Phase Behavior of a New Class of Anthraquinone-Based Discotic

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Phase behavior of a new class of anthraquinone-based discotic liquid crystals Joydip De, Santosh Prasad Gupta, Indu Bala, Sandeep Kumar, and Santanu Kumar Pal Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b03031 • Publication Date (Web): 08 Nov 2017 Downloaded from http://pubs.acs.org on November 9, 2017

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Phase behavior of a new class of anthraquinonebased discotic liquid crystals Joydip De, † Santosh Prasad Gupta, † Indu Bala, † Sandeep Kumar ‡ and Santanu Kumar Pala, † * †

Department of Chemical Sciences, Indian Institute of Science Education and Research (IISER) Mohali, Sector-81, SAS. Nagar, Knowledge City, Manauli-140306, India ‡

Raman Research Institute, C. V. Raman Avenue, Bangalore 560 080, India

ABSTRACT

Five novel columnar liquid crystalline compounds (4.1 - 4.5) consisting of a central anthraquinone core carrying four alkoxy chains (R = n-C6H13, n-C8H17, n-C10H21, n-C12H25, and 3,7-dimethyl octyl) with two diagonally opposite 1-ethynyl-4-pentylbenzene units were synthesized and their phase transitions were investigated between changes in the molecular structure & their self-assembly into the columnar mesophases. Small and wide angle X-ray scattering (SAXS/WAXS) studies were performed to deduce the exact nature of the mesophases and their corresponding electron density maps were derived from the intensities of the peaks observed in the diffraction patterns. A comparison of compounds with different alkoxy chains indicated that soft crystal columnar rectangular (Crcolrec) phase was stable at lower temperature for shortest peripheral alkoxy chain (4.1; R = n-C6H13) and found to exhibit columnar hexagonal (Colh) phase and then discotic nematic (ND) phase with increasing temperature. In contrast,

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increasing the peripheral chain length to n-C8H17 or branched one (4.2 and 4.5) stabilized the Colh phase at lower temperature and showed ND phase at higher temperature. Further increase in chain length (4.3 and 4.4; n-C10H21, n-C12H25) demonstrated the formation of ND phase. Conductivity measurement in Colh mesophase was found almost ten times higher in magnitude than corresponding Crcolrec phase. HOMO-LUMO band gap of all the compounds was found to be in the range of 2.79 eV to 2.82 eV which is quiet less and comparable with optical energy band gap.

Introduction In recent years, intensive research has focused on the design and synthesis of functionintegrated molecules leading to soft materials through molecular self-assembly. Among them, liquid crystals (LCs) consisting of self-organized molecules can potentially be used as new anisotropic functional materials which are easily processable without using advanced techniques.1 The generation of self-assembled one-dimensional supramolecular assemblies can be easily constructed through weak intermolecular interactions such as hydrogen bonding, π−π stacking, electrostatic interactions, van der Waals forces, and solvophobic interactions between pre-programmed building blocks.2-4 In this regard, discotic liquid crystals (DLCs) which can self-assemble into ordered columnar structures have been recognized as playing a key role as molecular wires in various electronic applications.5-11 Recent developments in the field of organic electronics include an increasingly significant role of DLCs based on anthraquinone (rufigallol) derivatives (AQs), revealed by the budding research activities in this field. Most of the discotics based on AQs form columnar mesophases probably due to strong π−π interactions of aromatic cores. Since, π−π interactions within the same column


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is much stronger than interactions between neighbouring columns, charge transport in these materials is expected to be quasi one-dimensional. Thus, the AQ derivatives can act as a n-type or p-type semi-conductor depending on the substituents. Except from the point of the energy and charge transportation, discotic mesogens are potential candidates for various device applications such as organic light-emitting diodes (OLEDs),12,13 organic field effect transistors (OFETs),14-16 organic light emitting transistors (OLETs),17,18 photovoltaic solar cells,19-23 gas sensors24 and supramolecular scaffolds for biological systems, such as bacteria25,26 due to their easier processability, spontaneous alignment between electrodes, self-healing of defects and so on. In addition, they could form highly stable room temperature mesophases over a wide temperature range suitable for such applications. As a matter of fact, the performances of such materials in electronic devices rely on the intermolecular order in the active layer. Therefore, providing valuable insights into the nature and strength of such non-covalent interactions responsible for different self-assembly into columns of discotic molecules facilitate to design new functional LC materials for electronic devices. Literature survey reveals a wide range of discotic LCs based on AQs.27-32 Unfortunately, only a few attempts have been made that relate phase behaviour with the different substituents attached to the AQ core and thus, limits the scope of structure-property relationships. To address this challenge, herein, a new series of compounds derived from the coupling of 1,5-dihydroxy2,3,6,7-tetraalkoxyanthraquinones with 1-ethynyl-4-pentylbenzene were prepared. The study was motivated by two goals. First, we sought to determine whether addition of two alkynylbenzene units lead to a change in the mesophases behaviour of the AQ derivatives. The change in the mesophases behaviour may be expected as alkynylbenzene units, in general, tend to form the nematic phase. Second, we sought to explore additional insight about the packing of these


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hybrids in the mesophase through simple variation in the number of alkoxy chains (R = n-C6H13, n-C8H17, n-C10H21, n-C12H25, and 3,7-dimethyl octyl) connecting to the AQ moiety. This is possible because of the unequal reactivity of the six phenolic groups of rufigallol, two of which are less reactive by virtue of being intra-molecularly hydrogen bonded to the adjacent quinone carbonyls. Interestingly, it was observed that compound with shortest hexylalkoxy chains (4.1; R= nC6H13) connecting to AQ units showed three mesophases; Crcolrec, Colh and ND phases as a function of increase in temperature. With increasing the peripheral octylalkoxy chain length (4.2, n-C8H17 or branched one, 4.5) Colh phase was found to get stabilized at lower temperature and ND phase at higher temperature. Further increase to decyl and dodecyl chains (4.3 and 4.4) lead to the stabilization of only ND phase. The types of mesophases in the compounds 4.1 - 4.5 are confirmed by detailed analysis of POM and X-ray scattering (SAXS/WAXS) data. Further, their corresponding electron density maps were derived from the intensities of the peaks observed in the diffraction patterns. The observed phase sequence is also supported by the measurement of dielectric behavior in the columnar self-assembly. Overall, the results in this study allow us to systematically probe the effects of different type of chains on the columnar self-assembly accompanied by change in the packing upon LC-LC phase transitions which could lead to the developments of new functional materials and improve their usability in devices.

Experimental Section Materials. Chemicals and solvents (AR quality) were used as received without any further purification. Column chromatographic separations were performed on silica gel (100–200 &


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230-400 mesh). Thin layer chromatography (TLC) was performed on aluminium sheets precoated with silica gel (Merck, Kieselgel 60, F254). Measurements and Characterization. Structural characterization of the compounds was carried out through a combination of infrared spectroscopy (Perkin Elmer Spectrum Two), 1H NMR and 13

C NMR (Bruker Biospin Switzerland Avance-iii 400 MHz and 100 MHz spectrometers

respectively), UV-vis-NIR spectrophotometers (Agilent Technologies, Cary 5000) and Mass spectrometry (Water Synapt G-2-s QTOF with MALDI ion source and α-cyano-4-hydroxycinnamic acid). IR spectra were recorded in neat form for target compounds. 1H NMR spectra were recorded using deuterated chloroform (CDCl3) as solvent and tetramethylsilane (TMS) as an internal standard. All the UV-vis experiments were performed in 1 µM CHCl3 solutions. Cyclic Voltammetry (CV) experiments were performed on CH Instruments, electrochemical workstation. The transition temperatures and associated enthalpy values were determined using a differential scanning calorimeter (Perkin Elmer DSC 8000 coupled to a controlled liquid nitrogen accessory (CLN 2)) which was operated at a scanning rate of 5 °C min-1 both on heating and cooling. Thermogravimetric analysis (TGA) was carried out from 25 to 500 °C (at a heating rate of 10 °C min−1) under nitrogen atmosphere on a Shimadzu DTG-60 instrument. Textural observations of the mesophase were performed with Nikon Eclipse LV100POL polarizing microscope provided with a Linkam heating stage (LTS 420). All images were captured using a Q-imaging camera. X-ray diffraction (XRD) was carried out by filling samples in glass capillaries using Cu-Kα (λ = 1.5418 Å) radiation from Xeuss (Model C HP100 fm) X-ray diffractometer from Xenocs equipped with GeniX 3D source operating at 50 kV and 0.6 mA in conjunction with a multilayer mirror and Pilatus 200 hybrid pixel detector from Dectris.


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The dielectric measurements were carried out using Solartron SI 1260 Impedance/gain-phase analyzer and Solartron dielectric interface 1296 with Mettler Toledo FP82HT temperature controller. The synthesis, procedures, the spectroscopic data of all the compounds, POM images, TGA data, X-ray data, Photophysical studies, electrochemical studies and conductivity data’s are given in detail in the supporting information.

Results and Discussion Synthesis and structural characterization: The synthetic strategy for the preparation of anthraquinone derivatives 4.1 - 4.5 is illustrated in Scheme 1. The synthesis started from the condensation of gallic acid in presence of concentrated sulfuric acid to give 1,2,3,5,6,7-hexahydroxy anthraquinone 1 in good yield.33 The controlled alkylation of 1 with 1-bromalkanes in presence of NaOH and DMSO gave the corresponding 1,5-dihydroxy-2,3,6,7-tetraalkoxyanthraquinones 2.1 - 2.5.34 Ditriflification of compounds 2.1 2.5 were carried out with trifluoromethanesulfonic anhydride and pyridine at room temperature to

get

2,3,6,7-tetrakis(alkyloxy)-9,10-dioxo-9,10-dihydroanthracene-1,5-diyl

bis

(trifluoromethanesulfonate) 3.1 - 3.5. These compounds were used in two fold Sonogashira cross-coupling in presence of 1-ethynyl-4-pentylbenzene to get the target molecules 4.1 - 4.5. The structures of all the intermediates and target molecules were confirmed using 1H NMR, 13C NMR and IR spectroscopic techniques (Figure S1-S25, see ESI). MALDI-MS spectra also confirmed the expected mass of all the final compounds (see ESI).


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Scheme 1. Synthesis of Anthraquinone derivatives 4. Reagents and Conditions: (i) NaOH, RBr, DMSO, 70 °C, 18h; (ii) Tf2O, Pyridine, dry DCM, RT, 24h; (iii) Pd(PPh3)2Cl2, PPh3, CuI, Et3N, Reflux, 18h. Thermal behaviour: The thermal behaviour of all the compounds is determined by polarized optical microscopy (POM) and differential scanning calorimetry (DSC) study. The phase transition temperatures of all the compounds together with transition enthalpy values are summarized in Table 1. The first compound of the series, 4.1, with shortest alkyl spacer (i.e. n = 6) displays room temperature LC


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mesophase. In DSC, on heating it exhibited three mesophases with transitions at 33°C and 65.6 °C, before it clears at 88.2 °C (Figure S26, ESI). However, on cooling these transitions were not perceived in DSC, may be because of low enthalpy change, but clearly observed under POM (Figure 1). Such type of behaviour from isotrpic to liquid crystal transition can be observed by POM, but not detected by DSC has also been realized for other mesogens.35 The peak at 33.2 °C (∆H = 0.54 kJ mol-1) is attributed to Crcolrec to Colh phase transition and the peak at 65.6 °C (∆H = 0.04 kJ mol-1) correspond to Colh to ND phase transition (vide infra). On cooling, under POM, the appearance of partially homeotropically aligned, movable birefringent texture was observed at 86.5 °C, indicating the existence of mesophase which was confirmed to be ND as suggested from detailed X-ray diffraction analysis (vide infra) (though it was not clear from texture). The texture of this mesophase is shown in Figure 1d at 73 °C. On further cooling, at 62.2 °C, the formation of well-defined texture with homeotropic domains and rectilinear defects was observed, characteristic of Colh mesophases (Table 1, Fig. 2c). The texture of the Colh phase was then transformed to broken, four-brush type texture at 31.4 °C indicating transition to a soft crystal phase which was confirmed by X-ray measurements to be Crcolrec phase36,37 (Table 1, Figure 2a, S28, see ESI). The coexistence of both the mesophases was clearly evident from the Figure 1b. Similarly, compound 4.2 exhibited columnar to columnar phase transition (Colh to ND as confirmed from X-ray diffraction) at 82.3 °C (∆H = 2.41 kJ mol-1) and a mesophase to isotropic (Iso) phase transition at 111.3 °C (∆H = 0.10 kJ mol-1). On cooling under POM, mesophase (ND) appears at 109.4 °C which then transformed to Colh phase at 81.4 °C (Figure S26, see ESI). Compound 4.5 with branched alkyl spacer’s shows similar type of textural transition as observed for compound 4.2 (see Table 1).


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Figure 1. Optical microscopy images of compound 4.1 at (a) 23 °C, (b) 32 °C, (c) 42 °C and (d) 73 °C on cooling from isotropic liquids, kept between a glass slide and a coverslip. (crossed polarizers, magnification X 200). Table 1. Experimental data of thermal properties of compounds 4.1 - 4.5.a Mesogen

Heating Scan

Cooling Scan

4.1

Crcolrec 33.2 (0.54) Colh 65.6 (0.04) ND 88.2 (0.10) Isob

Iso 86.5 ND 62.2 Colh 31.4 Crcolrec c

4.2

Colh 82.3 (2.41) ND 111.3 (0.10) Isob

Iso 109.4 (0.05) NDb 81.4 Colhc

4.3

ND 76.6 (0.25) Isob

Iso 73.2 NDc

4.4

ND 72.1 (0.13) Isob

Iso 70.3 (0.24)NDb

4.5

Colh101.5 (17.41) ND 115.4 (1.01) Isob

Iso 111.9 (0.36) NDb 90.2 Colhc


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Transition temperatures (peak, in °C) and the respective enthalpy changes in brackets in kJ mol-1. b Transition temperature from DSC. c Transition temperature from POM. Abbreviations: Crcolrec = soft crystal columnar rectangular phase, Colh = Columnar hexagonal, ND = Discotic nematic, Iso = isotropic liquid a

Compounds 4.3 and 4.4 exhibited ND phase at room temperature which transform into the isotropic phase on heating at 76.6 (∆H = 0.25 kJ mol-1) and 72.1 °C (∆H = 0.13 kJ mol-1), respectively (Figure S26, ESI). Under POM, compound 4.3 showed the appearance of the mesophase at 73.2 °C, on cooling from the isotropic phase. On the other hand, compound 4.4 showed the mesophase formation at 70.3 °C, with the transition enthalpy (∆H) of 0.24 kJ mol-1 (on cooling). It can be noted that in all the compounds the assignment of ND phase has been done purely on the basis of X-ray diffraction studies, while POM studies, indicate only presence of the mesophase. The thermal stability of all the compounds was measured using thermo gravimetric analysis (TGA) as shown in Figure S29 (see ESI). The compounds were found to be stable upto 146 to 227 °C depending on the chain length. With the increase of chain length the thermal stability of the compounds decreases from 4.1 - 4.4, but in case of 4.5 the thermal stability is greater than that of its n-alkyl chain analogue (4.2).

X-ray diffraction studies: In order to completely identify the structure of mesophases of compound 4.1 - 4.5, small and wide angle X-ray scattering (SAXS/WAXS) studies were carried out. The X-ray diffraction pattern of compound 4.1 below 31 °C shows a series of narrow reflection peaks in the low‐angle region (Figure 2a, b). These series of reflections are indexed on a columnar 2D centred rectangular (Crcolrec) lattice. Measured and calculated d‐spacing values are listed in Tables 2, S1, S2 and S3 (see ESI).


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Table 2.The observed and calculated d-spacings and planes (indices) & correlation lengths of the diffraction peaks of the soft crystal columnar rectangular (Crcolrec) phase observed at 22 °C in compound 4.1. The calculated lattice parameters are a = 33.60 Å, b = 29.92 Å and c = 3.63 Å. d-spacing

d-spacing

Correlation

Experimental

Calculated

Length

dobs (Å)

dcal (Å)

ξ (Å)

11

22.49

22.36

131.6

5.9±0.1

20

16.79

16.83

31.7

1.9±0.2

13

9.67

9.56

40

8.50

8.42

hs

7.18

7.33

83.2

11.6±0.7

44

5.63

5.59

62

5.15

5.25

26

4.80

4.78

64

4.55

4.49

55

4.40

4.47

82

4.04

4.05

66

3.82

3.73

ha

4.83

8.5

1.8±0.3

hc

3.63

94.3

26.0±1.6

Planes (hk)

ξ/dobs

The cell parameters (at 22 °C) are a = 33.60 Å, b = 29.92 Å. Interestingly, there are two sharp peaks hs and hc in the mid and wide angle region of spacings 7.18 Å and 3.63 Å, respectively. Whereas, hc originates due to core‐to‐core correlation (π−π interaction), indicative of columnar


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nature of the phase and hs peak appears because of flip-flop arrangement of compound inside the column (vide infra). The hc peak provides the other lattice parameter i.e. c is 3.63 Å, indicative of core to core (π−π) interaction. This compound also shows a broad background peak, ha of spacing about 4.83 Å which mainly reflects the liquid-like correlation of the molten chains. The correlation lengths of the (11), hs, hc and ha peaks and their corresponding number of correlated units are summarized in the Table 2. In the higher temperature range (31°C

Phase Behavior of a New Class of Anthraquinone-Based Discotic

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