Synthesis, characterization, and hole-transporting properties of pyrenyl N-substituted triazatruxenes

Abstract

Two new pyrenyl triazatruxene derivatives (TAT1 and TAT2) were successfully synthesized via Br₂-catalyzed cyclotrimerization of indole and Suzuki cross-coupling with pyrene-1-boronic acid. These compounds exhibited maximum absorption around 344–351 nm and maximum emission around 472–483 nm in CHCl₃ solution. The electrochemical investigation by cyclic voltammetry (CV) suggested that the HOMO and LUMO energy levels of these compounds were around −5.3 and −2.4 eV, respectively. The performance of TAT2 as a hole-transporting material was three-times better than that of the commercial compound N,N′-bis(3-methylphenyl)-N,N′-diphenylbenzidine (TPD). The electroluminescent device of the structure ITO/PEDOT:PSS/TAT2/Alq₃/LiF:Al exhibits a bright green emission with a maximum luminance of 31 971 cd m⁻² at 10.8 V and a turn-on voltage of 2.6 V. In addition, the use of spin-casted films of TAT1 or TAT2 doped with 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP) as greenish blue-emitting materials are demonstrated.

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1. Introduction

The development of optoelectronic materials used in organic light-emitting diodes (OLEDs) has gained much scientific interest during the last few decades.¹ This might have been influenced by the advantageous characteristics of OLED displays, for instance, the cost effectiveness, high power efficiencies, ease of device fabrication by solution processes, and opportunity to create curved or flexible devices. Because the operation of OLED involves the migration of excitons and creation of photons upon the recombination of holes and electrons, the device performances can be enhanced by the incorporation of hole-transporting materials (HTMs).² These materials are generally easily oxidized to form stable radical cations. They play a key role in transporting positive charge and deterring electron migration towards the opposite electrode without recombination with holes. A large number of carbazole and aromatic amine derivatives have been developed as HTMs due to the relatively small exchange energies associated with small orbital overlap in the n–π* transition of carbazole.³ Examples of commercially available HTMs are N,N′-diphenyl-N,N′-bis(1-naphthyl)-(1,1′-biphenyl)-4,4′-diamine (NPB) and N,N′-bis(3-methylphenyl)-N,N′-bis(phenyl)benzidine (TPD) which exhibit notable hole-mobility. However, their low glass transition temperatures (T_g) (100 and 65 °C for NPB and TPD, respectively) and poor morphological properties can lead to deterioration of devices upon long-term uses.

With their excellent thermal stabilities and highly emissive nature, truxenes (10,15-dihydro-5H-diindeno[1,2-α;1′,2′-c]-fluorene) and triazatruxenes (10,15-dihydro-5H-dihydro-5H-diindolo[3,2-a:3′,2′-c]carbazole) have been regarded as an interesting scaffold for functional materials.⁴ They had been used in HTM,⁵ blue fluorescent emitters,⁶ and C₃-symmetric materials.⁷ In our research project on design of optoelectronic materials, we recently reported the synthesis of new thermally stable truxene derivatives containing carbazole moieties and demonstrated their hole-transporting properties in OLED devices.⁸ With the advantages of solution processes such as their cost-effectiveness and potential for large surface fabrication, a number of elegant works on solution-processable materials have been published.⁹ In this paper, we report the synthesis and characterization of new solution-processable pyrenyl triazatruxene compounds (TAT1 and TAT2, Fig. 1). The alkyl substituents on the nitrogens should facilitate the solubility of both compounds in organic solvents, making it possible to fabricate these materials by spin-coating method. The performances of these compounds as hole-transporting and emissive materials in OLED devices will also be evaluated.

Fig. 1 Structure of TAT1 and TAT2.

2. Experimental

2.1 Chemical and instruments

All reagents were purchased from Aldrich, Fluka and used without further purification. All ¹H-NMR spectra were recorded on Varian Mercury 400 MHz NMR spectrometer (Varian, USA) using CDCl₃ and DMSO-d6. ¹³C-NMR spectra were recorded at 100 MHz on Bruker NMR spectrometer using the same solvent. Mass spectra were recorded on a Microflex MALDI-TOF mass spectrometer (Bruker Daltonics) using doubly recrystallized α-cyano-4-hydroxy cinnamic acid (CCA) as a matrix. Elemental (CHN) analyses were performed on Perkin-Elmer 2400 series II (Perkin-Elmer, USA). Absorption spectra were measured by a Varian Cary 50 UV-vis spectro photometer. Fluorescence spectra were obtained from a Varian Cary Eclipse spectrofluorometer. Absolute quantum yields were measured using the FLS980 spectrometer (Edinburgh Instrument). Thermal experiments with Differential Scanning Calorimeter (DSC) were performed on Mettler Toledo DSC 822e and Thermogravimetric Analysis (TGA) was studied using Simultaneous Thermal Analyzer Netzsch 409. Cyclic voltammetry was performed using an AUTOLAB spectrometer. All measurements were made at room temperature on sample solutions in freshly distilled dichloromethane with 0.1 M n-Bu₄NPF₆ as electrolyte. CH₂Cl₂ was distilled from calcium hydride and the electrolyte solutions were degassed by nitrogen bubbling. A glassy carbon working electrode, platinum wire counter electrode, and Ag/AgNO₃ (Sat.) reference electrode were used in all cyclic voltammetric experiments.

2.2 Synthetic procedures

3,8,13-Tribromo-10,15-dihydro-5H-diindolo [3,2-a:3′,2′-c]carbazole (2). A solution of N-bromosuccinimide (NBS) (0.28 g, 1.55 mmol) in dimethylformamide (2 mL) was added dropwise to a mixture of 1 (0.17 g, 0.5 mmol) in acetone (10 mL) at 0 °C. The mixture was slowly warmed to room temperature and stirred for an additional 30 min before it was poured into water. Then, the organic phase was separated and dried over anhydrous Na₂SO₄. After the solvent was evaporated, the crude product was purified by column chromatography using hexane/acetone (8 : 2) as the eluent to afford 2 as a pale white solid (0.22 g, 76%). ¹H NMR (400 MHz, acetone-d6) δ 11.34 (s, 3H), 8.39 (d, J = 8.3 Hz, 3H), 7.85 (s, 3H), 7.45 (d, J = 6.8 Hz, 3H). The data agree with the literature report.¹⁰

5,10,15-Triethylhexyl-10,15-dihydro-5H-diindolo[3,2-a:3′,2′-c]carbazole (3a). A mixture of 2 (0.15 g, 0.25 mmol) and KOH (0.28 g, 5 mmol) was stirred at room temperature, then a solution of ethylhexylbromide (0.27 mL, 1.5 mmol) was added slowly, the mixture was stirred overnight. The mixture was poured into water and extracted with EtOAc. The combined organic layer was dried over anhydrous Na₂SO₄, filtered, and concentrated under reduce pressure. The crude product was purified by chromatography (EtOAc : n-hexane, 5 : 95) to give the compound 3a as a yellow solid (0.22 g, 95%). ¹H NMR (400 MHz, CDCl₃) δ 7.86 (d, J = 8.5 Hz, 3H), 7.52 (s, 3H), 7.46 (d, J = 8.6 Hz, 3H), 4.40 (s, 6H), 1.79 (s, 3H), 1.10–0.44 (m, 42H). The data agree with the literature report.¹¹

5,10,15-Tribenzyl-10,15-dihydro-5H-diindolo[3,2-a:3′,2′-c]carbazole (3b). A mixture of 2 (0.15 g, 0.25 mmol), KOH (0.28 g, 5 mmol), and [CH₃(CH₂)₃]₄N(HSO₄) (0.0083 g, 0.025 mmol) was heated to reflux in acetone (10 mL). Benzyl bromide (0.2 mL, 1.68 mmol) was then added and the mixture was stirred for 3 h. The mixture was diluted with CH₂Cl₂, washed with 10% aqueous HCl and with saturated aqueous NaCl solution, and dried (Na₂SO₄), the solvent was then evaporated. The residue was triturated with hexanes to give 3b as a white solid (0.21 g, 98%). ¹H NMR (400 MHz, CDCl₃) δ 8.12 (s, 3H), 7.89 (d, J = 7.0 Hz, 3H), 7.74 (d, J = 8.5 Hz, 3H), 7.47 (m, 9H), 7.10 (d, J = 7.2 Hz, 6H), 5.96 (s, 6H). The data agree with the literature report.¹¹

TAT1. To a degassed (N₂) solution of 3a (0.09 g, 0.1 mmol) and Pd(PPh₃)₄ (0.012 g, 0.01 mmol) in toluene (5 mL), then pyreneboronic acid (0.1476 g, 0.6 mmol) and 2 M aqueous K₂CO₃ solution (1 mL) were added via syringe. The reaction mixture was stirred at 70 °C for 48 h. After cooling, the product was extracted with CH₂Cl₂, washed with water, and dried over anhydrous Na₂SO₄. The solvent was evaporated, affording the crude mixture. The crude product was purified by column chromatography (CH₂Cl₂ : hexanes, 1 : 9) to give TAT1 as a yellow solid (83 mg, 65%). This compound exists as a mixture of several diastereomers due to stereogenic center on the 2-ethylhexyl groups. ¹H NMR (400 MHz, CDCl₃) δ 8.47 (d, J = 9.2 Hz, 3H), 8.35–7.88 (m, 30H), 7.67 (s, 3H), 5.09 (br, s, 6H), 2.28 (d, J = 12.1 Hz, 3H), 1.70 (s, 6H), 1.41 (s, 3H), 1.20–0.55 (m, 33H). ¹³C NMR (100 MHz, CDCl₃) δ 141.4, 131.6, 131.14, 131.08, 130.5, 129.7, 128.9, 128.8, 128.1, 127.5, 127.4, 126.4, 126.0, 125.8, 125.1, 124.74, 124.68, 122.6, 122.0, 113.6, 51.0, 38.4, 37.1, 34.3, 33.8, 32.8, 32.1, 31.9, 30.2, 30.04, 29.98, 29.8, 29.70, 29.65, 29.4, 29.3, 28.4, 27.1, 26.7, 23.2, 22.8, 22.7, 20.8, 19.7, 14.1, 13.9, 10.4, 10.3. MALDI-TOF MS (m/z): calcd: 1281.6900; found: 1281.797 [M⁺]. Anal. calcd for C₉₆H₈₇N₃: C, 89.89; H, 6.84; N, 3.28%; found: C, 89.21; H, 6.56; N, 2.95%.

TAT2. To a degassed (N₂) solution of 3b (85.2 mg, 0.1 mmol) and Pd(PPh₃)₄ (11.6 mg, 0.01 mmol) in toluene (5 mL), then pyreneboronic acid (14.8 mg, 0.6 mmol) and 2 M aqueous K₂CO₃ solution (1 mL) were added via syringe. The reaction mixture was stirred at 70 °C for 48 h. After cooling, the product was extracted with CH₂Cl₂, washed with water, and dried over anhydrous Na₂SO₄. The solvent was evaporated, affording the crude mixture. The crude product was purified by column chromatography (CH₂Cl₂ : hexanes, 2 : 8) to give TAT2 as a yellow solid (60 mg, 49%). ¹H NMR (400 MHz, CDCl₃) δ 8.12 (d, J = 7.9 Hz, 3H), 7.90 (m, 30H), 7.70 (d, J = 9.2 Hz, 3H), 7.47 (s, 3H), 7.38 (d, J = 6.6 Hz, 6H), 7.28 (d, J = 7.2 Hz, 6H), 6.04 (s, 6H). ¹³C NMR (100 MHz, CDCl₃) δ 141.7, 140.2, 138.03, 137.96, 136.3, 131.5, 130.9, 130.3, 129.2, 128.5, 127.9, 127.6, 127.36, 127.34, 127.28, 127.2, 126.7, 125.9, 125.4, 124.89, 124.86, 124.6, 124.5, 123.3, 122.4, 121.6, 113.1, 103.4, 51.4. MALDI-TOF MS (m/z): calcd: 1215.4552; found: 1215.485 [M⁺]. Anal. calcd for C₉₃H₅₇N₃: C, 91.82; H, 4.72; N, 3.45%; found: C, 92.22; H, 4.34; N, 3.01%.

2.3 OLED fabrication and measurement

The patterned indium tin oxide (ITO) glass substrate with a sheet resistance 14 Ω sq⁻¹ (purchased from Kintec Company) was thoroughly cleaned by successive ultrasonic treatment in detergent, deionised water, and acetone, and then dried at 100 °C in a vacuum oven. A 50 nm thick PEDOT:PSS hole injection layer was spin-coated on top of ITO from a 0.75 wt% dispersion in water at a spin speed of 2500 rpm for 30 s and dried at 100 °C for 15 min under vacuum. Thin films of HTL were deposited on top of PEDOT:PSS layer by spin-coating THF:toluene solution of TPD or TAT2 (2% w/v) on an ITO glass substrate at a spin speed of 2500 rpm for 30 second to get a 30–40 nm thick film. Then Alq₃ was deposited onto the surface of the HTL film as light-emitting (EML) and electron-transporting layer (ETL) with a thickness of 50 nm by evaporation from resistively heated alumina crucibles at evaporation rate of 0.5–1.0 nm s⁻¹ in vacuum evaporator deposition (ES280, ANS Technology) under a base pressure of ∼10⁻⁵ mbar. The film thickness was monitored and recorded by quartz oscillator thickness meter (TM-350, MAXTEK). The chamber was vented with dry air to load the cathode materials and pumped back; a 0.5 nm thick LiF and a 150 nm thick aluminium layers were the subsequently deposited through a shadow mask on the top of EML film without braking vacuum to from an active diode areas of 4 mm². The measurement of device efficiency was performed according to M. E. Thomson's protocol and the device external quantum efficiencies were calculated using procedure reported previously. Current density–voltage–luminescence (J–V–L) characteristics were measured simultaneous by the use of a Keithley 2400 source meter and a Newport 1835C power meter equipped with a Newport 818-UV/CM calibrated silicon photodiode. The EL spectra were acquired by an Ocean Optics USB4000 multichannel spectrometer. All the measurements were performed under ambient atmosphere at room temperature.

3. Results and discussion

3.1 Synthesis of TAT1 and TAT2

The target triazatruxene derivatives, TAT1 and TAT2, were synthesized as outlined in Scheme 1. The starting triazatruxene core 1 was prepared according to the literature procedure.¹² Treatment of 1 with three molar equivalent of NBS in DMF and acetone at 0 °C provided tribromotriazatruxene 2 in good yield of 76%. To enhance the solubility of this material in organic solvents, the –NH group was then alkylated with 2-ethylhexyl bromide or benzyl bromide to afford 3a and 3b in excellent yields of 95–98%. The last step involved the Suzuki coupling between the tribromo 3a or 3b with pyrene-1-boronic acid using Pd(PPh₃)₄ as the catalyst which produced TAT1 and TAT2 in 65 and 49% yield, respectively. It should be noted that 3a and TAT1 exist as mixtures of various diastereomers since they derived from alkylation of 2 with racemic 2-ethylhexyl bromide.

Scheme 1 Synthesis of TAT1 and TAT2.

3.2 Photophysical properties

The photophysical properties of TAT1 and TAT2 were investigated in both solution and solid states are summarized in Table 1 and Fig. S1 and S2.† In CHCl₃ solution phase, the UV spectra exhibited maximum absorption bands at 344 and 351 nm for TAT1 and TAT2, respectively, with a relatively similar molar absorptivity (log ε = 4.52 and 4.88 M⁻¹ cm⁻¹). In solid state, the absorption maxima of both compounds shifted towards longer wavelengths (348 and 378 nm) as affected by the more effective conjugation induced by the solid-state packing. From the fluorescence spectra in solution phase, TAT1 and TAT2 exhibited maximum emission wavelength at 483 and 472 nm, respectively. TAT1 containing ethylhexyl group showed a slightly longer emission wavelength and lower quantum efficiency in comparison with TAT 2 containing benzyl group that might be attributed to the greater rotational relaxation of the alkyl chain. As thin films, the emission maxima for TAT1 and TAT 2 were virtually similar (472 and 471 nm), which is probably the result of the restriction of bond rotation in the solid state.

Table 1 Photophysical properties of TAT1 and TAT2

Cpd	λ^absmax [nm] (log ε [M^−1 cm^−1])		λ^emitmax [nm]		QY^f (%)
	Solution^a	Solid^b	Solution^c (ΦF^e)	Solid^d	Solution	Solid
a The absorption spectra from the UV-vis spectra was measured in CHCl3.b The absorption spectra from the UV-vis spectra measured in thin film.c The PL emission excited at the absorption maxima in dilute CHCl3 solution.d The PL emission excited at the absorption maxima in thin film.e PL quantum yield determined in CHCl3 solution (A < 0.1) at room temperature using quinine sulfate solution in 0.1 M H2SO4 (ΦF = 0.54) as a standard.f Absolute quantum yield.
TAT1	344 (4.52)	348	483 (0.52)	472	48.7	16.7
TAT2	351 (4.88)	378	472 (0.61)	471	55.1	17.2

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3.3 Electrochemical properties

The HOMO energy levels were estimated from the onset oxidation potential appeared in the cyclic voltammogram (Fig. S3 and S4†).¹³ The HOMO–LUMO energy gap could be estimated from the onset of the UV-vis absorption spectra. The LUMO energy levels could then be estimated from the HOMO levels and the band gap energy. The computationally calculated and experimental electronic properties are summarized in Table 2. The experimental HOMO and LUMO levels of two compounds were in the range of −5.15 to −5.16 eV and −1.68 to −1.74 eV, respectively. These energy levels obtained from the calculations using Gaussian 09 code¹⁴ with geometry optimizations using B3LYP/6-31G(d,p) method also agree with the experimental values. The orbital plots (Fig. 2) showed exclusive electron localizations at the triazatruxene core for the HOMOs of both TAT1 and TAT2, while the electron density in the LUMOs are delocalized at antibonding of two pyrene units.

Table 2 Electrochemical and thermal properties of TAT1 and TAT2

Cpds	Experimental			Calculated^d			Tg^e (°C)	Td^10%^f (°C)
	HOMO^a (eV)	LUMO^b (eV)	Eg^c (eV)	HOMO (eV)	LUMO (eV)	Eg (eV)
a Estimated by the empirical equation: HOMO = −(4.44 + Eonset).b Estimated from LUMO = HOMO + Eg.c The optical energy gap estimated from the onset of the absorption spectra (Eg = 1240/λonset).d All calculations were performed by Gaussian 09 code and geometry optimizations were done by B3LYP/6-31G(d,p) method.e Obtained from DSC measurements on the second heating cycle with a heating rate of 10 °C min^−1 under N2.f 10% decomposition temperature obtained from TGA measurement with a heat rate of 10 °C min^−1 under N2.
TAT1	−5.31	−2.41	2.90	−5.15	−1.74	3.41	305.1	310.7
TAT2	−5.34	−2.39	2.95	−5.16	−1.68	3.48	231.7	262.7

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Fig. 2 Frontier orbital plots for TAT1 (top) and TAT2 (bottom).

3.4 Thermal properties

For the materials used in optoelectronic devices, the thermal properties of TAT1 and TAT2 must be investigated. The TGA curves revealed that two compounds were thermally stable with high 10% weight-loss temperatures (T_d¹⁰) at 310.7 and 262.7 °C, respectively. The excellent thermal stability with high T_d can lead to high stability OLEDs, which is favorable to improve the performance and lifetime of OLEDs during operation. The thermograms from second heating cycle in DSC showed endothermic baseline shifts due to glass-transition temperatures (T_g) for both TAT1 (305.1 °C) and TAT2 (231.7 °C). These results suggested that both compounds could form molecular glass with T_g higher than most of the commercially available HTMs. The more conformationally flexible ethylhexyl groups in TAT1 may allow stacking of the triazatruxene segments in solid state, and contribute to the higher T_g and T_d compared to TAT2.

3.5 Fabrication of electroluminescent devices

Since the HOMO levels of TAT1 and TAT2 lied between the work function of ITO (−4.80 eV) and HOMO level of Alq₃ (−5.70 eV), both of these compounds could potentially be used as a hole transporting layer (HTL) in OLED. To investigate their hole-transporting properties, three multi-layer OLED devices with the structure of ITO/PEDOT:PSS (50 nm)/HTL (spin-coating)/Alq₃ (50 nm)/LiF (0.5 nM)/Al (150 nm) were fabricated (Fig. 3a). Device 1 with no HTL and device 2 with a commercial hole-transporting material (N,N′-bis(3-methylphenyl)-N,N′-diphenylbenzidine or TPD) were fabricated and used as reference devices, whereas device 3 used TAT2 as the hole-transporting material. It was found that all devices emitted a characteristic green light of Alq₃ (518 nm). From the voltage–luminance and voltage–current density characteristics of the devices shown in Fig. 3b and Table 3, device 3 with TAT2 as HTL exhibited the best performance with a maximum luminance of 31 971 cd m⁻² at 10.8 V, a turn-on voltage of 2.6 V and an external efficiency of 1.17%. In term of maximum luminance, it should be noted that the performance of TAT2 is approximately three-time greater than TPD. It should be mentioned that attempts to fabricate TAT1 as HTL were failed due to short circuits. These results may involve the poor film-forming ability of TAT1, which will be discussed later.

Fig. 3 (a) Configuration of device 1–3. (b) Current density and luminance vs. voltage (J–V–L) characteristics of device 1–3.

Table 3 Electroluminescent properties of device 1–7

Device	EML	HTL	ETL	Von^a	Vmax	Lmax^b	Jmax^c	LEmax^d	PE (lm W^−1)	% EQE^e	CIE^f
a Turn-on voltage (V).b Maximum luminance (cd m^−2).c Current density (mA m^−2).d Luminance efficiency (cd A^−1) (at applied potential V).e External efficiency (%).f Commission International d'Eclairage coordinates (x, y).
1	Alq3	None	—	4.0	9.6	4633	795	1.01/7.0 V	0.52/5.6 V	0.25/7.0 V	0.297, 0.532
2	Alq3	TPD	—	2.6	10.8	11 446	684	4.15/4.6 V	3.55/3.0 V	1.02/4.6 V	0.276, 0.515
3	Alq3	TAT2	—	2.6	10.8	31 971	1297	4.76/5.2 V	3.64/3.6 V	1.17/5.2 V	0.275, 0.515
4	5% TAT1 in CBP	—	BCP	8.6	20.8	3910	388	1.08/20.6 V	0.16/20.6 V	1.49/20.6 V	0.197, 0.300
5	10% TAT1 in CBP	—	BCP	10.0	21.8	2789	335	0.83/21.8 V	0.12/21.8 V	1.15/21.8 V	0.197, 0.283
6	5% TAT2 in CBP	—	BCP	8.1	20.2	5666	393	1.46/19.8 V	0.23/19.8 V	2.01/19.8 V	0.172, 0.252
7	10% TAT2 in CBP	—	BCP	6.8	19.8	5410	392	1.42/19.6 V	0.23/19.0 V	1.97/19.6 V	0.172, 0.253

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The high photoluminescence (PL) quantum yields of TAT1 and TAT2 motivated us to test these compounds as emitting layers (EML) in OLED. Device 4–7 were then fabricated using TAT1 and TAT2 doped at 5 and 10% in a commercially available host material (4,4′-bis(N-carbazolyl)-1,1′-biphenyl or CBP) (Fig. 4a). Bathocuproine (BCP) was chosen to be an electron-transporting layer (ETL) in order to improve device performance. The voltage–luminance and voltage–current density characteristics of the devices are shown in Fig. 4b and summarized in Table 3. From the results, device 6 with CBP doped with TAT2 at 5% exhibited the best performance with a maximum luminance of 5666 cd m⁻² at 19.8 V, a turn-on voltage at 8.1 V and an external efficiency 2.01%. Based on their electroluminescent spectra and CIE plots, device 4–7 emitted greenish blue light with the EL peaks at 478, 478, 473 and 473 nm, respectively (Fig. S5 and S7†). The photoluminescent spectra of pure TAT1 and TAT2 were at 472 and 471 nm, whereas the thin films of TAT1 and TAT2-doped CBP showed hypsochromic shift of about 20 nm (Fig. S6†). It is possible that intercalation of CBP with TAT1 and TAT2 in the doped films could decrease π-conjugation and lead to shorter emission wavelength. The large differences between LUMO levels of electron-injecting and emissive layers in device 4–7 could result in higher turn-on voltages as compared to those of device 1–3. In comparison to the blue-emitting devices fabricated from fluorene-substituted triazatruxene derivatives of equivalent sizes and shapes,^9d device 6 exhibits superior performances in terms of brightness and external quantum efficiencies.

Fig. 4 (a) Configuration of device 4–7. (b) Current density and luminance vs. voltage (J–V–L) characteristics of device 4–7.

The surface morphology of thin films was also studied by AFM and the results are shown in Fig. 5. All four spin coating films, TAT1, TAT1-doped CBP, TAT2 and TAT2-doped CBP were prepared from their 0.3% w/v solution in a 2 : 1 mixture between CHCl₃ and toluene. Rough surfaces of TAT1 and TAT2 were observed as the compounds may agglomerate and aggregate during the spin-casting process. On the other hand, the doping of TAT1 and TAT2 into CBP resulted in much smoother film, which presumably leads to more efficient exciton migration and better device performances.

Fig. 5 AFM images of TAT1, TAT1-doped CBP, TAT2, and TAT2-doped CBP spin-coated films.

4. Conclusions

In conclusion, two new symmetrical pyrenyl triazatruxene derivatives were successfully synthesized via Br₂-catalyzed cyclotrimerization of indole and Suzuki cross-coupling with pyrene-1-boronic acid. The N-substitution with 2-ethylhexyl (in TAT1) and benzyl groups (in TAT2) could prevent the pi-stacking aggregation. These compounds were well soluble in organic solvents and exhibited high fluorescence quantum efficiencies. In CHCl₃ solutions, these compounds exhibited maximum absorption and emission around 344–351 nm and 472–483 nm, respectively. The hypsochromic shifts of both absorption and emission bands along with narrower Stoke shifts in solid state spectra may result from the solid-state packing and more restricted structural geometry. The electrochemical investigation by cyclic voltammetry (CV) suggested that the HOMO energy level of TAT1 and TAT2 were at −5.31 and −5.34 eV, while the LUMO energy level were at −2.41 and −2.39 eV, respectively. Thermal property examination by DSC and TGA showed that these compounds have high glass transition temperatures (T_g > 230 °C) and 10%-decomposition temperatures (T_d > 305 °C). The performance of TAT2 as a hole-transporting material was three-time better than that of the commercial compound TPD as the device of structure ITO/PEDOT:PSS/TAT2/Alq₃/LiF:Al exhibited a bright green light with a maximum luminance of 31 971 cd m⁻² at 10.8 V and a turn-on voltage of 2.6 V. Both TAT1 and TAT2 could also be used as greenish blue-emitting materials when doped in CBP.

Acknowledgements

This research was supported by the Ratchadaphiseksomphot Endowment Fund (2013) of Chulalongkorn University (CU-56-425-AM). TT thanks the 90^th Anniversary of Chulalongkorn University Fund for his scholarship. PR thanks the Grant for Chula Research Scholar from the Ratchadapiseksomphot Endowment Fund of Chulalongkorn University.

References

1. (a) N. T. Kalyani and S. J. Dhoble, Renewable Sustainable Energy Rev., 2012, 16, 2696–2723; (b) D. S. Weiss and M. Abkowitz, Chem. Rev., 2010, 110, 479–526; (c) S. R. Forrest and M. E. Thompson, Chem. Rev., 2007, 107, 923–925; (d) F. Hide, M. A. Diaz-Garcia, B. J. Schartz and A. J. Heeger, Acc. Chem. Res., 1997, 30, 430–436; (e) J. H. Burroughes, D. D. C. Bradley, A. R. Brown, R. N. Marks, K. Mackey, R. H. Frien, P. L. Burns and A. B. Holmes, Nature, 1990, 347, 539–541.
2. G. Gustafsson, Y. Cao, G. M. Treacy, F. Klavetter, N. Colaneri and A. J. Heeger, Nature, 1992, 357, 477–479.
3. (a) J.-H. Pan, H.-L. Chiu and B.-C. Wang, J. Mol. Struct.: THEOCHEM, 2005, 725, 89–95; (b) N. Seidler, S. Reineke, K. Walzer, B. Lüssem, A. Tomkeviciene, J. V. Grazulevicius and K. Leo, Appl. Phys. Lett., 2010, 9, 093304; (c) A. Tomkeviciene, J. V. Grazulevicius, K. Kazlauskas, A. Gruodis, S. Jursenas, T.-H. Ke and C.-C. Wu, J. Phys. Chem. C, 2011, 115, 4887–4897; (d) Y. Zhang, T. Wada and H. Sasabe, J. Mater. Chem., 1998, 8, 809–828; (e) J. V. Grazulevicius, P. Strohriegl, J. Pielichowski and K. Pielichowski, Prog. Polym. Sci., 2003, 28, 1297–1353.
4. F. Goubard and F. Dumur, RSC Adv., 2015, 5, 3521–3551.
5. Z. Yang, B. Xu, J. He, L. Xue, Q. Guo, H. Xia and W. Tian, Org. Electron., 2009, 10, 954–959.
6. C. Yao, Y. Yu, X. Yang, H. Zhang, Z. Huang, X. Xu, G. Zhou, L. Yue and Z. Wu, J. Mater. Chem. C, 2015, 3, 5783–5794.
7. (a) L.-L. Li, P. Hu, B.-Q. Wang, W.-H. Yu, Y. Shimizu and K.-Q. Zhao, Liq. Cryst., 2010, 37, 499–506; (b) R. Misra, R. Sharma and S. M. Mobin, Dalton Trans., 2014, 43, 6891–6896.
8. D. Raksasorn, S. Namuangruk, N. Prachumrak, T. Sudyoadsuk, V. Promarak, M. Sukwattanasinitt and P. Rashatasakhon, RSC Adv., 2015, 5, 72841–72848.
9. (a) J.-H. Jou, S. Sahoo, S. Kumar, H.-H. Yu, P.-H. Fang, M. Singh, G. Krucaite, D. Volyniuk, J. V. Grazulevicius and S. Grigalevicius, J. Mater. Chem. C, 2015, 3, 12297–12307; (b) C. W. Lee and J. Y. Lee, Adv. Mater., 2013, 25, 596–600; (c) G. Zhou, W. Y. Wong, B. Yao, Z. Xie and L. Wang, Angew. Chem., 2007, 119, 1167–1169; (d) W.-Y. Lai, Q.-Y. He, R. Zhu, Q.-Q. Chen and W. Huang, Adv. Funct. Mater., 2008, 18, 265–276.
10. R. A. Valentine, A. Whyte, K. Awaga and N. Robertson, Tetrahedron Lett., 2012, 53, 657–660.
11. H. Hidetaka, K. Hironobu, O. Hideo, S. Takaaki and M. Shuntaro, Heterocycles, 2007, 72, 231–238.
12. M. Franceschin, L. Ginnari-Satriani, A. Alvino, G. Ortaggi and A. Bianco, Eur. J. Org. Chem., 2010, 134–141.
13. (a) R. Cervini, X.-C. Li, G. W. C. Spencer, A. B. Holmes, S. C. Moratti and R. H. Friend, Synth. Met., 1997, 84, 359–360; (b) B. W. D'Andrade, S. Datta, S. R. Forrest, P. Djurovich, E. Polikarpov and M. E. Thompson, Org. Electron., 2005, 6, 11–20.
14. M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J. A. Montgomery, J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski and D. J. Fox, Wallingford CT, 2009.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra09688c

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