AIE-active and reversible mechanochromic tetraphenylethene-tetradiphenylacrylonitrile hybrid luminogens with re-writable optical data storage application

Article abstract of DOI:10.1016/j.dyepig.2017.06.043

Two new tetraphenylethene-based diphenylacrylonitrile derivatives (TPEDPAN1 and TPEDPAN2) were synthesized in high yields and their reversible mechanochromic luminescence and aggregation-induced emission characteristics are reported. These luminogen cryst

Full text of DOI:10.1016/j.dyepig.2017.06.043

Accepted Manuscript
AIE-active and reversible mechanochromic tetraphenylethene-
tetradiphenylacrylonitrile hybrid luminogens with re-writable optical data storage
application
Srinivas Gundu, Myeongjin Kim, Naveen Mergu, Young-A. Son
PII:
S0143-7208(17)30991-9
10.1016/j.dyepig.2017.06.043
DYPI 6064
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 1 May 2017
Revised Date: 13 June 2017
Accepted Date: 15 June 2017
Please cite this article as: Gundu S, Kim M, Mergu N, Son Y-A, AIE-active and reversible
mechanochromic tetraphenylethene-tetradiphenylacrylonitrile hybrid luminogens with re-writable optical
data storage application, Dyes and Pigments (2017), doi: 10.1016/j.dyepig.2017.06.043.
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Graphical abstract

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AIE-Active and reversible mechanochromic tetraphenylethene-
tetradiphenylacrylonitrile hybrid luminogens with re-writable optical data
storage application
Srinivas Gundu, Myeongjin Kim, Naveen Mergu and Young-A. Son*
Department of Advanced Organic Materials Engineering, Chungnam National University, 220
Gung-dong, Yuseong-gu, Daejeon 305-764, South Korea
*Corresponding author: yason@cnu.ac.kr; Tel.: +82 42 821 6620; Fax: +82 42 821 8870
Abstract
Two new tetraphenylethene-based diphenylacrylonitrile derivatives (TPEDPAN1 and
TPEDPAN2) were synthesized in high yields and their reversible mechanochromic
luminescence and aggregation-induced emission characteristics are reported. These luminogen
crystals exhibiting green fluorescence could be transformed into powders emitting an orange-
yellow color upon grinding, and the green emission could be restored when the ground powders
were fumed with dichloromethane. To reveal the mechanochromic mechanism, a series of tests
including powder XRD, single-crystal X-ray, DSC, and solid state UV-visible absorbance were
carried out. These studies suggested that the reversible mechanochromism originated from the
transformation of a crystalline state into an amorphous state and vice versa. The single crystal
structure of TPEDPAN1 revealed that the poor intermolecular interactions of C-N (3.087 Å)
with in the dimer, would give loose packing of the molecular conformation, thus yielding
mechanochromic behavior in the crystals upon grinding.

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Keywords: Tetraphenylethene; Reversible mechanochromism; Grinding-fuming; Crystalline-
amorphous; Aggregation-induced emission
1. Introduction
The development of stimuli-responsive fluorescent organic materials has been of great
interest in recent years due to their potential practical applications such as in sensors,
photoelectronic devices, displays and memories [1-8]. The emission properties of
mechanochromic luminescent materials generally depend strongly on their molecular
arrangement motif and intermolecular interactions. As a result, it is sometimes possible to
change the mode of molecular packing by means of various external stimuli such as shearing,
grinding or rubbing, resulting in tuning or switching of photoluminescent properties including
emission intensity and color [9]. Generally, no chemical structural damage occurs during the
reversible emissive color change process, and only the orientation of molecular packing in the
solid state is changed under an external pressure stimulus in mechanofluorochromism (MFC). As
a kind of “smart material” [10-12], mechanochromic fluorescent materials are noteworthy
because of their promising applications in optical storage, pressure sensors, rewritable media,
and security ink [13].
Many organic materials are nonluminescent or weakly emissive in the solid state, but they
emit strongly in the solution state under UV irradiation. This is ascribed to the strong
intermolecular electronic interactions in the solid state, which in turn favor the formation of
excimers and exciplexes. This phenomenon is termed “aggregation-caused quenching” (ACQ)
[14]. In contrast, a milestone has been reported by Tang’s group finding that some organic dyes,
that were almost nonfluorescent in solution, were strong emitters in their aggregated state [15].

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This phenomenon is called aggregation-induced emission (AIE) and is exactly the inverse of
ACQ. The AIE phenomenon is attributed to the restriction of intramolecular rotations (RIR)
[16,17], which is favorable for the generation of highly emitting solid organic luminogens [18-
21]. It is very significative to synthesize mechanochromic luminogens possessing AIE property.
So far, more number of AIE-active materials have been reported in literature. In contrast, AIE-
active organic luminogens exhibiting mechanochromic behavior are still rare. It is fascinating
that some mechanochromic luminogens are AIE active [22-24]. For instance, Tang et al.
demonstrated that tetraphenylethene (TPE) derivatives were known to possess mechanochromic
properties originating from their loose molecular packing in crystal states, which could be easily
altered under mechanical stimuli and could lead to the transformation of the emitted color [25].
Therefore, it could be envisaged that the presence of extended nonplanar π-conjugation in
organic dyes is essential in potential mechanochromic materials because of their loose packing
[26,27]. By using TPE as a key scaffold, few efficient stimuli-responsive AIEgens have been
constructed, and they exhibit varying responses to pH, light, temperature, viscosity, pressure, and
mechanical force [28-30]. In addition, some cyanostilbene derivatives are demonstrated in
literature to exhibit intramolecular charge transfer (ICT), aggregation-induced emission,
crystallizationinduced emission enhancement (CIEE) and mechanochromic characteristics [31].
Carbazole-based triphenylacrylonitrile derivatives also known to possess AIE characteristics
with high solid-state efficiency and high quantum yield (ΦF 75.6%) in THF–H2O mixtures [32].
In this communication, we report synthesis of novel tetraphenylethene-based luminogens
TPEDPAN1 and TPEDPAN2 with a diphenylacrylonitrile skeleton in high yields. Then, we
investigated the aggregation-induced emission (AIE) property in THF–H2O mixtures and

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reversible mechanochromic luminescent property by grinding with mortar and pestle and fuming
with dichloromethane (DCM).
2. Experimental
2.1. Materials and methods
All chemicals and reagents were purchased from Sigma Aldrich and used without further
1
purification. The H and 13C nuclear magnetic resonance (NMR) spectra were recorded on a
Bruker AVANCE III 600 spectrometer, CDCl3 used as the solvent. The fluorescence spectra
were recorded on a Shimadzu RF-5301PC fluorescence spectrophotometer. Solid-state
absorption spectra were recorded on a SolidSpec-3700 diffuse reflectance UV-VIS-NIR
spectrophotometer. Solid-state fluorescence spectra were recorded on a HORIBA SCIENTIFIC-
Fluorolog3 high sensitivity spectrofluorometer. FT-IR spectra were recorded on an FTS-175C
(Bio-laboratories. Cambridge, USA) infrared spectrometer. Mass spectra (EI-MS) were recorded
on a JEOL MS station (JMS-DX303). Powder X-ray diffraction measurements were carried out
with an X-Pert PRO MPD high performance X-ray diffractometer. DSC measurements were
conducted using a Mettler-Toledo DSC thermal analyzer with a heating/cooling rate of 10˚C/min
under nitrogen atmosphere. Single-crystal X- ray diffraction was carried out using a SMART
APEX II single crystal diffractometer. Ground samples were prepared by grinding, using a
mortar and pestle. The solvent vapor fuming treatment method involved filling the ground
sample into a grooved glass, which was then placed in a large beaker saturated with
dichloromethane (DCM) for 5 min at room temperature. The ground-state geometries were
optimized using density functional theory (DFT) with the B3LYP hybrid functional at the basis

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set level of 6-31G(d,p). TD-DFT calculations were performed at the B3LYP/6-31G(d,p) level.
All the calculations were carried out in the gas phase using the Gaussian 09 program.
2.2. Synthesis
The synthetic route for TPEDPAN1 and TPEDPAN2 is shown in Scheme 1. Syntheses of
compounds 1, 2 and 4 were carried out according to previously published protocol [33,34].
Compounds TPEDPAN1 and TPEDPAN2 were prepared via Knoevenagel condensation of
4',4''',4''''',4'''''''-(ethene-1,1,2,2-tetrayl)tetrakis(([1,1'-biphenyl]-4-carbaldehyde)) (2) with phenyl
acetonitriles (3). Most of the reported Knoevenagel condensation reactions involved the usage of
hazardous base catalysts like piperidine [35] and di-n-propylamine [36]. To avoid the usage of
these bases, we have carried out the reaction in presence of simple base NaOMe in ethanol in
high yields.
2.3. Synthesis of TPEDPAN1 and TPEDPAN1:
To a well-stirred solution of 4',4''',4''''',4'''''''-(ethene-1,1,2,2-tetrayl)tetrakis(([1,1'-biphenyl]-4-
carbaldehyde)) (748 mg, 1 mmol) in ethanol (20 mL), phenylacetonitriles (4.5 mmol) was added.
To this mixture, sodium methoxide (540 mg, 10 mmol) was added in small portions with
constant stirring, and the reaction mixture was vigorously stirred for 24 h to complete the
reaction, which was monitored by TLC. After cooling to room temperature, the reaction mixture
was poured into ice-cold water, and the organic layer was extracted with DCM. The solvent was
removed under vacuum, and the residue was purified by silica gel column chromatography to
obtain the solid product.

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TPEDPAN1: Yield: 926 mg, 81%. FT-IR (KBr) cm-1: 3025 (C=C-H), 2211 (C≡N), 1591 (C=C);
1H NMR (600 MHz, CDCl3) δ(ppm): 7.89 (d, J = 8.25 Hz, 7H), 7.63-7.61 (m, 15H), 7.48 (s,
13
C=C-H, 4H), 7.42-7.37 (m, 17H), 7.32 (t, J = 8.25 Hz, 4H), 7.18-7.16 (m, 9H); C NMR (150
MHz, CDCl3) δ(ppm): 142.3, 141.5, 140.6, 137.0, 133.5, 131.6, 131.1, 128.8, 128.3, 128.1,
128.0, 126.2, 125.4, 124.9, 117.1, 110.3; HRMS (ESI): m/z calculated for C86H56N4 1144.45;
found 1168.43 [M+Na]+.
TPEDPAN2: Yield: 1.1 g, 76%. FT-IR (KBr) cm-1: 3025 (C=C-H), 2212 (C≡N), 1592 (C=C);
1H NMR (600 MHz, CDCl3) δ(ppm): 7.94 (d, J = 8.09 Hz, 8H), 7.70 (d, J = 8.28 Hz, 8H), 7.59-
7.53 (m, 20H), 7.48 (d, J = 8.28 Hz, 8H), 7.24 (d, J = 8.09 Hz, 8H); 13C NMR (150 MHz,
CDCl3) δ(ppm): 143.4, 142.8, 142.7, 141.9, 137.9, 133.5, 132.4, 132.2, 132.1, 129.8, 127.4,
127.2, 126.4, 123.4, 117.7, 110.1; ESI-MS 1464 [M+4]+.
3. Results and discussion
3.1. Quantum chemical calculations
To understand the geometry and electronic structure of TPEDPAN1 and TPEDPAN2,
density functional theory (DFT) calculations were performed. The optimized geometries of the
dyes show twisted conformation (Figure S1, ESI†). The spatial distribution and orbital energies
of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital
(LUMO) were also generated using the calculations, as displayed in Figure 1. The HOMO of
TPEDPAN1 was mainly distributed on the TPE unit, whereas the LUMO orbitals are distributed
over the phenyl acetonitrile moiety. The HOMO orbitals of TPEDPAN2 are located around Br,
and its LUMO orbitals are distributed over the TPE and phenyl acetonitrile units. The theoretical
band gaps for TPEDPAN1 and TPEDPAN2 are 3.033 and 2.989 eV, respectively. The lower

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band gap of TPEDPAN2 suggests a bathochromic shift in the electronic absorption spectra. The
TD-DFT calculations show good agreement with the experimental spectra, and reveal that the
main transition (λmax) in TPEDPAN1 and TPEDPAN2 is from H ‰ L (Table S1, ESI†).
3.2. Aggregation-induced emission (AIE) properties
The fluorescence spectra of TPEDPAN1 and TPEDPAN2 in THF solution are shown in
Figure 2. Complexes TPEDPAN1 and TPEDPAN2 are almost non-emissive in pure THF
solution. The poor fluorescent nature in solution is attributed to the intramolecular rotations of
the tetraphenylethene core. However, up on addition of a water fraction to this solution, a
dramatic increase in fluorescence intensity was observed due to the aggregation-induced
emission (AIE). The AIE properties of the two luminogens were investigated by measuring the
emission spectra of THF–H2O mixtures with increasing water fractions ranging from 0% to –
95%. As the water fraction increased from 0% to –80%, a gradual increase in emission intensity
was observed due to the formation of aggregates of molecules, which validates the proposed AIE
behavior for TPEDPAN1 and TPEDPAN2. In the aggregated state, the free molecular rotations
in the TPE unit decrease from restricted intramolecular rotation (RIR), resulting in an increase in
fluorescence intensity. The poor emission of TPEDPAN1 and TPEDPAN2 in THF solution was
enhanced in the aggregated suspensions (80% water fraction) by 17- and 10-fold, respectively.
When the water fraction increased above 80%, an unexpected decrease in emission intensity was
observed as more aggregates precipitated out of solution. However, despite the decrease in
emission intensity, remained emissive above 80% water fraction. The quantitative enhancement
in emission of TPEDPAN1 and TPEDPAN2 was evaluated by the fluorescence quantum yields,
using 9,10-diphenylanthracene as standard. In pure THF solution TPEDPAN1 and TPEDPAN2

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show quantum yields of 0.056 and 0.034, respectively, which increased to 0.727 and 0.643 in the
aggregated state in THF–H2O mixture (fw = 80%).
3.3. Mechanofluorochromic properties
To determine the mechanofluorochromic properties of TPEDPAN1 and TPEDPAN2, solid-
state absorption and fluorescence emission spectra were studied under different states including
pristine (as-prepared), ground, and fumed samples (Figure 3). The absorption spectra of
crystalline forms of TPEDPAN1 and TPEDPAN2 absorb at 383 nm and 400 nm, respectively.
Upon grinding TPEDPAN1 and TPEDPAN2 exhibit a bathochromic shift and absorb at 397 nm
and 404 nm, respectively (Figure S3, ESI†). The fluorescence spectra of pristine TPEDPAN1
exhibited green emission at 525 nm. Interestingly, after gentle grinding for 2 min using a mortar
and pestle, the emission wavelength red shifted and emitted at 552 nm, a 27 nm red shift of the
wavelength to orange-yellow fluorescence. In addition, the ground TPEDPAN1 could be easily
returned to the original luminescence via fuming with dichloromethane vapors for 2 min, proving
the excellent reproducibility of the mechanofluorochromic phenomenon (Figure 4). To examine
the effect Br substitution on mechanochromic property TPEDPAN2 was synthesized and
evaluated its mechanofluorochromism. The fluorescence spectra pristine TPEDPAN2 exhibited
green emission at 527 nm, which red shifted upon grinding and emitted at 556 nm with orange-
yellow emission. This exhibited a 29 nm bathochromic wavelength shift, which could be
reverted to the original fluorescence by fuming with dichloromethane vapors for 2 min. This
demonstrated that the Br substitution on core nucleus displayed negligible effect on
mechanochromism. This mechanochromic conversion could be repeated many times without
fatigue, as these stimuli do not cause any chemical change (Figure 5).

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To evaluate the origin and mechanism of the mechanochromic properties, the phase
transition characteristics of TPEDPAN1 and TPEDPAN2 were investigated by powder X-ray
diffraction (XRD) and differential scanning calorimetry (DSC). The XRD diffractograms of the
pristine samples showed intense and sharp reflection peaks, indicative of their well-ordered
crystalline natures. In contrast, after grinding, the diffraction peaks disappeared and only a
diffuse band was observed, which implied that grinding led to a crystalline to amorphous phase
conversion of solids (Figure 6). After exposing the ground samples to dichloromethane vapor,
the sharp and intense diffraction peaks reappeared. Their reappearance suggests that the
amorphous powders restored their crystallinity upon solvent fumigation. Thus, the powder XRD
results revealed that the mechanochromism phenomenon in TPEDPAN1 and TPEDPAN2 can
be ascribed to a morphological transition between crystalline and amorphous states upon the
grinding–fuming process. In addition, DSC thermograms for pristine and ground samples
showed that new exothermal recrystallization peaks at 156.9 °C and 161.6 °C were observed for
ground samples of TPEDPAN1 and TPEDPAN2, respectively (Figure 7). This result
demonstrates that at the above mentioned temperatures, thermal recrystallization begins for these
ground amorphous powders, and it can be concluded that the microcrystals of pristine samples
were destroyed and converted to the amorphous state by the grinding treatment.
To gain more insight into the mechanochromic mechanism of TPEDPAN1 and
TPEDPAN2, we attempted to generate their single-crystal packing structures. Fortunately,
single crystals of TPEDPAN1 suitable for X-ray structure analysis were obtained by
recrystallization (Figure 8). The crystal structure of TPEDPAN1 exhibits a twisted conformation
of the phenyl rings of the TPE unit. This twisted conformation supports active intramolecular
rotations. The TPEDPAN1 molecules were packed in a head-to-tail orientation in the crystal

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structure. Intermolecular van der Waals interactions were observed between the N45–C54 atoms
of the dimer separated by 3.087 Å. The elongated bond length (1.189 Å) of C≡N between N45–
C44 and the distorted bond angle of C36–C37–C38 (127.3°) with-in the molecule supported the
intermolecular van der Waals interactions. The lack of an intense intermolecular attractive force
gives rise to a loose packing motif, which allows slippage upon exposure to external mechanical
grinding and the exhibited red-shifts of the photoluminescence spectra.
By exploiting the mechanochromic response of complex TPEDPAN1, to develop a practical
real time application demonstrator for fast-responding safe information recording/erasing system,
we prepared a green-emitting film by casting THF solution of TPEDPAN1 on a filter paper
(Figure 9). Thus, we obtained the security paper with yellow color under ambient light and green
color under UV light. Then the letters ‘‘AIE’’ were written on the paper using a spatula. Faint
orange letters appeared on the green backgrounds, which can be clearly seen under UV light due
to the amorphization of compound in the written area caused by the shear stress on the writing
action. Under ambient light, the information on the paper was invisible. Erasing the letters
‘‘AIE’’ by exposing to DCM vapors for 2 min reinstalls the original green background. Thus,
various kinds of optical data patterns can be recorded by handwriting and subsequently be erased
with solvent.
4. Conclusions
In summary, we have designed and synthesized tetraphenylethene-based
diphenylacrylonitrile AIE-active luminophores (TPEDPAN1 and TPEDPAN2), which exhibited
promising highly reversible mechanochromic properties. The luminescence properties were
reversible upon grinding and fuming. The powder XRD results demonstrated that the transition

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of solid-state packing from the crystalline to the amorphous state upon the grinding–fuming
process is responsible for the mechanochromism. The results obtained here can assist in the
further design of new AIE-active and mechanochromic switching materials for various
rewritable optoelectronic applications.
Acknowledgments
This study was supported by the National Research Foundation of Korea (NRF) funded by
the Ministry of Science, ICT and Future Planning (Grant no. NRF-2014M3C1A9060739).
Appendix A. Supplementary data
† Electronic Supplementary Information (ESI) available: UV-vis absorption; DFT calculations;
Crystal data and details of TPEDPAN1 (CCDC No. 1511617), and copies NMR and Mass
spectra.

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Figure captions:
Scheme 1. Synthetic Route for TPEDPAN1 and TPEDPAN2.
Figure 1. HOMO and LUMO frontier molecular orbitals of TPEDPAN1 and TPEDPAN2
obtained at the DFT level using a B3LYP/6-31G(d,p) basis set.
Figure 2. PL spectra of TPEDPAN1 (a) and TPEDPAN2 (b) (1.0×10-5 mol L-1) in THF–H2O
mixtures with different water fractions (0–95% v/v) at room temperature. Changes in the
emission intensity of TPEDPAN1 (c) and TPEDPAN2 (d) at different water fractions (0–95%
v/v) in THF–H2O mixtures; Inset: emission images of luminogens in pure THF solution and
THF–H2O mixture (fw = 80%) under 365 nm UV illumination.
Figure 3. Molecular structures of TPEDPAN1 (a) and TPEDPAN2 (b) and mechanoresponsive
luminescence behavior of TPEDPAN1 (c) and TPEDPAN2 (d) with different processes;
photographs taken under visible light and UV illumination.
Figure 4. Photoluminescence spectra of TPEDPAN1 (a) and TPEDPAN2 (b) under different
treatments; pristine, ground and fumed samples.
Figure 5. Repeated switching of the solid-state fluorescence of TPEDPAN1 (a) and
TPEDPAN2 (b) by repeated grinding and fuming cycles.
Figure 6. XRD curves of TPEDPAN1 (a) and TPEDPAN2 (b) as a powder with different
treatments; pristine, ground and fumed samples.
Figure 7. DSC curves of Pristine and Ground samples of TPEDPAN1 (a) and TPEDPAN2 (b).

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Figure 8. (a) ORTEP drawing of the crystal structure of TPEDPAN1 (b) Molecules associate
into dimers by N45-C44 interactions (blue dotted bond). (c) Molecular packing pattern of
TPEDPAN1.
Figure 9. Photographic images of fluorescence re-writable data recording films on pieces of
paper in response to mechanical writing and erasing with DCM vapor fuming under UV light
(365 nm).

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Scheme 1.

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Figure 1.

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Figure 2.

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Figure 3.

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Figure 4.

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Figure 5.

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Figure 6.

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Figure 7.

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Figure 8.

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Figure 9.