Phase Behavior of a New Class of Anthraquinone ... - ACS Publications

Article Cite This: Langmuir XXXX, XXX, XXX-XXX

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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*,† †

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 S Supporting Information *

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 and 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 the soft crystal columnar rectangular (Crcolrec) phase was stable at lower temperature for the shortest peripheral alkoxy chain (4.1; R = n-C6H13) and was found to exhibit the columnar hexagonal (Colh) phase and then the discotic nematic (ND) phase with increasing temperature. In contrast, increasing the peripheral chain length to n-C8H17 or the branched one (4.2 and 4.5) stabilized the Colh phase at lower temperature and showed the ND phase at higher temperature. Further increase in chain length (4.3 and 4.4; n-C10H21, n-C12H25) demonstrated the formation of the ND phase. Conductivity measurement in the Colh mesophase was found to be almost 10 times higher in magnitude than the corresponding Crcolrec phase. The HOMO−LUMO band gap of all the compounds was found to be in the range from 2.79 to 2.82 eV, which is quite less and comparable with the optical energy band gap.

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INTRODUCTION In recent years, intensive research has focused on the design and synthesis of function-integrated 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 preprogrammed 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 © XXXX American Chemical Society

interactions within the same column are much stronger than interactions between neighboring columns, charge transport in these materials is expected to be quasi-one-dimensional. Thus, the AQ derivatives can act as an n-type or p-type semiconductor 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 sensors,24 and supramolecular scaffolds for biological systems, such as bacteria25,26 due to their easier processability, spontaneous alignment between electrodes, selfhealing 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, Received: August 27, 2017 Revised: November 7, 2017 Published: November 8, 2017 A

DOI: 10.1021/acs.langmuir.7b03031 Langmuir XXXX, XXX, XXX−XXX

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Langmuir Scheme 1. Synthesis of Anthraquinone Derivatives 4a

a Reagents and conditions: (i) NaOH, RBr, DMSO, 70 °C, 18 h; (ii) Tf2O, pyridine, dry DCM, RT, 24 h; (iii) Pd(PPh3)2Cl2, PPh3, CuI, Et3N, reflux, 18 h.

branched one, 4.5), the Colh phase was found to get stabilized at lower temperature and the ND phase at higher temperature. Further increase to decyl and dodecyl chains (4.3 and 4.4) leads to the stabilization of only the 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 types 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.

providing valuable insights into the nature and strength of such noncovalent interactions responsible for different self-assembly into columns of discotic molecules facilitates the design of 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 behavior 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-dihydroxy-2,3,6,7-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 leads to a change in the mesophase behavior of the AQ derivatives. The change in the mesophase behavior 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 hybrids in the mesophase through simple variation in the number of alkoxy chains (R = n-C6H13, n-C8H17, n-C10H21, n-C12H25, and 3,7dimethyl 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 intramolecularly hydrogen bonded to the adjacent quinone carbonyls. Interestingly, it was observed that the compound with the shortest hexylalkoxy chains (4.1; R= n-C6H13) 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

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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 and 230−400 mesh). Thin layer chromatography (TLC) was performed on aluminum 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 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 B


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Langmuir Table 1. Experimental Data of the Thermal Properties of Compounds 4.1−4.5a mesogen

heating scan

4.1 4.2 4.3 4.4 4.5

Crcolrec 33.2 (0.54) Colh 65.6 (0.04) ND 88.2 (0.10) Isob Colh 82.3 (2.41) ND 111.3 (0.10) Isob ND 76.6 (0.25) Isob ND 72.1 (0.13) Isob Colh 101.5 (17.41) ND 115.4 (1.01) Isob

cooling scan Iso Iso Iso Iso Iso

86.5 ND 62.2 Colh 31.4 Crcolrecc 109.4 (0.05) NDb 81.4 Colhc 73.2 NDc 70.3 (0.24) NDb 111.9 (0.36) NDb 90.2 Colhc

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

compounds. 1H NMR spectra were recorded using deuterated chloroform (CDCl3) as solvent and tetramethylsilane (TMS) as an internal standard. All of the UV−vis experiments were performed in 1 μM CHCl3 solutions. Cyclic voltammetry (CV) experiments were performed on a 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 upon both heating and cooling. Thermogravimetric analysis (TGA) was carried out from 25 to 500 °C (at a heating rate of 10 °C min−1) under a nitrogen atmosphere on a Shimadzu DTG-60 instrument. Textural observations of the mesophase were performed with a 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 a Xeuss (Model C HP100 fm) X-ray diffractometer from Xenocs equipped with a 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. The dielectric measurements were carried out using a Solartron SI 1260 Impedance/gain-phase analyzer and Solartron dielectric interface 1296 with Mettler Toledo FP82HT temperature controller. The synthesis, procedures, spectroscopic data of all of the compounds, POM images, TGA data, X-ray data, photophysical studies, electrochemical studies, and conductivity data are given in detail in the Supporting Information.

first compound of the series, 4.1, with the shortest alkyl spacer (i.e., n = 6) displays a room temperature LC mesophase. In DSC, on heating, it exhibited three mesophases with transitions at 33 and 65.6 °C, before it clears at 88.2 °C (Figure S26, Supporting Information). However, on cooling, these transitions were not perceived in DSC, maybe because of low enthalpy change, but clearly observed under POM (Figure 1).

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 the 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 the 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 was carried out with trifluoromethanesulfonic anhydride and pyridine at room temperature to get 2,3,6,7-tetrakis(alkyloxy)-9,10-dioxo-9,10dihydroanthracene-1,5-diyl bis(trifluoromethanesulfonate) 3.1−3.5. These compounds were used in twofold Sonogashira cross-coupling in the 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 (Figures S1−S25, see the Supporting Information). MALDI-MS spectra also confirmed the expected mass of all of the final compounds (see the Supporting Information). Thermal Behavior. The thermal behavior of all of the compounds is determined by polarized optical microscopy (POM) and differential scanning calorimetry (DSC) study. The phase transition temperatures of all of the compounds together with transition enthalpy values are summarized in Table 1. The

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 ×200).

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Such a type of behavior 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 (ΔH = 0.54 kJ mol−1) is attributed to Crcolrec to Colh phase transition, and the peak at 65.6 °C (ΔH = 0.04 kJ mol−1) corresponds to Colh to ND phase transition (vide inf ra). On cooling, under POM, the appearance of partially homeotropically aligned, movable birefringent texture was observed at 86.5 °C, indicating the existence of a mesophase which was confirmed to be ND, as suggested from detailed X-ray diffraction analysis (vide inf ra) (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, Figure 2c). The texture of the Colh phase was then transformed to broken, fourbrush type texture at 31.4 °C, indicating transition to a soft crystal phase which was confirmed by X-ray measurements to be the Crcolrec phase36,37 (Table 1, Figure 2a, Figure S28, see the Supporting Information). The coexistence of both of the mesophases was clearly evident from Figure 1b. C


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Figure 2. X-ray diffraction pattern of compound 4.1: (a) at temperature 22 °C with indexing for small and wide angle peaks (inset) in the soft crystal columnar rectangular (Crcolrec) phase and (b) corresponding 2D X-ray diffraction pattern. (c) Diffraction pattern of the hexagonal (Colh) phase (blue in color) at a temperature of 40 °C (the inset shows the zoomed view of the mid angle region) and nematic (ND) phase (red in color) at temperature 80 °C and (d, f) corresponding 2D X-ray diffraction pattern at 40 and 80 °C. (e) SAXS profile of the ND phase obtained at 70 °C in compound 4.1. The experimental data (blue color half-filled diamonds) and fitted Lorentzian curve (red line) superimposed over a linear background. (The inset shows the deconvolution of the wide angle peak. The half-filled blue sphere corresponds to the wide angle data. The curves in cyan and green colors are corresponding to the fluid chain−chain, ha, and core−core separation, hc, respectively. The red curve is the sum of the above-mentioned two curves.) ha - due to fluid chain−chain correlation, hc - due to core−core (face-to-face, π−π interactions) correlation, and hs due to flip-flop (...ABAB...) like arrangement of compound in column.

Similarly, compound 4.2 exhibited columnar to nematic phase transition (Colh to ND, as confirmed from X-ray diffraction) at 82.3 °C (ΔH = 2.41 kJ mol−1) and a mesophase to isotropic (Iso) phase transition at 111.3 °C (ΔH = 0.10 kJ mol−1). On cooling under POM, the mesophase (ND) appeared at 109.4 °C which then transformed to the Colh phase at 81.4 °C (Figure S26, see the Supporting Information). Compound 4.5 with branched alkyl spacers shows a similar type of textural transition, as observed for compound 4.2 (see Table 1). Compounds 4.3 and 4.4 exhibited the ND phase at room temperature which transforms into the isotropic phase on heating at 76.6 (ΔH = 0.25 kJ mol−1) and 72.1 °C (ΔH = 0.13 kJ mol−1), respectively (Figure S26, Supporting Information). 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 (ΔH) of 0.24 kJ mol−1 (on cooling). It can be noted that in all of the compounds the assignment of the ND phase has been done purely on the basis of X-ray diffraction studies, while POM studies indicate only the presence of the mesophase. The thermal stability of all of the compounds was measured using thermogravimetric analysis (TGA), as shown in Figure S29 (see the Supporting Information). The compounds were found to be stable up to 146−227 °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 the 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 compounds 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 centered rectangular (Crcolrec) lattice. Measured and calculated d-spacing values are listed in Tables 2, S1, S2, and S3 (see the Supporting Information). The cell parameters (at 22 °C) are a = 33.60 Å and b = 29.92 Å. Interestingly, there are two sharp peaks hs and hc in the mid and wide angle region of spacings 7.18 and 3.63 Å, respectively. hc originates due to core-to-core correlation (π−π interaction), indicative of the columnar nature of the phase, and the hs peak appears because of the flip-flop arrangement of the compound inside the column (vide inf ra). 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 of 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 Table 2. In the higher temperature range (31 °C < T < 62 °C) on cooling, two peaks (Figure 2c,d) in the small angle region are observed, within a q ratio of 1:√3, which confirms the occurrence of the columnar hexagonal (Colh) phase and the calculated lattice parameter, a, is found to be 25.30 Å at 40 °C (Table 3). hs, hc, and ha peaks are also observed in the wide angle region. The correlation lengths of the (10), hs, hc, and ha peaks and their corresponding number of correlated units are summarized in Table 3. D


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phase) is not observed, (ii) variation of correlation length (ξ)/ number of correlated column units (n) for the compound 4.1 clearly shows three-step melting, as shown in Figure 3a, and as also confirmed by DSC (Figure S26, Supporting Information), (iii) the peak’s profile fits to a Lorentzian curve superimposed over a linear background (Figure 2e) and the side-by-side correlation length (ξ) is about 50 Å which corresponds to about 2 correlated disc units alongside, (iv) further, the wide angle peak is the convolution of two peaks, as shown in Figure 2e (inset). The curves in cyan and green colors correspond to the fluid chain−chain and core−core (disc−disc) separation, respectively. The correlation length (ξ) corresponding to the core-to-core peak is found to be about 8 Å, which is equivalent to about 2 correlated disc units along face to face (for the columnar nature, this should be more than 8). Therefore, after considering all facts, the phase can be assigned as discotic nematic (ND) in nature. Similarly, compound 4.2 shows the Colh mesophase at low temperature which transforms into the ND phase at higher temperature (Figure 4a,b, Tables S4−S7, see the Supporting Information). However, the Crcolrec phase is not seen in this compound. Further, the behavior of compound 4.5 is very similar to 4.2 and exhibits the Colh and ND phases (Figure 4c,d, Tables S8−S10, see the Supporting Information). This compound also does not show the Crcolrec phase. Furthermore, compounds 4.3 and 4.4 exhibit only the ND phase; the details are given in Table S11 and Table S12 (see the Supporting Information). However, the ha peak in compounds 4.1−4.5 appears at about q ≈ 1.25 Å−1 which is mainly due to fluid chain−chain separation. However, for compound 4.5, this peak appears at slightly lower q value and is also a bit narrow in appearance. That is because compound 4.5 consists of a branched chain which leads to ordering of chain-to-chain correlation. As a result, the chain−chain separation is relatively higher and chains are more ordered. Therefore, the corresponding peak is narrow and appears at a slightly lower q value. Next, the variation of correlation length corresponding to column−column correlation (/number of correlated column units (n)) in the column plane for compounds 4.1−4.5 is shown in Figure 3a. This also reveals the three-step (Crcolrec → Colh → ND → I), two-step (Colh → ND → I), and one-step (ND → I) melting of compounds 4.1, 4.2, and 4.3, respectively. Compounds 4.5 and 4.4 show similar behavior as compounds 4.2 and 4.3, respectively. The number of correlated column units (n) (side by side) in the Colh phase is about 7 for compounds 4.2 and 4.5. However, this number is about 5 in the Colh phase for compound 4.1. It seems that compound 4.1 could not pack nicely in the Colh phase because of its inherent tendency to be in rectangular shape. Moreover, this value is about 6 in the Crcolrec phase. Further, the number of correlated disc units within the column along face to face is found to be about 12 and 26 in the Colh and Crcolrec phases, respectively (Tables 2, 3, and S4 (see the Supporting Information)), indicating their nice columnar nature. In contrast, the number of correlated disc units alongside is about 2 in the ND phase in compounds 4.1, 4.2, and 4.5 (Tables S7 and S10 (see the Supporting Information)). However, a comparatively lesser value (less than 2) of correlated disc units alongside is found in compounds 4.3 and 4.4 (Tables S11 and S12 (see the Supporting Information)). However, the reported value of n for ND in the literature is around 2.38 The lesser value (less than 2) in

Table 2. Observed and Calculated d-Spacings and Planes (Indices) and Correlation Lengths of the Diffraction Peaks of the Soft Crystal Columnar Rectangular (Crcolrec) Phase Observed at 22 °C in Compound 4.1a planes (hk)

d-spacing experimental dobs (Å)

d-spacing calculated dcal (Å)

11 20 13 40 hs 44 62 26 64 55 82 66 ha hc

22.49 16.79 9.67 8.50 7.18 5.63 5.15 4.80 4.55 4.40 4.04 3.82 4.83 3.63

22.36 16.83 9.56 8.42 7.33 5.59 5.25 4.78 4.49 4.47 4.05 3.73

correlation length ξ (Å)

ξ/dobs

131.6 31.7

5.9 ± 0.1 1.9 ± 0.2

83.2

11.6 ± 0.7

8.5 94.3

1.8 ± 0.3 26.0 ± 1.6

a

The calculated lattice parameters are a = 33.60 Å, b = 29.92 Å, and c = 3.63 Å.

Table 3. Observed and Calculated d-Spacings and Planes (Indices) and Correlation Lengths of the Diffraction Peaks of the Hexagonal (Colh) Lattice Observed at 40 °C in Compound 4.1a planes (hk)

d-spacing experimental dobs (Å)

d-spacing calculated dcal (Å)

10 11 hs ha hc

21.85 12.81 7.66 4.85 3.78

21.91 12.65

a

correlation length ξ (Å)

ξ/dobs

110.7

5.1 ± 0.1

44.2 7.7 46.5

5.8 ± 0.7 1.6 ± 0.4 12.3 ± 1.0

The calculated lattice parameters are a = 25.30 Å and c = 3.78 Å.

The phase at the high temperature range (62 °C < T < 86 °C) on cooling is birefringent and exhibits a broad peak in the small angle area, showing short-range positional order with a mean distance between scattering objects of 21.50 Å (Figure 2c,e,f and Table 4). This mesophase transforms into an isotropic phase on increasing the temperature. Further, the following facts are associated with this mesophase: (i) the phase is not the coexistence of isotropic (Iso) and hexagonal (Colh) phases, because superimposition of a broad (corresponding to the Iso phase) and a sharp peak (corresponding to the Colh Table 4. Variation of Phases and Respective Lattice Parameter/Column-to-Column Separation (cs) with Temperature in Compound 4.1 temperature (°C)

lattice parameter/column-to-column separation (Å)

25

a = 32.90 b = 30.44 a = 25.30 a = 25.21 cs = 21.56 cs = 21.42

40 48 70 80 110

phase Crcolrec Colh Colh ND ND I E


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Figure 3. (a) Variation of correlation length (ξ)/number of correlated column units (n) in compounds 4.1−4.5 with temperature on cooling. Black, red, blue, green, and magenta colors are used for compounds 4.1, 4.2, 4.5, 4.3, and 4.4, respectively. Crcolrec, Colh, ND, and Iso denote the columnar rectangular, hexagonal, nematic, and isotropic phases, respectively. Bars on the curves in pink, green, and cyan colors correspond to Crcolrec to Colh, Colh to ND, and ND to Iso phase transition temperatures, respectively. (b) X-ray diffraction pattern of compounds 4.1−4.5 at 25 °C.

Figure 4. (a) X-ray diffraction pattern of compound 4.2 at a temperature of 25 °C in the Colh phase with indexing for small angle and wide angle (inset) peaks and (b) corresponding 2D X-ray diffraction pattern. (c) X-ray diffraction pattern of compound 4.5 at temperature 25 °C in the hexagonal (Colh) phase with indexing for small angle and wide angle (inset) peaks and (d) corresponding 2D X-ray diffraction pattern.

and Figure S30, see the Supporting Information). The electron density map along with the molecular structure of compound 4.1 is also shown in Figure 5b for better clarity. The reconstructed electron density maps of the Colh phases are shown in Figure 5c,d and Figure S31 (see the Supporting Information). On the basis of indexing and a reconstructed electron density map, a layered structural model for the Crcolrec phase is proposed. As the spacing of the hs peak is observed to be double of the hc peak, we hypothesized an ...ABAB... kind of layering in the mesophase, as shown in Figure 6. We assumed two possible orientations of compound 4.1 and called them clockwise (CW) and anticlockwise (AW), as shown in Figure 6a(i) and (ii), respectively. In layer A, CW and AW units are separately arranged on a 2D rectangular lattice in such a way that the center of the CW rectangular lattice is surrounded by

compounds 4.3 and 4.4 is most probably due to their bulky peripheral chains which like to form a more disordered structure. Further, the number of correlated disc units along face to face is about 2, confirming its discotic nature. In summary, the key observation is that the Crcolrec phase transforms into the ND phase via the Colh phase with increasing temperature as well as peripheral chain length (Figure 3b). The electron density maps were reconstructed with the Miller indices and intensities of the reflections to determine the exact positions of the anthraquinone cores in the respective unit cell (Tables 2 and 3, Tables S2−S6, S8, and S9, see the Supporting Information). Red represents the highest electron density, and dark blue, the lowest. The reconstructed electron density maps of the Crcolrec phase show the packing of anthraquinone cores on the 2D rectangular lattice (Figure 5a F


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Figure 5. (a) Reconstructed electron density map of the Crcolrec phase of compound 4.1 at 22 °C along with the packing of the molecule. The rectangle on the map shows a unit cell of the Crcolrec phase. The molecule at the center of the rectangle is rotated by 180° with respect to the corner one. (b) Zoomed visualization of the electron density map along with the molecular structure of compound 4.1. Reconstructed electron density maps of the Colh phase of (c) compound 4.1 at 40 °C and (d) compound 4.2 at 25 °C. The parallelogram on the map (in black color) shows the unit cell of the Colh phase, and the set of the dotted parallel deep green and magenta lines correspond to the (10) and (11) planes of the 2D hexagonal lattice, respectively. The color bar for the electron density map is also given; dark blue corresponds to the lowest electron intensity, and red is the highest electron intensity, respectively. a,⃗ b⃗, and angle α are the lattice parameters. In the case of Colh, |a⃗| = |b|⃗ and α = 60°.

in the column is not fixed, but they have mobility in the column. As described above, compounds 4.1−4.5 exhibit three different columnar mesophases (Crcolrec, Colh, and ND). The phase sequence is found to be Crcolrec to Colh to ND with increasing temperature/periphery chain length. The schematic arrangment is shown in Figure 7a. The phase transition from Crcolrec to ND via the Colh phase could be most possibly understood in terms of the changes in the effective molecular shape from a smooth-corner rectangular to circular (Figure 7b) with increasing temperature/periphery chain length. That implicitly leads to a decrease in disc-to-disc interaction within the column which results in destruction of the column and hence gives rise to Crcolrec to ND transition via the Colh phase. Further, the observed decrease in the number of correlated cores (number of correlated disc units which correspond to the hc peak) from about 26 in the Crcolrec phase to about 12 in the Colh phase to about 2 in the ND phase is also consistent with the effective shape transformation (implicit decrease in disc-to-

the AW unit and vice versa. Thus, one CW unit is surrounded with four AW units and vice versa (Figure 6b). Similarly, layer B (Figure 6c) is identical to layer A but translated by (a/2, b/2) (where a and b are the lattice parameters of the Crcolrec phase) with respect to layer A. During formation of the Crcolrec ordering, layer B comes just below A and vice versa, leading to ...ABAB... layering. This gives rise to two kinds of mixed columns [AWCWAWCW... (1), CWAWCWAW... (2)]. However, these columns are translationally identical. Therefore, the arrangement of molecules in a column (AW to AW via CW or CW to CW via AW) is either flop to flop via flip orientations or vice versa. The schematic of such a f lip-f lop arrangement is shown in Figure 6d. The arrangement is also described as lamellae of the AWCW unit, and the d-spacing of the lamellae periodicity corresponds to the hs peak. The molecular assembly can be realized in terms of the arrangement of these columns [(1) and (2)] as follows. Columns (1) arranged on a rectangular lattice (a = 33.60 Å, b = 29.92 Å, and α = 90°, at 22 °C in compound 4.1), and the centers of the rectangles are decorated with columns (2), as shown in Figure 6e. The position of disc units G


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Figure 6. (a) Schematics of compound 4.1: (i) clockwise, CW, and (ii) anticlockwise, AW, orientations. Arrangement of compound 4.1 on the 2D rectangular lattice: (b) layer A and (c) layer B. (d) Flip-flop arrangement of compound in the columns. (e) Arrangement of compound 4.1 in the Crcolrec phase. Rectangles in blue and orange boundary colors show the unit cell of the Crcolrec phase centered on column (1) and column (2), respectively.

Figure 8. (a) UV−vis absorption spectra of 4.1. (b) PL spectra of 4.1 recorded at excitation at λmax. Solutions were prepared in CHCl3 at concentrations of ∼1 μM for both UV−vis and PL. The inset shows the corresponding absorption and emission spectra in the thin film state of compound 4.1.

Figure 7. (a) Schematic representation of the phase sequence observed in compounds 4.1−4.5. (b) Schematic of the change in the shape of the compound with increasing temperature/periphery chain length.

absorption and emission spectra have been recorded in ∼1 μM CHCl3 solution for all of the compounds. The details of photophysical properties are documented in Table 5. In the case of compound 4.1, a strong absorption band at 313 nm was observed (Figure 8) which corresponds to π−π* transition and a less intense peak at 403 nm indicates the n−π* (forbidden) transition of the central anthraquinone core.39 The other shoulder peak at 295 nm may be because of the π−π* transition of the benzene ring, as shown in Figure 8.

disc interaction within the column) of the compound along with the phase transitions from Crcolrec to ND via Colh. Photophysical Characterization. The photophysical properties of anthraquinone based compounds 4.1−4.5 have been evaluated by UV−vis and photoluminescence (PL) spectroscopy both in solution and in solid state, as shown in Figures 8, S32, and S33 (see the Supporting Information). The H


Article

Langmuir Table 5. Experimental Data of Photophysical Properties of Compounds 4.1−4.5a mesogen 4.1 4.2 4.3 4.4 4.5 a b

absorption (nm) 295, 296, 298, 292, 297,

313, 314, 311, 311, 314,

403 404 403 398 404

emissionb (nm) 547, 547, 544, 543, 542,

566 579 581 564 582

Stokes shiftb 234 265 270 272 268

Measured in CHCl3 solution at concentrations of ∼1 μM. Corresponding to excitation wavelength.

Figure 9. (a) Cyclic voltammogram of compound 4.1 in HPLC DCM solution of TBAP (0.1 M) at a scanning rate of 50 mV s−1. (b) The HOMO and LUMO energy gap (ΔEg) of compound 4.1 obtained from DFT calculation at the B3LYP/6-311g(d,p) level using Gaussian 09.

Compounds 4.2 and 4.5 exhibit a similar kind of spectra (Figure S32, see the Supporting Information), as observed for compound 4.1. Interestingly, for compound 4.4 with dodecyloxy chain length, the absorption maximum (λmax) was observed at 292 nm related to the π−π* transition of the benzene ring connected through a triple bond with the central anthraquinone core. The π−π* transition and n−π* (forbidden) transition of the anthraquinone core were seen at 311 and 398 nm, respectively (Figure S32, see the Supporting Information). In this compound (4.4), the λmax changes (compared to 4.1, 4.2, and 4.5) may be because of the presence of a longer alkyl chain and, thus, the stacking of the central anthraquinone core gets disturbed. In addition, in the case of 4.4, benzene rings of alkynylbenzenes (Scheme 1) come closer, which favors the π−π* transition of the benzene ring. Compound 4.3 revealed three peaks with 311 nm as absorption maxima similar to 4.4. The PL spectra for all of the compounds have been carried out by exciting the solutions of these compounds at their λmax (Figure 8 and Figure S33, see the Supporting Information). The two peaks were observed at 547 and 566 nm in the PL spectra of compound 4.1 with emission maxima at 547 nm, as shown in Figure 8b. All of the compounds show two peaks in their PL spectra (Table 5). A Stokes shift in the range of ∼234−272 nm has been observed for all compounds (4.1−4.5) (Table 5). In order to perform solid state absorption and emission spectra, a thin film of compound 4.1 was prepared by drop casting of ∼1 μM CHCl3 solution on a glass substrate and the solvent evaporated completely at room temperature. Interestingly, solid state UV−vis spectra of compound 4.1 are almost similar to their solution state spectra (with only a little blue shift (∼4 nm) in its absorption maxima), as shown in Figure 8a (inset). Surprisingly, solid state PL spectra of compound 4.1 also showed blue-shifted emission by 54 nm compared to its solution state (Figure 8b (inset)). This significant blue shift of 54 nm (from emission spectra) could be explained by the aggregation due to intermolecular interactions present in the solid state.40 There are two types of aggregation reported in the literature.41,42 One is the H aggregate which occurs due to the stacking of molecules one on top of another, and another is the J aggregate which is observed when the molecules are arranged in slip disc manner in the aggregated state. Since H aggregates lead to blue-shifted absorption, we hypothesized that in the present case the H-type of aggregation is likely getting stabilized over J aggregates. Electrochemical Characterization. In order to find out the electronic energy level that determines the electron transfer processes, cyclic voltammetry (CV) was carried out. CV studies were performed by using a 1 mM solution of compounds 4.1− 4.5 in dry dichloromethane (DCM), as shown in Figure 9 (see

also Figure S34 in the Supporting Information). Tetrabutylammonium hexafluorophosphate (TBAP) of 0.1 M was used as a supporting electrolyte during the experiment. A single compartment cell equipped with Ag/AgNO3 (0.1 M) as a reference electrode, platinum rod as a counter electrode, and glassy carbon as a working electrode was used for the experiments. The reference electrode was calibrated with the ferrocene/ ferrocenium (Fc/Fc+) redox couple (absolute energy level of −4.80 eV to vacuum) before performing the experiment. The CVs were recorded with a scanning rate of 50 mV s−1. All of the compounds (4.1−4.5) exhibited similar quasireversible oxidation and reduction waves. Figure 9a shows the representative CV spectra of compound 4.1. The optical band gap (ΔEg,UV) of all of the compounds was estimated from the red edge of the absorption spectra.43 In the HPLC DCM, AQs go through two consecutive one-electron reductions to the radical anion and then to the dianion, as depicted by eqs 1 and 2.44,45 +e −

AQ Hooo−I AQ− −e

(1)

+e −

AQ− Hooo−I AQ2 − −e

+e

2AQ− Hooo−I AQ + AQ2 − −e

(2)

−

(3)

Because of the presence of four alkoxy groups, compounds 4.1−4.5 are quite electron-rich, ensuing in a class of compounds that needs large negative potentials to be reduced. In the cathodic scan, two reduction peaks were observed and found to be well separated (peak 1 and peak 2, Figure 9a). Peak 1 is attributed to the first reduction to the corresponding radical anion and requires a reductive potential around −1.091 V (forward path of eq 1). Further one-electron reduction of the radical anion to the dianionic species (forward path of eq 2) is denoted by peak 2 which requires a reduction potential of −1.716 V. During oxidation, two peaks were observed (peaks 3 and 4, Figure 9a). Peak 3 can be assigned due to the oxidation of the AQ dianion to the AQ radical anion (backward path of eq 2). On the other hand, peak 4 is scarce because it is so anodically shifted, but its presence was seen in all of the compounds. Peak 4 is ascribed to a stabilized AQ radical anion, which might be because of the consequence of either an ionpairing interaction or a protonated AQ radical anion, as reported earlier.46−51 I


Article

Langmuir Table 6. Electrochemical Properties of Compounds 4.1−4.5a compounds

λmax,UV (nm)

λonset (nm)

ΔEg,UVb,c

E1 redd

E1 oxdd

EHOMOb,e

ELUMOb,f

ΔEg,CVb,g

ΔEg,DFTb,h

4.1 4.2 4.3 4.4 4.5

313 314 311 311 314

465 466 465 466 463

2.68 2.67 2.68 2.67 2.69

−1.019 −1.241 −1.128 −1.195 −0.963

−1.465 −1.637 −1.536 −1.570 −1.554

−5.41 −5.28 −5.35 −5.34 −5.30

−2.62 −2.47 −2.55 −2.54 −2.48

2.79 2.81 2.80 2.80 2.82

2.88 2.89 2.89 2.89 2.92

a

Experimental conditions: Ag/AgNO3 as reference electrode, platinum wire as counter electrode, glassy carbon as working electrode, tetrabutylammonium perchlorate (0.1 M) as supporting electrolyte, room temperature. bElectronvolts (eV). cBand gap determined from the red edge of the longest wavelength in the UV−visible absorption spectra. dIn volts (V). eEstimated from the formula EHOMO = −(4.8 − E1/2,Fc,Fc+ + Eoxd,onset) eV. fEstimated from the onset reduction peak values using ELUMO = −(4.8 − E1/2,Fc,Fc+ + Ered,onset) eV. gEstimated from the formula ΔEg,CV = ELUMO − EHOMO. hHOMO−LUMO energy gap calculated from DFT studies.

to obtain information on the conductivity of these compounds in the LC state by applying an alternate voltage and a frequency. Figure S37a shows the complex impedance plot [Zim(Ω) vs Zr(Ω)] in the Colh and Crcolrec phases of compound 4.1. The conductivity plot of compound 4.1 was shown in Figure S37b. It is observed that with an increase in temperature the conductivity (σ) values of 4.1 also increased as it changed from the Crcolrec phase to the Colh phase. The conductivity value is found to be ∼10 times higher in the hexagonal mesophase as compared to the rectangular one (Crcolrec/Colh = 4.10 × 10−9 S m−1/6.16 × 10−8 S m−1). The enhancement of the conductivity in the hexagonal phase can be ascribed due to the formation of a more 1-D ordered phase, i.e., an increase in charge migration, which can be explained by the hopping formalism as reported earlier.56 It is also possible that limiting the molecular rotation within the columns leads to a decrease in the degree of freedom within the Colh mesophase, and therefore, the charge carrier mobility within the columns increases.57 All of the experimental data was fitted with the NeMS software (Figure S37c as a representative at 27 °C), and from there, the resistance (RS) and conductivity (σ) values were calculated (see the Supporting Information). For data fitting, we have proposed one model in which resistance (RS) is in series with Wurburg impedance (W) and both of them is in parallel with a constant phase element (CPE) (Figure S37c inside). Variation of conductivity in Colh mesophases for compounds 4.1, 4.2, and 4.5 at 50 °C is shown in Figures S37d and S38. It should be noted that the conductivity values in the Colh mesophase decrease as we move from 4.1 to 4.2. This could be due to more disordered packing because of the higher alkoxy chain length in the case of compound 4.2. Interestingly, adding a branched chain (compound 4.5), a slight increase in conductivity was observed as compared to compound 4.2. The values obtained for the electrical conductivities in the Colh mesophase are 6.16 × 10−8, 1.78 × 10−8, and 2.04 × 10−8 S m−1 for compounds 4.1, 4.2, and 4.5, respectively.

In the anodic scan, an additional broad peak (just after peak 3) is observed for all of the compounds. The reoxidation of radical anion (nonstabilized) to the neutral AQ (backward path of eq 1) is probably the reason for this unusual phenomena, as reported earlier.46−51 In this case, one can consider another equilibrium because the integration of peak 2 is unequal to the charge consumed in peak 3, which indicates that there might be formation of different chemical species between the reactions of two AQ dianions. Literature survey reveals disproportionation (forward path of eq 3) and comproportionation (backward path of eq 3) redox reactions for quinone derivatives.52,53 It is observed that, upon formation of the AQ dianion (at a scan rate of 0.05 V s−1), a portion of the dianionic species might undergo a comproportionation reaction to form AQ radical anions. For compound 4.1, the dianion is formed as demonstrated by peak 2 (Figure 9a). After that, this dianionic species undergoes a partial comproportionation reaction to produce the AQ radical anion. The remaining amount of AQ dianions (that did not undergo the comproportionation reaction) is then reduced to the AQ radical anions, as shown in peak 3, which clarifies the unequal charge balance between peaks 2 and 3. Then, AQ radical anions are oxidized to neutral AQ which is clearly evident from peak 4 in Figure 9a. The HOMO and LUMO energy levels and the corresponding energy gap have been stated in Table 9. HOMO energy levels are calculated by using the formula EHOMO = −(4.8 − E1/2,Fc,Fc+ + Eoxd,onset) eV and LUMO energy levels by ELUMO = −(4.8 − E1/2,Fc,Fc+ + Ered,onset) eV.54 The energy gaps between the HOMO and LUMO of all of the compounds (4.1−4.5) are found to be almost similar. This clearly suggests that changing the alkyl chain length in the ether linkage has little influence on the stabilization of the HOMO and LUMO energy level. A DFT analysis (B3LYP/6-311G(d,p)) was carried out to investigate the HOMO and LUMO energy levels. The band gap of compounds 4.1−4.5 is mentioned in Figure 9b, Figures S35 and S36, and Table S13 (see the Supporting Information). A comparative study of band gaps via UV−vis, CV, and DFT calculation is given in Table 6. The calculated values of band gaps by CV are in good agreement with the optical energy band gap calculated from the red edge of the absorption spectra as well as with DFT study. Among the five compounds, 4.5 is found to be more stable because of the higher HOMO−LUMO energy gap.55 Electrical Conductivity Studies. One-dimensional conductivities have been measured with the complex impedance method by the indium tin oxide (ITO) coated glass sandwich cells (cross-sectional area = 0.66 cm2) with a separation of 20 μm. The samples were filled into the cell through capillarity action. The impedance spectroscopy provides a useful method

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CONCLUSIONS In summary, we have synthesized and characterized a new class of anthraquinone derivatives which exhibit three mesophases, Crcolrec, Colh, and ND, with respect to the change in the temperature and also in the peripheral chain length. X-ray scattering studies provide an insight of the phase transitions at different peripheral chain length consistent with the effective shape transformation of the compounds. The observed band gap values in the range 2.79−2.82 eV of all the compounds may J


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be suitable for possible potential applications in the device point of view.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.7b03031. Experimental section and characterization details, NMR spectra, POM images, TGA data, X-ray data, photophysical studies, electrochemical studies, and conductivity data (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; [email protected]. ORCID

Santanu Kumar Pal: 0000-0003-4101-4970 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was carried out with the financial support from IISER Mohali. We are grateful to the NMR, HRMS, and SAXS/ WAXS facility at IISER Mohali. J.D. and I.B. acknowledge the receipt of a graduate fellowship from IISER Mohali and CSIRNET, respectively. We thank Dr. S. Gayan from Dr. Gautam Sheet’s lab at IISER Mohali for useful discussions related to conductivity measurements and Dr. Vivek Bagchi and his student Ritu Rai from INST Mohali for cyclic voltammetry measurements. S.K.P. is grateful for INSA Medal for Young Scientist 2015 and the financial support from INSA bearing Sanction No. SP/YSP/124/2015/433. S.K.P. is grateful for the financial support from CSIR bearing Sanction No. 02(0311)/ 17/EMR-II for the project entitled “Synthesis and Characterization of Nanographenes based on hexabenzocoronene Discotics for Photovoltaics Applications”.

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