1,3-Bisdiphenylethenyl-substituted carbazolyl derivatives as charge transporting materials

Article abstract of DOI:10.3390/molecules171214846

Synthesis of 1,3-diphenylethenylcarbazolyl-based charge transporting materials involving electron donating hydrazone moieties and an electron withdrawing 1,3-indandione moiety is reported. The obtained materials were examined by various techniques, including differential scanning calorimetry, UV-Vis spectroscopy, xerographic time of flight technique and the electron photoemission in air method. Photoemission spectra of the amorphous films of the investigated compounds showed ionization potentials of 5.54-5.90 eV. The hole drift mobility was measured by the xerographic time of flight technique. The highest hole drift mobility, exceeding 10-5 cm2/V.s at 6.4 × 105 V/cm electric field, was observed for the 1,3-diphenylethenylcarbazolyl derivative molecularly doped with a N,N-diphenylhydrazone moiety in the polymeric host bisphenol-Z polycarbonate (PC-Z).

Full text of DOI:10.3390/molecules171214846

Molecules 2012, 17, 14846-14857; doi:10.3390/molecules171214846
OPEN ACCESS
molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Article
1,3-Bisdiphenylethenyl-substituted Carbazolyl Derivatives as
Charge Transporting Materials
Giedre Bubniene 1, Maryte Daskeviciene 1, Audrius Pukalskas 1, Vygintas Jankauskas 2 and
Vytautas Getautis 1,*
1
Faculty of Chemical Technology, Kaunas University of Technology, Radvilenu pl. 19,
Kaunas LT-50254, Lithuania
2
Department of Solid State Electronics, Vilnius University, Sauletekio 9-III,
Vilnius LT-10222, Lithuania
* Author to whom correspondence should be addressed; E-Mail: vytautas.getautis@ktu.lt;
Tel.: +370-37-300-196; Fax: +370-37-300-152.
Received: 19 November 2012; in revised form: 9 December 2012 / Accepted: 11 December 2012 /
Published: 13 December 2012
Abstract: Synthesis of 1,3-diphenylethenylcarbazolyl-based charge transporting materials
involving electron donating hydrazone moieties and an electron withdrawing 1,3-indandione
moiety is reported. The obtained materials were examined by various techniques, including
differential scanning calorimetry, UV-Vis spectroscopy, xerographic time of flight technique
and the electron photoemission in air method. Photoemission spectra of the amorphous
films of the investigated compounds showed ionization potentials of 5.54–5.90 eV. The
hole drift mobility was measured by the xerographic time of flight technique. The highest
hole drift mobility, exceeding 10−5 cm2/V·s at 6.4 × 105 V/cm electric field, was observed for
the 1,3-diphenylethenylcarbazolyl derivative molecularly doped with a N,N-diphenylhydrazone
moiety in the polymeric host bisphenol-Z polycarbonate (PC-Z).
Keywords: carbazole; hydrazone; 1,3-indandione; ionization potential; charge carrier mobility
1. Introduction
Charge transporting materials (CTMs) are key components for the fabrication of high-performance
electronic and optoelectronic devices using organic materials as active elements. Among CTMs
carbazole-based compounds represent one of the most widely used and studied class of substances.

Molecules 2012, 17
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Carbazole derivatives are well-known to exhibit good electro- and photo-active properties due to their
high hole transporting mobility, strong absorption in the ultraviolet spectral region and blue-light emission.
Since the discovery of photoconductivity in poly(9-vinylcarbazole) (PVK) [1] carbazole-containing
derivatives became the subject of numerous investigations for applications in electrophotography [2].
The second wave of interest in carbazole-based CTMs is connected mostly with the discovery of
organic light emitting diodes [3] and organic photorefractive materials [4]. Apart from electrophotographic
photoreceptors [5], light-emitting diodes and photorefractive materials, carbazole-containing transporting
materials are studied as the components of photovoltaic devices [6–8] and field-effect transistors [9].
Novel carbazole based derivatives possessing 1,3- or 3,6-diphenylethenyl fragments have been
reported by us recently [10]. Commercial availability and relative cheapness of the starting materials,
simple synthesis, number of sites available for easy functionalization, good charge drift mobility and
solubility in common organic solvents makes these precursors attractive building blocks for the
construction of more complex materials for optoelectronic applications. In the present work, synthesis
of 1,3-diphenylethenylcarbazolyl-based CTMs containing electron donating hydrazone moieties and
electron withdrawing 1,3-indandione moieties are reported. Optical, thermal, and photophysical properties
of the synthesized carbazole derivatives are reported.
2. Results and Discussion
2.1. Synthesis
Recently we have showed that condensation of 9H-carbazol-2-ol with diphenylacetaldehyde in the
presence of catalyst (±)-camphor-10-sulfonic acid (CSA, with the water generated during the course
of the reaction being removed with 4 Å molecular sieves) results in formation of 1,3-bis(2,2-
diphenylethenyl)-9H-carbazol-2-ol (1) [10]. In the present work, carbazol-2-ol derivative 1 (Scheme 1)
was synthesized according to the modified procedure, using a mixture of toluene and dioxane (3/2, v/v)
as solvent and water generated during the course of the reaction was removed using a Dean-Stark trap;
such a procedure modification significantly reduces the duration of the reaction.
Scheme 1. Synthesis route to the aldehyde 3.
O
O
n-C3H7Br
KOH
DMF
POCI3
CSA
N
H
OH
N
H
OH
N
O
N
O
3
1
2
A simple alkylation with 1-bromopropane was performed to yield 1,3-bis(2,2-diphenylethenyl)-9-
propyl-2-propoxycarbazole (2), which was used in Vilsmeier formylation to confirm that the 6-position
of the carbazole remained active for electrophilic substitution reactions. The intermediate compound
1,3-bis(2,2-diphenylethenyl)-6-methanoyl-9-propyl-2-propoxycarbazole (3) was successfully isolated.

Molecules 2012, 17
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The highest yield of aldehyde 3 was achieved when a large excess (6 equivalents) of the DMF/POCl3
complex was used.
Finally, 1,3-bisdiphenylethenylcarbazolyl-based charge transporting materials CTM1, CTM2 and
CTM3 were synthesized by the synthetic route shown in Scheme 2. Hole transporting materials
containing electron donating hydrazone moieties CTM1 and CTM2 were prepared by interaction of
aldehyde 3 with N-methyl-N-phenyl- or N,N-diphenylhydrazine, respectively. Meanwhile, in the
reaction of aldehyde 3 with 1,3-indandione, a charge transporting material possessing both electron
donating 1,3-bisdiphenylcarbazolyl moiety and an electron withdrawing 1,3-indandione moiety was
synthesized. We expected that this potentially bipolar material could transport both holes and electrons
as it comprises donor and acceptor segments, which demonstrate the good transporting properties of
the appropriate charge carriers [11].
Scheme 2. Synthesis routes to charge-transporting materials CTM1–3.
N
NH2
N
N
N
N
NH2
N
3
N
O
N
O
CTM2
CTM1
O
O
O
O
N
O
CTM3
2.2. Thermal Properties
The obtained compounds are soluble in common organic solvents, such as THF, acetone, MEK, and
toluene at room temperature. The thermal properties of the synthesized materials were determined by
differential scanning calorimetry (DSC). The melting (Tm) points and glass transition temperatures (Tg)
of the synthesized compounds are presented in Table 1. The data of the parent compound 2 was added
for the comparison.
Table 1. Thermal parameters of CTM1–3.
Compound
Tm (C)
2
CTM1 CTM2 CTM3
144
64
239
92
220
94
239
107
Tg (C)
The DSC analysis of the CTM1 reveals melting of the crystals at 239 C during the first heating
(Figure 1a). No recrystallization takes place during cooling or second heating, and only a glass transition at
92 C is observed in the second heating, indicating that CTM1 remains in glassy state after melting and

Molecules 2012, 17
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subsequent cooling. This is common feature for other 1,3-bisdiphenylethenylcarbazolyl-based charge
transporting material possessing hydrazone moieties investigated here. The glassy state of these
materials is quite stable, and no signs of crystallization were detected during over year’s storage at ambient
conditions. Meanwhile, indandione derivative CTM3 demonstrates different behavior (Figure 1b).
During the second heating a glass transition occurs at 107 C, followed by crystallization at 171 C and
subsequent melting of the crystals with an endothermic peak at 238 C. Such a DSC pattern indicates
that the amorphous state of the CTM3 isn’t stable and it recrystallizes easily.
Figure 1. (a) DSC curves of CTM1; (b) DSC curves of CTM3 (heating rate 10 K/min).
Tg values of the obtained compounds demonstrate that the hydrazone and indandione fragments
have a significant influence on the glass transition temperatures of the investigated derivatives. Tg was
found to be higher for CTM1–3 (92 °C for CTM1, 94 °C for CTM2 and 107 °C for CTM3) than it was
observed for the parent compound 2 (64 °C). Meanwhile, the Tg value for a hydrazone with the
N,N-diphenyl moiety (CTM2) is very similar that of the hydrazone possessing an N-methyl-N-phenyl
fragment (CTM1). The replacement of the hydrazone segment by the indandione moiety increases
the glass transition temperature, but decreases the stability of amorphous state of the bipolar
compound CTM3.

Molecules 2012, 17
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2.3. UV-Vis Spectra
Size of the conjugated -electrons system is very important for the charge transporting process in
the CTM structures, therefore, it was investigated using UV-Vis absorption spectroscopy. The absorption
spectrum of the parent compound 2 was given for the comparison. Electron transitions to the higher
energy states in the investigated derivatives give two main photon absorption maxima (Figure 2) at
ca. 240 nm and 325 nm. The comparison of UV/Vis spectra of CTM1 and CTM2 with the parent
compound 2 shows that the incorporation of hydrazone fragment into the structure of compound 2
doesn’t contribute too much to the existing π-conjugated system. The absorption spectra of both
CTM1 and CTM2 are bathochromically shifted by just 8–10 nm. Also there isn’t much difference in
the → * absorption band when comparing hydrazones CTM1 and CTM2 between each other. The
replacement of the methyl group in CTM1 by a phenyl group in CTM2 leads to a slight additional
bathochromic shift of 2 nm. In both cases this is most likely due to unfavorable arrangement of the
fragments in the molecule.
Figure 2. UV/Vis absorption spectra of 10−4 M THF solutions of CTM1–3 and 2.
CTM1
CTM2
1.0
CTM3
2
326 nm
0.8
0.6
0.4
0.2
0.0
328 nm
468 nm
316 nm
250
300
350
400
450
500
550
Wavelength (nm)
Presence of the donor and acceptor moieties in the structure of CTM3 results in the appearance of
the strong absorption band around 468 nm due to the intramolecular charge transfer process (ICT).
Due to this ICT compound CTM3 has an intense orange color.
2.4. Photophysical Properties
Photoemission spectra of the films of the investigated compounds CTM1–3 are presented in Figure 3.
The ionization potential values obtained by the electron photoemission in air method are presented in
Table 2. Usually the photoemission experiments are carried out under vacuum and high vacuum is one
of the main requirements for these measurements. If the vacuum is not high enough, the sample
surface oxidation and gas adsorption influence the measurement results. In our case, however, the
organic material investigated is stable enough to oxygen and the measurements may be carried out in air.

Molecules 2012, 17
Figure 3. Photoemission spectra of the amorphous films of CTM1–3 measured in air.
14851
5.87 eV
26
24
22
20
18
16
14
12
10
8
CTM3
CTM2
CTM1
5.54 eV
5.90 eV
6
4
2
0
5.2
5.4
5.6
5.8
6.0
6.2
6.4
h (eV)
Table 2. Ip and hole drift mobility data for mixtures of 2 and CTM1–3 with PC-Z.
Layer thickness
μo
μ
Compound Ip (eV)
Layer composition
(μm)
9
(cm2/V·s)
4·10−7
(cm2/V·s) 1
1.8·10−5
9·10−6
CTM1
CTM2
CTM3
5.90
5.87
5.54
Al+(CTM1+PC-Z, 1:1)
Al+(CTM2+PC-Z, 1:1)
Al+(CTM3+PC-Z, 1:1)
8
1.6·10−7
2·10−8
5.4
4·10−6
1 at 6.4 × 105 V/cm electric field.
The measured ionization energies vary in the range from 5.54 eV to 5.90 eV. Introduction of
hydrazone fragments significantly increases the ionization potential values by ~0.3 eV (compared to
5.6 eV for the parent compound 2). This is somewhat unusual—to our knowledge this is the highest Ip
values among hydrazone derivatives investigated as hole transporting materials [12]. The Ip for the
bipolar compound CTM3 possessing both electron donating 1,3-bisdiphenylcarbazolyl moiety and
electron windrawing 1,3-indandione moieties is 5.54 eV.
Crystalline CTMs usually are used for layer preparation in compositions based on a polymeric
binder such as PC-Z. The charge transport properties of the synthesized compounds CTM1–3 were
studied by the xerographic time of flight technique. The zero field hole drift mobility parameter μo as
well as the hole drift mobility μ value at the 6.4 × 105 V/cm field strength are given in Table 2. Figure 4
shows the room temperature dependencies of the hole drift mobility on the electric field in the films of
the investigated compounds molecularly doped in polymeric host PC-Z (50% solid solutions).
Hole drift mobilities of the investigated 1,3-bisdiphenylethenyl-substituted carbazolyl derivatives,
doped in PC-Z reach 10−5 cm2/Vs for strong electric fields. Comparing the charge carrier-transport
properties of CTM1 and CTM2, it is interesting to note that N,N-diphenylhydrazone derivative CTM2
shows higher mobility than the N-methyl-N-phenyl analogue CTM1. The slightly extended conjugation
network in compound CTM2 most probably results in stronger electronic coupling between molecules,
thereby facilitating hole hopping. The comparison of compound CTM3 containing an indandione
fragment, reveals that mobility values drop. On the other hand the hole mobility in CTM3 is two
orders of magnitude higher than in methylene-1,3-indandione derivatives reported in the literature [13].

Molecules 2012, 17
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The XTOF transients for the compound CTM3, were of disperse character, but the transit time was
well seen on lg-lg plots (Figure 5a). Meanwhile, hole transport transients are with well-defined transit
time on linear plots in case of CTM1 and CTM2 (Figure 5b).
Figure 4. Electric field dependencies of the hole-drift mobilites (μ) in charge transport
layers of compounds CTM1–3 doped in PC-Z (mass proportion 1:1).
10-4
10-5
10-6

10-7
Al+CTM1+(PC-Z), 1:1
Al+CTM2+(PC-Z), 1:1
Al+CTM3+(PC-Z), 1:1
10-8
0
200
400
600
E1/2 (V/cm)1/2
800
1000
1200
Figure 5. (a) XTOF transients for CTM2 (1:1 composition with PC-Z); (b) XTOF
transients for CTM3 (1:1 composition with PC-Z).
t=25oC
a)
CTM2 + (PC-Z), 1:1, d=9m
101
100
U0:
1060 V
892 V
644 V
460 V
360 V
240 V
178 V
90 V
10-1
10-2
10-3
1.5
1.0
0.5
0.0
UV flash
644 V
0.0000
0.0002
t (s)
0.0004
10-6
10-5
10-4
10-3
10-2
t (s)
101
100
b)
t =25oC
CTM3+PC-Z, 1:1, d=5.4m
U0:
750 V
690 V
596 V
520 V
488 V
392 V
316 V
10-1
10-2
10-3
0.4
0.2
0.0
UV flash
520 V
0.0000
0.0002
0.0004
t
(s)
10-6
10-5
10-4
10-3
t (s)

Molecules 2012, 17
14853
Unfortunately, we were unable to measure the electron drift mobility of the bipolar compound
CTM3 because of the too dispersive charge carrier transport.
3. Experimental
3.1. General
9H-carbazol-2ol, diphenylacetaldehyde, (±)-camphor-10-sulfonic acid, indan-1,3-dione, N-methyl-N-
phenylhydrazine and N,N-diphenylhydrazine hidrochloride were purchased from Aldrich Chemical Co.
(Milwaukee, WI, USA) and were used without purification. 1,3-bis(2,2-Diphenylethenyl)-9-propyl-2-
propoxycarbazole (2) and 1,3-bis(2,2-diphenyl-ethenyl)-6-methanoyl-9-propyl-2-propoxycarbazole (3)
were obtained according to the methods described in [10]. The course of the reactions were monitored
by TLC on ALUGRAM SIL/UV254 plates, using a mixture of acetone and n-hexane as eluent and
1
developed with iodine or UV radiation. The H-NMR spectra were taken on a Varian Unity Inova
(300 MHz) spectrometer in CDCl3. All the data are given as chemical shifts in  (ppm), (CH3)4Si
(TMS, 0 ppm) was used as an internal standard. The IR spectra of synthesized compounds were
recorded in KBr pellets on a Perkin Elmer Spectrum BX II FT-IR System spectrometer. MS analysis
was performed on a Maxis 4G Q-TOF instrument (Bruker Daltonik GmbH, Bremen, Germany).
Compounds were ionized in ESI positive mode. Capillary voltage was set to 4,500 V, N2 gas was used
as nebulizer at a pressure of 1.5 bar, and as drying gas at a flow rate of 8 L/min. Drying temperature was
set at 200 °C. Sample was injected with a 6-port valve equipped with 20 µL sample loop. The eluent
flow rate was 0.4 mL/min. No fragmentation was performed. Elemental analysis was performed with
an Exeter Analytical CE-440 Elemental Analyzer. Melting points were determined using Electrothermal
Mel-Temp melting point apparatus. The UV-vis spectra were recorded on a Perkin Elmer Lambda 35
spectrometer in THF (10−4 M) in a microcell with an internal width of 1 mm. The differential scanning
calorimetry (DSC) measurements were carried out on a Mettler DSC 30 calorimeter at a scan rate of
10 K/min under nitrogen atmosphere. The glass transition temperatures (Tg) were determined from the
second heating.
3.2. Synthesis Procedures
1,3-Bis(2,2-diphenylethenyl)-carbazol-2-ol (1). 9H-Carbazol-2-ol (15.0 g, 0.082 mol) was dissolved in
a mixture of toluene (75 mL) and 1,4-dioxane (50 mL), then (±)-camphor-10-sulfonic acid (19.04 g,
0.082 mol) was added and the mixture was heated at reflux for 20 min. Afterwards diphenylacetaldehyde
(40.2 g, 0.205 mol) was added and reflux continued (2 h) using a Dean-Stark trap. After termination of the
reaction (TLC control, acetone/n-hexane, 7:18), the mixture was treated with ethyl acetate and washed
with distilled water. The organic layer was dried over anhydrous MgSO4, filtered and solvents were
removed. The residue was dissolved in the mixture of toluene and 2-propanol (1/3, v/v). The crystals,
formed upon standing in RT, were filtered off, washed with 2-propanol and dried. The yield of 1 was
25.0 g (57%) as a pale yellow powder, mp: 193–195 °C (toluene, 194–195 °C [10]). 1H-NMR δ 7.57–7.50
(m, 1H, NH); 7.45–6.96 (m, 27H, Ar); 5.31 (s, 1H, OH). 13C-NMR δ 150.5, 145.7, 143.5, 143.1, 142.9,
140.4, 139.68, 139.62, 137.4, 130.7, 129.8, 128.96, 128.74, 128.66, 128.49, 128.37, 128.09, 127.65,
126.18, 124.6, 123.8, 121.6, 120.5, 119.57, 119.36, 117.99, 116.74, 110.28, 107.41. IR ν (cm−1): 3537

Molecules 2012, 17
14854
(OH); 3440 (NH); 3077, 3056, 3028 (aromatic CH); 1625, 1611, 1594, 1490, 1444 (CC, C–N); 1183
(CHt–OH); 806, 779, 769, 760, 736, 701, 654 (CHCH 1,2,3-trisubstituted carbazole and monosubstituted
benzene). Anal. Calcd for C40H29NO: C 89.02; H 5.42; N 2.60. Found: C 88.90; H 5.53; N 2.45.
1,3-Bis(2,2-diphenylethenyl)-6-(1-phenyl-1-methylhydrazon-2-yl)-9-propyl-2-propoxycarbazole (CTM1).
1,3-bis(2,2-Diphenylethenyl)-6-methanoyl-9-propyl-2-propoxycarbazole (2, 1.0 g, 1.53 mmol) was
dissolved in tetrahydrofuran (5 mL) and N-phenyl-N-methylhydrazine (0.19 g, 1.53 mmol) was added.
The mixture was refluxed until all compound 2 has reacted. The crystals formed upon standing of the
cooled reaction mixture, were filtered off, washed with 2-propanol and dried. The yield was 1.10 g
1
(95%), mp 235–236 C. H-NMR δ 7.76 (d, J = 1.3 Hz, 1H, 5-H carbazole), 7.69 (dd, J1 = 8.5 Hz,
J2 = 1.5 Hz, 1H, 7-H carbazole), 7.60 (s, 1H, CHN), 7.55–6.89 (m, 29H, Ar), 4.66–4.05 (m, 2H,
NCH2), 4.01–3.66 (m, 2H, OCH2), 3.43 (s, 3H, NCH3), 1.86–1.63 (m, 4H, NCH2CH2, OCH2CH2),
0.92 (t, J = 7.4 Hz, 3H, N(CH2)2CH3), 0.79 (t, J = 7.4 Hz, 3H, O(CH2)2CH3). IR ν (cm−1): 3078, 3054,
3024 (aromatic CH), 2957, 2931, 2872 (aliphatic CH), 1591, 1568, 1501, 1486, 1467, 1443 (CC,
C–N), 1203 (CHt–O), 1135, 1113, 1071 (C–O–C), 800, 772, 764, 747, 736, 699 (CHCH of
1,2,3,6,9-pentasubstituted carbazole, monosubstituted benzene). MS (APCI+), m/z: 756 ([M+H]+).
Anal. Calcd for C54H49N3O: C, 85.79; H, 6.53; N, 5.56. Found: C, 86.03; H, 6.74; N, 5.68.
1,3-Bis(2,2-diphenylethenyl)-6-(1,1-diphenylhydrazon-2-yl)-9-propyl-2-propoxycarbazole (CTM2). The
title compound was prepared according to the same procedure as described for CTM1, except that
N,N-diphenylhydrazine hydrochloride (0.73 g, 4 mmol) dissolved in methanol (10 mL) was added into
the solution of compound 2 (2.0 g, 3.07 mmol) in THF (3 mL). After termination of the reaction
(TLC control), the mixture was extracted with ethyl acetate and water. The organic layer was washed
with distilled water until the wash water was neutral, dried over anhydrous Mg2SO4, ethyl acetate was
removed and the residue was dissolved in acetone. The crystals formed upon standing were filtered off
1
and washed with diethyl ether. The yield was 1.04 g (41%), mp 220–221 C. H-NMR δ 7.75 (s, 1H,
CHN), 7.58–6.98 (m, 38H, Ar), 4.66–4.04 (m, 2H, NCH2), 4.00–3.69 (m, 2H, OCH2), 1.83–1.64
(m, 4H, NCH2CH2, OCH2CH2), 0.91 (t, J = 7.4 Hz, 3H, N(CH2)2CH3), 0.77 (t, J = 7.4 Hz, 3H,
O(CH2)2CH3). IR ν (cm−1): 3077, 3055, 3021 (aromatic CH), 2964, 2932, 2875 (aliphatic CH), 1591,
1494, 1460, 1444 (CC, C–N), 1204 (CHt–O), 1093, 1069, 1056 (C–O–C), 812, 764, 698 (CHCH of
1,2,3,6,9-pentasubstituted carbazole, monosubstituted benzene). MS (APCI+), m/z: 818 ([M+H]+).
Anal. Calcd for C59H51N3O: C, 86.62; H, 6.28; N, 5.14. Found: C, 86.40; H, 6.15; N, 5.26.
1,3-Bis(2,2-diphenylethenyl)-6-(indan-1,3-dione-2-methylen)-9-propyl-2-propoxycarbazole (CTM3). A
solution of aldehyde 2 (1.5 g, 2.3 mmol) and indan-1,3-dione (0.67 g, 4.6 mmol) in dioxane (8 mL)
was heated at reflux for 48 h. After termination of the reaction (TLC control), the product was isolated
similarly as compound 3. The yield was 1.3 g (72%), mp 238–239 C. 1H-NMR δ 8.86 (dd, J1 = 8.8 Hz,
J2 = 1.7 Hz, 1H, 7-H carbazole), 8.47 (d, 1H, J = 1.6 Hz, 5-H carbazole), 8.06–7.95(m, 3H, Ar),
7.84–7.74 (m, 2H, indan-1,3-dione), 7.53–6.98 (m, 24H, Ar), 4.74–4.13 (m, 2H, NCH2), 4.04–3.69 (m,
2H, OCH2), 1.89–1.67 (m, 4H, NCH2CH2, OCH2CH2), 0.93 (t, J = 7.4 Hz, 3H, N(CH2)2CH3), 0.83 (t,
J = 7.4 Hz, 3H, O(CH2)2CH3). IR ν (cm−1): 3078, 3054, 3022 (aromatic CH), 2957, 2929, 2874
(aliphatic CH), 1718, 1675 (CO), 1629, 1596, 1575, 1558, 1488, 1464, 1443 (CC, C–N), 1336 (CO),

Molecules 2012, 17
14855
1229, 1092, 1068 (C–O–C), 1200 (CHt–O); 816, 769, 737, 698 (CHCH of 1,2,3,6,9-pentasubstituted
carbazole, monosubstituted benzene). MS (ESI+), m/z: 780 ([M+H]+). Anal. Calcd for C56H45NO3: C,
86.24, H, 5.82, N, 1.80. Found: C, 86.02; H, 5.96; N, 1.69.
3.3. Measurements
3.3.1. Ionization Potential Measurement
Ionization potential (Ip) was measured by the electron photoemission in air method described in [14,15].
The samples for the ionization energy measurement were prepared by dissolving materials in THF and
then coating them on Al plates precoated with ~0.5 μm thick methylmethacrylate and methacrylic acid
copolymer adhesive layer. The thickness of the transporting material layer was 0.5–1 μm. The samples
were illuminated with monochromatic light from the quartz monochromator with a deuterium lamp.
The power of the incident light beam was (2–5) × 10−8 W. The negative voltage of I0.5 and hν near the
threshold. The linear part of this dependence was extrapolated to the hν axis and the Ip value was
determined as the photon energy at the interception point. The samples for hole drift mobility
measurements were prepared from 1:1 mass proportion compositions of compounds with PC-Z
(Iupilon Z-200 from Mitsubishi Gas Chemical Co. Tokyo, Japan). The sample substrate was polyester
film with conductive Al layer. The layer thickness was in the range of 5–12 μm.
3.3.2. Charge Transport Measurement
The sample for the measurement was prepared by drop casting of the solution of CTM in THF
(100 mg in 1 mL) onto a polyester film with a conductive Al layer. After coating the sample was heated
at 70 °C for 1 h. Thus the transporting layer of the sample was prepared. The thickness of the
transporting layer was ~5–9 μm. The hole drift mobility was measured by xerographic time of flight
technique (XTOF) [16]. Positive corona charging created electric field inside the transporting material
(TM) layer. The charge carriers were generated at the layer surface by illumination with pulses of
nitrogen laser (pulse duration was 2 ns, wavelength 337 nm). The layer surface potential decrease as a
result of pulse illumination was up to 1–5% of the initial potential before illumination. The capacitance
probe that was connected to the wide frequency band electrometer 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 linear scale. The drift mobility was calculated by the formula μ = d2/U0tt, where d is the
layer thickness and U0 is the surface potential at the moment of illumination.
4. Conclusions
1,3-Bisdiphenylethenyl-substituted carbazolyl derivatives containing electron donating hydrazone
moieties and an electron withdrawing 1,3-indandione moiety were synthesized and investigated as
potential charge transporting materials for optoelectronics. It was determined that compounds with
hydrazone moieties can function as hole transporting materials. The hole mobilities in the amorphous
films of the investigated compounds CTM1 and CTM2 molecularly doped in the polymeric host PC-Z
(50% solid solutions) reach 10−5 cm2/V·s in strong electric fields. It is intriguing that these hole TMs
show very high Ip values (ca. 5.9 eV). The investigated indandione derivative CTM3 exhibits dispersive

Molecules 2012, 17
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charge carrier transport. The molecular structures of the synthesized 1,3-bisdiphenylethenyl substituted
carbazolyl derivatives CTM1 and CTM2 allow them to exist in a stable amourphous state with glass
transition temperatures of 92 °C and 94 °C, respectively.
Acknowledgments
This research was supported by the European Social Fund under the Global Grant measure
(VP1-3.1-ŠMM-07-K 01-078). G. Bubniene acknowledges EU Structural Funds project “Postdoctoral
Fellowship Implementation in Lithuania” for funding her postdoctoral fellowship.
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Sample Availability: Samples of the compounds are available from the authors.
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