An electron-deficient tris(triazole)-based discotic liquid crystal that exhibits fast electron transport

Article abstract of DOI:10.1039/d1tc00866h

In this study, a C3-symmetric star-shaped discotic mesogen has been reported based on an electron-deficient tris(triazole) central rigid core functionalized with three peripheral trialkoxyphenyl units. The mesogen exhibits an enantiotropic columnar hexagonal (Colh) phase that can sustain over a significant temperature range. When used in space-charge limited current (SCLC) devices, it showed ambipolar charge transport behavior with high electron mobility (μe) of 1.61 × 10-2 cm2 V-1 s-1 at ambient temperature. The finding is sufficiently significant to warrant their use in organic electronic devices.

Full text of DOI:10.1039/d1tc00866h

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An electron-deficient tris(triazole)-based discotic
liquid crystal that exhibits fast electron transport†
Cite this: J. Mater. Chem. C, 2021,
9, 5628
a
a
a
Shallu Dhingra,
Indu Bala,
Joydip De,
Santosh Prasad Gupta,b
Upendra Kumar Pandey *c and Santanu Kumar Pal
*
a
Received 23rd February 2021,
Accepted 5th April 2021
DOI: 10.1039/d1tc00866h
rsc.li/materials-c
In this study, a C3-symmetric star-shaped discotic mesogen has semiconductors.3,9 The charge transport values depend highly
been reported based on an electron-deficient tris(triazole) central on the structural ordering of molecules and intracolumnar
rigid core functionalized with three peripheral trialkoxyphenyl orbital coupling to ensure a uniaxial pathway for charge
units. The mesogen exhibits an enantiotropic columnar hexagonal migration.10 The n-type aromatic systems,11,12 possessing
(Colh) phase that can sustain over a significant temperature range. nitrogen-rich cores, improve structural ordering because of
When used in space-charge limited current (SCLC) devices, it their excellent p–p stacking ability.13,14 This type of design
showed ambipolar charge transport behavior with high electron has been contrived to place the HOMO (LUMO) in the central
mobility (le) of 1.61 ꢀ 10ꢁ2 cm2 Vꢁ1 sꢁ1 at ambient temperature. The position of the molecules to increase the overlap of atomic
finding is sufficiently significant to warrant their use in organic orbitals that will improve the hole (electron) transport property.
electronic devices.
HATNA (5,6,11,12,17,18-hexaazatrinaphthylene), formed after
substituting six nitrogen atoms in a triphenylene core, showed
Organic semiconducting materials (OSCs) provide an alternate an electron mobility of 10ꢁ3 cm2 Vꢁ1 sꢁ1 15
The electron
.
solution to inorganic semiconductors as electron and hole- mobility of anthraquinone, an electron-deficient core was
transporting materials in various (opto)-electronic devices due found to be 3.2 ꢀ 10ꢁ4 cm2 Vꢁ1 sꢁ1 16
.
Truxenone and triaza-
to their low cost, less sophisticated, and low-temperature truxene are electron-deficient cores derived from truxene that
fabrication processes.1,2 The simple processing of OSC materi- show high electron mobility.17 Introduction of boronic esters in
als with liquid crystalline (LC) phases can be regarded as a new the triphenylene core makes it electron-deficient, however, it
generation of OSCs3 because of the organization of discotic showed only hole mobility.18 A heterocoronene core, an exten-
molecules into supramolecular columnar (Col) structures hav- sion of the coronene core using four electron-withdrawing
ing charge carrier migration properties in one-dimension.4,5 carboxaimide groups, shows an ambipolar charge transport.19
Discotic liquid crystals (DLCs) based on electron-rich systems Apart from these, discotic systems bearing nitrogen moieties
(p-type semiconductors) have been widely studied in compar- with ambipolar charge transport are still rather scarce, in
ison to electron-deficient (n-type) ones.6 The few reported respect to the present context. In this regard, tristriazolotria-
electron-deficient cores are designed from their electron-rich zines are new tetracyclic, nitrogen-rich electron-deficient cores
counterparts by substituting aromatic CH groups with nitrogen for DLCs, but to date, extensive research has been done on
atoms7 or by the introduction of electron-withdrawing groups8 compounds based on 1,2,4-triazole and 1,3,5-triazine.20
like carboxyl, imide and amide. It has been seen that n-type Besides, Muzik et al. first laid the foundation for the synthesis
discotics with low bandgap are excellent candidates for trans- of tris(triazole)-based molecules.21
portation of both positive and negative charges in organic
Motivated by this, the present work demonstrated the synth-
esis of a C3-symmetrical star-shaped DLC material (Compound
1, Scheme 1), with an electron-deficient tris(triazole) central
rigid core attached to which were electron-rich alkoxy units at
the periphery. Tait et al. reported a non-LC analogue that was
shown to grow into highly ordered structures.22 The mesogen
reported in this paper shows excellent homeotropic Col
alignment23,24 (the director is perpendicular to the substrate
surface) at room temperature over a macroscopic area which is
highly warranted for semiconducting devices. In addition to
a Department of Chemical Sciences, Indian Institute of Science Education and
Research (IISER) Mohali, Sector-81, SAS Nagar, Knowledge City, Manauli-140306,
India. E-mail: skpal@iisermohali.ac.in, santanupal.20@gmail.com
b Department of Physics, Patna University, Patna-800005, India
c Department of Electrical Engineering, School of Engineering, Shiv Nadar
University, Gautam Buddha Nagar, Uttar Pradesh-201314, India.
E-mail: upendra.pandey@snu.edu.in
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
d1tc00866h
5628 | J. Mater. Chem. C, 2021, 9, 5628–5632
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temperature studies were performed. Under POM, the com-
pound displayed characteristic Col textures with rectilinear
defects. A striking fan-like growth pattern (Fig. S15, ESI†) was
observed that gradually blends into a mosaic texture having
dispersed bright and dark regions as shown in Fig. 1a. This
shearable texture that persists to room temperature indicates
that compound 1 is a room temperature LC. The outcomes
corresponding to the phase transitions observed in DSC ther-
mograms (Fig. S16 and Table S1, ESI†) were similar to those of
the observations in POM. In the heating cycle of DSC from
ꢁ50 1C, one endothermic peak was observed at 50.4 1C (DH =
56.4 kJ molꢁ1) i.e., melting temperature, which corresponds to
crystalline to mesophase transition and another endothermic
peak at 163.3 1C (DH = 5.8 kJ molꢁ1) i.e., clearing temperature,
which corresponds to the mesophase to isotropic transition.
In addition, a crystal to crystal transition was observed at
28.5 1C (DH = 24.7 kJ molꢁ1). In the cooling cycle of DSC
down to ꢁ50 1C, the mesophase appeared at 159.5 1C (DH =
4.6 kJ molꢁ1) and notably, the optical texture remained stable
down to ambient temperature and at 16.0 1C (DH = 27.1 kJ molꢁ1),
it was finally changed to a solid crystalline state. The Col phase
was present over a wide temperature range including room
temperature. It was observed that the change in phase and their
corresponding phase transition temperatures in both optical
(POM) and calorimetric (DSC) studies can be replicable for several
heating/cooling cycles suggesting its high thermal stability. The
thermal stability was also confirmed by thermogravimetric analy-
sis (TGA) (Fig. S17, ESI†). Detert and co-workers20b reported a
structural isomer of 1 with trialkoxy phenyl substituents in the
tangential positions. The isomer exhibits short-range high-
temperature columnar disorder mesophase (Cr 128 1C Colhd
153 1C Iso). Interestingly, in the present tris(N-phenyl)triazole
derivative with trialkoxy phenyl substituents in the radial posi-
tions, the mesomorphic range tremendously widens down to
room temperature.
Scheme 1 Molecular design of tris(triazole)-based ambipolar DLCs with
planar aromatic structure.
this, the introduction of peripheral flexible alkyl chains ele-
gantly empowers the devices with a solution or melt processa-
bility. When mesogen 1 is fabricated in SCLC devices to
determine charge carrier mobility, the electron and hole mobi-
lities extracted from an average of at least five devices were
1.85 ꢂ 0.72 ꢀ 10ꢁ3 cm2 Vꢁ1 sꢁ1 (1.61 ꢀ 10ꢁ2 cm2 Vꢁ1 sꢁ1 for
champion device) and 1.38 ꢂ 0.50 ꢀ 10ꢁ6 cm2 Vꢁ1 sꢁ1
,
respectively.
The threefold symmetric target compound was readily pre-
pared by the reaction sequence depicted in Scheme 2. In basic
conditions, the azo-coupling reactions22,25 on all three bare
positions of 1,3,5-triaminobenzene with the diazotization pro-
duct derived from 3,4,5-tridodecyloxyaniline followed by its
cyclization reaction afforded the requisite product as a beige
solid in reasonably good yield (i.e. 72%). It was then purified
and fully characterized by 1H NMR, 13C NMR, and HRMS
(Fig. S1–S14, ESI†) techniques.
To determine the phase behavior of the synthesized com-
pound 1, differential scanning calorimetry (DSC), polarizing
optical microscopy (POM), and X-ray diffraction at variable
To deduce the detailed structure of the assembly of com-
pound 1, wide-angle X-ray scattering experiments have been
performed. The compound exhibits a columnar hexagonal
(Colh) phase where the cross-section of the columns is arranged
on two-dimensional hexagonal lattices. The pattern in X-ray
diffraction of the compound in the mesophase temperature
range exhibited four peaks in the small-angle region with a
pffiffi
pffiffiffi
ꢀ
pffiffi
ꢀ
ꢀ
d-spacing in a ratio 1=1: 1 3: 1 4: 1 9 corresponding to
reflection from the (10), (11), (20), and (30) planes of the
hexagonal lattice (Fig. 1b). Moreover, there is one broad peak,
ha, in a wide-angle region of spacing 4.93 Å, which appears due
to fluid alkyl chain–chain correlation and one broad hump hc,
with spacing 3.69 Å, indicative of the p–p interactions between
the cores. The calculated value of the lattice parameter at
temperature 60 1C is found to be a = 37.04 Å (Fig. 1c and Table
S2, ESI†). The structure remains Colh in the mesophase regime
and the lattice parameter is found to be more or less constant
with temperature. Furthermore, to have a better understanding
of the arrangement of molecules in the hexagonal phase,
electron density maps (EDMs)26 have been constructed by using
the information of the peak indexes and intensities. It is clear
Scheme 2 Synthesis of tris(triazole) derivative 1 (R: C12H25). Reagents and
conditions: (i) K2CO3, dodecylbromide, DMF, 80 1C, 48 h, yield = 75%; (ii)
NaNO2, Conc. HNO3, DCM, RT, 12 h, yield = 65%; (iii) Pd–C, dry THF, H2
atm, 12 h, yield = 90%; (iv) DPPA, triethylamine, toluene, tert-butanol,
80 1C, 4 h, followed by 110 1C, 2 h, yield = 68%; (v) Conc. HCl, ethanol, RT,
12 h, yield = 75%; (vi) TFA, tert-butyl nitrite, K2CO3, DCM, 0 1C, 12 h, yield =
79%; (vii) Cu(OAc)2ꢃH2O, pyridine, 80 1C, 1 h, yield = 72%.
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Fig. 1 Photomicrograph of the texture observed under POM for the Col phase of 1 kept between a glass slide and coverslip at (a) 59.1 1C, in the cooling
cycle. (b) XRD pattern of 1 at 60 1C exhibiting Colh phase. The inset showed the zoomed area of the pattern. (c) Electron density map in the Colh phase of
compound 1 at temperature 60 1C (a = b = 37.04 Å). The highest electron density is represented by red and the lowest electron density by deep blue.
from the EDM (Fig. 1c) that higher electron density regions other being non-ohmic, to ensure efficient charge injection. A
exhibit regular hexagonal lattices.
non-ohmic counter electrode ensures minimal or no contribu-
Due to the several chromophores and potential p- tion to the overall current measured during J–V characteristics,
conjugations present between the peripheral area and the from the opposite nature of the charge (for example hole in
central core, interesting photophysical properties are exhibited case of electron mobility extraction). Briefly, in the SCLC
by compound 1. In the solution state (Fig. S18a, ESI†), UV-vis technique, during J–V characteristics, at lower applied voltages,
spectra (dark grey trace) revealed two peaks assigned at 234 and current behaves linearly at applied voltages. This linear beha-
342 nm corresponding to p–p* and n–p* transitions, respec- vior is mainly because at lower applied voltages injected
tively, with a small shoulder at 300 nm. Irradiating the solution charges are much less than that of the intrinsic carrier concen-
with absorption maxima, viz., 342 nm, generated the blue tration of the material. However, at higher applied voltages, due
emission maxima at 447 nm (red traces). No change in the to sufficient charge injection from ohmic contact, charge build
emission spectra was observed by changing the excitation up takes place in the sample and the measured current is no
wavelength (lex) at 234, 300 and 342 nm (Fig. S19, ESI†). more governed by the intrinsic charges of the material, leading
Compound 1 was found to be extremely blue fluorescent in to quadratic dependence of the current with applied voltages.
the solution state with the corresponding quantum yield of Considering the material to be trap free, this SCLC behavior of
0.72 (Table S3 and Fig. S20, ESI†). On the other hand, in the J–V characteristics can be modeled by the Mott Gurney equa-
solid-state, compound 1 showed the absorption maxima at tion:28
339 nm while the emission maxima were observed at 445 nm
(Fig. S18b, ESI†).
9
V2
d3
J ¼ e0erm
8
Knowledge of the redox potentials of the material is impor-
tant to further our understanding of conduction in these
systems. For compound 1, in the oxidation cycle, two well-
resolved peaks were observed, whereas one reduction peak was
observed (Fig. S21, ESI†). The energy levels of the LUMO and
HOMO were estimated at ꢁ3.36 eV and ꢁ5.71 eV, based on the
reduction and first oxidation onsets, respectively (Table S4,
ESI†). The electrochemical bandgap value for compound 1
was calculated to be 2.35 eV. Moreover, the HOMO and LUMO
distribution of the molecular orbitals obtained by DFT (B3LYP/
6-31G(d,p) level) suggested the efficient overlap of the orbitals
throughout the molecules, prerequisites for high charge trans-
port behavior (Fig. S22, ESI†).
To evaluate the potential application of compound 1 in
organic devices, charge carrier transport studies were carried
out using the SCLC technique.27 Charge carrier mobility (hole/
electron) is considered as one of the performance parameters
to fabricate efficient devices. In the SCLC technique, compound
1 was sandwiched between two electrodes with one being an
Ohmic contact with the HOMO (for hole mobility extraction)
and with the LUMO (for electron mobility extraction) and the
where J is the current density measured at applied voltage V, e0
is the permittivity of free space, er is the dielectric constant
(4.18) of the material, m is the charge mobility of the material,
and
d is the thickness of the sample used for SCLC
measurements.
For electron mobility cells, Cs2CO3 (work function Bꢁ2.9 to
ꢁ3.1 eV) coated ITO was chosen, both injecting and non-
injecting electrodes to fabricate electron-only cells. Cs2CO3
was chosen because its work function matches well with the
LUMO of the compound 1.29 For hole-only devices gold (Au)
(work function Bꢁ5.1 eV) was chosen as the injecting electrode
with UV ozone treated ITO as a non-injecting electrode to
fabricate SCLC devices. A typical J–V characteristic obtained
for compound 1 for electron and hole mobility devices is shown
in Fig. 2e and f, respectively, where one can clearly distinguish
linear as well as the quadratic regime.
The average SCLC electron and hole mobilities extracted
from five devices were 1.85 ꢂ 0.72 ꢀ 10ꢁ3 cm2 Vꢁ1 sꢁ1 and
1.38 ꢂ 0.50 ꢀ 10ꢁ6 cm2 Vꢁ1
s
ꢁ1, respectively (Table S5, ESI†).
However, the electron mobility of 1.61 ꢀ 10ꢁ2 cm2 Vꢁ1 sꢁ1 was
5630 | J. Mater. Chem. C, 2021, 9, 5628–5632
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birefringent regions along with homeotropic domains. More-
over, nearly uniform homeotropic alignment was found to exist
over a wide region of electron-only cells suggesting better
mobility (Fig. 2a and Fig. S23, ESI†). Whereas, no such uniform
homeotropic alignment was observed for hole-only cells (Fig. 2c
and Fig. S24, ESI†). Besides, the existence of grain boundaries
was visualized after rotation of the polarizer to 451 angle
(Fig. 2b and d). The uniform homeotropic background was
observed for electron-only cells. In contrast, partial or no
homeotropic alignment in the case of the hole-only cell further
explains its lower mobility values.
Conclusion
We have presented trialkoxy-substituted triazoloarene DLC that
shows a stable Colh phase at room temperature. The charge
transport studies reveal fast electron transport of the order of
Fig. 2 Cross polarized optical microscopic images of the SCLC cell taken 10ꢁ2 cm2 Vꢁ1 sꢁ1 as measured using the SCLC technique. The
for electron mobility at an angle of (a) 901 and (b) 451 and for hole mobility
at an angle of (c) 901 and (d) 451 of compound 1. J–V curve for 1 (at 25 1C)
showing ideal ohmic (slope 1, green line) and SCLC regimes (slope 2, red
line) for the (e) electron mobility and (f) hole mobility cell.
finding is expected to guide the development of new high-
performance n-type materials for semiconducting devices.
Conflicts of interest
There are no conflicts to declare.
determined for the champion device which is one order of
magnitude higher than the average electron mobility. Higher
electron mobility (Bthree orders of magnitude) over hole
mobility for 1 is obvious since the core of 1 is designed by
Acknowledgements
utilizing a central tris(triazole) core (electron-deficient) along
with flexible trialkoxy units (electron-rich) at the periphery to
enhance their ability to increase the ordering of molecules
bearing p rings in donor–acceptor arrangement. Structural
ordering is one of the fundamental requirements to achieve
higher charge carrier mobility in p-stacked molecules.30
SKP is grateful to SERB project (CRG/2019/000901/OC) for
financial support. SD acknowledges PMRF-192002-352 for fel-
lowship. UKP is grateful to Shiv Nadar University for funding
and Dr R. Singh for J–V characterization facility.
Notes and references
The mobility in the SCLC technique is highly dependent on
the homeotropic alignment of the material in their respective
hole-only or electron-only cells. It can vary from three to
four orders of magnitude within the same cell having different
devices.31 Approximately, one order of higher electron mobility
for the champion device over the average value observed
can be attributed to its higher degree of homeotropic
alignment as compared to other samples. In DLCs, generally,
the homeotropic alignment can be achieved by introducing a
suitable surface anchoring layer,23,32 using various temperature
treatments,33,34 roughness and surface energy of the substrate35,36
and so on. However, in the present case, the homeotropic align-
ment is obtained spontaneously during the slow filling of the SCLC
cells through capillary action from the isotropic melt followed by
slow cooling of the samples up to room temperature.
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