 measured at 88ºC. For the sake of clarity, the static 13C spectrum is divided into two regions. The region from 130-210 ppm is contributed by the core unit carbons whereas peaks arising from hexyl chain as well as terminal octyloxy chains are noted in the range12-65 ppm. For the core unit methine carbons, the intense peaks in the range 134-148 ppm are observed whereas moderate to low intense peaks are noted in the range 179-210 ppm. Owing to the presence of hexyl chain on thiophene, the molecules do not have center symmetry. As a result, the biphenyl units of the core do not have chemical shift equivalence. In solution spectrum, both the biphenyl unit carbons show very low chemical shift differences whereas in the liquid crystalline phase, due to the alignment of the molecules, the anisotropic effects appear resulting in appreciable change in chemical shifts of some of the carbons. Since the long axis of the molecule is not making similar angle for both the biphenyl units due to the non-centro symmetric nature of the molecule, appreciable change in C2 and C19 carbons is noted. For these carbons in solution only one line is seen where as in nematic phase, the two peaks are observed at 134.0 (C2) and 136.1 (C19) ppm. Similar differences for other carbons of ring I and ring IV could be expected. However, due to the appearance of relatively broad peaks in the region, the overlap of other methine carbons is noticed. Among the methine carbons of phenyl rings, the peaks appeared at 145.6 - 147.6 are more intense than other peaks. The quaternary carbons of biphenyl as well as thiophene rings are witnessed in the range 179.0 -210.0 ppm. The lone methine carbon of central thiophene ring could not be identified in 1D static spectrum due to low intensity. For hexyl as well as terminal octyloxy chain carbons, the peaks are seen in the span of 12-65 ppm. Of them, the OCH2 (64.5 ppm) and CH3 (12.2 ppm) carbons are distinct 13 ACS Paragon Plus Environment


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while the other methylene carbons have appeared in the range 21-35 ppm. For further insights, the 2D experiments performed in nematic phase at 88 and 100ºC are considered. The 2D SLF spectrum of HT8 is shown in Figure 8. The spectrum is recorded in nematic phase at 100oC. It shows the

13

C chemical shifts and

13

C-1H dipolar couplings of the

molecules. As many as 14 contours arising from core unit carbons are seen in the range 130207 ppm. The intensities of them vary indicating the overlapping of some of the carbons as well as the presence of quaternary and methine carbons. Since the core unit has four phenyl rings, 8 methine carbons are expected. However, the spectrum shows 5 contours similar to static 1D spectrum. These carbons are seen in the range 130 -150 ppm in which 140 -145 ppm contours are high intense. The other two contours seen in the range 132-135 ppm are well resolved. Since both the arms of the mesogens have biphenyl units, the chemical shift difference of these carbons is expected due to the non-centro symmetric nature of the thiophene ring. Even in solution, despite the chemical shift differences are very narrow, the lines are resolved. In liquid crystalline phase, on the other hand, five contours accounting for eight carbons are noticed. The quaternary carbons of the biphenyl rings as well as thiophene ring appeared as low intense contours. The lone methine carbon of thiophene is noticed at 175.89 ppm. The assignment of the quaternary carbons is attempted by considering the location of long axis as well as comparing the known liquid crystalline molecules in which biphenyl moieties are present.39,40 In contrast to 1D static 13C spectrum, the peak at 176 ppm is well resolved in to two contours in 2D spectrum. Table 2 lists the assigned chemical shifts of all the core unit carbons of the mesogens in nematic mesophase. Order of core unit Generally, the 2D SLF experiments provide 13C-1H dipolar oscillation frequencies of resolved carbons for a given molecular moiety i.e. phenyl or thiophene ring. The


13

C-1H dipolar

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coupling information, however, need to be extracted from experimental

13

C-1H dipolar

oscillation frequencies as outlined in our earlier reports.41-44 By making use of 13C-1H dipolar couplings of individual carbons of the each ring which constitutes the core unit, the orientation order parameters can be calculated by the following equation ,45,46 DCH =K [Szz (3 cos2θz-1)/2 + (Sxx – Syy) (cos2θx - cos2θy)/2+ Sxz (cosθx cosθz )] where K = -hγHγC/4π2r3CH, with γH and γC are the gyromagnetic ratios of 1H and

(1) 13

C nuclei

respectively and rCH is the inter nuclear distance between them, θx, θy and θz are the angles formed by rCH with the respective coordinate axes of the molecular fragment under consideration. The equation is based on the fact that for 1-4 di substituted phenyl ring, the D2 symmetry is considered and accordingly two order parameters i.e. Szz and Sxx-Syy are sufficient. For these rings, the z, x and y axes are defined as respectively the para axis of the ring, an axis perpendicular to the z axis but in the plane of the ring and the axis perpendicular to the plane of the ring. During fitting, the standard bond distances rCH=1.1 Å for the C–H bond and rCC=1.4 Å for C–C bond are used. The C-C-H bond angles are also slightly varied around 120° to get the best fit.47 For the thiophene ring, owing to the asymmetry because of 2,3,5-tri substitution, three order parameters i.e., Szz, Sxx-Syy and Sxz are required for an arbitrary choice of the axis system.48,49 The bond angles and bond distances are taken from the energy minimized structure of thiophene for HT8 mesogen. (Figure S12 and S13 of Supporting Information). To begin with, the z-axis is considered along the C8-C9 bond by which the thiophene is directly connected to the biphenyl unit with the expectation that the order along this direction is likely to be the largest. Quality of the fit further improved by introducing an angle β between the z-axis and the C8-C9 bond around 18º. It implies that the z-axis is almost collinear to C10-C11 bond. Table 3 and 4, respectively lists the order parameter values of phenyl and thiophene rings computed by using equation 1 at 88 ºC and 100 ºC respectively. A slight variation in phenyl ring order parameters of biphenyl units of 15 ACS Paragon Plus Environment


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the both arms is noticed. Accordingly for rings I and II, the values are 0.56 while for rings III and IV, the value is found to be 0.55. These values are in consistent with the data normally noticed for rod-like nematogens.45-49 For thiophene ring, the calculated order parameters are 0.61 ( Szz ) 0.166 ( Sxx - Syy ) and 0.086 (Sxz) at 100oC. An important finding of the order parameter determination is the large variation between phenyl rings of the side arm and center thiophene ring. This observation is comprehensible if the geometry of the thiophene ring is taken in to consideration. Since, the biphenyl units are linked to thiophene at 2, 5position a bent is expected between the local biphenyl axis and the long axis (Figure 9). As a result, the central thiophene ring possesses more order than the side arm phenyl rings. In other words, the higher value for thiophene advocates that the long axis is passing through the thiophene ring and the local biphenyl axis make considerable angle α as shown in Figure 9. With increase in temperature, the order parameter showed decreasing trend due to increase in molecular dynamics. Ordering of lateral and terminal chains For the aliphatic carbons arising from the terminal octyloxy chain as well as lateral n-hexyl chain, the 2D PELF spectrum (Figure S14 and S15 of Supporting Information) measured at 100°C is considered. The peak appeared at 64.74 ppm in 1D static spectrum resolves in to two contours in 2D spectrum due to variation in

13

C-1H dipolar couplings. The methyl of

terminal octyloxy chain and hexyl chain, however, show one contour in 2D spectrum where as in 1D static spectrum, a shoulder peak is additionally noticed at 12.00 ppm. The shoulder peak is assigned to methyl of hexyl chain while the intense peak is attributed to methyl carbons of terminal octyloxy chain. The well resolution achieved by PELF spectrum facilitates the identification of almost all the carbons of both the chains in contrast to 1D spectrum where 9 peaks are only seen. Table 5 lists the assigned chemical shift values of all the carbons of lateral and terminal chains. The important feature of the aliphatic carbons of 16 ACS Paragon Plus Environment

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the spectrum is the chemical shift trend of terminal versus later chains. Among the carbons resolved from 2D spectrum, four carbons (A-D) showed increase in chemical shifts whereas the rest (10 carbons) showed decrease in chemical shifts as against to solution values. Generally, in liquid crystalline molecules, the aliphatic carbons of the terminal chain show lower chemical shifts than their solution spectra whereas the core unit carbons show increase in chemical shifts.50-52 This variance is due to the change in the orientational constraints of different moieties i.e. core versus terminal chains of the molecule owing to the alignment in the magnetic field. An increase in chemical shift values of aliphatic carbons is only noticed when they are located away from the local axis of the phenyl rings.53,54 In other words, the chains which are placed at the lateral position exhibit increase in chemical shifts while those located at terminal position follow reverse trend. Further, an increase in the chemical shifts of CH2 carbons is also governed by the mean orientation of them.55,56 In the present case, the nhexyl chain is located on the thiophene ring whereas octyloxy chains are placed at two ends of the molecule. As a result, the octyloxy chains are expected to show decrease in chemical shift values while the methylene carbons of hexyl chain can show increase in chemical shifts. From 1D and 2D spectral studies, it is observed that the four carbons (A-D) of lateral hexyl chain which are nearer to thiophene ring showed increase in chemical shifts whereas rest of the two carbons i.e. one methylene and one methyl showed a decrease. Fung et al. 57,58 as well as Bayle et al.

59,60

have extensively studied the

13

C NMR of mesogens in which aliphatic

chains are placed at lateral position of the core. Based on the chemical shifts in mesophase, they interpreted the data with respect to the mean orientation of the chain along the long molecular axis. In cases where CH2 group adopts trans conformation, the chemical shift values are found to decrease in contrast to their solution values whereas the methylene groups with preferred cis conformation showed reverse trend. Based on these literature studies,

53-60

and also taking in to the chemical shift trends of the chains of HT8, it is concluded that the



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most of the carbons of n-hexyl chain are in cis conformation while the E, F carbons are in trans conformation. For terminal alkoxy chains the methylene as well as methyl carbons are in fully extended trans confirmation since the alignment induced shift (AIS) values are found to be negative. A close examination of

13

C-1H dipolar couplings of methylene carbons of

hexyl chain reveal that A, C, E, F carbons show very low

13

C-1H dipolar coupling values

(0.33 to 0.89 kHz). This is in contrast to terminal octyloxy chain where the dipolar couplings are in the range of 1.46 to 4.00 kHz except for b carbon. These inferences indicates that the molecular confirmation more specifically those arising from aliphatic carbons can be investigated by 13C NMR using AIS as well as 13C-1H dipolar couplings as experienced in the present case. The change in order parameter values with increase in temperature is further verified by plotting AIS versus temperature which is shown in Figure 10. It is clear from the data that with increase in temperature, the AIS values follow decreasing trend since the order parameter drop due to increasing molecular dynamics.61-63 Thus the 2D solid state NMR experiments can provide complete information about the ordering of core unit moieties, the lateral alkyl chain as well as the terminal octyloxy chain. This provides direct information about the molecular organization as well as orientational constraints of different segments of mesogen. Conclusions 3-hexyl thiophene centered 4-alkoxy biphenyl containing π-conjugated mesogens were synthesized by palladium acetate catalyzed direct arylation. By varying the terminal alkoxy chain from 6-14 (even carbons), six mesogens were realized which displayed self-assembly in solution and enantiotropic nematic phase upon melting the crystalline solid. Remarkably, despite the presence of five rings in the core, the low melting transition noticed was attributed to non-centro symmetric nature of the molecule due to the presence of lateral n-hexyl chain. 18 ACS Paragon Plus Environment

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The AFM and FESEM studies confirm the formation of supramolecular fibers in solution due to nano-seggregation driven by π-π stacking and van der Waals interactions. The optical absorption and emission studies in solution confirmed that the absorption band was structure less while the fluorescence spectrum was structured with two maxima located at 422 nm and 445 nm. Further, the appearance of dual maxima was attributed to vibrational coupling with electronic transition involving C=C ring stretching. The solid state

13

C NMR investigations

mapped the orientational constraints of aromatic rings of the core as well as the lateral and terminal chains. Owing to the asymmetry of the thiophene ring, three order parameters Szz (0.61), Sxx-Syy (0.166) and Sxz (0.086) were essential to describe the ordering. For the phenyl rings of biphenyl units, the order parameters were estimated to be 0.56 and 0.55 (88°C) and the variation in contrast to thiophene ring was reflection of bent between the thiophene and side arm phenyl rings. The investigation discloses that despite the structural simplicity of the mesogens, the manifestation of molecular properties such as self-assembly and blue fluorescence emission in solution and enantiotropic nematic phase is indication of thiophenephenyl direct link and positioning of the flexible chains at lateral and terminal locations.

ASSOCIATED CONTENT SUPPORTING INFORMATION It contains Figures of SAMPI-4 pulse sequence, solution 1D and 2D NMR spectra for mesogens, thiophene model, PELF pulse sequence and experimental details. This material is available free of charge via the Internet at http://pubs.acs.org AUTHOR INFORMATION Corresponding Author *E-mail:[email protected], Phone: 91-44-24422059



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ACKNOWLEDGEMENTS The authors would like to acknowledge the support of Dr. B. Chandrasekaran, Director, CSIR-CLRI and Prof. K. V. Ramanathan, NMR Research Centre, Indian Institute of Science, Bangalore, India. We also thank B.V. N. Phani Kumar and R. Ravikanth Reddy, CSIR-CLRI for the solution 2D NMR measurements. The partial financial support from STRAIT CSC0201-WP2 is duly acknowledged. Srinivasan Sampath acknowledges the financial support in the form of DST-INSPIRE Faculty Award (IFA13-CH130). Y. Santosh Kumar Reddy thanks the financial support from Council of Scientific and Industrial Research (CSIR), New Delhi in the form of Senior Research Fellowship.


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Perez, F.; Berdague, P.; Bayle, J. P.; Brauniger, T.; Khan, M. A.; Fung, B. M. Orientational Ordering of Some Biforked Nematic Liquid Crystals. New J. Chem. 1997, 21, 1283-1290.

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Bayle, J. P.; Berdague, P.; Ho, M. S; Fung, B. M. Study of Orientational Ordering of Laterally Alkoxy Branched Nematics by One and Two Dimensional C-13 NMR. New J. Chem. 1994, 18, 715-720.

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Berdague, P.; Perez, F.; Bayle, J. P.; Ho, M. S; Fung, B. M. Influence of a Lateral Aliphatic Chain on the Ordering in Some Nematic Compounds. New J. Chem. 1995, 19, 383-390.

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Perez, F.; Bayle, J. P.; Fung, B. M. Orientational Ordering of Laterally Dialkoxy Branched Nematics Studied by One and Two Dimensional C-13 NMR. New J. Chem. 1996, 20, 537-544.

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Perez, F.; Judeinstein, P.; Bayle, J. P.; Fung, B. M. The Effect of a Lateral Aromatic Branch on the Orientational Ordering of Laterally Alkoxy Substituted Nematics. Liq. Cryst. 1997, 22, 711-719.

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Sinha, N.; Ramanathan, K. V.; Leblanc, K.; Judeinstein, P.; Bayle, J. P. Ordering of a Lateral Crown Ether and Terminal Short POE Chains in Some Symmetrical Nematogens by 13C NMR. Liq. Cryst. 2002, 29, 449–457.

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Perez, F.; Judeinstein, P.; Bayle, J. P.; Allouchi, H.; Cotrait, M.; Roussel, F.; Fung, B. M. Orientational Ordering in Some Nematogens Deviating from the Classical Rod-Shape. . Liq. Cryst. 1998, 24, 627-637.

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Ramamoorthy, A., Ed. Thermotropic Liquid Crystals: Recent Advances; Springer: Dordrecht, Netherlands, 2007.

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Dong, R. Y. Nuclear Magnetic Resonance Spectroscopy of Liquid Crystals; World Scientific: Singapore, 2010.



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Cifelli, M.; Domenici, V.; Veracini, C. A. Recent Advancements in Understanding Thermotropic Liquid Crystal Structure and Dynamics by Means of NMR Spectroscopy. Curr. Opin. Colloid Interface Sci. 2013, 18, 190– 200.


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Table 1: Transition Temperatures and Enthalpy values of Synthesized Mesogens from Second Heating Cycle Phase transition temperatures and enthalpy values (△H kcal/mol) Cr – N 85.1 ºC N - I 134.0 ºC HT6 (3.14) (0.16) Cr – N 85.5 ºC N – I 118.7 ºC HT8 (4.45) (0.10) Cr – N 83.1 ºC N – I 115.6 ºC HT10 (6.12) (0.13) Cr – N 79.3 º C N – I 101.0 ºC HT12 (8.60) (0.14) Cr – I [90.3 ºC] HT14 (9.70) Code

[ ] indicates monotropic transition; △H values are in parenthesis Cr: crystallisation; N: nematic; I: isotropic



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Table 2: 13C NMR Data for Core Unit of HT8 Mesogen in Solution and Nematic Mesophase

88 ºC Carbon Solution No. (ppm) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

158.86 114.87 129.47 139.81 132.87 126.99 127.87 141.81 133.01 137.10 125.63 139.75 132.85 126.71 125.86 139.82 132.86 128.00 114.87 158.89

100 ºC 13

CS (ppm)

AIS (ppm)

209.55 134.15 147.68 190.96 186.54 146.58 145.84 196.89 178.66 154.46 179.55 182.29 186.54 145.84 143.79 190.96 186.54 145.84 136.13 205.66

50.69 19.28 18.21 51.15 53.67 19.59 17.97 55.08 45.65 17.36 53.92 42.54 53.69 19.13 17.93 51.14 53.68 17.84 21.26 46.77

1

C- H DOF (kHz) 1.35 2.31 2.49 1.28 1.20 2.42 2.42 1.28 0.65 1.31 1.45 1.27 1.20 2.42 2.44 1.28 1.20 2.42 2.41 1.35

13

CS (ppm)

AIS (ppm)

204.86 132.27 145.14 185.85 181.56 145.14 143.85 191.49 174.78 151.71 175.89 177.59 181.56 143.85 141.95 185.85 181.56 143.85 134.07 201.19

46.00 17.40 15.67 46.04 48.69 18.15 15.98 49.68 41.77 14.61 50.26 37.84 48.71 17.14 16.09 46.03 48.70 15.85 19.20 42.30

C-1H DOF (kHz) 1.27 2.01 2.17 1.18 1.13 2.17 2.13 1.19 0.59 1.14 1.35 1.19 1.13 2.13 2.15 1.18 1.13 2.13 2.10 1.22

CS: chemical shift; AIS: alignment induced shift; DOF: dipolar oscillation frequencies


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Table 3: Orientational Order Parameters for Phenyl Rings of HT8 Mesogena c

a

d

T (ºC)

88

100

Phenyl Ring I II III IV I II III IV

CCH Bond Angles θb 120.4 119.5 119.5 119.7 120.9 119.7 119.8 120.1

θc 119.7 119.5 119.5 119.8 120.2 119.9 119.7 120.2

b

Szz

Sxx-Syy

0.56 0.56 0.55 0.55 0.52 0.52 0.51 0.51

0.062 0.046 0.050 0.058 0.054 0.038 0.040 0.050

Calculated Dipolar Oscillation Frequencies (kHz) b c a d 2.32 2.50 1.31 1.30 2.43 2.43 1.26 1.26 2.43 2.43 1.25 1.25 2.44 2.41 1.27 1.27 2.01 2.17 1.22 1.21 2.18 2.13 1.16 1.17 2.13 2.16 1.15 1.15 2.14 2.11 1.17 1.18

a

RMSD (kHz) 0.02 0.03 0.03 0.05 0.03 0.02 0.02 0.03

In the Figure above the Table, b and c are methine carbons, a and d are quaternary carbons for phenyl ring



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Table 4: Orientational Order Parameters for Thiophene Ring of HT8 Mesogen

a

T (ºC)

β

Szz

Sxx-Syy

Sxz

88 100

18.4 18.3

0.61 0.55

0.166 0.158

0.086 0.076

Calculated Dipolar Oscillation Frequencies (kHz) C-11 C-10 C-12 C-9 1.44 1.35

1.32 1.19

1.25 1.14

Angle between the long axis and side arm axis


0.64 0.57

RMSD (kHz)

Bent Anglea α

0.01 0.03

13.5 11.0

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Table 5:

13

C NMR data of Lateral and Terminal chains for HT8 Mesogen in solution and Nemaic Mesophase

88 ºC

100 ºC

Carbon Solution CS AIS DCH SCH CS AIS DCH SCH No (ppm) (ppm) (kHz) (ppm) (ppm) (ppm) (kHz) a,a’ 14.11 12.36 -1.75 0.98 -0.043 12.43 -1.68 0.80 -0.035 b

22.68

22.02

-0.66

2.86 -0.126 22.45

-0.23

2.40 -0.106

b'

22.68

21.07

-1.61

1.72 -0.076 21.11

-1.57

1.44 -0.063

b’

31.85

28.41

-3.44

2.54 -0.112 28.61

-3.24

2.10 -0.093

c,c’

29.40

25.35

-4.05

3.18 -0.140 25.82

-3.58

2.69 -0.119

d,d’

29.27

25.01

-4.26

2.50 -0.110 25.40

-3.87

2.07 -0.091

e,e’

26.09

24.63

-1.46

4.03 -0.178 25.07

-1.02

3.46 -0.153

f,f’

29.26

25.01

-4.25

2.50 -0.110 25.40

-3.86

2.07 -0.091

g,g’

14.11

12.36

-1.75

0.98 -0.043 12.43

-1.68

0.80 -0.035

h

68.15

64.13

-4.02

4.49 -0.198 64.42

-3.73

3.93 -0.173

h’

68.15

64.82

-3.33

4.08 -0.180 64.92

-3.23

3.58 -0.158

A

29.33

30.87

1.54

-0.55 0.024 30.63

1.30

-0.52 0.023

B

31.68

34.80

3.12

-2.58 0.114 34.17

2.49

-2.35 0.104

C

29.05

30.87

1.82

-0.55 0.024 30.63

1.58

-0.52 0.023

D

30.99

34.02

3.03

-2.06 0.091 33.16

2.17

-1.86 0.082

E

22.63

22.48

-0.15

0.89 -0.039 22.20

-0.43

0.82 -0.036

F

14.10

13.62

-0.48

0.33 -0.015 13.35

-0.75

0.30 -0.013

CS: chemical shift; AIS: alignment induced shift; DCH: Dipolar coupling; SCH: Bond order parameter



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure Captions 1) Scheme 1: Synthetic strategy for HT mesogens 2) AFM images of the Mesogens (a) HT6 in decane, (b) HT8 in decane, (c) HT10 in decane, (d) HT12 in heptane. In all studies concentration of mesogen is 5 x 10-5 M. All the samples were drop casted on a freshly cleaved mica surface and solvents were removed by keeping the samples under vacuum at room temperature for 24 h. 3) FESEM images of the mesogens of (a) HT6, (b) HT8, (c) HT10 and (d) HT12 in heptane at 3 x 10-3 M. All the samples were drop casted on an aluminum foil and solvents were removed by keeping the samples under vacuum at room temperature for 24h. 4) UV-visible (a) absorption (b) emission spectra of Mesogens in chloroform solution (1 x 10-6 M). The excitation wavelength (λex) at 344 nm and emission collected from 350 to 650 nm. 5) HOPM Textures of mesogens on cooling the isotropic phase - (a) birefringent droplets of nematic phase at 100°C for HT8, (b) birefringent droplets of nematic phase at 90°C for HT10, (c) threaded nematic phase of HT12 at 80ᵒC and (d) threaded nematic phase of HT14 70°C. 6) DSC scans of (a) HT8 and (b) HT12 mesogens in heating and cooling cycle. 7) Planar molecular structure of HT8 mesogen. 8) Proton-decoupled

13

C NMR spectra of HT8 in (a) solution and (b) nematic phase at

88°C. 9) 2D SAMPI-4 spectrum of HT8 mesogen at 100 °C in nematic mesophase. 10) Molecular structure of HT8 mesogen with the projection of long axis and side arm axis. 11) Plot of alignment induced chemical shifts (AIS) versus temperature for HT8 mesogen in nematic phase.


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Scheme 1

R = C6H13 (HT6), C8H17 (HT8), C10H21 (HT10), C12H25 (HT12), C14H29 (HT14)



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Page 36 of 46

Figure 1

(b)

(a)

4.5

n m

50 45 40 35 30

4.0

n 3.5 m 3.0 2.5 0

5

10

15

20

25

30

µm

40

35

(c)

2

0

4

6

µm

8

10

12

(d)

12 10 n 8 m6 4 2

n 109 m 8 7 6 5 4 0

2

4

6

8

µm

0

2

4


6

8 10 12 14 16

µm

14

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Figure 2

(a)

(b)

0.5 µm

5 µm (d)

(c)

5 µm

10 µm



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Figure 3

(a)

(b)


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Figure 4

(a)

(b)

100 μm

100 μm

(c)

(d)

100 μm

100 μm



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Figure 5

(a)

(b)


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Figure 6



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 7


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Figure 8



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Figure 9

C6H13

α S

OC8H17 H17C8O


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Figure 10



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Table of Contents


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Morphology, Mesophase, and Molecular Order of 3-Hexyl Thiophene

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