Room temperature columnar liquid crystals as an efficient pure deep

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Organic Electronic Devices

Room temperature columnar liquid crystals as an efficient pure deepblue emitter in OLEDs with an external quantum efficiency of 4.0% Joydip De, Wan-Yun Yang, INDU BALA, Santosh Prasad Gupta, Rohit Ashok Kumar Yadav, Deepak Kumar Dubey, Arjun Chowdhury, Jwo-Huei Jou, and Santanu Kumar Pal ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b18749 • Publication Date (Web): 01 Feb 2019 Downloaded from http://pubs.acs.org on February 3, 2019

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ACS Applied Materials & Interfaces

Room temperature columnar liquid crystals as an efficient pure deep-blue emitter in OLEDs with an external quantum efficiency of 4.0% Joydip De,† Wan-Yun Yang,# Indu Bala,† Santosh Prasad Gupta,‡ Rohit Ashok Kumar Yadav,# Deepak Kumar Dubey,# Arjun Chowdhury,† Jwo-Huei Jou,# 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 ‡

#

Department of Physics, Patna University, Patna-800005, India

Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan

KEYWORDS: Discotic Liquid Crystals, Columnar, deep-blue, OLEDs, AIEE

ABSTRACT

A novel design of aggregation-induced emission (AIE) active columnar (Col) luminomesogens are reported and demonstrated to act as a highly efficient deep-blue emitter in organic light-emitting diodes (OLEDs). All the derivatives exhibit columnar (Col) liquid crystalline (LC) behaviour at

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room temperature over a wide temperature range and desirable alignment properties which is very important in using them as materials for organic electronic devices. These new AIE active luminomesogens were found to act as a highly efficient emitter in OLEDs and unveiled a maximum external quantum efficiency (EQE) of 4.0% for the first time in Col LCs with Commission International de l´E´clairage (CIE) coordinates of (0.17, 0.07), closely matches to the National Televison System Committee (NTSC) standard, corresponding to the pure deep blue colour. The detailed supramolecular assembly of the compounds has been characterized by modelling in the mesophase derived from small and wide angle X-ray scattering results.

Introduction

In recent years, organic light emitting diodes (OLEDs) have captured the electronics technology owing to their tremendous uses in small mobile device to large flat-panel display with superb display imaging quality.1-3 In general, for producing either white light or colour display, red, green, and blue (RGB) colours are required and they should have comparable identical properties in terms of their stability, purity and the efficiency.4 The scarcity of blue emitters in comparison to red and green has triggered to uncover new stable pure blue emitters. Though, efficient inorganic blue LEDs bring major breakthrough in this area but, suffer from high cost and low endurance, which limits their widespread use in fabrication on a flexible substrate. In contrast, luminescent polymers provides high endurance but, they also have issues such as low solubility and in obtaining them as a high purity. To overcome these challenges, luminescent organic small molecules have been proposed 5 which can bring the promise of cheap production, flexibility, biocompatibility as well as high purity. In this regard, discotic liquid crystals (DLCs)6-17 being small luminescent organic molecules aided by their inherent self-organization features can control the polarization of light-


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emitting devices in an efficient manner and can increase the device efficiency.18 In addition, LC materials exhibits the tendency of self-healing from defects by means of thermal annealing which can increase the durability of the device.19 The use of DLCs benefitted with the long alkyl chains makes fabrication process very easy by solution processing technique which can reduce the high fabrication cost. It is well known that most of the luminescent DLCs exhibited excellent luminescence in solution. However, in the solid state this luminescence get quenched because of strong π-π stacking of discs within the columns.20-25 This effect is known as aggregation caused quenching (ACQ). The effective approach to fabricate OLEDs using luminescent DLCs is to design the molecular structure in such a way that there should be a unique balance between the π-π interaction and the quenching of luminescence efficiency in the neat state. In that case, the compounds can strongly exhibit emission in the solid state which is commonly recognised as aggregation-induced emission (AIE) effect, first reported by Tang et al. in 2001.26 Extensive efforts have been made to fabricate deep-blue OLEDs by using DLCs but external quantum efficiency of these devices are less. 27-29. However, recent attempts have been made to improve the blue OLEDs efficiency using small organic molecules. For example Li et al. synthesized imidazole-containing deep-blue emissive materials and reported 2.3% EQE.30 Liu’s group made a fluorene-based oligomer which also acts as a deep-blue emitter. They have achieved 2.7% EQE having CIE coordinates of (0.16, 0.07) in these materials.31 Chavez et al. developed new benzo[1,2-d:4,5-d’]bisoxazole core-based emitters which were shown to exhibit 2.9% EQE with CIE coordinates of (0.16, 0.05).32 Gao et al. synthesised materials for blue OLEDs with an EQE of 3.02%.33 Mullen et al. synthesized materials for deep blue OLEDs fabricated from pyrene based compounds. These compounds exhibit 3.1% EQE with CIE coordinates of (0.16, 0.024).34 Jeong et al. reported an indenophenanthrene core-


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based emitter showing an EQE of 3.27% and deep-blue emission consisting CIE coordinates of (0.158, 0.040).35 Recently, Lee and Tong’s group reported highly efficient deep-blue emissive devices based on donor-acceptor systems.36, 37 In the present study, a new series of AIE active luminomesogens have been synthesized and used as an emitter in deep-blue OLED devices. The device unveiled a maximum EQE of 4.0% for the first time in the field of DLCs with CIE coordinates of (0.17, 0.07) corresponding to pure deep-blue colour. These materials not only provide solitary proportion to the π-π interaction (to make them AIE-active systems) within the cores but also render expedient alignment in the neat state that greatly reinforce their usability in OLEDs devices. Experimental Section Materials. The purchased materials were used as they have received. The materials were purified by column chromatographic techniques. The silica gel (100–200 mesh) were used for purification. Thin layer chromatography (TLC) were performed by using aluminium sheets which are pre-coated with silica gel. Measurements and Characterization. The synthesized materials were structurally characterized by combining the spectroscopic techniques as reported eleswhere.29 Results and discussions Synthesis The synthetic scheme of the target compounds 4.1-4.5 is outlined in Scheme 1. The synthesis of compound 1, 2 and 3 was described elsewhere.29,

38

For the synthesis of target material 4,

compound 3 and 2 were reacted together. The reaction was done in presence of a strong base


4

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(potassium tert-butoxide) using anhydrous THF as a solvent. Yield for the final comounds is mentioned in the supporting information (SI). The detailed characterization of the compounds is described in the SI.

Scheme 1. (i) K2CO3, Propargyl Bromide, Acetone, 56 °C, 18h; (ii) RN3, CuI, DMF, 90°C, 18h; (iii) P(OEt)3, 150 °C, 24h; (iv) KOtBu, dry THF 18h. Thermal Behaviour Thermal and structural characterizations were fully elucidated by conventional analytical techniques (Figure S11-S13, see SI). All the compounds were found to exhibit room temperature columnar LC phase. In differential scanning calorimetry (DSC), compound 4.2 (i.e. R = -C12H25) exhibits two transitions on heating. The first transition at 149.2 °C corresponds to LC phase to another LC phase transition and the second transition at 174.5 °C denotes LC phase to isotropic


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transition (Figure S11b (see SI) and Table 1). Upon cooling, movable birefringent texture appears at 173.8 °C followed by another mesophase transition at 146.9 °C. The POM micrographs of 4.2 at 26 °C and 160 °C is shown in Figure S12a and S12b (see SI), respectively. The X-ray analysis (vide infra) confirms the existence of two different columnar rectangular (Colr) assemblies at those temperatures. Compound 4.1 shows similar behaviour as observed for 4.2. Compound 4.3 on heating, also exhibits two transitions at 173.1 °C and at 181.4 °C (Figure S11c (see SI) and Table 1) in DSC. On cooling, the peak appears at 164.5 and 131.6 °C, which corresponds to isotropic to columnar hexagonal (Colh) and Colh to Colr transition (vide infra). The POM images of compound 4.3 in Colr and Colh phase at 35 °C and 162 °C are represented in Figure S12c and S12d (see SI) upon cooling. The compound 4.4 (Figure S12e-f, see SI) and 4.5 showed similar kind of mesomorphic behaviour. Compound 4.5 display a transition at 153.1 °C followed by a second transition at 173.9 °C on heating. With the support of X-ray analysis (vide infra) we demonstrate that the first transition corresponds to Colh1 to Colh2 and the second one (Figure S11e (see SI) and Table 1) denotes from Colh2 to Isotropic transition. On cooling, the corresponding mesophases appear at 168.4 and 111.6 °C, respectively. The formation of two different Colh mesophases were also supported by microscopic studies. Under POM, 4.5 shows the growth of domains in two clearly different directions as depicted in Figure 1a (at 30 °C) and 1b (at 163 °C) (indicated by white arrows).


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b

a

Figure 1. POM image of 4.5 at (a) 30 °C and (b) 163 °C, on cooling from isotropic phase, magnification X 500. White arrows indicate growth of domains in two clearly different directions. Table 1. Phase transition temperatures of 4.1 - 4.5.a

TM

Heating cycle

Cooling cycle

4.1

Colr1 86.3 (15.39) Colr2 120.3 (18.35) I

I 111.8 (9.44) Colr2 85.3 (7.99) Colr1

4.2

Colr1 149.2 (28.29) Colr2 174.5 (41.50) I

I 173.8 (40.30) Colr2 146.9 (27.13) Colr1

4.3

Colr 173.1 (50.41) Colh 181.4 (12.30) I

I 164.5 (14.39) Colh 131.6 (40.23) Colr

4.4

Colh1 163.5 (56.83) Colh2 172.4 (13.84) I

I 165.4 (10.02) Colh2 112.1 (50.19) Colh1

4.5

Colh1 153.1 (53.84) Colh2 173.9 (8.50) I

I 168.4 (7.30) Colh2 111.6 (51.21) Colh1

a

The temperatures are the peak temperatures (in °C) obtained from DSC. Their respective enthalpies are in parenthesis (kJ mol-1). TM = Target materials. SAXS/WAXS studies The small and wide angle X-ray scattering (SAXS/WAXS) experiments have been conducted to deduce the detailed supramolecular assembly of compounds 4.1-4.5. The SAXS/WAXS pattern for compound 4.1 is shown in Figure 2a, at 25 °C. This pattern is well matched for a 2D rectangular lattice as detailed in Table S1 (see SI). The lattice parameters for 4.1 are found to be as a = 48.17 Å and b = 71.40 Å (represented as Colr1). The SAXS/WAXS pattern at higher temperature (at 110


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°C, Figure 2b) can also be correlated on a rectangular lattice (Table S2, see SI) with slight variation in the lattice parameters (a = 49.96 Å and b = 66.60 Å, represented as Colr2). Interestingly, the ha peak (correlations of the molten chains, 5.08 Å) is prominent at higher temperature (Figure 2b) whereas, it is found to be overlapped with the peaks in the wide angle regime at 25 °C (Figure 2a). Similarly, compound 4.2 exhibits two different Colr assemblies: Colr1 and Colr2 (as noted from DSC results) with little variations in the lattice parameters (Figure S14a-S14b & Table S3-S4, see SI).

0.2

0.3

0.4 0.5 -1 q (Å )

1.5

(42)

(33)

(04)

(21)

0.1

1.0

0.6

(80)

(40)

(30) 4

10

2.0

0.4 q (Å-1)

0.6 ha

3

10

2

10

0.7

0.2

1.0

0.3 -1 q (Å )

1.5

0.4

2.0

(30)

3

10

0.2

d

10

2

(11)

0.7

ha

1.5

(21)

0.6

1.0

(20)

0.5

(10)

Intensity (arb. units)

(42)

(33)

1.5

(11)

0.4 q (Å-1)

2

(10)

0.3

(11)

b

0.2

1.0

ha

10


0.1

(04) (30)

(21)

10

ha: back- hc

ground due to alkyl chain-alkyl chain correlation

(22)

2

c

(95) (97)

(82)

(11) 3

10

(02)


a


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

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0.5

Figure 2. SAXS/WAXS pattern of TM; (a) 4.1 at 25 °C, (b) 4.1 at 110 °C, (c) 4.4 at 25 °C and (d) 4.4 at 150 °C in the small angle region and corresponding pattern in the wide angle regime (inset). Interestingly, compound 4.3 showed two different phases. At lower temperature Colr phase was observed whereas, at higher temperature Colh was noted as evident from POM and detailed SAXS/WAXS studies (Figure S14c-S14d & Table S5-S6, see SI). In contrast, compound 4.4


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exhibit only Colh phase in their mesophase temperature range. At 25 °C, the SAXS/WAXS pattern display broad peaks in the small angle region and narrow peak in the wide angle regime (Figure 2c, denoted as Colh1) whereas at higher temperature (at 150 °C, Figure 2d, denoted as Colh2), narrow peaks were found in the small angle and broad peak in the wide angle regime. The dspacing’s are well corroborated with the Colh pattern (Table S7-S8, see SI) and clearly signify two different Colh assembly (Colh1 and Colh2) at those temperatures. The compound 4.5 showed the similar mesomorphic behaviour as that of compound 4.4. It also showed only Colh phase in their mesophase temperature range. The SAXS/WAXS pattern (Figure S14e-S14f & Table S9-S10, see SI) is also similar to that seen for compound 4.4, revealed the occurrence of two kinds of Colh phase at lower (Colh1) and higher temperature (Colh2). Electron density maps (EDM) are generated to understand the detailed molecular arrangements in their respective mesophases (Figure S17S19, see SI). In-order to have better understanding about the mesophases at lower and higher temperature regime, we analyzed the 2D SAXS/WAXS pattern of 4.5 at 25 °C and 140 °C. The SAXS/WAXS pattern at 25 °C (Figure 3a) is aligned in small and wide angle regime as well without any external effort (i.e. mechanical or magnetic method) which shows good alignment ability of the material. Further, the wide angle peak (attributing the alkyl chain ordering) is narrow and exhibits distorted six fold symmetry. It is distorted because the azimuthal peak positions are not in the multiple of 60 degree but they are at 0, 50, 130, 180, 230 and 310 degrees (Figure 3a). This result confirm that the alkyl chains are packed in an order manner at lower temperature (Colh1) whereas, the rigid part showed less ordered arrangement in the mesophase.


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a

b 1

2

equator

a*, b*

6

3

c*

1 50

2 100

meridian

5

4

2 angle (Degree)

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

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150

5

4

3 200

250

300

Azimuthal angle (Degree)

6 350

(30) (21)

(20) (10)

Figure 3. (a) 2D SAXS/WAXS patterns of compound 4.5 at 25 °C. Bottom picture of (a) shows the azimuthal plot of the wide angle peak. (b) 2D SAXS/WAXS patterns of compound 4.5 at 140 °C. Bottom picture of (b) shows the zoomed area of the small angle region.


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High

a

Low High

b

Low

Figure 4. (a) Electron density map (EDM) of compound 4.5 in hexagonal phase at temperature 25 °C along with sketch of compound (left). Arrangement of compound in the hexagonal phase (right). Here, the column of ordered chains is forming the hexagonal lattice. (b) EDM of compound 4.5 in hexagonal phase at temperature 140 °C along with sketch of compound (left). Arrangement of compound in the hexagonal lattice (right). Here the column of molecule is forming the hexagonal lattice. High electron density region depicted with red and deep blue is the lowest electron density region.


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Figure 5. Schematic representation of compound 4.1-4.5. It exhibits columnar rectangular phase for R (alkyl chain length) < C14H29 (4.1 and 4.2) and columnar hexagonal phase for R > C14H29 (4.4 and 4.5). For R = C14H29 (4.3), showed columnar rectangular phase at low temperature and columnar hexagonal phase at high temperature. Further, columnar hexagonal phase (Colh1) found at low temperature is due to arrangement of voids along with ordered chains (pink) whereas the columnar hexagonal phase (Colh2) found at high temperature is result of regular packing of molecule (rigid part) (green) on hexagonal lattice. The transformation among these phases can be realized by changing the temperature or chain length or both.


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However, at 140 °C the diffraction pattern showed broad peak in wide angle regime with no alignment (Figure 3b). But, in the small angle regime, the observed peaks are narrow and exhibited an aligned pattern. This result is consistent with the fact that at higher temperature, the chains are arranged in disordered manner whereas the rigid parts are packed properly (Colh2 arrangement). Thus, correlation length (calculated with respect to (10) peak) is noted to be comparatively larger at higher temperature (Figure S15-S16, see SI). Figure S18 shows the EDM of compound 4.5 in the Colh phase at 25 and 140 °C. At 25 °C, the voids along with ordered chains (result from the specific arrangement of molecule) form the hexagonal lattice whereas at 140 °C, molecule itself form the hexagonal lattice. The schematic of molecular arrangement is also given along with the EDM39, 40 in Figure 4a and Figure 4b for better clarity. The phase behavior of compound at various chain length and temperature is summarized in the Figure 5.

Absorption and Emission Behaviour The absorption and emission behaviour of all the compounds 4.1-4.5 is shown in Figure 6. The study was done for all the TM, both in dilute solution (10-6 M solution in CHCl3) and in neat form (thin film) (4.1-4.5). All data are represented in Table S11 (see SI). In solution the compound 4.1-4.5 showed two peak in the absorption spectra at 283 nm and 332 nm (with maximum intensity, λmax) as depicted in Figure 6a. The peak at 232 nm comes from * transition of triazole unit38 and the 332 nm peak arises from * transition of central phenylenevinylene part. The emission spectra of compound 4.1-4.5 exhibited two peaks at 402 nm and 420 nm (Figure 6a, vide infra).


13


5

a

Abs_4.1 Abs_4.2 Abs_4.3 Abs_4.4 Abs_4.5 Ems_4.1 Ems_4.2 Ems_4.3 Ems_4.4 Ems_4.5

2.0x10

Emission Intensity (CPS)

c

fw (vol %) 0% 10 % 30 % 50 % 70 % 80 % 90 %

5

1.5x10

5

1.0x10

4

5.0x10

0.0

300

b

375 450 525 Wavelength (nm)

400

600

Abs_4.5 Abs_4.4 Abs_4.3 Abs_4.2 Abs_4.1 Ems_4.5 Ems_4.4 Ems_4.3 Ems_4.2 Ems_4.1

50

d


0%

600

10% 30% 50% 70% 80% 90%

40

I/I0

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

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30 20 10 0

300


600

0

20 40 60 80 Water fraction (vol%)

100

Figure 6. UV-vis absorption and photoluminescence spectra of compounds 4.1-4.5 (a) in chloroform solvent (b) in thin film state. The inset shows blue emission in thin film state. (c) Emission spectra for compound 4.3 upon increasing water fractions fw (vol%). (d) I/I0 vs. water fractions (fw) plots. The I0 represents emission intensity in pure THF solution. I represents the emission intensity in presence of increasing amount of water fractions as described above. Inset shows the emission from the solutions with varying fw in presence of UV light (365 nm). To study the absorption and emission behaviour in the solid state, thin film was made on a glass surface by drop casting technique. 10-6 M CHCl3 solution of respective compound at room temperature was drop casted on the substrate for the experiment. In Figure 6b, it can be noted that there is very less change of λmax in neat state (330 nm) compared to solution state (332 nm). In


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contrast, the emission maxima is shifted from 402 nm (solution state) to 437 nm (solid state). Remarkably, all the derivatives show pure blue light emission in the pristine state (Figure 6b (inset)) which is very important for solid state emissive devices. Furthermore, we performed the fluorescence lifetime decay measurements for the respective compounds. Same concentration (106

M in chloroform) was used for the experiment (Figure S20, see SI). For all the compounds we

observed that the decay curves were properly fitted in bi-exponential function. The average lifetime value (Ʈav) lies in the range of 6.38-8.47 ns for the compounds 4.1-4.5 (Table S11, see SI). The fluorescence decay curve for compound 4.1 clearly shows a very good bi-exponential fit (Figure S21c, χ2= 1.03) compare to its mono-exponential one (Figure S21b, χ2= 4.38) (Table S12). To gain additional insight, we performed lifetime decay experiments by varying the emission wavelength from 390 nm to 470 nm, keeping the excitation wavelength constant. It was observed that both the lifetimes values (1 & 2) and their corresponding amplitudes (1 & 2) remained almost same as a function of emission wavelength as shown in figure S22a and b, respectively (see SI). Therefore, the bi-exponential nature of the fluorescence decay clearly suggests the existence of two different emissive species which are due to possible cis-trans isomerism of the molecule in the excited state with addition complications that may arise from vibrionic coupling. All the compounds showed aggregation induced emission (AIE) behaviour, with pure blue colour emission (Figure 6d (inset)). To study the AIE activity in THF solution of compound 4.3, water was added fraction wise. In beginning when there was no water fraction (f w) the emission intensity was very low. With increasing fw from 0% to 30% it starts to show blue emission when observed under 365 nm UV light. This is shown in the inset of Figure 6d. The corresponding fluorescence spectra (Figure 6c) corroboates the blue light emission as observed through THFwater experiment. At 50% fw, it shows maximum emission intensity and after that up to 90% fw it


15


shows increased intensity (Figure 6d) compared to 0%.41, 42 Moreover, the fluorescence emission peaks shifted towards the red end of the emission spectra with increasing amount of fw. The bathochromic shifting of the peaks in the present case can be explained due to formation of increased J-type aggregation with increase in the poor solvent (water) fraction.

Monomer species H-aggregate

1.0

Normalized Intensity

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

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fw (vol %)

0.8 0.6

388

J-aggregate

395 400

0% 10% 30% 50% 70% 80% 90%

Red shifting of J-aggregate peak

0.4 0.2 0.0 340

360

380

400

420

Wavelength (nm) Figure 7. Fluorescence excitation spectra of compound 4.3 with increasing water content (fw) from 0% to 90%. To get better insight of their aggregation behaviour with increasing water content (fw) from 0% to 90%, fluorescence excitation studies have been performed (Figure 7) for compound 4.3. From Figure 7, it can be clearly seen that in pure THF (fw = 0%) it gives a peak at 395 nm corresponding


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to the monomer species, which later on splits into two peaks, in lower water fraction (fw) of 10%, at 388 nm and 400 nm. The two peaks at 388 and 400 nm are consistent with the formation of H and J-aggregates, respectively, as observed also in prior reports.29 Upon increasing fw to 30%, compound 4.3 starts to show the J-aggregate peak more dominantly compared to its H-one, whereas at fw = 50%, it exhibits majorly J-aggregate peak (at 405 nm) along with the almost diminished H-aggregate peak. Furthermore, on increasing water fraction from 70% to 80% to 90%, it exhibits the J-aggregates peak only with little red-shifted. So, the shifting of fluorescence intensity peaks towards red end upon increasing water fraction has been explicitly explained with increase in the amount of J-aggregation.

Electrochemical Behaviour The electrochemical behaviour of compound 4.1-4.5 has been studied by cyclic voltammetry (CV) to explore their electronic properties. A representaive CV is shown in Figure 8a for compound 4.3 that shows reversible oxidation and reduction curve. It has been observed that all the compounds shows similar oxidation and reduction waves (Figure S23, see SI). The energy levels for ionization potentials (EHOMO) and electron affinities (ELUMO) are in the range of -5.59 eV to -5.29 eV and -3.50 eV to -3.38 eV, respectively, for the compound 4.1-4.5. The energy band gap is in the range of 2.18 eV to 1.80 eV (Table S13, see SI). To attain more insights into the electronic properties of compound 4.1-4.5, density functional theory (DFT) calculations were carried out (Figure 8b-8c and Figure S24, see SI). The optimized structure of 4.1-4.5 adopted planar core. The experimentally observed band gap values (ΔEg,CV), their respective optical band gap (ΔEg,UV) and DFT calculation data (ΔEg,DFT) are closely matches with each other (Table S13, see SI).


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Figure 8. (a) CV of compound 4.3; the experiment is done in 0.1 M DCM solution of TBAP and the scan rate is 50 mVs-1. Computationally calculated (b) HOMO and (c) LUMO of compounds 4.3 when the calculation was performed via the DFT method with B3LYP/6-31G (d,p) parameters. Electroluminescent Properties The electroluminescence (EL) performances of all the synthesized columnar luminomesogens 4.1-4.5 were investigated by using following multilayer solution-processed device structure: ITO (Indium tin oxide) (125 nm) /PEDOT:PSS (35 nm) /CBP doped with 0.5, 1, 3, 5, and 100 wt% of emitters 4.1-4.5 (20 nm) /TPBi (40 nm) /LiF (1 nm) /Al (100 nm), in which PEDOT:PSS was utilized as a hole injecting layer, CBP as a bipolar host material and TPBi as an electrontransporting and hole-blocking layer. The energy level drawing of the resultant OLED devices consisting of columnar luminomesogens as the emitters is revealed in Figures 9a and 9b. The key electroluminescence characteristic plots of the devices are displayed in Figures S25-S29 (see SI) and pertinent data are listed in Table S14 (see SI). After observing performance from the non-doped OLED devices, CBP host were employed to amplify the EL performance of synthesized emitters by balancing the charge-distribution in the emissive zone. Remarkably, the current densities of devices significantly reduced with dramatic enhancement in the brightness. The reason why the doped devices exhibited high driving voltage but concomitantly increased in the luminance may be attributed to four important factors in the device structures, which are (i) an


18

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effective energy transfer from host-to-guest because of employed CBP host, which enable balanced charge transport in the desired recombination zone, (ii) the excitons formed in the CBP layer were efficiently captured by the dopants, resulting in emission originating from the dopants, (iii) a low energy trap between emissive layer and electron transporting layer, which enable effective injection of electrons, and (iv) feasibility of excitons generation on both host and guest.4345

The doping concentration of the emitters strongly influences the device performance and lightbehaviour. For the doping concentration 0.5 wt%, the plots of current density-voltageluminescence (I-V-L) of all the studied devices are shown in Figure 9f and the electroluminescence parameters are mentioned in Table 2. The OLED device fabricated with 4.1 (0.5 wt%) showed the best performance among all the studied emitters. The resultant device exhibited supreme external quantum efficiency (EQE) of 4.0% with CIE coordinates of (0.17, 0.07) (Figure 9e and Table 2) and an EL emission peak at 392 nm. The maximum EQE of device decreased from 4.0% to 1.0% upon increasing the concentration of dopant from 0.5 wt% to 3 wt%. It is also notable that as the concentration of dopant increased to 5 wt%, the maximum EQE of device dropped to 0.8%. It can be elucidated that the dilution effect comes from the even dispersion of guest in the employed matrix at low doping concentration (1 wt%).44, 45 Moreover, high emitter concentration may lead to non-radiative excited state decay due to Auger recombination. It may also influence the charge carrier imbalance, the crystallinity of compounds and thin film morphology in the device, hence device performance.46-52 Furthermore, the resultant OLED device displayed a maximum brightness of 715 cd m-2 with near-UV or ultra deep-blue emission. In contrast, the OLED device containing emitter 4.2 (0.5 wt%) displayed the maximum current efficiency (CE) of 0.6 cd A-1, power efficiency (PE) of 0.4 lm W-1 and an EQE of 2.8% with CIE coordinates of (0.17, 0.07) as shown


19


in Figure 9d-9e & Table 2. The CE of the device increased from 0.6 cd A-1 to 1.1 cd A-1 as a function of increasing the doping concentration from 0.5 wt% to 1 wt% and attained a maximum luminescence of 951 cd m-2. We have also made the OLED device fabricated with 4.3 (0.5 wt%) emitter that shows a maximum PE of 0.6 lm W-1, CE of 0.9 cd A-1 and an EQE of 3.9% with CIEcoordinates of (0.17, 0.07) (Table 2). Remarkably, the power and current efficiency of the OLED

5.2

TPBi

3.38

4.5

3.50

4.4

3.46

4.3

3.41

4.2

3.29

4.1

PEDOT

3.30

4.3 LiF/Al

4.90

ITO

5.29 5.59

5.32

5.54

5.56 CBP host (2.9, 6.0)

0

-1

10

10

-2

10 -1

10

-3

1 Emitters 10

100

2

Luminance (cd/m )

6.2

10 Emitters

2

Luminance (cd/m )

3

c

2

10

1000

f

4.1 4.2 4.3 4.4 4.5

10

700 600 500 400 300

1

10

200 100

10

3

4

5

6

7

Voltage (V)

8

0 9

e

2

0

Current density (A/cm )

b

Emitters 1 4.1 10 4.2 4.3 4.4 100 4.5

d

1

10

Normalized intensity

2.7

a

Current efficiency (cd/A)

devices consisting of emitter 4.3 are progressively amplified as compared to other counterparts. Power efficiency (Im/W)

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 31

1.0

g

0.8 0.6

Emitters 4.1 4.2 4.3 4.4 4.5

0.4 0.2 0.0

400

450

500

550

600

Wavelength (nm)

Figure 9. (a) Energy-level diagram, (b) schematic illustration of OLED devices composing five different emitters (4.1-4.5) in a molecular host CBP. (c) Photograph of an electricity-driven deepblue OLED device based on emitter 4.1. (d) Luminance-voltage-current density, (e) CIE chromatogram,

(f)

current

efficiency-luminance-power

efficiency

and

(g)

Resultant

electroluminescence (EL) spectra of near-UV emission OLED devices consisting of 0.5 wt% emitters (4.1-4.5) doped in CBP host.

Table 2. Data Summary of OLED devices consisting of 0.5 wt % emitters 4.1-4.5 doped in CBP host.a Dopant

Dopant (wt%)

OV (Von)

PEmax/ CEmax/ EQEmax (lm W-1/ cd A-1/ %)

PE100/ CE100/ EQE100 (lm W-1/ cd A-1/ %)

CIExy coordinates

Maxium Luminiscence


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(cd m-2) 4.1

0.5

4.7

0.6/ 0.9/ 4.0

0.5/ 0.7/ 3.2

(0.17, 0.07)

715

4.2

0.5

4.7

0.4/ 0.6/ 2.8

0.4/ 0.5/ 2.4

(0.17, 0.07)

615

4.3

0.5

4.7

0.6/ 0.9/ 3.9

0.5/ 0.7/ 3.0

(0.17, 0.07)

836

4.4

0.5

4.6

0.5/ 0.7/ 2.5

0.3/ 0.5/ 2.0

(0.17, 0.08)

674

4.5

0.5

5.1

0.3/ 0.5/ 1.7

0.3/ 0.5/ 1.7

(0.17, 0.07)

573

a

Von = turn on voltage. Turn-on voltage and CIE coordinates recorded at the luminance of 1 and 100 cd m-2, respectively It is noteworthy that OLED devices fabricated with emitter 4.3 displayed low turn-on voltage and high luminance when compared with other dyes. The reason beside this may be shallow and deep lying levels of LUMO and HOMO, respectively, which facilitated balanced and easy injection of charge carriers in the emissive layer. The OLED device consisting of dyes 4.4 and 4.5 revealed maximum EQE of 2.8% and 2.7% with CIE coordinates of (0.17, 0.1) and (0.17, 0.11), respectively. Figure 9g shows the EL spectra of all the composed OLED devices consisting of 0.5 wt% of the emitters. An image of electricity-driven deep-blue OLED device based on emitter 4.1 is shown in Figure 9c. Near-UV emission was perceived for all emitters (4.1-4.5) with peaks centered at 395405 nm with CIE coordinates of (0.17±0.01, 0.07±0.01) at lower doping concentration, i.e. 0.5 wt%, matching closely with the NTSC standard pure blue coordinates of (0.14, 0.08). The EL spectra of devices with higher doping concentrations shifted towards higher wavelength. It is also notable that EL spectra of all the doped devices show no emission associated with CBP, indicating that an efficient energy transfer has occurred from host to synthesized emitter. Furthermore, the high colour purity of all the doped devices employing these emitters is attributed to narrow fullwidth half maxima (FWHM) of the electroluminescence spectrum. The emission maxima (EL spectra) of devices were found to be almost similar to the respective solid-state PL of the emitters.


21


Page 22 of 31

It indicates the proper dispersion of dopant into the host material with suppression of aggregation in thin films and the origin of the emission from dye molecules.53, 54 Also, we have measured the PL spectra of all the synthesized compounds in CBP host for comparison with their EL spectra (0.5 wt%) as presented in Figure S30 (see SI). It shows that the EL spectra of all the compounds closely resemble to their corresponding PL spectra. It indicates that the EL possibly arises from the radiative decay of singlet excitons55 which are confined on the guest molecules and there aggregation is absent between the molecules in the emissive film. Furthermore, we have chosen the compound 4.1 (as it shows the maximum EQE value) for the crystallinity dependent EL studies. In order to analyze the crystallinity dependent EL intensity of compound 4.1, the emissive layer of the device annealed at four different temperatures: 25 C, 60 C, 100 C and 140 C for 15 min was used for the measurements. The recorded EL spectra are displayed in the Figure S31 (see SI). It is notable that there are no significant changes observed in the EL peak maxima while the EL intensity dropped with increasing temperature. The EL spectra of all the fabricated devices centered around 395-405 nm with slightly varying in the CIE coordinates. The OLED device fabricated at room temperature (25 C) shows CIE coordinates of (0.17, 0.07). While, OLED devices in which emissive layer annealed at 60, 100 and 140 C show CIE coordinates of (0.17,0.08), (0.17, 0.08) and (0.17, 0.09), respectively, at the practical need of brightness for display i.e. 100 cd m-2. Moreover, these results also confirmed the thermal stability of the synthesized materials and fabricated OLED device based on it.

Conclusions In summary, we have developed a new strategy to design materials for room temperature Col LCs via click chemistry. These materials act as a highly pure deep blue emitter in OLEDs with a


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maximum EQE of 4.0% for the first time in Col LCs with CIE coordinates of (0.17, 0.07) corresponding to the pure blue colour. Therefore, the new AIE active derivatives have strong potential to the development of columnar LC based OLEDs technology and to the realization for commercial applications as new class of emitters with higher efficiency. This study may serve as a starting point for future innovations in research and development of luminescent columnar liquid crystals as efficient emitters in OLEDs.

ASSOCIATED CONTENT Supporting Information. Supporting Information includes: Synthesis and characterization details, NMR Spectral data, Thermal Behaviour studies by POM, DSC, TGA and X-ray, Photophysical studies, Electrochemical studies, computational studies by DFT and OLED device fabrication data. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]; [email protected]. ACKNOWLEDGMENT We thank INSA (sanction No. SP/YSP/124/2015/433), IISER Mohali for the financial support. We also acknowledge CSIR for funding bearing sanction No. (02(0311)/17/EMR-II). We acknowledge X-ray scattering facility, HRMS and NMR facility at IISER Mohali.

The

instrumental facilities at Department of Chemistry, IISER Mohali are also acknowledged. I. Bala thanks the CSIR-NET, File No. 09/947(0061)/2015-EMR-1 for financial support. J. De thanks the


23


Page 24 of 31

IISER Mohali for the receipt of a graduate fellowship. J. De thanks to Dr. Manisha Devi for helping in flourescene experiments. We sincerely acknowledge Dr. Samrat Mukhopadhyay, Dr. Sanchita Sengupta and Dr. Arijit Kumar De for many helpful discussions in fluorescence experiments. REFERENCES (1) Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.; Mackay, K.; Friend, R. H.; Burns, P. L.; Holmes, A. B. Light-Emitting Diodes Based on Conjugated Polymers. Nature 1990, 347, 539-541. (2) Middleton, A. J.; Marshall, W. J.; Radu, N. S. Elucidation of the Structure of a Highly Efficient Blue Electroluminescent Material. J. Am. Chem. Soc. 2003, 125, 880-881. (3) Tang, C. W.; VanSlyke, S. A. Organic Electroluminescent Diodes. Appl. Phys. Lett. 1987, 51, 913-915. (4) Zhu, M.; Yang, C. Blue Fluorescent Emitters: Design Tactics and Applications in Organic Light-Emitting Diodes. Chem. Soc. Rev. 2013, 42, 4963-4976. (5) Schubert, E. F.; Kim, J. K. Solid-State Light Sources Getting Smart. Science 2005, 308, 1274-1278. (6) Kumar, S. Self-Organization of Disc-Like Molecules: Chemical Aspects. Chem. Soc. Rev. 2006, 35, 83-109. (7) Bisoyi, H. K.; Kumar, S. Discotic Nematic Liquid Crystals: Science and Technology. Chem. Soc. Rev. 2010, 39, 264-285. (8) Imrie, C. T.; Henderson, P. A. Liquid Crystal Dimers and Higher Oligomers: Between Monomers and Polymers. Chem. Soc. Rev. 2007, 36, 2096-2124.


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10% 30% 50% 70% 80% 90%

Al

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EQE : 4%

LiF

TPBi Host:Emitter PEDOT:PSS ITO Glass substrate

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Colr (30) (21) (06) (22) (02) (11)

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Room temperature columnar liquid crystals as an efficient pure deep

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