Triphenylene-Based Room-Temperature Discotic ... - ACS Publications

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Triphenylene-Based Room-Temperature Discotic Liquid Crystals: A New Class of Blue-Light-Emitting Materials with Long-Range Columnar Self-Assembly Monika Gupta† and Santanu Kumar Pal*,† †

Department of Chemical Sciences, Indian Institute of Science Education and Research (IISER) Mohali, Knowledge City, Manauli 140306, India S Supporting Information *

ABSTRACT: A straightforward synthesis of multialkynylbenzene-bridged triphenylenebased dyad systems (via flexible alkyl spacers) that self-organize into room-temperature columnar structures over a long range is reported. The compounds with spacer lengths (n) of 8 and 10 exhibit a columnar rectangular mesophase whereas a compound with n = 6 shows a columnar rectangular plastic phase. Interestingly, the later compound (n = 6) shows the formation of well-nucleated spherulites of about several hundred micrometers that suggest the existence of a long-range uniform self-assembly of columns. All of these compounds show blue luminescence in solution and in the thin-film state under longwavelength (365 nm) UV light. These compounds fulfill the described demands such as long-range columnar self-assembly at room temperature, a good yield with high purity, and blue-light emitters under the neat condition for possible potential applications in semiconductor devices. They also match the criteria of facile processing from the isotropic state because of their low isotropization temperature. This new class of materials is promising, considering the emissive nature and stabilization of the columnar mesophase at ambient temperature.

1. INTRODUCTION Emissive organic semiconductors are receiving increasing attention nowadays in the development of electronic devices, such as in fabricating white organic light-emitting diodes (WOLEDs) for next-generation solid-state light sources, fieldeffect transistors, and display applications.1−6 Only the triads of red-, green-, and blue-emissive materials can produce the white light that can revolutionize lighting technology.7−9 In the literature, red/green-emissive materials are widely reported, but blue-emitting materials are limited and have remained a challenge for three decades. The invention of an efficient blue LED based on inorganic semiconductors has contributed to the creation of white light in an entirely new way to us.10 However, most of the inorganic semiconductors suffer from high cost and low efficiency and limit fabrication in a flexible substrate. The latter option can be solved by using luminescent polymers, but they also experience trouble with low solubility, high purity, and stability. In this regard, luminescent organic small molecules have advantages in terms of all aspects described above.11 In addition, the combination of luminescence with columnar self-assembly formed by discotic liquid crystals (DLCs) can enhance the applicability of emissive molecules in OLED applications.12−15 The interest in DLCs as smart materials for OLED devices is due to their advantageous properties such as high intrinsic charge-carrier mobility, ease of processability, and high purity that reduces possible tapping sites for charge carriers.16−18 Inherent charge-carrier mobility is important because, in an OLED, the opposite charges from the two electrodes undergo recombination in the emissive layer to generate the light. © 2016 American Chemical Society

Furthermore, a pronounced and defect-free long-range order of columnar (col) self-assembly is favorable for efficient charge transport. In the literature, there are only a few reports that described long-range self-assembly in molecules such as terphenyls, crowded arenes, hexa-peri-hexabenzocoronene, and graphite oxide LCs.19−22 Some examples of blue-light-emitting molecules based on discotics have been reported recently, but most of them exhibit photoluminescence in solution, which is not suitable for device fabrication.23−26 Longo et al. reported a new core for luminescent DLCs based on tristriazolotriazines.27 There are some reports on luminescent materials based on tristriazolotriazine, 1,3,4-oxadiazole, and thiadiazole that exhibit blue luminescence in the solid state.28−33 However, in most of these cases, materials exhibit a columnar phase at very high temperature without showing any long-range columnar selfassembly. In some systems, although they show glassy characteristics with the retention of columnar order (on cooling from an isotropic liquid), they are not suitable to use as pristine because of their crystalline nature at room temperature. They also exhibit very high isotropization temperatures and thus low processability, which limits their widespread use in various optoelectronic devices. Therefore, the design of new materials combining long-range columnar self-assembly around room temperature and strong photoluminescent behavior in the solid state will serve as promising candidates in the future in the field of reliable and Received: September 5, 2015 Revised: December 13, 2015 Published: January 8, 2016 1120

DOI: 10.1021/acs.langmuir.5b03353 Langmuir 2016, 32, 1120−1126

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alkylation methods as a result of the rapid formation of quinone. The synthesis of compound 4, therefore, was carried out using solvent-free conditions. Tetrabromohydroquinone 3 (1 equiv) was finely ground with KOH (2.5 equiv), and a small amount of tetrahexylammonium bromide (0.09 equiv) was also mixed, followed by the addition of compound 2 (2.5 equiv). The mixture was transferred to a 10 mL roundbottomed flask and was heated to 80 °C for 12 h with continuous stirring. On completion of the reaction, as monitored by TLC, the mixture was diluted with water and the product was extracted with dichloromethane, which was further purified by column chromatography to obtain target compound 4. For the synthesis of compound 5, 30 mL of dry triethylamine was taken in a round-bottomed flask that had been degassed, followed by the addition of Pd(PPh3)Cl2 (50 mg), CuI (50 mg), PPh3 (100 mg), and compound 4 (0.63 mmol). The mixture was stirred for 15 min, followed by the gradual addition of 1-ethynyl-4pentyl benzene (6.3 mmol). The reaction mixture was stirred at 100 °C for 24 h under nitrogen, and after cooling to room temperature it was poured into 30 mL of 5 M HCl. The product was extracted with DCM and was purified by column chromatography to obtain target compounds 5. The synthesized compounds were characterized by 1H NMR, 13C NMR, FT-IR, UV−vis, and mass spectrometry as shown below (the Supporting Information, Figures S1−S7): 2.2.1. Compound 5a. FT-IR (cm−1): 3035.0, 2954.59, 2929.83, 2857.83, 2209.8, 1616.87, 1516.30, 1467.64, 1434.78, 1388.36, 1330.31, 1262.88, 1173.22, 1043.52, 838.30, 726.50, 601.28. UV−vis (nm): 262, 272, 280, 318, 377. 1 H NMR (400 MHz, CDCl3, δ): δ 7.85 (d, 12H, J = 12 Hz), 7.51 (d, 8H, J = 8 Hz), 7.13 (d, 8H, J = 8 Hz), 4.36 (t, 4H, J = 4, 8 Hz), 4.24 (m, 24H), 2.53 (t, 8H, J = 8 Hz), 1.97 (m, 28H), 1.76 (m, 8H), 1.59 (m, 28H), 1.35 (m, 60H), 0.92 (m, 42 H). 13 C NMR (400 MHz, CDCl3, δ): 157.28, 157.12, 149.0, 148.94, 143.94, 131.63, 128.53, 123.63, 121.09, 120.45, 107.29, 99.38, 84.12, 69.70, 35.88, 31.71, 31.46, 30.87, 29.45, 26.29, 25.88, 22.69, 22.50, 14.09. MS: m/z for C166H266O14, 2444.7006; found, 2444.7664. 2.2.2. Compound 5b. FT-IR (cm−1): 2960.8, 2929.12, 2857.44, 2214.8, 1616.27, 1512.91, 1467.79, 1434.76, 1386.01, 1262.61, 1171.09, 1039.87, 838.88, 726.28, 600.67. UV−vis (nm): 262, 272, 280, 318, 373. 1 H NMR (400 MHz, CDCl3, δ): δ 7.86 (s, 12H), 7.51 (d, 8H, J = 8 Hz), 7.17 (d, 8H, J = 8 Hz), 4.32 (t, 4H, J = 4, 8 Hz), 4.25 (m, 24H), 2.59 (t, 8H, J = 8 Hz), 1.96 (m, 28H), 1.6 (m, 28H), 1.36 (m, 92H), 0.93 (m, 42 H). 13 C NMR (400 MHz, CDCl3, δ): 157.15, 148.93, 143.91, 131.93, 131.58, 128.54, 123.59, 121.07, 120.52, 107.25, 99.34, 98.02, 97.50, 84.16, 74.71, 69.69, 69.07, 64.64, 35.94, 31.72, 31.48, 30.96, 29.44, 25.88, 22.70, 22.54, 14.11, 14.06. MS: m/z for C170H234O14, 2500.7632; found, 2500.7691. 2.2.3. Compound 5c. FT-IR (cm−1): 3026.21, 2954.02, 2928.65, 2856.77, 2210.06, 1616.65, 1514.01, 1467.59, 1434.93, 1387.78, 1330.08, 1262.57, 1170.96, 1041.82, 928.52, 838.08, 725.79, 600.93, 550.16, 528.02. UV−vis (nm): 237, 262, 271, 280, 318, 373. 1 H NMR (400 MHz, CDCl3, δ): δ 7.85 (s, 12H), 7.51 (d, 8H, J = 12 Hz), 7.18 (d, 8H, J = 8 Hz), 4.30 (t, 4H, J = 8, 4 Hz), 4.25 (m, 24H), 2.63 (t, 8H, J = 8 Hz), 1.96 (m, 28H), 1.6 (m, 28H), 1.36 (m, 84H), 0.93 (m, 42 H). 13 C NMR (400 MHz, CDCl3, δ): 157.16, 148.95, 143.86, 131.98, 131.59, 128.52, 123.61, 121.05, 120.55, 107.32, 99.33, 84.18, 74.75, 69.70, 35.96, 31.72, 31.48, 30.96, 29.73, 29.45, 26.49, 26.28, 25.88, 22.69, 22.55, 14.11, 14.09, 14.05. MS: m/z for C174H242O14, 2556.8258; found, 2556.8027. 2.3. Instrumental. 2.3.1. Structural Characterization. Structural characterization of the compound was carried out through a combination of infrared spectroscopy (PerkinElmer Spectrum AX3), 1 H NMR and 13C NMR (Bruker Biospin Switzerland Avance-iii 400 and 100 MHz spectrometers, respectively), UV−vis−NIR spectrophotometry (Agilent Technologies UV−vis−NIR spectrophotometer), and mass spectrometry (Waters synapt G2s). NMR spectra were recorded

cost-efficient WOLED manufacturing. In fact, the combination of long-range self-assembly and photoluminescence makes them very promising for application in the emerging field of organic light-emitting field-effect transistors (OLEFETs).1 Earlier, we reported that a triphenylene-pentaalkynylbenzenebased dimer exhibited a room-temperature colh mesophase with blue-light emission in solution, whereas in the solid state, green emission was observed.34 Herein, a new derivative of the multialkynylbenzene-bridged triphenylene-based dyad system is presented. This compound fulfils the above-described demands such as long-range columnar self-assembly at room temperature, good yield with high purity, blue-light emitters in the neat state suitable for devices, and so on. This compound also matches the criteria of facile processing from the isotropic state because of its low isotropization temperature.

2. EXPERIMENTAL SECTION 2.1. Materials and Reagents. Chemicals and solvents were all of AR quality and were used without further purification. Tetrabromohydroquinone, 1,2-dihydroxybenzene, bromohexane, ferric chloride, Bbromocatecholborane, 1-ethynyl-4-pentylbenzene, copper iodide, bis(triphenylphosphine) palladium(II) dichloride, triphenylphosphine, triethylamine, cesium carbonate, dibromohexane, dibromooctane, dibromodecane, potassium hydroxide, and tetraoctyl ammonium bromide were all purchased from Sigma-Aldrich (Bangalore, India). Column chromatographic separations were performed on silica gel (60−120 and 230−400 mesh). Thin-layer chromatography (TLC) was performed on aluminum sheets precoated with silica gel (Merck, Kieselgel 60, F254). 2.2. Synthesis and Characterization of the Dimeric Mesogens. Syntheses of monohydroxytriphenylene 1, and ω-bromosubstituted triphenylene 2 (Scheme 1) have been reported earlier.34 The synthesis of compound 4 was not feasible using the conventional

Scheme 1. Synthesis of Target Compounds 5a

a

Reagents and conditions: (i) Cs2CO3, KI, Br-(CH2)n-Br, butanone, 80 °C, 18 h, 88%; (ii) KOH, THAB, 80 °C, 12 h, 60%; (iii) Pd(PPh3)2Cl2, 4-pentylphenylacetylene, PPh3, CuI, Et3N, 100 °C, 24 h, 80%. 1121


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Surprisingly, at a slow cooling rate (1 °C/min) well-ordered randomly nucleated spherulite-like domains over the whole sample area were observed, indicating high anisotropy and longrange self-assembly (Figure 1b,c). This behavior is unusual for discotics and has been found for some low- molar-mass LCs, and only a few reports for high-molar-mass LCs have been reported so far.20−22 These spherulites can be demonstrated as Maltese crosses where the isogyres followed the extinction of the polarizer/analyzer, indicating an edge-on arrangement of the molecules.40 This observation was further confirmed by AFM imaging depicting the topography of the spherulites with growth originating from the nucleation center that expanded radially up to several hundred micrometers (Figure 1d). The morphology of spherulitic texture was further examined by SEM studies. The radius of a typically grown spherulite of compound 5a was found to be approximately 230 μm as shown in Figure 2a. The SEM micrograph also indicates plate-like fibers

using deuterated chloroform (CDCl3) as the solvent and tetramethylsilane (TMS) as an internal standard. 2.3.2. Differential Scanning Calorimetry. DSC measurements were performed on a PerkinElmer DSC 8000 coupled to a controlled liquid nitrogen accessory (CLN 2) at a scan rate of 5 °C/min. 2.3.3. Polarized Optical Microscopy. 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. 2.3.4. X-ray Diffraction. X-ray diffraction (XRD) was carried out on powder samples using Cu Kα (λ = 1.54 Å) radiation from a source (GeniX 3D, Xenocs) operating at 50 kV and 0.6 mA. The diffraction patterns were collected on a two-module Pilatus detector. 2.3.5. Photophysical Studies. Fluorescence emission spectra and steady-state anisotropy experiments were performed on Horiba Scientific Fluoromax spectrofluorometer 4. Time-resolved lifetime measurements were made on a time-correlated single photon counter from Horiba Jobin Yvon. For time-resolved experiments, excitation was carried out with a 375 nm laser diode.

3. RESULTS AND DISCUSSION Scheme 1 shows the straightforward synthesis route for tetraalkynylbenzene-bridged triphenylene (TP) derivatives. ωBromo-substituted triphenylene 2, obtained from monohydroxy hexaalkoxytriphenylene 1, was reacted with tetrabromohydroquinone 3 under solvent-free conditions to achieve dyad structure 4, which was subsequently coupled in a 4-fold Sonogashira reaction to obtain the final target material 5 on a multigram scale (see detailed synthesis procedures and characterization details in the Experimental Section). The structures were confirmed by 1H NMR, 13C NMR, and FT-IR (Supporting Information, Figures S1−S7). The thermal behavior of compounds 5 as investigated by polarizing optical microscopy (POM) and differential scanning calorimetry revealed a phase transition from the LC phase to the isotropic melt. We found that this new class of TP derivatives, in comparison with most of other TP-based dyads, forms an enantiotropic LC at room temperature with a wide LC temperature range and lower isotropization temperatures.35−39 As shown in Figure 1(a), POM of 5a on cooling from the isotropic melt (75 °C), under crossed polarizing conditions, displayed a texture characteristic of a columnar mesophase.

Figure 2. SEM micrograph of (a) a grown spherulite of compound 5a showing the radius of the spherulite to be around 230 μm and (b) the plate-like fibril showing a smooth outer surface.

diverging from a common center having a smooth outer surface as shown in Figure 2b. To study the extent of ordering present in the mesophase, the correlation length in the plastic phase of compound 5a was calculated from the Scherrer equation, ξ = 0.89λ/β cos θ, where 0.89 is the correction factor, β represents the full width at half-maximum (fwhm) of the peak obtained for core−core spacing, and λ and θ represent the wavelength of Xrays and Bragg’s angle, respectively. The correlation length was calculated to be around ∼256 Å, which denotes approximately 75−80 nearest discotic neighbors and is higher than that for other columnar phases.41 In general, the correlation lengths were the result of about 14 to 18 stacked discotic units, depending on the mesophase composition. In our case, the correlation length is found to be much higher in the mesophase with the formation of well-ordered spherulites that nucleated randomly over the whole sample during the cooling scan. These spherulites revealed high anisotropy and thus cogent long-range columnar self-assembly as convincingly supported by the POM and SEM observations. Consequently, the columns are arranged in the spherulite growth direction in an edge-on arrangement of the molecules up to several hundred micrometers as also observed in earlier studies.20,22 All of these factors denote a long-range columnar self-assembly of compound 5a. Compounds 5b and 5c with longer alkyl spacers had lower isotropization temperatures at 66 and 64 °C, respectively. They showed typical focal conic texture of the columnar mesophase on cooling, although no spherulitic growth was observed in the POM (Figure 3). A possible reason for the formation of longrange order for compound 5a could be due to its smallest alkyl spacer that allows the molecule to be much more sterically hindered as compared to the other longer spacer derivatives. The greater steric hindrance can lead to weaker intermolecular

Figure 1. Polarized optical micrographs of compound 5a at (a) 50 °C on cooling at a rate of 5 °C/min (crossed polarizers, magnification 200×). (b, c) Spherulitic domains at 55 °C on cooling at a rate of 1 °C/min from the isotropic phase (crossed polarizers, magnification 50×). (d) AFM image revealing the radial growth of the spherulitic domains. 1122


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observed that corresponds to core−core separation (hc) of the col stacks. A broad reflection at 4.29 Å is associated with the lateral alkyl chain separation (ha). Compound 5c (Figure 4c) also showed the colr phase as detailed in Table 1. In this case, the lattice constants calculated from the d spacings of d200 and d110 were found to be a = 34.54 Å and b = 16.21 Å. The calculated values of the lattice area (S), molecular volume (V), and number of molecules present in a columnar slice of height hc (Z) for all three compounds is documented in Table 1 (details in Supporting Information). All of the compounds had a light-yellow tinge in solution. Figure 5a displays the representative UV−vis absorption spectra of compound 5c in 5 μM dichloromethane solution, which shows absorption maxima centered at 280, 318, and 373 nm with two shoulder peaks at 262 and 272 nm. Emission spectra of all of the compounds were recorded by exciting the solutions of these compounds at their absorption maxima, which showed blue-light emission with maxima centered at 454 nm (Figure 5b,c, Figure S9−S12) for all of the fluorophores. A Stokes’ shift of 76−81 nm was observed for all of the compounds. A thin film of compound 5c was prepared by the drop-casting method on a glass slide. The absorption spectrum of the film showed absorption maxima at 255, 303, and 407 nm (Figure 5d), whereas the emission spectrum showed a maximum at 467 nm. A blue shift in the λmax of absorption in the solid state indicates the possibility of the formation of H aggregates. A red shift of 13 nm was observed for the emission of a neat sample as compared to that in the solution state. A Stokes’ shift of 60 nm was observed from the absorption and emission spectra of the thin film. As evident from the emission spectrum, the thin film showed blue luminescence when irradiated with UV light of 365 nm wavelength (inset in Figure 5e). Figure 5f shows blue luminescence from neat compound 5c under 365 nm UV light. A thin film of compound 5b also showed similar characteristics. However, for compound 5a, the emission spectra in the thin film became broader (indicating increased intermolecular interactions), with a peak at 465 nm and a shoulder peak at 498 nm. The film displayed cyan photoluminescence under 365 nm UV light (Figure S12). The quantum yields relative to quinine sulfate in 0.1 N H2SO4 (Figure S13) for compounds 5a−5c in micromolar solutions in dichloromethane were found to be in the range of 0.20−0.24 (Table 2), which is comparable to some of the 1,3,4-oxadiazoleand thiadiazole-based LC blue-light emitters.10d−f The optical band gap for compounds 5a−5c calculated from the red edge of the absorption spectra was found to be in the range of 2.87−2.9 eV (Table 2). To gain insight into the nanoenvironment of the fluorophores, the fluorescence lifetime and steady-state anisotropy were measured in dilute solutions (5 μM in dichloromethane). All of the compounds showed a single-exponential decay in the fluorescence decay spectra (Figure S14). The average fluorescence lifetimes for compounds 5a−5c were about 1.39, 1.60, and 2.42 ns, respectively (Table 2). Compound 5a due to restricted molecular motion exhibits a larger degree of association between the molecules, leading to lesser rotational diffusion and a faster relaxation of the S1 state and thus a decreased lifetime. Similarly, the increased steady-state anisotropy for 5a can be attributed to the smaller displacement of the emission dipole of the fluorophores. The estimation of frontier orbitals, i.e., HOMO and LUMO levels, and the electrical band gap for compound 5c was done from cyclic voltammetry studies (Figure S15) in a micromolar dichloromethane solution of compound 5c. For these studies, a

Figure 3. Polarized optical micrographs of compounds (a) 5b at 52 °C and (b) 5c at 31 °C on cooling at a rate of 5 °C/min from the isotropic phase (crossed polarizers, magnification 200×).

interaction, resulting in a slower growth rate and therefore the formation of long-range self-assembled domains. The supramolecular organization of these dyads in the columnar mesophase was studied by X-ray diffraction techniques (Figure 4). The results of indexing the one-dimensional (1D)

Figure 4. XRD patterns of compounds (a) 5a at 50 °C, (b) 5b at 42 °C, and (c) 5c at 30 °C on cooling from the isotropic liquid.

intensity vs 2θ profile obtained from the powder 2D pattern (Supporting Information, Figure S8) of the columnar phases on cooling from the isotropic melt are summarized in Table 1. Compound 5a, at larger scattering angles (as shown in Figure 4a), exhibited a doublet peak that corresponds to (002) and (102) reflections, indicating a columnar plastic phase. At a small angle, the diffraction profile showed reflections that can be assigned to a two-dimensional rectangular lattice with lattice constants of a = 31.04 Å and b = 20.87 Å (Table 1) calculated from the spacings of d200 and d110, respectively. Therefore, this phase can be assigned as a colrp phase, which is a new entrant in the family of col LCs as described recently by Wang et al.42 Compound 5b (Figure 4b) showed two peaks of roughly equal intensity at the small angle and several additional peaks at wide angles of lower intensity, and these peaks were characterized as (200), (110), (020), (220), and (330) for a 2D rectangular lattice with lattice spacings of a = 36.5 Å and b = 17.02 Å. At the larger scattering angle (2θ > 25°), a sharp reflection at 3.3 Å was 1123


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Langmuir Table 1. X-ray Diffraction Data for Compounds 5 Compound

Temperature/ °C

d spacing/Å obsd (calcd)

5a

50

5b

42

5c

30

35.69 (34.64) 21.55 (20.87) 17.32 15.52 12.02 (12.45) 9.33 (9.26) 8.53 (8.66) 7.73 (7.76) 7.31 (7.34) 6.65 (6.54) 3.85 3.70 (3.82) 18.25 15.43 8.79 (8.51) 7.98 (7.71) 4.84 (5.14) 4.29 3.33 17.27 14.68 10.52 (11.82) 9.44 (9.38) 8.47 (8.10) 7.44 (7.33) 4.84 (4.89) 4.11 3.48

Miller indices (hkl)

Mesophase (lattice spacing/Å), lattice area S (Å2), molecular volume V (Å3), no. of molecules in a columnar slice of height hc (Z)

1

/2 1/2 0 010 200 110 210 310 220 400 320 211 002 102 200 110 020 220 330 ha hc 200 110 210 310 020 220 330 ha hc

colrp (a = 31.04, b = 20.87, c = 7.70), S = 647.80, V = 4988.09, Z = 1.22

colr (a = 36.5, b = 17.02), S= 621.23, V = 2497.34, Z = 0.60

colr (a = 34.54, b = 16.21), S = 560.13, V = 2246.14, Z = 0.52

Figure 5. Representative (a, d) UV−vis absorption and (b, e) emission spectra of compound 5c in solution (5 μM in dichloromethane) and of a thin film, respectively. (c) Picture of compound 5c in solution (5 μM in dichloromethane) and (f) under neat conditions via UV illumination at wavelength 365 nm showing blue-light emission. The inset in (e) shows blue-light emission from the thin film of compound 5c.

platinum wire counter electrode, and a glassy carbon working electrode was used for experiment. The HOMO and LUMO energy levels (Figure S16) were estimated to be −5.20 and −3.46

0.1 M solution of tetrabutylammonium hexafluorophosphate was used as the supporting electrolyte in dichloromethane. A singlecompartment cell fitted with a Ag/AgNO3 reference electrode, a 1124


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Langmuir Table 2. Optical Data of the Compounds Synthesized in the Studya

a

compound

absorption (nm)

emission (nm)

ΔEg(eV)b

ΦFLc

τav (ns)d

steady-state anisotropy

5a 5b 5c

280, 318, 377 280, 318, 373 280, 318, 373

453 454 454

2.87 2.90 2.90

0.20 0.24 0.24

1.39 1.60 2.42

13.04 × 10−3 7.17 × 10−3 5.26 × 10−3

In micromolar solutions in CH2Cl2. bOptical band gap calculated from the λonset. cRelative to quinine sulfate in 0.1 N H2SO4 (ΦFL = 0.54). Fluorescence lifetime.

d

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eV, respectively. The electrical band gap was calculated to be 1.74 eV, which is smaller as compared to the optical band gap.

4. CONCLUSIONS Multi-alkynyl-bridged triphenylene-based dimers having different spacer lengths have been synthesized and characterized. All of these compounds exhibited LC properties even at room temperature as confirmed by POM and XRD. Compounds with spacer lengths of n = 8 and 10 exhibited columnar rectangular mesophase, whereas the compound with the shortest spacer length, i.e., n = 6, exhibited a columnar rectangular plastic phase. In addition to this, the compound exhibited long-range col self-assembly showing the formation of well-nucleated spherulites of several hundred micrometers. All of the compounds exhibited blue-light emission on irradiation at their absorption maxima in solution as well as under neat conditions. The properties of long-range room-temperature columnar selfassembly together with blue-light emission in the neat state make these compounds very promising for possible potential applications in semiconductor devices.

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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.5b03353. 1 H and 13C NMR of the synthesized dimers, 2D X-ray diffraction patterns, detailed photophysical data, cyclic voltammetry, and pictorial representation of HOMO and LUMO levels (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel: +91-172-2240266. Fax: +91-172-2240266. E-mail: skpal@ iisermohali.ac.in. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was carried out with financial support from the IISER Mohali and Department of Science and Technology, Key Project SB/FT/CS-47/2011 “Liquid Crystal Nanocrystal - A New Resource of Functional Soft Materials for Nanosciences”. We are grateful to the NMR, HRMS, and SAXS/WAXS facility at IISER Mohali. M.G. acknowledges the receipt of a graduate fellowship from IISER Mohali.

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REFERENCES

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