Triazine-based aromatic amines as new glass-forming charge transport materials

Article abstract of DOI:10.1080/15421400701229776

Various diarylamino-substituted 1,3,5-triazine derivatives have been synthesized by Cu catalyzed Ullmann-type arylamino-aryl coupling reaction. Full characterization of the compounds structures by mass spectrometry, IR and electronic absorption, as well as 1H NMR spectroscopy is presented. Some of the compounds represent amorphous materials with glass transition temperatures exceeding 84C and with thermal decomposition starting at temperatures330C. The electron photoemission spectra of the materials were recorded and the ionisation potentials of ca. 5.3-5.8eV were established. Time-of-flight hole drift mobility of some diarylamino-substituted triazines molecularly dispersed in bisphenol Z polycarbonate approached 10-5cm2/Vs at high electric field.

Full text of DOI:10.1080/15421400701229776

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Triazine-Based Aromatic
Amines as New Glass-Forming
Charge Transport Materials
V. Vaitkeviciene a , J. V. Grazulevicius a , V.
Peciuraite a , S. Grigalevicius a & V. Jankauskas b
a Department of Organic Technology , Kaunas
University of Technology , Kaunas, Lithuania
b Department of Solid State Electronics , Vilnius
University , Vilnius, Lithuania
Published online: 22 Sep 2010.
To cite this article: V. Vaitkeviciene , J. V. Grazulevicius , V. Peciuraite , S.
Grigalevicius & V. Jankauskas (2007) Triazine-Based Aromatic Amines as New Glass-
Forming Charge Transport Materials, Molecular Crystals and Liquid Crystals, 468:1,
141/[493]-150/[502], DOI: 10.1080/15421400701229776
To link to this article: http://dx.doi.org/10.1080/15421400701229776
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Mol. Cryst. Liq. Cryst., Vol. 468, pp. 141=[493]–150=[502], 2007
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400701229776
Triazine–Based Aromatic Amines as New Glass–Forming
Charge Transport Materials
V. Vaitkeviciene
J. V. Grazulevicius
V. Peciuraite
S. Grigalevicius
Department of Organic Technology, Kaunas University of Technology,
Kaunas, Lithuania
V. Jankauskas
Department of Solid State Electronics, Vilnius University,
Vilnius, Lithuania
Various diarylamino-substituted 1,3,5-triazine derivatives have been synthesized
by Cu catalyzed Ullmann-type arylamino-aryl coupling reaction. Full characteri-
zation of the compounds structures by mass spectrometry, IR and electronic
absorption, as well as 1H NMR spectroscopy is presented. Some of the compounds
represent amorphous materials with glass transition temperatures exceeding 84ꢀC
and with thermal decomposition starting at temperatures >330ꢀC. The electron
photoemission spectra of the materials were recorded and the ionisation potentials
of ca. 5.3–5.8 eV were established. Time-of-flight hole drift mobility of some diary-
lamino-substituted triazines molecularly dispersed in bisphenol Z polycarbonate
approached 10ꢁ5 cm2=Vs at high electric field.
Keywords: aromatic amine; glass transition; hole drift mobility; ionisation potential;
triazine
Financial support of this research by the Lithuanian Science and Studies Foundation
is gratefully acknowledged. We thank dr.habil V. Gaidelis for the help in recording of the
electron photoemission spectra.
Address correspondence to J. V. Grazulevicius, Department of Organic Technology,
Kaunas University of Technology, Radvilenu plentas 19, LT 50254, Kaunas, Lithuania.
E-mail: juozas.grazulevicius@ktu.lt
141=[493]

142=[494]
V. Vaitkeviciene et al.
1. INTRODUCTION
Low molecular weight organic compounds that readily form stable
glasses are called molecular glasses or amorphous molecular materi-
als. They show excellent processability and homogeneous properties,
and receive growing attention both in terms of academic interest
and technological applications. Arylamine and=or triazine based mol-
ecular glasses belong to charge-transporting materials and are known
for various applications such as electrophotographic photoreceptors,
organic light-emitting diodes, photovoltaic cells etc [1–3].
Tris(diphenylamino)-substituted symmetric triazine was discovered
more than a century ago [4] and since then several of its chemical
behaviours [5,6] and applications to organic light emitting diodes as
an electron transport layer [7,8] have been studied. However, most
symmetric triazines formed exciplexes or charge transfer complexes
between their and the adjacent organic layers, demonstrating that
the triazine core has strong electron-accepting characteristics. On
the other hand tris(diphenylamino)-substituted symmetric triazine
had no clear glass transition temperature (Tg) and its thin films were
found to have a polycrystalline morphology [9].
In the present study we report on the synthesis and properties of new
unsymmetrical 6-phenyl-1,3,5-triazine core having derivatives substi-
tuted with electron rich diarylamino fragments. Tris(ditolylamino)-
substituted symmetric triazine was prepared for comparison of their
properties. We expected that modifying the triazine electron affinity
by introducing electron-donating substitutes will provide compounds
with the well-tuned electronic properties required for charge transport
in electro-photographic photoreceptors and electroluminescent devices.
2. EXPERIMENTAL
2.1. Instrumentation
1H NMR spectra were recorded using 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 (FL) spectra were recorded
with a MPF-4 spectrometer. Differential scanning calorimetry (DSC)
measurements were carried out using a Perkin Elmer Pyris Diamond
calorimeter. Thermogravimetric analysis (TGA) was performed on a
Netzsch STA 409. The TGA and DSC curves were recorded in a nitro-
gen atmosphere at a heating rate of 10ꢀC=min.

Triazine Derivatives As Charge Transport Materials
143=[495]
The ionisation potentials of the layers of the compounds synthesized
were measured by the electron photoemission method in air, which
was described earlier [10,11]. The samples for the ionisation potential
measurement were prepared as described previously [12].
The charge drift mobility was measured by the xerographic time
of flight technique [13]. The samples for the charge carrier mobility
measurements were prepared by casting the mixtures in weight pro-
portion 1:1 with polycarbonate-Z (PC-Z) on polyester films with Al
layer. The thickness of the charge transport layer varied in the range
of 5–9 mm.
2.2. Materials
9H-Carbazole (1), 2,4-diamino-6-phenyl-1,3,5-triazine (4), 2,4,6-triamino-
1,3,5-triazine (5) and 4-iodotoluene (6) were purchased from Aldrich and
used as received.
3-Iodo-9H-carbazole (2) was obtained by a modified procedure of
Tucker [14]. 3-Iodo-9-(2-ethylhexyl)carbazole (3) was prepared by alky-
lation of 3-iodo-9H-carbazole (2) in the presence of a phase transfer
catalyst [15].
2,4-Di{di[9-(2-ethylhexyl)carbazol-3-yl]amino}-6-phenyl-1,3,5-triazine
(7). 0.19 g (1 mmol) of 2,4-diamino-6-phenyl-1,3,5-triazine (4), 3.24 g
(8 mmol) of 3-iodo-9-(2-ethylhexyl)carbazole (3), powdered potassium car-
bonate (2.2 g, 16 mmol), copper powder (0.51 g, 8 mmol) and 18-crown-6
(0.06g, 0.24 mmol) were stirred in o-dichlorobenzene (10 ml) at 175ꢀC
under nitrogen for 30 h. Copper powder and inorganic salts were then
removed by filtration of the hot reaction mixture. The solvent was dis-
tilled under reduced pressure and the crude product was purified by silica
gel column using hexane=acetone (vol. ratio 75:1) as an eluent. Yield of
the compound 7 was 0.5 g.
IR nmax (KBr): 3052, 2957, 2928, 2858, 1600, 1589, 1537, 1489, 1381.
1
MS (eV): m=z ¼ 1297([M þ 1]þ). H NMR (300 MHz, CDCl3), d (ppm):
8.12–7.89 (m, 6H, CHAr), 7.52–6.98 (m, 27H, CHAr), 4.16–3.72 (m, 8H,
NCH2CH), 2.11–1.87 (m, 4H, NCH2CH(CH2ꢁ)2), 1.58–1.12 (m, 32H,
ꢁCH2ꢁ), 1.02–0.73 (m, 6H, ꢁCH3).
2,4-Di[di(4-methylphenyl)amino]-6-phenyl-1,3,5-triazine (8). 1 g
(5.3 mmol) of 2,4-diamino-6-phenyl-1,3,5-triazine (4), 5.82 g (26.7 mmol)
of 4-iodotoluene (6), powdered potassium carbonate (4.5 g, 42.46 mmol),
copper powder (1.36 g, 21.4 mmol) and 18-crown-6 (0.3 g, 1.1 mmol) were
stirred in o-dichlorobenzene (10 ml) at 180ꢀC under nitrogen for 25 h.
Copper powder and inorganic salts were then removed by filtration of
the hot reaction mixture. The solvent was distilled under reduced
pressure and the crude product was purified by silica gel column

144=[496]
V. Vaitkeviciene et al.
using hexane=acetone (vol. ratio 5:1) as an eluent. Compound 7 was
crystallized from the eluent to yield 0.4 g of white crystals (m.p.:152ꢀC).
IR nmax (KBr): 3060, 3028, 2919, 2860, 1589, 1486, 1447. MS (eV):
1
m=z ¼ 548([M þ 1]þ). H NMR (300 MHz, CDCl3), d (ppm): 8.16–8.11
(m, 2H, CHAr), 7.46–7.33 (m, 3H, CHAr), 7.19 (d, 8H, CHAr, J ¼ 8.4),
7.11 (d, 8H, CHAr, J ¼ 8.4), 2.39 (s, 12H, ꢁCH3).
2,4,6-Tri[di(4-methylphenyl)amino]-1,3,5-triazine (9). 0.3 g (2.3 mmol)
of 2,4,6-triamino-1,3,5-triazine (5), 6 g (27.7 mmol) of 4-iodotoluene (6),
powdered potassium carbonate (7.7 g, 55.7 mmol), copper powder
(1.75 g, 27.6 mmol) and 18-crown-6 (0.37 g, 1.4 mmol) were stirred in
o-dichlorobenzene (10 ml) at 175ꢀC under nitrogen for 96 h. Copper
powder and inorganic salts were then removed by filtration of the hot
reaction mixture. The solvent was distilled under reduced pressure
and the crude product was purified by silica gel column using hexa-
ne=THF (vol. ratio 4:1) as an eluent. Compound 7 was crystallized from
the eluent to yield 1.3 g of white crystals (m.p.: 267ꢀC).
IR nmax (KBr): 3053, 2958, 2928, 2858, 1589, 1540, 747. MS (eV):
1
m=z ¼ 667 ([M þ 1]þ). H NMR (300 MHz, CDCl3), d (ppm): 7.04 (d,
12H, CHAr, J ¼ 8.4), 6.96 (d, 12H, CHAr, J ¼ 8.4), 2.33 (s, 18H, ꢁCH3).
3. RESULTS AND DISCUSSION
The synthetic route towards the diarylamino substituted 1,3,5-
triazines (7–9) is shown in Scheme 1. 3-Iodo-9-(2-ethylhexyl)carbazole
(3) as key starting material was synthesized from 3-iodo-9H-carbazole
(2) by alkylation with 2-ethylhexylbromide under basic conditions.
The iodo-derivative 2 was prepared from commercially available 9H-
carbazole (1) by Tucker iodination [14] with KI=KIO3 in acetic acid.
Di(diarylamino)-substituted phenyl-1,3,5-triazines (7–8) were
synthesized via the modified Ullmann coupling [16] reaction of
2,4-diamino-6-phenyl-1,3,5-triazine (4) with an excess of 3-iodo-9-
(2-ethylhexyl)carbazole (3) or 4-iodotoluene (6), respectively. Tris
(ditolylamino)-substituted derivative (9) was prepared by the Ullmann
reaction of melamine (5) with an excess of 4-iodotoluene (6). All the
newly synthesized compounds were identified by mass spectrometry,
1
IR and electronic absorption, as well as H NMR spectroscopy. The
data were found to be in good agreement with the proposed structures.
Thermal properties of the synthesized materials were examined
using DSC and TGA analysis under a nitrogen atmosphere. TGA
measurements revealed that the materials exhibit sufficient thermal
stability. The onset of decomposition for the materials was observed
at temperatures >330ꢀC. Especially high thermal stability was

Triazine Derivatives As Charge Transport Materials
145=[497]
SCHEME 1 Synthesis of materials under study.
observed for compound 7. The temperature of 5% weight loss was
detected at ca. 465ꢀC.
The derivative 7 having long alkyl chains was obtained as amorph-
ous material as confirmed by DSC in the temperature range of
0–300ꢀC. During the heating cycles the glass-transitions of the
material were observed at 85ꢀC, and no peaks due to crystallization
or melting appeared. Repeated thermal cycling of the samples (several
cooling and heating cycles) revealed only the glass transition. The dito-
lylamino substituted triazines (8–9) both were obtained as polycrys-
tals by recrystallization from solution. However the derivative 8
formed a glass when its melt sample was cooled. The DSC thermo-
grams of 8 are shown in Figure 1. When the crystalline sample was
heated, the endothermic peak due to melting was observed. The
melted substance formed a glass upon cooling, i.e., no crystallization
peak was observed in the cooling scan. When the glassy sample was
heated again, the glass-transition phenomenon was observed at
84ꢀC, and on further heating an exothermic peak due to crystallization

146=[498]
V. Vaitkeviciene et al.
FIGURE 1 DSC curves of 8. Heating rate: 10ꢀC=min.
at 137ꢀC and an endothermic peak due to melting at 152ꢀC were
observed.
UV absorption (Fig. 2) and FL spectra of dilute solutions of com-
pounds 7–9 were recorded. The electronic absorption energy of these
compounds is similar, and the kmax values are in the range of 230–
315 nm. Their FL emission maxima appear at 530, 497 and 443 nm
for 7, 8 and 9, respectively. The absorption bands of the lowest energy
and the photoluminescence band of the compound 7 having di(9-
alkylcarbazol-3-yl)amino moieties attached to phenyltriazine core are
slightly red shifted with respect of the corresponding bands of com-
pound 8, in which ditolylamino fragments are linked by phenyltria-
zine. The bathochromic shifts indicate that the 9-alkylcarbazol-3-yl
groups make a considerable effect on the conjugation of molecules of
the compounds, and a solid layer of the material 7 is expected to show
lower ionization potential (Ip) than that of material 8. The lowest
energy absorption bands of the symmetric compound 9 are similar
with respect of the corresponding bands of derivative 7, however FL
emission maxima of 9 is blue shifted to 443 nm. This observation
indicates that conjugations between the central triazine ring and the
substituents in the compound 9 are rather small due to the steric
hindrance of ditolylamino substituents.

Triazine Derivatives As Charge Transport Materials
147=[499]
FIGURE 2 UV absorption spectra of the derivatives (7–9).
The electron photoemission spectra of films of compounds 7–9 are
presented in Figure 3. The values of Ip for the films of compounds
are 5.36, 5.83 and 5.97 eV for 7, 8 and 9, respectively. As it could be
expected from UV and FL data, Ip of the compound 7 having di(9-alkyl-
carbazol-3-yl)amino substituents is lower than of the ditolylamino-
substituted triazines (8–9).
Material 7 is potential charge transporting component for electrolu-
minescent devices. Ip of amorphous layers prepared using 7 is rather
close to that of indium-tin oxide (ITO), which is widely used as anode
in the latter devices [17]. The injection barrier of holes from the elec-
trode into the layers of 7 would be ca 0.5 eV. The layers of 7–8 could
also be applied for electrophotographic photoreceptors. Holes would
be easily injected into their layers from a charge generation layer or
a conductive anode with Ip or work function close to 5.3–5.8 eV. The
Ip values for charge generation materials, including those widely used
in electro-photographic photoreceptors pigments, such as titanyl
phthalocyanines [18,19], perylene pigments [20] and bisazo pigments
[21] are in the range of 5.1–5.6 eV.
The compounds 7–8 having the suitable Ip values and good solu-
bility in PC-Z solutions were used for preliminary charge carrier
mobility studies. Time of flight measurements were used to character-
ise the magnitude of the charge drift mobility for the materials
dispersed in the latter polymeric binder (Fig. 4).

148=[500]
V. Vaitkeviciene et al.
FIGURE 3 The electron photoemission spectra of the compounds 7–9.
The experiments revealed that hole transport is characteristic for
the materials 7–8 dispersed in PC-Z. The room temperature hole drift
mobility shows a linear dependence on the square root of the electric
FIGURE 4 The electric field dependencies of the hole drift mobility in charge
transport layers of the materials 7 and 8 doped in PC-Z (50%).

Triazine Derivatives As Charge Transport Materials
149=[501]
field in films of the compositions. This characteristic dependence is
observed for the majority of non-crystalline organic systems and can
be attributed to the effects of disorder on charge transport [22]. The
PC-Z compositions with 7 or 8 demonstrated hole drift mobility values
reaching 2 ꢂ 10ꢁ6 cm2=(Vs) (E ¼ 1.1 ꢂ 106 V=cm) or 1.4 ꢂ 10ꢁ5 cm2=(Vs)
(E ¼ 1.9 ꢂ 106 V=cm), respectively, at room temperature. These values
are noticeably higher than those observed in layers of classical photocon-
ductor poly(N-vinylcarbazole) [22].
In conclusion, we have synthesized new diarylamino-substituted
1,3,5-triazine derivatives, which exhibit good thermal stability and
form amorphous films with glass transition temperatures exceeding
84ꢀC. The values of ionisation potentials (5.3 eV–5.8 eV) and the
preliminary hole drift mobility studies show that these materials
are potential components for electrophotographic photoreceptors and
electroluminescent devices.
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