Room-temperature columnar liquid crystal based on tetrathiafulvalene

Article abstract of DOI:10.1039/b805023f

A series of novel TTF derivatives has been synthesized. Polarized optical microscopy (POM), differential scanning calorimetry (DSC) and X-ray diffraction (XRD) techniques reveal the first room temperature columnar liquid crystal based on TTF. The Royal Society of Chemistry 2008.

Full text of DOI:10.1039/b805023f

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COMMUNICATION
www.rsc.org/materials | Journal of Materials Chemistry
Room-temperature columnar liquid crystal based on tetrathiafulvalene†
*
Lei Wang, Kwang-Un Jeong and Myong-Hoon Lee
Received 25th March 2008, Accepted 29th April 2008
First published as an Advance Article on the web 8th May 2008
DOI: 10.1039/b805023f
long alkyl peripherals as shown in Scheme 1. In particular, 5c and 5d
show mesomorphic states in a wide temperature range including
room temperature. A TTF-based compound showing a LC phase at
room temperature is reported for the first time to our knowledge.
In order to create TTF-based mesomorphic states, a series of
4-alkyloxybenzoic acid (2) with various alkyl chain lengths was
introduced onto both sides of the TTF core. The electron-with-
drawing ester linkage was adopted to increase the air/light stability by
decreasing the HOMO levels of the TTF derivatives. The syntheses of
these compounds were accomplished by 2-step reactions: an esterifi-
cation reaction in the presence of 1,3-dicyclohexyl carbodiimide
(DCC)–4-dimethylaminopyridine (DMAP) followed by a phosphate-
induced cross-coupling reaction. The detailed synthetic procedures
are shown in Scheme 1. The cross-coupling reaction of 3 with an
excess of 4 in P(OEt)3 at 120 ꢀC for 4 h yielded an asymmetric
mesogenic core structure. The TTF derivatives from 5a to 5d were
extensively purified by column chromatography several times. Their
A series of novel TTF derivatives has been synthesized. Polarized
optical microscopy (POM), differential scanning calorimetry
(DSC) and X-ray diffraction (XRD) techniques reveal the first room
temperature columnar liquid crystal based on TTF.
Discotic liquid crystalline (LC) materials have attracted a lot of
attention as promising charge-transfer organic candidates in opto-
electronic devices.1 Due to the intermolecular p-orbital overlapping
between discotic building blocks within ordered 2-dimensional (2D)
columnar phases, electrical/photo conductivity can be significantly
enhanced along the long axis of columns.1e Additionally, mesomor-
phic states of discotic columnar phases allow us to control the
molecular alignment and minimize the defects, such as grain
boundaries, which are inevitable drawbacks for charge trans-
portation in crystalline materials.2 Moreover, the good solubility of
LC materials in organic solvents make them solution-processable,
which is technically favorable. Based on these advantages of meso-
morphic states of discotic columnar LC phases, a variety of columnar
LC materials including perylenetetracarboxylic acid diimides
(PDIs)3,4 and hexabenzocoronenes (HBCs)1c,1e,5 have been introduced
as promising materials for organic field-effect transistors (OFETs),
organic light-emitting devices (OLEDs), and photovoltaic cells.
Among the various candidates of organic materials for optoelec-
tronic applications,6–11 tetrathiafulvalene (TTF) is most attractive due
to its excellent electron-donating properties. However, most research
into TTF and its derivatives has been focused on their crystalline
phases. In order to enhance their electron mobility, it is necessary to
control their structures and morphologies to get rid of grain
boundaries between the domains and to obtain a macroscopically
oriented single domain. The question is: how can we obtain defect-
free single crystal-like TTF domains? One of the possible answers is
the columnar LC phase. Surprisingly, to the best of our knowledge,
there are only a few reports of LC compounds based on TTF, and
none of them shows a LC phase at room temperature.12 Additionally,
because of the strong electron-donating property of TTF, its deriv-
atives are usually sensitive to air and light, which is one of the main
drawbacks of TTF for practical applications. In order to overcome
this barrier, electron-withdrawing groups can be introduced to
decrease its HOMO energy level, which generally results in improved
air/light stability.13
In this communication, we report a new series of TTF derivatives
(5a–5d) having an asymmetric structure of a TTF mesogenic core and
Department of Polymer/Nano Science and Technology, Chonbuk National
University, Chonju, Chonbuk, 561-756, Korea. E-mail: mhlee2@chonbuk.
ac.kr
† Electronic supplementary information (ESI) available: Full
experimental details; DSC traces; 1D WAXD pattern; UV spectra;
cyclic voltammograms. See DOI: 10.1039/b805023f
Scheme 1 Synthesis route to TTF derivatives.
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Table 1 Phase-transition temperature (ꢀC) and enthalpies (kJ molꢂ1) of
TTF derivatives 5a–5d determined by POM and DSC in the first cooling
a
scan (cooling rate: 5 ꢀC minꢂ1
)
Phase transition temperatures
(onset) [ꢀC]/transition
Compound
enthalpies [kJ molꢂ1
]
5a
5b
5c
5d
Cr 120/25.8 I
Cr 93/43.1 I
Cr 57/5.1 Col 96/39.1 I
Cr 9/13.2 Col 96/25.7 I
a
Cr ¼ crystal, Col ¼ columnar phase, I ¼ isotropic phase.
Fig. 2 Model of the highly ordered columnar liquid crystal phase of 5d.
ꢀ
mechanically sheared at 95 C, and the deformed morphology was
1
chemical structures and purities were confirmed by H NMR, 13C
observed under the POM. A macroscopically oriented single domain
was obtained due to the alignment of LC molecules. Furthermore,
the onset temperature of Ti and its heat of transition for 5c and 5d did
not change significantly with respect to the cooling and heating rates
in DSC, which is often observed in the I 4 LC transition.14 More
detailed structural analyses including unit cell dimensions and
molecular packing symmetry are under investigation utilizing 2D
WAXD of oriented samples and selected area electron diffraction of
single domains.
NMR, Fourier-transform infrared spectroscopy, elemental analyses
and high-resolution mass spectrometry (see ESI†).
The phase structural evolutions of TTF derivatives 5a–5d were
investigated by a combination of techniques including differential
scanning calorimetry (DSC, Fig. S1 in ESI†), cross-polarized optical
microscopy (POM) and 1D wide angle X-ray diffraction (WAXD).
The DSC results are summarized in Table 1. The listed temperatures
are the onset temperatures of the first cooling scan. Whereas
compounds 5a and 5b do not show any mesomorphic LC states, 5c
and 5d having longer chain lengths show LC phases below the
isotropic temperature (Ti). The isotropic state above Ti was
confirmed by the dark state under POM as well as scattering halos in
1D WAXD. In particular, 5d having the longest alkyl chain (n ¼ 12)
exhibited a LC phase in a particularly wide temperature window
All the TTF derivatives 5a–5d are soluble in common organic
solvents, such as dichloromethane, chloroform, toluene and tetra-
hydrofuran, which can allow us to fabricate thin films by solution
processing. In order to investigate the optical properties of the
derivatives, UV absorption was measured in chloroform and the data
are listed in Table 2 (also refer to Fig. S3 in ESI†). Absorption
maxima of 5a–5d in chloroform were found at ꢁ390 nm. The optical
band gaps were calculated to be 2.72–2.77 eV, significantly higher
than that of pentacene (2.2 eV), indicating better air stability.15
In order to evaluate electrochemical properties, the cyclic voltam-
metry (CV) measurements were performed in a dry dichloromethane
solution of Bu4NBF4 (0.1 M) with a scan rate of 50 mV sꢂ1 at room
temperature (Table 2, Fig. S4–7 in ESI†). TTF derivatives 5a–5d
showed two reversible single-electron oxidation peaks at ꢁ1.06 and
1.37 V, corresponding to the formation of radical cations and dica-
tions, respectively. All compounds displayed similar oxidation
potentials, suggesting the lengths of alkyl chains do not appreciably
influence the oxidation processes. Both oxidation potentials of 5a–5d
are higher than those of TTF probably due to the existence of elec-
tron-withdrawing ester linkages. Their HOMO levels were estimated
from the onset of oxidation potentials to be between ꢂ5.37 and
ꢀ
between 9 and 96 C, even below room temperature. Based on the
experimental results of POM observation (Fig. 1) and 1D WAXD
measurement (Fig. S2 in ESI†) of 5d at 95ꢀC, it was identified that the
phase is a highly ordered discotic columnar LC phase. In the POM
image, a mosaic texture which is typical in columnar LC phases was
observed. The three Bragg reflections in the low angle region of 1D
WAXD at 2q ¼ 2.57ꢀ, 3.80ꢀ and 5.30ꢀ allowed us to build a 2D a*b*
lattice based on the triangulation method.14 The calculated g was not
60ꢀ so that the 2D lattice of the highly ordered columnar phase
perpendicular to the columnar long axis is considered to be oblique
(Fig. 2). Specifically, the WAXD results tell us that the 2D ab unit cell
is an oblique columnar structure, which is common in LC
compounds with asymmetric discotic mesogens. In order to prove
this is truly a mesomorphic LC phase, compound 5d was
Table 2 Optical, electrochemical potentials and energy levels of
compounds 5a–5d
E1a/V
E2a/V
lUV/nm
Egb/eV
HOMOc/eV
TTF
5a
5b
5c
5d
0.34d
1.07
1.06
1.04
1.04
0.73d
1.34
1.39
1.37
1.37
—
376
391
388
386
—
—
2.77
2.74
2.74
2.72
ꢂ5.39
ꢂ5.39
ꢂ5.37
ꢂ5.37
a
E1 and E2 are the half wave oxidation potentials. b Eg ¼ hc/l0.1 max. Eg is
the optical band gap energy, l0.1 max is the wavelength at which the
c
absorption coefficient drops to 10% of the peak value.17 EHOMO
¼
ꢂ(Eoxonset + 4.4 eV), where Eox
is the onset potential for the
onset
d
oxidation.18 From ref. 19.
Fig. 1 POM image of 5d at 95 ꢀC.
2658 | J. Mater. Chem., 2008, 18, 2657–2659
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was monitored as shown in Fig. S8 (ESI†). As 5d is chemically
oxidized to its radical (5dc+) by FeCl3, the absorption bands at 302,
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solution for 300 min (Fig. S9 in ESI†). These compounds can be kept
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