Photoconductivity of liquid crystalline derivatives of pyrene and carbazole

Article abstract of DOI:10.1039/b612253a

Two structurally related discogens containing either pyrene (1a) or carbazole (2a) were investigated by thermal, XRD, spectroscopic, and time-of-flight (TOF) methods. Experiments demonstrated for 1a a narrow range Col_h phase, which easily forms a glass state at ambient temperature. TOF measurements showed an ambipolar charge transport for 1a with the mobilities on the order of 10^-3 cm² V^-1 s^-1. The carbazole 2a has two enantiotropic phases (Cr_col and Col _h) and behaves as a p-type semiconductor. The activation energy for positive charge mobility in 1a was found to be 0.10 ± 0.01 eV. The Royal Society of Chemistry.

Full text of DOI:10.1039/b612253a

View Article Online / Journal Homepage / Table of Contents for this issue
PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Photoconductivity of liquid crystalline derivatives of pyrene and carbazole{
Monika J. Sienkowska,a Hirosato Monobe,b Piotr Kaszynski*a and Yo Shimizub
Received 25th August 2006, Accepted 11th December 2006
First published as an Advance Article on the web 15th January 2007
DOI: 10.1039/b612253a
Two structurally related discogens containing either pyrene (1a) or carbazole (2a) were
investigated by thermal, XRD, spectroscopic, and time-of-flight (TOF) methods. Experiments
demonstrated for 1a a narrow range Colh phase, which easily forms a glass state at ambient
temperature. TOF measurements showed an ambipolar charge transport for 1a with the mobilities
on the order of 1023 cm2 V21 s21. The carbazole 2a has two enantiotropic phases (Crcol and Colh)
and behaves as a p-type semiconductor. The activation energy for positive charge mobility in 1a
was found to be 0.10 ¡ 0.01 eV.
Introduction
Photogeneration of an electron–hole pair and one dimensional
charge transport1–8 along the self-assembled columns in
aromatic discotic liquid crystals9,10 has been recognized as an
attractive property for construction of light emitting diodes,11
photovoltaic cells,12 and field-effect transistors.13,14 The initial
focus was on triphenylene-based discotics,1 however other
classes of discotics, including perylenes,15 phthalocyanines,16,17
hexabenzocoronenes,18 and other polycyclic aromatics,19 have
been recently investigated for their photophysical properties.
Typical charge mobility in these systems has been found to be in
the range of 1024–1021 cm2 V21 s21 9,20
which is comparable
,
with those of polycrystalline organics21,22 and attractive for
device applications. In this context, we set out to investigate
the photophysical behavior of discotic derivatives of pyrene
and carbazole.
Photoconductive properties of pyrene were investigated in
pure crystals,23 in thin films,24 and also in solutions of
liquid hydrocarbons.25 Carbazole derivatives are attractive
for technological applications and they have been intensively
studied in polymers26–29 and as components of a discotic
triphenylene derivative.30 To date, no photoconduction studies
of discotic liquid crystals solely based on pyrene or carbazole
have been reported.
Recently, we prepared discotic derivative of pyrene 1a.31
Here, we report the preparation of the carbazole analog 2a,
and also XRD phase characterization and photoconductive
properties of both pyrene and carbazole discotics 1a and 2a,
which contain four 3,4-dioctyloxyphenyl substituents each.
For comparison with other structurally similar discotics,31
we also prepared and characterized carbazole derivatives 2b
and 2c.
Results and discussion
Synthesis
Tetraarylcarbazoles 2 were prepared from 1,3,6,8-tetrabromo-
carbazole32 (3) and appropriate boronic esters 4 according to
the general Suzuki coupling procedure33 (Scheme 1). Biphenyls
5 were obtained as the homocoupling byproducts. The
aOrganic Materials Research Group Department of Chemistry,
Vanderbilt University, Nashville, TN, 37235, USA
bNanotechnology Research Institute, National Institute of Advanced
Industrial Science and Technology, AIST Kansai Centre, Midorigaoka
1-8-31, Ikeda, Osaka, 536-8577, Japan
{ Electronic supplementary information (ESI) available: UV, XRD,
and photoconductivity data. See DOI: 10.1039/b612253a
Scheme 1
1392 | J. Mater. Chem., 2007, 17, 1392–1398
This journal is ß The Royal Society of Chemistry 2007

View Article Online
desired tetraarylated carbazoles 2 were separated by column
chromatography from partially arylated arenes and biphenyls
5 using a gradient polarity eluent. Chromatographic purifica-
tion of 2a and 2b was followed by recrystallization from a
hexane–ethanol mixture. Derivative 2c was found to be an
isotropic liquid at room temperature. Tetrabromide 3 was
obtained by direct bromination of the parent carbazole
according to a literature procedure.32,34 Preparation of boronic
esters 4, characterization of biphenyls 5, and preparation of
pyrene derivative 1a are described elsewhere.31
crystalline and crystalline phases. These textures remained
virtually the same after cooling to room temperature.
Pyrene derivative 1a melts to a discotic hexagonal phase at
73 uC, which is followed by isotropization at 87 uC (Fig. 2a).31
On cooling, the isotropic–discotic transition is supercooled by
8 uC and no crystallization peak is observed at a scanning
rate of 10 uC min21. The optical texture remained practically
unchanged indicating the formation of an oriented glass at
room temperature. This was confirmed by an attempt to shear
the cover slip.
The DSC trace for carbazole derivative 2a exhibits three
transitions (Fig. 2b). The first endotherm at 60 uC with a small
enthalpy of 3 kJ mol21 was assigned as a transition to a
crystalline columnar phase (Crcol) as identified by XRD
(vide infra). This was followed by melting to a hexagonal
columnar mesophase at 109 uC and isotropization at 126 uC
with an enthalpy of 20 kJ mol21. The cooling trace showed all
three transitions progressively shifted to lower temperatures as
the viscosity of the phases increased. Thus, the lowest transition
temperature was supercooled by 16 uC. The second heating
curve reproduced the transitions obtained for the virgin sample.
The formation of the crystalline columnar phase by 2a and
glassification of the Colh in 1a are consistent with observations
for other discogens with the ‘‘pinwheel’’ substitution patterns
and are desired for opto-electronic applications.37–39 This can
be rationalized by the relatively large core–core distance in
the mesophase (vide infra) and consequently ineffective p–p
interactions which typically drive the crystallization and
molecular tilting of small aromatics.40
Liquid crystalline properties
The investigation of carbazoles 2a–2c using polarized optical
microscopy and differential scanning calorimetry (DSC)
showed that only 2a forms
a liquid crystalline phase.
Compound 2b showed only melting to an isotropic phase
upon heating at 114 uC (38 kJ mol21) and crystallization upon
cooling, and derivative 2c is an isotropic liquid at ambient
temperature. These results are consistent with the observed
behavior of pyrene and other analogs of 2.31 For instance, the
benzo[c]cinnoline analog of 2c was also found to exist as an
isotropic liquid at ambient temperature. This was ascribed to
the poor core–core interactions resulting from steric hindrance
of the 3,5-dioctyloxyphenyl substituent.31
The mesophases obtained on cooling of compounds 1a
and 2a displayed fan-shaped textures that are characteristic
for columnar mesophases35,36 (Fig. 1). The texture for 1a
was highly birefringent. In contrast, carbazole derivative 2a
showed large homeotropic domains with discotic columnar
axes oriented perpendicular to the substrate in both liquid
Fig. 1 Optical textures of (a) discotic hexagonal phase of 1a at 75 uC
(magnification 606), (b) glass phase, retaining structural features of
the preceding hexagonal mesophase of 1a at room temperature
(magnification 3006), (c) crystalline columnar phase of 2a at 80 uC;
the texture is identical with that of the discotic hexagonal phase
obtained at higher temperature (magnification 3006).
Fig. 2 DSC traces of a) pyrene 1a and b) carbazole 2a. Heating rate
10 uC min21. Transition temperatures (uC) and enthalpies (in italics,
kJ mol21): Cr = crystalline, Crcol = crystalline columnar, Colh
hexagonal columnar, Iso = isotropic.
=
This journal is ß The Royal Society of Chemistry 2007
J. Mater. Chem., 2007, 17, 1392–1398 | 1393

View Article Online
Fig. 3 XRD pattern for the Colh mesophase of 1a at 80 uC obtained
by cooling from the isotropic phase. The boxed region is expanded in
the inset.
Powder X-ray diffraction (XRD)
Liquid crystalline derivatives 1a and 2a were investigated by
variable temperature powder X-ray diffraction to determine
their phase structure. The results are shown in Fig. 3 and 4 and
tabularized data is provided in the ESI.{ The cell parameter a
˚
˚
was calculated to be 28.9 A for 1a and 29.8 A for 2a using
eqn (1) and assuming a hexagonal columnar mesophase
structure for both compounds. The cell parameter a is smaller
˚
than the van der Waals diameter of the molecule (37 A) in the
most extended conformation. This indicates either interdigita-
tion or partial folding of the chains in both 1a and 2a.
.
pffiffi
a~d200|4
3
(1)
Fig. 4 XRD patterns for 2a: a) a Colh phase at 120 uC, and b) a Crcol
phase at 70 uC obtained by cooling from the isotropic phase. The
boxed region is shown expanded in the inset.
Diffractograms obtained for both compounds in the tem-
perature regions identified to be liquid crystalline (vide supra)
showed reflection patterns that can be ascribed to a two-
dimensional hexagonal lattice of a columnar discotic liquid
crystal.41 Graphs of one-dimensional intensity as a function of
the diffraction angle 2h show the strong (100) peak and weak
higher-order reflections (hk0) in the approximate spacing ratio
of d100/!3, d100/!4, d100/!7, etc. (Fig. 3 and 4a). For both
compounds the (110) peak is surprisingly weak albeit detect-
able, and together with the more intense (210) and (310) reflec-
tions support the phase assignment to columnar hexagonal
(Colh). Both diffractograms are dominated by the (h00) reflec-
tions, with the (200) reflection being the second strongest. This
rather unusual diffraction pattern observed for both com-
pounds may result from incomplete powder averaging and, in
consequence, unreliable relative intensities of the reflections.
The wide-angle region of the diffractograms shows a broad
A comparison of results obtained for carbazole 2a at 120 uC
and 70 uC shows similar X-ray diffraction patterns in the low
angle, but significant differences in the wide-angle areas
(Fig. 4). The wide-angle X-ray pattern for the hexagonal
columnar mesophase at 120 uC shows only the broad halo,
while the pattern at 70 uC displays additional sharp reflections
in the range of 4.2 A–5.9 A. Thus, the X-ray diffraction
pattern recorded at 70 uC appears to arise from periodic
arrangement of molecules within each column, incorporating
column-to-column registry, i.e. a three-dimensional crystal.
However, alkyl chains in this phase are still significantly
˚
˚
˚
disordered, as indicated by the broad halo at about 4.5 A. This
pattern can be tentatively assigned to a crystalline columnar
phase (Crcol) based on the fact that the phase is formed upon
cooling of the Colh mesophase with unnoticeable texture
change (vide supra).
˚
halo at around 4.6 A indicating the mean distance between the
molten alkyl chains within one column. The peak on the right
Absorption spectroscopy
side of the diffused feature arises from the core–core correla-
˚
tion. For 1a the core-core mean distance is 3.9 A at 80 uC,
The UV absorption spectra for pyrene 1a and carbazole 2a
derivatives in cyclohexane are shown in Fig. 5. Substitution of
the parent carbazole with four 3,4-dioctyloxyphenyl groups
results in a red-shift of its lowest energy transition by 33 nm to
˚
which is larger than the 3.7 A measured for 2a at 120 uC, both
in the Colh phase. Both of these values are larger than a typical
41
˚
3.5 A mean core–core separation in other discotics.
1394 | J. Mater. Chem., 2007, 17, 1392–1398
This journal is ß The Royal Society of Chemistry 2007

View Article Online
Fig. 5 UV-vis spectra for 3,4-dioctyloxyphenyl derivatives of pyrene
1a and carbazole 2a recorded in cyclohexane.
363 nm in 2a. The lowest energy absorption band for 1a has a
maximum at 391 nm, which is shifted by 56 nm to the red
1
relative to the allowed La transition or 36 nm relative to the
forbidden 1Lb band in pyrene.42 The absorption at 391 nm
tails into the visible range, which results in a light yellow
color for 1a.
Fig. 7 The temperature dependence of positive carrier mobilities
obtained by the TOF method for pyrene 1a (top) and carbazole 2a
(bottom). Electric field strength: 30 kV cm21
.
Photoconductivity
Charge carrier mobilities of pyrene 1a and carbazole 2a
derivatives were measured by a standard TOF (time-of-flight)
method43 in 17.0 mm or 30.8 mm thick ITO-coated glass cells,
respectively (for details see the Experimental section and
ESI{). Natural textures of the samples were grown from the
isotropic phase (see Fig. 1).
logarithmic plot gave the transit time tT,44 which was used to
calculate the charge carrier mobility m according to eqn (2) (l is
the sample thickness and V is the applied voltage).
m = l2/tTV
(2)
Raw data for photocurrent as a function of time were
analyzed in double logarithmic plots. A typical presentation of
data is shown in Fig. 6 for positive and negative photocurrents
generated in the Colh phase of pyrene 1a at 60 uC.
Photocurrent generation was investigated over a range of
temperatures and results for positive charge carriers (holes)
mobility as a function of temperature are shown in Fig. 7. The
full tabularized data are listed in the ESI.{
The intercept of the two lines corresponding to the pre-
In general, the charge mobility recorded within the same
phase was decreasing with decreasing temperature, and was
bias-independent, which is characteristic for liquid crystalline
semiconductors. Analysis showed that the charge transport
in columnar mesophases of pyrene 1a and carbazole 2a
derivatives and also in the crystalline columnar phase Crcol
of 2a was non-dispersive and the bundle of charge carriers,
electrons (when the top electrode was negative) and holes
(when the top electrode was positive), was moving through the
sample with a constant velocity. This was indicated by the
well-defined plateau on the photocurrent curves (Fig. 6). At
the transit time tT the photocurrent decay broadened, when the
charge carriers arrived at the bottom electrode.
and post-transit slopes of the photocurrent in the double
Positive charge (hole) mobilities in unaligned samples
of both compounds were found to be on the order of
1023 cm2 V21 s21, which is typical for many other discotic
mesogens in the Colh phase.45,46 Measurements of the negative
charge mobility for pyrene 1a at several temperatures gave
values that are close to the hole mobility. Such ambipolar
charge transport has been observed for other well-purified46–48
discogens including phthalocyanines17 and triphenylenes,48
and is consistent with a hopping transport mechanism.49 In
Fig. 6 Photocurrent
I transient decay curves in the hexagonal
columnar mesophase of pyrene 1a at 60 uC for the positive (h+) and
negative (e2) carriers obtained by the TOF method. Wavelength of
laser pulse: 337 nm, electric field strength: 20 kV cm21, sample
thickness: 17.0 mm, bias: 34.0 V. The intercept of the two lines shows
the transit time tT. The stripe pattern of the graph results from
digitalization of the signal.
This journal is ß The Royal Society of Chemistry 2007
J. Mater. Chem., 2007, 17, 1392–1398 | 1395

View Article Online
contrast, measurements of negative photocurrents in both Colh
and Crcol phases of carbazole 2a were less unequivocal, and
precise values for charge mobility could not be obtained. This
is presumably due to the presence of small ionic impurities
which obscure the analysis and determination of me47 especially
in a thick cell (30.8 mm).46
0.1 cm2 V21 s21 have been reported for large p-electron
deficient discotics.53
Conclusions
TOF results indicate that the charge transport in pyrene 1a is
ambipolar, while the carbazole derivative 2a behaves as a
p-type semiconductor. Positive (hole) carrier mobilities are on
the order of 1023 cm2 V21 s21 and are typical for a hexagonal
mesophase. The formation of an ordered glass phase rather
than a crystal lattice in 1a results in a broad range Arrhenius-
type behavior of charge mobilities. Phase behavior of 1a and
2a allows for the formation rigid phases (frozen Colh and
Crcol) with charge mobilities typical for a fluid mesogen.
Upon cooling from the Colh phase, carbazole 2a shows an
abrupt increase of charge mobility by a factor of 3 at the phase
transition to the Crcol phase (Fig. 7). This is consistent with a
transition to a more organized phase and better intermolecular
contact within the columns. Support for this is provided by
XRD results, which show a decrease of the mean core–core
˚
distance by 0.2 A upon Colh to Crcol transition (Fig. 4).
Measurements conducted at 30 uC for the crystalline phase of
carbazole 2a showed completely dispersive charge transport.
This can be interpreted as a disruption in hexagonally
arranged transport tunnels in the Crcol phase by creating the
grain boundaries during crystallization.2 Consequently, charge
carriers were deeply trapped before they were able to reach the
counter electrode.
Experimental
All NMR spectra were obtained at 300 MHz (1H) and 75 MHz
(13C) in CDCl3. Chemical shifts were referenced to TMS (1H)
or solvent (13C). IR spectra were recorded for neat samples
(liquid or microcrystalline) on NaCl plates. Elemental analysis
was provided by Atlantic Microlabs.
The behavior of pyrene 1a is different from that of 2a, and
the charge mobility follows a monotonous decrease with
decreasing temperature (Fig. 7). Pyrene 1a supercools to room
temperature forming a rigid glass (vide supra) with structural
features of the preceding Colh phase and without disruption of
the charge transport tunnels. An Arrhenius plot of the positive
charge mobility data gave an activation energy for 1a
of 0.10 ¡ 0.01 eV (Fig. 8). This low Ea value and weak
temperature dependence of mh are consistent with the charge
hopping mechanism and intracolumn charge transport.7,50
Ionic charge transport shows larger temperature dependence
and Ea in the range of 0.2–0.4 eV.48
Optical microscopy was performed using a PZO Biolar
microscope equipped with an HS1 Instec hot stage. Phase
transition temperatures were reported as onset of the peak and
were obtained from a TA Instruments DSC 2920. Samples
were heated and cooled with the temperature rate of
10 uC min21. UV spectra were recorded in cyclohexane and
molar absorption was obtained from a Beer’s law plot of 4–5
concentrations. XRD measurements were carried out using a
Rigaku RINT2000 X-ray diffractometer with a homemade
heater.
The charge carrier mobilities of liquid crystals were mea-
sured by the TOF (time-of-flight) method43 using a nitrogen
gas laser (Nippon Laser, l = 337 nm) and a polarizing micro-
scope with a hot stage. The cells were mounted on a
commercially available hot stage and electric bias was applied
by a stabilized DC power supply. Depending on the polarity of
the applied field, positive (holes) or negative (electrons) charge
carriers were moving through the sample, causing displace-
ment photocurrent, which was detected on a digital oscillo-
scope with a homemade preamplifier. A schematic diagram of
the experimental apparatus is shown in Fig. 9.
The observed charge mobilities in 1a and 2a are comparable
to those obtained for typical materials20 with natural home-
otropic alignment such as triphenylenes51 in which large
homeotropic domains are formed by slow cooling from the
isotropic phase. The alignment of samples by slow cooling in a
magnetic field does not significantly affect the photoconduc-
tivity results.3 Higher charge mobilities close to 1 cm2 V21 s21
are observed for large planar systems52 such as hexabenzo-
coronenes and superphenalenes,19 and other highly ordered
systems.2 Similarly, high electron mobilities on the order of
The cell thickness was 17.0 mm for pyrene derivative 1a
and 30.8 mm for carbazole derivative 2a. The cells were filled
with isotropic material by capillary forces. The material was
Fig. 8 Arrhenius plot for positive charge mobility in 1a in the
temperature range of 45 uC–85 uC.
Fig. 9 Schematic diagram of the experimental apparatus (TOF
method).
1396 | J. Mater. Chem., 2007, 17, 1392–1398
This journal is ß The Royal Society of Chemistry 2007

View Article Online
slowly cooled to a discotic hexagonal phase and stabilized.
No magnetic field was used for sample alignment. Polarized
optical microscopy showed a highly birefringent texture for 1a
practically with no homeotropic domains, while the texture
of the carbazole derivative 2a exhibited large homeotropic
domains with discotic columnar axes perpendicular to the
electrodes.
We are also in debt to Dr Wiktor A. Piecek, a visiting NATO
Fellow on leave from Military University of Technology
(Warsaw, Poland), for his technical assistance.
References
1 D. Adam, F. Closs, T. Frey, D. Funhoff, D. Haarer, H. Ringsdorf,
P. Schuhmacher and K. Siemensmeyer, Phys. Rev. Lett., 1993, 70,
457–460.
1,3,6,8-Tetraarylcarbazole (2). General procedure
2 D. Adam, P. Schuhmacher, J. Simmerer, L. Haeussling,
K. Siemensmeyer, K. H. Etzbach, H. Ringsdorf and D. Haarer,
Nature, 1994, 371, 141–143.
3 N. Boden, R. J. Bushby, J. Clements, B. Movaghar, K. J. Fonobsn
and T. Kreouzis, Phys. Rev. B, 1995, 52, 13274–13280.
4 D. Adam, W. Roemhildt and D. Haarer, Jpn. J. Appl. Phys., 1996,
35, 1826–1831.
5 A. Ochse, A. Kettner, J. Kopitzke, J. H. Wendorff and H. Bassler,
Phys. Chem. Chem. Phys., 1999, 1, 1757–1760.
6 S. Chandrasekhar and S. K. Prasad, Contemp. Phys., 1999, 40,
237–245.
7 T. Kreouzis, K. J. Donovan, N. Boden, R. J. Bushby, O. R. Lozman
and Q. Liu, J. Chem. Phys., 2001, 114, 1797–1802.
8 K. J. Donovan, T. Kreouzis, K. Scott, J. C. Bunning, R. J. Bushby,
N. Boden, O. R. Lozman and B. Movaghar, Mol. Cryst. Liq.
Cryst., 2003, 396, 91–112.
9 S. Chandrasekhar, in Handbook of Liquid Crystals, ed. D. Demus,
J. W. Goodby, G. W. Gray, H.-W. Spiess and V. Vill, Wiley-VCH,
New York, 1998, vol. 2B, pp. 749–780; N. Boden and B. Movaghar,
in Handbook of Liquid Crystals, ed. D. Demus, J. W. Goodby,
G. W. Gray, H.-W. Spiess and V. Vill, Wiley-VCH, New York,
1998, vol. 2B, pp. 781–798; R. J. Bushby and O. R. Lozman, Curr.
Opin. Colloid Interface Sci., 2002, 7, 343–354.
A mixture of 1,3,6,8-tetrabromocarbazole32 (4, 482 mg,
1 mmol), appropriate boronic ester 5 (4.4 mmol), (Ph3P)4Pd
(0.2 mmol), 2 M aqueous solution of Na2CO3 (4 mL), EtOH
(4 mL), and toluene (40 mL) was refluxed for 24–72 h under
N2 until a single major product was observed by TLC. The
reaction mixture was cooled, Et2O was added, and the organic
layer was separated and dried (Na2SO4). The solvent was
evaporated and the crude product was passed through a silica
gel plug with hexanes–CH2Cl2 (2 : 1 ratio). The product was
isolated by column chromatography (pure hexanes followed by
increasing ratio of hexanes–CH2Cl2) followed by recrystalliza-
tion from a hexanes–ethanol mixture.
1,3,6,8-Tetrakis(3,4-dioctyloxyphenyl)carbazole (2a). Cr 61
(3 kJ mol21) Crcol 109 (24 kJ mol21) Colh 126 (20 kJ mol21
)
Iso; 1H NMR d 0.78–0.85 (m, 24H), 1.22–1.44 (m, 80H), 1.73–
1.85 (m, 16H), 3.95–4.07 (m, 16H), 6.92 (d, J = 1.6 Hz, 2H),
6.95 (s, 2H), 7.15–7.22 (m, 8H), 7.55 (d, J = 1.6 Hz, 2H), 8.16
(d, J = 1.5 Hz, 2H), 8.44 (s, 1H); IR 3438 (NH), 1514 (CLC)
1248 (C–O–C) cm21; UV-vis lmax (log e): 209 (4.99), 267
(4.91), 358 (3.83), 368 (3.83). Anal. Calcd for C100H153NO8: C,
80.22; H, 10.30; N, 0.94. Found: C, 80.09; H, 10. 32; N, 0.94%.
10 S. Kumar, Chem. Soc. Rev., 2006, 35, 83–109.
11 I. H. Stapff, V. Stumpflen, J. H. Wendorff, D. B. Spohn and
D. Mobius, Liq. Cryst., 1997, 23, 613–617; I. Seguy, P. Jolinat,
P. Destruel, J. Farenc, R. Mamy, H. Bock, J. Ip and T. P. Nguyen,
J. Appl. Phys., 2001, 89, 5442–5448.
12 L. Schmidt-Mende, A. Fechtenkotter, K. Mu¨llen, E. Moons,
R. H. Friend and J. D. Mackenzie, Science, 2001, 293, 1119–1122.
13 A. M. van de Craats, N. Stutzmann, O. Bunk, M. M. Nielsen,
M. Watson, K. Mullen, H. D. Chanzy, H. Sirringhaus and
R. H. Friend, Adv. Mater., 2003, 15, 495–499.
1,3,6,8-Tetrakis(4-octyloxyphenyl)carbazole (2b). Cr1 104
1
(6 kJ mol21) Cr2 114 (38 kJ mol21) Iso; H NMR d 0.83 (t,
14 I. McCulloch, W. Zhang, M. Heeney, C. Bailey, M. Giles,
D. Graham, M. Shkunov, D. Sparrowe and M. Tierney,
J. Mater. Chem., 2003, 13, 2436–2444.
15 C. W. Struijk, A. B. Sieval, J. E. J. Dakhorst, M. van Dijk,
P. Kimkes, R. B. M. Koehorst, H. Donker, T. J. Schaafsma,
S. J. Picken, A. M. van de Craats, J. M. Warman, H. Zuilhof and
E. J. R. Sudholter, J. Am. Chem. Soc., 2000, 122, 11057–11066.
16 H. Fujikake, T. Murashige, M. Sugibayashi and K. Ohta, Appl.
Phys. Lett., 2004, 85, 3474–3476.
J = 6.5 Hz, 12H), 1.19–1.45 (m, 40H), 1.71–1.81 (m, 8H), 3.96
(t, J = 6.5 Hz, 4H), 3.97 (t, J = 6.5 Hz, 4H), 6.94 (d, J = 8.7 Hz,
4H), 6.99 (d, J = 8.7 Hz, 4H), 7.54 (s, 2H), 7.55 (d, J = 8.3 Hz,
4H), 7.60 (d, J = 8.7 Hz, 4H), 8.18 (s, 2H), 8.35 (s, 1H); 13C
NMR d 14.1, 22.7, 26.1, 29.27, 29.32, 29.36, 29.40, 31.8, 68.1,
114.8, 115.3, 117.1, 124.7, 125.0, 125.1, 128.3, 129.2, 131.0,
133.4, 134.4, 136.8, 158.3, 158.7; IR 3479 (NH), 1511 (CLC)
1250 (C–O–C) cm21. Anal. Calcd for C68H89NO4: C, 82.96; H,
9.11; N, 1.42. Found: C, 82.78; H, 9.11; N, 1.42%.
17 H. Iino, J. Hanna, R. J. Bushby, B. Movaghar, B. J. Whitaker and
M. J. Cook, Appl. Phys. Lett., 2005, 87, 132102.
18 A. M. van de Craats, J. M. Warman, A. Fechtenkotter, J. D. Brand,
M. A. Harbison and K. Mullen, Adv. Mater., 1999, 11, 1469–1472.
19 M. G. Debije, J. Piris, M. P. de Haas, J. M. Warman, Z. Tomovic,
C. D. Simpson, M. D. Watson and K. Muellen, J. Am. Chem. Soc.,
2004, 126, 4641–4645.
20 R. J. Bushby and O. R. Lozman, Curr. Opin. Solid State Mater.
Sci., 2003, 6, 569–578.
21 S. F. Nelson, Y.-Y. Lin, D. J. Gundlach and T. N. Jackson, Appl.
Phys. Lett., 1998, 72, 1854–1856; F. Granier, G. Horowitz, X. Peng
and D. Fichou, Adv. Mater., 1990, 2, 592–594.
1,3,6,8-Tetrakis(3,5-dioctyloxyphenyl)carbazole (2c). Liquid:
1H NMR d 0.86 (t, J = 6.4 Hz, 12H), 0.89 (t, J = 6.5 Hz, 12H),
(m, 24H), 1.30–1.50 (m, 80H), 1.74–1.85 (m, 16H), 3.98 (t, J =
6.6 Hz, 8H), 4.05 (t, J = 6.5 Hz, 8H), 6.48 (t, J = 2.1 Hz, 2H),
6.51 (t, J = 2.1 Hz, 2H), 6.81 (d, J = 2.2 Hz, 4H), 6.98 (d, J =
2.2 Hz, 4H), 7.71 (d, J = 1.6 Hz, 2H), 8.32 (d, J = 1.4 Hz, 2H),
8.66 (s, 1H); IR 3459 (NH), 1590 (CLC) 1166 (C–O–C) cm21
.
22 N. Karl, K.-H. Kraft, J. Marktanner, M. Munch, F. Schatz,
R. Stehle and H.-M. Uhde, J. Vac. Sci. Technol., A, 1999, 17,
2318–2328.
Anal. Calcd for C100H153NO8: C, 80.22; H, 10.30; N, 0.94.
Found: C, 80.37; H, 10.37; N, 0.90%.
23 R. F. Chaiken and D. R. Kearns, J. Chem. Phys., 1968, 49,
2846–2850.
24 M. Tierney and D. Lubman, Appl. Spectrosc., 1987, 41, 880–886.
25 R. A. Holroyd, J. M. Preses, E. H. Boettcher and W. F. Schmidt,
J. Phys. Chem., 1984, 88, 744–749.
Acknowledgements
This project was supported by NSF grants (CHE-9528029 and
CHE-0096827). We thank Prof. Paul A. Heiney for valuable
suggestions concerning the interpretation of the XRD results.
26 R. Oshima, T. Uryu and M. Seno, Macromolecules, 1985, 18,
1043–1045.
This journal is ß The Royal Society of Chemistry 2007
J. Mater. Chem., 2007, 17, 1392–1398 | 1397

View Article Online
27 T. Uryu, H. Ohkawa and R. Oshima, Macromolecules, 1987, 20,
712–716.
42 A. C. Benniston, A. Harriman, D. J. Lawrie and S. A. Rostron,
Eur. J. Org. Chem., 2004, 2272–2276.
43 E. Muller-Horsche, D. Haarer and H. Scher, Phys. Rev. B, 1987,
35, 1273–1280.
28 D. W. Kim, H. Moon, S. Y. Park and S. I. Hong, React. Funct.
Polym., 1999, 42, 73–86.
44 H. Scher and E. W. Montroll, Phys. Rev. B, 1975, 12, 2455–2477.
45 N. Boden and B. Movaghar, in Handbook of Liquid Crystals, ed.
D. Demus, J. W. Goodby, G. W. Gray, H.-W. Spiess and V. Vill,
Wiley-VCH, New York, 1998, vol. 2B, pp. 781–798.
46 H. Iino, Y. Takayashiki, J. Hanna, R. J. Bushby and D. Haarer,
Proc. SPIE–Int. Soc. Opt. Eng., 2005, 5940, 594008.
47 H. Iino, Y. Takayashiki, J. Hanna, R. J. Bushby and D. Haarer,
Appl. Phys. Lett., 2005, 87, 192105.
29 K.-Y. Law, Chem. Rev., 1993, 93, 449–486.
30 K. J. Donovan, K. Scott, M. Somerton, J. Preece and
M. Manickam, Chem. Phys., 2006, 322, 471–476.
31 M. J. Sienkowska, J. M. Farrar, F. Zhang, S. Kusuma, P. A. Heiney
and P. Kaszynski, J. Mater. Chem., 2007, 17, DOI: 10.1039/
b615545f.
32 K. Smith, D. M. James, A. G. Mistry, M. R. Bye and
D. J. Faulkner, Tetrahedron, 1992, 48, 7479–7488.
33 N. Miyaura, T. Yanagi and A. Suzuki, Synth. Commun., 1981, 11,
513–519.
48 H. Iino, J. Hanna, D. Haarer and R. J. Bushby, Jpn. J. Appl. Phys.,
2006, 45, 430–433.
49 N. Boden, R. J. Bushby and J. Clements, J. Chem. Phys., 1993, 98,
5920–5931; L. Lever, R. Kelsall and R. J. Bushby, J. Comput.
Electron., 2005, 4, 101–104; V. Lemaur, D. A. Da Silva Filho,
V. Coropceanu, M. Lehmann, Y. Geerts, J. Piris, M. G. Debije,
A. M. van de Craats, K. Senthilkumar, L. D. A. Siebbeles,
J. M. Warman, J.-L. Bredas and J. Cornil, J. Am. Chem. Soc.,
2004, 126, 3271–3279.
34 K. Ogino, S. Iwashima, H. Inokuchi and Y. Harada, Bull. Chem.
Soc. Jpn., 1965, 38, 473–477.
35 S. Chandrasekhar, B. K. Sadashiva and K. A. Suresh, Pramana,
1977, 9, 471–480.
36 I. Dierking, Textures of Liquid Crystals, Wiley-VCH, Weinheim,
2003.
37 N. Boden, R. J. Bushby, A. N. Cammidge and P. S. Martin,
J. Mater. Chem., 1995, 5, 1857–1860.
50 H. Nakayama, M. Ozaki, W. F. Schmidt and K. Yoshino, Jpn. J.
Appl. Phys., 1999, 38, L1038–L1041.
38 N. Boden, R. J. Bushby, A. N. Cammidge, A. El-Mansoury,
P. S. Martin and Z. Lu, J. Mater. Chem., 1999, 9, 1391–1402.
39 B. Glu¨sen, A. Kettner and J. H. Wendorff, Mol. Cryst. Liq. Cryst.,
1997, 303, 115–120.
40 G. R. Desiraju and A. Gavezzotti, Acta Crystallogr., Sect. B, 1989,
45, 473–482; C. A. Hunter, K. R. Lawson, J. Perkins and C. J. Urch,
J. Chem. Soc., Perkin Trans. 2, 2001, 651–669.
41 S. K. Prasad, D. S. S. Rao, S. Chandrasekhar and S. Kumar, Mol.
Cryst. Liq. Cryst., 2003, 396, 121–139.
51 J. Simmerer, B. Glu¨sen, W. Paulus, A. Kettner, P. Schuhmacher,
D. Adam, K. H. Etzbach, K. Siemensmeyer, J. H. Wendorff,
H. Ringsdorf and D. Haarer, Adv. Mater., 1996, 8, 815–819.
52 A. M. van de Craats and J. M. Warman, Adv. Mater., 2001, 13,
130–133.
53 M. Lehmann, G. Kestemont, R. G. Aspe, C. Buess-Herman,
M. H. J. Koch, M. G. Debije, J. Piris, M. P. de Haas, J. M.
Warman, M. D. Watson, V. Lemaur, J. Cornil, Y. H. Geerts,
R. Gearba and D. A. Ivanov, Chem.–Eur. J., 2005, 11, 3349–3362.
1398 | J. Mater. Chem., 2007, 17, 1392–1398
This journal is ß The Royal Society of Chemistry 2007