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transport value exhibits some increase. This was attributed to
the formation of poly-crystalline domains at the initial crystalliz-
ing with close intermolecular π-π stacking and slow molecular
dynamics, therefore increasing mobility.[43] Further, the cooling
run curve shows a slightly higher hole mobility than during the
heating run (Figure S12).
fused phthalocyanine-Zn complex (10À 2 cm2 VÀ 1 sÀ 1),[16] whereas
a X-shaped pyrene-oligothiophenes[45] showed mobility in the
range from 10À 4 to 10À 5 cm2 VÀ 1 sÀ 1, but could be largely
improved to 10À 3 cm2 VÀ 1 sÀ 1 when mixed in a physical gel
blend. Let us as well mention some ambipolar octupolar π-
conjugated columnar LCs with a mobility in the range 10À 2–
The other π-extended mesogen, BDTDd, exhibits even
higher hole as well as electron mobility rates, with μh =4.0×
10À 3 cm2 VÀ 1 sÀ 1 [19] For comparison, with PR-TRMC technique, a
.
propeller-shaped fused oligothiophene columnar mesogen
displayed a short-range mobility of 0.18 cm2 VÀ 1 sÀ 1,[30] while its
long-range TOF mobility was 0.02 cm2 VÀ 1 sÀ 1, one order of
magnitude lowered. Similarly, an H-bonded benzotristhiophene
derivative was reported to exhibit a mobility of 0.02 cm2 VÀ 1 sÀ 1,
while TOF mobility was found in the range of 10À 3 cm2 VÀ 1 sÀ 1.[46]
The most important factors in determining conductivity of
LC semiconductors remain thus essentially the extension of
their π-conjugated aromatic core and the order degree of
stacking in the self-organized mesophases in order to maximize
orbital overlaps. Columnar mesogens with enlarged π-conjugat-
ing cores, such as hexabenzocoronenes,[47] phthalocyanines,[48]
and dibenzocoronene diimides,[49] indeed display very high
conductivities. These new butterfly-shaped thiophene-fused
discogens, as well as some reported previously by us,[25–27] stand
out therefore as effective concurrents, and can be foreseen as
promising candidates for future applications in thin film
electronic devices.
10À 3 cmÀ 2 VÀ 1 sÀ 1 and μe =5.1×10À 3 cmÀ 2 VÀ 1 sÀ 1 at 220 C, re-
°
spectively (Figure 10c). The electron transport behaviour exhib-
its a stable value of (5.1–6.6)×10À 3 cmÀ 2 VÀ 1 sÀ 1 between 110—
°
220 C, while the hole mobility rate μh lies in the range of 4.0–
4.5×10À 3 cmÀ 2 VÀ 1 sÀ 1, before declining to 2.2×10À 3 cmÀ 2 VÀ 1 sÀ 1
°
below 160 C. The charge mobility of BDTDd in the Colhex
mesophase reveals an important feature: both electrons and
holes are transported by this materials, with μe higher than μh.
Therefore, the annulated π-extended benzodithiophene-4,8-
dione core can be considered as an ambipolar mesogenic
semiconductor. It is noteworthy that such high ambipolar TOF
drift mobility is uncommon in dynamic columnar mesophases.
In summary, these novel butterfly-shaped fused-thiophene
columnar mesogens with polar cores (BDTD and CDTO) show
outstanding charge carrier transport properties largely due to
their larger π-electron systems. The results can be interpreted
by the Marcus theory: larger π-conjugated aromatics possess
lower reorganization energies and higher electronic transfer
integrals and therefore cause higher rates of charge hopping.[44]
The trend observed is in good agreement with the stacking Conclusion
values measured by S/WAXS (hπ, Figure S4), whose most intense
signal, which indicates a regular and long-range stacking, is
observed for BDTDd, whereas it is weaker, and thus corre-
sponds to a shorter stacking range for CDTOd, and almost
imperceptible for 4d. Unfortunately, compounds with the
electron-rich BDT core did not display satisfactory photocurrent
decay curves for the drift time readout, the laser-induced
photo-oxidation of the aromatic core might cause the failure of
TOF measurement for this molecule.
We have demonstrated an efficient synthetic process to access
new thiophene-fused butterfly-shaped columnar liquid crystals
from accessible starting materials. The tandem of reactions
used, Suzuki-Miyaura cross-coupling and intramolecular Scholl
aromatic oxidative cyclo-dehydrogenation, respectively, pro-
ceeded in high overall yields. As expected, the electronic and
stereo effects of the molecules have significant impact on their
self-organizational behaviours, photophysical properties, gelati-
on and semiconducting properties.
For the tetraphenyl-substituted thiophene-containing aro-
matic hydrocarbons, electron-deficient (ketone 4d and quinone
3d) systems exhibit advantageous effect on the formation and
stabilization of Colhex mesophases by enhanced intermolecular
interactions, while the electron-rich systems (1 and 2) with 10
or 12 alkyl chains do not show any mesomorphism. The
intramolecular oxidative aromatic cyclization permitted the
extension and hardening of the π-conjugated systems by
annulating the peripheral phenyls, therefore affording materials
showing broader Colhex mesophases and higher phase transition
temperatures.
The electron-withdrawing ketone and diketone (quinone)
functional groups separate the molecules into electron-donat-
ing and electron-accepting segments. Thus, the molecules
based on CDTO and BDTD display the classical intramolecular
charge transfer (ICT) UV/vis absorption band, which conse-
quently quenches the fluorescence emission, while molecules
based on BDT (and the previously reported DTT) exhibit blue
light fluorescence emission in both solutions and films. On the
Usually, three techniques are applied for the measurements
of charge carrier mobility (μ) in columnar LC materials: pulse-
radiolysis/flash photolysis time-resolved microwave conductivity
(PR/FP-TRMC), time-of-flight photoconductivity, and space-
charge limited conductivity (SCLC).[6] SCLC is very sensitive to
ionic impurities of the sample-cell interface and to the history
of sample thermal and electrical-field annealing, and the
measured conduction from the I/V curve of a same sample may
[6]
surprisingly vary by 5 to 6 orders of magnitude. The TRMC
technique measures the short-range, ideal charge transporta-
tion of LC samples, usually yielding high μ values. On the other
hand, TOF technique measures the long-range (10–20 μm)
charge carrier drift mobility, which is sensitive to ionic
impurities and sample alignment in LC cells. Thus, from the
above, comparison of LC conducting values measured from
these different techniques may not be rational nor appropriate.
In this context, the results obtained here compared quite
well with those of other columnar LC thiophene derivatives
reported in the literature, and thus are quite relevant. Indeed,
one of the highest TOF mobility was reported for a thiophene-
Chem Asian J. 2021, 16, 1106–1117
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