Di(9-alkylcarbazol-3-yl)arylamines as electroactive amorphous materials for optoelectronics

Article abstract of DOI:10.1007/s00706-006-0509-x

Star-shaped molecules of di(9-alkylcarbazol-3-yl)arylamines were synthesized and found to constitute new glass-forming materials with glass transition temperatures ranging from 40 to 147°C. The electron photoemission spectra of the molecular glasses were

Full text of DOI:10.1007/s00706-006-0509-x

Monatshefte f€ur Chemie 137, 1053–1062 (2006)
DOI 10.1007/s00706-006-0509-x
Di(9-alkylcarbazol-3-yl)arylamines
as Electroactive Amorphous Materials
for Optoelectronics
Vida Peciuraite1, Violeta Vaitkeviciene1, Saulius Grigalevicius1,
Juozas V. Grazulevicius1;ꢀ, and Vygintas Jankauskas2
1
Department of Organic Technology, Kaunas University of Technology, Kaunas, Lithuania
2
Department of Solid State Electronics, Vilnius University, Vilnius, Lithuania
Received September 19, 2005; accepted (revised) November 7, 2005
Published online August 3, 2006 # Springer-Verlag 2006
Summary. Star-shaped molecules of di(9-alkylcarbazol-3-yl)arylamines were synthesized and found
to constitute new glass-forming materials with glass transition temperatures ranging from 40 to 147ꢁC.
The electron photoemission spectra of the molecular glasses were recorded and ionisation potentials of
4.9–5.0 eV were established. Room temperature time of flight hole drift mobilities of the di(9-alkyl-
carbazol-3-yl)phenylamines molecularly dispersed in polycarbonate-Z approached 5ꢂ 10ꢃ6 cm2=Vs at
high electric fields. Some of the compounds were converted to cross-linkable derivatives, which are
potential components for insoluble charge transport layers.
Keywords. Aromatic amine; Molecular glasses; Ionisation potential; Charge transport.
Introduction
Film-forming electroactive organic materials are known for various applications
such as electrophotographic photoreceptors [1], organic light emitting diodes [2, 3],
photovoltaic cells [4], photorefractive materials [5], and organic transistors [6].
Amorphous organic electroactive materials are divided into two categories. One
is low-molar-mass molecules; the devices are generally fabricated by vacuum-
deposition. The other is polymers; the devices are fabricated by the casting or
spin-coating methods. Among the low-molar-mass derivatives, triphenylamine, tri-
phenyldiamine (TPD), and their derivatives are the most extensively used as hole
transport materials. They are easily oxidized to form stable radical cations [7, 8].
However, the low-molar-mass triphenylamine and TPD derivatives have some
shortcomings, such as low morphological stability of their amorphous films as well
as rather high ionization potentials [9, 10]. We have reported previously on carbazol-
9-yl substituted TPD derivatives which formed amorphous films with glass transition
ꢀ
Corresponding author. E-mail: juozas.grazulevicius@ktu.lt

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V. Peciuraite et al.
temperatures (Tg) higher than 100ꢁC [11]. Electron photoemission spectroscopy of
thin films of the compounds showed ionisation potentials of ca. 5.4 eV.
In this work, we have designed and synthesised new triarylamines having the
bulky carbazolyl species, which enhances the amorphous nature of low-molar-mass
compounds. These branched structures are expected to form amorphous films with
good morphological stability due to the geometry of these molecules, which does
not favour close packing. We also report on the synthesis and properties of cross-
linkable triarylamine derivatives, which could be applied in manifold ways for
optoelectronic devices [12–14]. Photo-cross-linking is a useful method of protect-
ing thin organic systems against solvents [15, 16].
Results and Discussion
Di(9-alkylcarbazol-3-yl)arylamines 4–7 were generally prepared as described in
Scheme 1 via the modified Ullmann coupling reaction [17, 18] of an excess of
3-iodo-9-alkylcarbazole 2 or 3 with 3-amino-9-ethylcarbazole, 4-toluidine, or 4-
anisidine. The iodo-derivatives 2 and 3 were prepared by alkylation of 3-iodo-9H-
carbazole (1) which was synthesized by iodination of 9H-carbazole.
N-(4-Methoxyphenyl)-N,N-di[9-(2-ethylhexyl)carbazol-3-yl]amine (7)wasused
for the preparation of the reactive di(9-alkylcarbazol-3-yl)phenylamines 8 and 9 by
Scheme 1

Electroactive Amorphous Materials for Optoelectronics
1055
Table 1. Thermal characteristics of 4–8
Compound
Tg=ꢁC
Tm=ꢁC
TID=ꢁC
4
5
6
7
8
147
66
40
39
48
323
a
432
444
367
404
369
–
a
–
a
–
111
a
Obtained as an amorphous substance
two-step reaction as shown in Scheme 1. The first step involved cleavage of the
methyl hydroxy-protecting group using BBr3 [19] to get 8. The second step was
condensation of the hydroxyl substituted aromatic amine 8 with epichlorohydrin in
the presence of benzyltrimethylammonium cations.
All the new compounds were identified by mass spectrometry, IR, and electronic
1
absorption, as well as H NMR spectroscopy. The data were found to be in good
agreement with the proposed structures. The di(9-alkylcarbazol-3-yl)arylamines
4–9 were soluble in common organic solvents, such as tetrachloroethane, CHCl3,
THF, and acetone at room temperature. Transparent and stable thin amorphous films
of these materials could be prepared by casting or spin coating from solutions.
The behaviour of 4–8 under heating was studied by DSC and TGA under N2
atmosphere. The values of Tg, melting points (Tm), and the temperatures at which
initial loss of mass was observed (TID) are summarised in Table 1. All the deriva-
tives demonstrate high thermal stability. The mass loss occurs at temperatures higher
than 367ꢁC as confirmed by TGA with a heating rate of 10ꢁC=min.
It is evident that the thermal stability of 4–8 decreases with the introduction of
a 4-substituted phenyl group (cf. TID of 4 and 5 with that of 6–8). TID only slightly
depends on the substituent at the p-position of the phenyl group (cf. TID of 7 with
that of 6 and 8).
Derivatives 4 and 8 were obtained as polycrystals by crystallisation from solu-
tions, however, they readily formed glasses when the melt samples were cooled on
standing in air or with liquid N2. The DSC thermograms of 4 are shown in Fig. 1a.
When the crystalline sample was heated, the endothermic peak due to melting was
observed at 323ꢁC. When the melted sample was cooled down and heated again,
the glass-transition phenomenon was observed at 147ꢁC and on further heating no
peaks due to crystallisation and melting appeared. The crystalline sample of 8 dem-
onstrated a similar behaviour. It melts on first heating at 111ꢁC and forms a glass
(Tg ¼ 48ꢁC) upon cooling.
Compounds 5–7 were obtained as amorphous materials as confirmed by DSC.
As an example, the DSC thermograms of 5 are shown in Fig. 1b. When a sample of
5 was heated Tg was observed at 66ꢁC, and no peaks due to crystallisation and
melting appeared. Cooling down and repeated heating revealed the glass transition
again only.
UV absorption and emission spectra of dilute solutions of 4–9 were recorded.
Selected spectra are presented in Fig. 2. For comparison the corresponding spectra
of 9-ethylcarbazole are given. The carbazole trimers 4 and 5 as well as the series of

1056
V. Peciuraite et al.
Fig. 1. DSC curves of 4 (a) and 5 (b); heating rate 10ꢁC=min
di(9-alkylcarbazol-3-yl)phenylamines 6–9 exhibit very similar UV and fluores-
cence spectra, therefore only the spectra of 5 and 6 are shown. The lowest energy
absorption maxima of these compounds exhibit bathochromic shifts with respect to
the corresponding absorption maxima of 9-ethylcarbazole. This circumstance influ-
ences fluorescence spectra, which are considerably red-shifted with respect of the
fluorescence spectrum of 9-ethylcarbazole. The emission maxima of 4–9 appear in
the region from 448 to 466 nm.
The red shifts of UVabsorption and emission, which are interpreted as extended
conjugation, stabilizing the oxidized state of the compounds, are expected to influ-
ence the values of ionization potentials (Ip).
The electron photoemission spectra of 5 and 6 are presented in Fig. 3. For
comparison the spectrum of poly(vinylcarbazole) (PVK) containing unsubstituted
carbazole rings as chromophores is shown. The values of Ip are 4.9 for 4 and 5, ca.
5.0 for 6–9, and 5.77 eV for PVK. Thus, the Ips of the newly synthesized com-
pounds are much lower than those of PVK and other polymers having electroni-
cally isolated carbazole electrophores [20].
Holes would be easily injected into the layers of compounds 4–9 from a con-
ductive anode or charge generation layer with a work function or Ip close to 5.0 eV.
The Ip values for charge generation materials, including those widely used in
electrophotographic photoreceptors, e.g. titanyl phthalocyanines [21, 22], perylene

Electroactive Amorphous Materials for Optoelectronics
1057
Fig. 2. UV absorption (a) and PL emission (b) spectra of dilute THF solutions of 5 (dotted line), 6
(dashed line), and 9-ethylcarbazole (solid line)
Fig. 3. The electron photoemission spectra of 5, 6, and PVK
pigments [23], and bisazo pigments [24] are in the range of 5.1–5.6 eV. The Ip
values of 4–9 are very close to that of indium-tin oxide (4.8 eV), which is widely
used as an anode in electroluminescent devices [25]. The injection barrier of

1058
V. Peciuraite et al.
holes from the electrode into the layers of the materials synthesized would be only
0.1–0.2 eV.
Compounds 6 and 7 having the best solubility in PC-Z solutions were used for
charge transport studies. Xerographic time of flight (XTOF) measurements were
used to characterize the magnitudes of the hole drift mobility (ꢀh) for the com-
pounds molecularly dispersed in PC-Z. Transit time for the samples with the
transporting materials was determined by a kink on the dU=dt transient in double
logarithmic scale (Fig. 4).
The room temperature ꢀh showed a linear dependence on the square root of the
electric field (Fig. 5). Such dependence is observed for the majority of non-crystal-
line organic systems and is attributed to the effects of disorder on the charge
transport [26]. Solid solutions of 6 or 7 in PC-Z (50%) demonstrated hole drift
mobility values reaching 2 ꢂ 10ꢃ6 cm2=Vs at 8.1 ꢂ 105 V=cm.
Fig. 4. XTOF transients ot 7 doped in PC-Z (50%); insert shows one of the transient curves in linear plot
Fig. 5. The electric field dependencies of the hole drift mobility in charge transport layers of 6 and 7
doped in PC-Z

Electroactive Amorphous Materials for Optoelectronics
1059
In conclusion, we have synthesized a series of di(9-alkylcarbazol-3-yl)arylamine
based compounds which exhibit high thermal stability and form amorphous films
with glass transition temperatures in the range of 40–147ꢁC. Some of the com-
pounds were converted into reactive derivatives, which are potential components
for the preparation of insoluble electroactive layers. The values of ionisation poten-
tials (4.9–5.0 eV) and the results of hole drift mobility studies have shown that
these compounds are potential materials for electrophotographic photoreceptors
and multilayer electroluminescent devices.
Materials and Methods
1H NMR spectra were recorded using the Varian Unity Inova and JOEL JNM-FX
100 apparatuses. Mass spectra were obtained on a Waters ZQ 2000 spectrometer.
FTIR spectra were recorded using Perkin Elmer FT-IR System. UV spectra were
measured with a Spectronic GenesysTM 8 spectrometer. Fluorescence spectra were
recorded with a MPF-4 spectrometer. DSC measurements were carried out using a
Perkin Elmer Pyris Diamond calorimeter. TGA was performed on a Netzsch STA
409. The TGA and DSC curves were recorded in a N2 atmosphere at a heating rate
of 10ꢁC=min.
The ionisation potentials of the layers of 4–9 were measured by the electron
photoemission method in air described earlier [27, 28].
The hole drift mobility was measured by the xerographic time of flight technique
[29, 30]. The samples for the charge carrier mobility measurements were prepared
by casting solutions of the mixtures of 6 and 7 with bisphenol Z polycarbonate
(PC-Z) in weight proportion 1:1 on polyester films with Al layer. The thickness of
the charge transport layers varied in the range of 4–10 ꢀm. Positive corona char-
ging created electric field in the sample. Charge carriers were generated at the layer
surface by illumination with pulses of UV flash (pulse duration was 1 ꢀs, strong
absorption). The layer surface potential decrease as a result of pulse illumination
was up to 2–5% of the initial potential before illumination. The capacitance probe
measured the rate of the surface potential decrease dU=dt. The transit time tt was
determined by the kink on the curve of the dU=dt transient in double logarithmic
scale. The drift mobility was calculated by the formula ꢀh ¼ d2=U0tt, where d is the
layer thickness and U0 is the surface potential at the moment of illumination.
9H-Carbazole, 3-amino-9-ethylcarbazole, 4-anisidine, 4-toluidine, alkyliodides,
and epichlorohydrin were purchased from Aldrich and used as received. 3-Iodo-
9H-carbazole (1) was obtained by a modified procedure of Tucker [31]. 3-Iodo-9-
alkylcarbazoles 2 and 3 were prepared by alkylation of 1 in the presence of a phase
transfer catalyst [32].
N,N,N-Tris(9-ethylcarbazol-3-yl)amine (4, C43H39N4)
A mixture of 2 g (10.2 mmol) 3-amino-9-ethylcarbazole, 15.6 g (50.8 mmol) 3-iodo-
9-ethylcarbazole (2), powdered K2CO3 (11.2 g, 81.5 mmol), Cu powder (2.64 g,
40.9 mmol), and 18-crown-6 (0.53 g, 2 mmol) were refluxed in 20 cm3 o-dichloro-
benzene under N2 for 24 h. Cu and inorganic salts were filtered and the solvent
was removed by distillation. The crude product was purified by silica gel column

1060
V. Peciuraite et al.
chromatography using ethyl acetate=n-hexane (v=v 1=4) as eluent. The product was
crystallized from the eluent to yield 1.2 g of yellow crystals (mp 323ꢁC). IR (KBr):
ꢁꢀ¼ 3050 (CHar), 2973, 2981, 2890 (CHaliphatic), 1627, 1601, 1482, 1470, 1460
1
(C¼Car) cmꢃ1; MS (70 eV): m=z ¼ 597 (Mþ), 403; H NMR (300 MHz, CDCl3):
ꢂ ¼ 1.49 (t, 3CH3, J ¼ 4.0 Hz), 4.44 (q, 3CH2–N, J ¼ 8.0 Hz), 7.25–8.43 (m, 21CHar)
ppm.
N-(9-Ethylcarbazol-3-yl)-N,N-di[9-(2-ethylhexyl)carbazol-3-yl]amine
(5, C54H60N4)
Prepared from 1 g (2.6 mmol) 3-amino-9-ethylcarbazole and 9.9 g (13.2 mmol) 3-
iodo-9-(2-ethylhexyl)carbazole (3) using powdered K2CO3 (5.4 g, 21.1 mmol), Cu
powder (1.23 g, 19.4 mmol), and 18-crown-6 (0.26 g, 1 mmol) in the same way as
for 4. The compound was purified by column chromatography with silica gel using
ethyl acetate=n-hexane (v=v 1=5) as eluent. Yield 1 g amorphous yellow powder. IR
(KBr): ꢁꢀ¼ 3049 (CHar), 2957, 2928, 2871, 2858 (CHaliphatic), 1627, 1601, 1464,
1458 (C¼Car) cmꢃ1; MS (70 eV): m=z ¼ 765 (Mþ), 113, 167; 1H NMR (300 MHz,
DMSO-d6): ꢂ ¼ 0.78 (t, 2CH3, J ¼ 6.6 Hz), 0.86 (t, 2CH3, J ¼ 7.0 Hz), 1.17 (t, CH3,
J ¼ 4.8 Hz), 1.20–1.33 (m, 8CH2), 1.92–2.00 (m, 2CH), 2.51–2.53 (m, 2CH2), 4.19
(d, 2CH2–N, J ¼ 7.3 Hz), 4.38 (q, CH2–N, J ¼ 7.0 Hz), 6.97–7.84 (m, 21CHar) ppm.
N-(4-Methylphenyl)-N,N-di[9-(2-ethylhexyl)carbazol-3-yl]amine (6, C47H55N3)
A mixture of 1 g (9.45 mmol) p-toluidine, 11.5 g (28.34 mmol) 3-iodo-9-(2-ethyl-
hexyl)carbazole (3), powdered K2CO3 (10.4 g, 75.6 mmol), Cu powder (2.4 g,
37.8 mmol), and 18-crown-6 (0.5 g, 1.9 mmol) were refluxed in 10 cm3 o-dichlor-
obenzene under N2 for 20 h. Cu and inorganic salts were filtered off and the solvent
was removed by distillation. The residual product was purified by silica gel column
chromatography using CHCl3=n-hexane (v=v 1=5) as eluent. After separation the
product was dissolved in THF and precipitated into methanol to provide 3 g of an
amorphous yellow powder. IR (KBr): ꢁꢀ¼ 3051, 3023 (CHar), 2957, 2928, 2871,
2857 (CHaliphatic), 1628, 1611, 1486, 1465 (C¼Car) cmꢃ1; MS (70 eV): m=z ¼ 662
1
(Mþ); H NMR (300 MHz, CDCl3): ꢂ ¼ 0.93 (t, 2CH3, J ¼ 6.8 Hz), 0.99 (t, 2CH3,
J ¼ 7.3 Hz), 1.30–1.50 (m, 10CH2), 2.09–2.16 (m, 2CH), 2.22 (s, CH3), 7.18–7.99
(m, 18CHar) ppm.
N-(4-Methoxyphenyl)-N,N-di[9-(2-ethylhexyl)carbazol-3-yl]amine
(7, C47H55N3O)
Prepared from 2.6 g (20.78 mmol) p-anisidine and 25.25 g (62.33 mmol) 3 using
powdered K2CO3 (22.9 g, 166.2 mmol), Cu powder (5.2 g, 83.1 mmol), and 18-
crown-6 (1.1 g, 4.2 mmol) in the same way as for 6. The compound was purified
by column chromatography with silica gel using ethyl acetate=n-hexane (v=v 1=5)
as eluent. After separation the product was dissolved in THF and precipitated into
methanol to provide 6 g of an amorphous yellow powder. IR (KBr): ꢁꢀ¼ 3049
(CHar), 2956, 2928, 2870, 2857 (CHaliphatic), 1627, 1601, 1482, 1465 (C¼Car),
1
1239 (C–O–C) cmꢃ1; MS (70 eV): m=z ¼ 678 (Mþ), 113; H NMR (300 MHz,

Electroactive Amorphous Materials for Optoelectronics
1061
DMSO-d6): ꢂ ¼ 0.77 (t, 2CH3, J ¼ 7.0 Hz), 0.85 (t, 2CH3, J ¼ 7.0 Hz), 1.16–1.34
(m, 8CH2), 1.92–2.0 (m, 2CH), 3.69 (s, O–CH3), 4.17–4.19 (d, 2CH2–N,
J ¼ 5.9 Hz), 6.78–7.90 (m, 18CHar) ppm.
N-(4-Hydroxyphenyl)-N,N-di[9-(2-ethylhexyl)carbazol-3-yl]amine
(8, C46H53N3O)
N-(4-Methoxyphenyl)-N,N-di[9-(2-ethylhexyl)carbazol-3-yl]amine (7) (3 g,
4.4 mmol) was dissolved in 27 cm3 dry CH2Cl2 at room temperature and cooled to
ꢃ80ꢁC under N2. Boron tribromide (22 cm3 of 1 M CH2Cl2 solution, 22.1 mmol)
was added slowly to the mixture and it was stirred at ꢃ80ꢁC for 30 min and at room
temperature for 12 h. The reaction mixture was poured into H2O and the product
was extracted with CHCl3. The organic phase was dried (Na2SO4), filtered, and
concentrated under vacuum. The crude product was purified by silica gel column
chromatography with silica gel using ethyl acetate=n-hexane (v=v 1=10) as eluent.
The product 8 was crystallized from the eluent to yield 1.16 g of green crystals
(mp 111ꢁC). IR (KBr): ꢁꢀ¼ 3502 (OH), 3049 (CHar), 2957, 2927, 2870, 2858
(CHaliphatic), 1627, 1600, 1483, 1465 (C¼Car) cmꢃ1; MS (70 eV): m=z ¼ 664
1
(Mþ), 391, 149; H NMR (300 MHz, DMSO-d6): ꢂ ¼ 0.78 (t, 2CH3, J ¼ 7.3 Hz),
0.86 (t, 2CH3, J ¼ 7.3 Hz), 1.17–1.35 (m, 8CH2), 1.94–2.02 (m, 2CH), 3.36 (d,
2CH2, J ¼ 9 Hz), 4.20 (s, OH), 6.69–9.14 (m, 18CHar) ppm.
N-[4-(2,3-Epoxypropyl)oxyphenyl]-N,N-di[9-(2-ethylhexyl)carbazol-3-yl]amine
(9, C49H57N3O2)
N-(4-Hydroxyphenyl)-N,N-di[9-(2-ethylhexyl)carbazol-3-yl]amine (8) (0.95 g,
1.43 mmol), epichlorohydrin (4.3 cm3), and 0.2 g benzyltrimethylammonium chlor-
ide were refluxed for 1 h. The excess of epichlorohydrin was removed by distilla-
tion and the crude product was dissolved in CHCl3 and washed with H2O.
Purification of 9 was achieved by silica gel column chromatography using acet-
one=n-hexane (v=v 1=15) as eluent. Yield 0.3 g of brown oil. IR (KBr): ꢁꢀ¼ 3053
(CHar), 2959, 2929, 2870, 2872 (CHaliphatic), 1482, 1465 (C¼Car), 1217 (C–O–C)
cmꢃ1; MS (70 eV): m=z ¼ 720 (Mþ), 149; 1H NMR (300 MHz, CDCl3): ꢂ ¼ 0.91 (t,
2CH3, J ¼ 7 Hz), 0.97 (t, 2CH3, J ¼ 7.3 Hz), 1.29–1.45 (m, 10CH2), 2.05–2.15 (m,
2CH), 2.61 (d, CH2, J ¼ 8.0 Hz), 2.93–2.96 (m, 2CH), 3.41 (d, CH2, J ¼ 5.9 Hz),
6.75–8.24 (m, 18CHar) ppm.
Acknowledgements
Financial support of this research by the Lithuanian Science and Studies Foundation is gratefully
acknowledged. We thank habil. Dr. V. Gaidelis for help in ionisation potential measurements.
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