9,10-Bis((Z)-2-phenyl-2-(pyridin-2-yl)vinyl)anthracene: Aggregation-induced emission, mechanochromic luminescence, and reversible volatile acids-amines switching

Article abstract of DOI:10.1016/j.dyepig.2017.10.012

Pyridine-containing luminophores have been attracting much attention due to pyridine functions in carrier transport and cation complexation. Here we prepared a butterfly-like molecule 9,10-bis(2-phenyl-2-(pyridin-2-yl)vinyl)anthracene (PyVA) for developing new multifunctional dyes. The multiple rotatable aryl groups enabled PyVA to exhibit a strong aggregation-induced emission effect, and the seriously twisted butterfly-like backbone rendered the solid weak intermolecular π-π interactions and mechanofluorochromism. Quantum chemical calculations indicated that protonation of the pyridyl units could narrow the band gap of PyVA, and X-ray crystallography revealed that PyVA molecules packed in a Z configuration in which the pyridyl units adopted a more twisted conformation with lone N-electron pairs stabilized by intra- and intermolecular CH···N and CH···π interactions. It was observed that both fluorescent crystalline and amorphous PyVA solids could sense volatile acids with a remarkably red-shifted emission, but the amorphous state was more sensitive. Moreover, the acidichromic PyVA solid could further sense volatile amines, and this alternative pH sensitive chromic process was reversible and applicable to fluorescence patterning.

Full text of DOI:10.1016/j.dyepig.2017.10.012

Accepted Manuscript
9,10-Bis((Z)-2-phenyl-2-(pyridin-2-yl)vinyl)anthracene: Aggregation-induced emission,
mechanochromic luminescence, and reversible volatile acids-amines switching
Qikun Sun, Honghong Wang, Xin Xu, Yuansheng Lu, Shanfeng Xue, Haichang
Zhang, Wenjun Yang
PII:
S0143-7208(17)31690-X
10.1016/j.dyepig.2017.10.012
DYPI 6310
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 6 August 2017
Revised Date: 9 October 2017
Accepted Date: 9 October 2017
Please cite this article as: Sun Q, Wang H, Xu X, Lu Y, Xue S, Zhang H, Yang W, 9,10-Bis((Z)-2-
phenyl-2-(pyridin-2-yl)vinyl)anthracene: Aggregation-induced emission, mechanochromic
luminescence, and reversible volatile acids-amines switching, Dyes and Pigments (2017), doi: 10.1016/
j.dyepig.2017.10.012.
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ACCEPTED MANUSCRIPT
Graphical abstract

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9,10-Bis((Z)-2-phenyl-2-(pyridin-2-yl)vinyl)anthracene: Aggregation-induced
Emission, Mechanochromic Luminescence, and Reversible Volatile
Acids-Amines Switching
Qikun Sun, Honghong Wang, Xin Xu, Yuansheng Lu, Shanfeng Xue, Haichang
Zhang,* and Wenjun Yang*
Key Laboratory of Rubber-Plastics of Ministry of Education/Shandong Province
(QUST), School of Polymer Science & Engineering, Qingdao University of Science &
Technology, 53-Zhengzhou Road, Qingdao 266042, China. E-mail address:
ywjph2004@qust.edu.cn.
Abstract:
Pyridine-containing luminophores have been attracting much attention due to
pyridine functions in carrier transport and cation complexation. Here we prepared a
butterfly-like molecule 9,10-bis(2-phenyl-2-(pyridin-2-yl)vinyl)anthracene (PyVA)
for developing new multifunctional dyes. The multiple rotatable aryl groups enabled
PyVA to exhibit a strong aggregation-induced emission effect, and the seriously
twisted butterfly-like backbone rendered the solid weak intermolecular π-π
interactions and mechanofluorochromism. Quantum chemical calculations indicated
that protonation of the pyridyl units could narrow the band gap of PyVA, and X-ray
crystallography revealed that PyVA molecules packed in a Z configuration in which
the pyridyl units adopted a more twisted conformation with lone N-electron pairs
stabilized by intra- and intermolecular CH···N and CH···π interactions. It was
observed that both fluorescent crystalline and amorphous PyVA solids could sense
volatile acids with a remarkably red-shifted emission, but the amorphous state was
more sensitive. Moreover, the acidichromic PyVA solid could further sense volatile
amines, and this alternative pH sensitive chromic process was reversible and
applicable to fluorescence patterning.
Keywords:
anthrylvinyl
derivatives;
aggregation-induced
emission;
mechanofluorochromic; amorphous state; acidochromism; volatile amine-induced
recovery.

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1. Introduction
Stimuli responsive fluorescence solid materials whose fluorescence emissions
could be switched between “On” and “Off” states or shifted among different emission
colors upon external stimuli such as light, electricity, heat, mechanical force and
chemical vapor have been regarded as a class of “smart” materials[1-3]. Recently,
increasing researches have paid attention to this field due to their potential
applications in sensors[4], chemical detection[5], memory storage[6], display devices[7],
security ink[8], and optical recording[9]. Among them, mechanofluorochromic (MFC)
and vapofluorochromic (VFC) materials have become of special interest because of
the convenient realization and the reversible use[1–9]. As for organic solid fluorescence
sensors, two major aspects should first be considered: the strong solid fluorescence
and the workable active unit. However, most fluorescent organic materials showed an
aggregation-caused quenching (ACQ) effect due to strong π-π stacking interactions, a
typical problem for common organic chromophores. Thanks to the discovery by Tang
et al., of some conjugated organic molecules with a twisted skeleton bearing rotatable
peripheral aryls have been demonstrated to exhibit aggregation-induced emission
(AIE) or crystallization-enhanced emission (CEE)[10,11]. The emergence of these new
π-conjugated organic materials have drawn serious research attention and greatly
widened the scope of molecular candidates applicable in optics and optoelectronics.
Since Prasad et al. reported the AIE phenomenon and the two-photon fluorescence
imaging and photodynamic therapy applications of 9,10-bis[4′-(4″-aminostyryl)-
styryl]anthracene[12], 9,10-bis(arylvinyl)anthracene (9,10-DVA) derivatives have
become a class of promising and attractive AIE/CEE molecules. In recent years, Tian,
Chi, and Yang, et al. have designed and synthesized a lot of 9,10-DVA derivatives and
demonstrated their unique aggregation behavior and stimuli-responsive properties
[13-15,1d]. These studies revealed that subtle manipulation of peripheral aryl and alkyl
groups could endow 9,10-DVAs with novel optical properties, such as self-assembly
behavior, amplified spontaneous emission, mechanochromism, vapochromism, and
thermochromism. Moreover, when some functional moieties such as active H- and
N-containing heterocyclic groups were introduced into anthracene-based molecules,

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more supramolecular interactions and crystal polymorphs were formed, which could
render materials with diverse optical properties to serve as functional organic solids
and to study the influence of aggregate structure on photo-physical properties [1d,15e,16]
.
Recently, pyridine-containing luminophores have attracted much attention due to
pyridine functions in carrier transport and cation complexation[17]. However, reports
on multi-stimuli responsive pyridine-containing anthracene-based fluorescent
materials are still rare, and it is very interesting to combine MFC and VFC properties
to develop multifunctional pyridine-containing dyes. In this context, if pyridyl groups
were used as peripheral functional units, three different positional (ortho, meta, and
para) isomers could be chosen. Further, when phenyl-pyridyl-methanone was reacted
with 9,10-bis(diethylphosphorylmethyl)anthracene, the obtained 9,10-bis(2-phenyl-2-
(pyridin-2-yl)vinyl)anthracene could form either Z- or E-configuration. These
complicate factors would all affect molecular aggregation behavior and optical
properties. Additionally, almost MFC-active 9,10-DVAs were characterized by
mechano-induced amorphization, and the stimuli responses of crystalline and
amorphous states to chemical vapors could be different but lacked investigation.
Herein we employed the commercially available phenyl-(pyridin-2-yl)methanone to
react with 9,10-bis- (diethylphosphorylmethyl)anthracene to readily afford a new
9,10-divinyl- anthracene derivative (PyAV, Scheme 1). It was found that PyVA
adopted a twisted butterfly-like Z-configuration with the fairly weak intermolecular
π-π interactions and the exposed sp2 lone N-electron pairs. We now report that the
amorphous state of the typical AIE- and MFC- active PyVA solid exhibits quicker and
remarkable acidochromism and volatile amine-induced recovery Moreover, this
alternate and reversible process is applicable to fluorescence patterning.
2. Experimental section
2.1 Materials
9,10-Bis(diethoxylphosphorylmethyl)anthracene was described in a previous
work[15f]. Tetrahydrofuran (THF) was distilled over sodium before use.
Potassium
t-butoxide,
triethyl
phosphite,
phosphoryl
trichloride,
paraformaldehyde, phenyl(pyridin-2-yl)methanone and others were all

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commercial available analytical-grade chemicals and used without further
purification.
2.2 Measurements
1H (500 MHz) and 13C NMR (125 MHz) spectra were recorded on a
Bruker-AC500 spectrometer in CDCl3 with tetramethylsilane (TMS) as the
internal standard. UV–visible absorption and fluorescence emission spectra
were recorded on an Hitachi U-4100 spectrophotometer and an Hitachi F-4600
spectrophotometer, respectively. Powder X-ray diffraction (PXRD) experiments
operating at 3 kW were performed on a Powder X-ray Diffractometry (INCA
Energy, Oxford Instruments). Differential scanning calorimetry (DSC)
experiments were carried out on a Netzsch DSC204F1 at a heating (or cooling)
rate of 10 oC min–1.
The ground-state geometry was optimized at the level of B3LYP/6-31G(d,p),
and spatial distributions of the HOMO and LUMO of the compound were
obtained from the optimized ground state structure.
2.3 Synthesis
9,10-Bis(chloromethyl)anthracene (1)
To a stirred solution of anthracene (1.78 g, 10 mmol), anhydrous ZnCl2 (1.64
g, 12 mmol), paraformaldehyde (1.50 g, 50 mmol) in dioxane (20 mL) was
slowly added concentrated aqueous hydrochloric acid (40 mL) at room
temperature. After addition, the mixture was gentle refluxed for 3 h and allowed
to stand for 16 h at room temperature. The resulting fine granular solid was
separated by filtration and washed with H2O and dioxane to afford a crude
product. The crude product was recrystallized from toluene to give
9,10-bis(chloromethyl)anthracene as a yellowish solid (1.8 g, 64.1%). 1H NMR
(500 MHz, CDCl3; ppm) δ 8.40 (m, 4H), 7.62 (m, 4H), 5.61 (s, 4H).[15c]
9,10-Bis(diethoxylphosphorylmethyl)anthracene (2)
A solution of 9,10-bis-(chloromethyl)anthracene (7.8 g, 28.3 mmol) and
triethyl phosphite (30 mL) was stirred vigorously overnight at gentle reflux.
The excess triethyl phosphate was removed by distillation under reduced

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pressure. The crude product was separated by silica gel column
chromatography using ethyl acetate/petroleum ether (1/1, v/v) as the eluent. A
1
yellow solid was obtained (10.4 g, 76.8 %). H NMR (500 MHz, CDCl3; ppm)
δ 8.32 (4H, d, J = 9.5 Hz), 7.53 (4H, d, J = 8.0 Hz), 4.21 (8H, d, J = 8.5 Hz),
3.78 (4H, d, J = 9.0 Hz), 1.11 (12H, t, J = 7.5 Hz).13C NMR (125 MHz, CDCl3;
ppm) δ 130.07, 125.55, 125.35, 124.01, 61.99, 28.04, 26.20, 16.04.[15c]
9,10-bis((Z)-2-phenyl-2-(pyridin-2-yl)vinyl)anthracene (PyVA)
Compound 2 (1.00 g, 478.45 mmol) was stirred with potassium tert-butoxide (2 g,
224.4 mmol) and Phenyl(pyridin-2-yl)methanone(1.53g, 280.31 mmol) in THF (50
mL) under Nitrogen. Then the mixture was stirred for 6 h at room temperature.
Methanol was added into the mixture and a yellow solid was collected by filtration.
The crude product was purified by column chromatography(silica gel, ethyl
acetate/dichloromethane, 1/1, v/v). A yellow solid was obtained (0.72 g, 64.3 %). Mp
265 ; FT-IR (KBr): ʋ (cm-1) 2959, 2919, 2850, 1739, 1599, 1539, 1521, 1464, 1414,
1262, 1105, 1045, 807, 698, 618. 1H NMR (500 MHz, CDCl3; ppm) δ 8.77 (2H, d, J =
4.7 Hz), 8.33 (2H, t, J = 17.8 Hz), 8.17 (4H, dd, J = 6.8, 3.3 Hz), 7.67 (2H, td, J = 7.7,
1.9 Hz), 7.28 (8H, q, J = 3.8, 2.9 Hz), 6.98–6.79 (10H, m).13C NMR (125 MHz,
CDCl3; ppm) δ 157.56, 149.57, 149.29, 145.16, 136.47, 135.61, 134.97, 131.81,
130.42, 128.91, 126.53, 125.34, 122.97, 122.60, 121.85, 121.57. Anal. Found: C, 89.5;
H, 5.3; N, 5.2%. Calcd for C40H28N2: C, 89.5; H, 5.3; N, 5.2%. MALDI-TOF MS: m/z
Calcd. for C40H28N2: 536.2252; found 559.2296 [M+Na]+.
Results and discussion
3.1 Aggregation-induced emission
The structure and synthetic route to PyVA was shown in Scheme 1, and its
1
13
chemical structure and composition was unambiguously proven by H and C
NMR spectroscopy and elemental analysis. It was known that, when aryl
groups and arylvinyl groups were linked at the 9,10-position of anthracene ring,
the large internal steric hindrance resulting from the 1,4,5,8-positions of
anthracene ring could force the aryls and arylvinyls to strongly twist. However,
backbone-twisted molecules did not necessarily show AIE effect, such as

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9,10-diarylanthracenes and 10-arylvinyl-anthracenes. Fig. 1 showed the
absorption and emission spectra of PyVA in THF solution and crystalline state.
Like other 9,10-divinylanthracene derivatives, PyVA was a strongly AIE–active
molecule―very weak fluorescence in solution with the fluorescence quantum
yield of 0.21% but stronger fluorescence in the solid with the fluorescence
efficiency of 62%. The much weaker, broader, and red-shifted emission
observed in solution relative to that observed in crystalline state implied the
existence of large amplitude relaxation motion in the excited state facilitating
non-radiative relaxation pathways. When molecules were aggregated together,
the intramolecular vibrational and rotational motions were restricted by
intermolecular interactions and decreased free volume. Thus, the high
fluorescence efficiency was observed in the solid. Fig. 2 showed the emission
spectra of PyVA in dilute THF/water mixtures. Water is the anti-solvent for
PyVA, and the molecules must have aggregated and dispersed in the
solvent/water mixtures with high water contents. Thus, when a large amount of
water was added into the THF solution of PyVA, the fluorescence emission was
turned on and boosted by about 43-folds in THF/water (1/9) dispersion.
3.2 Mechanofluorochromism
Mechanochromism of organic conjugated materials greatly depends on
molecular conformation and packing associated with intermolecular
interactions such as π-π, hydrogen bond, and dipole-dipole interactions.
However, organic small molecule materials with mechanochromic properties
have often appeared as a single isolated event owing to the inherent intricacies
of molecular structural factors and intermolecular forces, and it is still difficult
to predict the mechanofluorochromic (MFC) behavior of new organic
chromophores to date[1-9,16]. Therefore, it was necessary to examine whether
PyVA was MFC active. Fig. 3 showed the stimuli responses of PyVA solid
under different external stimuli. The grinding obviously changed the
fluorescence color from pristine green to ground yellow states (upper inset),
and the solvent-fuming over dichloromethane and thermal annealing below the

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melting point could recover the fluorescence color. Meanwhile, the emission
spectra for these fluorescence color changes were recorded and depicted in Fig.
3
. If the peak-wavelength difference between ground and annealed states was
denoted as the grinding-induced spectral shift (∆λ), the ∆λ value was
determined to be 28 nm. As expected, regrinding could again change the
fluorescence emission colour.
3.3 X-ray diffraction and DSC analysis
Powder wide-angle X-ray diffraction (PWXD) and differential scanning
calorimetry (DSC) measurements on pristine and ground PyVA solid were used
to gain insight into what happened upon grinding. As shown in Fig. 4, the
pristine solid showed multiple sharp reflections, indicating the well-ordered
crystalline state. In contrast, the ground solid displayed a broad and featureless
diffractogram, reflecting the appearance of a notable amorphous state. Fig. 5
indicated that the pristine solid only had an endothermic peak of isotropic melt
transition; however, the ground solid exhibited an additional exothermic peak at
o
107 C, besides the isotropic melt transition. This exothermic peak could be
ascribed to the cold-crystallization transition of the amorphized solid induced
by grinding. Thus, a DSC experiment also confirmed the formation of
amorphous state upon grinding. It could be concluded that the phase
transformation between crystalline and amorphous states was responsible for
the MFC behavior.
3.4 Crystal structure analysis
To understand the molecular confirmation and the intermolecular packing
and interactions of PyVA in crystalline state, a single crystal was grown and
analyzed. X-ray crystallography indicated a monoclinic unit cell for PyVA with
the space group P21/c (Table S1, see the Supporting Information,CCDC
1537375). All PyVA molecules adopted the same twisted conformation with
Z-configuration (Fig. 6a) and assembled together in a parallel fashion (Fig. 6b).
There was a large torsion angle (68.6o) between the anthracene ring and
vinylene substituted at 9,10-positions because of the large internal steric

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hindrance. The torsion angle between vinylene and phenyl ring was only 28.6o,
and that between vinylene and pyridyl ring was up to 54.7o. Although there
were multiple intermolecular CH···π interactions in the ac plane, only two
kinds of CH/π hydrogen bonds were strong with the interaction distances of
2.764 Å and 2.809 Å (Fig. 6c) where the phenyl-CH of one molecule acted as a
H-donor and the pyridyl ring of another adjacent molecule played the role of
H-acceptor. The molecules pack in parallel into molecular columns along the b
axis, where one molecule slid to its neighbors along the long axis of anthracene
ring with an angle of 152.3o and the interaction distance between edge phenyls
was 4.383 Å, implying small π–π overlap and weak π–π interactions. These
supramolecular interactions in the crystals prevent the free torsional motions
around the double bonds, and the molecules are tightly stabilized in the crystal
lattice but still distorted resulting from the internal steric hindrance, even after
stacking, disturbing the close packing, to diminish the intermolecular
quenching effects.
3.5 Acidochromism and amine-induced recovery and FMO feature
Controlling the self-assembly of low molecular weight π-conjugated
molecules via supramolecular interactions required molecular level intelligence.
Further challenges exist in making these self-assemblies functional. Until now,
a number of small organic molecules, metal complexes, and dendrimers with
MFC properties have been reported. However, MFC materials responsive to
chemical ion and molecule species and giving observable signals were highly
desirable, but only several examples have been demonstrated. Strong
fluorescent PyVA solid with exposure lone N-electron pair might have the
potential for sensing volatile acids. Qualitatively, we studied the volatile acid
sensing or proton sensing ability of PyVA solid. Since PyVA solid could exist in
crystalline and amorphous states, we exposed them to a volatile inorganic acid
(HCl) and a volatile Lewis acid (BF3·Et2O) at room temperature. With exposed
lone to HCl for 5 min and to BF3·Et2O for 2 h, both the crystalline and the
amorphous states bathochromically changed their fluorescence colors,

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accompanying with the decreased fluorescence intensity. On keeping the
samples for 24h, the fluorescence colors hardly changed, implying a
pronounced and stable protonation effect. To investigate the sensing sensitivity
of crystalline and amorphous states to volatile acids, the time-dependent
fluorescence color changes were conducted. As shown in Fig. 7, the ground
amorphous state could respond faster upon volatile acid fumigation, which was
about 4 times faster than the crystalline state, regardless of volatile acid types.
This was probably due to the looser stacking and the weaker intermolecular
interactions in the amorphous state. These features indicated that PyVA may be
a potential candidate for sensing acidic vapours.
The other volatile acids such as strong (CF3COOH, TFA) and weak
(CH3COOH and HCOOH) organic acids was the same employed to check the
stimuli responsive property of the amorphous PyVA solid. The proton donating
ability of volatile acids has a dramatic effect on the fluorescence color and
responsive time. Upon fuming with TFA, the effectiveness was same as HCl
and BF3·Et2O (40 nm of emission red-shift), but CH3COOH and HCOOH could
only red-shift the fluorescence color of about 20 nm within 1 hour, regardless of
emission intensity. The fluorescence photos and peak emission wavelengths
were depicted in Fig. 8, and emission spectra were shown in Fig. S3. It has
been shown that the interactions between the fluorescent molecule with a
proton acceptor and an acid could vary from moderate hydrogen bond, very
strong hydrogen bond to covalent ionic bond, depending on the acidity (pKa) of
volatile acids. It seemed that the interaction between PyVA and CH3COOH (or
HCOOH) might be of a typical hydrogen bond, and the interaction between
PyVA and other strong acids might exhibit ionic character or covalent bonding,
which could be responsible for the different acidichromic behaviors.
Volatile acids could interact with the pyridine moiety of PyVA and enable
PyVA solid to show red-shifted emission spectra (540-580 nm) and intense
colors. Thus the volatile strong bases such as triethylamine (TEA) and
ammonia hydroxide (NH3·H2O) should destroy the acid function and lead to the

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recovery of the original emission. As shown in Fig. 8, upon exposure to the
volatile bases, all the protonated PyVA solids with deep colors became strong
green emission under 365 nm UV illumination. This process was very rapid and
realized within minutes at most. Moreover, almost all of the base fumed
samples exhibited the fluorescence emissions peaking at about 500 nm,
implying base-fuming has stimulate the amorphous state to recover to the
unground crystalline state as shown in Fig. S4
.
To gain an insight into the effect of the protonation on the fluorescence
behavior, the frontier molecular orbital (FMO) distributions and energy levels
of PyVA before and after protonation under the optimized geometries were
calculated using density functional theory (DFT) with the Gaussian program
package. As shown in Fig. 9, before protonation, the highest occupied
molecular orbital (HOMO) was entirely located at the anthracene ring, and the
lowest unoccupied molecular orbital (LUMO) was also mainly spread over the
anthracene ring with a small amount of electron density on pyridyl ring. This
was a symmetrically convergent and locally excited FMO distribution with the
relatively high energy levels and large band gap (3.14 eV). After protonation,
the HOMO was distributed along the pyridyl–anthracene–pyridyl backbone,
and the LUMO was located on the pyridylvinyl noieties and the central phenyl
of anthracene ring. Since the nitrogen protonation could enhance the electron-
withdrawing ability of pyridine, the protonated PyVA should be more
delocalized. Generally, the delocalization of the electron cloud is beneficial for
deceasing the molecular energy levels and band gap (Fig. 9), leading to
red-shifts in the fluorescence color and emission (Fig. 7 and Fig. 8). Therefore,
the fluorescence switching of the fumed powder upon stimulation with volatile
acids and amines arose mostly from the change in the electron cloud density of
the PyVA molecule.
Finally, since PyVA exhibited fast acidichromism and amine-induced
recovery character with high contrast, we have covered mask of ring or five-star
on the ground PyVA film to check its potential in patterning. After fuming with

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HCl or BF3·Et2O, the covered place showed the green emission when the
rubber masks were removed (Fig. 10). The patterns were almost intact and
formed a high contrast with fumed regions. Further, we fumed the patterned
samples with volatile amines, and the patterns disappeared and the whole
surface exhibited a green emission. Therefore, PyVA exhibited a typical volatile
acid–base controlled emission switching feature. It is noteworthy that both HCl
and NH3 gas were directly obtained from their concentrated aqueous solution
without drying or diluting, which indicates the acid responsive behavior may be
stable in such a formidable environment and immune to humidity disturbance.
3. Conclusions
A novel pyridine-containing 9,10-divinylanthracence derivative (PyVA) have
been readily synthesised, and it showed the typical AIE feature and could be
mechanically ground to an amorphous state accompanying a moderate spectral
shift. Although crystalline and amorphous PyVA solids could all interact with
the volatile acids to show red-shifted emission spectra and intense colors, the
ground amorphous state could response faster probably due to the looser
stacking and the weaker intermolecular interactions. It was also found that all
the acidichromic PyVA solids could rapidly switch to the green-emitting state
upon fuming with volatile amines, implying the re-crystallization of the ground
amorphous state. This fast acidichromism and amine-induced recovery
character with high color contrast may be applicable in sensing, detection and
patterning.
Acknowledgements
We thank the financial support from the NSF of China (51473084 and
51673105), the NSF of Shandong Province (ZR2014EMQ013), and the
Specialized Research Fund for the Doctoral Program of Higher Education
(201333719120005). We are also grateful for China Postdoctoral Science Fund
(2015M582061, 2014M560537), and Open Project of State Key Laboratory of
Supramolecular Structure and Materials of Jilin University (SKLSSM201728).
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Scheme and Figures
Scheme 1 Synthesis and structure of PyVA.
Fig. 1 Normalized absorption (UV) and fluorescence emission (PL) spectra of PyVA
in THF solution and crystalline state.

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Fig. 2 Emission spectra of PyVA in THF/water mixtures. The inset is the
corresponding fluorescence images under a 365 nm UV lamp.
Fig. 3 Normalized emission spectra of PyVA solid upon grinding, solvent-fuming,
re-grinding, and annealing. The upper inset is the corresponding fluorescence images
under a 365 nm UV lamp.
Fig. 4 Powder X-ray diffraction patterns of pristine and ground PyVA solids at room
temperature.
Fig. 5 DSC curves of pristine and ground PyVA solids.
Fig. 6 Molecular conformation (a), intermolecular packing (b) and interactions (c) on
ac plane, and intermolecular packing and interactions along b axle (d).
Fig. 7 Acidochromic evolution process of the ground and annealed PyVA solids at
room temperature.
Fig. 8 Acidochromism of ground PyVA solid and the alkali-induced recovery at room
temperature.
Fig. 9 Frontier molecular orbital plots of PyVA before and after protonation.
Fig. 10 Fluorescence patterning images based on acidochromism of ground PyVA
solid and then amine-induced recovery at room temperature.

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Fig. 1 Normalized absorption (UV) and fluorescence emission (PL) spectra of PyVA in
THF solution and crystalline state.

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Fig. 10 Fluorescence patterning images based on acidochromism of ground PyVA
solid and then amine-induced recovery at room temperature.

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Fig. 2 Emission spectra of PyVA in THF/water mixtures. The inset is the
corresponding fluorescence images under a 365 nm UV lamp.

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Fig. 3 Normalized emission spectra of PyVA solid upon grinding, solvent-fuming,
re-grinding, and annealing. The upper inset is the corresponding fluorescence images
under a 365 nm UV lamp.

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Fig. 4 Powder X-ray diffraction patterns of pristine and ground PyVA solids at room
temperature.

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Fig. 5 DSC curves of pristine and ground PyVA solids.

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Fig. 6 Molecular conformation (a), intermolecular packing (b) and interactions (c) on
ac plane, and intermolecular packing and interactions along b axle (d).

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Fig. 7 Acidochromic evolution process of the ground and annealed PyVA solids at
room temperature.

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Fig. 8 Acidochromism of ground PyVA solid and the alkali-induced recovery at room
temperature.

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Fig. 9 Frontier molecular orbital plots of PyVA before and after protonation.