Synthesis and hole transport properties of highly soluble pyrene-based discotic liquid crystals with trialkylsilylethynyl groups

Article abstract of DOI:10.1080/15421406.2011.536485

Four pyrene-based discotic compounds with trialkylsilylethynyl groups in the side chains were prepared and characterized by differential scanning calorimetry, polarizing optical microscopy, and X-ray diffraction measurements. All of the compounds were hig

Full text of DOI:10.1080/15421406.2011.536485

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Synthesis and Hole Transport Properties
of Highly Soluble Pyrene-Based Discotic
Liquid Crystals with Trialkylsilylethynyl
Groups
Takuji Hirose a , Yuki Shibano a , Yutaro Miyazaki a , Norihito Sogoshi
a , Seiichiro Nakabayashi a & Mikio Yasutake a
a Graduate School of Science and Engineering, Saitama University,
Sakura, Saitama, Japan
Published online: 18 Jan 2011.
To cite this article: Takuji Hirose , Yuki Shibano , Yutaro Miyazaki , Norihito Sogoshi , Seiichiro
Nakabayashi & Mikio Yasutake (2011) Synthesis and Hole Transport Properties of Highly Soluble
Pyrene-Based Discotic Liquid Crystals with Trialkylsilylethynyl Groups, Molecular Crystals and Liquid
Crystals, 534:1, 81-92, DOI: 10.1080/15421406.2011.536485
To link to this article: http://dx.doi.org/10.1080/15421406.2011.536485
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Mol. Cryst. Liq. Cryst., Vol. 534: pp. 81–92, 2011
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421406.2011.536485
Synthesis and Hole Transport Properties of Highly
Soluble Pyrene-Based Discotic Liquid Crystals
with Trialkylsilylethynyl Groups
TAKUJI HIROSE, YUKI SHIBANO, YUTARO
MIYAZAKI, NORIHITO SOGOSHI, SEIICHIRO
NAKABAYASHI, AND MIKIO YASUTAKE
Graduate School of Science and Engineering, Saitama University,
Sakura, Saitama, Japan
Four pyrene-based discotic compounds with trialkylsilylethynyl groups in the side
chains were prepared and characterized by differential scanning calorimetry, polar-
izing optical microscopy, and X-ray diffraction measurements. All of the compounds
were highly soluble in organic solvents, and two of the compounds exhibited a
columnar phase, Colr, and the others were crystalline. The hole transport ability
in the liquid-crystalline phases was determined by the time-of-flight (TOF)
method to be on the order of 10ꢀ2 cm2 Vꢀ1 sꢀ1, the highest hole mobilities for pyrene
derivatives yet observed.
Keywords Discotic liquid crystal; hole transport; pyrene derivatives; synthesis;
trialkylsilylethynyl group
Introduction
Recently, organic semiconductors have received increasing attention because of their
potential applications in organic light-emitting diodes (OLEDs) [1], organic
field-effect transistors (OFETs) [2], and so on [3]. Flexible, lightweight, and low-cost
electronic devices are expected from organic conductive materials; in fact, some
pentacene derivatives are found to have higher charge mobilities than amorphous
silicon FETs [4]. However, crystalline organic semiconductors, such as polyacenes,
typically have low solubility in organic solvents and expensive high-vacuum
deposition is necessary for thin-film preparation in device processing [5]. On the
other hand, amorphous organic materials, such as those used in photocopiers and
laser printers, have charge mobilities of only 10ꢀ6–10ꢀ5 cm2 Vꢀ1 sꢀ1, and therefore
their performance is not sufficient for OLEDs and OFETs [6].
In liquid-crystalline semiconductors, problems with processing and the decrease
in charge mobilities at the grain boundary are expected to become irrelevant [7].
Therefore, an appropriate molecular design is of great interest and importance for
Address correspondence to Takuji Hirose, Graduate School of Science and Engineering,
Saitama University, 255 Shimo-ohkubo, Sakura, Saitama 338-8570, Japan. E-mail: hirose@
apc.saitama-u.ac.jp
81

82
T. Hirose et al.
obtaining conductive liquid-crystalline compounds that exceed the performance of
amorphous silicons [8]. One-dimensional charge transport through columns of disco-
tic liquid crystals is a fascinating property of OFET and OLED devices, and charge
mobilities up to ꢁ1 cm2 Vꢀ1 sꢀ1 have been reported [9–12].
High solubility in common organic solvents, which would allow the material to
be processed in solution, is another requirement [8,13]. We have attempted to
develop columnar liquid-crystalline materials with pyrene as a relatively small
p-conjugated core in order to obtain high solubility [14]. However, the particular
molecular design necessary for high charge mobility remained unclear. It is known
that the r-orbitals of silicon interact strongly with adjacent p-electron systems
(r-p conjugation), thus enhancing the conjugation of the system [15–18]. In
addition, it has been reported that introduction of trialkylsilyl groups enhances
the solubility of organic compounds and the ability of polymer materials to form
films [19]. However, to the best of our knowledge, the electrical properties of
liquid-crystalline compounds bearing trialkylsilyl groups have not yet been
reported [20]. In this article, we report the synthesis of novel columnar liquid
crystals having a pyrene core bearing trialkylsilylethynyl groups as flexible side
chains, as well as the hole transport properties of these materials as determined
in time-of-flight (TOF) studies.
Experimental
General
1H nuclear magetic resonance (NMR) spectra were recorded on Bruker AC-200,
Bruker AC-300P, and Bruker DRX-400 spectrometers (Molecular Analysis and Life
Science Center [MALS], Saitama University). The chemical shift was reported in
parts per million using tetramethylsilane as an internal standard. Matrix-assisted
laser desorption=ionization time-of-flight (MALDI-TOF) mass spectra were mea-
sured with a Bruker Daltonics autoflex III (MALS). UV-Vis spectra were recorded
as a hexane solution on a JASCO V-550.
Phase transition temperatures were determined using a Mac Science DSC-3100
differential scanning calorimeter (DSC). Polarizing optical microscopy (POM) was
performed using a Nikon OPTIPHOT2-POL equipped with a Mettler FP-82 hot
stage and a Mettler FP-90 central processor to verify thermal transitions and char-
acterize anisotropic textures. X-ray diffraction (XRD) analysis was performed on
quartz substrate using a Rigake model Ultima III X-ray diffractometer with a mono-
chromated CuKa radiation source.
The carrier transport was characterized using a conventional TOF setup
equipped with an Nd:YAG laser (k ¼ 355 nm, pulse width ¼ 20 ns, 0.5 mJ pulseꢀ1).
A bias voltage was applied across the sample by a dc power supply and the transient
photocurrents were monitored with a digital oscilloscope at a given temperature
maintained by a homemade hot stage. The transit time for carriers was determined
from the shoulder in the transient photocurrent.
The pyrene derivatives were injected into the cell (l ¼ 5.0-mm-thick ITO-coated
glass cells of 100 mm2 electrode area) by capillary action, and then the samples were
slowly cooled to room temperature from their isotropic temperatures. The applied
electric field, E, was 4.4–5.5 kV cmꢀ1 for A5 and 3.0 kV cmꢀ1 for A6 [11,26]. The
mobility, l, was calculated using the relation l ¼ l=(sT ꢂ E), where sT is the transit

Hole Transport Properties of Pyrene-Based LCs
83
time. The POM observation showed birefringent textures for both A5 and A6,
indicating the polydomain alignments.
Synthesis
3,5-Bis(trimethylsilylethynyl)bromobenzene 1 [21]. Under nitrogen, PdCl2(PPh3)2
(362 mg, 0.516 mmol), PPh3 (271 mg, 1.03 mmol), i-Pr2NH (10 mL),
trimethylsilylacetylene (TMSA); (2.23 g, 22.7 mmol), and CuI (99 mg, 0.52 mmol)
were added to 1,3,5-tribromobenzene (3.25 g, 10.3 mmol) in dry THF (30 mL).
After stirring at 50ꢃC for 20 h, the reaction mixture was cooled to room
temperature and then filtered through Celite. The filtrate was concentrated and the
residue was extracted with chloroform and water. The organic layer was dried over
anhydrous Na2SO4 and concentrated to give a brown liquid. The crude product
was purified by silica gel column chromatography with hexane and purified by
thin-layer chromatography (TLC) on silica gel with hexane to give a colorless
1
liquid 1 (56.4%, 2.04 g, 5.82 mmol). H NMR (300 MHz, CDCl3): d (ppm) ¼ 0.236
(s, 18H, CH3), 7.49 (t, 1H, J ¼ 1.5 Hz, Ar-H), 7.53 (d, 2H, J ¼ 1.5 Hz, Ar-H).
3,5-Diethylbromobenzene 2. To the solution of 1 (1.77 g, 5.08 mmol) in THF (10 mL)
was added 6 N KOH (6.68 ml, 40.1 mmol) and the reaction mixture was stirred at
room temperature for 12 h [21]. The solution was concentrated and the residue
was extracted with chloroform and water. The organic layer was dried over
anhydrous Na2SO4 and concentrated to give a yellow solid. The crude product
was purified by silica gel column chromatography with hexane to give a white
1
solid 2 (87.6%, 913 mg, 4.45 mmol). H NMR (300 MHz, CDCl3): d (ppm) ¼ 3.13
(s, 2H, CH), 7.52 (t, 1H, J ¼ 1.5 Hz, Ar-H), 7.60 (d, 2H, J ¼ 1.5 Hz, Ar-H).
3,5-Bis(dimethylpentylsilylethynyl)bromobenzene B5. Under nitrogen, bromoethane
(1.64 g, 15.0 mmol) was added to magnesium (401 mg, 16.5 mmol) in dry THF
(40 mL) and stirred at room temperature for 2 h. Magnesium ethylbromide was
added drop wise to the solution of 2 (913 mg, 4.45 mmol) in dry THF (40 mL) for
15 min. After stirring at room temperature for 1 h, dimethylpentylsilylchloride
(1.83 g, 11.4 mmol) was added and the reaction mixture was refluxed for 12 h. The
reaction was quenched with 1 N HCl and the solution was concentrated. The
residue was extracted with hexane and water. The organic layer was dried over
anhydrous Na2SO4 and concentrated to give a yellow liquid. The crude product
was purified by silica gel column chromatography with hexane to give a colorless
liquid B5 (66.1%, 1.36 g, 2.94 mmol). 1H NMR (300 MHz, CDCl3): d (ppm) ¼
0.170 (s, 12H, CH3), 0.635 (t, 4H, J ¼ 8.1 Hz, CH2), 0.867 (t, 6H, J ¼ 6.8 Hz,
CH3), 1.29–1.38 (m, 12H, CH2), 7.44 (t, 1H, J ¼ 1.5 Hz, Ar-H), 7.49 (d, 2H,
J ¼ 1.5 Hz, Ar-H).
Other Bn (n ¼ 6–8) were prepared following the same procedure as B5.
3,5-Bis(dimethylhexylsilylethynyl)bromobenzene B6. Yield: 65.4%. 1H NMR
(300 MHz, CDCl3): d (ppm) ¼ 0.200 (s, 12H, CH3), 0.675 (t, 4H, J ¼ 7.9 Hz, CH2),
0.893 (t, 6H, J ¼ 6.8 Hz, CH3), 1.30–1.37 (m, 16H, CH2), 7.47 (t, 1H, J ¼ 1.5 Hz,
Ar-H), 7.53 (d, 2H, J ¼ 1.5 Hz, Ar-H).
3,5-Bis(dimethylheptylsilylethynyl)bromobenzene B7. Yield: 70.1%. 1H NMR
(300 MHz, CDCl3): d (ppm) ¼ 0.200 (s, 12H, CH3), 0.671 (t, 4H, J ¼ 7.9 Hz, CH2),

84
T. Hirose et al.
0.887 (t, 6H, J ¼ 6.8 Hz, CH3), 1.28–1.37 (m, 20H, CH2), 7.47 (t, 1H, J ¼ 1.5 Hz,
Ar-H), 7.53 (d, 2H, J ¼ 1.5 Hz, Ar-H).
3,5-Bis(dimethyloctylsilylethynyl)bromobenzene B8. Yield: 45.3%. 1H NMR (300
MHz, CDCl3): d (ppm) ¼ 0.200 (s, 12H, CH3), 0.671 (t, 4H, J ¼ 7.9 Hz, CH2), 0.881
(t, 6H, J ¼ 6.7 Hz, CH3), 1.28–1.37 (m, 24H, CH2), 7.47 (t, 1H, J ¼ 1.4 Hz, Ar-H),
7.53 (d, 2H, J ¼ 1.4 Hz, Ar-H).
1,3,6,8-Tetrakis(trimethylsilylethynyl)pyrene 3. Under nitrogen, PdCl2(PPh3)2 (83.5
mg, 0.12 mmol), PPh3 (62.4 mg, 0.24 mmol), i-Pr2NH (15 mL), TMSA (2.34 g,
23.8 mmol), and CuI (22.7 mg, 0.12 mmol) were added to 1,3,6,8-tetrabromopyrene
(1.23 g, 2.38 mmol) in dry THF (70 mL). After stirring at 50ꢃC for 20 h, the
reaction mixture was cooled to room temperature and filtered through Celite. The
filtrate was concentrated and extracted with CHCl3 and water. The organic layer
was dried over anhydrous Na2SO4 and concentrated to give a reddish-yellow solid.
The crude product was purified by silica gel column chromatography with hexane
1
to give a red solid 3 (91.3%, 1.28 g, 2.17 mmol). H NMR (300 MHz, CDCl3): d
(ppm) ¼ 0.383 (s, 36H, CH3), 8.31 (s, 2H, H2 and H7), 8.60 (s, 4H, H4, H5, H9, H10).
1,3,6,8-Tetraethynylpyrene 4 [22]. To the solution of 3 (720 mg, 1.23 mmol) in THF
(20 mL) was added 6 N KOH 25 mL (150 mmol) and the reaction mixture was stirred
at room temperature for 12 h. The solution was concentrated and the residue was
washed with chloroform and water to give a yellow solid 4 (97.6%, 358 mg,
1.20 mmol). 1H NMR (300 MHz, DMSO-d6): d (ppm) ¼ 5.01 (s, 4H, CH), 8.43
(s, 2H, H2 and H7), 8.70 (s, 4H, H4, H5, H9, H10).
1,3,6,8-Tetrakis[3,5-bis(dimethylpentylsilylethynyl)phenylethynyl]pyrene A5 [21,22].
Under nitrogen, PdCl2(PPh3)2 (11.9 mg, 0.0170 mmol), PPh3 (8.9 mg, 0.034 mmol),
i-Pr2NH (20 mL), B5 (630 mg, 1.37 mmol), and CuI (3.2 mg, 0.017 mmol) were
added to 4 (102 mg, 0.342 mmol) in dry toluene (40 mL). After stirring at 50ꢃC for
48 h, the reaction mixture was cooled to room temperature and filtered through
Celite. The filtrate was concentrated and extracted with chloroform and water.
The organic layer was dried over anhydrous Na2SO4 and concentrated to give a
red liquid. The crude product was purified by silica gel column chromatography
with hexane, by TLC on silica gel with hexane, and by molecular sieve
chromatography with hexane. The eluent was concentrated and washed with
acetone to afford A5 as an orange solid (26.2%, 163 mg, 0.0896 mmol). mp:
1
122.0–123.6ꢃC. H NMR (400 MHz, CDCl3): d (ppm) ¼ 0.251 (s, 48H, CH3), 0.713
(t, 16H, J ¼ 8.0 Hz, CH2), 0.924 (t, 24H, J ¼ 7.0 Hz, CH3), 1.36–1.47 (m, 48H,
CH2), 7.60 (t, 4H, J ¼ 1.5 Hz, Ar-H), 7.75 (d, 8H, J ¼ 1.5 Hz, Ar-H), 8.41 (s, 2H,
H2 and H7), 8.74 (s, 4H, H4, H5, H9, H10). MALDI-TOF MS m=z (%): Anal.
Calc. for C120H154Si8 1819.0199; Found: 1819.0189 (Mþ, 43%), 1820.0329 (100%),
1821.0388 (99%), 1822.0384 (72%), 1823.0393 (34%), 1824.0394 (10%). Anal. Calc.
for C120H154Si8: C, 79.14; H, 8.52. Found: C, 78.83; H, 8.56.
Other An (n ¼ 6–8) were prepared following the same procedure as A5.
1,3,6,8-Tetrakis[3,5-bis(dimethylhexylsilylethynyl)phenylethynyl]pyrene A6. Yield:
1
8.1%. mp: 110.1–112.1ꢃC. H NMR (300 MHz, CDCl3): d (ppm) ¼ 0.243 (s, 48H,
CH3), 0.715 (t, 16H, J ¼ 7.9 Hz, CH2), 0.905 (t, 24H, J ¼ 6.8 Hz, CH3), 1.26–1.41
(m, 64H, CH2), 7.60 (t, 4H, J ¼ 1.5 Hz, Ar-H), 7.75 (d, 8H, J ¼ 1.5 Hz, Ar-H),
8.41 (s, 2H, H2 and H7), 8.75 (s, 4H, H4, H5, H9, H10). MALDI-TOF MS m=z

Hole Transport Properties of Pyrene-Based LCs
85
(%): Anal. Calc. for C128H170Si8 1931.1451; Found: 1931.1444 (Mþ, 36%), 1932.1379
(86%), 1933.1315 (100%), 1934.1253 (71%), 1935.1193 (33%), 1936.1512 (13%). Anal.
Calc. for C128H170Si8: C, 79.52; H, 8.86. Found: C, 79.40; H, 8.91.
1,3,6,8-Tetrakis[3,5-bis(dimethylheptylsilylethynyl)phenylethynyl]pyrene A7. Yield:
1
10.1%. mp: 90.4–91.4ꢃC. H NMR (300 MHz, CDCl3): d (ppm) ¼ 0.241 (s, 48H,
CH3), 0.711 (t, 16H, J ¼ 7.9 Hz, CH2), 0.888 (t, 24H, J ¼ 6.8 Hz, CH3), 1.26–1.42
(m, 80H, CH2), 7.60 (t, 4H, J ¼ 1.8 Hz, Ar-H), 7.75 (d, 8H, J ¼ 1.8 Hz, Ar-H), 8.41
(s, 2H, H2 and H7), 8.75 (s, 4H, H4, H5, H9, H10). MALDI-TOF MS m=z (%):
Anal. Calc. for C136H186Si8 2043.2703; Found: 2043.2664 (Mþ, 42%), 2044.2481
(89%), 2045.2702 (100%), 2046.2532 (78%), 2047.2758 (45%), 2048.2637 (19%),
2049.2809 (7%). Anal. Calc. for C136H186Si8: C, 79.85; H, 9.17. Found: C, 78.90;
H, 9.29.
1,3,6,8-Tetrakis[3,5-bis(dimethyloctylsilylethynyl)phenylethynyl]pyrene A8. Yield:
1
19.3%. mp: 48.8 ꢀ 52.7ꢃC. H NMR (400 MHz, CDCl3): d (ppm) ¼ 0.243 (s, 48H,
CH3), 0.710 (t, 16H, J ¼ 8.0 Hz, CH2), 0.870 (t, 24H, J ¼ 6.8 Hz, CH3), 1.28–1.43 (m,
96H, CH2), 7.60 (t, 4H, J ¼ 1.6 Hz, Ar-H), 7.75 (d, 8H, J ¼ 1.6 Hz, Ar-H), 8.41 (s,
2H, H2 and H7), 8.75 (s, 4H, H4, H5, H9, H10). MALDI-TOF MS m=z (%): Anal.
Calc. for C144H202Si8 2155.955; Found: 2155.3926 (Mþ, 31%), 2156.3985 (85%),
2157.3997 (100%), 2158.4047 (79%), 2159.3972 (44%), 2160.3953 (19%), 2161.3989
(6%). Anal. Calc. for C144H202Si8: C, 80.15; H, 9.44. Found: C, 80.27; H, 9.71.
Results and Discussion
Synthesis
To prepare highly conjugated pyrene derivatives, phenylacetylene moieties were
introduced onto the pyrene core not only to increase p-conjugation but also to
reduce the steric hindrance of the trialkylsilyl groups (Scheme 1). As a result, a
novel class of pyrene derivatives, An (n ¼ 5–8), was designed and synthesized in a
Scheme 1. 1,3,6,8-Tetrakis[3,5-bis(dimethylalkylsilylethynyl)phenylethynyl]pyrene An.

86
T. Hirose et al.
Scheme 2. Synthetic route of 1,3,6,8-tetrakis[3,5-bis(dimethylalkylsilylethynyl)phenylethynyl]-
pyrene An. (a) 2 eq. TMSA, cat. PdCl2(PPh3)2, cat. CuI, cat. PPh3=dry THF-i-Pr2NH, 50ꢃC,
20 h; (b) 6 N KOH aq.=THF, rt, 12 h; (c) C2H5MgBr=dry THF, rt, 1 h; (d) 2 eq.
ClSi(CH3)2CnH2nþ1=dry THF, reflux, 12 h; (e) 4 eq. TMSA, cat. PdCl2(PPh3)2, cat. CuI,
cat. PPh3=dry THF-i-Pr2NH, 50ꢃC, 20 h; (f) 6 N KOH aq.=THF, rt, 12 h; (g) 4 eq. Bn
(n ¼ 5–8), cat. PdCl2(PPh3)2, cat. CuI, cat. PPh3=dry toluene-i-Pr2NH, 50ꢃC, 48 h. TMSA ¼
trimethylsilylacetylene.
straightforward manner, as shown in Scheme 2. The total yield was between 7% for
A6 and 23% for A5 starting from 1,3,6,8-tetrabromopyrene and 3,5-bis(alkyldi-
methylsilylethynyl)bromobenzene, respectively, which were prepared in 45% (R8)
to 70% (R7) yield from 1,3,5-tribromobenzene. The yield of the last step, Sonoga-
shira coupling, strongly depends on the reaction conditions and requires further
optimization. All An (n ¼ 5–8) were soluble in CHCl3, toluene, and even hexane
and were purified by repeated chromatography using hexane. The high solubility
of these compounds in common organic solvents arises from the eight alkyldimethyl-
silyl groups as well as the small pyrene core.
Thermal and UV-Vis Absorption Properties
The thermal behavior of the compounds was investigated by DSC and POM, and A5
and A6 were found to exhibit liquid-crystalline phases (Table 1). The mesophases
Table 1. Phase transition behavior of An
Phase transition temperatures (ꢃC) and
enthalpy changes (kJ=mol)
Compound
Heating
Cooling
A5
Cr1 105 (8.8) Cr2 116 (2.4)
Colr 150 (6.8) I
I 144 (7.1) Colr 89 (7.1) Cr
A6
A7
Cr 86 (12.1) Colr 114 (6.1) I
Cr1 41 (3.5) Cr2 72 (14.3) Cr3
88 (10.8) I
I 105 (7.1) Colr 69 (10.5) Cr
I 53 (4.7) Cr4 31 (2.6) Cr5
a
A8
Cr 41 (9.3) I
—
Transition temperatures were determined by DSC. Scan rate was 5ꢃC=min.
Abbreviations: Cr ¼ crystalline phase; I ¼ isotropic phase; Colr ¼ rectangular columnar
phase.
aNot crystallized during the DSC measurement.

Hole Transport Properties of Pyrene-Based LCs
87
Figure 1. Optical textures of (a) A5 at 140ꢃC, (b) A6 at 100ꢃC, and (c) A6 at 60ꢃC during the
cooling process.
obtained upon cooling of their isotropic phases displayed dendritic textures (Figs. 1a
and 1b) characteristic of a Colr phase. On the other hand, A7 and A8 were crystalline
as observed by POM and showed only melting behavior.
The structures of A5 and A6 in the mesophases were investigated by XRD analy-
sis. The diffraction pattern of A5 at 120ꢃC during the cooling process is shown in
˚
Fig. 2. Two sharp peaks were observed at 24.7 and 22.9 A, and small ones at 10.2,
˚
9.4, 8.7, 8.4, 8.0, and 7.6 A were observed. A broad halo and a faint shoulder were
˚
observed in the wide-angle region corresponding to the molten chains (hch at 5.0 A)
˚
and to the stacking of the molecular cores (h at 3.6 A), respectively. These reflection
patterns are indicative of a liquid-crystalline phase in this temperature range.
Although these were assigned to neither a simple lamellar nor hexagonal structure,
the two sharp peaks in the small-angle region suggested a rectangular structure
(Colr). Based on the assumption that the two intense peaks were (110) and (200)
reflections, respectively, other peaks could be indexed as (230), (420), (510), (140),
(240), and (600). In the same manner, the structure of A6 was assigned as Colr,
and the results for A5 and A6 are summarized in Table 2.
Figure 2. X-ray diffraction pattern of A5 at 120ꢃC during the cooling process.

88
T. Hirose et al.
Table 2. X-ray diffraction data of A5 (120ꢃC) on the cooling process and A6 (85ꢃC)
on the heating process
˚
Spacing (A)
Lattice
constant (A)
Miller
indices (h k l)
˚
Compound
Phase
dobs
dcalc
A5
120ꢃC
Colr
P2i=a
a ¼ 45.8
b ¼ 34.1
Z ¼ 2
27.4
22.9
10.2
9.4
8.7
8.4
8.0
7.6
6.9
6.7
27.4
22.9
10.2
9.5
8.9
8.4
8.0
7.6
7.0
6.8
6.3
6.1
5.9
110
200
230
420
510
140
240
600
620
150
630
720
450
hch
h
6.4
6.1
5.9
5.0
3.6
A6
85ꢃC
Colr
P2x=a
a ¼ 47.7
b ¼ 34.4
Z ¼ 2
27.8
23.6
19.7
10.2
9.5
8.6
8.0
5.0
3.7
27.9
23.9
19.6
10.3
9.3
110
200
210
230
330
040
600
hch
H
8.6
8.0
Z ¼ (q ꢂ a ꢂ b ꢂ h ꢂ NA)=M. Z ¼ Molecular numbers per unit cell, NA ¼ Avogadro’s number, a
and b ¼ lattice constant, h ¼ stacking distance, q ¼ density (assumed to be 1.0 g cmꢀ3),
M ¼ molecular weight of the compound.
The UV-Vis absorption spectrum of A5 in hexane features absorption in the
range of 380–480 nm, which is in the visible light region (Fig. 3). Substitution of
the pyrene core with ethynyl and phenyl groups contributed to a large red-shift of
the lowest absorption by ꢁ130 nm, which resulted in an orange color [18].
Hole Transport Properties
The positive charge (hole) photocurrents were measured as a function of time over
the temperature range of the Colr and Cr phases by the TOF method (Fig. 4 for
A5). The hole mobility was confirmed to be bias independent for both phases of each
sample, as shown for A5 at 125ꢃC in Fig. 5. On the other hand, in the isotropic
phase, negligible charge mobility was observed, probably due to the decrease of
p-p stacking in the isotropic state. The results were analyzed in double logarithmic
plots as shown in the inset of Fig. 4 at 135ꢃC, 125ꢃC (Colr), and 75ꢃC (Cr) because

Hole Transport Properties of Pyrene-Based LCs
89
Figure 3. UV-Vis spectrum of A6 (1.0 ꢄ 10ꢀ5 mol=L) in n-C6H14.
the raw data in the Cr phase were dispersive, which was attributed to disruption in
molecular alignment at the grain boundaries during crystallisation and trapping of
charge carriers. The intercept of the two lines corresponding to the pre- and
post-stransit slopes gave the transit time, sT.
The results for hole mobility as a function of temperature are shown for A5 and
A6 in Fig. 6. It was found that the hole transport in the columnar phase slightly
decreases as temperature decreases. On the other hand, upon cooling from the Colr
to the Cr phases, hole mobility was observed to decrease by factor of ꢁ2 for A5 and
Figure 4. Typical transient photocurrents for the hole mobility during the cooling process of
A5 in an electric field of 5.0 kV cmꢀ1 at 135ꢃC, 125ꢃC, and 75ꢃC. The inset shows the double
logarithmic plots. The cell thickness was 5.0 mm.

90
T. Hirose et al.
Figure 5. Electric field dependence of hole mobility for A5 at 125ꢃC. The cell thickness was
5.0 mm. R2 ¼ 0.9849.
Figure 6. Temperature dependence of hole mobilities for (a) A5 and (b) A6 during the cooling
processes. The data were collected at 75ꢃC–140ꢃC for A5 and at 60ꢃC–103ꢃC for A6. The
electric field was 5.0 kV cmꢀ1 and the cell thickness was 5.0 mm for both A5 and A6.

Hole Transport Properties of Pyrene-Based LCs
91
ꢁ3 for A6 around the transition. These decreases can be understood from the texture
changes shown in Figs. 1b and 1c for A6; specifically, because some columnar liquid
crystals exhibit higher mobility in the crystalline phases [23] or super cooled state
[24], some columnar structure was considered to remain. For other discotic liquid
crystals, similar decreases in the hole mobility at the Col to Cr phase transition have
been observed [25–27]. In the present system, the bulky trialkylsilyl groups might
have disturbed the molecular alignment and the intermolecular interactions.
The hole mobilities of A5 and A6 were found to be on the order of 10ꢀ2 cm2
V
ꢀ1 sꢀ1 in the liquid-crystalline phase and 10ꢀ3 cm2 Vꢀ1 sꢀ1 in the crystalline phase.
The values for the mesophases are rather high in comparison with other discotic
liquid-crystalline compounds, especially considering that both samples are pyrene
derivatives and in the Colr phase [23–30]. The transport ability is thought to arise
from the highly conjugated system with r-p conjugation of the terminal trialkylsily-
lethynyl groups. To the best of our knowledge, A5 and A6 are the first discotic liquid
crystals bearing trialkylsilyl groups and exhibit the highest reported hole mobilities
among pyrene derivatives determined by the TOF method.
Conclusions
Highly soluble pyrene derivatives were prepared by introduction of trialkylsilylethy-
nyl groups with phenylacetylene moieties. TOF measurements indicated that the hole
transport abilities in the Colr phase for two compounds, A5 and A6, are on the order
of 10ꢀ2 cm2 Vꢀ1 sꢀ1 and two to three times higher than their hole transport abilities
in the Cr phase. These hole mobilities, which are the highest yet observed for pyrene
derivatives in the mesophase determined by the TOF method [29,30], and the trans-
port characteristics of these compounds are thought to arise from unique r-p conju-
gation properties and the bulkiness of the trialkylsilyl groups in the conjugated side
chains.
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