Near-room-temperature phase-change fluorescent molecular rotor and its hybrids

Article abstract of DOI:10.1016/j.molliq.2018.05.128

A unique molecular rotor consisting of a tetraphenylethylene derivative with a paraffin wax-like long alkyl chain (TPE18) is synthesized and compared to other derivatives containing shorter alkyl chains. In differential scanning calorimetry, TPE18 shows a

Full text of DOI:10.1016/j.molliq.2018.05.128

Journal of Molecular Liquids 265 (2018) 260–268
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Near-room-temperature phase-change fluorescent molecular rotor and
its hybrids
⁎
Young-Jae Jin, Young-Ghil Choi, Hyosang Park, Giseop Kwak
Department of Polymer Science & Engineering, Polymeric Nanomaterials Laboratory, School of Applied Chemical Engineering, Kyungpook National University, 1370 Sankyuk-dong, Buk-ku, Daegu
702–701, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
A unique molecular rotor consisting of a tetraphenylethylene derivative with a paraffin wax-like long alkyl chain
(TPE18) is synthesized and compared to other derivatives containing shorter alkyl chains. In differential scanning
calorimetry, TPE18 shows an endothermic peak (enthalpy of fusion, ΔHfus = 55.3 J g−1) at 45 °C during the
heating process and an exothermic peak at 38.6 °C during the cooling process. TPE18 exhibits critical changes
in the fluorescence intensity and color during the crystal–melt phase change. TPE18 can be dissolved by an
octadecane melt and then absorbed by filter paper. When the waxed paper is exposed to the heat sources of
an IR laser or a finger, high-resolution fluorescence images of the characters and fingerprints appear on the
paper. TPE18 also serves as a host matrix in a melt state that was easily mixed with the guest fluorophores of
Alq3 and rubrene. Homogeneous hybrid solids with defined features are obtained without any significant segre-
gation after cooling to room temperature. Fluorescence images with various colors are drawn onto the surfaces of
glass slides or papers using the hybrids as crayons.
Received 13 March 2018
Accepted 29 May 2018
Available online 30 May 2018
Keywords:
Phase change
Molecular rotors
Fluorophores
Hybrids
Fluorescence imaging
© 2018 Elsevier B.V. All rights reserved.
1. Introduction
thermo- or hydrodynamically induced fluorescence attenuation should
be pronounced in the melt (or solution). These static and dynamic fluo-
Organic fluorophores have recently become ubiquitous as high-tech
materials for applications such as optoelectronic device materials, (bio)
sensors, and image probes [1–9]. These functional active materials will
be more intelligent if the fluorescence emission can be varied in re-
sponse to external stimuli, such as heat, light, electric fields, and
chemicals [10–18]. A thorough understanding of the static and dynamic
fluorescence emission behaviors is essential to design such materials
[19]. Since most fluorophores are π-conjugated systems with planar ge-
ometry, an intermolecular (in solid phase) or intramolecular (in liquid
phase) stack structure has the most significant influence on static fluo-
rescence quenching. As the degree of stacking between the fluorophore
molecules increases, the probability of the occurrence of an excimer that
exhibits remarkable fluorescence attenuation will increase. Conversely,
if the stack structure is relaxed in an ambient environment, the excimer
will be removed, and significant fluorescence enhancement will take
place. In this case, fluorescence behavior associated with molecular dy-
namics should be carefully considered. Molecular fluctuation according
to the internal and external environment is a primary factor that influ-
ences the dynamic fluorescence quenching. As vibrational relaxation
and collisional quenching in the process of heating (or solvation) in-
creases, non-radiative fluorescence decay becomes dominant. The
rescence quenching processes always compete with each other [20].
Using this knowledge, we aim to develop highly advanced functional
materials that exhibit unique thermodynamic fluorescence emission
behavior. The target materials were designed to be more emissive in
the crystal state than in the melt state and to undergo reversible
crystal-to-melt phase changes near room temperature. Nakanishi et al.
recently developed highly emissive liquid fluorophores by covalently
connecting branched long alkyl chains to typical crystalline π-
conjugated core systems [21–23]. In a bulk state, the intermolecular π-
π stacking of the rigid core with a planar geometry was completely
avoided due to randomly arranged alkyl chains. The original crystallin-
ity was disrupted and led to highly emissive liquid phase at room tem-
perature. The bulk liquid served as fluorescent matrices for other
components in composite systems to result in white-light emission at
appropriate composition ratios. Although Nakanishi's finding was
promising in the development of softened bulk fluorophores with
high fluorescence quantum efficiency, the practical application was lim-
ited because the materials exist in a permanent liquid state around
room temperature [24]. Therefore, development of liquid fluorophores
that can be readily converted into solid phase near room temperature
with significant fluorescence enhancement is greatly desired.
Organic compounds that exhibit stronger fluorescence emission in
the solid state than in the solution state have been extensively devel-
oped by Tang's research group with a well-established mechanism
⁎
Corresponding author.
E-mail address: gkwak@knu.ac.kr (G. Kwak).
https://doi.org/10.1016/j.molliq.2018.05.128
0167-7322/© 2018 Elsevier B.V. All rights reserved.

Y.-J. Jin et al. / Journal of Molecular Liquids 265 (2018) 260–268
261
aromaticity, the thermodynamic phase transition temperatures are
often N100 °C. This property may limit the processability and practical
uses. Therefore, by using simple molecular design to lower the phase
transition temperature near room temperature, a material with a re-
versible crystal–melt phase change and dramatic fluorescence change
can be developed for advanced functions and extensive applications.
We synthesized a novel molecular rotor containing a paraffin wax-
like long alkyl chain via a simple substitution reaction. We found that
this material underwent a critical phase change near room temperature
and showed remarkable simultaneous changes in fluorescence intensity
and color. This material was also readily hybridized with other
fluorophores in the melt state that became highly emissive solids after
cooling to room temperature. Fluorescence images with various colors,
including visible wavelengths of blue, green, orange, and even white,
could be drawn on the surfaces of glass slides or paper by using the
solid hybrids as crayons. The material was also readily dissolved in par-
affin wax melts and absorbed into filter papers. When the fluorescent
waxed paper was brought into contact with heat sources, high-
resolution fluorescence images appeared on the paper.
Fig. 1. Chemical structures of TPE derivatives.
[25–28]. These compounds, often called molecular rotors, exhibit very
weak fluorescence in solution, but the emission is remarkably enhanced
by an aggregation process induced by the non-solvent. Some molecular
rotors react with carbon dioxide and biomolecules in solution to show
significant fluorescence enhancement due to the formation of nano-
sized aggregate complexes [29–34]. This phenomenon is termed
aggregation-induced emission enhancement (AIEE) and is intrinsically
based on the restriction of intramolecular rotation (RIR) [35–37]. In-
creasing the ambient medium (solvent) viscosity or substantially low-
ering the ambient temperature may also be effective strategies to
intensify the RIR activity [38]. Similar to the case of solutions, the degree
of RIR will significantly change during the crystal–melt phase change in
the bulk state with an associated change in fluorescence emission. How-
ever, since molecular rotors are highly crystalline compounds with full
2. Results and discussion
Tetraphenylethylene (TPE) derivatives with different-length alkyl
side chains were synthesized by a substitution reaction between four-
armed lithiated TPE and the corresponding alkyldimethylchlorosilanes
(TPE1, TPE8, and TPE18 in Fig. 1 and Scheme S1). The thermodynamic
properties of the TPE derivatives were examined using differential scan-
ning calorimetry (DSC). Fig. 2 shows the DSC curves of the TPE deriva-
tives and the results are summarized in Table 1. TPE18 did not show
b)
a)
Exo ↑
Exo ↑
cooling
cooling
heating
heating
Endo ↓
Exo ↑
Endo ↓
Exo ↑
-40 -20
0
20 40 60 80
36 38 40 42 44 46 48
Temperature / rC
Temperature / rC
c)
d)
heating
heating
Endo ↓
100
Endo ↓
120
150
180
210
240
150
200
250
Temperature / rC
Temperature / rC
Fig. 2. DSC thermograms of a) TPE18 (heating rate: 0.1 °C min−1 in N2 atmosphere), b) TPE8, c) TPE1 and d) TPE (heating rate: 5 °C min−1).

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Y.-J. Jin et al. / Journal of Molecular Liquids 265 (2018) 260–268
Table 1
Thermodynamic, photophysical properties, and solubility of TPE derivatives.
TPEs
Thermodynamic propertiesa
UV–Vis absorption and FL emission properties
Solubility propertyb
Tm [°C]
Tc [°C]
ΔHfus [J g−1
]
In solutiond
In bulk
Solubility in hexadecane [mg/mL]
λ
max,abs [nm]
λmax,FLe (nm)
FLQYe [%]
λmax,FLe [nm]
FLQYe [%]
TPE
225.2
234.1
N.D.
45
N.D.c
N.D.c
N.D.
127.4
4.8
N.D.
55.3
309
322
323
322
468
476
476
478
0.02
0.03
0.04
0.05
441
442
26.9f
b0.5
3
N800
N800
TPE1
TPE8
TPE18
33.3f
478
16.9g
38.6
447h (487)i
29.8h (13.7)i
a
Determined by DSC.
b
c
d
e
f
Judged by adding 1.0 mg TPEs into 10 mL hexadecane at 25 °C with stirring overnight.
Not detected due to sublimation during the heating cycle.
Determined in THF solution with 1 × 10−4 M at 25 °C.
Excited at 350 nm.
Determined in a dispersed state in PMMA matrix (1 × 10−2 M).
In bulk liquid.
In bulk solid at 25 °C.
g
h
i
In bulk liquid at 40 °C relative to reference materials.
any phase transition peak near the melting temperature (Tm) of 225 °C
that was ascribed to the TPE core. A new endothermic peak (enthalpy of
fusion, ΔHfus = 55.3 J g−1) was observed at 45 °C during the heating pro-
cess, and an exothermic peak was observed at the crystallization temper-
ature (Tc) of 38.6 °C during the cooling process. These new phase
transition temperatures were close to room temperature. Eventually,
the crystalline structure of the aromatic core collapsed, and the paraffin-
like long alkyl groups contributed crystallinity that drastically lowered
the phase-change temperatures. TPE8, with a shorter alkyl chain, did
not show any thermodynamic transition in a wide temperature range
from −50 °C to 100 °C. TPE8 lost additional crystallinity due to the ab-
sence of crystallinity in the side alkyl chains of octyl groups once it lost
crystallinity ascribed to the aromatic TPE core structure. TPE1, with the
shortest alkyl chain, exhibited a phase transition near the Tm of TPE, al-
though the ΔHfus was very small at 4.8 J g−1 compared to that of TPE
(127.4 J g−1) [39]. TPE1 molecules were not arranged randomly enough
to disrupt the crystalline structure of the aromatic core completely.
The morphologies were confirmed by X-ray diffraction (XRD) analy-
sis at room temperature in comparison with TPE (Fig. 3). TPE18 and
TPE1 show relatively weak but sharp diffraction peaks with patterns
that were different from that of TPE. Only the broad amorphous halo
peak was observed in TPE8. These morphologies agreed well with the
DSC results. TPE18 appeared as a plate-like paraffin wax at room tem-
perature, TPE8 existed in liquid form, and TPE1 appeared in a needle-
shape powdered clump. TPE was well formed as hemihedral crystals
with a cluster that was 50–100 μm in diameter (Fig. 4). The solubility
also reflected the effect of alkyl chain length in paraffin waxes such as
hexadecane and octadecane (Table 1). TPE18 and TPE8 were readily dis-
solved in hexadecane at room temperature and with greatly enhanced
solubility (N800 mg mL−1). TPE1 did not dissolve readily in hexadecane
(~3 mg mL−1), although its solubility was slightly enhanced relative to
TPE (b0.5 mg mL−1). The long alkyl groups of TPE18 and TPE8 improved
the miscibility in paraffin waxes by reducing aromaticity. As will be de-
scribed later in detail, the hybridization of TPE derivatives with paraffin
waxes advanced their functions and applications. The hybrid materials
demonstrated more dramatic fluorescence changes and higher-
resolution fluorescence images than TPE derivatives when exposed to
heat sources.
approximately 476 nm. As the length of the alkyl chain increased, the
shoulder band at a longer wavelength of 530 nm also increased. The tor-
sional angle of four side phenyl rings relative to the plane of ethylene moi-
ety likely decreased as the hydrodynamic volume of the alkyl groups
increased, which enhanced the coplanarity and conjugation length [41].
In the bulk state at room temperature, TPE18 and TPE1 existed in crystals
as previously described and exhibited maximum fluorescence emission at
shorter wavelengths (λmax, FL ~ 447 and 442 nm, respectively) relative to
the solutions, which is similar to the case of TPE. TPE8 existed in the liquid
phase at room temperature and showed a nearly identical λmax, FL to the
solution. The fluorescence quantum yields (FLQY) of the two crystal TPE
derivatives were 29.8 and 33.3%, respectively, which was larger than
that of the liquid TPE8 (16.9%) (Table 1). When the crystal TPE18 was
melted by heating at 40 °C, the fluorescence band shifted to a longer
wavelength (λmax, FL ~ 487 nm) while the fluorescence intensity de-
creased (FLQY ~ 13.7%) and was similar to that of the liquid TPE8. The
shorter fluorescence wavelength and larger FLQY of the crystal phase rel-
ative to liquid phase were due to the larger torsional angle and RIR activity
in crystal phase.
TPE18 exhibited unique thermodynamic fluorescence emission be-
havior in the bulk state during the cooling and heating process. The
22ƒ, 4 Å
TPE18
TPE8
5.5ƒ, 16 Å
TPE1
TPE
The ultraviolet-visible (UV–Vis) absorption and fluorescence emission
spectra of the TPE derivatives were measured and compared (Fig. 5 and
Table 1). The absorption bands of the TPE derivatives in solution were
slightly shifted to a longer wavelength by approximately 10 nm relative
to TPE and showed a nearly identical maximum absorption wavelength
(λmax, abs) at approximately 322 nm. This red-shift was probably due to
σ-π conjugation based on the silylene linkage between the aromatic
core and alkyl chains [40]. The fluorescence emission bands were also
shifted to a longer wavelength by about 10 nm relative to TPE and exhib-
ited a nearly identical maximum fluorescence wavelength (λmax, FL) at
5
10 15 20 25 30 35 40
2theta / degree
Fig. 3. XRD patterns of TPE derivatives at 25 °C.

Y.-J. Jin et al. / Journal of Molecular Liquids 265 (2018) 260–268
263
TPE
TPE1
TPE8
TPE18
100µm
200µm
FL microscope images
bulk liquid
Fig. 4. Photographs of TPE derivatives in bulk state at room temperature (excited at N365 nm).
temperature-variable fluorescence emission spectra are shown in Fig. 6a
and b. During cooling over the temperature range from 60 to 0 °C, the
fluorescence intensity abruptly increased within the narrow temperature
range of 40–35 °C due to the exothermic volume contraction, and the
fluorescence band simultaneously shifted to a shorter wavelength
(green in melt → blue in crystal) by N50 nm. By contrast, significant fluo-
rescence attenuation was observed as the fluorescence color was reverted
within the narrow temperature range of 45–50 °C due to endothermic
volume expansion during the heating process. This critical fluorescence
change was directly visualized and was reversed by repeated temperature
cycling between 0 (crystal) and 60 (melt) °C as shown in Fig. 6c. The tem-
perature range over which the critical fluorescence changes occurred was
consistent with the phase transition temperature obtained by DSC analy-
sis (Fig. 6d). TPE and other TPE derivatives with shorter alkyl chains were
also tested for the critical fluorescence change over the same temperature
range and exhibited a monotonous change in fluorescence intensity with-
out any band shift upon cooling or heating due to the lack of a phase tran-
sition in the same temperature range (Figs. 6e and S1).
those of TPE18 during the phase change of the paraffin wax (Fig. 7).
When octadecane was used as the hybrid paraffin wax, the fluorescence
emission was enhanced, and the fluorescence color changed from green
to blue near 25 °C during the cooling process. Significant fluorescence
attenuation was observed with the green color recovered near 30 °C
during heating process (Figs. 7a and S2a). The temperatures at which
the critical fluorescence changes occurred were consistent with the Tc
(24.4 °C) and Tm (28.0 °C) [42] of octadecane. The fluorescence change
of the hybrid was more dramatic than that of TPE18 due to the increased
crystallinity of octadecane (ΔHfus = 192.4 J g−1) [42] relative to that of
TPE18 (Fig. 7c). Similarly, when hexadecane (Tm = 17.9 °C, Tc = 15.2 °C,
ΔHfus = 184.8 J g−1) [42] was used as a paraffin wax instead of
octadecane, the TPE18-in-hexadecane (1.0 wt%) hybrid also showed a
significant fluorescence change at a relatively lower temperature
(Figs. 7b, d, and S2b). TPE8 readily dissolved in the paraffin wax melts
to give hybrid materials with good features, but the TPE8-in-paraffin
wax hybrids did not show a critical fluorescence change in the same
temperature range during the cooling or heating process (Figs. 8 and
S3). The absence of crystallinity in TPE8 resulted in the monotonous
fluorescence change as expected from the DSC results and the
temperature-variable fluorescence emission spectra of the bulk TPE8.
As described previously, TPE18 readily dissolved in paraffin wax
melts to give well-featured hybrids. The TPE18-in-paraffin wax
(1.0 wt%) hybrids also showed critical fluorescence changes similar to
a)
b)
TPE
TPE (crystal)
TPE1
TPE1 (crystal)
TPE8 (liquid)
Abs
TPE8
TPE18
TPE18 (crystal)
FL
300 350 400 450 500 550 600 650
400
450
500
550
600
650
Wavelength / nm
Wavelength / nm
Fig. 5. UV–Vis absorption and fluorescence emission spectra of TPE derivatives a) in THF solution (1.0 × 10−4 M), b) in bulk state (excited at 350 nm).

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Y.-J. Jin et al. / Journal of Molecular Liquids 265 (2018) 260–268
The TPE18-in-octadecane hybrid melt was absorbed by porous filter
cooled through the entire surface, the fluorescence images were readily
erased. On the contrary, when the fluorescent waxed paper at 50 °C was
brought into contact with a cooled metal rod, a high-resolution turn-on
fluorescence image of the characters could be drawn as desired (Fig. 9a).
When the waxed paper was subsequently heated through the entire
surface, the fluorescence images were readily erased. When the fluores-
cent waxed paper was brought into contact with a finger at body
paper, and the waxed paper was nearly permanently resistant to re-
peated heating and cooling without significant leakage (Figs. 9a and
S4). When the fluorescent waxed paper at 25 °C was brought into con-
tact with a warmed metal rod or exposed to an IR laser, a high-
resolution turn-off fluorescence image of the characters could be
drawn as desired (Fig. 9a). Subsequently, when the waxed paper was
Fig. 6. Fluorescence emission spectra of TPE18 with a) cooling and b) heating at a rate of 1 °C min−1 over the temperature range of 0–60 °C (excitation at 350 nm, spectra are shown every 5
°C). c) Variation in fluorescence intensity and λmax, FL of TPE18 during the heating and cooling cycle. d) Comparison between changes in fluorescence intensity and heat flow of TPE18
during the heating and cooling process (red line: heating process, blue line: cooling process). e) Variation in fluorescence intensity ratio (I/I0, where I0 is fluorescence intensity at 0 °C)
and λmax, FL of TPE, TPE1, and TPE8 over the temperature range of 0–60 °C. (Excitation at 350 nm, monitored at fluorescence maximum wavelength). (For interpretation of the
references to color in this figure legend, the reader is referred to the web version of this article.)

Y.-J. Jin et al. / Journal of Molecular Liquids 265 (2018) 260–268
265
temperature, a high-resolution fluorescence image of the fingerprint
appeared on the paper under a common hand-held UV lamp (Fig. 9b).
It is because normal body temperature is 36.5 °C and octadecane
melts at this temperature. Octadecane melted only at the portions in
contact with the ridges of the fingerprint, so a turn-off fluorescence
image pattern with fine features was generated. This high-resolution
fingerprint image suggested the potential use of the fluorescent
waxed paper for biometric identification in the fields of security and fo-
rensic science [43].
TPE18 melt also served as a host matrix that was easily hybridized
with other fluorophores (Fig. 10a). Alq3 and rubrene guest dopants
were used as green- and red-light emitting sources, respectively.
These guest materials did not appear fully dissolved in the TPE18 melt
at 50 °C due to the high aromaticity and crystallinity. However, they
were well-dispersed in the melt to produce homogeneous hybrids
with good features and no significant segregation when cooled to
room temperature. This three-component hybrid solid emitted very
strong white light at a mole ratio of TPE18:Alq3:rubrene = 1:0.6:0.15.
The fluorescence emission spectrum was very broad over the full visible
region (Fig. 10b). The hybrid melt was able to be readily applied to a
glass surface. When the hybrid melt was applied to the surface of a com-
mercial UV-LED chip, white LED was easily obtained, and the hybrid did
not flow down at room temperature (Fig. 10c). Similar to white light
emission, other fluorescence colors such as green (TPE18:Alq3:rubrene
= 1:2:0.1) and orange (TPE18:Alq3:rubrene = 1:0.2:2) were obtained
by adjusting the amounts of guest dopants in the hybrids. Crayons were
prepared by molding the TPE18 melt or the hybrid melts in micropi-
pette tips. Fluorescence images with various fluorescence colors were
drawn onto the surfaces of glass slides or paper using the crayons
(Figs. 10d, S5). The blue-light fluorescence image, which was drawn
with TPE18 crayon, was readily changed to a green-light fluorescence
image by heating to 50 °C and then reverted to the original image by
cooling to room temperature (Fig. 10e). Similarly, other fluorescence
images obtained from the hybrids showed fluorescence color changes
during the heating and cooling cycle.
3. Conclusions
We developed a unique molecular rotor, TPE18, with a paraffin wax-
like, long alkyl chain and compared it to other derivatives containing
shorter alkyl chains. TPE18 lost crystallinity of the aromatic core entirely
but revealed new crystallinity due to the paraffin wax-like alkyl chains,
unlike the other derivatives. As a result, the phase change temperature
was significantly lowered near room temperature. TPE18 and TPE18-
in-paraffin wax hybrids showed a significant fluorescence response to
temperature in the bulk state. The hybrid-absorbed fluorescent wax
Fig. 7. Fluorescence emission spectra of a) 1 wt% TPE18-in-octadecane and b) TPE18-in-hexadecane hybrids during cooling from 50 to 0 °C (excitation at 350 nm, spectra are shown every 5
°C). Inset: fluorescence photographs at 25 °C and 50 °C (excitation at 365 nm). Comparison between changes in fluorescence intensity and heat flow of c) the TPE18-in-octadecane and
d) the TPE18-in-hexadecane during the heating and cooling process (excitation at 350 nm, monitored at fluorescence maximum wavelength; red line: heating process, blue line: cooling
process). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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Y.-J. Jin et al. / Journal of Molecular Liquids 265 (2018) 260–268
Fig. 8. Fluorescence emission spectra of a) 1 wt% TPE8-in-octadecane and b) TPE8-in-hexadecane during cooling from 50 to 0 °C (excitation at 350 nm, spectra are shown every 5 °C). Inset:
fluorescence photographs at 25 °C and 50 °C (excitation at 365 nm).
paper experienced a critical fluorescence change during the crystal—
melt phase change near room temperature. When in contact with a
human finger or exposed to an IR laser, high-resolution fluorescence
images of fingerprints or characters appeared on the surfaces. In the
melt state, TPE18 served as a host matrix for facile hybridization with
guest fluorophores and highly dispersed homogeneous hybrids were
obtained without any segregation after reversion to the crystal state.
The desired fluorescence color was tuned by adjusting the molar ratio
of the components of the hybrid in this simple hybridization method.
Fluorescence images with various colors were drawn on the surfaces
of glass slides or paper using the hybrids as crayons. The fluorescence
colors significantly changed during the repeated heating and cooling
cycle. Our findings characterized the phase changes and thermody-
namic fluorescence emission behaviors of molecular rotor and its hy-
brids toward the design of novel near-room-temperature phase-
change fluorophores for advanced applications.
Butyllithium (3 mL, 5.0 mmol) was added dropwise to the solution, and
the reaction mixture was stirred for 1 h. Chlorotrimethylsilane (0.54 g,
5.0 mmol) was added, and the mixture solution was stirred for 2 h.
After filtration of salt and removal of the solvent by evaporation, the
a)
4. Experimental section
4.1. Materials
TPE, Alq3, rubrene and paraffin waxes (hexadecane and octadecane,
purities N98.0%) were purchased from Tokyo Chemical Industry (TCI).
Other chemical reagents were purchased from Aldrich, TCI and used
without further purification. Filter paper was purchased from
ADVANTEC Co., Ltd., Taiwan.
b)
4.2. Synthesis
4.2.1. Tetrakis(4-bromophenyl)ethene (1, scheme S1)
Compound 1 was synthesized by modifying the previously reported
procedures [44]. TPE (6.0 g, 18.0 mmol) was treated with bromine
(7.5 mL, 0.15 mol) in a flask for 24 h at room temperature. The resulting
solid was dissolved in hot toluene (120 mL), and the solution was iso-
lated in hexane. The resulting crude product was purified by flash
silica-gel column chromatography (eluent: hexane) to yield 1 (5.27 g,
50%) as white solid. 1H NMR (500 MHz, CDCl3, δ): 7.26 (d, J = 8.6 Hz,
8H), 6.84 (d, J = 8.6 Hz, 8H).
4.2.2. TPE1
Fig. 9. a) Fluorescence photographs of the 1 wt% TPE18-in-octadecane waxed paper at 25
°C and 50 °C, respectively, and the fluorescence images of the characters written on the
paper by using a warmed or cooled metal rod, and an IR laser. b) Fluorescence
fingerprint image left on the paper after contact with the finger.
A three-necked flask was equipped with a dropping funnel and mag-
netic stirring bar and then flushed with dry nitrogen. 1 (0.97 g, 1.0 mmol)
and THF (15 mL) were placed into the flask at 0 °C (i.e., ice bath). n-

Y.-J. Jin et al. / Journal of Molecular Liquids 265 (2018) 260–268
267
crude residue was extracted with diethyl ether. The ether solution was
4.2.4. TPE18
washed with 2 N HCl aq., sat. NaHCO3 aq., and brine, then dried over an-
hydrous MgSO4. The solvent was evaporated, and the obtained crude
product was purified by flash silica-gel column chromatography (eluent:
hexane/CH2Cl2, 10/1 v/v) to give a white solid. 1H NMR (500 MHz, CDCl3,
δ): 7.22 (d, 8H), 6.98 (d, 8H), 0.20 (s, 36H).
Chlorodimethyl(octadecyl)silane (1.73 g, 5.0 mmol) was used as re-
agent instead of chlorotrimethylsilane. The reaction and purification
were conducted by following the same procedure as that for the synthe-
sis of TPE1. The final product was obtained as a white solid. 1H NMR
(500 MHz, CDCl3, δ): 7.19 (d, 8H), 6.97 (d, 8H), 1.26–0.66 (m, 148H),
0.18 (s, 24H).
4.2.3. TPE8
Chlorodimethyl(octyl)silane (1.03 g, 5.0 mmol) was used as reagent
instead of chlorotrimethylsilane. The reaction and purification were
conducted by following the same procedure as that for the synthesis
of TPE1. The final product was obtained as a slightly colorless liquid.
1H NMR (500 MHz, CDCl3, δ): 7.19 (d, 8H), 6.96 (d, 8H), 1.25–0.66 (m,
68H), 0.18 (s, 24H).
4.3. Instruments
DSC (TA Instruments Q2000) was performed in pure nitrogen gas at
heating and cooling rates of 10 °C min−1. XRD measurements were per-
formed at room temperature using an X-ray diffractometer (PANalytical
X'Pert PRO-MPD) at the Korea Basic Science Institute (Daegu). The
Fig. 10. a) Preparation and b) fluorescence emission spectrum (excitation at 365 nm) of white-light-emitting three-component hybrid (TPE18:Alq3:rubrene = 1:0.6:0.15 mol). c) The
white-light emitting hybrid coated on a glass slide and a commercial UV-LED chip (excitation at 365 nm). d) Green-, orange-, and white-light emitting crayons obtained from the
molding of hybrids with different composition ratios and the fluorescence images of the characters written on a glass slide using the crayons. e) Blue-light emitting crayon obtained
from the molding of TPE18, the fluorescence image of the characters written on a glass slide using the crayon and its color change according to temperature. (For interpretation of the
references to color in this figure legend, the reader is referred to the web version of this article.)

268
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Acknowledgements
This work was supported by the Basic Science Research Program
of National Research Foundation of Korea (NRF) grants funded
by the Korea government (MEST) (2017R1A2B4007348 and
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Appendix A. Supplementary data
Additional data, including synthetic scheme, FL emission spectra and
photographs (Scheme S1, Figs. S1–S5). Supplementary data related to
this article can be found at doi:https://doi.org/10.1016/j.molliq.2018.
05.128.
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