Efficient Blue and Yellow Organic Light-Emitting Diodes Enabled by

Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Efficient Blue and Yellow Organic Light-Emitting Diodes Enabled by Aggregation-Induced Emission N. Venkatramaiah, G. Dinesh Kumar, Yogesh Chandrasekaran, Ramesh Ganduri, and Satish Patil* Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012, India S Supporting Information *

ABSTRACT: A new class of donor−bridge−acceptor (D−π−A) πconjugated light-emitting molecules comprising carbazole as donor and maleimide (Cbz-MI, Cbz-MI(d)), phthalimide (Cbz-Pth) as acceptor units with phenyl ring as spacer have been synthesized in good yields. These compounds exhibit high quantum yield with three distinct emission colors yellowish-green (Cbz-MI), bright yellow Cbz-MI(d), and sky blue (CbzPth) in the solid state. Single-crystal X-ray and quantum chemical calculations reveals that twisting of the phenyl rings with high torsional angle on maleimide and phthalimide units reduce the effective interchromophore electronic coupling, furnish dramatic changes in their photophysical properties in solution and solid states. Intriguingly, CbzMI(d) and Cbz-Pth exhibits a unique aggregation-induced blue-shifted emission (AIBSE) due to restricted intramolecular rotation (RIR) process, while Cbz-MI shows red-shifted emission in the solid state. The solvatochromic study reveal that combined RIR and excited state migration augment AIE (aggregation-induced emission) properties. The electrochemical properties reveal that Cbz-MI exhibits high oxidation propensity while Cbz-Pth shows low reduction values. Subsequently, organic light-emitting diodes (OLEDs) were fabricated with a simple three-layer device containing Cbz-Pth and Cbz-MI(d) as emitting layers. Cbz-MI(d) exhibits high performance yellow OLED with an external quantum efficiency exceeding ∼4.1% and a brightness exceeding ∼73915 cd/m2, which is among the best performance reported for bright yellow fluorescence organic light-emitting diodes. KEYWORDS: carbazole, aggregation-induced emission (AIE), OLEDs, donor−acceptor conjugates (D−A), restricted intramolecular rotation (RIR), photoluminescence

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INTRODUCTION Organic light-emitting diodes (OLEDs) are poised to rapidly gain market in display industry. However, OLEDs encountered several challenges to become a mainstream in large display applications, such as television. The key challenges for the OLEDs include color purity, stability, and limited external quantum efficiency (EQE).1 A high quantum efficiency in the solid-state is a prerequisite to fabricate OLED with improved EQE. The molecular arrangement of fluorophores in the solid state has a significant influence on their photoluminescence quantum yield (PLQY) and subsequently EQE of the device.2 The inherent nature of molecular materials to aggregate while casting a thin film often quenches the emission by notorious aggregation-caused quenching (ACQ).3−5 Pioneering research work of Tang and co-workers has shown an intriguing aggregation-induced emission (AIE) and developed number of AIE/AIEE-based materials by weak intermolecular and π−π interactions through restricted intramolecular rotation (RIR) process. Design of such novel AIE based multifunctional luminescent materials offer tunable emission and creates new avenue with improved EQE in OLEDs.6−9 Several AIE-based πconjugated materials, such as tetraphenylethene (TPE) derivatives, 9,10-distyrylanthracene (DSA), diphenyldibenzo© XXXX American Chemical Society

fulvenes and silole derivatives were developed. In this context, TPE was considered as a unique and archetypal AIE material owing to its synthetic feasibility and easy chemical derivatization.10−15 Grafting of TPE units with different functional fluorophores, such as pyrene, carbazole, or triphenylamine, led to efficient solid-state emitters with significantly high EQE values as a non-doped OLEDs.16 To date, the design of stable blue-emitting materials and devices based on AIE is long-standing problem and has received much attention.17−19 In this regard, carbazole-based molecules have emerged as useful building block for blue OLEDs because of their high structural rigidity and wide band gap.20−23 Adachi and co-workers systematically functionalized carbazole unit at different positions to tune the emission of colors and developed OLEDs with promising EQE.24 Carbazole−triphenylamine and carbazole−carbazole conjugates with varied number of triphenylamine and carbazole units show deep blue electroluminescence with EQE of 3.0% at 100 cd m−2.25 Carbazolebased compounds with exciplex-forming properties were Received: July 26, 2017 Accepted: December 21, 2017

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DOI: 10.1021/acsami.7b11025 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

ACS Applied Materials & Interfaces

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developed for efficient and color tunable OLEDs with EQE of 3.3%.26 Similarly, to improve the charge balance and device efficiency, the conjugated π-systems having donor (D) and acceptor (A) backbone architecture was found to be an effective method to tune the band gap and enhance charge carrier mobility.27 The outstanding optical and electronic properties of this class of materials also simplify device fabrication strategies by replacing complicated multilayer device with comparatively simple device architectures. However, the fine control over push−pull effect is necessary to tune the band gap and color purity. Adachi’s group has developed bis(4-fluorophenyl)sulfone derivatives based on thermally activated delayed fluorescence (TADF) phenomena and achieved remarkable EQE of ∼9.9% with push−pull π-conjugated chromophores.28 Yasuda et al. have developed pre-twisted acridan−pyrimidine (D−A) based molecules as TADF emitters with small singlet− triplet energy gap (ΔEST), exhibits efficient deep-blue emission with remarkable EQE ∼ 20.4%, along with maximum current efficiencies of 41.7 cd A−1 and power efficiencies of 37.2 lm/ W.29 A twisted π-conjugated phthalimide and maleimide based molecules with acridan exhibited green delayed fluorescence owing to its ΔEST and showed EQE of ∼11.5%.30 Fluorophores tethered with unsubstituted maleimides (NPAMLMe) were developed as red host emitter. The performance of the nondoped red-light-emitting devices were examined as a function of thickness and reveals that the device exhibits pure red EL with a brightness ∼4600 cd m−2 and an external quantum efficiency as high as 1.6%.31 Recently, our group reported maleimide flanked flexible carbazole donor-π-acceptor based material as emitting layer and demonstrated EQE of ∼2.5% in the yellow region with emission maxima at ∼550 nm.32 To date, the majority of the fluorescent materials developed for OLED applications exhibit red-shifted emission behavior in solution or solid state upon direct conjugation with acceptor groups. This leads to a shift in the spectral pattern and severely damages color purity. Hence, it remains a quite challenging task to develop fine-tunable donor and acceptor push−pull systems for enhanced charge transport properties to achieve high EL efficiency, stability, and high color purity by preventing undesirable intermolecular and π−π interactions. Design of π-conjugated molecular ensembles with highly twisted propeller units along with steric hindrance by bulky units provides a unique method to generate restricted intramolecular rotation with enhanced nonradiative pathway impart AIE active materials with high quantum yields. Besides, the choice of wider partnered functional units of π-conjugated molecular materials endows control over the color purity. Herein, we report a new family of donor−bridge−acceptor (D−π−A) molecular materials in which a maleimide/ phthalimide acceptor unit was covalently linked in conjugation with carbazole donor moiety (Cbz-MI(d) and Cbz-Pth) through phenyl and biphenyl (Cbz-MI) π-spacer. The large twist and steric repulsion between carbazole unit and maleimide/phthalimide creates large variation in the torsional angle leading to effective RIR processes, enabling AIE behavior. The influence of the molecular packing and conformation on optical properties were correlated. Multilayer electroluminescent devices were fabricated with three developed compounds as emitting layer. Among all, Cbz-MI(d) exhibits bright yellow color with external quantum efficiency of ∼4.1% and remarkably high brightness of ∼73915 cd/m2 with negligible roll-off at high current.

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MATERIALS AND METHODS

All the chemicals and reagents were obtained from Sigma-Aldrich. The solvents were purified with standard procedures. 1H and 13C NMR spectra were recorded using a Bruker NMR spectrometers (400 and 100 MHz) in CDCl3. The UV−visible spectra were recorded on a PerkinElmer (Lambda 35) UV−visible spectrometer. Solution based spectra were recorded in chloroform, THF (1 × 10−5 M). Thin films on quartz substrate were used to collect the UV−visible spectra in solid state. Photoluminescence (PL) studies were carried with Spex FluoroLog-3 spectrofluorometer (Jobin-Yvon, Inc.) with slit width of 2 nm. Fluorescence lifetime measurements were performed in solution and thin films with Horiba Jovin Yvon lifetime spectrometer by TCSPC method with diode laser (369 nm, 25 ps). Thermal properties were measured on Mettler Toledo STARe system with a heating rate of 5 °C/min and N2 flow of 40 mL/min. Electrochemical studies were performed using CH electrochemical analyzer with a scan rate of 50 mV/s. HOMO and LUMO energies were calculated from the oxidation and reduction peak potentials.34 The ground state geometry optimization was carried out in Gaussian 09 software.35 Electron density surfaces were obtained in Gauss View 5.0.8. Oxford Xcalibur (Mova) diffractometer equipped with an EOS CCD detector with Mo Kα radiation (λ = 0.71073 Å) source is used to collect single crystal Xray diffraction data. Structure determination was carried out with Win Gx program.36,37 Morphological studies were carried on drop-cast thin films on silicon wafer using a Zeiss scanning field emission electron microscope at 5 kV. Synthesis. Synthesis of 3,4-Bis(4-(9H-carbazol-9-yl)phenyl)-1hexyl-1H-pyrrole-2,5-dione (Cbz-MI(d)). 3,4-Bis(4-bromophenyl)-1hexyl-1H-pyrrole-2,5-dione (1) was synthesized according to the procedure reported in literature.32 In a round-bottom flask, 3,4-bis(4bromophenyl)-1-hexyl-1H-pyrrole-2,5-dione (0.5 g, 1.12 mmol) and carbazole (0.51 g, 3.05 mmol) were dissolved in toluene (10 mL). The reaction mixture was degassed with Argon gas for 10 min and BINAP (0.23g, 0.36 mmol), K2CO3 (1.26 g, 9.1 mmol), and catalytic amount of Pd2(dba)3 was added and refluxed at 110 °C for 24 h. The reaction mixture was cooled to ambient temperature and filtered through sintered funnel using Celite. The crude product was purified by silica gel column chromatography. The compound was recrystallized with methanol gave yellow color solid. Yield: 60% (400 mg). 1H NMR (400 MHz, CDCl3, ppm): δ 8.04 (d, J = 8 Hz, 4H), 7.74 (d, J = 8 Hz, 4H), 7.56 (d, J = 4 Hz, 4H), 7.42 (d, J = 8 Hz,), 7.32 (s, 4H), 7.21(s, 4H), 3.63 (s, 2H), 1.66 (br, 2H), 1.29−1.17 (m, 6H), 0.84 (br, 3H). 13C NMR: (100 MHz, CDCl3, ppm): δ 170.7, 140.3, 139.4, 135.3, 131.6, 127.2, 126.7, 126.2, 123.8, 120.5, 109.9, 38.7, 31.4, 28.7, 26.6, 22.6, 14.1. MALDI-TOF-MS: m/z calculated for C46H37N3O2: 663.2885, obtained 686.2786 [M + Na]+. Synthesis of 5,6-Bis(4-(9H-carbazol-9-yl)-hexylisoindoline-1,3dione (Cbz-Pth). In a round-bottom flask, N-hexyl-4,5-diiodophthalimide 6 (0.6 g, 1.24 mmol) and 9-(4-(4,4,5,5-tetramethyl-1,3,2dioxaborolan-2-yl)phenyl)-9H-carbazole (1.03 g, 2.8 mmol) was dissolved in toluene (10 mL). The reaction mixture was degassed with argon for 10 min, and catalytic amount of freshly prepared Pd(PPh3)4 was added and refluxed at 110 °C for 48 h. The resultant final reaction mixture was cooled to room temperature and filtered through sintered funnel using Celite. The crude product was purified through silica gel column chromatography with a solvent mixture of ethyl acetate: hexane (6:4, v/v) as eluent and gave a light-yellow solid with yield of 52%. 1H NMR (400 MHz, CDCl3, ppm): δ 8.15 (d, J = 8 Hz, 4H), 8.10 (s, 2H), 7.57 (d, J = 8 Hz, 4H), 7.47 (d, J = 8 Hz, 4H), 7.40−7.37 (m, 8H), 7.31−7.26 (m, 4H), 3.77 (t, J = 12 Hz, 2H), 1.75 (t, J = 16 Hz, 2H), 1.39−1.34 (m, 6H), 0.91 (t, J = 12 Hz, 3H). 13C NMR: (100 MHz, CDCl3, ppm): δ 168.17, 145.86, 140.67, 138.73, 137.55, 131.62, 131.29, 126.95, 126.18, 125.32, 123.60, 123.43, 120.25, 109.56, 38.40, 31.43, 28.65, 26.62, 22.57, 14.04. MALDI-TOF-MS: m/ z calculated for C50H39N3O2: 713.3042, obtained 713.191 [M+]. Device Fabrication. Indium tin oxide (ITO) coated glass substrates were obtained from DELTA technologies, USA and used for device fabrications. ITO substrates were treated in different solutions such as isopropanol, acetone and in deionized water for 20 min each in ultrasonic sonication bath. Finally, the treated substrates B


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ACS Applied Materials & Interfaces were dried under nitrogen flow and kept in oven at 80 °C for 30 min. The pre-cleaned ITO glass substrates were treated with ozone for 15 min. ITO substrates were deposited with a thick layer of α-NPD layer (40 nm) in an inert chamber under a pressure of Cbz-MI > Cbz-Pth. The optical band gaps of three derivatives were measured from the onset of absorption bands at lower energy region and were found to be ∼2.16, 2.24, and 2.74 eV for Cbz-MI(d), Cbz-MI, and Cbz-pth, respectively. The emission spectra of three derivatives in chloroform exhibits emission maxima at 552, 626, and 548 nm with Stokes shift of 152, 205, and 201 nm for Cbz-MI, CbzMI(d), and Cbz-Pth, respectively (Figure 1b). The observed emission maxima in solution are in good agreement with absorption spectra as described. The emission spectra in thin films are in complete contrast to the solution behavior. Table 1 summarizes the photophysical data of the developed molecules in solution and thin films. We have observed three notable features in the emission spectra of thin films in comparison with that of solution. (i) Cbz-MI shows emission maxima ∼580 nm with a red shift of ∼32 nm and Stokes shift of ∼168 nm indicates the formation of molecular aggregates. (ii) In contrast to that of Cbz-MI, CbzMI(d) shows emission maxima of ∼591 nm with a blue shift of ∼35 nm and Stokes shift of ∼165 nm. (iii) Cbz-Pth shows emission maxima of ∼481 nm with large blue shift of ∼71 nm with Stokes shift of ∼106 nm. The observed unique blue shift in the emission of Cbz-MI(d) and Cbz-Pth in thin films indicates aggregation induced blue shift emission (AIBSE) in these materials. The direct covalent linking of carbazole with imidazole and phthalimide facilitate in realizing RIR process by non-radiative pathway which leads to AIBSE behavior. The observed AIBSE effect is more pronounced in the case of CbzPth compared to that of Cbz-MI(d). Figure 1c shows the photograph of the three compounds dissolved in chloroform solutions and illuminated under UV light at 365 nm. Figure 1d shows the images of pristine powders of three derivatives under day light (i), under the illumination at 365 nm (ii), thin films cast by drop cast method (iii). It clearly shows that, Cbz-Pth and Cbz-MI(d) exhibits bright emission than Cbz-MI.

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RESULTS AND DISCUSSION Synthesis of carbazole appended maleimide and phthalimide derivatives were accomplished by following synthetic procedure as described in Scheme 1. The precursors 1 and 6 were Scheme 1. Synthetic Route for Preparation of CarbazoleBased Maleimide/Phthalimide Derivatives

obtained by using the procedure reported in literature.32 Stille coupling between compound 1 and 9-(4-(tributylstannyl)phenyl)-9H-carbazole yielded the desired product Cbz-MI in 77% yield. Buchwald coupling was carried out between 1 and carbazole to obtain the product Cbz-MI(d) in 65% yield as yellow solid. Cbz-Pth was obtained by coupling of 9-(4-(4,4,5,5-tetramethyl1,3,2-dioxaborolan-2-yl)phenyl)-9H-carbazole and phthalimide derivative 6 with 52% yield. The desired final compounds CbzMI(d) and Cbz-Pth were purified by column chromatography. Further, structure of the compounds were characterized by C


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Figure 1. Absorption (a) and emission spectra (b) of three compounds in chloroform and in thin films. (c) Change in the emission behavior of compounds in solution (d) in solid state (i) under normal light (ii) under UV light at (λex= 365 nm) and in thin films (iii) at (λex= 365 nm).

Table 1. Summary on the Photophysical Data of Cbz-MI, Cbz-MI(d), and Cbz-Pth in Solution and in Thin Film compound Cbz-MI Cbz-MI(d) Cbz-Pth a

states solution film solution film solution film

absorption (nm) 290, 295, 341, 341, 328, 375

400 412 421 426 347

λem

Φf (%)

lifetime (ns)

Stokes shift (nm)

kr (ns−1)

knr (ns−1)

552 580 626 591 548 481

84 57a 42 66a 78 70a

6.4 5.5 5.4 4.0 11.2 10.0

152 168 205 165 201 106

0.131 0.103 0.077 0.165 0.069 0.070

0.025 0.078 0.107 0.085 0.019 0.03

PLQY were measured with emitters (10%) doped with CBP films.

rings are −130.48° and 126.95° reveals that both carbazole and phenyl rings were highly twisted with respect to phthalimide unit in the solid state. The molecules were held together with C−H···O interactions (2.671 Å) between CO of the phthalimide with hydrogens of the spacer phenyl ring through heterogenic catamer hydrogen bonding pattern resulted in the formation of molecular dimer with center of inversion (Figure S15). The molecular dimers held together by C−H···π interactions (3.048 Å) with carbazole unit and alkyl chain at C2 position results the formation of 2D structure (Figure S16). In addition to C−H···O and C−H···π interactions, Cbz-Pth exhibits π−π interactions of 3.648 and 3.713 Å indicative of strong π−π overlap in the molecular ensemble. Cbz-MI(d) also crystallizes in monoclinic crystal system with space group of P21/n with Z = 4. The torsional angles of maleimide with spacer phenyl and carbazole units are −144.60° and 132.38°, respectively. In comparison to Cbz-Pth, Cbz-MI(d) exhibits less twist in the torsional angle indicating better planarity and higher conjugation degree. The variation in the twist of the phenyl ring reflects strongly on the photophysical properties in the solid state. In Cbz-MI(d) unit cell, the molecules were arranged head to tail and resembles like molecular column with C−H···O interactions (2.482 and 2.5503 Å). The molecular

The fluorescence lifetime by TCSPC method is found to be 6.4, 5.4, and 11.2 ns with quantum yield of 84%, 42%, and 78% for Cbz-MI, Cbz-MI(d), and Cbz-Pth, respectively (Figure S12). The nonradiative constant (knr) of Cbz-MI(d) (0.107 ns−1) is much higher than that of Cbz-MI (0.025 ns−1) and Cbz-Pth (0.019 ns−1) resulting in the lower quantum yield (Φf) in solution. In solid state, Cbz-MI, Cbz-MI(d), and Cbz-Pth are strongly emissive at 580, 591, and 481 nm with quantum yield of 57%, 66%, and 70%, respectively. The non-radiative constant (knr) of Cbz-Pth (0.03 ns−1) is found to be smaller in comparison with Cbz-MI (0.078 ns−1) and Cbz-MI(d) (0.085 ns−1). These results reveal that non-radiative pathway plays a significant role on the emission behavior of these compounds in the solid state. To further understand the solid-state properties, single crystals of three derivatives were obtained by slow solvent evaporation method at room temperature. Figure 2 shows the single crystal X-ray determined (SXRD) structures of the three compounds. The molecular packing, ORTEP of individual molecules are shown in the Figures S13−S23. Cbz-Pth crystallized in monoclinic crystal system with space group of P21/C and Z = 4. The crystal structure shows an intramolecular torsional angle of phthalimide ring with phenyl and carbazole D


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Figure 2. Single crystal X-ray structures of Cbz-MI, Cbz-MI(d), and Cbz-Pth and their molecular arrangements in the unit cell. The crystal structure of Cbz-MI was adopted from our previous report for comparison.32

Figure 3. Frontier molecular orbitals (HOMO and LUMO) for the model compounds.

columns were held together with C−H···π (2.696 Å) and π−π interactions (3.904 and 3.889 Å) respectively (Figure S19). The π−π distance is found to be 0.19 Å higher than Cbz-Pth indicating that decrease in the conjugation of the acceptor unit furnishes weaker π−π interactions. To understand this behavior, we have correlated the π−π interactions by fixing acceptor unit and introduced biphenyl unit as spacer between maleimide and carbazole units. The crystal structure of Cbz-MI was reported by our group and the comparison study was made for the ease of correlation.32 CbzMI crystallizes in monoclinic system with space group of I2/a with Z = 6. The torsional angles of the maleimide unit with biphenyl and carbazole units are −145.48° and 122.44°. The carbazole units attain more planarity compared with other two derivatives. The biphenyl units show torsional angle of 160.76°

between the benzene units. Cbz-MI shows two different kinds of C−H···O interactions formed by hydrogen bonding with one carbonyl units of maleimide with adjacent carbazole while the other carbonyl group show interaction with biphenyl unit. The molecular packing shows 2D molecular chain assembly with four different C−H···π interactions at 2.689, 2.705, 2.779, and 2.779 Å, respectively (Figure S22). The extension of conjugation in Cbz-MI exhibits very weak π−π interaction at a distance of 5.026 Å. Owing to weak π−π interactions, high torsional angles, and presence of cavities in the crystalline solids, the structures are very feeble and can readily undergo structural deformations. The decrease in the crystal lattice binding force leads to planarization of molecular conformation and shift in the spectral pattern. Strong π−π interactions, weak C−H···π interactions and high twist in the torsional angles of E


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Figure 4. PL spectra of (a) Cbz-Pth and (b) Cbz-MI(d) in THF: H2O mixtures with different water fractions, (conc. 1.6 × 10−5 M). The inset photograph shows change in the emission behavior under UV light at 365 nm.

THF and water fractions at fixed concentrations of the three derivatives (Figure 4). For Cbz-Pth, the fluorescence intensity at 520 nm ( f w= 0%) is quenched dramatically for addition of f w = 10% to f w = 50%. At f w = 60%, a new emission peak appears at 466 nm and show maximum emission at f w = 70% (Figure 4a). Further increase in the water fraction, the emission spectra slightly red-shifted with decrease in the emission intensity. The decrease in the emission intensity at high water fraction ( f w > 90%) arises due to precipitation of Cbz-Pth. A blue shift of ∼54 nm observed in the emission behavior at f w = 70% demonstrates the strong intermolecular self-assembly behavior restricts the intramolecular rotation process. Figure 4b shows the change in the emission spectra of CbzMI(d) at different THF:H2O fractions. Cbz-MI(d) shows decrease in the emission intensity f w < 60% as similar to CbzPth. From f w = 70%, the emission intensity is blue-shifted by ∼12 nm and the emission intensity increases gradually with further increase in the ratio of THF: H2O. It reveals that f w> 70%, Cbz-MI(d) exhibits aggregation induced emission enhancement by formation of molecular aggregates. These combined results imply that Cbz-Pth exhibits strong AIBSE behavior while Cbz-MI(d) shows AIBSE behavior along with aggregation-induced emission enhancement (AIEE). The observed dual behavior in Cbz-MI(d) is a rare example not known among existing systems of AIBSE. The emission intensity of Cbz-MI is quenched completely until f w = 60% and a new emission peak appears at f w = 70% without change in the emission maxima. Further increase in the concentration f w > 70%, we have observed decrease in the emission intensity by formation of molecular aggregates (Figure S24). Figure S25 shows the change in the emission intensities of Cbz-MI(d) and Cbz-Pth at different concentrations of THF: H2O ratio and corresponding change in the color of the solutions are shown in Figure S26. The concentration-dependent PL studies were carried out to understand the ACQ in these materials. Figure S27 reveals that, the emission intensity gradually increases without change in the emission maxima with increase in the concentration from 1 × 10−6 M to 4.5 × 10−4 M. These studies further support that formation of non-covalent interactions at higher water fractions furnish the formation of spontaneous self-assembled molecular aggregates and favors the restricted intramolecular rotations leading to observed unusual blue shift in the solid state. To have better understanding on AIBSE, viscosity dependent emission studies were carried out in a series of solvents. The absorption spectra of the compounds did not show any significant change in different polar solvents. However, the emission spectra showed solvatochromic shift upon increasing

Cbz-Pth and Cbz-MI contribute to the formation of high aggregation induced emission behavior in the solid state. In solution, the presence of active intramolecular rotations leads to twist in molecular geometry triggers nonplanar confirmations. However, locking of such molecular rotations in the crystal lattice renders the efficient π−π stacking and furnish the radiative deactivation of excitons and contributes to the AIE effect. Because of less torsional and more planar confirmation at spacer units, Cbz-MI shows red-shifted emission, while twisted conformation of the spacer unit in Cbz-Pth and Cbz-MI(d) shows blue-shifted emission. Summary of the crystallographic data is provided in Table S1. To gain deeper insight into the origin of blue-shifted emission, we have performed computational calculations in the ground and excited states. The structural features were correlated with SXRD. Density functional theory (DFT) with B3LYP/6-31g* basis set were used to perform quantum chemical calculations. TD-DFT studies were carried out with optimized geometries with same basis set. The optimized structures reveal that substitution of phenyl units on maleimide and phthalimide rings undergo twist from the central plane. Carbazole units show torsional angles of 128, 127, and 125 Å for Cbz-Pth, Cbz-MI(d), and Cbz-MI respectively. Cbz-Pth shows torsional angle of 131 Å at phthalimide moiety while Cbz-MI and Cbz-MI(d) were twisted by ∼147 Å. The variations in the torsional angles are in good agreement with single crystal X-ray structures. The large variation in the torsional angles renders the uniform progression of π-electron density on the molecular backbone. For all the derivatives, the electron density of HOMO localized on the donor carbazole units and LUMO electron density localized on acceptor maleimide and phthalimide units indicate favorable donor− acceptor π → π* transitions (Figure 3). TD-DFT simulated absorption spectral pattern of three Cbz derivatives show wellseparated low-energy intense absorption bands in the range of 532−563 nm (Cbz-MI and Cbz-MI(d)) and 461 nm (CbzPth) and less intense bands in the range of 250−355 nm. The simulated absorption properties were found to be in good agreement with the experimental results. This study reveals that the electronic transitions occur from HOMO → LUMO energy levels. The estimated lowest energy vertical electronic transitions, oscillator strengths and orbital contributions are presented in Table S2. The oscillator strengths were found to be higher for Cbz-MI(d) and Cbz-MI indicates greater in absorption. Cbz-Pth exhibits high dipole moment of 0.978 D indicating the propensity to form efficient π−π stacking. To investigate the unique blue shift in Cbz-MI(d) and CbzPth, aggregation studies were carried out in different ratios of F


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2.29 eV. Cbz-MI exhibits HOMO and LUMO energy levels −5.69 and −3.48 eV, respectively. The oxidation potentials were found to be higher for CbzMI, which arises due to increase in the electron density of the donor unit in the molecular backbone. Extending the conjugation on the acceptor unit, the reduction propensity increases with increase in the conjugation of acceptor unit (phthalimide) furnishes decrease in the LUMO energy levels dramatically. The measured energy levels are well matched with the energy levels obtained from the DFT and onset of the UV− visible measurements. Thermal properties of the three derivatives were characterized by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). In DSC, Cbz-Pth exhibits a glass transition temperature (Tg) at 105 °C, crystallization temperature at ∼200 °C, and the melting temperature at ∼234 °C. Cbz-MI(d) and Cbz-MI exhibits only melting temperature ∼188 and 318 °C, respectively (Figure S31a). The observed Tc for Cbz-Pth indicates that extending the conjugation on acceptor unit induces the crystallization. TGA of three derivatives (Figure S31b) show high thermal decomposition at 400, 430, and 400 °C for Cbz-MI, Cbz-MI(d), and Cbz-Pth, respectively. The high thermal stability of Cbz-MI(d) suggests that more rigid arrangement of carbazole units in the molecular backbone. The improved thermal stability of these compounds is important for the stability of the organic light emitting diode performance. Electroluminescence Properties. Electroluminescence characteristics were investigated by fabricating a multilayer layer device comprising 10% of Cbz-MI(d) (device-1), CbzPth (device-2), and Cbz-MI (device-3) in CBP, as emissive layer, with device configuration ITO/NPB (40 nm)/10 wt % Cbz-MI(d): CBP (15 nm)/TPBi (40 nm)/LiF (0.7 nm)/Al (100 nm). In this device architecture, NPB (N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)benzidine) is used as hole transporting layer, CBP (4,4′-bis(N-carbazolyl)-1,10-biphenyl) is host, and TPBi (2,2′,2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1H-benzimidazole) served as electron-transporting layer and hole-blocking layer. Figure 6 shows the energy level diagram and device architecture. The PL and EL spectra, current efficiency, current density−voltage−luminance and external quantum efficiency of the fabricated devices are shown in Figure 7 and pertinent data is summarized in Table 2. Figure 7a shows thin film photoluminescence spectra of three compounds in CBP host. The PL emission maxima obtained for Cbz-MI(d), Cbz-MI, and Cbz-Pth are 561, 550, and 465 nm, respectively. These values found to be slightly blue-shifted in comparison with pristine thin films. The EL spectra of CbzMI(d), Cbz-MI, and Cbz-Pth shows EL maxima at 584, 563, and 490 nm respectively (Figure 7b). The EL spectral pattern is resembled to the emission spectrum of the materials in thin film and reveals that EL is indeed from the emitting layer. Device-1 shows turn on bias voltage at 5.85 V exhibits bright yellow light emission with maximum luminance (Lmax) of 73915 cd/m2 and maximum current efficacy (ηC,max) 13.8 cd/A. The EQE (ηext,max) is calculated ∼4.1%, which is close to the theoretical limit (5%) for organic luminescent materials.39 The observed high brightness is one among the best performance reported for yellow fluorescence organic light-emitting diodes (Table S3). At 1000 cd/m2, the device exhibits high ηext of ∼3.7% with small roll-off efficiency. The measured CIE coordinates are (0.44, 0.51), which fall in the super yellow emission (0.44, 0.54).40,41

the solvent polarity (Figure S28). It reveals that in the excited state, these molecules exhibit more polar nature than in the ground state. As evidenced from the chemical structures, the compounds possess donor−acceptor functionalities; the observed shift in the emission may arise due to charge-transfer excited states. For Cbz-Pth, the emission maxima was observed at 560 nm in dichloromethane. As we increase the polarity of the solvent medium containing Cbz-Pth, the emission maxima exhibit strong blue shift. This effect is more evident in DMSO, glycerol and show emission maxima at ∼460 nm due to high viscous nature of solvent medium favors restricted IMR process.33 The emission maxima in viscous solvents is found to be similar to the emission maxima in thin films with a blue shift of ∼100 nm. The observed blue shift in the emission spectra in high viscous liquids indicate that non-radiative decay pathway of the excited fluorophore is hampered by intramolecular rotation of aryl substituents on phthalimide and maleimide units. Because of solvent relaxation, we have observed an abrupt decrease in the emission intensity in polar solvents like methanol and ethanol. Interestingly, we have observed large blue shift of ∼95 nm in toluene medium. This arises due to its high insolubility nature of Cbz-Pth in toluene medium leads to formation of spontaneous self-assembled molecular aggregates. Similar behavior is observed for CbzMI(d) in different solvents. Figure S29 shows the fluorescence lifetime decay of Cbz-Pth in different THF: H2O concentrations. The lifetime values were gradually increased with increase in the formation of aggregates reveal that existence of different molecular conformations. The SEM of Cbz-Pth (Figure S30) further corroborates morphological changes at different THF: H2O ratio. At f w = 20%, the aggregates show fibrillar structures, which transformed into crystalline at f w = 50%. The more ordered dense crystals were observed at f w = 70%. Further, increase in the water concentration, the aggregates were dissociated by changing crystalline to amorphous morphology. Such a morphological change brought a significant impact on their molecular packing and photophysical properties as described earlier. The electrochemical studies were carried out in cyclic voltammetry (CV) with an internal standard of ferrocene/ ferrocenium (Fc/Fc+) couple. The HOMO and LUMO energy values were determined from the oxidation and reduction onsets, respectively. The onset oxidation potential of Cbz-Pth and Cbz-MI(d) is found to be 0.78 and 0.89 eV, respectively (Figure 5). The measured HOMO and LUMO energy levels are −5.36 and −2.65 eV for Cbz-Pth and −5.43 and −3.14 eV for Cbz-MI(d) with HOMO−LUMO band gap of 2.71 and

Figure 5. Cyclic-voltammograms of Cbz-MI(d), Cbz-Pth, and CbzMI. G


Research Article


Figure 6. Energy level diagram (a) and device configuration (b) of the developed three devices.

Figure 7. (a) PL spectra of three emitters doped in CBP film. (b) EL spectra of three emitters doped in CBP film. (C) EQE−current density characteristics. (d) Current density−voltage−luminance characteristics of EL devices.

A. The EQE with ηext,max is determined to be 3.7% with CIE coordinates (0.48, 0.50). Under the current device architecture, the performance of the device-3 showed an improved current efficiency and EQE than previously reported architecture by our group.32 The exceptional EL performance of Cbz-MI(d) over Cbz-Pth might be due to variation in the acceptor unit and good charge injection property. On close look at the energy level diagram (Figure 6a), the electron injection barrier at the TPBi/Cbz-MI(d) junction is only 0.51 and 0.84 eV TPBi/CbzPth junction. Small difference in the energy of electron injection at barrier enables the high emitting nature for CbzMI(d) at low bias and TPBi layer efficiently blocks the flow of holes from the emitting layer.7 The complementary energy levels of hole and electron injection layers are helpful for the charge balance and exciton recombination at the emitting layer furnishes improved EL efficiency. Using 10% weight of Cbz derivatives in CBP matrix could also favors the observed high brightness in Cbz-Pth and CbzMI(d) devices. In general, high doping concentrations of AIE

Table 2. Summary of the Electroluminescence Properties of Carbazole-Appended Maleimide and Phthalimide Derivatives compound CbzMI(d) Cbz-Pth Cbz-MI

PL (nm)a

EL (nm)

Von (V)

Lmax (cd/m2)

current efficiency (cd/A)

EQE (%)

561

584

5.85

73915

13.8

4.1

465 550

490 563

5.81 5.88

39568 18303

6.4 12.9

2.6 3.7

a

Photoluminescence maxima of three emitters were obtained in CBP host.

Device-2 shows turn on bias voltage at 5.81 V with maximum luminance (Lmax) of 39568 cd/m2 and maximum current efficacy (ηC,max) of 6.4 cd/A. The EQE with ηext,max is determined to be 2.6% with CIE coordinates (0.21, 0.36) which falls in the CIE bright sky blue light. Device-3 shows turn on bias voltage at 5.88 V with maximum luminance (Lmax) of 18303 cd/m2 and maximum current efficiency (ηC,max) 12.9 cd/ H


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and Engineering Research Board (SERB), Department of Science and Technology (DST), India, through the project EMR/2015/000969.

luminogen increases the crystallinity and leads to aggregation of the compounds in the solid-state device fabrication. The formation of high crystalline and aggregated morphology enables AIE effect furnishes high brightness in these materials. These results evident that carbazole containing AIE luminogens are promising molecular materials for the fabrication of efficient organic light-emitting diodes.

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(1) Wong, M. Y.; Zysman-Colman, E. Purely Organic Thermally Activated Delayed Fluorescence Materials for Organic Light-Emitting Diodes. Adv. Mater. 2017, 29, 1605444. (2) Endo, A.; Sato, K.; Yoshimura, K.; Kai, T.; Kawada, A.; Miyazaki, H.; Adachi, C. Efficient Up-conversion of Triplet Excitons into a Singlet State and its Application for Organic Light Emitting Diodes. Appl. Phys. Lett. 2011, 98, 083302. (3) Chiang, C. L.; Tseng, S. M.; Chen, C. T.; Hsu, C. P.; Shu, C. F. Influence of Molecular Dipoles on the Photoluminescence and Electroluminescence of Dipolar Spirobifluorenes. Adv. Funct. Mater. 2008, 18, 248−257. (4) Gaylord, B. S.; Wang, S.; Heeger, A. J.; Bazan, G. C. WaterSoluble Conjugated Oligomers: Effect of Chain Length and Aggregation on Photoluminescence-Quenching Efficiencies. J. Am. Chem. Soc. 2001, 123, 6417−6418. (5) Moorthy, J. N.; Natarajan, P.; Venkatakrishnan, P.; Huang, D. F.; Chow, T. J. Steric Inhibition of π-Stacking: 1,3,6,8-Tetraarylpyrenes as Efficient Blue Emitters in Organic Light Emitting Diodes (OLEDs). Org. Lett. 2007, 9, 5215−5218. (6) Mei, J.; Leung, N. L. C.; Kwok, R. T. K.; Lam, J. W. Y.; Tang, B. Z. Aggregation-Induced Emission: Together We Shine, United We Soar. Chem. Rev. 2015, 115, 11718−11940. (7) Qin, W.; Yang, Z. Y.; Jiang, Y. B.; Lam, J. W. Y.; Liang, G. D.; Kwok, H. S.; Tang, B. Z. Construction of Efficient Deep Blue Aggregation-Induced Emission Luminogen from Triphenylethene for Non-doped Organic Light-Emitting Diodes. Chem. Mater. 2015, 27, 3892−3901. (8) Li, J.; Jiang, Y.; Cheng, J.; Zhang, Y.; Su, H.; Lam, J. W.; Sung, H. H.; Wong, K. S.; Kwok, H. S.; Tang, B. Z. Tuning the Singlet−Triplet Energy gap of AIE Luminogens: Crystallization-Induced Room Temperature Phosphorescence and Delay Fluorescence, Tuneable Temperature Response, Highly efficient non-doped Organic LightEmitting Diodes. Phys. Chem. Chem. Phys. 2015, 17, 1134−1141. (9) Gan, S.; Luo, W.; He, B.; Chen, L.; Nie, H.; Hu, R.; Qin, A.; Zhao, Z.; Tang, B. Z. Integration of Aggregation-Induced Emission and Delayed Fluorescence into Electronic Donor−Acceptor Conjugates. J. Mater. Chem. C 2016, 4, 3705−3708. (10) Yoon, S. J.; Chung, J. W.; Gierschner, J.; Kim, K. S.; Choi, M.G.; Kim, D.; Park, S. Y. Multi-stimuli Two-Color Luminescence Switching via Different Slip-Stacking of Highly Fluorescent Molecular Sheets. J. Am. Chem. Soc. 2010, 132, 13675−13683. (11) He, J. T.; Xu, B.; Chen, F. P.; Xia, H. J.; Li, K. P.; Ye, L.; Tian, W. J. Aggregation-Induced Emission in the Crystals of 9,10Distyrylanthracene Derivatives: The Essential Role of Restricted Intramolecular Torsion. J. Phys. Chem. C 2009, 113, 9892−9899. (12) Kim, S.; Zheng, Q. D.; He, G. S.; Bharali, D. J.; Pudavar, H. E.; Baev, A.; Prasad, P. N. Aggregation-Enhanced Fluorescence and TwoPhoton Absorption in Nanoaggregates of a 9,10-Bis[4′-(4″aminostyryl)styryl]anthracene Derivative. Adv. Funct. Mater. 2006, 16, 2317−2323. (13) Yu, G.; Yin, S.; Liu, Y.; Chen, J.; Xu, X.; Sun, X.; Ma, D.; Zhan, X.; Peng, Q.; Shuai; Tang, B. Z.; Zhu, D.; Fang, W.; Luo, Y. Structures, Electronic States, Photoluminescence, and Carrier Transport Properties of 1,1-Disubstituted 2,3,4,5-Trtraphenylsiloles. J. Am. Chem. Soc. 2005, 127, 6335−6346. (14) Dong, Y.; Lam, J. W.; Qin, A.; Li, Z.; Sun, J.; Sung, H. H.; Williams, I. D.; Tang, B. Z. Switching the Light Emission of (4biphenylyl)phenyldibenzofulvene by Morphological Modulation: Crystallization-Induced Emission Enhancement. Chem. Commun. 2007, 40−42. (15) Gong, Y.; Liu, J.; Zhang, Y.; He, G.; Lu, Y.; Fan, W. B.; Yuan, W. Z.; Sun, J. Z.; Zhang, Y. AIE-active, Highly Thermally and Morphologically Stable, Mechanochromic and Efficient Solid Emitters

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CONCLUSIONS In conclusion, we have developed a new series of carbazolebased molecular materials and demonstrated the role of aggregation to tune the color and efficiency of OLED. Tuning the conjugation of both acceptor and donor units furnish dramatic changes in the photophysical properties with unusual shift in emission spectra. This study reveals that a twist in the phenyl rings enable large torsional angle provokes ordered structures in the solid state and show red-shifted emission (∼32 nm) for Cbz-MI, while Cbz-MI(d) and Cbz-Pth exhibits blueshifted emission (∼35 and 71 nm) in comparison with emission spectra in solution. Cbz-MI(d) exhibits bright yellow electroluminescence (584 nm) with remarkable brightness as well as an EQE of 4.1%. The high AIBSE observed for Cbz-Pth device shows blue color with EL maxima at 492 nm with EQE of ∼2.6%. Notably both Cbz-MI(d) and Cbz-Pth devices exhibits very low roll-off efficiency with increasing luminance brightness. These results reveal that introduction of bulky donor conjugate molecules on acceptor units of D−A conjugates is a practical strategy to develop enhanced solid-state emitters based on AIE for achieving efficient OLEDs with desired color purity.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b11025. Detailed synthetic procedures of precursor compounds and structural characterization (NMR (1H and 13C), HRMS, single crystal X-ray analysis, fluorescence lifetime, and aggregation behavior and morphology (PDF) Crystallographic information file for Cbz-MI(d) (CIF) Crystallographic information file for Cbz-Pth (CIF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Tel: +91-80-22932651. Fax: +91-80-23601310. E-mail: [email protected]. ORCID

Satish Patil: 0000-0003-3884-114X Notes

The authors declare no competing financial interest. Crystallographic data for CCDC-1538232 (Cbz-MI(d)) and CCDC-1538231 (Cbz-Pth) can be obtained free of charge from the Cambridge Crystallographic Data Centre via https:// www.ccdc.cam.ac.uk/structures/?.

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ACKNOWLEDGMENTS The authors thank Dr. Qisheng Zhang, Zhejiang University, China for providing OLED device fabrication and characterization facilities. Authors also thank the Proteomics facility, MBU, IISc, for MALDI and NMR Research Centre, IISc, for NMR studies. Y.C. thanks UGC for D. S. Kothari Postdoctoral fellowship. This research was financially supported by Science I


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Efficient Blue and Yellow Organic Light-Emitting Diodes Enabled by

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