Versatile Fluorinated Derivatives of Triphenylamine as Hole

Article pubs.acs.org/JPCC

Versatile Fluorinated Derivatives of Triphenylamine as HoleTransporters and Blue-Violet Emitters in Organic Light-Emitting Devices Zhanfeng Li,† Zhaoxin Wu,*,† Wen Fu,† Peng Liu,† Bo Jiao,† Dongdong Wang,‡ Guijiang Zhou,*,‡ and Xun Hou† †

Key Laboratory of Photonics Technology for Information, School of Electronic and Information Engineering, Xi’an Jiaotong University, Xi’an, 710049, People's Republic of China ‡ Department of Chemistry, School of Science, Xi’an Jiaotong University, Xi’an, 710049, People's Republic of China S Supporting Information *

ABSTRACT: A series of triphenylamine derivatives endcapped with various fluorinated phenyl (TPAF) have been designed and synthesized for the application in organic lightemitting devices (OLEDs). By changing the substitution pattern of electron-withdrawing groups, such as F and CF3, the ability of hole-transport, energy levels, and thermal stability of these, TPAF are tuned, which are supported by density functional study of their geometry and electronic structure. TPAF can be used as either hole-transporters or blue-violet emitters in OLEDs. Among TPAF, the device with TPA-(2)-F as hole-transport material achieved the maximum current efficiency of 4.7 cd A−1, which was much higher than that of the typical N,N′-di(1-naphthalenyl)-N,N′-diphenyl-4,4′-diamine-based device. This good performance of the TPA-(2)-F-based device was attributed to the more balanceable injected carriers in the device by tuning hole injection and transport. More importantly, nondoped blue OLEDs utilizing TPAF as the emitters exhibited blue-violet emissions peaking between 408 and 428 nm with Commission Internationale de L’Eclairage coordinates in a range of (0.16−0.18, 0.06−0.12), which were also expected to be a new material class with an enhanced current efficiency/color purity compromise for future blue light-emitting devices. mobility.7,18−20 In general, the hole drift mobility of HTMs is much higher than electron mobility of typical electrontransporting materials (ETMs). For example, the hole mobility in commonly used HTMs such as N,N′-di(1-naphthalenyl)N,N′-diphenyl-4,4′-diamine (NPB) or 1,1-bis[(di-4-tolylamino)- phenyl]cyclohexane (TAPC) is about 2 orders of magnitude higher than the electron mobility in tris-(8hydroxyquinoline)aluminum (Alq 3). Consequently, holes from the anode tend to transport easily to the ETM and even further to the cathode without recombining efficiently with electrons in the emitting layer. This will largely decrease the current efficiency of the devices. It is well-known that the incorporation of the electron-donor or electron-withdrawing moieties into hole-transporting (p-type) semiconductors may contribute to tuning the electronic properties and to altering charge transport properties.21−25 In particular, fluorination has been used in the past decade as a route to induce stability and electron transport or ambipolar transport in organics by lowering the energy levels (both the highest-occupied

1. INTRODUCTION Organic light-emitting devices (OLEDs) have attracted much attention because of their wide applications in full-color flatpanel displays and solid-state lightings.1−3 In the past decade, triphenylamine (TPA) and its derivatives (simple, branched, and star-shaped) have attracted much interest, owing to their application as excellent hole-transporting materials (HTMs) and electroluminescence (EL) materials in OLEDs.4−17 Maintaining the balance between electron and hole currents in OLEDs is an important issue for achieving good device performance. However, an imbalance in charge transport for most OLEDs leads to the accumulation of charges in a heterostructure, thereby resulting in a loss of efficiency and lifetime. Moreover, high-performance blue-emitting materials are still relatively rare due to the intrinsic wide band gap, which makes it hard to inject charges into emitters; therefore the performance of blue light emitting devices is often inferior to that of their red and green counterparts. As to the first issue, there are many strategies that have been developed to meet the requirement of charge balance, such as the employment of multilayered structures, the introduction of bipolar or multifunctional molecules, and enhancement of electron mobility and/or a judicial reduction of hole © 2012 American Chemical Society

Received: March 26, 2012 Revised: August 28, 2012 Published: September 5, 2012 20504

dx.doi.org/10.1021/jp3028929 | J. Phys. Chem. C 2012, 116, 20504−20512

The Journal of Physical Chemistry C

Article

groups on the absorption and fluorescence spectra, energy levels (HOMO/LUMO), and the hole-transport ability of the materials in experiment and theory, especially by theoretical calculations employing the B3LYP functional. The device with TPA-(2)-F as HTM showed the better performance than that of the typical NPB-based device. More importantly, nondoped blue OLEDs utilizing TPAF as the emitters presented an enhanced current efficiency/color purity compromise for blueviolet light-emitting devices.

molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels) in the molecule or polymer, especially for OLEDs.26−31 For example, electronwithdrawing groups, such as F and CF3, in direct conjunction with the electron-donor N atoms of TPA and the well-known hole-transport material N,N′-bis(tolyl)-N,N′-diphenyl-1,1′- biphenyl-4,4′-diamine (TPD), lowered the HOMO levels and the hole mobility and tuned the molecular properties, such as solubility or stability, which leaded to better charge balance and higher charge recombination efficiency.21,22,24,32 In our previous research, we have designed and synthesized 2,4difluorophenyl-functionalized triphenylamine and its dimer and found that the incorporation of the strong electron-withdrawing fluorinated substituents into the arylamine moiety can reduce holes mobility of HTMs.33 Therefore, one of the simplest way to obtain HTMs with suitable hole transport is to introduce strong electron-withdrawing substituents onto a π-conjugated core, which lowers both the HOMO and LUMO energy levels so that the electron injection is made easier and the materials display a greater resistance against the degradative oxidation processes. As to the second issue, we need further improvement in blueemitting materials and devices to match the National Television System Committee (NTSC) standard blue Commission Internationale de L’Eclairage (CIE) coordinates of (0.14, 0.08).34−37 Such a device not only can effectively reduce the power consumption in the device but also can be utilized to generate light of various colors either by irradiation of luminescent dyes or by excitation energy transfer to luminescent dopants including emissive phosphors. Although some work has been reported on nondoped blue-violetemitting devices that emitted light peaking at wavelengths shorter than 430 nm, few reports show the considerable efficiencies.16,36,38−42 As a result, the preparation of novel pure/ deep-blue-light-emitting materials exhibiting high efficiencies with a very-good CIE coordinate of y < 0.10 is still greatly desirable. In this work, a series of EL materials based on fluorinated triphenylamine derivatives (TPAF) was prepared as shown in Scheme 1. Electron-withdrawing groups, such as F and CF3, were introduced into the periphery of the aryl substituents on TPA. We systematically studied the effect of different fluorine

2. EXPERIMENTAL SECTION 2.1. Materials and Instruments. The manipulation involving air-sensitive reagents was performed under an inert atmosphere of dry nitrogen. Tris(4-bromophenyl)amine, Pd(PPh3)4, o-tolylboronic acid, 2-fluorophenylboronic acid, 3fluorophenylboronic acid, 4-fluorophenylboronic acid, 3,4,5trifluorophenylboronic acid, and 3,5-bis(trifluoro-methyl)phenylboronic acid were purchased and used as received. The synthesis of tris(2′,4′-difluorobiphenyl-4-yl)amine has been described in our previously work,33 and the abbreviation TPA(2,4)-F is the systematic name based on the definitions in this paper. Differential scanning calorimetry (DSC) measurements and thermogravimetric analysis (TGA) were performed on a NETZSCH (DSC200PC) and a TG209C analyzer under a N2 atmosphere, respectively. Photoluminescence (PL) and absorption spectra were recorded by a Horiba Jobin Yvon Fluoromax4 spectrophotometer and a Hitachi UV 3010 spectrophotometer, respectively. Quinine sulfate in 1.0 M H2SO4 (ΦPL = 0.56 at 334 nm) was used as a reference.43 To obtain the quantum-yield data as precisely as possible, the concentration was adjusted so that the absorbance of the solution would be lower than 0.1. The difference between the refractive index of the solvent and that of the standard should also be counted. The equation ⎛ Ts × A r ⎞⎡ ns ⎤2 Φs = Φr × ⎜ ⎟⎢ ⎥ ⎝ Tr × A s ⎠⎣ nr ⎦

was used to calculate quantum yields where the subscripts s and r refer, respectively, to the sample and reference. T represents the integrated emission intensity (i.e., the area of the fluorescence spectrum), A the absorbance of the solution, and ns and nr the refraction indexes of the solvents in which the sample and the reference were dissolved.44 Cyclic voltammetry was performed using a Princeton Applied Research model 273 A potentiostat at a scan rate of 100 mV s−1. All experiments were carried out in a three-electrode compartment cell with a Pt-sheet counter electrode, a glassy carbon working electrode, and a Pt-wire reference electrode. The supporting electrolyte used was 0.1 M tetrabutylammonium perchlorate ([Bu4N]ClO4) solution in dry acetonitrile. The cell containing the solution of the sample (1 mM) and the supporting electrolyte was purged with a nitrogen gas thoroughly before scanning for its oxidation and reduction properties. Ferrocene was used for potential calibration in each measurement. All the potentials were reported relative to ferrocene-ferrocenium (Fc/Fc+) couple, whose oxidation potential was +0.22 V relative to the reference electrode. The oxidation and reduction potentials were determined by taking the average of the anodic and cathodic peak potentials. The HOMO and LUMO values were estimated by using the following general equation: EHOMO = −(qEox + 4.8) eV;28,45 ELUMO = EHOMO + Egopt,46−48 which were

Scheme 1. Synthesis of TPAF

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2.3.5. Tris(3′,4′,5′-trifluorobiphenyl-4-yl)amine (TPA(3,4,5)-F). A procedure analogous to the preparation of compound TPA-T was used but instead starting from 3,4,5trifluorophenylboronic acid (1.76 g, 10 mmol). TPA-(3,4,5)-F was obtained as a white powder. Yield: 82% (1.56 g). Mp: 179 °C. 1H NMR (CDCl3, 400 MHz): δ 7.02−7.04 (d, J = 8.8 Hz, 3H), 7.14−7.18 (m, 9H), 7.39−7.42 (d, 6H). Anal. Calcd for C36H18NF9: C, 68.04%; H, 2.85%; N, 2.20%. Found: C, 62.53%; H, 2.00%; N, 2.67%. ESI-MS (m/z): 659.2853 [M + H + Na]+ (Calcd: 659.1267). 2.3.6. Tris(3′,5′-bis(trifluoromethyl)biphenyl-4-yl)amine (TPA-(3,5)-CF3). A procedure analogous to the preparation of compound TPA-T was used but instead starting from 3,5bis(trifluoromethyl)phenylboronic acid (2.58 g, 10 mmol). TPA-(3,5)-CF3 was obtained as a white powder. Yield: 84% (2.22 g). Mp: 241 °C. 1H NMR (CDCl3, 400 MHz): δ 7.28− 7.30 (d, J = 8.4 Hz, 6H), 7.56−7.58 (d, J = 8.4 Hz, 6H), 7.84 (s, 3H), 8.01 (s, 6H). Anal. Calcd for C42H21NF18: C, 57.22%; H, 2.40%; N, 1.59%. Found: C, 57.43%; H, 1.90%; N, 1.94%. ESIMS (m/z): 881.1350 [M]+ (Calcd: 881.1381).

calculated using the internal standard ferrocene value of −4.8 eV with respect to the vacuum level.49 2.2. Devices Fabrication and Characterizations. The devices were fabricated by conventional vacuum deposition of the organic layers, LiF and Al, of the cathode onto an indium tin oxide (ITO)-coated glass substrate under a base pressure lower than 1 × 10−3 Pa at the evaporation rates of 0.04, 0.025, and 0.5 nm s−1, respectively. The thickness of each layer was determined by a quartz thickness monitor. The effective size of the OLED was 14 mm2. The voltage−current density (V−J), voltage−brightness (V−L), and brightness−current efficiency (L−η) curves of devices were measured with a computercontrolled Keithley 2602 Source-Meter under ambient condition. 2.3. Synthesis of Materials. 2.3.1. Tris(2′-methylbiphenyl-4-yl)amine (TPA-T). Tris(4-bromophenyl) amine (TPA-Br) (1.45 g, 3 mmol), o-tolylboronic acid (1.36 g, 10 mmol), and Pd (PPh3)4 (0.30 g, 0.27 mmol) were added to a mixture of aqueous Na2CO3 (2.0 M, 15 mL), ethanol (10 mL), and toluene (30 mL) under nitrogen. The mixture was heated to reflux and maintained at this temperature overnight. When the reaction was completed (judging from thin-layer chromatography), water was added to quench the reaction. Then, the products were extracted with CH2Cl2. The organic layer was collected, dried over anhydrous MgSO4, and evaporated under vacuum. The solid was absorbed on silica gel and purified by column chromatography using light petrol ether/ethyl acetate as the eluent to give compound TPA-T as a white solid (1.33 g, 86%). Mp: 169 °C. 1H NMR (CDCl3, 400 MHz): δ 2.53 (s, 9H), 7.10−7.58 (m, 15H), 7.26−7.28 (m, 9H). Anal. Calcd for C39H33N: C, 90.83%; H, 6.45%; N, 2.72%. Found: C, 92.46%; H, 4.14%; N, 3.52%. ESI-MS (m/z): 516.2664 [M]+ (Calcd: 516.2686). 2.3.2. Tris(2′-fluorobiphenyl-4-yl)amine (TPA-(2)-F). A procedure analogous to the preparation of compound TPA-T was used but instead starting from 2-fluorophenylboronic acid (1.40 g, 10 mmol). TPA-(2)-F was obtained as a white powder. Yield: 88% (1.39 g). Mp: 241 °C. 1H NMR (CDCl3, 400 MHz): δ 7.08−7.17 (m, 3H), 7.24−7.26(t, 3H), 7.27−7.28 (m, 3H), 7.44−7.47 (d, J = 8.0 Hz, 3H), 7.48−7.51 (d, J = 8.0 Hz, 9H). Anal. Calcd for C36H24NF3: C, 81.96%; H, 4.59%; N, 2.65%. Found: C, 81.94%; H, 3.63%; N, 2.97%. ESI-MS (m/z): 527.1847 [M]+ (Calcd: 527.1855). 2.3.3. Tris(3′-fluorobiphenyl-4-yl)amine (TPA-(3)-F). A procedure analogous to the preparation of compound TPA-T was used but instead starting from 3-fluorophenylboronic acid (1.40 g, 10 mmol). TPA-(3)-F was obtained as a white powder. Yield: 84% (1.33 g). Mp: 187 °C. 1H NMR (CDCl3, 400 MHz): δ 6.98−7.05 (t, 3H), 7.19−7.25 (t, 6H), 7.28−7.31 (s, 3H), 7.34−7.41 (m, 6H), 7.48−7.53 (d, J = 8.4 Hz, 6H). Anal. Calcd for C36H24NF3: C, 81.96%; H, 4.59%; N, 2.65%. Found: C, 82.05%; H, 3.70%; N, 2.90%. ESI-MS (m/z): 527.1846 [M]+ (Calcd: 527.1855). 2.3.4. Tris(4′-fluorobiphenyl-4-yl)amine (TPA-(4)-F). A procedure analogous to the preparation of compound TPA-T was used but instead starting from 4-fluorophenylboronic acid (1.40 g, 10 mmol). TPA-(4)-F was obtained as a white powder. Yield: 90% (1.42 g). Mp: 173 °C. 1H NMR (CDCl3, 400 MHz): δ 7.10−7.14 (m, 9H), 7.46−7.53 (m, 15H). Anal. Calcd for C36H24NF3: C, 81.96%; H, 4.59%; N, 2.65%. Found: C, 82.01%; H, 3.80%; N, 2.89%. ESI-MS (m/z): 527.1867 [M]+ (Calcd: 527.1855).

3. RESULTS AND DISCUSSION 3.1. Synthesis and Structures. A series of TPAF were readily obtained with one-step Suzuki coupling between tris(4bromophenyl)amine and the respective fluorinated phenylboronic in the presence of palladium catalyst with yields ranging from 82 to 90%. The chemical structures and the synthetic routes of fluorinated compounds (TPAF) prepared for this study were shown in Scheme 1, and the details of synthesis were given in the Experimental Section. Three kinds of substituents (i.e., CH3, F, and CF3) were introduced into the periphery of phenyl substituents on TPAF. By changing the number and position of the substituents, we systematically synthesized seven compounds together with TPA-(2,4)-F reported previously.33 All compounds were isolated by column chromatography on silica gel, and their molecular structures were confirmed with 1H nuclear magnetic resonance (H NMR), mass spectrometry (MS), and element analysis. Theoretical calculations using DFT//B3LYP/6-31G level in the Gaussian 03 program were carried out to characterize the three-dimensional structures and the frontier molecular orbital energy levels of each material.50 As illustrated in Figure 1, all of the TPAF show the three-dimensional propeller structures, which result in good solution processability. Particularly, the peripheral substituted phenyl are twisted with respect to the adjacent phenyl of TPA core by an angle 50−79° because of a steric repulsion of substituted phenyl peri-H (F, CH3, or CF3) atoms with hydrogen atoms of the phenyl of the TPA unit. Such structural characteristics can influence some of their electronic and physical properties such as suppression of the conjugation.51 The HOMO orbitals are mostly developed in the TPA core with vanishing influence from the peripheral substituted phenyl groups. It can be seen that, after three additional rings, the space extension of the total HOMO is almost negligible indicating that the major influence of the peripheral substituted phenyl on the HOMO energy is predominantly inductive in nature.52 The LUMOs of the Fsubstituted TPAF are located predominantly on the two quasidegenerate virtual orbitals, whereas that of CH3 substituted analogue (TPA-T) distributes with higher density on the phenyl of the TPA unit. It should be noted that the LUMO of TPA-(3,5)-CF3 is dominated predominantly on the peripheral CF3-substituted phenyl, which can be explained by the 20506



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3.2. Thermal Properties. The thermal properties of TPAF were measured by TGA and DSC, and were listed in Table 1 together with those of TPA-(2,4)-F reported before. The onset decomposition temperatures (Td), which correspond to a 5% weight loss upon heating during TGA, of all the compounds were in the range of 332−399 °C (see Figure S1 in the Supporting Information), demonstrating the good thermal stability of the materials. In Figure 2, the glass temperatures (Tg) of compounds TPA-T, TPA-(3)-F, TPA-(4)-F, and TPA(3,5)-CF3 were determined to be 62, 59, 73, and 126 °C, respectively, by DSC in the second heating scans, while TPA(2)-F and TPA-(3,4,5)-F did not exhibit a glass transition up to the melting temperature (Tm). The more rigid structure and bulkier size of the CF3 moiety compared to the F and CH3 units explained the highest Tg shown by TPA-(3,5)-CF3 among the TPAF. What a pleasant surprise is TPA-(2)-F, whose Tm is much higher than that of its monosubstituted analogue (Figure 2a). This result may be attributed to the stronger intramolecular F···H interactions with the nearest distance of 2.65 Å between the F atom at the ortho position of the peripheral substituted phenyl and the H atom at the ortho position of phenyl group of the core in TPA-(2)-F than those of other TPAF (see Figure 1). It clearly shows that compounds TPA(2)-F and TPA-(3,5)-CF3 have good thermal stability and very desirable characteristics for OLEDs stability. Two crystalline transition temperatures (Tc) for TPA-T were detected at 101 and 128 °C on the second heating process, probably due to the formation of different crystals during the solid−solid phase transitions. And the Tc for TPA-(3)-F and TPA-(4)-F were 89 and 105 °C, respectively. 3.3. Photophysical Properties. The UV−visible absorption and PL spectra of TPAF in CH2Cl2 dilute solutions and films (∼50 nm) on a precleaned quartz substrate are shown in Figure 3, and the spectral parameters are summarized in Table 1. As shown in Figure 3a, the absorption maxima of TPAF lie in the range of 322−359 nm due to the π−π* transition of the conjugated aromatic rings. A red shift in the absorption spectra is observed upon conversion of the substituents onto the phenyl group attached to the TPA core from −CH3 to −F and to −CF3, which can be explained by the increased solvation due to the increased molecular polarity of the compounds substituted by higher electronegative F and the strong electron-withdrawing fluorinated substituents CF3. Interestingly, the absorption spectra of the meta position on the phenyl

Figure 1. The optimized geometries and the molecular orbital surfaces of the HOMOs and LUMOs for TPAF obtained at the B3LYP/6-31G level.

compounds substituted by the strong electron-withdrawing fluorinated substituents CF3. As a result, it is reasonable to expect that the influence of the last rings in the side arms should be more significant for the LUMO energies, meaning that they are more influential on the spectroscopic properties and eventually on the electron mobility.52 The calculated HOMOs and LUMOs of all TPAF are listed in Table 1.

Table 1. Thermal, Photophysical, and Electrochemical Properties of TPAF λAbsmax (nm) solna/filmb

λPLmax (nm) solna/filmb

ΦPLc

Eoxd (V)

TPA-T

322/336

398/390

0.21

0.42

TPA-(2)-F

341/349

414/408

0.23

0.51

TPA-(3)-F

349/352

424/417

0.27

0.53

TPA-(4)-F

339/344

412/411

0.27

0.47

TPA-(2,4)-F TPA(3,4,5)-F TPA-(3,5)CF3

337/340 334/341

399/400 421/401

0.24 0.11

0.48 0.64

359/362

436/419

0.35

0.69

compound

a

Tg/Tm/Td (°C)

HOMO/LUMOexp (Eg) (eV)

HOMO/LUMOcal (ΔEHOMO−LUMO) (eV)

−1.48

−5.22/−1.81 (3.41)

−5.28/−0.93 (4.35)

−1.41, −1.89 −1.44, −1.93 −1.36, −1.84 −1.31 −1.51

−5.31/−2.01 (3.30)

−5.21/−1.30 (3.91)

62/160, 169/380 NA/241/368

−5.33/−2.11 (3.22)

−5.43/−1.50 (3.93)

59/187/394

−5.27/−1.98 (3.29)

−5.39/−1.38 (4.01)

73/173/399

−5.28/−2.00 (3.28) −5.44/−2.19 (3.25)

−5.47/−1.53 (3.94) −5.39/−1.47 (3.92)

69/180/374 NA/179/351

−5.49/−2.36 (3.13)

−6.11/−2.16 (3.95)

126/241/332

Eredd (V)

−1.38, −1.85

Measured in CH2Cl2. bMeasured in film. cDetermined in CH2Cl2 using quinine sulfate (ΦPL = 0.56 in 1.0 M H2SO4 solution) as standard. Measured vs Fc/Fc+ in CH3CN.

d

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Figure 2. DSC scans of TPAF recorded under nitrogen during the second heating cycle at a scan rate of 10 °C min−1.

Figure 3. Absorption of TPAF in CH2Cl2 (a) and films (c) and PL spectra of TPAF in CH2Cl2 (b) and films (d).

ring with respect to fluorine atom in monosubstituted molecule TPA-(3)-F appear to be significantly red-shifted with the most prominent peak appearing at 349 nm when compared with the nonfluorinated analogue (TPA-T: 322 nm). It is the most electron deficient for the meta position on the phenyl ring because it is affected only by the inductive electron-withdrawing character of the fluorine atom. Moreover, the combination of the double nature of the electronic effect of the fluorine atom, which is inductive and mesomeric, causes only a lesser bathochromic shift effect on the spectroscopic properties of TPA-(2)-F and TPA-(4)-F compared with the nonfluorinated analogue (TPA-T). As in case of absorption spectra in CH2Cl2, a red shift in fluorescence maxima of TPAF is observed (TPAT: 398 nm, TPA-(2)-F: 414 nm, TPA-(3,5)-CF3: 436 nm) upon replacing phenyl groups with electron-withdrawing F and CF3 groups (Figure 3b). The absorption spectra of these thinfilm samples all show a broad band at 336−362 nm (Figure 3c), which is slightly red-shifted by approximately 3 nm relative to

those in solution. The optical band gaps are calculated from the onset of absorption spectra in solid thin films deposited on quartz plates and are listed in Table 1. The thin film PL emission spectra of TPAF, having three-dimensional structures, display a bright blue emission peak with maxima in the range of 390 and 419 nm (Figure 3d), which exhibits a hypsochromic shift of about 1−20 nm compared to their solution spectra. This result may also be attributed to the aforementioned solid state packing force which prohibits the electron-vibration coupling between the TPA core and the peripheral substituted phenyl unit.51 We measured the fluorescence quantum yields (ΦPL) of the TPAF in dilute CH2Cl2 solution using quinine sulfate as standard (ΦPL = 0.56 in 1.0 M H2SO4 solution) at room temperature. Our blue emitters TPAF, except for TPA-(3,4,5)F, exhibited high quantum yields in CH2Cl2 solution between 0.21 and 0.35 comparing their emissions with that of the wellknown blue compound 3-tert-butyl-9,10-di(naphth-2-yl)20508



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Figure 4. Cyclic voltammograms of TPAF in CH3CN with a Pt wire used as reference electrode.

anthracene (TBADN: ΦPL = 0.24) under the same condition. It has been observed that the fluorescence quantum yield may be increased or decreased upon fluorination, depending on both the molecular framework and the degree of fluorination. 3.4. Electrochemical Properties. Cyclic voltammograms of TPAF show only one electro-oxidation process but one or two electroreduction processes (Figure 4). Their HOMO energy levels were calculated based on the electrochemical oxidation potentials. A positive change in the oxidation potentials relative to ferrocene from 0.42 to 0.69 V was observed from TPA-T to TPA-(3,5)-CF3 with the highest value observed for TPA-(3,5)-CF3. All of the oxidations should be attributed to the removal of electrons from the TPA groups. The relative HOMO level depends on the type and position of substituent. The introduction of a more electronegative F group at the ortho position of the phenyl group in TPA-(2)-F lowered the HOMO levels by 0.09 eV as compared to its CH3substituted counterpart (TPA-T) with the same position. TPA(3)-F with a mono-F substituent at the meta position has a slightly lower HOMO level (−5.33 eV) than that at the ortho or para positions (−5.31 eV for TPA-(2)-F and −5.27 eV for TPA-(4)-F). By increasing the number of the highly electronegative fluorine substituents on the phenyl group, the HOMO level was significantly lowered (TPA-(4)-F, −5.27 eV vs TPA(3,4,5)-F, −5.44 eV). CF3 substitution at the meta position (TPA-(3,5)-CF3) resulted in the lowest HOMO level (−5.49 eV). The LUMO energy levels were estimated to be in the range of −1.81 and −2.36 eV by the difference of the HOMO level and the absorption edge of the optical absorption spectra of TPAF. The LUMO levels decreased with increasing the highly electronegative fluorine substituents to a maximum difference of 0.55 eV, whereas the HOMO levels were less sensitive to electronic variations by only ∼0.27 eV. The electrochemical properties as well as the energy level parameters of TPAF are listed in Table 1. As shown in Table 1, the experimental HOMOs and LUMOs are in excellent agreement with the calculated values. 3.5. Hole-Transporting Properties. To investigate the hole-transport ability of TPAF, the hole-only devices with structure ITO/TPAF (100 nm)/Al (80 nm) were fabricated and characterized. Their current density−voltage characteristics are shown in Figure 5; it can be seen that the current density at a given applied voltage exhibits TPA-(2)-F > TPA-(3)-F ≈ TPA-(4)-F > TPA-T > TPA-(3,4,5)-F > TPA-(2,4)-F > TPA(3,5)-CF3, suggesting their hole mobility decreased in turn. In our previous research, we have reported TPA-(2,4)-F as the hole-transporting layer in the typical two-layer device and

Figure 5. Current density−voltage curves for the hole-only devices, ITO/TPAF (100 nm)/Al (80 nm).

found that TPA-(2,4)-F exhibited poorer performance. This was attributed to the incorporation of much more electronegative F groups into one electron-donor moiety in the molecule, which lowered significantly the hole mobility. Similarly, TPA-(3,5)-CF3 is the poorest because of the compound substituted by the strong electron-withdrawing fluorinated substituents CF3. The interesting thing is that the hole mobility of the CH3-substituted counterpart (TPA-T) is much lower than that of each mono-F substituents, which may be attributed to a more twisting conformation between the peripheral CH3-substituted phenyl at the ortho position and TPA core in TPA-T than that in mono-F substituents (see Figure 1). These results show that the mono-F substituents may serve as a better HTM with suitable hole mobility compared with other TPAF. The typical two-layer devices with the structure of ITO/ TPAF (60 nm)/Alq3 (60 nm)/LiF (1 nm)/Al (70 nm) were fabricated with TPAF as HTL and Alq3 as an electron transporting and light-emitting layer. NPB-based device (ITO/ NPB (60 nm)/Alq3 (60 nm)/LiF (1 nm)/Al (70 nm)) was also prepared for the comparison. Figure 6 shows current density− voltage, brightness−voltage, and current efficiency−brightness for the OLEDs containing the HTL of NPB and TPAF. Especially for the TPA-(2)-F-based device, the highest efficiency reached 4.7 cd A−1, much higher than that of the device with NPB as the HTL (3.8 cd A−1), and the highest current density and luminance were 650 mA cm−2 and 13039 cd m−2, respectively. The better performance of TPA-(2)-Fbased device was attributed to the more balanceable injected carriers in the Alq3 layer, which is due to the suitable hole mobility in mono-F substituted analogue. The maximum 20509



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Table 2. Performance of the Devices Containing HTL of NPB and TPAF HTL

VDa (V)

Lmaxb (cd m−2)

ηc,maxc (cd A−1)

NPB TPA-T TPA-(2)-F TPA-(3)-F TPA-(4)-F TPA-(2,4)-F TPA-(3,5)-CF3

5.5 9.2 6.0 5.8 8.8 8.3 14.2

19222 7782 13039 12787 2003 8387 825

3.8 3.8 4.7 3.4 2.4 4.1 0.4

Driving voltage at 10 cd m−2. bMaximum luminance. cMaximum current efficiency.

a

transporters, and 1,3,5-tris(1-phenyl-1H- benzimidazol-2-yl)benzene (TPBi) as an electron transporters and hole-blockers, ITO/NPB (40 nm)/TcTa (20 nm)/TPAF (40 nm)/TPBi (40 nm)/LiF (1 nm)/Al (70 nm), were fabricated. Table 3 Table 3. Performance of Blue Emission Devices λELmaxa (nm)

fwhm of EL at 14 V (nm)

VDb (V)

Lmaxc (cd m−2)

ηc,maxd (cd A−1)

TPA-T

424

70

11.0

1355

0.43

TPA(2)-F TPA(3)-F TPA(4)-F TPA(2,4)-F TPA(3,5)CF3

408

41

16.4

81

0.38

428

64

10.2

354

0.39

416

67

8.1

1600

0.48

424

87

11.1

975

0.48

416

49

11.8

693

0.60

blue emitter

CIE (x, y)e (0.18, 0.12) (0.17, 0.10) (0.16, 0.06) (0.16, 0.07) (0.17, 0.08) (0.17, 0.08)

EL was measured at 14 V. bDriving voltage at 10 cd m−2. cMaximum luminance. dMaximum current efficiency. eAt a current density of 20 mA cm−2.

a

summarizes the device performances, and Figure 7 presents the EL spectra of the blue devices. All devices exhibited blueviolet emission with emission peaks between 408 and 428 nm, and the full width at half-maximum (fwhm) values vary from 41 to 87 nm, nearly identical to the corresponding PL spectra obtained in the thin-film state, which indicates that the EL emissions are mainly contributed from the fluorescence of the

Figure 6. Current density−voltage curves (a), brightness−voltage curves (b), and current efficiency−brightness curves (c) for the ITO/ NPB or TPAF (60 nm)/Alq3 (60 nm)/LiF (1 nm)/Al (70 nm) devices.

current efficiency of the mono-F substituents TPA-(3)-F and TPA-(4)-F-based device were 3.4 and 2.4 cd A−1, respectively (Figure 6c). Among the devices, the TPA-(3,5)-CF3-based device shows the poorest performances; its current efficiency and maximum luminance are only 0.44 cd A−1 and 825 cd m−2, respectively. This may be contributed to the difficulty for holes to be injected into TPA-(3,5)-CF3, which has the lowest HOMO level of the TPAF, and to the lowest mobility for hole transporting in TPAF (see Figure 5). All the electroluminescence characteristics of the devices containing HTL of NPB and TPAF are summarized in Table 2. 3.6. Blue-Violet-Emitting Properties. TPAF were found to exhibit blue-violet fluorescence with relatively high quantum efficiency and the fine reversible redox properties. Therefore, it is expected to serve as blue-violet emitter in OLED. The OLEDs using TPAF as emitters, 4,4′,4″-tris(carbazol-9-yl)triphenylamine (TcTa) as electron-blockers, NPB as a hole

Figure 7. The EL spectra of blue-light emitting devices ITO/NPB (40 nm)/TcTa (20 nm)/TPAF (40 nm)/TPBi (40 nm)/LiF (1 nm)/Al (70 nm). 20510



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luminances were reached (0.27 cd A−1 at 1500 cd m−2). This means that the efficiency does not roll off under high luminance. The TPA-T-based device shows slightly lower performances (1355 cd m−2 at 17.6 V and 0.43 cd A−1 at 2.13 mA cm−2). For TPA-(3,5)-CF3 the maximum luminance of 693 cd m−2 with a blue-violet emission spectrum with CIExy = (0.17, 0.08), and a maximum efficiency of 0.60 cd A−1 were achieved. The high performances of the devices were comparable to those previously reported for nondoped fluorescence blue-light-emitting OLEDs.41,42,53,54 This was attributed to the relatively high fluorescence quantum efficiencies and the balance of electron and hole currents inside the emissive layer because of the effective electron and hole-blocking ability by using a low EA of TcTa (2.7 eV) and a high IP of TPBi (6.2 eV), respectively. In addition, the CF3 groups in TPA-(3,5)-CF3 influence the packing behavior of the molecules that has an influence on the device performance.55 In view of the fact that the blue-light-emitting layer was nondoped, the devices achieved very high purity and high efficiency and may give an enlarged color gamut for color displays.

TPAF. It is worthy to note that the nondoped devices show pure blue emission with chromatic coordinates (CIE 1931) in a range of (0.16−0.18, 0.06−0.12), which almost perfectly match to the NTSC blue standard of (0.14, 0.08). The current density−voltage−luminance−efficiency (J−V− L−η) characteristics of the blue-violet emission fluorescent OLEDs with TPAF as blue emitters are shown in Figure 8 and

4. CONCLUSIONS In summary, a series of fluorinated derivatives of triphenylamine (TPAF) have been successfully prepared by one-step Suzuki coupling reactions in high yields. We tuned the absorption and fluorescence spectra, energy levels (HOMO/ LUMO), and the hole-transport ability by changing the substitution pattern, which were supported by theoretical calculations employing the B3LYP functional. By adjusting the hole-transport ability, the TPA-(2)-F-based device showed the current-efficiency of 4.7 cd A−1, which was much higher than that of the typical NPB-based device (3.8 cd A−1). More importantly, OLEDs utilizing TPAF as the emitters exhibited blue-violet emissions with CIE coordinates in a range of (0.16− 0.18, 0.06−0.12), which were nearest to the NTSC blue standard of (0.14, 0.08). These TPAF showed an enhanced current efficiency/color purity and allowed for further device optimization.

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ASSOCIATED CONTENT

S Supporting Information *

TGA curves for compounds TPA-T, TPA-(2)-F, TPA-(3)-F, TPA-(4)-F, TPA-(2,4)-F, TPA-(3,4,5)-F, and TPA-(3,5)-CF3. This information is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Fax: +86-029-82664867 (Z.W.). E-mail: [email protected]. edu.cn (Z.W.); [email protected] (G.Z.).

Figure 8. Current density−voltage curves (a), brightness−voltage curves (b), and current efficiency−brightness curves (c) for the ITO/ NPB (40 nm)/TcTa (20 nm)/TPAF (40 nm)/TPBi (40 nm)/LiF (1 nm)/Al (70 nm) devices.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by Basic Research Program of China (2013CB328701-2013CB328706), National Natural Science Foundation of China (Grant Nos. 61275034 and 61106123), Fundamental Research Funds for the Central Universities (Grant No. xjj2012087) and China Postdoctoral Science Foundation (Grant No. 20110491653).

are summarized in Table 3. The device with TPA-(4)-F as blue emitter shows the lowest driving voltage of 8.1 V at 10 cd m−2, while with TPA-(2)-F 16.4 V and with TPA-(3)-F 10.2 V, respectively. The highest luminance was 1600 cd m−2 at 17.5 V (534 mA cm−2), and the efficiency reached more than 0.48 cd A−1 at 8.35 mA cm−2 in a TPA-(4)-F-based device, and the efficiency−luminance curve remained stable until very-high 20511



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Versatile Fluorinated Derivatives of Triphenylamine as Hole

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