2-Phenylbenzothiophene-based liquid crystalline semiconductors

Article abstract of DOI:10.1016/j.dyepig.2019.107964

We report herein the design and synthesis of some novel liquid crystalline semiconductors constructed from a biologically and pharmacologically active building block molecule, namely 6-methoxy-2-(4-methoxyphenyl)benzo[b]thiophene, presenting efficient luminescence and medium charge mobility rate. A first series of mesogenic 2-phenylbenzothiophene derivatives (nPBT) was simply and rapidly obtained in good yields by successive demethylation/alkylation reactions of the available methoxy precursor. The further stepwise oxidations moreover resulted in two new sets of the corresponding sulfoxide (nPBTO) and sulfone (nPBTO₂) derivatives, respectively, that were also mesomorphic. The liquid crystalline behaviour was comprehensively characterized by DSC, POM and SAXS: all compounds exhibit smectic-like behaviour in agreement with their calamitic shape. More specifically, mesogens nPBT showed a SmA phase, with in addition, a higher ordered smectic phase at lower temperature. As for the oxidized mesogens, they displayed a SmA phase only, and over particularly large temperature ranges for the nPBTO with longer chain lengths (n ≥ 8). The photo-physical properties have also been studied both in solution and thin films, and the molecules were found to display strong absorption in the UV/vis domain and intense luminescence in the range of 400–650 nm (yellowish green light) in high quantum yields (up to 62%). Both absorption and luminescence were also found to be affected by the oxidation of the benzothiophene moiety. Finally, the semi-conducting behaviour of three PBT compounds in the various mesophases was investigated by photocurrent TOF technique. Hole mobility rates of ca. 4 × 10^?3 cm² V^?1 s^?1 were measured in the lower temperature ordered mesophase for all of them, with the best performances (temperature range and mobility values) however obtained for the shortest homolog (n = 6). With such highly reasonable mesomorphic, light-emitting and semiconducting functional features, as well as being cheap and easy to synthesize and to process, these materials become very attractive and may be incorporated into various kinds of electronic devices (OFET and OLED).

Full text of DOI:10.1016/j.dyepig.2019.107964

Dyes and Pigments 173 (2020) 107964
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: http://www.elsevier.com/locate/dyepig
2-Phenylbenzothiophene-based liquid crystalline semiconductors
,
,**
Yu-Jie Zhonga, Ke-Qing Zhaoa ***, Bi-Qin Wanga, Ping Hua, Hirosato Monobeb
,
Benoît Heinrichc, Bertrand Donnioc
,
*
a College of Chemistry and Material Science, Sichuan Normal University, Chengdu, 610066, China
b Inorganic Functional Materials Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Ikeda, Osaka, 5638577, Japan
c
�
�
Institut de Physique et Chimie des Materiaux de Strasbourg (IPCMS), CNRS-Universite de Strasbourg (UMR 7504), 67034, Strasbourg, France
A R T I C L E I N F O
A B S T R A C T
Keywords:
We report herein the design and synthesis of some novel liquid crystalline semiconductors constructed from a
biologically and pharmacologically active building block molecule, namely 6-methoxy-2-(4-methoxyphenyl)
benzo[b]thiophene, presenting efficient luminescence and medium charge mobility rate. A first series of
mesogenic 2-phenylbenzothiophene derivatives (nPBT) was simply and rapidly obtained in good yields by
successive demethylation/alkylation reactions of the available methoxy precursor. The further stepwise oxida-
tions moreover resulted in two new sets of the corresponding sulfoxide (nPBTO) and sulfone (nPBTO2) de-
rivatives, respectively, that were also mesomorphic. The liquid crystalline behaviour was comprehensively
characterized by DSC, POM and SAXS: all compounds exhibit smectic-like behaviour in agreement with their
calamitic shape. More specifically, mesogens nPBT showed a SmA phase, with in addition, a higher ordered
smectic phase at lower temperature. As for the oxidized mesogens, they displayed a SmA phase only, and over
particularly large temperature ranges for the nPBTO with longer chain lengths (n � 8). The photo-physical
properties have also been studied both in solution and thin films, and the molecules were found to display
strong absorption in the UV/vis domain and intense luminescence in the range of 400–650 nm (yellowish green
light) in high quantum yields (up to 62%). Both absorption and luminescence were also found to be affected by
the oxidation of the benzothiophene moiety. Finally, the semi-conducting behaviour of three PBT compounds in
the various mesophases was investigated by photocurrent TOF technique. Hole mobility rates of ca.
4 � 10À 3 cm2 VÀ 1 sÀ 1 were measured in the lower temperature ordered mesophase for all of them, with the best
performances (temperature range and mobility values) however obtained for the shortest homolog (n ¼ 6). With
such highly reasonable mesomorphic, light-emitting and semiconducting functional features, as well as being
cheap and easy to synthesize and to process, these materials become very attractive and may be incorporated into
various kinds of electronic devices (OFET and OLED).
Organic semiconductor
Time-of-flight photocurrent technique
2-Phenylbenzothiophene
Smectic mesophase
1. Introduction
possess many more biological activities [1].
Therefore, as important pharmaceutical intermediate, it is produced
The benzo[b]thiophene core exhibits a wide range of biological and
pharmacological activities thanks to structural similarities with many
natural and synthetic molecules in drug design [1]. For instance, the
2-phenylbenzo[b]thiophene motif is the main parent structure of cancer
drugs like raloxifene and arzoxifene [2], and it is also used in the
treatment of Alzheimer’s disease [3]. Benzothiophene and its de-
rivatives have now became indispensable building blocks for chemistry
and for the development of the drug industry as it has been found to
in large scale, and thus is abundant, cheap and widely commercially
available. However, despite this economical advantage, its natural rod-
like shape, easy synthetic derivatization and appealing electronic
structure, 2-phenylbenzo[b]thiophene has not yet been considered in
the design of liquid crystalline materials with potentially interesting
semiconducting behaviour.
Thiophene-containing liquid crystalline (LC) semiconductors (Fig. 1)
perform a high degree of π-delocalization and optical tunability due to
* Corresponding author.
** Corresponding author.
*** Corresponding author.
E-mail addresses: kqzhao@sicnu.edu.cn (K.-Q. Zhao), monobe-hirosato@aist.go.jp (H. Monobe), bdonnio@ipcms.unistra.fr (B. Donnio).
https://doi.org/10.1016/j.dyepig.2019.107964
Received 12 September 2019; Received in revised form 11 October 2019; Accepted 11 October 2019
Available online 12 October 2019
0143-7208/© 2019 Elsevier Ltd. All rights reserved.

Y.-J. Zhong et al.
Dyes and Pigments 173 (2020) 107964
the combination of their intermolecular well-ordered morphology and
unique electronic structure, which is an essential requirement for ap-
plications involving optoelectronic and photonic devices [4–9]. In
addition, such materials usually show good charge carrier mobility,
which depends strongly on the degree of ordering of the self-organized
mesophases and on the orientation of the LC molecules, indicating the
promising prospects of thiophene-based organic semiconductors for
realizing cheap, flexible and printed electronics applications [10–14].
Among the promising thiophene-based candidates for OFETs are the
benzothienobenzothiophene (BTBT) derivatives (Fig. 1) [15–18]. The
core of BTBT consists of two fused benzothiophene motifs and due to the
presence of lateral aliphatic chains, the corresponding molecules present
good solubility in organic solvents as well as form well-ordered crys-
talline thin films, resulting in high carrier mobility [19–22]. Recently,
our group has reported such BTBT-based semiconductors containing
disk and calamitic LC molecules, which exhibit bipolar carrier mobility
in the range of 10À 3 cm2 VÀ 1 sÀ 1 in the columnar mesophase, and carrier
mobility of hole approaching 0.07 cm2 VÀ 1 sÀ 1 in the smectic phase, as
measured by TOF technique [23,24]. Today, a large range of new
thiophene-containing semiconductors of various shapes and structures,
and differing in the sequences and connections between the thiophene
units, are being investigated and their semiconducting performances
evaluated in ordered mesophases (Fig. 1). Charge mobilities higher than
10 cm2 VÀ 1 sÀ 1 [25–28], have been achieved with various
thiophene-based LC materials, even in solution process [29,30], proving
that they are expected to be promising materials to display good OTFTs
device performances.
or after thermal annealing of the liquid crystal phase, leading to effective
charge carrier transport in organic field-effect transistors (OFETs)
[32–38]. Thus, since they combine features of both crystalline and
amorphous materials, the molecular design of novel LC semiconductors,
supported by time-saving and efficient synthetic strategies, is highly
desirable (self-organization, fluidity, and self-healing abilities, high
control of the molecular orientation, one- or two-dimensional charge
transport properties) for the discovery of new materials and the modu-
lation of their physical properties. Yet, few works of LC materials con-
sisting of a very simple benzothiophene system have been described in
patents and literature, though it has already been proved versatile in the
design of various mesogens [39–41].
In this work, a series of 2-phenylbenzo[b]thiophene (nPBT) meso-
genic derivatives was synthesized in two easy steps and in good yields
from abundant available starting material, i.e. 6-methoxy-2-(4-methox-
yphenyl)benzo[b]thiophene, which already possesses a natural rod-
like and
π-conjugated core, prerequisite of LC phase formation and
semiconducting behaviour. Further stepwise chemical oxidation of the
benzothiophene moiety resulted in two additional sets of functional
derivatives, namely the corresponding sulfoxide homologues (nPBTO)
and one sulfone derivative (10PBTO2), which were also found to be
mesomorphous. The liquid crystalline properties, photo-physical
behaviour and charge transport properties were influenced greatly by
adjusting the length of terminal alkyl chain and by the degree of
oxidation of the benzothiophene moiety. All nPBT showed a SmA phase
in addition to a low-temperature, high-ordered smectic phase, while
only a SmA phase was observed for the oxidized derivatives nPBTO and
10PBTO2. The photo-physical properties, highly depending on the
oxidation degree, displayed strong UV/Vis absorption and intense
yellowish green luminescence, with high quantum yields, in the range of
38–62%. The charge transport carrier of three PBT compounds was also
precisely measured in various mesophases as a function of chain-length
by photocurrent TOF technique, and found to exhibit hole mobility rate
of about 4.2 � 10À 3 cm2 VÀ 1 sÀ 1, in the low temperature ordered
mesophase.
However, some economic constraints and environmental drawbacks
need to be tackled and considered in this active field of research. First,
from a synthetic view point, the chemistry of such organic semi-
conducting molecules is usually not straightforward, and often requires
metal-catalysed coupling methods for connecting the different active
parts through polluting, metal-contamination, expensive, time-
consuming and sometimes low-yields synthetic procedures [31]. Thus,
the easy access to an abundant precursor resource is always beneficial
and cost- and time-saving in materials science, especially to carry out
systematic studies on structure-property relationships. Secondly, the
other fundamental prerequisite that drives the good functioning of the
organic devices, and which has been extensively investigated in LC
materials, is the ability to prepare high quality organic thin crystalline
films (defect-free), with good molecular orientation, either as-deposited
2. Results and discussion
2.1. Synthesis and characterization
The synthesis of the benzo[b]thiophene derivatives was straightfor-
ward, and is shown in Scheme 1. The dihydroxy compound 2 was pre-
pared by demethylation of the readily available precursory compound 1
with pyridine hydrochloride at 190 �C, and obtained as a white solid in
almost quantitative yield (87%). Then, Williamson etherification of 2
with n-alkylbromide at both ends gave the desired nPBT derivatives
(white crystalline solids) in 80–92% yields.
The synthesis of the nPBTO derivatives was based on the general
procedure described in literature, consisting of the oxidation of nPBT by
addition of trifluoroacetic acid and a 30% aqueous solution of H2O2 at
0 �C [42]. The sulfoxide compounds were obtained as yellow-green
powders in yields ranging between 50 and 73%. The sulfone deriva-
tive 10PBTO2 could be directly obtained from 10PBT in 70% yield, by
simply increasing the amount of trifluoroacetic acid and extending the
reaction time. It is worth noting that as the alkyl chain-length was
increased, the solubility of nPBT compounds deteriorated in chloroform,
whereas compounds nPBTO and 10PBTO2 showed almost the same good
solubility. The structures of all synthesized compounds were charac-
terized by means of 1H and 13C NMR (Fig. S11), elemental analysis, and
high resolution mass spectroscopy (HRMS, Fig. S12). Details on the
synthesis are given in the Supplementary Information.
2.2. Mesomorphic properties
The mesomorphic behaviour of all compounds was investigated by
polarized optical microscopy (POM, Fig. S1) and differential scanning
Fig. 1. Representative thiophene-based LC semiconductors reported in
the literature.
2

Y.-J. Zhong et al.
Dyes and Pigments 173 (2020) 107964
of the fans and focal conics to a spherulitic texture (Figs. 2 and S2),
which is reminiscent of a higher molecular re-organization within the
layers [43]. Both nPBTO and 10PBTO2 showed a single SmA phase
(Fig. 2) recognized by the characteristic fan-shaped optical textures. All
the transition temperatures were confirmed by DSC (Fig. S2).
With the increase of the alkyl chain-length, the clearing temperature
of the nPBT terms is decreased (Fig. 3). The range of the SmA phase
however expands gradually to ca. 25 �C (maximum SmA range for
10PBT). Meanwhile, the interval of the low-temperature phase (referred
to as SmX, see below) decreased at the expense of the expansion of the
crystalline phase (Cr). This highly ordered phase has completely dis-
appeared in the longest homolog 14PBT.
The thermal behaviour of the next series is simpler, with the rapid
enlargement of the SmA range with the increase of the alkyl chain length
to reach a 40 �C range for the longer homologues (maximum SmA range
for 10PBTO), due essentially to a smooth decrease of the clearing
Scheme 1. Synthesis, yields and nomenclature of the benzo[b]thiophene de-
rivatives (nPBT) and corresponding oxidized homologues (nPBTO and
10PBTO2). Reagents and conditions: (i) pyridine hydrochloride, reflux, 1 h,
190 �C (87%). (ii) n-Bromoalkane, K2CO3, 2-butanone, 24 h, 100 �C, (80–92%).
(iii) Trifluoroacetic acid, anhydrous CH2Cl2, 30% H2O2, 2–6 h, 0 �C (50–73%).
(iv) Excess of trifluoroacetic acid, anhydrous CH2Cl2, 30% H2O2, 8 h,
0 �C (70%).
calorimetry (DSC, Fig. S2, Table S1). From POM observations, all the
phenylbenzothiophenes and the oxidized derivatives are liquid crystal-
line and solely show smectic phases consistent with their calamitic
shape. The nPBT compounds exhibit all a high temperature smectic A
(SmA), which was unequivocally recognized by the formation of
smooth, focal conic, typical fan-shaped textures, as observed for all of
them on slow cooling from the isotropic liquid (Figs. 2 and S1). For two
short homologues of this series, an additional phase was also detected by
POM and DSC (Figs. S1 and S2, Table S1), monotropic for 5PBT and
enantiotropic for 6PBT, assigned as SmB by POM. On further cooling,
another less fluid, higher-ordered smectic-like phase was detected, prior
to crystallization, by an abrupt textural change with the transformation
Fig. 3. Bar graph summarizing the thermal behaviour of nPBT, nPBTO and
10PBTO2 derivatives (2nd heating).
Fig. 2. Representative POM images, observed on cooling from the isotropic liquid: clockwise from top left: 10PBT at 120 �C in SmA; 10PBT at 114 �C in SmX;
10PBTO at 145 �C in SmA; 10PBTO2 at 77 �C in SmA.
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Y.-J. Zhong et al.
Dyes and Pigments 173 (2020) 107964
temperatures (for n � 8) and an average melting between 100 and
110 �C. The isotropization temperatures of nPBTO are higher than of
their corresponding nPBT homologues (n � 8), and likely dependent on
the stronger dipolar interactions in the latter [44,45]. The reason for the
appearance of the highly ordered phase (SmX) in PBT series may be
attributed to the presence of the aromatic-rich π-electron system and a
strong permanent dipole on the sulfur atom in the thiophene ring, which
endows molecules with high anisotropic polarizability and occurrence of
spontaneous polarization [46–48]. The electron-withdrawing ability of
the sulfur atom in sulfoxide nPBTO is stronger than in nPBT resulting in
the increase of the isotropization temperature. Further oxidation is
detrimental to mesophase stability, which, when compared to the two
other decyl derivatives, 10PBT and 10PBTO, is reduced down to 10 �C
only for 10PBTO2, between ca. 110–120 �C.
The smectic nature of the mesophases of all benzo[b]thiophene de-
rivatives was confirmed by small-angle X-ray scattering (Figs. 4 and S3,
Table S2). On cooling from the isotropic liquid, in the upper temperature
phase, most compounds show a single, intense fundamental reflection
(001) along a few higher order peaks (002, 003, … ), in the ratio 1:2:3:
…, characterizing the lamellar arrangement of the molecules, along a
diffuse scattering at high Bragg angles, corresponding to the molten
chains (Fig. 4, hch). The SAXS patterns of the nPBTO terms, of lesser
quality (less peaks and of lower intensity) than those of the nPBT series
and that of 10PBTO2, but still are in agreement with the formation of a
smectic phase. Combined with POM, the mesophase can be readily
assigned to as SmA in all cases. As expected, the d-spacings in the SmA
increase almost linearly with the alkyl chain length (Fig. 4) for both
series nPBT and nPBTO, respectively, whereas the molecular area, ob-
tained from the ratio between the molecular volume (Vmol) and the
smectic periodicity (Table S2), do not greatly vary with n, but logically
increase upon oxidation of the thiophene ring along the sequence nPBT
(A � 27.5–30 Å2) → nPBTO (A � 32.5–35 Å2) → nPBTO2 (A � 34.5 Å2).
Upon further cooling, the SAXS patterns of the PBT terms revealed
numerous sharp and equidistant (00l) reflections over the whole angular
range (up to 13 for 14PBT, Fig. S3) confirming the resilience of the long-
range lamellar layering, with very sharp interfaces, which persists even
in the lower temperature crystalline phase (Cr). The scattering halo
corresponding to the molten chains is no more detectable on the SAXS
patterns, suggesting that the chains are in a crystalized or pseudo crys-
talized state, and thus that the phase is not a true liquid crystalline
phase. The supramolecular organizations within the SmX (standing for
crystalline phase with a highly well-defined smectic structure) and
crystalline (Cr) phases appear very similar to each other (similar
spacing) and close to the SmA phase.
Fig. 4. Two representative SAXS patterns (6PBT, top, and 10PBTO2, middle).
Variation of the lamellar periodicity, d, and molecular area, A, with the alkyl
chain length, n, for nPBT, nPBTO and 10PBTO2 derivatives (bottom).
2.3. Photophysical properties – UV/Vis absorption and
photoluminescence properties
of 62% (Tables 1 and S3). The length of the alkyl chains has almost no
influence on the absorption and emission spectra which are fully su-
perimposable (Fig. S4).
The impact of the oxidation of the thiophene ring on the photo-
physical properties was evaluated in dilute THF solutions
(10À 5 mol LÀ 1), and in thin films for all benzo[b]thiophene derivatives,
nPBT, nPBTO and 10PBTO2. The absorption spectra recorded for solu-
tions of nPBT exhibit two main absorption maxima at 240 and 320 nm,
while for those of nPBTO and 10PBTO2 showed three strong absorption
maxima centred at 235, 305 and 375 nm, and at 252, 312 and 373 nm,
respectively (Figs. 5 and S4). Solutions of nPBT showed a maximum
emission at 365 nm with two shoulder peaks at 350 and 382 nm, and a
double maximum emission in thin film, at ca. 372 and 393 nm.
2.4. Charge transport properties
Time-of-flight (TOF) technique was used to investigate the charge
carrier mobility of the various liquid crystalline materials. For this study,
both the hole and negative charge mobilities of three representative
benzothiophene derivatives, nPBT with n ¼ 6, 8, 10, were studied as a
function of chain-length. The transient photocurrent decay curves ob-
tained at different temperatures from the isotropic down to the crys-
talline phase showed reasonable shapes of decay. The textures of the
three samples sandwiched in ITO cells, filled by capillary attraction from
the isotropic liquid phase, were observed by POM of at various tem-
peratures (Figs. S6, S8 and S10). All of compounds exhibited a typical
fan-shaped texture in the SmA/B phase(s) as well as a spherulitic texture
in the SmX phase. When the temperature was decreased from the
Emission spectra for the solutions of nPBTO exhibited one broad
emission maximum at 477 nm while centred at 480 nm in thin film,
whereas 10PBTO2 showed a similar emission maximum slightly shifted
at 459 nm in THF solution and at 476 nm in thin film. It is worth noting
that nPBTO showed yellow-green light emission in THF and thin film
under the illumination of UV light of 365 nm (Fig. S4). Quantum yields
calculated for nPBT were around 50%, slightly lower for nPBTO, at
around 40%; 10PBTO2 exhibited the highest quantum yield with a value
4

Y.-J. Zhong et al.
Dyes and Pigments 173 (2020) 107964
Table 1
Photo-physical properties of the three decyl representative benzo[b]thiophene
derivatives. Quantum yields (QY) were measured with solution concentration of
5 � 10À 5 in THF excited at 310 nm (10PBT), at 365 nm (10PBTO) and at 350 nm
(10PBTO2). The data for the other compounds are in Table S3.
Mesogens
λabs
(nm)
ε
� 104
λem (nm)
λem (nm)
QY
(L molÀ 1cmÀ 1
)
solution
film
[%]
10PBT
238
321
1.28
1.95
350
366
382
478
374
393
50.8
41.9
62.6
10PBTO
244
313
379
252
312
373
1.83
160
483
1.07
2.00
1.61
1.44
10PBTO2
459
476
field, thus proving that the transient photocurrent curves of three
mesogens are non-dispersive. With the order degree of mesophase raised
on cooling, the transit time of charge carriers decreased for all samples
(Figs. S5, S7 and S9).
The positive charge carrier mobility of three mesogens is shown in
the cooling process without electric field dependence (Fig. 6), while
electron mobility was not observed. The hole transport of 6PBT shows
mobilities in the order of 2 � 10À 4 cm2 VÀ 1 sÀ 1 in the isotropic liquid. In
the SmA/SmB phase (~150 �C) the mobility rate increases to
5.5 � 10À 4 cm2 VÀ 1 sÀ 1, followed by a second sharp increase up to
4.2 � 10À 3 cm2 VÀ 1 sÀ 1 at the transition to SmX phase (142 �C). Mean-
while, the corresponding hole mobility values exhibited a slight tem-
perature dependence both in the isotropic and smectic phases, it
decreases very smoothly with the temperature falling. The charge car-
rier mobility in crystalline phase was not observed owing to cracks at
domain boundaries. Similarly to 6PBT, the positive charge mobilities of
8PBT and 10PBT display lower values in the SmA phase than in the
higher ordered SmX phase. The charge mobility of holes shows a slight
dependence on the alkyl chain length, decreasing slowly with increasing
length. These results are similar to several literature reported [49], both
in discotic and calamitic molecules. In contrast, TOF measurements for
the sulfoxide (PBTO) and sulfone (PBTO2) derivatives did not yield any
transit photo current.
The presence of this more ordered, low-temperature mesophase in
the phase sequence, thus results in a significant improvement of the
charge transport properties thanks to the pre-organizing templating ef-
fect of the SmA mesophase above, favoured by slow cooling [43].
3. Conclusion
We have successfully synthesized a series of benzothiophene-based
liquid crystalline semiconductors and further oxidized derivatives
from an abundant commercially available pharmaceutical precursory
intermediate. The facile grafting of linear alkyl chains was indeed suf-
ficient to promote the formation of smectic mesophases, showing the
strong mesogenic character of this molecular species. The length of the
alkyl chain and the degree of oxidation of the benzothiophene moiety
were found to greatly influence the phase transition temperatures and
thermal behaviour. Furthermore, the optical properties, emitting blue or
yellow light with high quantum yields in the range of 38–62%, also are
dramatically affected by the oxidation of the sulfur atom of the thio-
phene moiety. The charge carrier mobility of compound 6PBT measured
by the photocurrent TOF technique revealed highest mobility range
(about 4.2 � 10À 3 cm2 VÀ 1 sÀ 1) in the SmX phase, depending strongly on
the order degree of the mesophase. Therefore, the utilization of this
available pharmaceutical intermediate to design organic semi-
conducting and luminescent functional materials shows high promising
applicative potentials, providing some additional structural molecular
adjustments. For instance, PBT compounds have potential applications
Fig. 5. (a) Normalized absorption (straight lines) and emission spectra (dotted
lines) in micromolar THF solution obtained for 10PBT, 10PBTO and 10PBTO2
(chosen as representative compounds); (b) Normalized emission spectra of thin
films of compounds 10PBT, 10PBTO and 10PBTO2. (c) Images of the micro-
molar THF solutions (top panel) and thin films (bottom panel) of nPBTO
(n ¼ 4–14) under UV light (365 nm).
isotropic to the crystalline phase, the POM images showed spherulitic
texture with the presence of grain boundary on the substrate, which
indicated that the molecules still retained the layer structure of the SmX
phase in the crystalline phase. The charge carrier mobility was per-
formed by TOF technique for the representative samples in a sandwich
type ITO cell. The transient photocurrent measurements were performed
on cooling at a very slow rate from the isotropic liquid to crystalline
phase, at different temperatures with an applied electric field strength of
20000–50000 V cmÀ 1
.
The double-logarithm plot of positive transport decay curves for
6PBT, 8PBT and 10PBT, respectively, showed in Fig. 6, enable the ac-
curate read out of the transit time of charge carriers at various electric
5

Y.-J. Zhong et al.
Dyes and Pigments 173 (2020) 107964
Fig. 6. Representative charge carrier mobility
measured by the TOF technique. (Left) Electric
field dependency of positive photocurrent decay
curves (normal plot; Inset double-logarithm plot) at
120 �C (6PBT, with cell thickness is 17.7
μ
m),
m) and
m),
100 �C (8PBT, with cell thickness is 17.7
μ
100 �C (10PBT, with cell thickness is 18.2
μ
from top to bottom. (Right) Temperature depen-
dence of hole mobility (positive charge carrier
transports), measured in the different phases on
cooling for 6PBT (cell thickness is 17.7
μ
m), 8PBT
(cell thickness is 17.7
ness is 18.2 m).
μm) and 10PBT (cell thick-
μ
in the field of organic field-effect transistors (OFETs), as the TOF
mobility results have been demonstrated, while PBTO and PBTO2 de-
rivatives can be used in fluorescent and light-emitting applications, as
they emit blue and blue to green light, with mid-to-high quantum yields.
The design of new mesogens based on 2-phenylbenzo[b]thiophene
maybe be further improved by straightforward molecular derivatiza-
tion, for example, through the substitution of the aromatic core by
electron-donating or accepting groups, or by the extension of the mo-
luminescence spectra, TOF tables, 1H and 13C NMR spectra and HMRS
spectra.
Declaration of competing interest
The authors declare no conflict of interest.
Acknowledgements
lecular anisotropy and consequently enhanced
π conjugation to boost
the electronic properties. Moreover, the inclusion of such moieties into
larger oligomeric, dendritic and polymeric structures would further
facilitate their processing and incorporation into corresponding field-
effect transistors and fluorescent sensors.
This research was financially supported by the National Natural
Science Foundation of China (NSFC, Fund numbers: 51773140,
21772135). KQZ and HM thank the support of NSFC-JSPS (Japan So-
ciety for the Promotion of Science, Joint Project 50811140156). BD and
BH thank CNRS and University of Strasbourg for support.
Supporting information
Appendix A. Supplementary data
The file contains experimental techniques, synthetic details, POM
images, DSC traces, SAXS patterns and indexation, UV–Vis and
Supplementary data to this article can be found online at https://doi.
6

Y.-J. Zhong et al.
Dyes and Pigments 173 (2020) 107964
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