Phenylethenyl-substituted triphenylamines: Efficient, easily obtainable, and inexpensive hole-transporting materials

Article abstract of DOI:10.1002/chem.201204064

Star-shaped charge-transporting materials with a triphenylamine (TPA) core and various phenylethenyl side arm(s) were obtained in a one-step synthetic procedure from commercially available and relatively inexpensive starting materials. Crystallinity, glas

Full text of DOI:10.1002/chem.201204064

DOI: 10.1002/chem.201204064
Phenylethenyl-Substituted Triphenylamines: Efficient, Easily Obtainable,
and Inexpensive Hole-Transporting Materials
Tadas Malinauskas,[a] Maryte Daskeviciene,[a] Giedre Bubniene,[a] Ieva Petrikyte,[a]
Steponas Raisys,[b] Karolis Kazlauskas,*[b] Valentas Gaidelis,[c] Vygintas Jankauskas,[c]
Robertas Maldzius,[c] Saulius Jursenas,[b] and Vytautas Getautis*[a]
Abstract: Star-shaped charge-transport-
ing materials with a triphenylamine
(TPA) core and various phenylethenyl
side arm(s) were obtained in a one-
step synthetic procedure from commer-
cially available and relatively inexpen-
sive starting materials. Crystallinity,
glass-transition temperature, size of the
p-conjugated system, energy levels, and
the way molecules pack in the solid
state can be significantly influenced by
variation of the structure of these side
arm(s). An increase in the number of
phenylethenyl side arms was found to
hinder intramolecular motions of the
TPA core, and thereby provide signifi-
cant enhancement of the fluorescence
quantum yield of the TPA derivatives
in solution. On the other hand, a larger
number of side arms facilitated exciton
migration through the dense side-arm
network formed in the solid state and,
thus, considerably reduces fluorescence
efficiency by migration-assisted nonra-
diative relaxation. This dense network
enables charges to move more rapidly
through the hole-transport material
layer, which results in very good
charge drift mobility (m up to
0.017 cm2 VÀ1 sÀ1).
Keywords: amorphous materials ·
fluorescence spectroscopy · ioniza-
tion potentials
semiconductors
· luminescence ·
Introduction
proper charge-transporting materials. Therefore, the devel-
opment of high-performance, charge-transporting materials
is a key issue for the fabrication of high-performance devi-
ces.
Electronic and optoelectronic devices that use organic mate-
rials as active elements, for example, organic light-emitting
diodes (OLED), organic photovoltaic devices (OPV), organ-
ic field-effect transistors (OFET), organic photoreceptors,
and organic photorefractive devices, have received a great
deal of attention from the standpoint of potential technolog-
ical applications as well as fundamental science.[1] Organic
material devices are attractive because they can take ad-
vantage of low molecular weights, potentially low cost, and
capability for thin-film, large-area, flexible device fabrica-
tion. All the devices described above involve charge trans-
port as an essential operation process, and hence, require
Realization of the potential of organic electronics for
simple processing requires the ability to form devices by sol-
ution deposition, preferably by using simple, inexpensive,
easily purified materials. Among the low-molecular-weight
organic compounds the highest charge-carrier mobilities in a
single crystal were measured for rubrene (m=
20 cm2 VÀ1 sÀ1).[2] Pentacene showed the highest charge-carri-
er mobility in a polycrystalline film (mꢀ5 cm2 VÀ1 sÀ1).[3]
However, pentacene is almost completely insoluble in
common organic solvents, and therefore is processed solely
by vacuum deposition. Thiophene derivatives have demon-
strated
very
good
charge
mobilities
(m=0.01–
[a] Dr. T. Malinauskas, Dr. M. Daskeviciene, Dr. G. Bubniene,
I. Petrikyte, Prof. V. Getautis
1 cm2 VÀ1 sÀ1),[4] solubility, and processability. Their main
drawback is relatively complicated and expensive multistep
syntheses that involve organotin, organolithium, or organo-
magnesium reagents, expensive palladium or nickel catalysts,
and rigorously anhydrous and oxygen-free conditions.[2,5]
Another large group of charge-transporting materials are ni-
trogen-containing triarylamines. These compounds have
long been known as hole-transport materials (HTMs) for
the active layer of organic photoreceptors, OLEDs, and or-
ganic solar cells. They show very good solubility in common
organic solvents, stability towards air and humidity,[2,5,6] and
Department of Organic Chemistry
Kaunas University of Technology
Radvilenu pl. 19, Kaunas, 50254 (Lithuania)
Fax : (+370)37-300152
E-mail: vytautas.getautis@ktu.lt
[b] S. Raisys, Dr. K. Kazlauskas, Prof. S. Jursenas
Institute of Applied Research
Vilnius University
Sauletekio 9-III, 10222, Vilnius (Lithuania)
E-mail: karolis.kazlauskas@ff.vu.lt
[c] Dr. V. Gaidelis, Dr. V. Jankauskas, Dr. R. Maldzius
Department of Solid State Electronics
Vilnius University
adequately
high
charge-carrier
mobilities
(m=
0.01 cm2 VÀ1 sÀ1 in vapor-deposited,[7] and spin-coated[8]
films). That said, however, it is important to emphasize that
mobility results for the spin-coated films were obtained by
Sauletekio 9, Vilnius, 10222 (Lithuania)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201204064.
15044
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Chem. Eur. J. 2013, 19, 15044 – 15056

FULL PAPER
using conjugated polymers of triphenylamine (TPA). Poly-
meric materials have a number of important properties, such
as ease of processing, normally very good film-forming
properties, and high flexibility of the films. However, batch-
to-batch variations in solubility, molecular weight, polydis-
persity, and purity can lead to different processing proper-
ties and performance. Small molecules contrast polymers in
that they have well-defined molecular structures and defi-
nite molecular weights without any distribution. They can
be purified by column chromatography, crystallization from
solution, or by vacuum sublimation. All these factors allow
for more reproducible fabrication protocols and a better un-
derstanding of molecular structure–property relationships.
Currently, solution-processed, low-molecular-weight triaryla-
mine derivatives demonstrate significantly lower results (m~
10À4 cm2 VÀ1 sÀ1).[9] Therefore, development of simple, inex-
pensive, easily purified, highly soluble triarylamine-based,
low-molecular-weight compounds with optimal electronic
properties remains an attractive and important goal.
lity.[10] In this paper, we explore this concept in more depth.
A number of new star-shaped charge-transporting materials
with a triphenylamine core and a varied number of different
phenylethenyl side arms have been synthesized. These hole-
transporting organic semiconductors were obtained by the
same one-step procedure reported earlier, from relatively in-
expensive, commercially available or easily synthesized
starting materials. The semiconductors can be solution pro-
AHCTUNGTREGcNNUN essed and possess comparatively high charge drift mobility
(m=0.017 cm2 VÀ1 sÀ1). Thermal, optical, and electronic
properties of the TPA compounds were thoroughly evaluat-
ed and compared as a function of the number and type of
phenylethenyl side arms.
Results and Discussion
Synthesis: The reported compounds were synthesized by
condensation of the appropriate TPA derivative and rele-
vant acetaldehyde in toluene (water generated in the course
of the reaction was removed with a Dean–Stark trap). The
mixture was heated at reflux in the presence of (Æ)-cam-
phor-10-sulfonic acid (CSA) [Scheme 1]. Although com-
pounds 3a–c have already been reported in the literature[11]
they are included in this paper as reference materials and to
Earlier, we have briefly reported the synthesis of star-
shaped charge-transporting materials with a triphenylamine
core and a variable number of diphenylethenyl side arms.
These materials were obtained by simple synthetic proce-
dures from inexpensive, commercially available starting ma-
terials, and demonstrated very good hole-transporting mobi-
Scheme 1. Synthesis of phenylethenyl-substituted triphenylamines 1–3.
Chem. Eur. J. 2013, 19, 15044 – 15056
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15045

K. Kazlauskas, V. Getautis et al.
demonstrate the scope and potential of the method. Similar
compounds that contain phenylethenyl or diphenylethenyl
fragments have been reported in the literature. However,
they are obtained either by a multistep synthetic procedure
that involves a McMurry cross-coupling reaction[12a] or by
palladium-catalyzed cross-coupling facilitated by a tailor-
made ligand.[12b]
perature. Due to their very low Tg values, 1c and 2c could
be applied as plasticizers or as HTMs in applications in
which good pore filling is needed, for example, solid-state
dye-sensitized solar cells.[13]
Monosubstituted TPA derivatives 3a–c behave similarly
to HTMs with two or three side arms. Diphenylethenyl-sub-
stituted compound 3a is fully crystalline and does not form
a stable amorphous state. Introduction of the methoxy
groups in 3b allows existence in both crystalline and amor-
phous states, whereas exchange of one phenyl ring for a
methyl group in 3c yields a fully amorphous material.
Thermal properties: Isolated derivatives 1–3 are soluble in
common organic solvents, such as THF, toluene, and chloro-
form. The thermal properties of 1–3 were determined by dif-
ferential scanning calorimetry (DSC) (Figure 1, Table 1).
The DSC measurements indicate that additional bulky phe-
Optical properties: The absorption bands of compounds 1–
3, measured in THF, are shown in Figure 2 and summarized
in Table 2.
Figure 1. DSC second-heating curves for the TPA derivatives 1a–c.
Table 1. Thermal characteristics of the synthesized derivatives.
[a]
Compound
Tg [8C]
M.p.[b] [8C]
1a
2a
3a
1b
2b
3b
1c
2c
3c
98
73
–
96
77
48
30
1
201
–
136
–
–
172
–
–
–
1
[a] Determined by DSC: scan rate=10 KminÀ1; N2 atmosphere; second
run. [b] M.p. was only detected during the first heating; the compound
vitrified on cooling to rt with scan rate=10 KminÀ1
.
nylethenyl fragments increase the glass-transition tempera-
ture (Tg) in all of the compounds investigated. Addition of
methoxy groups has a very noticeable effect on the thermal
characteristics of 3b; its melting point is increased by 368C
relative to 3a, which allows it to form stable molecular glass.
Methoxy groups also introduce more structural disorder
into 1b relative to 1a; 1b is fully amorphous. Interestingly,
however, the Tg values of 1b and 2b remain almost un-
changed. Substitution of one phenyl ring with a methyl
group lowers Tg close to (1c) or even below (2c) room tem-
Figure 2. Absorption spectra of the TPA derivatives as solutions in THF
(c~10À4 m). a) TPA (N), 1a (&), 2a ( ), 3a ( ); b) 1b (&), 2b ( ), 3b
~
*
*
~
~
*
(
); c) 1c (&), 2c ( ), 3c ( ).
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Phenylethenyl-Substituted Triphenylamines
FULL PAPER
Table 2. Optical characteristics of the TPA derivatives.
Compound
Absorption
Fluorescence
Toluene solution
PS film (0.1 wt%)
Neat film
[a]
Absmax
[nm]
e
Flmax
[nm]
FF
t[b]
Absmax
[nm]
Flmax
[nm]
FF
t[b]
Absmax
[nm]
Flmax
[nm]
FF
t[b]
A
]
[%]
[ns]
[%]
[ns]
[%]
[ns]
1a
2a
3a
1b
2b
3b
1c
2c
3c
392
6.97ꢁ104
4.45ꢁ104
4.25ꢁ104
5.77ꢁ104
3.97ꢁ104
4.02ꢁ104
4.96ꢁ104
3.26ꢁ104
2.62ꢁ104
474
469
476
462
464
477
428
424
424
10
0.27 (81)
2.34 (19)
401
394
380
383
382
377
367
357
349
461
457
453
451
451
451
424
422
422
54
53
45
55
62
62
58
63
47
1.33 (24)
2.28 (76)
395
396
377
382
384
374
366
353
349
481
480
478
480
479
479
455
445
446
4
0.07 (84)
0.40 (13)
3.00 (3)
390
370
380
381
369
361
354
343
5
0.20 (68)
0.21 (23)
2.1 (9)
0.23 (3)
1.49 (30)
2.29 (67)
8
0.08 (85)
0.35 (13)
2.68 (2)
1
0.01 (74)
1.69 (5)
5.05 (21)
0.55 (3)
1.92 (60)
2.87 (37)
11
6
0.20 (56)
0.67 (41)
2.72 (3)
13
12
5
0.08 (14)
0.51 (79)
2.19 (7)
0.19 (3)
1.68 (65)
2.63 (32)
0.10 (75)
0.46 (21)
2.82 (4)
0.03 (21)
0.47 (72)
2.63 (7)
0.80 (6)
1.97 (87)
3.39 (7)
12
18
5
0.12 (70)
0.49 (26)
2.84 (4)
0.17 (93)
4.73 (7)
0.52 (2)
1.97 (68)
3.03 (30)
0.19 (49)
0.80 (45)
3.40 (6)
48
44
16
0.74 (22)
1.42 (78)
1.89 (98)
10.2 (2)
0.10 (62)
0.56 (30)
2.05 (8)
1.25
1.06 (34)
1.80 (66)
11
23
0.15 (55)
0.52 (39)
2.03 (6)
0.61 (91)
4.55 (9)
1.66 (87)
4.05 (13)
0.30 (24)
1.06 (61)
4.08 (15)
[a] Measured in solution in THF (c~10À4 m). [b] Fluorescence decay time constant estimated at Flmax; the fractional contribution to the total fluorescence
intensity [%] is given in parentheses.
The bathochromic shift of the absorption of 2a and 3a
compared to that of TPA is attributed to extension of the
core p-conjugated system by one and two diphenylethenyl
units, respectively (Figure 2a). The maximum of the absorp-
tion spectra of 3a is shifted by about 71 nm with respect to
TPA and is located at l=370 nm. In line with expectations,
the absorption of 2a is shifted even more bathochromically,
by about 91 nm relative to TPA. The additional diphenyl-
affects the overall p-conjugated system quite noticeably; a
hypsochromic shift of around 30 nm relative to 1a–3a is ob-
served. However, TPA derivatives 1c and 2c are slightly less
bulky than their analogues 1a and 1b, and 2a and 2b, re-
spectively, therefore, it is easier for these molecules to adopt
a more planar conformation. Addition of the third side arm
in 1a and 1b had almost no effect on the position of the p–
p* transition band, however, in the case of 1c, the batho-
chromic shift is more noticeable (ꢁ7 nm).
ACHTUNGTRENNUNGethenyl fragment in 1a has a very limited effect on the over-
all p-conjugated system of the molecule; the bathochromic
shift on going from the di- to tri-substituted TPA derivative
is very small (ꢁ2 nm). However, absorption intensity in-
creases quite noticeably. This indicates that the transition is
accompanied by a larger change in the electronic charge dis-
tribution upon excitation. Addition of the methoxy groups
in 1b and 2b (Figure 2b) generally yields smaller p-conju-
gated systems (hypsochromic shift of ꢁ10 nm) than 1a and
2a, which indicates that 1b and 2b adopt less planar config-
urations. As expected, substitution of the aromatic phenyl
ring with a methyl group in compounds 1c–3c (Figure 2c)
The propeller-like structure of the TPA core with highly
À
twisted (ꢁ508) phenyl rings around the C N bonds implies
À
effective C N stretching vibrations and phenyl torsions,
which greatly affects n–p conjugation and, thus, the spectro-
scopic properties of TPA and the TPA-derived com-
pounds.[14] On the other hand, the nonplanar TPA geometry
predefines intricate molecular packing in the solid state,
which results in nontrivial optical and charge-transport
properties of the TPA compounds. Generally, the attach-
ment of phenylethenyl side arms to the phenyl groups of the
TPA core is expected to hinder phenyl vibrations/torsions
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K. Kazlauskas, V. Getautis et al.
and, consequently, by systematic variation of the number
and type of the side arms, tune the optical and charge-trans-
port properties of the compounds. To differentiate between
the intramolecular motions and intermolecular effects, the
TPA compounds were investigated in different media (solu-
tion, rigid polymer matrix, and neat film). Figure 3 displays
denced by a spectral shift to longer wavelengths, the absence
of this shift in the fluorescence spectra of the compounds in
solution in toluene indicates that planarization is improba-
ble. A similar argument rules out possible alteration of the
electron–vibron coupling with an increasing number of stil-
bene-like side arms.
In spite of the similar fluorescence spectra, fluorescence
quantum yield (FF) of the TPA compounds in dilute solu-
tions increases with the number of side arms from 16% for
compound 3c with one side arm to 48% for compound 1c
with three side arms (Figure 4a). Evidently, the enhance-
Figure 4. FF of the TPA derivatives versus number of TPA side arms
a) as a solution in toluene (c~10À5 m), b) as PS matrixes (c=0.1 wt%),
!
&
*
and c) as neat films. 1a–3a ( ); 1b–3b ( ); 1c–3c ( ).
Figure 3. Absorption and fluorescence spectra of TPA derivatives a) 3c,
b) 2c, and c) 1c that contain a varied number of phenylethenyl side arms
as solutions in toluene (c~10À5 m) [c], as PS matrixes (c=0.1 wt%)
[a], and as neat films (c). FF values indicated. Absorption spectra
of the compounds in PS matrixes and neat films are normalized to the
spectra of the compounds in solution, which are presented in absolute
values.
ment of FF is invoked by hindrance of the vibrations/tor-
sions of the phenyl groups of the TPA core by the side arms
and, thus, the torsion-activated nonradiative relaxation path-
way is considerably reduced. This result is confirmed by the
high value of FF obtained for compounds 1c–3c in rigid PS
matrixes, in which intramolecular vibrational/torsional mo-
tions of the phenyl groups are suppressed irrespective of the
presence of adjoining side arms (Figure 4b). Consequently,
FF varies insignificantly (47–63%) with respect to the
number of stilbene-like side arms attached. Note, however,
that at least one stilbene-like side arm is required to attain
such high FF values; the unsubstituted TPA molecule incor-
porated in a PS matrix exhibits a FF of only around 4%.
Fluorescence spectra of the neat films of the TPA deriva-
tives 1c–3c are broadened and redshifted relative to those
of the compounds in solution in toluene, which corresponds
with enhanced intermolecular interactions in the solid state
(Figure 4). These features are most pronounced for com-
pound 1c, which bears three side arms; this suggests that
stilbene-like side arms play a major role in the molecular
packing that leads to increased intermolecular interaction.
Unlike in solution, an increase in the number of side arms
diminishes FF values in the TPA neat films (Figure 4c). The
FF values continuously decrease from 23% for compound
3c with one side arm to 5% for compound 1c with three
side arms. Intuitively, the incorporation of more branches
the absorption and fluorescence spectra of the structurally
modified TPA derivatives with one, two, and three phenyl-
ACHTUNGTRENNUNGethenyl (stilbene-like) side arms. The details of the optical
properties of the compounds are summarized in Table 2.
In contrast to the absorption spectra dynamics, the fluo-
rescence-band maximum of the TPA derivatives 1c–3c is
found to be independent of the number of stilbene-like side
arms (lꢁ426 nm). Nearly identical spectra of the com-
pounds in solution in toluene and those of the compounds
dispersed in polystyrene (PS) matrixes point out that planar-
ACHTUNGTRENNUNGization of the noninteracting molecules 1c–3c is unlikely in
solution in the excited state.[15] The stilbene-like side arms
À
are twisted out of the plane of the C N bonds (like the
phenyl moieties in the propeller-shaped unsubstituted TPA
molecule) and the TPA derivatives can undergo planariza-
tion in solution, which is suppressed in a rigid polymer
matrix. Because the planarization is accompanied by an ex-
tension of the p-conjugated system, experimentally evi-
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Phenylethenyl-Substituted Triphenylamines
FULL PAPER
into the TPA molecule is supposed to prevent molecule ag-
gregation in the solid state and, thus, result in enhanced FF.
However, in this particular case, the stilbene-like side arms
most likely arrange in a densely overlapping pattern, which
facilitates excitation migration and migration-induced relax-
ation at nonradiative decay sites. Similar trends in the opti-
cal properties versus the number of side arms were also ob-
served for the rest of the TPA compound series 1a–3a and
1b–3b.
Introduction of a second phenyl group into the stilbene-
like side arms (compounds 1a–3a) resulted in redshifted (by
20–50 nm) absorption and fluorescence bands. Similar be-
havior was also observed in the presence of diphenylethenyl
side arms with methoxy substituents (compounds 1b–3b).
Although the extended conjugation conditioned nearly two-
fold enhanced absorbance, the additional phenyl moiety det-
rimentally affected the FF of 1a–3a and 1b–3b in solution
in toluene (Figure 4a). For the materials with one side arm,
FF dropped from 16% (3c) to 1 and 5% for 3a and 3b, re-
spectively (Figure 5). This drastic reduction of FF is attribut-
ed to induced steric hindrance from the second phenyl
arms and are roughly independent of the number of side
arms attached (Figure 4b).
Fluorescence spectra of the neat films of 1a–3a and 1b–
3b were found to be slightly redshifted relative to the spec-
tra of the same compounds in toluene solution or PS matrix-
es. However, the spectral shape remained unchanged and
contained no traces of vibronic structure, which verified an
amorphous character of the films formed. A monotonous
decrease of FF (from 11 to 4%, from 18 to 6%, and from
23 to 5% in the neat films of 1a–3a, 1b–3b, and 1c–3c, re-
spectively) with an increasing number of side arms high-
lighted the key role of the diphenylethenyl fragments in ex-
citon migration and migration-induced nonradiative deacti-
vation at quenching sites (distortions, defects, etc.; Fig-
ure 4c).
Generally, the fluorescence transients of the TPA deriva-
tives 1c–3c (Figure 6), as well as of 1a–3a and 1b-3b, dem-
onstrate multi-exponential decays that result from various
molecular conformations, which are feasible due to the
Figure 5. Absorption and fluorescence spectra of TPA derivatives with
one side arm in solution in toluene (c~10À5 m). 3a (c), 3b (a), and
3c (c); FF values indicated.
group, which activates phenyl vibrations/torsions and, thus,
promotes radiationless decay.[16] Note that the reduction of
FF for compounds 1b–3b is smaller than for 1a–3a because
the phenyl motions are somewhat impeded by the presence
of the methoxy moieties. Likewise, for 1c–3c, an increased
number of side arms led to increased FF (Figure 4a) and
caused greater redshift of the absorption bands relative to
the fluorescence bands. However, in contrast to compounds
1c–3c, the fluorescence spectra of compounds 1a–3a and
1b–3b dispersed in PS matrixes are blueshifted relative to
the spectra of the compounds in solution in toluene (see
Table 2). This result illustrates that the additional phenyl
group in the diphenylethenyl side arms provokes planariza-
tion of the molecules in the excited state.[15] Incorporation
of compounds 1a–3a and 1b–3b in PS matrixes strongly di-
minished the nonradiative relaxation rate by suppressing
phenyl torsions and, therefore, boosted FF up to 45–54 and
55–62%, respectively. These FF values are very similar to
those obtained for compounds 1c–3c with stilbene-like side
Figure 6. Fluorescence transients of TPA derivatives a) 3c, b) 2c, and
c) 1c that contain a varied number of phenylethenyl side arms as a solu-
tion in toluene (c~10À5 m; empty circles), as PS matrixes (c=0.1 wt%;
grey filled circles), and as neat films (black filled circles). Lines mark
single and double exponential fits to the experimental data. Fluorescence
decay time constants indicated.
labile phenyl groups. For a quantitative comparison of the
decay rates of multi-exponential transients, the dominant
decay-time component (t) with the largest fractional intensi-
ty was used. The transients reflected fairly well the tenden-
cies of FF evaluated in different media (dilute solution in
toluene, PS film, or neat film) as a function of the number
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K. Kazlauskas, V. Getautis et al.
and type of phenylethenyl side arms. An increase in the
number of side arms caused prolonged t in solution media,
in agreement with the enhanced FF values obtained, which
implies suppression of the phenyl torsions in the TPA core.
The increase of t with the number of side arms was ob-
served irrespectively of the type of phenylethenyl side arms
attached, though the presence of a second phenyl group in
the diphenylethenyl moieties (compounds 1a–3a and 1b–
3b) strongly activated phenyl torsions and, thus, torsion-in-
duced nonradiative decay, thereby t was reduced consider-
crease of FF when the compound contains a greater number
of side arms, observed in the neat films due to enhanced ex-
citon migration (Figure 4c).
From a practical and fundamental point of view it is im-
portant to investigate fluorescence concentration quenching
of the TPA derivatives and elucidate the impact of the
number of phenylethenyl side arms on the quenching. Con-
centration quenching of the compounds was explored by dis-
persion in a rigid transparent PS matrix and evaluating FF
dynamics versus concentration in the range c=0.1–100 wt%
(Figure 8). At concentrations up to 1 wt%, the quenching
ACHTUNGTRENNUNGably. The reduction of t from 0.6 (3c) to 0.1 ns (3a) for com-
pounds that contain one side arm is evidenced in Figure 7.
Initial fluorescence decay of compounds 3a and 3b, which
Figure 7. Fluorescence transients of TPA derivatives 3a (empty circles),
3b (filled grey circles), and 3c (filled black circles) with a single side arm
(c~10À5 m in toluene). IRF=instrument-response function. Lines mark
double exponential fits to the experimental data. Fluorescence decay
time constants indicated.
Figure 8. Normalized FF values of the TPA derivatives as a function of
*
*
*
concentration in PS matrix. a) 1c (N), 2c ( ), 3c ( ); b) 1a (N), 2a ( ),
bear sterically hindered phenyl groups, is so rapid that it ap-
proaches the temporal resolution (0.07 ns) of our experi-
mental setup. On another hand, suppression of phenyl tor-
sions by incorporation of the TPA derivatives in rigid PS
matrixes significantly increases t up to 1.7–2.4 ns, regardless
of the number and type of side arms used (see Figure 6 and
Table 2). Again, this behavior perfectly correlates with the
enhanced FF observed upon incorporation of the derivatives
in PS matrixes and also with the independence of this en-
hancement on the type and number of phenylethenyl side
arms (Figure 4b).
In the neat films of the TPA compounds that contain both
two and three side arms fluorescence transients become
very fast (they are close to the instrument-response function
(IRF) of the experimental setup). As found in dilute
(0.1 wt%) PS matrixes, phenyl torsions are severely inhibit-
ed in the neat films of the compounds, however, as opposed
to the polymer matrix, the condensed phase enables excita-
tion migration, which facilitates excitation quenching at non-
radiative decay sites. This is the main cause of the fast excit-
ed-state decay observed in the neat film samples. Moreover,
a general tendency for t to decrease with an increase in the
number of phenylethenyl side arms, irrespective of their
type, indicates a more favorable arrangement of the mole-
cules with more arms that result in more efficient excitation
migration. The latter result is in agreement with the de-
*
*
*
3a ( ); c) 1b (N), 2b ( ), 3b ( ).
was negligible, whereas a further increase of the concentra-
tion lowered the FF by up to ten times. Clearly, the quench-
ing became more pronounced with an increased number of
phenylethenyl side arms, regardless of the type of side arm
linked to the TPA core. These findings support the data that
demonstrate the most remarkable drop of FF and t in the
neat films of TPA compounds that feature the largest
number of side arms (1a–1c), and proves a significance of
the phenylethenyl side arms in the enhancement of exciton
migration.
Charge-transport properties: Charge-transport properties of
the synthesized TPA derivatives 1–3 were studied by the xe-
rographic time-of-flight (XTOF) technique[17] (Figure 9).
The values of the parameters that define charge mobility
[zero field mobility (m0), Poole–Frenkel parameter (a)] and
the mobility (mh) at the electric field strength of 6.4ꢁ
105 VcmÀ1 are given in Table 3 for compounds 1–3.
The room temperature hole drift mobility of the as-spun
film of the trisubstituted TPA derivative 1a was mh =
0.017 cm2 VÀ1 sÀ1 at strong electric fields and is approximate-
ly four-times higher than that of the monosubstituted ana-
logue 3a. Compound 2a demonstrated similar performance
15050
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Chem. Eur. J. 2013, 19, 15044 – 15056

Phenylethenyl-Substituted Triphenylamines
FULL PAPER
of the methoxy groups. Even though compounds 1c and 2c
possess the smallest p-conjugated systems of all the HTMs
investigated, a streamlined structure, which is less bulky
than 1a and 2a or 1b and 2b, allows easier arrangement of
the molecules into conformations more favorable for charge
transport. As a result, hole mobilities of 1c and 2c reach
mh =0.013 cm2 VÀ1 sÀ1 and are very similar to those demon-
strated by 1a and 2a.
It has been recognized that solution-processed materials
offer a better ease of processing than vacuum-deposited ma-
terials, but generally have lower carrier mobility. The lower
mobility is generally attributed to the lower structural order
that reduces intermolecular p-orbital overlap. However,
compared with other low-molecular-weight triarylamines
(Table 4), diphenylethenyl-substituted TPA derivatives 1a,
1c, 2a, and 2c demonstrate relatively high hole drift mobili-
ty that is an order of magnitude higher than very well
known HTM, N,Nꢂ-bis(3-methylphenyl)-N,Nꢂ-diphenylbenzi-
dine (TPD). Note that these results were obtained from so-
ACHUTNGRENlNUG uACTHUGNRTENNUtG ion-processed films, whereas the majority of the best-per-
forming low-molecular-weight triarylamines were vacuum
deposited.
The electronic properties of molecular-transport materials
depend on the manner in which the molecules are organized
in the solid state.[19] In particular, p-stacking, although not a
guarantee of good wavefunction overlap,[20] is a feature of
many high-mobility materials.[21]
Analysis of the optical properties of the investigated ma-
terials indicates that side arms play a major role in the mo-
lecular packing that leads to increased intermolecular inter-
action. Intuitively, the incorporation of more branches into
the TPA molecule is supposed to prevent molecule aggrega-
tion in the solid state and, thus, result in enhanced FF, how-
ever, in this particular case, the stilbene-like side arms most
likely arrange in a densely overlapping pattern.
Figure 9. Electric-field dependencies of the hole drift mobilities (m) in
~
&
*
charge-transport layers of TPA derivatives. a) 1a ( ), 2a ( ), 3a ( );
~
~
&
&
*
*
b) 1b ( ), 2b ( ), 3b ( ); c) 1c ( ), 2c ( ), 3c ( ).
to 1a with mh as high as 0.014 cm2 VÀ1 sÀ1. Hole mobility in
methoxy-substituted HTMs 1b and 2b was two orders of
magnitude lower than in the corresponding unsubstituted
analogues 1a and 2a. Most likely the manner in which mole-
cules organize in the layer is altered unfavorably by addition
Energy spectra: To elucidate the energetic conditions for
energy and electron transfer in dilute solutions, the EHOMO
and ELUMO values were determined by cyclic voltammetry
(CV) [Table 3]. These values do not represent any absolute
solid-state or gas-phase ionization potentials, but can be
Table 3. EHOMO, ELUMO, Ip, bandgap energies, and hole mobility data for 1–3.[a]
[c]
[d]
[e]
[f]
Compound
Eogpt[b]
[eV]
EHOMO
[V vs. NHE]
ELUMO
[V vs. NHE]
Ip
d
[mm]
m0
mh
a
G
G
[eV]
[cm2 VÀ1 sÀ1
]
[cm2 VÀ1 sÀ1
]
[cm1/2 VÀ1/2
]
1a
2a
3a
1b
2b
3b
1c
2c
3c
2.76
2.78
2.88
2.81
2.82
2.92
2.97
2.98
3.06
0.92
0.92
0.94
0.86
0.84
0.85
0.93
0.93
0.94
À1.84
À1.86
À1.94
À1.95
À1.98
À2.07
À2.04
À2.05
À2.12
5.41
5.35
5.43
5.27
5.23
5.35
5.45
5.47
5.45
6.3
2.9
4.9
3.4
4.8
2.3
6
3ꢁ10À3
0.017
0.014
0.0022
0.0025
0.0045
0.0053
0.0041
0.0045
0.0027
0.0027
0.0032
2ꢁ10À3
5.6ꢁ10À4
5.6ꢁ10À6
2.2ꢁ10À5
3ꢁ10À5
4.3ꢁ10À4
3.9ꢁ10À4
6ꢁ10À4
1.1ꢁ10À3
0.013
1.5ꢁ10À3
1.4ꢁ10À3
1.1ꢁ10À3
6.4
6.3
0.012
0.014
[a] The CV measurements were carried out at a glassy carbon electrode in CH2Cl2 that contained 0.1m tetrabutylammonium hexafluorophosphate as the
electrolyte and Ag/AgNO3 as the reference electrode. Each measurement was calibrated with ferrocene (Fc). Potentials measured versus Fc+/Fc were
converted to the normal hydrogen electrode (NHE) by addition of +0.63 V. [b] The optical bandgaps (Eogpt) were estimated from the edges of the elec-
tronic absorption spectra. [c] ELUMO =EHOMOÀEogpt. [d] Ionization potential was measured by the photoemission-in-air method from films. [e] Mobility
value at zero field strength. [f] Mobility value at 6.4ꢁ105 VcmÀ1 field strength.
Chem. Eur. J. 2013, 19, 15044 – 15056
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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15051

K. Kazlauskas, V. Getautis et al.
used to compare different compounds relative to one anoth-
er. The cyclic voltammograms of the synthesized compounds
in dichloromethane show one quasireversible oxidation
couple and no reduction waves (see the Supporting Informa-
tion; Figure S1 and Table S1).
The additional side arms in 1a and 2a did not influence
energy levels much; the energy levels remained quite similar
to those demonstrated by monosubstituted TPA 3a, and
only a very slight decrease in EHOMO was observed. Very
similar EHOMO values were also determined for 1c–3c, al-
though substitution of the
Table 4. Hole mobility of various low-molecular-weight triarylamines.
phenyl group with
a methyl
group decreased ELUMO by
about 0.2 V. The methoxy
groups in 1b–3b promote a de-
crease of EHOMO by about 0.1 V
relative to 1a–3a. However, as
with all other examples, there is
almost no difference between
the EHOMO levels in mono-, di-,
and trisubstituted TPA deriva-
tives. Additional phenylethenyl
groups have a more substantial
effect on ELUMO, which increases
by approximately 0.1 V with ad-
dition of the second fragment.
When an organic material is
considered for use in optoelec-
tronic applications it is impor-
tant to have an understanding
of its solid-state ionization po-
tentials (Ip). This understanding
can help with identification of
suitable partner organic trans-
port materials and inorganic
electrode materials. The ioniza-
tion potential was measured by
the ꢃelectron photoemission in
airꢂ method (Figure 10) and the
results are presented in Table 3;
the measurement error is evalu-
ated as 0.03 eV.[22]
Compound
m
Layer-formation method
Reference
[cm2 VÀ1 sÀ1
]
1ꢁ10À2
vacuum deposition
[7]
1.1ꢁ10À3
vacuum deposition
vacuum deposition
[18a]
[18b]
1ꢁ10À3
6.9ꢁ10À3
vacuum deposition
[18c]
3ꢁ10À5
vacuum deposition
[18d]
Usually the photoemission
experiments are carried out in a
vacuum; more specifically, high
vacuum is one of the main re-
quirements for these measure-
ments. If the vacuum is not
high enough sample surface ox-
idation and gas adsorption in-
fluence the measurements. In
our case, however, the organic
materials investigated are stable
enough to oxygen that the
measurements may be carried
out in the air. The measured Ip
values are about 0.3 eV higher
than the EHOMO levels found in
the CV experiments. The differ-
ence may result from different
measurement techniques and
conditions (solution samples for
1ꢁ10À4
spin coating
[9]
3ꢁ10À4
spin coating
[9]
15052
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Chem. Eur. J. 2013, 19, 15044 – 15056

Phenylethenyl-Substituted Triphenylamines
Table 4. (Continued)
FULL PAPER
Organic photoreceptor device
fabrication and parameters:
HTMs with three side arms,
1a–c, were used in the con-
struction of organic photorecep-
tor devices (Figure 11). The de-
vices consisted of a conducting
Al layer, 0.5 mm thick charge-
generation layer (CGL) com-
posed of a 2:1 mass proportion
of titanyl phthalocyanine and
polyvinyl butyral BX-1, and
Compound
m
Layer-formation method
Reference
[cm2 VÀ1 sÀ1
]
5ꢁ10À5
vacuum deposition
[18e]
charge-transporting
layer
(CTL), which consisted of a 1:1
CV and solid-film samples for the photoemission method).
Compounds 1c–3c demonstrate the highest Ip values of all
three groups investigated. Interestingly, 1a–3a have similar
EHOMO levels in solution, however, their ionization potentials
are slightly lower. Most likely this is caused by differences
in the way these molecules pack in the solid state. The pres-
ence of methoxy groups in 1b–3b lowers the Ip by about
0.1 eV relative to 1a–3a.
mass proportion mixtures of the transporting material and
polycarbonate Z-200 (PC-Z).
Figure 11. Structure of the organic photoreceptor devices.
Characteristics of the devices constructed from investigat-
ed materials 1a–1c were compared with well-known and ef-
ficient HTM, TPD (Table 5). Photosensitivity (S) of the syn-
thesized TPA derivatives with phenylethenyl side arms is
very similar or better than that of TPD. Relative residual
potential (UR/U0) is lowest for 1a, whereas the device creat-
ed with 1b demonstrates similar results to one constructed
with TPD. In the case of 1c, despite higher mobility than
1b, the remaining residual potential is larger; this could
likely be attributed to a higher rate of trap formation.
Figure 12 shows the spectral distribution of the photosensi-
tivity of an organic photoreceptor constructed from 1b or
TPD. Both compositions exhibit good xerographic photosen-
sitivity across most of the visible-light region of the spec-
trum, although somewhat better results are observed for 1b.
Similar tendencies to those observed with residual poten-
tial are noted on analysis of the potential decay curves after
illumination with a l=780 nm light pulse (Figure 13). The
potential drops rapidly in the devices that contain HTMs
with diphenylethenyl- and methoxy-substituted side arms
(1a and 1b), whereas 1c performs slightly worse for the
same reasons explained above.
Despite considerable reduction of the HTM concentration
in the CTL layer, the hole mobility remains very good. In
all cases it is better than the mh of TPD under identical con-
ditions (Figure 14, Table 5). The best results (mh =7.5ꢁ
10À4 cm2 VÀ1 sÀ1 at strong electric fields) are observed for the
device constructed from TPA derivative 1a with three di-
phenylethenyl side arms.
~
*
Figure 10. Photoemission-in-air spectra of 1–3. a) 1a ( ), 2a ( ), 3a (&);
~
~
&
*
*
b) 1b ( ), 2b ( ), 3b (&); c) 1c ( ), 2c ( ), 3c ( )
Chem. Eur. J. 2013, 19, 15044 – 15056
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
15053

K. Kazlauskas, V. Getautis et al.
Table 5. Characteristics of the organic photoreceptor devices that contain 1a–c.
[b]
Compound
d
[mm]
U0
[V]
S[a]
[m2 JÀ1
UR
[V]
UR/U0
t1/2
[s]
m0
m[c]
[cm2 VÀ1 sÀ1
a
G
]
[cm2 VÀ1 sÀ1
]
]
[cm1/2 VÀ1/2
]
1a
1b
1c
TPD
7.2
9.9
7.5
7.2
À348
À478
À495
À461
478
396
384
368
À6.6
À14.3
À30
0.019
0.03
0.061
0.029
0.082
0.094
0.104
0.102
1.4ꢁ10À5
7.2ꢁ10À7
3.1ꢁ10À5
4.6ꢁ10À6
7.5ꢁ10À4
1.8ꢁ10À4
5.3ꢁ10À4
1.3ꢁ10À4
0.005
0.0069
0.0035
0.0042
À13.2
[a] l=780 nm. [b] Mobility value at zero field strength. [c] Mobility value at 6.4ꢁ105 VcmÀ1 field strength.
Figure 14. The mobility field dependencies of the electrophotographic
Figure 12. Electrophotographic spectral photosensitivity of 1b (c) and
TPD (a).
~
!
^
samples. TPD ( ), 1a ( ), 1b ( ), 1c (N).
core in solution, whereas in the solid state it facilitated exci-
ton migration through the dense side-arm network formed.
In turn, this dense network enables charges to move more
rapidly through the HTM layer and results in very good
charge drift mobilities of up to 0.017 cm2 V À1 sÀ1 at strong
electric fields. Organic photoreceptor devices were con-
structed using HTMs 1a–1c. The test results demonstrate
that these materials outperform well-known HTM, TPD.
Due to their low Tg values, 1c and 2c could be used as
HTMs in solid-state dye-sensitized solar cells, in which good
pore filling is needed. Out of the HTMs investigated, com-
pounds 1a and 1c are especially promising candidates for
practical applications because they can be handled in air, re-
quire no high-temperature annealing steps, can be solution
deposited, possess comparatively high mobility, and can be
synthesized in one step from commercially available and
cheap starting material TPA.
Figure 13. The potential decay curves of the photoreceptor devices after
1 ms of illumination with a xenon flash lamp (l=780 nm light pulse). The
exposition H=1.7 mJcmÀ2. Negative initial potential: TPD ( ) À431 V,
*
~
!
1a ( ) À383 V, 1b ( ) À443 V, 1c (&) À438 V.
Conclusion
We have demonstrated a simple one-step synthesis to obtain
solution processable star-shaped charge-transport materials
with a TPA core and a varied number of different phenyl-
Experimental Section
General: Chemicals were purchased from Sigma–Aldrich and TCI
Europe and used as received without further purification. Compounds
1a–3a[10] and bis(4-methoxyphenyl)acetaldehyde[23] were synthesized by
procedures reported previously. 1H and 13C NMR spectra were recorded
on a Varian Unity Inova (300 MHz) spectrometer at room temperature
with chemical shifts (d) reported in ppm and TMS (d=0 ppm) as an in-
ternal standard. The reactions were monitored by TLC on ALUGRAM
SIL G/UV254 plates and plates were developed with I2 or viewed under
UV light. Silica gel (grade 9385, 230–400 mesh, 60 ꢄ, Aldrich) was used
for column chromatography. Elemental analysis was performed with an
Exeter Analytical CE-440 Elemental instrument. DSC measurements
were performed with a Q10 calorimeter (TA Instruments) at a scan rate
ACHTUNGTRENNUNGethenyl side arms from commercially available and relative-
ly inexpensive starting materials. Substitutions in the phenyl-
ethenyl fragment could noticeably influence the HTMs Tg
values and tendency to crystallize. By structural modifica-
tion of the side arms, the size of the p-conjugated system,
energy levels, and the way the molecules pack in the solid
state can also be significantly influenced. Optical measure-
ments revealed that an increase in the number of side arms
effectively suppressed intramolecular motions of the TPA
15054
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Chem. Eur. J. 2013, 19, 15044 – 15056

Phenylethenyl-Substituted Triphenylamines
FULL PAPER
of 10 KminÀ1 under a nitrogen atmosphere. The Tg values for the investi-
gated compounds were determined during the second heating scan.
by illumination with pulses of a nitrogen laser (pulse duration=2 ns, l=
337 nm). The layer surface potential decrease as a result of pulse illumi-
nation was up to 1–5% of the initial potential before illumination. The
capacitance probe that was connected to the wide-frequency band elec-
trometer measured the speed of the surface potential decrease (dU/dt).
The transit time (tt) was determined by the kink on the curve of the
dU/dt transient on a linear or double logarithmic scale. The drift mobility
was calculated by the formula m=d2/U0tt (d is the layer thickness, U0 is
the surface potential at the moment of illumination).
Optical measurements: Absorption spectra of the dilute solutions in THF
were recorded on
a UV/Vis–NIR spectrophotometer, Lambda 950
(Perkin–Elmer). Fluorescence of the investigated compounds as PS- or
neat films was initiated by a light-emitting diode (l=365 nm; Nichia
NSHU590-B) and measured by using a back-thinned charge-coupled
device (CCD) spectrophotometer PMA-11 (Hamamatsu). For these
measurements, a dilute solution of the required compound was prepared
by dissolution in spectral-grade toluene (c=1ꢁ10À5 m). PS films with con-
centrations that ranged from 0.1–100 wt% were prepared by mixing the
dissolved compound with a solution of PS in toluene at the appropriate
ratio and casting the solutions on quartz substrates under ambient condi-
tions. Drop casting from solutions in toluene (1ꢁ10À3 m) was also em-
ployed to prepare neat films of the compounds. The FF of the solutions
were estimated by comparison of the wavelength-integrated photolumi-
nescence (PL) intensity of the compound solutions to that of quinine sul-
fate in a 0.1m aqueous solution of H2SO4 (FF =53Æ2.3) as a reference.[24]
Optical densities of the reference and sample solutions were kept below
0.05 to avoid reabsorption effects. Estimated quantum yields of the com-
pound solutions were verified by using the integrated-sphere method.[25]
An integrating sphere (Sphere Optics) coupled to the CCD spectrometer
by optical fiber was also employed to measure the FF of the neat and PS
films. Fluorescence transients of the samples were measured by using a
time-correlated single-photon-counting system PicoHarp 300 (Pico-
Quant), which utilizes a semiconductor diode laser (repetition rate=
1 MHz, pulse duration=70 ps, lemission =375 nm) as an excitation source.
Construction of the organic photoreceptor: Hole-transporting material
TPD was purchased from Sigma–Aldrich. The samples of electrophoto-
graphic layers were prepared on PS film substrates with a conducting Al
layer and CGL (0.5 mm thickness) composed of 2:1 mass proportion com-
position of titanyl phthalocyanine ELA7051 (H. W. Sands Co.) and poly-
vinyl butyral BX-1 (Sekisui). The HTM layers were coated by the doctor-
blade method from the 1:1 mass proportion mixtures of the transporting
materials and polycarbonate Z-200 (Mitsubishi Gas Chemical Co.) dis-
solved in THF. After coating, the samples were heated at 808C for 1 h to
dry. The thickness of the transporting layers was approximately 8 mm.
The electrophotographic parameters of the samples were measured as
described in the literature.[26]
General synthetic procedure: The appropriate triarylamine was dissolved
in toluene (20 mLgÀ1+volume of the Dean–Stark trap), CSA (1.0 equiv)
was added, and the mixture was heated at reflux for 20 min. Afterwards,
the required phenylacetaldehyde (1.2 equiv per reactive functional
group) was added, and heating at reflux was continued in apparatus
fitted with a Dean–Stark trap. After termination of the reaction, the mix-
ture was extracted with toluene. The organic layer was dried over anhy-
drous MgSO4, filtered, the solvent was removed, and the residue purified
by column chromatography.
CV measurements: The electrochemical studies were carried out by a
three-electrode assembly cell from Bio-Logic SAS and a micro-AUTO-
LAB Type III potentiostat–galvanostat. The measurements were carried
out with a glassy carbon electrode in dichloromethane solutions that con-
tained tetrabutylammonium hexafluorophosphate (0.1m) as the electro-
lyte, Ag/AgNO3 as the reference electrode, and a Pt wire counter elec-
trode. Each measurement was calibrated with ferrocene (Fc). Oxidation
Compound 1b: TPA (0.5 g, 2.04 mmol), CSA (0.47 g, 2.04 mmol), and
bis(4-methoxyphenyl)acetaldehyde (1.88 g, 7.34 mmol) were used (reac-
tion time=10 h). After extraction (toluene), the crude product was puri-
fied by column chromatography (2:23 acetone/n-hexane) to give 1b
(1.5 g, 77%) as a bright-yellow powder. 1H NMR (300 MHz, CDCl3,
258C, TMS): d=7.34–6.73 (m, 35H), 6.67 (d, J=7.2 Hz, 4H), 3.87–
3.68 ppm (m, 18H); 13C NMR (75 MHz, CDCl3, 258C, TMS): d=145.7,
140.4, 136.7, 133.2, 132.5, 131.8, 131.6, 130.9, 130.4, 128.8, 128.3, 126.4,
125.9, 124.5, 124.0, 123.6, 114.3, 113.7, 113.6, 55.4 ppm; elemental analysis
calcd (%) for C66H57NO6: C 82.56, H 5.98, N 1.46; found: C 82.43, H
5.85, N 1.59.
potentials were obtained as an average value between each anodic (pa)
red=ox
1=2
and corresponding cathodic potential (pc) [E
=1/2ACHTUNGETRN(NUG Epc+Epa)]. The
optical bandgaps (Eogpt [eV]) were estimated from the edges of the elec-
tronic absorption spectra, ELUMO values by using the equation ELUMO
EHOMOÀEg.
=
Ionization-potential measurements: The ionization potential (Ip) of the
layers of the synthesized compounds was measured by electron photoem-
ission in air.[22] The samples were prepared by dissolution in CHCl3 and
the solutions were coated on Al plates pre-coated with approximately
0.5 mm thickness of a methylmethacrylate and methacrylic acid copoly-
mer adhesive layer. The thickness of the transporting material layer was
0.5–1 mm. The organic materials investigated are stable enough to oxygen
that the measurements may be carried out in the presence of air. The
samples were illuminated with monochromatic light from a quartz mono-
chromator fitted with a deuterium lamp. The power of the incident light
beam was 2–5ꢁ10À8 W. A negative voltage (À300 V) was supplied to the
sample substrate. The counter electrode with a 4.5ꢁ15 mm2 slit for illu-
mination was placed 8 mm from the sample surface. The counter elec-
trode was connected to the input of the BK2–16 type electrometer, work-
ing in the open input regime, for the photocurrent measurement. A
10À15–10À12 A photocurrent (I) flowed in the circuit under illumination.
The value of I is strongly dependent on the incident-light photon energy
(hn). The dependence I0.5 on incident-light quanta energy hn was plotted
from the experiment results. Usually the dependence of I on the incident
light quantum energy is described well by the linear relationship between
I0.5 and hn near the threshold.[22b,c] The linear part of this dependence was
extrapolated to the hn axis and the Ip value was determined as the
photon energy at the interception point.
Compound 2b: 4-Methyltriphenylamine (0.5 g, 1.93 mmol), CSA (0.45 g,
1.93 mmol), and bis(4-methoxyphenyl)acetaldehyde (1.19 g, 4.63 mmol)
were used (reaction time=6 h). After extraction (toluene), the crude
product was purified by column chromatography (3:22 acetone/n-hexane)
to give 2b (0.99 g, 70%) as a bright-yellow powder. 1H NMR (300 MHz,
CDCl3, 258C, TMS): d=7.29–6.94 (m, 16H), 6.92–6.84 (m, 4H), 6.82 (s,
2H), 6.80–6.74 (m, 8H), 3.81–3.72 (m, 12H), 2.31–2.27 ppm (m, 3H);
13C NMR (75 MHz, CDCl3, 258C, TMS): d=159.2, 158.3, 146.1, 144.8,
140.1, 137.9, 136.8, 133.2, 131.6, 130.8, 130.2, 128.7, 128.2, 125.9, 125.6,
123.2, 122.7, 114.3, 113.8, 113.6, 55.4, 21.1 ppm; elemental analysis calcd
(%) for C51H45NO4: C 83.24, H 6.16, N 1.90; found: C 83.39, H 6.07, N
1.72.
Compound 3b: 4,4ꢂ-Dimethyltriphenylamine (0.5 g, 1.83 mmol), CSA
(0.42 g, 1.83 mmol), and bis(4-methoxyphenyl)acetaldehyde (0.56 g,
2.2 mmol) were used (reaction time=6 h). After extraction (toluene), the
crude product was purified by column chromatography (3:22 acetone/n-
hexane) to give 3b (0.7 g, 75%) as a bright-yellow powder. M.p. 167–
1698C; 1H NMR (300 MHz, CDCl3, 258C, TMS): d=7.23 (d, J=8.9 Hz,
2H), 7.13 (d, J=8.7 Hz, 2H), 6.99 (dd, J=22.7, 8.4 Hz, 8H), 6.91–6.71
(m, 9H), 3.80 (d, J=6.5 Hz, 6H), 2.28 ppm (s, 6H); 13C NMR (75 MHz,
CDCl3, 258C, TMS): d=159.1, 159.0, 146.7, 145.2, 139.7, 136.8, 133.3,
132.8, 131.6, 131.0, 130.2, 130.0, 128.7, 126.1, 125.0, 121.5, 114.3, 113.7,
55.4, 21.0 ppm; elemental analysis calcd (%) for C36H33NO2: C 84.51, H
6.50, N 2.74; found: C 84.34, H 6.63, N 2.89.
Hole-drift-mobility measurements: The samples for the hole-drift-mobili-
ty measurements were prepared by spin coating solutions of the synthe-
sized compounds in toluene onto PS films with a conductive Al layer.
The layer thickness was in the range 5–10 mm. The hole drift mobility
was measured by XTOF.[17] An electric field was created by positive
corona charging. The charge carriers were generated at the layer surface
Compound 1c: TPA (0.5 g, 2.04 mmol), CSA (0.47 g, 2.04 mmol), and 2-
phenylpropionaldehyde (0.98 g, 7.34 mmol) were used (reaction time=
5 h). After extraction (toluene), the crude product was purified by
Chem. Eur. J. 2013, 19, 15044 – 15056
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
15055

K. Kazlauskas, V. Getautis et al.
column chromatography (1:49 THF/n-hexane) to give 1c (0.86 g, 71%)
as a yellow powder. 1H NMR (300 MHz, CDCl3, 258C, TMS): d=7.59–
7.43 (m, 5H), 7.41–7.19 (m, 15H), 7.16 (d, J=8.6 Hz, 3H), 7.06 (d, J=
8.5 Hz, 2H), 6.91–6.69 (m, 5H), 2.38–2.13 ppm (m, 9H); 13C NMR
(75 MHz, CDCl3, 258C, TMS): d=146.0, 144.5, 137.9, 136.7, 133.1, 130.3,
129.9, 128.8, 128.5, 127.5, 126.2, 123.9, 123.6, 17.9 ppm; elemental analysis
calcd (%) for C45H39N: C 91.02, H 6.62, N 2.36; found: C 91.27, H 6.51,
N 2.22.
Adachi, C. Tanaka, A. Katayama, N. Tamoto, K. Kishida, M. Anzai,
A. Imai (Ricoh Company Ltd., Hodogaya Chemical Co., Ltd.), Jpn.
Kokai Tokkyo Koho JP 09278723, 1997; c) M. Sasaki (Ricoh Compa-
ny Ltd.), Ger. Offen. DE 3414141, 1984.
[12] a) J. M. Plater, T. Jackson, Tetrahedron 2003, 59, 4673; b) M. V. Nan-
dakumar, J. G. Verkade, Angew. Chem. 2005, 117, 3175; Angew.
Chem. Int. Ed. 2005, 44, 3115.
[13] a) H. J. Snaith, L. Schmidt-Mende, Adv. Mater. 2007, 19, 3187; b) J.-
H. Yum, P. Chen, M. Grꢅtzel, M. K. Nazeeruddin, ChemSusChem
2008, 1, 699; c) G. K. Mor, S. Kim, M. Paulose, O. K. Varghese, K.
Shankar, J. Basham, C. A. Grimes, Nano Lett. 2009, 9, 4250; d) H. J.
Snaith, A. J. Moule, C. Klein, K. Meerholz, R. H. Friend, M. Grꢅt-
zel, Nano Lett. 2007, 7, 3372.
Compound 2c: 4-Methyltriphenylamine (0.5 g, 1.93 mmol), CSA (0.45 g,
1.93 mmol), and 2-phenylpropionaldehyde (0.62 g, 4.63 mmol) were used
(reaction time=5 h). After extraction (toluene), the crude product was
purified by column chromatography (1:249 acetone/n-hexane) to give 2b
1
(0.54 g, 57%) as yellow oil. H NMR (300 MHz, CDCl3, 258C, TMS): d=
[14] a) G. Meijer, G. Berden, W. L. Meerts, H. E. Hunziker, M. S. de V-
ries, H. R. Wendt, Chem. Phys. 1992, 163, 209; b) A. K. Bansal, A.
Penzkofer, Chem. Phys. 2008, 352, 48.
[15] R. Karpicz, S. Puzinas, S. Krotkus, K. Kazlauskas, S. Jursenas, J. V.
Grazulevicius, S. Grigalevicius, V. Gulbinas, J. Chem. Phys. 2011,
134, 204508.
7.55–7.45 (m, 3H), 7.35 (t, J=7.4 Hz, 3H), 7.30–7.16 (m, 7H), 7.14–6.94
(m, 8H), 6.82–6.69 (m, 3H), 2.38–2.12 ppm (m, 9H); 13C NMR (75 MHz,
CDCl3, 258C, TMS): d=146.4, 145.1, 144.5, 136.4, 133.4, 132.5, 130.2,
130.1, 129.8, 128.8, 128.5, 128.3, 127.6, 127.2, 126.2, 125.5, 123.2, 122.8,
21.1, 17.9 ppm; elemental analysis calcd (%) for C37H33N: C 90.39, H
6.77, N 2.85; found: C 90.53, H 6.49, N 2.98.
[16] a) K. Itami, Y. Ohashi, J. Yoshida, J. Org. Chem. 2005, 70, 2778;
b) H. Tong, Y. Dong, Y. Hong, M. Haussler, J. W. Y. Lam, H. H.-Y.
Sung, X. Yu, J. Sun, I. D. Williams, H. S. Kwok, B. Z. Tang, J. Phys.
Chem. C 2007, 111, 2287.
[17] a) E. Montrimas, V. Gaidelis, A. Pazera, Lith. J. Phys. 1966, 6, 569;
b) S. M. Vaezi-Nejad, Int. J. Electron. 1987, 62, 361; c) Y. Archie, C.
Chan, C. Juhasz, Int. J. Electron. 1987, 62, 625.
[18] a) Y. Shirota, K. Okumoto, H. Inada, Synth. Met. 2000, 111–112,
387; b) K. Okumoto, K. Wayaku, T. Noda, H. Kageyama, Y. Shirota,
Synth. Met. 2000, 111–112, 473; c) A. Higuchi, K. Ohnishi, S.
Nomura, H. Inada, Y. Shirota, J. Mater. Chem. 1992, 2, 1109; d) Y.
Shirota, Y. Kuwabara, H. Inada, T. Wakimoto, H. Nakada, Y. Yone-
moto, S. Kawami, K. Imai, Appl. Phys. Lett. 1994, 65, 807; e) Q.-X.
Tong, S.-L. Lai, M.-Y. Chan, K.-H. Lai, J.-X. Tang, H.-L. Kwong, C.-
S. Lee, S.-T. Lee, Chem. Mater. 2007, 19, 5851.
[19] a) H. E. Katz, Z. Bao, J. Phys. Chem. B 2000, 104, 671; b) R. C.
Haddon, X. Chi, M. E. Itkis, J. E. Anthony, D. L. Eaton, T. Siegrist,
C. C. Mattheus, T. T. M. Palstra, J. Phys. Chem. B 2002, 106, 8288.
[20] Y. Olivier, V. Lemaur, J. L. Bredas, J. Cornil, J. Phys. Chem. A 2006,
110, 6356.
Compound 3c: 4,4ꢂ-Dimethyltriphenylamine (0.5 g, 1.83 mmol), CSA
(0.42 g, 1.83 mmol), and 2-phenylpropionaldehyde (0.29 g, 2.2 mmol)
were used (reaction time=3 h). After extraction (toluene), the crude
product was purified by column chromatography (1:249 acetone/n-
hexane) to give 3c (0.52 g, 73%) as yellow oil. 1H NMR (300 MHz,
CDCl3, 258C, TMS): d=7.53–7.46 (m, 2H), 7.35 (t, J=7.4 Hz, 2H), 7.29–
7.18 (m, 3H), 7.09–6.98 (m, 8H), 6.95–6.90 (m, 1H), 6.76–6.71 (m, 2H),
2.33–2.14 ppm (m, 9H); 13C NMR (75 MHz, CDCl3, 258C, TMS): d=
146.8, 145.4, 144.6, 136.1, 132.7, 131.8, 130.1, 130.0, 128.5, 127.7, 127.1,
126.1, 124.9, 122.3, 21.0, 17.9 ppm; elemental analysis calcd (%) for
C29H27N: C 89.42, H 6.99, N 3.60; found: C 89.50, H 7.10, N 3.40.
Acknowledgements
This research was supported by the European Social Fund under the
ˇ
Global Grant measure (VP1–3.1-SMM-07K 01–078). G.B. acknowledges
EU Structural Funds project “Postdoctoral Fellowship Implementation in
Lithuania” for funding her postdoctoral fellowship.
[21] a) A. M. van de Craats, N. Stutzmann, O. Bunk, M. M. Nielsen, M.
Watson, K. Mꢆllen, H. D. Chanzy, H. Sirringhaus, R. H. Friend,
Adv. Mater. 2003, 15, 495; b) Z. An, J. Yu, S. C. Jones, S. Barlow, S.
Yoo, B. Domercq, P. Prins, L. D. A. Siebbeles, B. Kippelen, S. R.
Marder, Adv. Mater. 2005, 17, 2580; c) K. O. Sylvester-Hvid, J. Phys.
Chem. B 2006, 110, 2618; d) A. R. Murphy, J. S. Liu, C. Luscombe,
D. Kavulak, J. M. J. Frꢇchet, R. J. Kline, M. D. McGehee, Chem.
Mater. 2005, 17, 4892.
[22] a) M. Kirkus, M. H. Tsai, J. V. Grazulevicius, C. C. Wu, L. C. Chi,
K. T. Wong, Synth. Met. 2009, 159, 729; b) E. Miyamoto, Y. Yamagu-
chi, M. Yokoyama, Electrophotography 1989, 28, 364; c) M. Cardo-
na, L. Ley, Top. Appl. Phys. 1978, 26, 1.
[23] T. Kodera, K. Torizuka (Mitsubishi Paper Mills, Ltd.), Jpn. Kokai
Tokkyo Koho JP 11043458, 1999.
[24] M. J. Adams, J. G. Highfield, G. F. Kirkbright, Anal. Chem. 1977, 49,
1850.
[25] J. C. de Mello, H. F. Wittmann, R. H. Friend, Adv. Mater. 1997, 9,
230.
[1] a) S. R. Forrest, Nature 2004, 428, 911; b) U. Mitschke, P. Bꢅuerle, J.
Mater. Chem. 2000, 10, 1471; c) H. Spanggaard, F. C. Krebs, Solar
Energy Mater. Solar Cells 2004, 83, 125; d) C. D. Dimitrakopoulos,
P. R. L. Malenfant, Adv. Mater. 2002, 14, 99; e) D. S. Weiss, M. Ab-
kowitz, Chem. Rev. 2010, 110, 479.
[2] S. Allard, M. Forster, B. Souharce, H. Thiem, U. Scherf, Angew.
Chem. 2008, 120, 4138; Angew. Chem. Int. Ed. 2008, 47, 4070.
[3] E. Menard, V. Podzorov, S.-H. Hur, A. Gaur, M. E. Gershenson,
J. A. Rogers, Adv. Mater. 2004, 16, 2097.
[4] a) M. Halik, H. Klauk, U. Zschieschang, G. Schmid, W. Radlik, S.
Ponomarenko, S. Kirchmeyer, W. Weber, J. Appl. Phys. 2003, 93,
2977; b) M. Halik, H. Klauk, U. Zschieschang, G. Schmid, W.
Radlik, S. Ponomarenko, S. Kirchmeyer, W. Weber, Adv. Mater.
2003, 15, 917.
[5] Y. Shirota, H. Kageyama, Chem. Rev. 2007, 107, 953.
[6] P. Strohriegl, J. V. Grazulevicius, Adv. Mater. 2002, 14, 1439.
[7] P. M. Borsenberger, L. Pautmeier, R. Richert, H. Bꢅssler, J. Chem.
Phys. 1991, 94, 8276.
[26] a) S. Urnikaite, T. Malinauskas, V. Gaidelis, R. Maldzius, V. Jankaus-
kas, V. Getautis, Carbon 2011, 49, 320; b) D. Tomkute-Luksiene, T.
Malinauskas, M. Daskeviciene, V. Gaidelis, R. Maldzius, J. Sidaravi-
cius, V. Getautis, Synth. Met. 2011, 161, 1177; c) J. Kalade, E. Mon-
trimas, R. Maldzius, Z. Tokarski, J. Imaging Sci. Technol. 2005, 49,
28.
[8] J. Veres, S. D. Ogier, S. W. Leeming, D. C. Cupertino, S. M. Khaffaf,
Adv. Funct. Mater. 2003, 13, 199.
[9] M. Sonntag, K. Kreger, D. Hanft, P. Strohriegl, S. Setayesh, D. de L-
eeuw, Chem. Mater. 2005, 17, 3031.
[10] T. Malinauskas, D. Tomkute-Luksiene, M. Daskeviciene, V. Jankaus-
kas, G. Juska, V. Gaidelis, K. Arlauskas, V. Getautis, Chem.
Commun. 2011, 47, 7770.
[11] a) M. Sasaki (Ricoh Company Ltd.), US Patent US 4859556, 1989;
b) K. Nagai, M. Sasaki, H. Tamura, T. Suzuki, T. Shimada, C.
Received: November 13, 2012
Revised: July 29, 2012
Published online: September 17, 2013
15056
www.chemeurj.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 15044 – 15056