AIE-active mechanoluminescent discotic liquid crystals for applications in OLEDs and bio-imaging

Article abstract of DOI:10.1039/d0cc05813k

A multifunctional molecular design of fluorescent discotic liquid crystal (DLC) consisting of a tetraphenylethylene core is reported, which is found to serve as an excellent solid-state emitter in OLED devices with EQE of 4.4% and tunable mechanochromism.

Full text of DOI:10.1039/d0cc05813k

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AIE-Active Mechanoluminescent Discotic Liquid Crystals for
Applications in OLEDs and Bio-Imaging
Received 00th January 20xx,
Accepted 00th January 20xx
Joydip De,a Abdul Haseeb M. M.,a Rohit Ashok Kumar Yadav,b Santosh Prasad Gupta,c Indu Bala,a
Prateek Chawla,d Kiran Kishore Kesavan,b Jwo-Huei Jou,b and Santanu Kumar Pal*a
DOI: 10.1039/x0xx00000x
 Columnar Liquid Crystal
A multifunctional molecular design of fluorescent discotic liquid
crystal (DLC) consisting of tetraphenylethylene core is reported
which is found to serve as an excellent solid-state emitter in OLED
devices with EQE of 4.4% and tunable mechanochromism. X-ray
diffraction studies unveiled that change in supramolecular self-
assembly is the physical origin of mechanochromism. The
luminescent DLC molecule has been shown to act as a highly
selective probe for labelling acidic cellular compartments (such as
lysosomes) in bio-imaging using HeLa cells.
 Aggregation-Induced Emission
 Mechanochromic Photoluminescence
 Electroluminescence
 Bio-Imaging
Scheme 1. Design of multi-functional columnar DLCs.
task to obtain materials that render the chasm between these
two phenomena (AIE vs ACQ) that will add benefit for cutting-
edge applications. Herein, we have developed a new approach
to a design of tetraphenylethylene (TPE) based luminescent
columnar discotics (Scheme 1) for advanced applications such
as bio-imaging and highly efficient organic light-emitting
diodes (OLEDs) employing them as solid-state emitters. Cell
imaging of HeLa cells demonstrated the utility of TPE discotic
as a novel probe for labelling acidic cellular compartments
such as lysosomes with good stability. In OLED devices,
maximum brightness of 1768 cd m-2 with the emitter 1.1
having an external quantum efficiency of 4.4% has been
perceived. The columnar assembly of DLCs facilitates the faster
movement of charge carrier1d,8a,8b, prerequisite to efficient
recombination process in OLEDs. In addition, these DLC
materials can serve as potent mechano-responsive
luminogens.
Construction of fascinating columnar supramolecular
architecture formed by molecular self-assembly has drawn
incredible attention in recent years due to its various intriguing
applications.1 However, the field remains challenging in terms
of designing new functional materials for superior optical
applications,2 especially in bio-imaging and organic light-
emitting devices. Hereof, liquid crystals (LC) that form
columnar phases can lead to dynamic soft materials,3 the
properties of which can be tuned to meet the aforesaid
desired applications. Most traditional columnar phases formed
by discotic liquid crystals (DLCs) show strong luminescence in
dilute solutions; however, they experience poor emission at
concentrated solutions or in the solid-state due to strong -
interactions of the cores4 leading to the formation of excimers
or exciplexes5 (aggegation caused quenching, ACQ).
Nevertheless, this phenomena curbed their rampant
applications in most solid-state emitting devices. In contrast,
aggregation-induced emission (AIE) active luminogen, first
discovered by Tang et al.6 uncovers a new area7 in which
molecules show high emission in the aggregated state can act
as a potential alternative in solid-state devices. Certainly, it is
in acute demand and challenging
Detailed synthesis and characterizations of the compounds
(1.1-1.3) are discussed in Scheme S1 and Figure S1-S6. All the
compounds were found to exhibit high thermal stability as
observed from their thermogravimetric analysis (TGA) (Figure
S7) data. In differential scanning calorimetry (DSC), all the
materials during heating and cooling scan exhibited a single
peak that corresponds to mesophase to isotropic transition
and vice versa. For example, compound 1.1 displayed
isotropization at 52.1
℃
(with heat of transition ΔH of 12.43
kJmol-1) and a peak at 43.2
℃
(ΔH = 9.30 kJmol-1), on cooling
from the isotropic liquid. All the compounds show similar
behaviour in DSC, as mentioned in details in ESI (Table S1,
Figure S8). In all cases, under polarizing optical microscopy
(POM), the appearance of a well-defined texture (Figure 1a,
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1b, Figure S9) was observed which remained stable down to 1.1-1.3 (Figure 1c (inset), 1d (inset) and S11) represents the
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DOI: 10.1039/D0CC05813K
room temperature (even down to -40
℃). The mesophases of clear visualization of nano segregated columnar assembly in
1.2 and 1.3 were deduced as columnar hexagonal (Colh) on the their respective mesophases.
basis of typical textures observed by POM and of their X-ray In order to study the photophysical behaviour of compound
diffraction experiments. However, for 1.1, the obtained X-ray 1.1-1.3, UV-vis and photoluminescence (PL) spectroscopy were
pattern is consistent with the columnar oblique (Colob) phase performed in solution (10-6 M in THF) and solid-state. In
(see below).
solution, compound 1.1 showed three absorption peaks having
To deduce the exact nature of the mesophase assembly peak maxima (λmax) at 330 nm and only one emission peak at
small and wide angle X-ray Scattering (SAXS/WAXS) 534 nm (Figure S12a). On the contrary, it showed two
experiments have been performed. The SAXS/WAXS pattern of absorption peaks with maxima at 337 nm and one emission
1.1 at temperature 25 C is shown in Figure 1c. It exhibits peak at 505 nm in solid-state (Figure S13a). Compound 1.2 and
many narrow peaks in small-angle region, whereas, in the 1.3 exhibited similar kind of absorption and emission
wide-angle regime, it shows one broad peak (ha) and one weak behaviour as represented in Figure S12-S13. Interestingly, TPE-
peak (hc). The wide-angle peak, ha (d-spacing of ~4.84 Å) is derivatives
typical of liquid-like order of the aliphatic chains, and hc (d- phenomena, with yellow-green colour emission. To study the
spacing of ~3.53 Å) is indicative of interaction of the cores. AIE behaviour different fraction of water (in increasing order)
, exhibited the aggregation-induced emission (AIE)
-
In small angle, the diffraction pattern could be indexed on an was added into the solution of material 1.1 in THF (Figure 2a
oblique lattice with lattice parameters a = 58.38 Å, b = 43.65 Å and S14).9 It is observed that at 0 % water fraction (fw) it
and angle α = 40.42
is Colob in the mesophase of 1.1. The SAXS/WAXS pattern of increasing fw up to 20%, the emission intensity remains the
1.2 at 25 C exhibits many narrow peaks in small-angle region same. After that, it increased upon adding more amount fw at
with d-spacing in ratio and correspond 30%-70%. Then at 90 % fw maximum emission was observed
 (Figure 1c, Table S2). Thus, the assembly exhibits emission with significantly less intensity. With

√
√
√
to reflection from the planes (10), (11), (21), and (31), of (Figure S14a-S14b). Compound 1.2 and 1.3 also exhibited
hexagonal lattices, respectively, with lattice parameter a = similar AIE behaviour as observed in Figure S15. To gain more
45.34 Å (Figure 1d , Table S3). Further, in wide-angle region, insight into the luminescence efficacy, fluorescence quantum
one narrow peak (hac) with d-spacing 4.50 Å, appears due to yield was measured for all the TPE derivatives (1.1-1.3) and
partially crystallized alkyl chain and there is also a backgroud found to be 76.89%, 72.91% and 70.18% for 1.1
, 1.2 and 1.3,
(ha), originated mainly due to the correlations of fluid chains. respectively in the technologically important solid-state.
Moreover, hc peak with d-spacing of ~3.53 Å (indicative of π-π Excitingly, compound 1.1 is sensitive to external stimuli and
interaction) is observed in the wide angle region. Overall, the exhibited tunable mechanochromic fluorescence property. The
structure remains Colh in the mesophase of 1.2. Similar to 1.2
compound 1.3 exhibited Colh nature in its mesophase (Figure grinding changed to cyan colour as observed under 365 nm UV
S10, Table S4) as inferred from SAXS/WAXS analysis. light (Figure 2b inset). The PL peak is hypsochromically shifted
Further, to have a better understanding of the arrangement of from 505 nm to 487 nm upon grinding (Figure 2b). While after
,
compound showed yellowish-green colour, which upon
molecules in their respective phases, electron density map
fumed in dichloromethane (DCM), the ground film recovered
its original luminescence, i.e. it exhibits reversible
h
a
c
a
a 75k
pristine
ground
fumed
103
fw (vol%)
0%
50%
b
1.00
487 nm
505 nm
h
c
60k
45k
30k
15k
0
90%
0.75
0.50
0.25
0.00
ground
fumed
0.9
1.2
1.5
1.8
2.1
102
20 um
0.15
0.20
0.25
0.30
0.35
0.40
0.45
q (Å)
450
500
550
600
650
450 500 550 600 650
Wavelength (nm)
h
ac
Wavelength (nm)
b
d
c
103
ha:
background due
to fluid chain-chain
correlation
h
c
0.9
1.2
1.5
1.8
Figure 2. (a) PL spectra of 1.1 with different water fractions, fw (vol %). Inset represents
the images of the solutions of 1.1 with different fw under 365 nm UV light. (b) PL
spectra of 1.1 before and after grinding, also after exposing it to DCM vapour. λex = 330
nm. Inset shows the corresponding images of 1.1 with different conditions under UV
light. (c) Image of writing and erasing of letter “IISER” on a filter paper by using 1.1,
under UV light.
102
20 um
0.2
0.3
0.4
0.5
0.6
0.7
q (Å)
Figure 1. POM image of (a) 1.1 at 26.2 C and (b) 1.2 at 25.7 C under crossed polarizers
on cooling. SAXS/WAXS pattern of (c) 1.1 in Colob and (d) 1.2 in Colh mesophase at
25C. Inset of (c) and (d) shows EDM of 1.1 in Colob and 1.2 in Colh mesophase,
respectively. Deep red and blue represents the highest and lowest electron density,
respectively.
mechanochromism. In addition, Figure S16 represents the
repeated switching of the mechanochromic luminescence
behaviour of compound 1.1 upon mechanical grinding and
subsequently fumed with DCM for five cycles. Similarly,
(EDM)8 has been constructed by using the information of the
peaks indexes and intensities. The EDM for the compounds
2 | J. Name., 2012, 00, 1-3
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102
102
101
100
104
103
102
101
100
10-1
10-2
250
a
Emitters
Emitters
1.1
1.2
1.3
b
compound 1.2 and 1.3 also exhibited mechanochromic
behaviour (Figure S17). In order to study the physical origin of
the mechanochromism, X-ray diffraction (XRD) measurement
of 1.1-1.3 was performed in the film state before (pristine) and
after grinding. From Figure S18 and Table S4-S10, it is clear
that changes in molecular self-assembly upon grinding i.e.
from Colob to Colh for 1.1 and Colh to Colr for 1.2 and 1.3 is
responsible for mechanochromism.10 Moreover, the electron
density maps derived from the XRD patterns of 1.2 obtained
before and after grinding provided clear visualization of the
change in their assembly as shown in the Figure S19. This
mechanochromic PL behaviour is further studied for practical
application such as rewritable paper. In order to do so, a filter
paper is immersed in the DCM solution of 1.1. After drying, it
showed yellowish-green colour, on which the word “IISER” was
written by glass rod, and it appeared in cyan colour under 365
nm UV light (Figure 2c). After exposing this filter paper in DCM
for a few minutes, the appearance of the written letter “IISER”
vanished. Thus, compound 1.1 can be potentially applicable to
security inks.
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1.2
200
150
100
50
101
DOI: 10.1039/D10.3CC05813K
100
10-1
10-2
0
100
101
102
103
104
2
3
4
5
6
7
8
9
Luminance (cd m-2
)
Voltage (V)
Figure 3. (a) Current density-voltage-luminescence and (b) current efficiency-
luminance-power efficiency plots of OLEDs consist of 3 wt% emitters (1.1-1.3) doped in
CBP host.
Table 1. Electroluminescence behaviour of 1.1-1.3
Turn-on
Voltage
(V)
3.1
3.5
3.1
PEmax/CEmax
/
CIE
Coordinat
esb
(0.23,0.44)
(0.23,0.41)
(0.22,0.43)
Max.
Lum.
a
Emitter
EQEmax
(lmW-1/cdA-1/%)
10.6/11.8/4.4
(cd m-2)
1768
1.1
1.2
1.3
1324
2777
6.2/7.7/3.4
6.5/7.2/2.7
a
Maximum power efficiency (PE), current efficiency (CE), and EQE. bCIE
coordinates at 100 cd m-2.
favourable
balance
between
two
complementary
To get an insight about the electrochemical behaviour of
1.1-1.3, cyclic voltammetry (CV) has been carried out. In the
CV spectra (Figure S20), all the derivatives (1.1-1.3) exhibit
similar oxidation and reduction behaviour. The HOMO/LUMO
values are -5.53 eV/-3.39 eV, -5.48 eV/-3.43 eV and -5.50 eV/-
characteristics, i.e. luminescent and charge transport which
can be further revealed by the low turn-on voltages of the
device (Table 1). To analyze the charge transport in the OLED
devices, hole-only device (HOD) and electron-only device
(EOD) have been fabricated. The current density-voltage (J-V)
characteristics of the HOD, EOD, and reference device (CBP:1.1
3.0 wt%) are displays in Figure S27. The reference device
exhibits high current density at a low voltage compared to the
HOD and EOD, which indicates better injection of charge
carriers from the respective electrodes and favourable charge
transport across the layers for the recombination process12,
hence better device performance. In addition, the
photoluminescence quantum yield (PLQY, ΦPL) measurements
of the best emitter (i.e. 1.1) doped at different concentrations
(1.0, 3.0 and 5.0 wt%) into CBP host demonstrates that the
emitter 1.1 with 3.0 wt% exhibited a PLQY of 81.75% and
achieves a maximum EQE of 4.4% (Table S12 and S13). This
fact suggests that the PLQY of the system (host:guest) is one of
a key parameter that possesses a significant contribution in
evaluating the EQEs of the devices.13 Further, atomic force
microscopy (AFM) measurement of the films of 1.1:CBP with
different doping concentrations (1.0, 3.0 and 5.0 wt%) (Figure
S28) was performed and the films exhibited uniform surface
morphology with no observable cracks or pinholes with very
low surface roughness which is a prerequisite to achieve
efficient OLEDs. The EL spectra of the devices closely resemble
corresponding PL spectra (thin-film state) of the emitters
(Figure S29), suggesting that the EL emission arises from the
intrinsic emission and excited states of the emitters.
3.38 eV, for 1.1, 1.2 and 1.3, respectively (Table S11). The DFT
calculations of 1.1-1.3 were performed to observe the
molecular geometry and electronic delocalization in their
corresponding frontier molecular orbitals. It is observed that
both the HOMO and LUMO are mainly localized over the
central TPE core with little contribution from peripheral alkynyl
benzene units (Figure S21). In such kind of systems, the charge
transport is expected to be high, due to an increased overlap
among the frontier molecular orbitals of the discotic cores11,
which will lead to the better performance in devices.
The excellent photophysical and thermal characteristics of the
TPE based DLC molecules inspired us to study its suitability as
potential electroluminescent materials. For this, solution-
processed OLED devices have been fabricated with the
configuration of ITO/ PEDOT:PSS (35 nm)/ CBP: x wt%
emitter(x = 1.0, 3.0, 5.0, and 100) (25nm)/ TPBi (40 nm)/ LiF
(1.5 nm)/ Al (150 nm) using compound 1.1-1.3 as emitter
(Figure S22a). The OLED devices exhibited bright blueish-green
emission (λmax = 500-508 nm) in electroluminescence (EL)
spectra for emitters (Figure 22b inset) and their corresponding
CIE coordinates are provided in Figure S22b. The current
density- voltage-luminance (I-V-L) and current efficiency-
luminance-power efficiency characteristics of the devices are
illustrated in Figure 3a-3b, and pertinent data are summarized
in Table 1 and Table S12. It is observed that OLED device
fabricated with compound 1.1 (3.0 wt%) exhibited the best
performance among all of the studied devices (Figure S23-S25,
Table S12). The EQE-luminance plots of the devices are
displayed in Figure S26. The device performance shown by the
columnar TPE mesogens (as emitter) displays
The interesting fluorescence pattern displayed by the present
DLCs prompted us to assess their application in cellular
imaging. To test the efficacy of the compound as a potential
fluorophore, HeLa cells were incubated with varying
concentrations of 1.1. Based on the expression pattern, 7.5
μM was selected as a suitable concentration for further
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J. Name., 2013, 00, 1-3 | 3
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experiments (representative image Figure 4a). Investigating
the cells using confocal laser
Conflicts of interest
“There are no conflicts to declare”.
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DOI: 10.1039/D0CC05813K
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and
4 | J. Name., 2012, 00, 1-3
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ChemComm
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DOI: 10.1039/D0CC05813K
A multifunctional molecular design of fluorescent discotic liquid crystal consisting of tetraphenylethylene core is
reported for OLEDs, mechanochromism and bio-imaging applications.