Aggregation induction of tetraphenylethylene AIEgen and its supramolecular aggregates toward light-emitting diodes

Article abstract of DOI:10.1016/j.jssc.2019.121122

Aggregation behavior of luminophores is a subject of great interest over more than half a century. Aggregation generally causes quenching of luminescence of luminophores, which is detrimental for the solid state electroluminescence device fabrication. In

Full text of DOI:10.1016/j.jssc.2019.121122

Journal of Solid State Chemistry 282 (2020) 121122
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Journal of Solid State Chemistry
journal homepage: www.elsevier.com/locate/jssc
Aggregation induction of tetraphenylethylene AIEgen and its
supramolecular aggregates toward light-emitting diodes
,
,
Pramita Mondal a 1, Sandip Biswas a 1, Debabrata Jana b, Bidisa Das c, Uttam K. Ghorai d,
Binay K. Ghorai b, Somobrata Acharya a
,
*
a School of Applied & Interdisciplinary Sciences (SAIS), Indian Association for the Cultivation of Science, Jadavpur, Kolkata, 700032, India
b Department of Chemistry, Indian Institute of Engineering Science and Technology, Shibpur, Howrah, 711103, India
c Technical Research Center (TRC), Indian Association for the Cultivation of Science, Jadavpur, Kolkata, 700032, India
d Department of Industrial Chemistry and Applied Chemistry, Swami Vivekananda Research Center, Ramakrishna Mission Vidyamandira, Belurmath, Howrah, 711202,
India
A R T I C L E I N F O
A B S T R A C T
Keywords:
Aggregation behavior of luminophores is a subject of great interest over more than half a century. Aggregation
generally causes quenching of luminescence of luminophores, which is detrimental for the solid state electrolu-
minescence device fabrication. In oppose to the aggregation caused quenching, aggregation-induced emission
phenomenon leads to highly luminescence state of luminophores upon aggregate formation. However,
aggregation-induced emission behavior of the luminophores is commonly observed in irregular bulk solids, by
adding antisolvents into a solution or in the biological cells, where the state of aggregation remains uncontrolled.
Here we report on the synthesis of a new tailor made luminogen, 40,4000-(2,2-bis(4-methoxyphenyl)ethene-1,1-
diyl)bis([1,10-biphenyl]-4-carbaldehyde) (TPE 2) using Suzuki coupling reaction with high yield. We form the
well-controlled supramolecular aggregates of TPE 2 at the airꢀwater interface of a Langmuir trough. As a
consequence, the mono- and multilayer LangmuirꢀBlodgett films of TPE 2 show a large enhancement of lumi-
nescence in comparison to the solution phase owing to the aggregation-induced emission phenomenon. We reveal
inherent interaction of TPE 2 complexes within the supramolecular aggregates using density functional theory.
This unique feature of luminescence enhancement is utilized for the fabrication of light-emitting diodes with low
threshold voltage using TPE 2 as active layer. This study opens up the advantage of ordered supramolecular
assembly to achieve optimal luminescence performances.
Aggregation-induced emission
airꢀwater interface
Spherical shape aggregate
Luminescence
Light emitting diodes
1. Introduction
function in the solid state. In contrary to the ACQ effect, there exists a
class of molecules which are weakly emissive or nonemissive in diluted
Since the discovery of organic electroluminescence (EL) device, a
number of luminophores have been attempted for the fabrication of light-
emitting diodes (LEDs). However, most of the luminophores exhibit
strong photoluminescence (PL) in dilute solutions while their PL is often
weakened or even completely quenched in concentrated solutions or in
the solid state due to the well-known fluorescence quenching caused by
the aggregation. The phenomenon is called aggregation caused quench-
ing (ACQ) [1–6]. The aromatic rings of neighboring luminophores
solution, however, show strong luminescence upon aggregation forma-
tion. The recently discovered phenomenon is called aggregation-induced
emission (AIE) effect [7–14]. Successful exploration of AIE was pio-
neered in 2001, where a propeller-shaped silole molecule, 1,2,3,4,5-pen-
taphenylsilole showed non-fluorescent in dilute solution, however, was
highly emissive upon aggregation formation [7,8]. Since the discovery of
the AIE phenomenon, a number of AIEgens were reported till date with
different functionalities [10–17]. A common structural similarity of the
AIEgens relies on the rotatable phenyl rings decorated over a chromo-
phoric stator. In solution phase, active rotation and vibration of the rotor
takes place around the stators, which results in non-radiative decay of the
experience strong π–π stacking interactions which promote the excimer
formation that quenches the luminescence [1–3]. The ACQ effect is
detrimental for practical applications since the EL devices generally
* Corresponding author.
E-mail address: camsa2@iacs.res.in (S. Acharya).
These authors contributed equally.
1
https://doi.org/10.1016/j.jssc.2019.121122
Received 30 September 2019; Received in revised form 29 November 2019; Accepted 8 December 2019
Available online 16 December 2019
0022-4596/© 2019 Published by Elsevier Inc.

P. Mondal et al.
Journal of Solid State Chemistry 282 (2020) 121122
excited states. However, in the aggregated state, movement of the rotors
is restricted thereby opening the radiative channels for the excited states.
Based on these observations, two different qualitative mechanisms for
the AIE phenomenon have been proposed, which are restricted intra-
molecular rotation (RIR) and restricted of intramolecular vibration (RIV)
[7–19]. The aggregations of the AIEgens have been attempted in a
various way. Example include addition of antisolvents into the solution of
AIEgens [7,8], inclusion of AIEgens into the hydrophobic cavities [20],
introduction of AIEgens into layered materials [21,22] and aggregation
of AIEgens within the biological cells [23–25]. However, most of these
techniques lead to the uncontrolled state of aggregation of the, AIEgens,
which is detrimental for the complete exploration of luminescence
properties. In this regard Langmuir-Blodgett (LB) technique is an ad-
vantageous technique since well-controlled molecular level thick film
can be formed at air-water interface followed by dynamic manipulation
of the monolayer by compression or expansion [15,26–31]. The LB
technique allows better controllability of the packing and aggregation of
molecules, which is advantageous to fabricate the well-controlled su-
pramolecular aggregates of the AIEgens.
employed to prepare TPE di-aldehyde derivative 2. The synthetic
approach is outlined in Scheme 1. TPE 1 was prepared following our
previous report [46]. 1H NMR, 13C NMR and MS spectrum confirms the
formation of compound 1 (Figs. S1–S3). Double-fold Suzuki coupling of
TPE 1 with 4-formylboronic acid under given condition afforded desire
product 2 in good yield (80%). The structure of the final compound TPE
2 was confirmed by 1H and 13C NMR spectroscopic techniques (Figs. S4
and S5) (Supporting Information, synthesis details). MALDI-TOF MS
spectrum shows three different peaks in the range of m/z ¼ 500 to 750
(Fig. 1a). The formula weight of the compound is calculated to be 600.70.
MALDI-TOF MS spectra shows a peak at m/z 599.1 (Mþ ꢀ 1), which
matches with the molecular weight of TPE 2. The other peaks at m/z
653.03 (Mþ ꢀ 1) and m/z 653.03 (Mþ ꢀ 1) appears due to the frag-
mentation or polymerization of TPE 2 in high energy beam. Furthermore,
High-resolution mass spectrometry (HRMS) data of TPE 2 suggests syn-
thesis of pure TPE 2 (Figs. S6 and S7).
Owing to the hydrophobic nature, TPE 2 is insoluble in water and can
float at the air–water interface. The presence of –CHO and –OCH3 groups
in the TPE 2 reinforces the interaction with the water subphase imparting
stability to the monolayer at the air–water interface. The surface pressure
In recent years, the tetraphenylethene (TPE) core has been one of the
most studied luminogenic compound in the AIE community [10–15,
(
π
)–area per molecule (A) isotherm shows a transition from the liquid-
–condensed phase to solid phase with the compression of the barrier
(Fig. 1b). The –A isotherm shows that the monolayer collapses slightly
above 30 mN mꢀ1 (Fig. 1b). A limiting molecular area of ~27 Å2 is
extrapolated from the solid phase of the –A isotherm curve (Fig. 1b),
32–37]. Propeller-like conformation effectively prevents direct
π–π
stacking interactions that tend to induce nonradiative recombination.
Decorating TPE with electron donating or electron accepting moieties
leads to excellent luminogenic capability with different colors ranging
from blue to red in the visible region. Some of the luminogens with TPE
moiety show mechanoluminescence, which are known as piezo-
fluorochromic or mechanochromic (MCL) AIE materials [15,34–41]. The
packing of TPE within aggregated configuration induces mechanores-
ponsive PL behavior by changing the molecular conformation under
external stimuli [15,34,37]. Till date, various aggregated morphologies
of TPE derivatives have been reported with the aid of addition of anti-
solvents into the solutions of TPE derivatives, using the closely packed
proximal effect in the solid state or in biological cells [16,23–25,42–45].
However, attempts to fabricate highly ordered supramolecular aggregate
of TPE derivatives is indeed limited. Herein, we report on the synthesis of
a new AIE luminogen 40,4000-(2,2-bis(4-methoxyphenyl)ethene-1,1-diyl)
bis([1,10-biphenyl]-4-carbaldehyde) (TPE 2, compound 2 in Scheme 1)
using a facile synthesis route. Fabrication of supramolecular aggregates
TPE 2 in a controllable and efficient way at the airꢀwater interface using
the LB technique is demonstrated. The structural investigation of the
supramolecular aggregates at the airꢀwater interface reveals prevention
π
π
which is much lower than the geometric area of a single TPE 2 molecule
(~167.3 Å2) calculated in the minimum energy conformation using
density functional theory (DFT) [47] (Fig. 1c). A Lowering of the mo-
lecular area suggests non-planar conformation and possible formation of
aggregates of TPE 2 at the air–water interface. The formation of aggre-
gates of TPE 2 at the air-water interface is further supported from the
successive compression–expansion isotherm cycles below the collapse
pressure (Fig. 1d). A large hysteresis loop is observed between the first
compression and expansion cycle which suggests the irreversible nature
of the TPE 2 monolayer at the air-water interface upon compression. The
hysteresis reduces for further successive compression and expansion
isotherm cycles, which suggests that the major structural change of the
monolayer took place during the first compression isotherm. The irre-
versibility nature of the TPE 2 monolayer during the first compression
cycle indicates the existence of strong intermolecular interactions upon
spreading of the monolayer and formation of aggregates at the air–water
interface.
The surface potential (ΔV)–area per molecule (A) isotherm during the
uniaxial compression process represents the net apparent dipole moment
for the monolayer at the air–water interface (Fig. 1b) [48]. Surface po-
tential of the monolayer reaches maximum value when the molecules
arrange themselves at air-water interface in such a manner that the
effective dipole moment is in perpendicular direction to the air-water
interface. The ΔV–A isotherm does not show change at the initial stage
of compression owing to the random orientation of TPE 2 with a large
separation at the air-water interface. The ΔV starts rising from a mean
molecular area of ~35 Å2, which is higher than the limiting area per
of
π
ꢀ
π
stacking due to the propeller shaped geometry of TPE 2. As a
result, of luminescence intensity enhances by orders of magnitude in
comparison to the solution phase. The rapid enhancement of lumines-
cence is utilized for fabrication of LEDs with a low threshold voltage
using TPE 2 as the active layer. The devices show a maximum luminance
of ~3486 Cd mꢀ2 and current efficiency of 2 Cd Aꢀ1
.
2. Results and discussions
Pd-catalyzed multifold Suzuki coupling reaction technique was
Scheme 1. Steps for the Synthesis of TPE 2a.
aReagents and conditions: (i) Zn (powder), TiCl4, THF, reflux, 2 h; then 4,40-dimethoxybenzophenone, reflux, 5 h; (ii) 4-formylphenylboronic acid, Pd(PPh3)4, K2CO3,
toluene, H2O, Aliquat® 336, 90 ꢁC, 12 h.
2

P. Mondal et al.
Journal of Solid State Chemistry 282 (2020) 121122
Fig. 1. (a) MALDI-TOF MS spectrum of TPE 2. Inset showing the molecular formula and mass of the compound. (b) Surface pressure (
π)–area per molecule (A)
isotherm (red curve) and surface potential (ΔV)–area per molecule (A) isotherm (blue curve) of TPE 2 at air-water interface at 22 ꢁC. (c) Geometry optimized structure
of TPE 2 obtained using DFT. The calculated area of TPE 2 assuming a trapezoidal structure from DFT is 167.3 Å2. (d)
π–A compression-expansion cycles of TPE 2 at
the air-water interface at 22 ꢁC. Three successive cycles are shown in the figure. (For interpretation of the references to color in this figure legend, the reader is referred
to the Web version of this article.)
molecule of TPE 2 observed in the
π
–A isotherm curve. The difference
Further increase in surface pressure above the collapse pressure leads to
the formation of 2D sheets (Fig. 2f). Cross-sectional analysis of the 2D
sheets (Fig. S14) reveals a height of ~4.5 ꢂ 0.2 nm, which is slightly
lesser than that of the spherical aggregates obtained at different stages of
the compression isotherm. Thus, the 2D surface pressure prevalent in the
Langmuir monolayer uniquely merges the spherical aggregates of TPE 2
into flat 2D sheets.
originates from the fact that the –A isotherm is governed by short range
π
interactions, while the ΔV–A isotherm is dominated by the long-range
dipole–dipole interactions [48]. An increase of the surface potential is
observed upon increasing the surface pressures following the trend of the
π
–A isotherm suggesting the gradual orientation of the dipoles of TPE 2
perpendicular to direction of the air–water interface.
3. Morphological analysis
4. Spectroscopy
We have studied the aggregation behavior of the TPE 2 by lifting LB
monolayer films at different surface pressures following the π–A isotherm
curve. Formation of spherical shaped aggregates is evidenced from the
atomic force microscopy (AFM) topographic image even at a low surface
We have compared the UV–vis absorption spectra of monolayer LB
films deposited at various surface pressures with the solution spectrum of
TPE 2 (Fig. 3a). The solution absorption spectra by varying concentra-
tions ranging from 10ꢀ6 to 10ꢀ4 M show prominent peaks at ~281 and
~357 nm respectively. A red shift of ~5 nm of the absorption spectra of
monolayer LB films in comparison to the solution spectrum is observed at
different surface pressures suggesting formation of aggregates. Interest-
ingly, the solution absorption spectrum shows a stronger absorbance in
comparison to the monolayer LB films lifted at different surface pres-
sures. An increase in the absorbance is observed for the monolayer LB
films with the increase in the surface pressure. However, the absorption
peak position remains the same for the monolayer LB films deposited at
different surface pressures suggesting that the nature of the aggregates of
TPE 2 remains the same at the air–water interface for different surface
pressures (Fig. 3a). The aggregates come into the closer proximity with
aid of the surface pressure forming a compact monolayer. Conversely, the
PL spectra for monolayer LB films deposited at different surface pressures
show a rapid enhancement of the PL peak intensity in comparison to the
solution PL spectrum (Fig. 3b). In contrary to the absorption spectrum,
the solution PL shows a weak peak intensity even at higher concentra-
tions (10ꢀ4 M) with a single broad peak at 495 nm. For instance, the
pressure of
π
¼ 5 mN mꢀ1 (Fig. 2a). Zoom-in AFM image (Fig. S8) shows
that the spherical aggregates are of ~175 ꢂ 25 nm in diameter with a
uniform height of 6 ꢂ 0.5 nm. The spherical aggregates remain in the
interconnected configuration even at the low surface pressure. An in-
crease in the interconnectivity and packing density of the spherical ag-
gregates is observed at higher surface pressures of
(Fig. 2b). A closed packed interconnected network of spherical aggre-
gates is evidenced at
higher surface pressure of
and interconnectivity between the spherical shaped TPE 2 aggregates is
supported by the scanning electron microscopy (SEM) image of mono-
layer LB film lifted at 5 mN mꢀ1 (Fig. S13). Finally, a complete closed
π
¼ 10 mN mꢀ1
π
¼ 15 mN mꢀ1 (Fig. 2c), which becomes denser at
π
¼ 20 mN mꢀ1 (Fig. 2d). The spherical shape
packed uniform film is obtained at surface pressure of
π
¼ 25 mN mꢀ1
(Fig. 2e). Interestingly, the height of the aggregates within the monolayer
remains the same at different surface pressures (Figs. S8–S12) suggesting
that the increase in the surface pressure effectively fills void present
within the in-plane aggregates thus rigidifying the Langmuir monolayer.
3

P. Mondal et al.
Journal of Solid State Chemistry 282 (2020) 121122
Fig. 2. AFM topography images of TPE 2 at the air-water interface showing spherical aggregates at surface pressures (a) 5 mN mꢀ1, (b) 10 mN mꢀ1, (c) 15 mN mꢀ1
(d) 20 mN mꢀ1, and (e) 25 mN mꢀ1. (f) AFM topography image lifted after collapse pressure showing the formation of flat 2D sheets.
,
Fig. 3. (a) Comparison of UV–vis absorption spectra of TPE 2 in solution at different concentrations and monolayer LB films deposited lifted at different surface
pressure. (b) PL spectra of TPE 2 in solution at different concentrations and monolayer LB films deposited at different surface pressures. PL is obtained with an
excitation wavelength of 360 nm. (c) Comparisoꢀn1of UV–vis absorption spectrum of concentrated solution (10ꢀ4 M) of TPE 2 with different layer numbered LB films
deposited at a fixed surface pressure of 20 mN m . (d) Comparison of PL spectrum of concentrated solution (10ꢀ4 M) of TPE 2 with different layer numbered LB films
deposited at a fixed surface pressure of 20 mN mꢀ1. The color codes of the spectra are denoted in the inset of the figures. (For interpretation of the references to color
in this figure legend, the reader is referred to the Web version of this article.)
4

P. Mondal et al.
Journal of Solid State Chemistry 282 (2020) 121122
monolayer LB film deposited at a surface pressure of
π
¼ 5 mN mꢀ1 shows
films is found to be lower in comparison to the concentrated solution
absorbance spectrum (10ꢀ4 M). In contrary, a rapid enhancement of the
PL intensity of the multilayer LB films is observed in comparison to the
solution spectrum (Fig. 3d). The PL spectra for the multilayer LB films
contain a single broad peak at 520 nm, which is energetically the same as
the monolayer LB films. The retention of the same peak positions in the
absorption and PL spectra of the LB films suggests that the morphology of
the TPE 2 aggregates remains identical in the Langmuir monolayer and
multilayer LB films. An enhancement of the PL peak intensity of the
multilayer LB films within the surface pressure range of 5–25 mN mꢀ1 is
observed, although the peak intensities do not show a drastic change
within this surface pressure range. However, the peak intensity of the
multilayer LB films is drastically enhanced in comparison to the solution
spectrum (Fig. 3d). We have measured the absolute PL quantum yield
(PLQY) of the multilayer LB films by using a calibrated integrated sphere.
A low PLQY of 0.5% is recorded for the solution of TPE 2. The PL QYs of
the multilayer LB films are measured to be ~8.23% for 5 layers, 12.55%
for 9 layers, 20.40% for 15 layers, 26.46% for 19 layers and 33.38% for
23 layers respectively (Table S2, supporting information). In comparison,
a low QY of ~7% is recorded for dropcast film of TPE 2.
~3 times enhancement in the peak intensity in comparison to the 10ꢀ4 M
solution (Fig. S15). A single broad peak centered at 520 nm is observed
for monolayer LB films deposited at different surface pressures. A
prominent red shift of ~25 nm of the monolayer LB films in comparison
to the solution PL strongly suggests formation of the aggregates within
the monolayer (Fig. S15). We have checked the AIE activity of TPE 2 in a
mixture of good-solvent (THF)/bad-solvent (water) at different ratios.
The PL remains nearly unchanged until 90% water is added into the
solution (Fig. S16). Notably, water is added to the good-solvent to induce
aggregation of TPE 2. In case of water contents ꢃ80%, TPE 2 shows very
weak PL peaking at 450 nm. Shifting of PL peak to 520 nm occurs at 90%
water content in the mixture due to the aggregate formation which
matches with the PL of TPE 2 LB film (Fig. S15). A gradual increase of the
PL intensity upon increasing the surface pressure is observed (Fig. 3b).
Rapid enhancement of overall PL intensity of the monolayer LB films in
comparison to the solution PL suggests AIE phenomenon to occur within
the monolayer by activating RIR and RIV processes [7–14].
We have further enhanced the AIE effect by depositing multilayer LB
films of TPE 2 at a fixed surface pressure of
π
¼ 20 mN mꢀ1. The UV–vis
absorption spectra of multilayer films show prominent peaks at ~281
and ~357 nm respectively, which remain energetically in the same po-
sition as the monolayer films. The absorbance of the multilayer LB films
increases gradually with number of layers reveling a uniform LB film
transfer process (Fig. 3c). However, the absorbance of the multilayer LB
We have quantified the AIE effect by calculating the AIE factor, which
is the ratio of the QYs of the aggregated state and solution. A drastic
increase in the αAIE is observed with the increase in the number of the
monolayer, and ultimately it reaches to an optimum level for a higher
number of layers (Fig. 4a). The solution of TPE 2 shows a low QY of 0.5%
Fig. 4. (a) Plot of AIE factor versus the number of layers. Inset: Photographs of LB films with different number of layers deposited on quartz substrate at a fixed surface
pressure of 20 mN mꢀ1 under 365 nm UV illumination. (b) Confocal laser scanning microscopy (CLSM) image of monolayer LB film lifted at 25 mN mꢀ1 showing
intense green luminescence. (c) Comparison of PL decay of LB film (blue curve) and drop cast film (red curve) of TPE 2. Multilayer (39 layers) LB film was deposited at
π
¼ 20 mN mꢀ1 surface pressure. The excitation wavelength is λex ¼ 360 nm. For time resolved measurements, the emission was monitored at the corresponding PL
maxima. Solid lines represent the fitted curve. (d) Plot of on-trough PL spectra, recorded at different compression stages of the isotherm ranging from 0 to 32 mN mꢀ1
surface. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
5

P. Mondal et al.
Journal of Solid State Chemistry 282 (2020) 121122
owing to the allowed intermolecular rotation through which the energy
is dissipated. However, the αAIE increases drastically with the increase in
the number of layers of the LB films till to an optimum level (Fig. 4a). The
increase in the αAIE in the multilayer LB films is attributed to the
enhanced RIR and RIV processes in the aggregated state [7–14]. In
multilayer LB films, the rotation of the phenyl rotors is restricted due to
the rigid packing density activating the RIR and RIV processes, which
intersystem crossing from the singlet to the triplet states owing to the
reduced energy gap by preventing active molecular vibrations and ro-
tations, which facilitates the efficient intersystem crossing leading to
phosphorescence at room temperature.
In order to confirm that the supramolecular aggregates of TPE 2 oc-
curs within the Langmuir monolayer upon spreading; we have measured
"on-trough" PL at different surface pressure following the π–A isotherm.
enhances the PL intensities. Additionally, the interlayer C–H⋅⋅⋅
π
in-
The experimental set-up for measuring the on-trough PL of Langmuir
monolayer is described in the supporting information (Fig. S19). Before a
monolayer was deposited on the subphase, the baseline was corrected
before the PL measurements. As the monolayer was deposited at the air-
water interface, the PL was recorded as a function of the surface pressure
during the compression stage. The PL spectrum was detected from the
teractions [49,50] and van der Waals forces within the multilayer stiffen
molecular conformation within the aggregated structure that facilitates
strong AIE behavior. Owing to the rapid enhancement of PL intensity in
comparison to the solution phase, solid state PL can be obtained from the
multilayer LB films. The photographs of the multilayer LB films deposited
on quartz substrates shows green color under the 365 nm UV illumina-
tion (inset, Fig. 4a). Confocal laser scanning microscopy (CLSM) image of
the monolayer LB film shows homogeneous green fluorescence upon
excitation with 405 nm laser (Fig. 4b). The solid state green emission
from the monolayer and multilayer LB films are in line with the single
peak PL observed for LB films with different number of layers. The for-
mation of supramolecular aggregates within the Langmuir monolayer is
likely to be the leading reason for the observed green emission from the
LB films. It appears that propeller-shaped TPE 2 undergoes further rigid
conformation within the supramolecular aggregates in the Langmuir
monolayer at the air-water interface which restricts the intramolecular
rotation and vibration in an efficient way. This feature is retained in the
multilayered LB films deposited at different surface pressures.
beginning of the compression at
π
¼ 0 mN mꢀ1. The PL spectrum taken
across the Langmuir monolayer shows a single broad peak at 520 nm,
which the same as the monolayer or multilayer LB film (Fig. 4d). This
observation confirms that the supramolecular organization of TPE 2
takes place at the air-water interface immediately after spreading. A
monotonous increase in the PL intensity is observed by increasing the
surface pressure. The increase in the PL intensity with surface pressure
can be attributed to the effective fulfillment of the voids in the Langmuir
film by close packing of the spherical aggregates thereby enhancing the
AIE effect.
5. Modeling and interactions
Increasing the number of monolayer in the multilayer LB films further
rigidify the packing density thereby hindering the vibration and rotation
of TPE 2 in an efficient way. We have also compared the time-resolved PL
decays of the solution and LB films of TPE 2. A fast lifetime ~405 ps is
obtained from the TPE 2 solution (Fig. S18). However, the multilayer LB
To understand the internal structures of the spherical aggregates, we
have carried out X-ray diffraction (XRD) measurements of multilayer LB
film (Fig. 5a). A periodicity along the thickness of the multilayer LB films
is reflected from the peak at 1.70ꢁ (d ¼ 5.2 nm), which closely matches
with the thickness measured using AFM at different surface pressures
(Figs. S8–S12). The peak at 4.5ꢁ (d ¼ 1.9 nm) corroborates with the end-
to-end distance (~1.8 nm) of the adjacent –CHO groups of TPE 2 (Figs. 1c
and 5b, Front view). Broad peak at 23.46ꢁ (d ¼ 0.37 nm) can be assigned
films show a much longer lifetime of ~2.24 μs for the 39-layer LB film
deposited at
π
¼ 20 mN mꢀ1 (Fig. 4c). Additionally, we have carried out
control experiments with the drop cast film of TPE 2. The AFM image of
the drop cast film of TPE 2 shows aggregates with irregular morphologies
to multiple scattering from aromatic stacking including the C–H⋅⋅⋅
π
(Fig. S17). However, a longer lifetime of ~2.16
μs similar to the LB films
interaction distance as shown in the top view of Fig. 5b [26]. We have
further illumined the aggregation behavior using DFT calculations [47].
We observed earlier that the phenyl rings of TPE 2 remain in highly
twisted configuration (Fig. 1c) due to steric and electronic effects.
Furthermore, we have used DFT calculations for the dimeric assembly of
TPE 2 in order to understand the interaction between the adjacent
molecules within the aggregates. We observe that the dimeric assembly
of TPE 2 involves two prominent C–H⋅⋅⋅π interactions between the phenyl
rings containing –OCH3 group and adjacent hydrogen atom of adjacent
TPE 2 (Fig. 5b, top view). The phenyl rings containing –OCH3 groups
is obtained from the drop cast films (Fig. 4c). Such longer PL lifetime is
reported earlier for the AIEgens in the aggregated form [15,26,50,51].
Additionally, earlier theoretical reports on the aggregates of AIE mole-
cules suggested modulate energy gap between excited singlet and triplet
states (ΔESꢀT) [52]. Guozhen Zhang et al. showed that ΔESꢀT decreases
due to aggregate formation of AIEgens which causes aggregation induced
intersystem crossing (AI-ISC) for the aggregates thereby increasing the PL
lifetime [53]. In our case, the observed long-lived excited-state lifetime in
the of the aggregated structure within the LB films or drop cast films in
the solid state points out towards phosphorescent origin in ambient
only participates in the C–H⋅⋅⋅
molecules. In the front view of Fig. 5b of the dimeric assembly, the
π interactions between the adjacent TPE 2
temperature. The aggregated structure of TPE
2 facilitates the
Fig. 5. (a) XRD pattern of 30 layered LB film of TPE 2 lifted at a fixed surface pressure of 15 mN mꢀ1. (b) Energy-minimized dimeric structure of TPE 2 using DFT. The
perpendicular C–H⋅⋅⋅ hydrogen bond distance (Å) for the H atom interacting with the electron cloud of phenyl ring is marked by red lines in the top view. In the front
view two phenyl rings engaged in the C–H⋅⋅⋅ interactions are shown by solid fill. The end-to-end distance between two –CHO groups of TPE 2 is marked in the figure.
π
π
(For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
6

P. Mondal et al.
Journal of Solid State Chemistry 282 (2020) 121122
Fig. 6. (a) Energy-level diagram of TPE 2 based multilayer organic light-emitting diode (OLED). (b) EL spectra of the device at different bias voltages along with the
solid-state PL. (c) CIE chromaticity co-ordinates of the EL and PL spectra. (d) Current density vs voltage curve of the LED. Inset: Photograph of the working device at
16 V. (e) Luminance vs voltage characteristics. (f) Current efficiency and power efficiency vs current density characteristics of TPE 2 LED.
marked phenyl rings remain almost in parallel conformation with the
adjacent TPE 2 with a slightly lateral displaced condition. In the aggre-
energy-level matching of the components used in the device (Fig. 6a).
The devices show stable electroluminescence (EL) over a wide range of
operating voltages (Fig. 6b). The EL retraces the PL suggesting that the EL
occurs from the active TPE 2 layer (Fig. 6b). The corresponding Com-
gated state, the propeller shape of the TPE 2 prevents direct
interactions. Adjacent TPE 2 molecules stack via C–H⋅⋅⋅
between hydrogen atoms in the phenyl rings of one TPE 2 molecule and
the electron cloud of the phenyl rings of the adjacent TPE 2 molecule
(Fig. 5b). The distances in the C–H⋅⋅⋅ interactions shown in the top view
of Fig. 5b appears within the range of 3.4–3.75 Å. Although this distance
is slightly higher for effective C–H⋅⋅⋅ interaction, however, sufficiently
π
ꢀ
π
stacking
interactions
π
ꢀ
mission Internationale de l’eclairage (CIE) chromaticity coordinate dia-
π
gram indicates a nearly bias-independent cyan color of the LED at
different voltages (Fig. 6c). The CIE chromaticity coordinates of the LED
is (0.23, 0.64) at different voltages in comparison to CIE coordinates
(0.23, 0.65) of the PL spectrum. The currentꢀvoltage characteristic of the
working device is presented in Fig. 6d. Subsequently, the recombination
of the charge carrier takes place within the active TPE 2 layers by
external bias. A turn-on voltage of ~4 V is obtained from the device.
Luminance is detectable above the threshold voltage showing ~3486 cd
π
π
high binding energy of ꢀ21.2 Kcal (theoretical calculation, Supporting
Information) is obtained for the dimeric assembly suggesting a strong
interaction among the TPE 2 molecules in the aggregated state. These
intermolecular interactions further stiffen the molecular conformation in
the aggregated state and finally spherical aggregates are formed with
higher units of TPE 2 at lower surface pressure. Surface pressure brings
these spherical aggregates into the closer proximity, as evidenced from
the AFM observation (Fig. 2). Higher surface pressure causes intercon-
nection between these spherical aggregates and fulfills the voids of
monolayer LB film by densely packing of the aggregates. All these col-
lective phenomena lead to the high PL of the TPE 2 in the aggregated
state justifying the AIE effect.
m
ꢀ2 at 16 V (Fig. 6e). A photograph of the working device is presented in
the inset of Fig. 6d. The LED displays uniform green color over the entire
active device area without any inhomogeneity demonstrating that the
emission from TPE 2 is uniformly distributed in the working device. The
device attains a maximum current efficiency and a power efficiency of
~2 cd Aꢀ1 and ~0.6 lmWꢀ1 respectively (Fig. 6f).
7. Conclusion
6. LED fabrication
In conclusion, we report on the We report on the synthesis of a new
tailor made AIEgen, TPE 2, using Suzuki coupling reaction with high
yield. We have shown a unique supramolecular strategy to assemble of
AIE active TPE 2 using LB technique. We observe that the TPE 2 forms
spherical shaped aggregates at low surface pressure at the air-water
interface. The increase in the surface pressure eventually leads to a
closed packed monolayer film of the spherical shaped aggregates. For-
mation of the aggregates of TPE 2 enhances the AIE phenomenon by
restricting the rotation and vibration of TPE 2. As a result, a large
enhancement of PL is obtained from the mono- and multilayer LB films in
comparison to the solution phase. The resultant LB films showed efficient
solid-state PL and high absolute PL quantum efficiency. Multilayer LB
film exhibited excellent AIE properties with an increase in the AIE factor
of 67-fold compared to the solution phase. The multilayers LB films
consisting of much lesser amount of TPE 2 show luminescence equivalent
to the drop casted films, which is beneficial for fabricating LED. LEDs
Since TPE 2 shows bright PL in the solid state, we have fabricated an
LED using TPE 2 as an active element. First, we have calculated the
electronic structure of TPE 2 using DFT in order to estimate the HOMO
and LUMO levels, which helps in fabricating the device architecture. The
molecular geometry is optimized using the B3LYP/6-31G** (Supporting
Information) as implemented in the G09 program package [47]. We have
calculated the HOMO/LUMO energies from DFT to be ꢀ1.95 eV and
ꢀ5.18 eV respectively (Fig. S20). This implies a HOMOꢀLUMO gap of
3.23 eV, which is used to select hole transport and the electron trans-
porting layers within the device. However, the HOMOꢀLUMO gap is
expected to decrease in the aggregate state as expected for aggregated
systems (Table S1). We have used N,N0-di(1-napthayl)-N,N0- diphenyl-(1,
10-biphenyl)-4,40-diamine (NPD) as the hole transport layer and Bath-
ophenanthroline (Bphen) as the electron-transporting layer following the
7

P. Mondal et al.
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Declaration of competing interest
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The authors thank SERB grant # EMR/2014/000664, DST, India for
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Appendix A. Supplementary data
Supplementary data to this article can be found online at https://do
i.org/10.1016/j.jssc.2019.121122.
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