Photoconductive liquid-crystalline derivatives of 6-oxoverdazyl

Article abstract of DOI:10.1021/ja209467h

1,3,5-Triphenyl-6-oxoverdazyl radicals 1[n], in which each phenyl group is substituted with three alkylsulfanyl groups (n = 6, 8, 10), exhibit a monotropic columnar rectangular (Col_r) phase below 60 °C. Detailed analysis of 1[n] revealed a broa

Full text of DOI:10.1021/ja209467h

Communication
pubs.acs.org/JACS
Photoconductive Liquid-Crystalline Derivatives of 6-Oxoverdazyl
Aleksandra Jankowiak,† Damian Pociecha,‡ Jacek Szczytko,§ Hirosato Monobe,∥ and Piotr Kaszynski*,†,⊥
́
†Organic Materials Research Group, Department of Chemistry, Vanderbilt University, Nashville, Tennessee 37235, United States
‡Department of Chemistry, University of Warsaw, 02-089 Warsaw, Poland
§Institute of Experimental Physics, Faculty of Physics, University of Warsaw, Hoza 69, 00-681 Warsaw, Poland
̇
∥Research Institute for Ubiquitous Energy Devices, National Institute of Advanced Industrial Science and Technology, AIST Kansai
Centre, Ikeda, Osaka 563-8577, Japan
⊥Faculty of Chemistry, University of Łod
́
z, Tamka 12, 91-403 Łod
́
z, Poland
́
́
S
* Supporting Information
spectrum and an electrochemical window of ∼1.5 V,17 both
controllable by judicious choice of substituents. Some verdazyl
derivatives have been investigated as components of para-
magnetic semiconductors18 and in photoexcited molecular
systems19 intended for photomagnetic devices.20 Here we
report the first examples of discotic derivatives of verdazyl 1[n]
(Chart 1) and investigate their liquid-crystalline, electro-
chemical, magnetic, and photovoltaic properties.
ABSTRACT: 1,3,5-Triphenyl-6-oxoverdazyl radicals
1[n], in which each phenyl group is substituted with
three alkylsulfanyl groups (n = 6, 8, 10), exhibit a
monotropic columnar rectangular (Colr) phase below 60
°C. Detailed analysis of 1[n] revealed a broad absorption
band in the visible region with maxima at 540 and 610 nm
0/+1
0/−1
and redox potentials E
= +0.99 V and E
= −0.45 V
1/2
1/2
vs SCE. Photovoltaic studies of 1[8] demonstrated a hole
mobility of 1.52 × 10−3 cm2 V−1 s−1 in the mesophase with
Chart 1
an activation energy of 0.06
0.01 eV. Magnetization
studies of 1[8] revealed nearly ideal paramagnetic behavior
in either the solid or fluid phase above 200 K and weak
antiferromagnetic interactions at low temperatures.
ne-dimensional charge transport1 along the self-
O
assembled columns in discotic liquid crystals2 has been
recognized as a desirable property for applications in
photovoltaic cells,3,4 light-emitting diodes,5 and field-effect
transistors.6 Therefore, a number of discotics, including those
based on triphenylene,7 pyrene,8 carbazole,8 phthalocyanine,9
hexabenzocoronene,10 and other polycyclic aromatics have
been investigated for their photophysical properties.11
Radicals 1[n] were obtained from hydrazines 2[n]21 and
benzaldehydes 3[n]22 according to the Milcent method23
(Scheme 1). Crude hydrazines were converted to hydrazones
4[n], which were reacted with triphosgene to give carbonyl
chlorides 5[n]. After isolation and purification, the chlorides
were reacted with hydrazine 2[n] in benzene.24 The resulting
tetrazanes 6[n] were partially purified and oxidized with
K3Fe(CN)6 under phase-transfer catalysis conditions to give
radicals 1[n] in ∼20% overall yield. For magnetization and
differential scanning calorimetry (DSC) studies, radicals 1[n]
were obtained in comparable yields by oxidation of 6[n] with
PbO2.
All four radicals exhibited low-intensity absorption bands in
the visible range (maxima at 540 and 610 nm; Figure 1) that
according to time-dependent density functional theory results
are due to several electronic transitions originating mainly from
the four highest occupied MOs, which are localized on the
benzene rings, to the LUMO, which is localized on the verdazyl
More facile generation of an electron−hole pair and higher
current is expected in neutral π-radicals than in closed-shell
aromatics because of their relatively high lying singly occupied
molecular orbital (SOMO) and low ionization potential.12 In
this context, a number of thiazyl and thiazinyl derivatives have
been synthesized and investigated as unidimensional semi-
conductors.13 Attempts to generate liquid-crystalline π-
delocalized stable radicals were unsuccessful14 until recently.
In 2009, Valesco and co-workers15 reported a carbazole
derivative of triphenylmethyl radical that exhibits two columnar
phases, quasi-reversible redox pairs with an electrochemical
window of 1.5 V, a broad absorption spectrum in the visible
region, and phase-dependent magnetic interactions between the
cores. This very exciting discovery provided new impetus for
the development of other materials of this type in which
magnetic properties can be coupled with anisotropic charge
transport, dielectric switching, chirality, and photoalignment.
Verdazyls,16 including 6-oxoverdazyls, are among a handful of
π-delocalized radicals that are stable under ambient conditions.
They exhibit broad absorption bands in the visible part of the
Received: October 8, 2011
Published: January 20, 2012
© 2012 American Chemical Society
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Communication
a
Scheme 1. Synthesis of 1[n]
Figure 2. DSC traces for 1[8]. The heating and cooling rates were 5 K
min−1. For a description, see the text.
a transition to an isotropic (I) phase at 60 °C upon heating
(green line) with an enthalpy comparable to that recorded on
cooling. The crystalline polymorph of 1[8] obtained from the
melt was different from that obtained from solutions. It melted
with a peak at 52 °C (red line), forming an enantiotropic phase
with a transition to the I phase at 60 °C (Figure 2). The
position of the I → M transition peak was cooling-rate-
dependent.
Optical microscopy revealed that slow cooling of the sample
from the isotropic liquid resulted in the growth of crystallites
(Figure 3a), while quenching the sample at a temperature
a
Reagents and conditions: (a) cat. AcOH, EtOH, reflux, ∼80%; (b)
CO(OCCl3)2, pyridine, CH2Cl2, rt, ∼65%; (c) 2[n], Et3N, benzene,
50 °C, ∼45%; (d) K3Fe(CN)6, Na2CO3 0.5 M, cat. [Bu4N]+Br−,
CH2Cl2, rt or PbO2, K2CO3, toluene/MeCN, rt, ∼80%.
Figure 1. Electronic absorption spectrum of 1[8] in hexane.
Figure 3. Optical textures of 1[8] obtained upon (a) slow cooling, (b)
fast cooling, and (c) after 2 h at 30 °C [8× longer exposure than in
(b)].
unit.25 The excitation involving the SOMO-to-LUMO
transition was calculated to occur at 529 nm (f = 0.03).
Thermal analysis of a sample of 1[8] freshly crystallized from
AcOEt/MeCN revealed a melting transition with a peak at 65
°C (Table 1 and the black line in Figure 2). Upon cooling (blue
below 50 °C led to the formation of a mosaic texture typical of
discotic phases (Figure 3b). The phase slowly lost birefringence
(Figure 3c) and the ability to diffract (see below).
Similar mesogenic behavior was observed for the 1[6] and
1[10] homologues (Table 1), but compound 1[12] did not
exhibit mesomorphic properties. Thus, 1[12] showed only
melting at 63 °C and crystallization at 52 °C, both
characterized by large enthalpy changes. Fast cooling of an
isotropic sample of 1[12] did not give a birefringent phase such
as that in Figure 3b but instead led to the formation of a dark
phase similar to that in Figure 3c.
Powder X-ray diffraction (XRD) analysis of the three lower
homologues of 1[n] revealed formation of a columnar
rectangular (Colr) phase. The fast-cooled samples gave XRD
patterns with two sharp diffraction signals in the small-angle
region that can be indexed as (11) and (02) and a typical broad
halo at 4.2 Å in the wide-angle region (Figure 4 and Table 2).
Table 1. Transition Temperatures (°C) and Enthalpies (kJ/
a
mol, in Italics) for 1[n]
n
6
8
Cr 39 (54.2) Colr 50 (7.9) I
Cr 62 (108) [Colr 60 (15.1)] I
Cr 62 (125.6) [Colr 55 (14.0)] I
Cr 63 (116.6) I
10
12
a
Determined by DSC (5 K min−1) in the heating mode as onset. Cr =
crystalline; Colr = columnar rectangular; I = isotropic.
line), the sample underwent a transition at 56 °C with a
relatively small enthalpy (12.1 kJ/mol) to a phase that could be
supercooled to −40 °C. The monotropic (M) phase underwent
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Figure 4. XRD pattern for 1[8] obtained at 25 °C. The second broad
halo at 30° (3.0 Å) is due to diffraction on the glass substrate.
Figure 5. Plot of μeff vs T obtained for 1[8] at 200 Oe (rate of 1 K
min−1). The vertical line represents the melting transition. The inset
shows μeff vs T for a sample cooled from the I phase to 2 K.
Table 2. XRD Data for the Colr Phase of 1[n] at 30 °C
n
Miller indices hk
d spacing (Å)
lattice parameters (Å)
prevent effective intermolecular close π−π contacts, and the
unpaired electrons remain largely isolated in both the solid and
fluid phases.
6
11
02
11
02
11
02
19.7
17.1
21.9
18.8
23.5
19.6
a = 24.1
b = 34.2
a = 26.9
b = 37.6
a = 29.4
b = 39.2
Time-of-flight (TOF) measurements8,26 found the positive
charge carrier (hole) mobility (μh) in a fast-cooled, unaligned,
multidomain sample of 1[8] to be 1.52 × 10−3 cm2 V−1 s−1 at 40
°C (Figure 6), which is in the range of typical values (10−4−
8
10
The signal positions were almost temperature-independent.
Further cooling led to sample crystallization (Figure 3c), which
manifested itself in significant changes in the XRD patterns: the
sharp signals disappeared, and pronounced small-angle
scattering of the X-ray beam was observed instead. This
indicates that the crystalline phase grew in the form of
nanometer-sized grains. Larger crystallites (or another crystal-
line polymorph), however, could be observed for slowly cooled
samples; in this case, the diffraction patterns exhibited features
similar to those of the fast-cooled sample in the Colr phase,
with additional low-intensity sharp reflections over the entire
range of the diffractogram. In contrast, all of the patterns
obtained for 1[12] showed only small-angle X-ray scattering
but not the Bragg reflections, regardless of the sample cooling
rate.
Figure 6. Temperature dependence of the positive carrier mobility for
1[8] obtained by the TOF method at 337 nm. The electric field
strength was 40 kV cm−1.
Molecular modeling revealed that 1[8], in which the octyl
groups are in all-trans conformations, can be inscribed into a
circle with a diameter of 31 Å, while the unit cell parameters of
1[8] calculated from the (11) and (02) reflections are a = 26.9
Å and b = 37.6 Å (Table 2). This indicates significant reshaping
of the molecule in the Colr phase, which likely involves a
number of gauche conformations of the alkyl chain and/or
interdigitation of the chains between neighboring columns.
Magnetization studies at 200 Oe revealed paramagnetic
behavior of 1[8] in both the crystalline and fluid phases (Figure
5). The effective magnetic moment (μeff) in the crystalline
phase was found to be 1.620 0.03 over the temperature range
310−345 K, which is close to the value of 1.732 for an ideal
10−1 cm2 V−1 s−1) and attractive for device applications.11 The
observed value is consistent with a relatively low disproportio-
nation energy27 calculated for 1[1] (rigid, 106.6 kcal mol−1;
relaxed, 97.0 kcal mol−1).25 Arrhenius analysis of μh at several
temperatures gave an activation energy of Ea = 0.06 0.01 eV,
consistent with a hopping transport mechanism.28 In the I
phase, the charge mobility was lower by a factor of 10 (e.g., μh =
1.25 × 10−4 cm2 V−1 s−1 at 70 °C), which is typical for other
discotics.11
Electrochemical analysis of 1[6] in CH2Cl2 (10−3 M)
revealed two quasi-reversible redox pairs. The measured
paramagnet and corresponds to 87
number of spins increased to 95
3% of the spins. The
2% at ∼65 °C, which
0/+1
0/−1
potentials, E
= +0.99 V vs SCE and E
= −0.45 V vs
1/2
1/2
coincides with the Cr → I transition. No abrupt changes of
magnetization were observed at the phase transitions upon
cooling (I → Colr) or heating (Colr → I), and the number of
spins remained approximately constant at 95 2%. Cooling of
the sample from the I phase to 2 K at 200 Oe showed that the
molar susceptibility χm is well-described by the Curie law down
to 200 K. At lower temperatures, antiferromagnetic interactions
gradually diminished the observed magnetization (Figure 5
inset). Thus, the results indicate that the alkylsulfanyl groups
SCE, and the cell potential, Ecell = 1.44 V, are comparable to
those reported for the parent 1,3,5-triphenyl derivative in
MeCN.16b
In conclusion, the first series of liquid-crystalline derivatives
of the verdazyl radical has been synthesized. These compounds,
in which the unpaired electron is delocalized in the center of
the disk-like molecule, exhibit a monotropic rectangular
columnar mesophase above ambient temperature. For the
first time, we have demonstrated charge photogeneration and
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Communication
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its transport in a liquid-crystalline radical. The properties of
these radicals, such as a broad absorption spectrum and
mobility of the photogenerated charge, are desirable for
applications in photovoltaic devices and warrant further studies
of this class of materials. It is expected that the molecular and
bulk properties of these radicals, such as thermal behavior,
alignment, disproportionation energy, and charge transport, can
be controlled by the judicious choice of substituents and
dissymmetrization of the molecule using the synthetic method
shown in Scheme 1. This work is currently in progress in our
laboratory.
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ASSOCIATED CONTENT
* Supporting Information
■
S
Full experimental details and data for photoconductivity,
magnetization, electrochemical, UV, XRD and thermal
analyses; molecular modeling and computations; and synthetic
procedures. This material is available free of charge via the
Internet at http://pubs.acs.org.
(14) (a) Rawson, J. M.; Clarke, C. S.; Bruce, D. W. Magn. Reson.
Chem. 2009, 47, 3. (b) Zienkiewicz, J.; Fryszkowska, A.; Zienkiewicz,
K.; Guo, F.; Kaszynski, P.; Januszko, A.; Jones, D. J. Org. Chem. 2007,
72, 3510. (c) Sienkowska, M. J.; Farrar, J. M.; Kaszynski, P. Liq. Cryst.
2007, 34, 19. (d) Kaszynski, P. In Magnetic Properties of Organic
Materials; Lahti, P. M., Ed.; Marcel Dekker: New York, 1999; pp 305−
324 and references therein.
AUTHOR INFORMATION
Corresponding Author
piotr.kaszynski@vanderbilt.edu
■
Notes
The authors declare no competing financial interest.
(15) Castellanos, S.; Lop
Velasco, D. Angew. Chem., Int. Ed. 2009, 48, 6516.
́ ́
ez-Calahorra, F.; Brillas, E.; Julia, L.;
ACKNOWLEDGMENTS
■
(16) (a) Wiley, P. F. In Chemistry of 1,2,3-Triazines and 1,2,4-
Triazines, Tetrazines, and Pentazines; Wiley: New York, 1978; pp
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249, 2612 andreferences therein. (c) Hicks, R. G. In Stable Radicals:
Fundamentals and Applied Aspects of Odd-Electron Compounds; Hicks,
R. G., Ed.; Wiley: Chichester, U.K., 2010; pp 245−280 and references
therein.
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This project was supported by a Vanderbilt University
Discovery Grant. We thank Prof. Andrzej Twardowski for
funding SQUID measurements. XRD experiments were
performed at The Structural Research Laboratory at Warsaw
University, established under the European Regional Develop-
ment Fund Program (WKP-1/1.4.3/1/2004/72/72/165/
2005). We thank Ms. Ola Kruczkowska for her assistance
with preparation of the manuscript.
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