Hexaaryl-benzodipyrroles: Properties and application as amorphous carrier-transporting materials

Article abstract of DOI:10.3987/COM-14-S(K)16

We have newly synthesized 1,2,3,5,6,7-hexaarylbenzo- [1,2-b:4,5-b]dipyrroles (HABDPs) and studied their properties. The HABDPs showed UV absorption maxima at 377-390 nm, indicating their extended "-conjugation system. They were also found to serve as effi

Full text of DOI:10.3987/COM-14-S(K)16

HETEROCYCLES, Vol. 90, No. 1, 2015
261
HETEROCYCLES, Vol. 90, No. 1, 2015, pp. 261 - 270. © 2015 The Japan Institute of Heterocyclic Chemistry
Received, 4th March, 2014, Accepted, 21st April, 2014, Published online, 1st May, 2014
DOI: 10.3987/COM-14-S(K)16
HEXAARYL-BENZODIPYRROLES: PROPERTIES AND APPLICATION
AS AMORPHOUS CARRIER-TRANSPORTING MATERIALS
Hayato Tsuji,a,b* Yuki Yokoi,a Shunsuke Furukawa,a and Eiichi Nakamuraa,*
aDepartment of Chemistry, School of Science, The University of Tokyo, Hongo,
b
Bunkyo-ku, Tokyo 113-0033, Japan JST-PRESTO, 4-1-8 Honcho, Kawaguchi,
Saitama 332-0012, Japan
Email: tsuji@chem.s.u-tokyo.ac.jp, nakamura@chem.s.u-tokyo.ac.jp
This paper is dedicated to Professor Isao Kuwajima on the occasion of his 77th
birthday.
Abstract
–
We have newly synthesized 1,2,3,5,6,7-hexaarylbenzo-
[1,2-b:4,5-b!]dipyrroles (HABDPs) and studied their properties. The HABDPs
showed UV absorption maxima at 377–390 nm, indicating their extended
"-conjugation system. They were also found to serve as efficient amorphous
materials, featuring high electrochemical and thermal stability with high carrier
mobility of up to 10–3 cm2/Vs using the time-of-flight technique. Application of
the HABDPs to organic light-emitting diodes resulted in significantly improved
performance in driving voltage and efficiency compared with #-NPD, a
commonly used hole-transporting material.
INTRODUCTION
Pyrrole derivatives are acknowledged as an important class of compounds in a broad range of fields, such
as natural products and bioactive molecular chemistry,1,2 supramolecular and coordination chemistry,3 and
materials science.4 We have so far developed novel fused polycyclic heteroaromatic compounds,5–11
including N,N!-HexBn-protected (HexBn = (4-hexylphenyl)methyl) benzo[1,2-b:4,5-b!]dipyrroles
(HexBn-BDPs), and found that the HexBn-BDPs serve as solution-processable hole injection materials in
organic light-emitting diodes (OLEDs) and afforded mechanistic insights for the hole injection. In this
manuscript,
we
report
newly
synthesized
N,N!-diaryl-BDPs,
that
is,
1,2,3,5,6,7-
hexaarylbenzo[1,2-b:4,5-b!]dipyrroles (HABDPs), which show intriguing properties, such as high
stability and high carrier mobility, and are suitable to serve as a novel class of hole-transporting materials

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in OLEDs to achieve significant improvement in driving voltage and efficiency.
RESULTS AND DISCUSSION
Synthesis of the HABDPs is shown in Scheme 1. Using our previously developed zinc-mediated
cyclization reaction, N,N!-HexBn-protected BDPs 2 were prepared, which was followed by deprotection
using AlCl3 in anisole12 to obtain N,N!-protonated BDPs 3 in moderate yield. A palladium-catalyzed
N-arylation reaction13 afforded the new class of compounds HABDPs 4a–d in 55%, 52%, 58%, and 38%
yields, respectively.
Scheme 1
Photophysical and electrochemical property measurements of the HABDPs 4 in solution demonstrated
their extended "-conjugation character (Table 1). The UV absorption spectra of 4a–c (R1 = H) showed
similar longest absorption maxima and onset wavelengths at 377–382 nm and 424–430 nm, respectively
(Figure 1), while that of 4d (R1 = NPh2) is somewhat redshifted with shoulder-like transition around 390
nm and onset at 449 nm, indicating the extension of "-conjugation because of the additional
diphenylamino group at the 2,6-positions. These absorption maximum wavelengths are similar to those of
the benzo[1,2-b:4,5-b!]difurans (BDFs), which we have previously reported,5 showing a small difference
in the absorption nature from the oxygen counterparts. The cyclic voltammogram of these BDPs in
CH2Cl2 showed a reversible first oxidation wave with potentials of 0.23, 0.18, 0.25, and 0.12 V (vs

HETEROCYCLES, Vol. 90, No. 1, 2015
263
Fc/Fc+) for 4a–d, respectively, indicating electrochemical stability toward oxidation. No reduction wave
was observed within the solvent window. The HOMO energy levels were estimated to be –5.03, –4.98,
–5.05, and –4.92 eV, respectively. The LUMO energy levels, calculated from the HOMO energy and the
optical gap (Eg), were –2.15, –2.10, –2.13, and –2.16 eV, respectively. These HOMO and LUMO energy
levels are somewhat higher than those of the corresponding BDFs (e.g., HOMO: –5.49 eV; LUMO: –2.54
eV for 2,3,6,7-tetraphenylbenzo[1,2-b:4,5-b!]difuran), representing the increased electron-donating nature
of the incorporated nitrogen atoms compared with oxygen.
Figure 1. UV Absorption spectra of HABDPs 4a–d in CH2Cl2 at room temperature
Table 1. Summary of photophysical and electrochemical properties of HABDPs 4a–d.
$max/nma
$
onset/nma
Cmpd R1
R2
Eg/eVb
HOMO/eVc LUMO/eVd
4a
4b
4c
4d
H
H
301, 335, 377
308, 382
430
430
424
2.88
–5.03
–4.98
–5.05
–4.92
–2.15
–2.10
–2.13
–2.16
H
NPh2
2.88
H
9-carbazolyl 293, 315, 377
2.92
NPh2
H
306, 357, 390(sh)e 449
2.76
aIn CH2Cl2 at room temperature. bDeduced from the equation: Eg [eV] = 1240/$onset [nm]. cHOMO energy
level deduced from the equation: HOMO [eV] = –(4.8 + Eox [V]), where Eox refers to oxidation potential
(vs Fc/Fc+) in CH2Cl2. dDeduced from the equation: LUMO [eV] = HOMO [eV] – Eg. eShoulder.

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The thermal stabilities of the BDPs are high: thermogravimetric analysis (TGA) showed 5% weight loss
(Td) at 424 °C for 4a because of sublimation and 489, >500, and 486 °C for 4b–d, respectively. The BDP
4d exhibited a glass transition temperature (Tg) at 121 °C seen by differential scanning calorimetry (DSC).
The observed Tg values are substantially higher than those for a commonly used p-type material #-NPD
(95 °C. Structure is shown in Figure 3),14 indicating that the compounds show high stability in their
amorphous state. The BDP 4b did not show perceivable Tg, but the amorphous state was secured by the
X-ray diffraction measurement of the powder sample, whereas amorphous samples of BDPs 4a and 4c
were not successfully prepared.
The carrier mobilities of the BDPs 4a–c were very high for their amorphous state (Figure 2).
Time-of-flight (TOF) technique was applied for the vacuum deposited µm-thick amorphous film of BDPs.
The BDPs 4a and 4b showed nondispersive transient currents for hole and no current for electron,
indicating p-type noncarrier trap transportation. The hole mobility at room temperature of these
compounds was 2 % 10–3 cm2/Vs for both compounds at an electric field of 2.5 % 105 V/cm with little
electric field dependence (Figure 2a). Interestingly, the carbazole-substituted BDP 4c displayed transient
currents for both hole and electron, i.e., ambipolar character,15 similar to the carbazole-substituted BDF
(CZBDF) that we previously reported5 or commonly known 1,1!-di(N-carbazolyl)-4,4!-biphenyl (CBP).16
The hole and electron mobilities were 6 % 10–4 and 2 % 10–3 cm2/Vs, respectively, at an electric field of 2.5
% 105 V/cm.17
Figure 2. Carrier mobility of HABDPs 4a–c plotted against the square root of the applied electric field.
(a) Hole mobility of the p-type BDPs 4a and 4b. (b) Hole and electron mobilities of the ambipolar BDP 4c.
Based on the measurements above, we have revealed the advantageous character of BDPs for their
application to OLEDs: transparency in the visible region, appropriate orbital energy (HOMO level for

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265
carrier injection from an indium–tin oxide (ITO) anode, and LUMO for electron-blocking ability), high
electrochemical and thermal stability, and high carrier mobility. Taking advantage of these features, we
applied the p-type BDPs 4a and 4b as a hole transport layer (HTL) in OLEDs. The application of
ambipolar BDP 4c is under investigation and will be reported elsewhere. OLEDs have found practical
uses as flexible, lightweight, and power-saving display and illumination devices.18,19 For further
improvement of device performance in driving voltage and efficiency, development of new materials for
various parts of these devices has been extensively explored.20
The OLEDs have a standard configuration (Figure 3a): ITO (anode)/PEDOT:PSS (hole injection
layer)/HTL (45 nm)/Alq3 (emitting and electron transport layer; 60 nm)/lithium fluoride (LiF) (electron
injection layer; 1 nm)/Al, where the PEDOT:PSS21 layer was fabricated by spin-coating, and the
following layers by vacuum deposition. We fabricated OLEDs using #-NPD as a HTL as a reference. The
OLED performance was evaluated for three standard criteria at a luminance of 1,000 cd/m2: driving
voltage (V1000), luminance efficiency (!1000), and current efficiency (L/J1000). The data are summarized in
Table 2. These devices showed green electroluminescence from Alq3, indicating that the carriers are
effectively confined within the Alq3 layer. The driving voltage of the device using 4a was 7.0 V, which is
1.5 V lower than that of the reference device (i.e., 8.5 V for the #-NPD-based device), while that of the
4b-based device was the same as for the reference (8.5 V). The luminance and the current efficiencies
were also significantly improved: !1000 = 0.81, 0.84, and 0.59 lm/W; L/J1000 = 1.80, 2.29, and 1.60 cd/A for
4a, 4b, and #-NPD, respectively. We attribute the improved performance of the BDP-based devices to
their HOMO energy levels, which are suitable for carrier injection processes both from the anode and to
Alq3, and for facile carrier transport because of their high carrier mobility.
Table 2. Summary of OLED performance using either BDPs 4a and 4b or #-NPD as a hole transport
layera
Material
4a
V1000 [V]b
7.0
!
1000 [lm/W]c L/J1000 [cd/A]d
0.81
0.84
0.59
1.80
2.29
1.60
4b
8.5
#-NPD
8.5
aAll OLED performance data were collected at a luminance of 1,000 cd/m2. bDriving voltage. cLuminance
efficiency. dCurrent efficiency.
In summary, we found that HABDPs serve as excellent amorphous materials with high stability. The
carrier mobility measurement and application to OLEDs unambiguously demonstrated that BDP itself has

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hole-transporting ability. Incorporation of substituents would synergistically improve the device
performance by tuning the material properties.
Figure 3. Evaluation of the performance of OLEDs composed of BDPs 4a and 4b for a hole-transporting
material, together with that of #-NPD as a reference: (a) device configuration, where HTL stands for hole
transport layer, (b) J–V characteristics, and (c) L–V characteristics.
EXPERIMENTAL
Synthesis.
General. Proton nuclear magnetic resonance (1H NMR) and carbon nuclear magnetic resonance (13C
NMR) spectra were recorded using a JEOL ECA-500 (500 MHz) NMR spectrometer. High-vacuum train
sublimation (pressure below 30 mTorr) was performed using a P-100HT instrument (ALS Technology
Company). Melting points of solid materials were determined using a Mel-Temp II capillary
melting-point apparatus and are uncorrected. Mass spectra were obtained on a JEOL Accu TOF
JMS-T100LC spectrometer. N,N!-HexBn-protected BDPs 2 were prepared according to our previously
reported method.7
2,3,6,7-Tetraphenylbenzo[1,2-b:4,5-b!]dipyrrole
(3a).
To
a
suspension
of
1,5-bis[(4-
hexylphenyl)methyl]-2,3,6,7-tetraphenylbenzo[1,2-b:4,5-b!]dipyrrole (167 mg, 0.206 mmol) in anisole (1
mL) was added AlCl3 (440 mg, 3.30 mmol) at 0 °C. The mixture was stirred at 100 °C for 12 h. After
cooling to room temperature, the reaction was quenched with H2O. The organic layer was separated and

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the aqueous layer was extracted with CH2Cl2. The combined organic layers were concentrated in vacuo.
After dissolving in toluene, 2-propanol was added to precipitate. The precipitate was collected by
filtration and washed with 2-propanol to afford the title compound as a yellow solid (66.7 mg, 0.145
1
mmol, 70%). Mp 340–341 °C. H NMR (500 MHz, THF-d8) & 7.19–7.29 (m, 8H, ArH), 7.36 (t, J = 7.6
13
Hz, 4H, ArH), 7.48–7.50 (m, 8H, ArH), 7.55 (s, 2H, ArH). C NMR (125 MHz, THF-d8) & 99.2, 114.0,
126.4, 127.7, 128.6, 129.04, 129.06, 129.13, 131.0, 134.7, 135.4, 135.9, 137.8. MS (APCI+): 461 (M+1).
Anal. Calcd for C34H24N2: C, 88.67; H, 5.25; N, 6.08. Found: C, 88.43; H, 5.29; N, 5.90.
2,6-Bis(4-N,N-diphenylaminophenyl)-3,7-diphenylbenzo[1,2-b:4,5-b!]dipyrrole (3b). The reaction
was conducted at 100 °C. Mp 344–345 °C. 1H NMR (500 MHz, THF-d8) & 7.00–7.03 (m, 8H, ArH), 7.07
(d, J = 8.3 Hz, 4H, ArH), 7.14–7.18 (m, 116H, ArH), 7.23–7.28 (m, 12H, ArH), 7.35 (d, J = 7.5 Hz, 4H,
13
ArH), 7.63 (s, 2H, ArH). C NMR (125 MHz, 1,1,2,2-tetrachloroethane-d2) & 98.88, 113.62, 123.66,
123.85, 125.42, 126.35, 128.61, 128.80, 129.13, 129.49, 130.06, 131.10, 135.24, 135.41, 138.07, 147.67,
148.64. MS (APCI+): 795 (M+1). Anal. Calcd for C58H42N4: C, 87.63; H, 5.33; N, 7.05. Found: C, 87.33;
H, 5.56; N, 6.86.
Typical procedure for palladium-catalyzed N-arylation: 1,2,3,5,6,7-Hexaphenylbenzo-
[1,2-b:4,5-b!]dipyrrole (4a). To a suspension of 2,3,6,7-hexaphenylbenzo[1,2-b:4,5-b!]dipyrrole (1.38 g,
3.00 mmol) in toluene (15 mL) were added Pd2(dba)3•CHCl3 (124 mg, 0.120 mmol), P(t-Bu)3 in toluene
(0.24 mL, 1.0 M, 0.24 mmol), sodium tert-butoxide (1.15 g, 12.0 mmol), and iodobenzene (1.01 mL, 14.4
mmol). The mixture was stirred at 120 °C for 24 h. After cooling to room temperature, the reaction was
quenched with H2O. The mixture was filtered and the precipitate was washed several times with toluene
and EtOAc. The resulting crude solid was purified by high vacuum (pressure under 20 mTorr) train
sublimation at 300–320 °C to afford the title compound as a yellowish white solid (1.01 g, 1.65 mmol,
1
55%). Mp: sublimation. H NMR (500 MHz, 1,1,2,2-tetrachloroethane-d2) & 7.08–7.09 (m, 10H, ArH),
7.15 (t, J = 7.2 Hz, 2H, ArH), 7.23–7.26 (m, 10H, ArH), 7.30–7.35 (m, 8H, ArH), 7.59 (s, 2H, ArH). 13C
NMR (125 MHz, 1,1,2,2-tetrachloroethane-d2) & 99.0, 116.5, 125.7, 126.4, 126.8, 127.1, 127.7, 128.2,
128.5, 129.0, 130.3, 131.3, 132.3, 135,6, 136.4, 138.6, 139. MS (APCI+): 613 (M+1). Anal. Calcd for
C46H32N2: C, 90.16; H, 5.26; N, 4.57. Found: C, 90.13; H, 5.39; N, 4.62.
1,5-Bis(4-N,N-diphenylaminophenyl)-2,3,6,7-tetraphenylbenzo[1,2-b:4,5-b!]dipyrrole (4b). Yield:
52%. Mp 360–361 °C. 1H NMR (500 MHz, THF-d8) & 7.00–7.03 (m, 8H, ArH), 7.07 (d, J = 8.3 Hz, 4H,
ArH), 7.14–7.18 (m, 116H, ArH), 7.23–7.28 (m, 12H, ArH), 7.35 (d, J = 7.5 Hz, 4H, ArH), 7.63 (s, 2H,
ArH). MS (APCI+): 947 (M+1). Anal. Calcd for C70H50N4: C, 88.76; H, 5.32; N, 5.92. Found: C, 88.69; H,
5.60; N, 5.82.
1,5-Bis(carbazolyl)-2,3,6,7-tetraphenylbenzo[1,2-b:4,5-b!]dipyrrole (4c). Yield: 58%. Mp 410–411 °C.
1H NMR (500 MHz, CDCl3) & 7.22–7.25 (m, 10H, ArH), 7.29–7.33 (m, 4H, ArH), 7.35–7.38 (m, 4H,

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ArH), 7.42–7.44 (m, 6H, ArH), 7.46–7.48 (m, 6H, ArH), 7.50–7.52 (m, 4H, ArH), 7.58–7.59 (m, 4H,
ArH), 7.85 (s, 2H, ArH), 8.15–8.17 (m, 4H, ArH). MS (APCI+): 943 (M+1). Anal. Calcd for C70H46N4: C,
89.14; H, 4.92; N, 5.94. Found: C, 88.86; H, 5.03; N, 5.77.
2,6-Bis(4-N,N-diphenylaminophenyl)-1,3,5,7-tetraphenylbenzo[1,2-b:4,5-b!]dipyrrole (4d). Yield:
38%. Mp 363–364 °C. 1H NMR (500 MHz, THF-d8) & 6.77 (d, J = 8.9 Hz, 4H, ArH), 6.94–7.00 (m, 16H,
ArH), 7.14 (t, J = 7.5 Hz, 2H, ArH), 7.19–7.22 (m, 8H, ArH), 7.25–7.31 (m, 10H, ArH), 7.37–7.42 (m,
8H, ArH), 7.61 (s, 2H, ArH). 13C NMR (125 MHz, 1,1,2,2-tetrachloroethane-d2) & 99.12, 116.68, 122.79,
123.91, 125.47, 126.39, 126.78, 127.23, 127.51, 128.94, 129.30, 129.80, 130.01, 130.93, 132.87, 136.80,
137.14, 138.53, 140.25, 147.73, 148.46. MS (APCI+): 947 (M+1). Anal. Calcd for C70H50N4: C, 88.76; H,
5.32; N, 5.92. Found: C, 88.80; H, 5.57; N, 5.91.
Property measurements. UV–visible absorption spectra were recorded using a JASCO V-570
spectrophotometer at a resolution of 0.5 nm. Spectroscopy-grade CH2Cl2 was used as a solvent. Ca. 10–5
M sample solutions in a 1-cm square quartz cell were used for the measurements. All measurements were
conducted at room temperature.
Cyclic voltammetry (CV) measurements were conducted using a Hokuto Denko HZ-5000 voltammetric
analyzer. All measurements were conducted at room temperature in a one-compartment cell under N2 gas,
equipped with a glassy-carbon working electrode, a platinum wire counter electrode, and an Ag/Ag+
reference electrode (10 mM AgClO4 in 0.1 M Bu4NPF6).
Differential scanning calorimetry (DSC) was performed using a Netzsch DSC 204/F1 thermal analyzer.
The glassy state sample was heated at a rate of 5 K min–1 under N2 gas, at a flow rate of 18 mL min–1.
Thermogravimetry-differential thermal analysis (TG-DTA) was performed using a Rigaku TG8120
analyzer. Temperature was raised at 10 K min–1 under N2 using Al2O3 as a reference.
Carrier mobility measurement using time-of-flight (TOF) technique. Films were prepared on
ITO-coated (145 nm) glass substrates by vacuum sublimation under heating (3–4 % 10–4 Pa and 2–4 % 10–4
Pa for 4a and 4b, respectively), with an average deposition rate of 4.0 nm/s. An ITO-coated glass
substrate was spaced at 100 mm from the sample crucible, and it was maintained at 25 °C. The thickness
of the film obtained was 4.57 and 3.83 µm for 4a and 4b, respectively. After Al cathode deposition (80
nm), the device was encapsulated under nitrogen atmosphere. The vacuum deposition was performed
using a Ulvac Kiko VPC-260 system. The TOF measurement was conducted using a Sumitomo TOF-401
instrument.
Device fabrication and evaluation. Compounds of analytical purity were further purified by train
sublimation. All the other materials were commercially available and were used as purchased. The typical
fabrication procedure for a phosphorescent green OLED is described for the device using either
compound 4a or 4b as a hole-transporting material. An ITO-coated glass substrate treated by O3-plasma

HETEROCYCLES, Vol. 90, No. 1, 2015
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was used as the anode. A PEDOT:PSS (poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)) water
dispersion was spin-coated into this substrate, it was dried at 120 °C, then it was annealed at 180 °C under
nitrogen. Next, a HTL with the thickness of 45 nm was fabricated by vacuum deposition. An emitting
layer was subsequently fabricated by vacuum deposition of Alq3 with the thickness of 60 nm. An ultrathin
layer (1 nm) of LiF was deposited at the cathode interface to enhance electron injection. After Al cathode
deposition (80 nm), the device was sealed by encapsulation with a fresh desiccant under nitrogen. The
emissive area of the device was 2 % 2 mm2.
Electroluminescence spectra and luminance-voltage characteristics of the OLED devices were measured
using a Precise Gauges EL 1003 instrument. All measurements were conducted under ambient
atmosphere.
ACKNOWLEDGEMENTS
We thank financial support (Strategic Promotion of Innovative Research and Development Programs)
from the Japan Science and Technology Agency (JST) for E.N. Financial support in part by Kakenhi for
E.N. (No. 22000008) and H.T. (No. 20685005) is also acknowledged.
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