Triphenylene-Based Room-Temperature Discotic Liquid Crystals: A

Subscriber access provided by UNIV OF CALIFORNIA SAN DIEGO LIBRARIES

Article

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 Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.5b03353 • Publication Date (Web): 08 Jan 2016 Downloaded from http://pubs.acs.org on January 14, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

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

Page 1 of 24

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

Langmuir

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 KEYWORDS: Liquid crystals, blue light-emitting, long range self-assembly, electronic devices ABSTRACT

A straightforward synthesis of multialkynylbenzene-bridged triphenylene-based dyad systems (via flexible alkyl spacers) that self-organize into room temperature columnar structures over long range is reported. The compounds with spacer length (n) 8 and 10 exhibit columnar rectangular mesophase whereas compound with n = 6 shows columnar rectangular plastic phase. Interestingly, the later compound (n = 6) shows the formation of well-nucleated spherulites of about several hundred micrometres that suggest the existence of a long-range uniform selfassembly of columns. All these compounds show blue luminescence in solution and in thin film state under long wavelength (365 nm) UV light. These compounds fulfils the described demands such as long-range columnar self-assembly at room temperature, good yield with high purity,

ACS Paragon Plus Environment

1

Langmuir

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

Page 2 of 24

blue light emitters in the neat condition etc. for possible potential applications in semiconductor devices. They also match the criteria of facile processing from the isotropic state due to their low isotropization temperature. This new class of materials is promising, considering the emissive nature and stabilization of 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 the next generation solid-state light sources, field effect transistors and display applications.1-6 Only the triads of red, green and blue emissive materials can produce the white light that can revolutionize the lighting technology.7-9 In literature, red/green emissive materials are widely reported, but blue emitting materials are limited and remained a challenge for three decades. The invention of the efficient blue LED based on inorganic semiconductors has contributed for creating white light in an entirely new way to us.10 However, most of the inorganic semiconductors suffer from high cost, low efficiency and limits 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, 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 mobilities, ease of processability and high purity


2

Page 3 of 24

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

Langmuir

that reduce 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. Furthermore, a pronounced and defectfree long range order of Columnar (col) self-assembly is favourable for efficient charge transport. In literature, there are only a few reports that described long range self-assembly in molecules such as terphenyls, crowded arenes, hexa-peri-hexabenzocoronene, graphite oxide LCs, etc.19-22 Some examples of blue light emitting molecules based on discotics are reported recently but, most of them exhibited 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 Some reports on luminescent materials based-on tristriazolotriazine, 1, 3, 4-oxadiazole and thiadiazole are there that exhibit blue luminescence in solid state.28-33 But, in most of these cases, materials exhibit columnar phase at very high temperature without showing any long range columnar self-assembly. In some of the system, although they show glassy characteristics with retention of columnar order (on cooling from 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 opto-electronic devices. Therefore, design of new materials combining long-range columnar self-assembly around room temperature and strong photo-luminescent behaviour in the solid state will serve as promising candidates in future in the field of reliable and cost-efficient WOLED manufacturing. Infact, 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


3

Langmuir

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

Page 4 of 24

Earlier, we have reported that a triphenylene-pentaalkynylbenzene based dimer exhibited room temperature Colh mesophase with blue light emission in solution whereas, in solid state, a green emission was observed.34 Herein, a new derivative of multialkynylbenzene-bridged triphenylenebased 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. They also match the criteria of facile processing from the isotropic state due to 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,

B-Bromocatecholborane,

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 & 230-400 mesh). Thin layer chromatography (TLC) was performed on aluminium sheets pre-coated with silica gel (Merck, Kieselgel 60, F254).

2.2. Synthesis and Characterization of the dimeric mesogens Synthesis of Hexaalkoxytriphenylene 1, monohydroxytriphenylene 2, and ω-bromo-substituted triphenylene 3 (Scheme 1) have been reported earlier.34 The synthesis of compound 4 was not feasible using the conventional alkylation methods due to the rapid formation of quinone. The synthesis of compound 4, therefore, was carried out using solvent-free conditions.


4

Page 5 of 24

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

Langmuir

Tetrabromohydroquinone (1 equivalent) was finely grounded with KOH (2.5 equivalents) and a small amount of tetraoctylammonium bromide (0.09 equivalents) was also mixed followed by the addition of compound 3 (2.5 equivalents). The mixture was transferred to a 10 ml round bottom flask and was heated to 80 °C for 12 hours 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 bottom flask which was degassed followed by the addition of Pd(PPh3)Cl2 (50 mg), CuI (50 mg) PPh3 (100 mg) & compound 4 (0.63 mmol). The mixture was stirred for 15 minutes followed by the gradual addition of 1-ethynyl-4-pentyl benzene (6.3 mmol). The reaction mixture was stirred at 100°C for 24 h under nitrogen & after cooling to room temperature it was poured into 30 ml of 5M HCl. The product was extracted with DCM & was purified by column chromatography to obtain the target compounds 5.


5

Langmuir

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

HO

OC6H13

Page 6 of 24

Br n(H2C) O

OC6H13

i C6H13O C6H13O

OC6H13

Br

+

OC6H13 Br

OC6H13 C6H13O 1

OH Br

C6H13O

OC6H13

2

Br OH

3

ii C6H13O

OC6H13

C6H13O Br

C6H13O C6H13O

O (CH2)n OC6H13

OC6H13

Br

O

O (CH2)n O

Br

OC6H13

C6H13O

Br

OC6H13

4 5a: n = 6 5b: n = 8 5c: n = 10

iii C5H11 C6H13O

C6H13O C6H13O

C5H11

OC6H13

C6H13O

O (CH2)n

O

O (CH2)n O

OC6H13

C5H11

OC6H13

C6H13O

5

OC6H13

OC6H13

C5H11

Scheme 1. Synthesis of the target compounds 5. Reagents and conditions: (i) Cs2CO3, KI, Br(CH2)n-Br, butanone, 80 °C, 18h, 88%; (ii) KOH, THAB, 80 °C, 12h, 60%; (iii) Pd(PPh3)2Cl2, 4pentylphenylacetylene, PPh3, CuI, Et3N,100 °C, 24h, 80%. The synthesized compounds were characterized by 1H NMR,

13

C NMR, FT-IR, UV-Vis and

mass spectrometry as shown below (see the supporting information, Figure S1-S7): 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 in ppm): 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).


6

Page 7 of 24

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

Langmuir

13

C NMR (400 MHz, CDCl3, δ in ppm): 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.

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 in ppm): 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 in ppm): 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.

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 in ppm): 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 in ppm): 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 Structural characterization. Structural characterization of the compound was carried out through a

combination of infrared spectroscopy (Perkin Elmer Spectrum AX3), 1H NMR and


13

C NMR

7

Langmuir

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

Page 8 of 24

(Bruker Biospin Switzerland Avance-iii 400 MHz and 100 MHz spectrometers respectively), UV-VIS-NIR spectrophotometer (Agilent Technologies UV-Vis-NIR Spectrophotometer) and mass spectrometry (Waters synapt G2s). NMR spectra were recorded using deuterated chloroform (CDCl3) as solvent and tetramethylsilane (TMS) as an internal standard. Differential Scanning Calorimetry. DSC measurements were performed on Perkin Elmer DSC

8000 coupled to a Controlled Liquid Nitrogen Accessory (CLN 2) with a scan rate of 5 °C/min. Polarized Optical Microscopy. Textural observations of the mesophase were performed with Nikon

Eclipse LV100POL polarising microscope provided with a Linkam heating stage (LTS 420). All images were captured using a Q-imaging camera. 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. Photophysical Studies. Fluorescence emission spectra and steady State anisotropy experiments

were performed on Horiba Scientific Fluoromax spectrofluorometer 4. Time resolved lifetime measurements were done on time correlated single photon counter from Horiba Jobin Yvon. For time resolved experiment, excitation was done by 375 nm laser diode. 3. Results and Discussion Scheme 1 shows the straightforward synthetic route for tetra-alkynylbenzene-bridged triphenylene (TP) derivatives. The ω-bromo-substituted triphenylene 2, obtained from monohydroxy Hexaalkoxytriphenylene 1, was reacted with tetrabromohydroquinone 3 under solvent free conditions to achieve the dyad structure 4, which was subsequently coupled in a


8

Page 9 of 24

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

Langmuir

four-fold sonogashira reaction to get the final target material 5 on multigram scale (see experimental section for detailed synthetic procedures and characterization details). The structures were confirmed by 1H NMR, 13C NMR and FT-IR (see supporting information, Figure S1-S7). The thermal behavior of all the compounds 5 as investigated by polarizing optical microscopy (POM) and differential scanning calorimetry revealed a phase transition from LC phase to the isotropic melt. We found that this new class of TP derivative, 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 isotropic melt (75 °C), under crossed polarizing conditions displayed a texture characteristic of a columnar mesophase. Surprisingly, at a slow cooling rate (1 °C/min) wellordered randomly nucleated spherulite like domains over the whole sample area were observed indicating high anisotropy and long-range self-assembly (Figure 1b & c). This behaviour 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 cross 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 centre that expanded radially up to several hundred micrometres (Figure 1d).


9

Langmuir

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

Page 10 of 24

Figure 1. Polarized optical micrographs of the compound 5a at (a) 50 °C on cooling at a rate of 5 °C/min (crossed polarizers, magnification ൈ 200). (b) & (c) shows spherulitic domains at 55 °C on cooling at a rate of 1 °C/min. from the isotropic (crossed polarizers, magnification ൈ 50). (d) AFM image revealing the radial growth of the spherulitic domains. The morphology of spherulitic texture was further examined by SEM studies. The radius of a typical grown spherulite of compound 5a was found to be approximately 230 µm as shown in figure 2a. SEM micrograph also indicates plate-like fibres diverging from a common centre 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 / βCosθ, where 0.89 is the correction factor, β represents full width half maxima (FWHM) of the peak obtained for core-core spacing, λ and θ represents the wavelength of X-rays and Bragg’s angle, respectively. The correlation length was calculated to be around ~256 Å which denotes approximately 75-80 nearest discotic neighbours and is higher as compared to other columnar phases.41 In general, the correlation lengths were the result of about 14 to 18 stacked discotic units, depending upon the mesophase composition. In our case,


10

Page 11 of 24

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

Langmuir

correlation length is found to be much higher in the mesophase with the formation of wellordered 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 from the POM and SEM observations. Consequently, the columns are arranged in the spherulite growth direction in an edge-on arrangement of the molecules upto several hundred micrometres as observed also by earlier studies.20,22 All these factors denotes a long-range columnar self-assembly of the compound 5a.

Figure 2. SEM Micrograph of (a) a grown spherulite of compound 5a showing the radius of the spherulite to be round 230 µm and (b) the plate like fibril showing smooth outer surface. Compound 5b and 5c with longer alkyl spacers showed lower isotropization temperatures at 66 °C and 64 °C, respectively. They showed typical focal conic texture of columnar mesophase on cooling, although no spherulitic growth was observed in the POM (Figure 3).A possible reason for the formation of long-range order for compound 5a could be due to its smallest alkyl spacer which allows the molecule to be much more sterically hindered as compared to the other longer spacer derivatives. The more steric hindrance can lead to weaker intermolecular interaction, resulting in lower growth rate and therefore the formation of long-range self-assemble domains.


11

Langmuir

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

Page 12 of 24

Figure 3. Polarized optical micrographs of the compounds (a) 5b at 52 °C and (b) 5c at 31 °C on cooling at a rate of 5 °C/min. from the isotropic. (crossed polarizers, magnification ൈ 200). The supramolecular organization of these dyads in the columnar mesophase was studied by Xray diffraction techniques (Figure 4). The results of indexing of one-dimensional (1D) intensity vs. 2θ profile obtained from the powder 2D pattern (see Supporting Information, Figure S8) of the columnar phases on cooling from isotropic melt are summarized in Table 1. Compound 5a, at larger scattering angles (as shown in Figure 4a) exhibited a doublet peak which corresponds to (002) and (102) reflections, indicating a columnar plastic phase. In the small angle, the diffraction profile showed reflections that can be assigned to a two-dimensional rectangular lattice with lattice constants a = 31.04 Å and b = 20.87 Å (see Table 1) calculated from the spacings of d200 & 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 in the small angle and several additional peaks at wide angles of lower intensity, these peaks were characterized as (200), (110), (020), (220) and (330) of a two dimensional rectangular lattice with lattice spacings a = 36.5 Å and b = 17.02 Å . At the larger scattering angle (2θ > 25 °), a sharp reflection at 3.3 Å was 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


12

Page 13 of 24

showed colr phase as detailed in Table 1. In this case, the lattice constants calculated from the dspacings of d200 & d110 were found to be a = 34.54 Å & b = 16.21 Å. The calculated values oflattice area (S), molecular volume (V) and number of molecules present in a columnar slice of height hc (Z) for all the three compounds is documented in Table 1. (see supporting information

(010)

for details).

(002) (102)

(310) (220) (400) (320) (211)

(200) (110) (210) (200)

(1/21/20)

(a)

Intensity (a.u.)

(020) (220)

(ha)

(hc)

(210) (310) (020) (220)

(330)

(110)

(c)

(hc)

(330)

(ha)

(110)

(b)

(200)

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

Langmuir

0

5

10 15 20 2θ (degrees)

25

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


13

Langmuir

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

Page 14 of 24

Table 1. X-ray diffraction data for the compounds 5.

Compound Temperature/°C

5a

50

5b

42

5c

30

d-spacing/Å Observed (calculated)

Miller indices (hkl)

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

½½0 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

Mesophase(lattice spacing/Å), Lattice area S (Å2), Molecular Volume V (Å3), No. of molecules in columnar slice of height hc (Z) 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

All the compounds were showing 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 &272 nm. Emission spectra of all the compounds were recorded by exciting the solutions of


14

Page 15 of 24

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

Langmuir

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 the fluorophores. A stoke’s shift of 76-81 nm was observed for all the compounds. A thin film of compound 5c was prepared by dropcasting method on a glass slide. The absorption spectrum of the film showed absorption maxima at 255, 303 & 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 a possibility of the formation of H-aggregates. A red shift of 13 nm was observed for the emission of neat sample as compared to that in the solution state. A stoke’s shift of 60 nm was observed from the absorption & emission spectra of 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 the neat compound 5c under 365 nm UV light. Thin film of compound 5b also showed similar characteristics. However, for compound 5a, the emission spectra in 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 coloured photoluminescence under 365 nm UV light (Figure S12).

Figure 5. Representative (a), (d) UV-Vis absorption & (b), (e) emission spectra of compound 5c in solution (5 µM in dichloromethane) &.of thin film respectively. (c) Picture of compound 5c in


15

Langmuir

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

Page 16 of 24

solution (5 µM in dichloromethane) & (f) in neat condition under UV illumination of wavelength 365 nm showing blue light emission. The inset picture in (e) shows blue light emission from the thin film of compound 5c. The quantum yields relative to quinine sulfate in 0.1 N H2SO4 (Fig. S13), for compounds 5a-c in micromolar solutions in dichloromethane were found to be in the range of 0.20 to 0.24 (Table 2) which is comparable to some of the 1,3,4-oxadiazole and thiadiazole based LC blue light emitters.10d-f The optical band gap for compound 5a-c calculated from the red edge of absorption spectra was found to be in the range of 2.87-2.9 eV (Table 2). In order to get insights into the nano environment of the fluorophores, fluorescence lifetime and steady-state anisotropy were measured in dilute solutions (5 µM in dichloromethane). All the compounds showed single exponential decay in the fluorescence decay spectra (Figure S14). The average fluorescence lifetime for compounds 5a, 5b and 5c was about 1.39, 1.60 and 2.42 ns respectively (Table 2). The 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 S1 state and thus decreased lifetime. Similarly, the increased steady-state anisotropy for 5a can be attributed to the lesser displacement of emission dipole of the fluorophores.


16

Page 17 of 24

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

Langmuir

Table 2 Optical data of the compounds synthesized in the studya

a

∆Egb ΦFLc (eV)

τavd (ns)

Steady State Anisotropy

453

2.87

0.20

1.39

13.04 ൈ 10-3

280, 318, 373

454

2.90

0.24

1.60

7.17 ൈ 10-3

280, 318, 373

454

2.90

0.24

2.42

5.26 ൈ 10-3

Compound

Absorption (nm)

Emission (nm)

5a

280, 318, 377

5b 5c

in µm solutions in CH2Cl2;

b

optical bandgap calculated from the λonset;

c

relative to quinine

sulfate in 0.1 N H2SO4 (ΦFL = 0.54); d fluorescence lifetime. The estimation of frontier orbitals i.e. HOMO and LUMO levels and electrical band gap for compound 5c was done from cyclic Voltammetry studies (Figure S15)

in micromolar

dichloromethane solution of compound 5c. For these studies a 0.1 M solution of tetrabutylammonium

hexafluorophosphate

was

used

as

supporting

electrolyte

in

dichloromethane. A single compartment cell fitted with Ag/AgNO3 reference electrode, platinum wire counter electrode and glassy carbon working electrode was used for experiment. The HOMO and LUMO energy levels (Fig. S16) were estimated to be -5.20 and -3.46 eV, respectively. The electrical band gap was calculated to be 1.74 eV which is lesser as compare to optical bandgap. CONCLUSION Multialkynyl bridged triphenylene based dimers having different length of spacer have been synthesized and characterized. All these compounds exhibited LC properties even at room temperature as confirmed by POM and XRD. The compounds with spacer length, n = 8, 10


17

Langmuir

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

Page 18 of 24

exhibited columnar rectangular mesophase whereas the compound with the shortest spacer length i.e. n = 6 exhibited columnar rectangular plastic phase. In addition to this, the compound exhibited a long-range self-assembly showing the formation of well-nucleated spherulites of about several hundred micrometres. All the compounds exhibited blue light emission on irradiation at their absorption maxima in solution as well as in neat conditions. The properties of room temperature columnar self-assembly over long range together with blue light emission in neat state make these compounds very promising for possible potential applications in semiconductor devices. ASSOCIATED CONTENT Supporting Information 1

H and

13

C NMR of the synthesized dimers, 2D X-ray diffraction patterns, detailed

photophysical data, cyclic voltammetry and pictorial representation of HOMO and LUMO levels. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * Corresponding author: [email protected]

Tel.:

+91-172-2240266;

fax:

+91-172-2240266;

E-mail:

Notes The authors declare no competing financial interest.


18

Page 19 of 24

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

Langmuir

ACKNOWLEDGEMENT This work was carried out with the financial support from 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 NMR, HRMS and SAXS/WAXS facility at IISER Mohali. M. Gupta acknowledges the receipt of a graduate fellowship from IISER Mohali.

REFERENCES (1) Muccini, M. A Bright Future for Organic Field-Effect Transistors. Nat Mater 2006, 5, 605613. (2) Meerholz, K.; Müller, C.-D.; Nuyken, O. Crosslinkable Organic Semiconductors for Use in Organic Light-Emitting Diodes (OLEDs). In Organic Light Emitting Devices, Wiley-VCH Verlag GmbH & Co. KGaA: 2006, 293-318. (3) Forrest, S. R.; Thompson, M. E. Introduction: Organic Electronics and Optoelectronics. Chem. Rev. 2007, 107, 923-925. (4) Jian, W.; Gang, Y. Organic Light-Emitting Devices and Their Applications for Flat-Panel Displays. In Organic Light-Emitting Materials and Devices, CRC Press: 2006. (5) Norman, H.; Hong, M. Organic Small Molecule Materials for Organic Light Emitting Diodes. In Organic Light-Emitting Materials and Devices, CRC Press: 2006. (6) Köhler, A.; Wilson, J. S.; Friend, R. H. Fluorescence and Phosphorescence in Organic Materials. Adv. Mater. 2002, 14, 701-707.


19

Langmuir

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

Page 20 of 24

(7) Yook, K. S.; Lee, J. Y. Organic Materials for Deep Blue Phosphorescent Organic LightEmitting Diodes. Adv. Mater. 2012, 24, 3169-3190. (8) D’Andrade, B. W.; Brooks, J.; Adamovich, V.; Thompson, M. E.; Forrest, S. R. White Light Emission Using Triplet Excimers in Electrophosphorescent Organic Light-Emitting Devices. Adv. Mater. 2002, 14, 1032-1036. (9) Xiao, L.; Chen, Z.; Qu, B.; Luo, J.; Kong, S.; Gong, Q.; Kido, J. Recent Progresses on Materials for Electrophosphorescent Organic Light-Emitting Devices. Adv. Mater. 2011, 23, 926-952. (10) Schubert, E. F.; Kim, J. K. Solid-State Light Sources Getting Smart. Science 2005, 308, 1274-1278. (11) Kovac, J.; Peternai, L.; Lengyel, O. Advanced Light Emitting Diodes Structures For Optoelectronic Applications. Thin Solid Films 2003, 433, 22-26. (12) Varghese, S.; Kumar, N. S. S.; Krishna, A.; Rao, D. S. S.; Prasad, S. K.; Das, S. Formation of Highly Luminescent Supramolecular Architectures Possessing Columnar Order from Octupolar Oxadiazole Derivatives: Hierarchical Self-Assembly from Nanospheres to Fibrous Gels. Adv. Funct. Mater. 2009, 19, 2064-2073. (13) Eccher, J.; Faria, G. C.; Bock, H.; von Seggern, H.; Bechtold, I. H. Order Induced Charge Carrier Mobility Enhancement in Columnar Liquid Crystal Diodes. ACS Appl. Mater. Interfaces 2013, 5, 11935-11943. (14) Hassheider, T.; Benning, S. A.; Kitzerow, H.-S.; Achard, M.-F.; Bock, H. Color-Tuned Electroluminescence from Columnar Liquid Crystalline Alkyl Arenecarboxylates.

Angew

Chem., Int. Ed. 2001, 40, 2060-2063.


20

Page 21 of 24

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

Langmuir

(15) Seguy, I.; Jolinat, P.; Destruel, P.; Mamy, R.; Allouchi, H.; Courseille, C.; Cotrait, M.; Bock, H. Crystal and Electronic Structure of a Fluorescent Columnar Liquid Crystalline Electron Transport Material. ChemPhysChem 2001, 2, 448-452. (16) Torgova, S.; Strigazzi, A. Organic Electroluminescent Materials. Mol. Cryst. Liq. Cryst. 2002, 375, 61-72. (17) Boden, N.; Movaghar, B.; Demus, D.; Goodby, J.; Gray, G. W.; Spiess, H. W.; Vill, V. Applicable Properties of Columnar Discotic Liquid Crystals. In Handbook of Liquid Crystals Set, Wiley-VCH Verlag GmbH: 2008, 781-798. (18) Markovitsi, D.; Marguet, S.; Bondkowski, J.; Kumar, S. Triplet Excitation Transfer in Triphenylene Columnar Phases. J. Phys. Chem. B 2001, 105,1299-1306. (19) Bushey, M. L.; Hwang, A.; Stephens, P. W.; Nuckolls, C. Enforced Stacking in Crowded Arenes. J. Am. Chem. Soc. 2001, 123, 8157-8158. (20) Pisula, W.; Kastler, M.; Wasserfallen, D.; Pakula, T.; Müllen, K. Exceptionally Long-Range Self-Assembly of Hexa-peri-hexabenzocoronene with Dove-Tailed Alkyl Substituents. J. Am. Chem. Soc. 2004, 126, 8074-8075. (21) Tong, L.; Qi, W.; Wang, M.; Huang, R.; Su, R.; He, Z. Long-range ordered graphite oxide liquid crystals. Chem. Commun. 2014, 50, 7776-7779. (22) Pisula, W.; Dierschke, F.; Mullen, K. Long-range self-assembly of bis(imidazole)-annulated terphenyls. J. Mater. Chem. 2006, 16, 4058-4064. (23) Cristiano, R.; Santos, D. M. P. d. O.; Gallardo, H. Synthesis and Characterization of Low Molecular Mass Luminescent Liquid Crystalline Materials with 1,3,4‐Oxadiazole Units. Liq. Cryst. 2005, 32, 7-14.


21

Langmuir

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

Page 22 of 24

(24) Wen, C.-R.; Wang, Y.-J.; Wang, H.-C.; Sheu, H.-S.; Lee, G.-H.; Lai, C. K. Columnar Metallomesogens

Derived

from

1,3,4-Oxadiazoles

and

X-ray

Crystal

Dichlorobis[2,5-bis(3,4,5-trimethoxyphenyl)-1,3,4-oxadiazole]palladium(II).

Structure

Chem.

of

Mater.

2005, 17, 1646-1654. (25) Vinayakumara, D. R.; Kumar, M.; Sreekanth, P.; Philip, R.; Kumar, S. Synthesis, Characterization and Nonlinear Optical Studies of Novel Blue-Light Emitting Room Temperature Truxene Discotic Liquid Crystals. RSC Adv. 2015, 5, 26596-26603. (26) Gupta, R. K.; Pradhan, B.; Pathak, S. K.; Gupta, M.; Pal, S. K.; Ammathnadu Sudhakar, A. Perylo[1,12-b,c,d] Thiophene Tetraesters: A New Class of Luminescent Columnar Liquid Crystals. Langmuir 2015, 31, 8092-8100. (27) Cristiano, R.; Gallardo, H.; Bortoluzzi, A. J.; Bechtold, I. H.; Campos, C. E. M.; Longo, R. L. Tristriazolotriazines: A Core for Luminescent Discotic Liquid Crystals. Chem. Commun. 2008, 5134-5136. (28) Vieira, A. A.; Gallardo, H.; Barbera, J.; Romero, P.; Serrano, J. L.; Sierra, T. Luminescent Columnar Liquid Crystals Generated by Self-Assembly of 1,3,4-Oxadiazole Derivatives. J. Mater. Chem. 2011, 21, 5916-5922. (29) Srivastava, R. M.; Neves Filho, R. A. W.; Schneider, R.; Vieira, A. A.; Gallardo, H. Synthesis, Optical Properties and Thermal Behaviour of 1,3,4‐Oxadiazole‐based Twin Dimers. Liq. Cryst. 2008, 35, 737-742. (30) Cristiano, R.; Eccher, J.; Bechtold, I. H.; Tironi, C. N.; Vieira, A. A.; Molin, F.; Gallardo, H. Luminescent Columnar Liquid Crystals Based on Tristriazolotriazine. Langmuir 2012, 28, 11590-11598.


22

Page 23 of 24

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

Langmuir

(31) Westphal, E.; Prehm, M.; Bechtold, I. H.; Tschierske, C.; Gallardo, H. Room Temperature Columnar Liquid Crystalline Phases of Luminescent Non-Symmetric Star Shaped Molecules Containing Two 1,3,4-Oxadiazole Units. J. Mater. Chem. C 2013, 1, 8011-8022. (32) Pathak, S. K.; Gupta, R. K.; Nath, S.; Rao, D. S. S.; Prasad, S. K.; Achalkumar, A. S. Columnar Self-Assembly of Star-Shaped Luminescent Oxadiazole and Thiadiazole Derivatives. J. Mater. Chem. C 2015, 3, 2940-2952. (33) Pathak, S. K.; Nath, S.; Gupta, R. K.; Rao, D. S. S.; Prasad, S. K.; Achalkumar, A. S. Effect of Regioisomerism on the Self-Assembly and Photophysical Behavior of 1,3,4 Thiadiazole-based Polycatenars. J. Mater. Chem. C 2015. (34) Gupta, M.; Bala, I.; Pal, S. K. A Room Temperature Discotic Mesogenic Dyad Based-on Triphenylene and Pentaalkynylbenzene. Tetrahedron Lett. 2014, 55, 5836-5840. (35) Kimura, M.; Moriyama, M.; Kishimoto, K.; Yoshio, M.; Kato, T. Self‐Assembly of Liquid Crystalline Triphenylene–Oligo(ethylene oxide)–Triphenylene Molecules and Their Complexes with Lithium Triflate. Liq. Cryst. 2007, 34, 107-112. (36) Zhao, K. Q.; Bai, Y. F.; Hu, P.; Wang, B. Q.; Shimizu, Y. Synthesis of Triphenylene Discotic Liquid Crystal Dimers: Click Chemistry As An Efficient Tool. Mol. Cryst. Liq. Cryst. 2009, 509, 77/[819]-88/[830]. (37) Pal, S. K.; Kumar, S. Microwave-Assisted Synthesis of Novel Imidazolium-based Ionic Liquid Crystalline Dimers. Tetrahedron Lett. 2006, 47, 8993-8997. (38) Kumar, S.; Gupta, S. K. The First Examples of Discotic Liquid Crystalline Gemini Surfactants. Tetrahedron Lett. 2010, 51, 5459-5462. (39) Gupta, S. K.; Kumar, S. Novel Triphenylene−Ammonium−Ammonium−Triphenylene based Discotic Ionic Liquid Crystalline Dimers. Liq. Cryst. 2012, 39, 1443-1449.


23

Langmuir

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

Page 24 of 24

(40) Hutter, J. L.; Bechhoefer, J. Banded Spherulitic Growth in a Liquid Crystal. J. Cryst. Growth 2000, 217, 332-343. (41) Prasad, S. K.; Rao, D. S. S.; Chandrasekhar, S.; Kumar, S. X-RAY Studies on the Columnar Structures of Discotic Liquid Crystals. Mol. Cryst. Liq. Cryst. 2003, 396, 121-139. (42) Wang, Y.; Zhang, C.; Wu, H.; Pu, J. Synthesis and Mesomorphism of Triphenylenebased Dimers with a Highly Ordered Columnar Plastic Phase. J. Mater. Chem. C 2014, 2, 16671674.

TOC Graphic

C 5 H 11 C 6 H 13 O

C 6 H 13 O C 6 H 13 O

C 5 H 11

O C 6 H 13

C 6 H 13 O

O (CH 2 ) n O C 6 H 13

C 5 H 11

O

O C 6 H 13

O (CH 2 ) n O C 6 H 13 O

O C 6 H 13 O C 6 H 13

C 5 H 11


24

Triphenylene-Based Room-Temperature Discotic Liquid Crystals: A

Recommend Documents