Synthesis and characterization of spiro(adamantane-2,9 '-fluorene)-based triaryldiamines: thermally stable hole-transporting materials

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Synthesis and characterization of spiro(adamantane-2,9

-fluorene)-based

triaryldiamines: thermally stable hole-transporting materials

Ching-Hsin Chen, Wen-Jian Shen, Kavitha Jakka, Ching-Fong Shu

∗ Department of Applied Chemistry, National Chiao Tung University, Hsin-Chu 30035, Taiwan Received 10 November 2003; received in revised form 4 December 2003; accepted 4 December 2003

Abstract

New triaryldiamines, Adm-TPD and Adm-NPD, which contain a spiro(adamantane-2,9-fluorene) core, have been synthesized and characterized. We believe that because of the presence of the rigid spiro-fluorene core, these triaryldiamines exhibit higher glass transition temperatures and have a less tendency to crystallize relative to their biphenyl analogs. Additionally, electrochemical studies showed that the planarity of the fluorene core leads to lower ionization potentials and greater degrees of␲ conjugation between the two amino groups. AFM measurements demonstrated that high-quality amorphous films of Adm-NPD with good morphological stability were prepared by vapor deposition. Electroluminescent devices using Adm-NPD as a hole-transporting layer—i.e., having the configuration ITO/Adm-NPD/Alq3/LiF/Al—emit a bright green light that is characteristic of Alq3.

Keywords: Adamantane; Fluorene; Triaryldiamine; Hole-transporting material; OLED

1. Introduction

Since the discovery of multi-layered organic light-emitting diodes (OLEDs) by Tang and Van Slyke[1] electrolumines-cent devices have been the subject of intensive investiga-tions because they have applicainvestiga-tions in full-color displays [2–6]. In addition to attempts at improving device efficiency, many studies have focused on enhancing the durability of OLEDs. Thermal stress can cause amorphous organic films to crystallize, forming polycrystalline films, which leads to device failure [7–9]. A considerable amount of evidence indicates that an amorphous thin film (in OLEDs) with a high glass transition temperature (Tg) is less vulnerable to

heat damage and, hence, the corresponding device’s perfor-mance is more stable[10–17]. Thus, high Tgmaterials are

always desirable in OLED applications.

Because of their capability to form good-quality amor-phous solid films, various triarylamine derivatives have been utilized as hole-transporting materials (HTM)[5,6,17]. Re-cently, many efforts have been devoted to the development of new amorphous triarylamines possessing high morpho-logic stability[12–14,18–25]. It has been demonstrated that the presence of a rigid fluorene (or derivatized fluorene) core, rather than a biphenyl group, leads to triarylamines

∗_{Corresponding author. Tel.:}_{+886-3-5712121; fax: +886-3-5723744.}

E-mail address: [email protected] (C.-F. Shu).

with enhanced thermal stability[12,22–25]. Herein, we re-port the synthesis and characterization of spiro(adamantane-2,9-fluorene) derivatives with analogous diamine structures to the well-known biphenyl HTMs, TPD (N,N -diphenyl-N,N-bis(3-methylphenyl)-1,1-diphenyl-4,4-diamine) and NPD (N,N-diphenyl-N,N-bis(1-naphthyl)-1,1 -diphenyl-4,4-diamine). Because of its rigid and near-spherical struc-ture, adamantane has been incorporated into both polymers and small molecules to provide unusual physical and ther-mal properties[26–30]. We expected that incorporating the spiro(adamantane-fluorene) group in these hole-transporting materials would increase their glass transition temperatures (Tg), thermal stabilities, and molecular rigidities, as well as

to reduce their crystallinity.

2. Experimental 2.1. General directions

The solvents were dried using standard procedures. All other reagents were used as-received from commercial sources, unless otherwise stated.1H and 13C NMR spectra were recorded on a Varian Unity 300 MHz and a Bruker-DRX 300 MHz spectrometer. Mass spectra were obtained on a JEOL JMS-SX 102A mass spectrometer. Differential scanning calorimetry (DSC) was performed on a SEIKO

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EXSTAR 6000DSC unit using a heating rate of 10◦C min−1 and a cooling rate of 40◦C min−1. Samples were scanned from 30 to 300◦C, cooled to 0◦C, and then scanned again from 30 to 300◦C. The glass transition temperatures (Tg)

were determined from the second heating scan. Thermo-gravimetric analysis (TGA) was undertaken on a DuPont TGA 2950 instrument. The thermal stability of the samples was determined under a nitrogen atmosphere, by measur-ing weight loss while heatmeasur-ing at a rate of 20◦C min−1. UV-visible spectra were measured with an HP 8453 diode-array spectrophotometer. Photoluminescence (PL) spectra were obtained on a Hitachi F-4500 luminescence spectrometer. Cyclic voltammetry (CV) measurements were performed on a BAS 100 B/W electrochemical analyzer with anhydrous CH2Cl2 containing 0.1 M

tetrabutylam-monium hexafluorophosphate (TBAPF6) as the

support-ing electrolyte. The potentials were measured against an Ag/Ag+ (0.01 M AgNO3) reference electrode. The onset

potentials were determined from the intersection of two tangents drawn at the rising current and background current of the cyclic voltammogram. Organic EL devices were fab-ricated under a base vacuum of 1.3 × 10−4Pa in a thin-film evaporation coater and characterized following a published protocol [31]. AFM images were recorded with a Digital Instruments Nanoscope E scanning probe microscope. 2.2. 2-(2-Biphenyl)adamantanol (1)

n-BuLi (2.5 M in hexane, 2.24 ml, 5.60 mmol) was added dropwise to a stirred solution of 2-bromobiphenyl (1.00 g, 4.29 mmol) in dry THF (3.0 ml) at−78◦C under a nitrogen atmosphere. After the addition was complete, the solution was warmed to room temperature over 30 min. The mixture was cooled to −78◦C and a solution of 2-adamantanone (0.75 g, 5.0 mmol) in dry THF (3 ml) was added dropwise. The solution was then warmed to room temperature and stirred overnight. The reaction contents were quenched with water and diluted with ethyl acetate. The organic phase was washed with water, dried over magnesium sulfate, evap-orated, and recrystallized from a mixture of hexane and ethyl acetate to afford the alcohol 1 (1.08 g, 82.6%). 1H NMR (CDCl3),δ (ppm): 1.43–1.86 (m, 11H), 2.13 (d, J = 12.3 Hz, 2H), 2.21 (s, 1H), 7.09 (dd, J = 7.7, 1.5 Hz, 1H), 7.23–7.36 (m, 7H), 7.62 (d, J = 7.7 Hz, 1H). 13C NMR (CDCl3),δ (ppm): 26.7, 27.6, 33.2, 35.1, 35.4, 37.9, 78.1, 126.8, 127.0, 127.2, 127.5, 128.0, 129.3, 133.8, 142.1, 142.7, 145.2. HRMS (m/z): [M+] calcd. for C22H24O, 304.1827; found, 304.1826. 2.3. Spiro(adamantane-2,9-fluorene) (2)

A catalytic amount of p-toluenesulfonic acid was added to a solution of 1 (1.00 g, 3.29 mmol) in acetic acid (5.0 ml) and then the mixture was heated under reflux for 30 min. The reaction mixture was poured into cold water and the precip-itated was filtered and dried to give 2 (0.91 g, 96.7%).1H

NMR (CDCl3),δ (ppm): 1.63 (s, 2H), 1.81 (d, J = 13.5 Hz, 4H), 2.01 (s, 2H), 2.23 (s, 2H), 2.96 (d,J = 12.9 Hz, 4H), 7.31 (ddd,J = 7.7, 7.2, 1.4 Hz, 2H), 7.39 (ddd, J = 7.7, 7.2, 1.2 Hz, 2H), 7.82 (d, J = 7.2 Hz, 2H), 8.13 (d, J = 7.7 Hz, 2H).13C NMR (CDCl3),δ (ppm): 27.7, 34.6, 35.2, 41.8, 58.8, 119.7, 126.2, 127.1, 129.6, 141.1, 151.2. HRMS (m/z): [M+] calcd. for C22H22, 286.1722; found, 286.1717.

2.4. Spiro[adamantane-2,9-(2,7-dibromofluorene)] (3) FeCl3 (catalytic amount) and bromine (0.39 ml, 7.50

mmol) were added in the dark to a stirred solution of 2 (1.00 g, 3.50 mmol) in chloroform (20 ml) at 0◦C. The so-lution was stirred at room temperature for 15 h and then poured into aqueous sodium thiosulfate solution and ex-tracted with CHCl3. The organic layer was washed with

water, dried over magnesium sulfate and evaporated. The residue was recrystallized from a mixture of hexane and CH2Cl2 to yield 3 (1.40 g, 89.7%). 1H NMR (CDCl3), δ (ppm): 1.59 (s, 2H), 1.80 (d,J = 13.8 Hz, 4H), 1.98 (s, 2H), 2.22 (s, 2H), 2.80 (d,J = 12.9 Hz, 4H), 7.50 (dd, J = 8.1, 1.6 Hz, 2H), 7.60 (d,J = 8.1 Hz, 2H), 8.21 (d, J = 1.6 Hz, 2H). 13C NMR (CDCl3),δ (ppm): 27.3, 34.3, 34.9, 41.5, 59.4, 120.5, 120.8, 130.4, 132.6, 139.0, 152.8. HRMS (m/z): [M+] calcd. for C22H2081Br2, 445.9891; found, 445.9883;

calcd. for C22H2079Br81Br, 443.9911; found, 443.9893;

calcd. for C22H2479Br2, 441.9931; found, 441.9907.

2.5. Preparation of Adm-TPD

A mixture of compound 3 (800 mg, 1.80 mmol), Pd(OAc)2 (20.2 mg, 90␮mol), and sodium tert-butoxide

(415 mg, 4.32 mmol) was flushed with nitrogen sev-eral times. 3-Methyldiphenylamine (0.62 ml, 3.60 mmol), P(t-Bu)3 (0.5 M in o-xylene, 0.72 ml, 0.36 mmol), and dry

o-xylene (6.0 ml) were added sequentially into the flask under nitrogen. The reaction mixture was then heated at 120◦C for 5 h, cooled to room temperature, diluted with H2O, and extracted with ethyl acetate. The combined

or-ganic solution was washed with brine, dried over MgSO4,

evaporated, and purified by column chromatography using hexane/ethyl acetate (100:1) as the eluent to afford the amine Adm-TPD (1.08 g, 92.5%). 1H NMR (CDCl3), δ (ppm): 1.62–1.66 (m, 6H), 1.86 (d,J = 19.4 Hz, 4H), 2.29 (s, 6H), 2.57 (d, J = 12.7 Hz, 4H), 7.00–7.88 (m, 24H). 13_{C NMR (CDCl} 3),δ (ppm): 21.4, 27.0, 34.0, 35.1, 41.4, 58.4, 118.2, 119.0, 119.5, 122.3, 122.6, 123.4, 124.2, 125.2, 125.5, 126.0, 129.0, 138.9, 145.2, 151.9. HRMS (m/z): [M+] calcd. for C48H44N2, 648.3505; found, 648.3507.

Anal. calcd. for C48H44N2: C, 88.85; H, 6.83; N, 4.32.

Found: C, 88.80; H, 6.95; N, 4.20. 2.6. Preparation of Adm-NPD

Adm-NPD was prepared (94.6%) from compound 3 and N-phenyl-1-naphthylamine following the procedure

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Br OH Br Br N N N N NH NH iv iv iii ii i 1

+

2 3 3

+

3 Adm-TPD Adm-NPD a

Reagents: (i)n-BuLi, THF, 2-adamantanone; (ii) AcOH, p-TsOH (cat);

(iii) Br2, FeCl3(cat), CHCl3; (iv) Pd(OAc)2(cat), P(t-Bu)3, NaOtBu,o-xylene.

Scheme 1. Reagents: (i) n-BuLi, THF, 2-adamantanone; (ii) AcOH, p-TsOH (cat); (iii) Br2, FeCl3(cat), CHCl3; (iv) Pd(OAc)2(cat), P(t-Bu)3, NaOtBu, o-xylene.

described for the preparation of Adm-TPD. 1HNMR (CDCl3),δ (ppm): 1.47–1.52 (m, 6H), 1.72 (d, J = 6.8 Hz, 4H), 2.35 (d, J = 12.8 Hz, 4H), 6.94–7.24 (m, 12H), 7.34–7.52 (m, 10H), 7.78 (d,J = 7.8 Hz, 4H), 7.91 (d, J = 8.1 Hz, 2H), 8.04 (d, J = 8.4 Hz, 2H).13CNMR (CDCl3), δ (ppm): 27.1, 34.0, 35.1, 41.5, 58.2, 119.1, 121.3, 121.5, 124.2, 124.5, 126.2, 12.4, 126.5, 127.3, 128.5, 129.2, 135.4, 145.8, 151.9. HRMS (m/z): [M+] calcd. for C54H44N2,

720.3505; found, 720.3502. Anal. calcd. for C54H44N2: C,

89.96; H, 6.15; N, 3.89. Found: C, 89.72; H, 6.31; N, 3.87.

3. Results and discussion

3.1. Synthesis and characterization

Scheme 1shows the synthetic route to the spiro(adaman-tane-2,9-fluorene)-based triaryldiamines, Adm-TPD and Adm-NPD. Spiro(adamantane-2,9-fluorene) (2) was prepa-red by the reaction of 2-lithiobiphenyl with 2-adamantan-one, followed by dehydrative ring closure of the

result-Table 1

Physical properties of triaryldiamines

Tg(◦C)a Td (◦C)b λmax (nm)c λem (nm)c Eox1/2(Eoxonset) (mV)d HOMO (eV)

Adm-TPD 94 390 371 396 632, 945 (561) 5.5

Adm-NPD 125 430 352 465 651, 992 (587) 5.5

TPD 60[25] 350 396 810, 1059 (742) 5.7

NPD 95[25] 336 448 845, 1115 (771) 5.7

a_{Measured under nitrogen at a heating rate of 10}◦_{C min}−1 _{and a cooling rate of 40}◦_{C min}−1_.

b_{The temperature corresponding to a 5% weight loss measured under nitrogen at a heating rate of 20}◦_{C min}−1_. c_{Measured in EtOAc; the photoluminescence maximum was recorded upon irradiation at the absorption maximum.} d_{Relative to a Ag/Ag}+_reference.

50 100 150 200 250 Endothermic Temperature (oC) (b) (a) 1st heating 2nd heating

Fig. 1. DSC thermograms obtained upon sequential heating and cooling cycles of: (a) Adm-TPD and (b) Adm-NPD.

ing alcohol 1 in the presence of a catalytic amount of p-toluenesulfonic acid. Bromination of 2 with Br2 in the

presence of FeCl3 resulted the dibromo compound 3. The

P(t-Bu)3–palladium complex catalyzed the amination of

3 with diarylamines gave the target triaryldiamines, each containing a spiro(adamantane-2,9-fluorene) core [32]. The chemical structures of Adm-TPD and Adm-NPD were confirmed by 1H and 13C NMR, high resolution mass spectrometry, and elemental analysis.

3.2. Thermal properties

The thermal properties of Adm-TPD and Adm-NPD were investigated by differential scanning calorimetry and ther-mogravimetric analysis; the results are presented inTable 1. Fig. 1 displays their DSC curves. DSC was performed in the temperature range between 30 and 300◦C. The poly-crystalline samples of Adm-TPD and Adm-NPD that are obtained upon sublimation melt only on their first heating, at 195 and 217◦C, respectively, and then they change into a glassy state upon cooling from the melt. When these amor-phous glassy samples are reheated, the former has a Tg at

94◦C, while the latter has a Tgat 125◦C; no exothermic peak

resulting from crystallization is observed up to 300◦C. As shown inTable 1, the spiro(adamantane-2,9-fluorene)-based triaryldiamines have higher values of Tg relative to their

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morphological stability of the fluorene-based materials to the presence of the rigid spiro-adamantane substituent at the central linkage, which may hinder the crystallization process[29,30]. It is important that OLEDs be constructed from materials having a relatively high value of Tg to

avoid problems associated with grain boundaries in poly-crystalline films [7–9]. Additionally, both Adm-TPD and Adm-NPD exhibit high thermochemical stability, as ev-idenced by thermogravimetric analysis, with 5% weight loss temperatures under nitrogen atmosphere of up to 390◦C.

To further investigate the morphological stability of Adm-NPD, we deposited a thin film on an indium tin ox-ide (ITO) substrate and measured its surface morphology by using atomic force microscopy (AFM) before and after annealing at 95◦C for 3 days under a nitrogen atmosphere. For the sake of comparison, the same experiment was con-ducted with a film of NPD. AsFig. 2indicates, the thin layer of Adm-NPD displays a fairly good surface morphology that is not changed upon annealing. In the case of the NPD film, the annealing induced a degradation of the surface morphology and, in addition, large crystals were observed on the annealed layer. It is apparent that the presence of the spiro(adamantane-fluorene) core inhibits the crystallization of Adm-NPD, which maintains its morphological stability in the glassy state.

Fig. 2. AFM images (top and angled views) of: (a) an Adm-NPD layer and (b) an NPD layer, after annealing at 95◦C for 3 days under a nitrogen atmosphere.

3.3. Optical properties

The absorption and emission spectral data of the fluorene-based triaryldiamines are summarized inTable 1. In EtOAc solutions, Adm-TPD and Adm-NPD have lowest-energy transitions, with ␭max at 371 and 352 nm,

respectively, that we attribute largely to electron transfer from a nitrogen lone pair to the␲∗ orbital of the biphenyl (fluorene) group [13]. The emission maxima are 396 nm for the former and 465 nm for the latter. The absorption bands of the fluorene-based triaryldiamines are signifi-cantly red-shifted (16–21 nm) with respect to those of their biphenyl analogs. This result indicates that the planar flu-orenyl linkage plays an important role in enhancing the ␲ conjugation and reducing the HOMO–LUMO energy gap. 3.4. Electrochemistry

The electrochemical behavior of Adm-TPD and Adm-NPD was investigated by cyclic voltammetry using an Ag/Ag+ reference electrode, as shown in Fig. 3, and the data are tabulated in Table 1. During the anodic scan, the triaryl-diamines exhibit two reversible oxidations, each of which corresponds to the removal of one electron from a tri-arylamine group. Relative to their biphenyl analogs, the spiro(adamantane-fluorene)-based materials have lower

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1. 2 1. 0 0. 8 0. 6 0. 4 0. 2 0. 0 E (V vs. Ag/Ag+) (a) (b) Current ( A)

Fig. 3. Cyclic voltammograms of: (a) Adm-TPD and (b) Adm-NPD in CH2Cl2containing 0.1 M TBAPF6.

oxidation potentials. This feature is due to the planarity of the fluorene core, which leads to better ␲ conjugation be-tween the two amino groups [24]. Additionally, the larger potential difference between the first and the second oxida-tion steps in the fluorene-based triaryldiamines reveals that the radical cation that is formed is delocalized more effi-ciently by the fluorene-linkage than by the biphenyl one. Al-though the electrochemical data do not yield an exact value of the HOMO energy level, we observe trends that can be compared with the oxidation potentials of well-studied HTL materials with similar structures. The HOMO energy levels of Adm-TPD and Adm-NPD are calculated from the cyclic voltammetry studies to be 5.19 and 5.22 eV, respectively, when compared with the value obtained for NPD (5.7 eV, estimated from the photoemission spectroscopy)[33]. 3.5. Electroluminescent devices

To investigate the hole-transporting properties of the spiro-fluorene-based materials, Adm-NPD, which has the higher thermal stability of the two, was selected as the hole-transporting layer in the fabrication of light-emitting diodes. Multilayer devices with the configuration ITO/Adm-NPD (60 nm)/Alq3(75 nm)/LiF (1.0 nm)/Al (200 nm) were

fabri-cated by sequential vapor deposition of the materials onto a glass substrate coated with ITO. In this structure, the ITO and Al layers are the anode and cathode, respectively, Alq3

is used as both the electron-transporting layer and as the emitter, and LiF is the electron-injecting layer. A reference device composed of NPD/Alq3with the same thickness was

also constructed for comparison.

Table 2

EL performance of triaryldiaminesa

Driving voltage (V) Current efficiency (cd A−1) Power efficiency (lm W−1) Maximum luminance (cd m−2) Peak wavelength (nm) Adm-NPD 6.12 (7.97) 2.21 (2.44) 1.14 (0.96) 28900 524 NPD 5.84 (7.74) 2.94 (3.10) 1.58 (1.26) 32700 524

a_{Measured at a current density of 20 mA cm}−2_{. The data in the parentheses were taken at a current density of 100 mA cm}−2_.

0 200 400 600 800 1000 0 10000 20000 30000 40000

Current Density (mA/cm2)

Luminance (Cd/m 2 ) Adm- NPD (60nm) / Alq(75nm) NPD (60nm) / Alq(75nm) (b) 0 2 4 6 8 10 12 0 200 400 600 800 1000 _{Adm-NPD (60nm) / Alq(75nm)} NPD (60nm) / Alq(75nm) Voltage (V)

Current Density (mA/cm

2 ) (a)

Fig. 4. (a) Current density–voltage characteristics and (b) luminescence– current density characteristics of Adm-NPD- and NPD-based OLEDs, having the configurations ITO/HTM (60 nm)/Alq3(75 nm)/LiF (10 nm)/Al (200 nm).

The current–voltage–luminance (I–V–L) characteristics of the devices are shown inFig. 4, and their performances are listed inTable 2. The OLED prepared with an Adm-NPD HTL has current–voltage characteristics very similar to the analogous NPD-based device. A bright green emission (λmax = 524 nm) from Alq3 was observed in both cases

with turn-on voltages at ca. 3.3 V. The Adm-NPD-based de-vice luminous efficiency achieves 2.44 cd A−1 (correspond-ing to a power efficiency of 0.96 lm W−1) at 100 mA cm−2 and exhibits a maximum luminescence of 28,900 cd m−2, which are values that are marginally lower than those of an NPD-based device. This outcome may be due to an inefficient carrier recombination in the Adm-NPD/Alq3

device. The energies of the HOMOs of Adm-NPD, NPD, and Alq3are 5.5, 5.7, and 6.0 eV, respectively. The greater

energy offset between the HOMOs of Adm-NPD and Alq3

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Adm-NPD to Alq3and an inefficient recombination of holes

and electrons in the Alq3layer, relative to these process in

NPD/Alq3[20,25].

In summary, we have synthesized new triaryldiamines, Adm-TPD and Adm-NPD, which each contain a spiro(ada-mantane-2,9-fluorene) core. These compounds, which do not crystallize upon cooling from their melts or on heating above their values of Tg, form amorphous films by vapor

de-position. Relative to their known biphenyl analogs, the new fluorene derivatives possess higher glass transition temper-atures and high HOMO energy levels, which we attribute to the presence of the rigid and planar spiro-fluorenyl core. AFM measurements reveal that the incorporation of the rigid spiro-core depresses the crystallization of Adm-NPD and allows it to maintain its morphological stability in the glassy state. A multilayer organic EL device using Adm-NPD as a hole-transporting layer—i.e., having the configura-tion ITO/Adm-NPD/Alq3/LiF/Al—produces a bright green

emission with reasonably good EL performance. Further modification and optimization of the structure and device are in progress.

Acknowledgements

We thank the National Science Council of the Republic of China for financial support. We also thank Professor Chin H. Chen for assistance with the fabrication and measurement of OLED devices.

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