Alkoxy substituted D-π-A dimethyl-4-pyrone derivatives: Aggregation induced emission enhancement, mechanochromic and solvatochromic properties

Article abstract of DOI:10.1016/j.dyepig.2016.09.063

Three series of D-π-A 2,6-dimethyl-4-pyrone-cored derivatives were designed and synthesized. Evaluation by spectroscopic methods indicated that all the compounds exhibited aggregation-induced emission enhancement (AIEE) effect and solvatochromism. The experimental results revealed that the restriction of intramolecular rotation is the key element for showcasing the AIEE phenomenon. Moreover, all of the compounds presented red-shifted mechanochromic behavior, the extent of the red-shift was mainly dependent on the position and length of alkoxyl group. The PXRD and DSC profiles demonstrated a transformation from crystalline to amorphous state upon grinding. The D-A strategy introduced in this work could facilitate the development and application of ingenious mechanochromic materials.

Full text of DOI:10.1016/j.dyepig.2016.09.063

Accepted Manuscript
Alkoxy substituted D-π-A dimethyl-4-pyrone derivatives: Aggregation induced
emission enhancement, mechanochromic and solvatochromic properties
YuQi Cao, Ye Xi, XinYu Teng, Yang Li, Xilong Yan, Ligong Chen
PII:
S0143-7208(16)30864-6
10.1016/j.dyepig.2016.09.063
DYPI 5515
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 18 July 2016
Revised Date: 28 September 2016
Accepted Date: 29 September 2016
Please cite this article as: Cao Y, Xi Y, Teng X, Li Y, Yan X, Chen L, Alkoxy substituted D-π-A
dimethyl-4-pyrone derivatives: Aggregation induced emission enhancement, mechanochromic and
solvatochromic properties, Dyes and Pigments (2016), doi: 10.1016/j.dyepig.2016.09.063.
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ACCEPTED MANUSCRIPT
Alkoxy substitued D-π-A dimethyl-4-pyrone derivatives: Aggregation induced
emission enhancement, Mechanochromic and Solvatochromic properties
YuQi Caoa b, Ye Xi a b, XinYu Teng a b, Yang Lia b c, Xilong Yan*a b c, Ligong Chen*a b c
a School of Chemical Engineering and Technology, Tianjin University, Tianjin, P. R. China. E-mail:
lgchen@tju.edu.cn, yan@tju.edu.cn; Fax: +86-022-27406314; Tel: +86-022-27406314
b Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), P. R. China
c Tianjin Engineering Research Center of Functional Fine Chemicals, Tianjin, P. R. China
Abstract
Three series of D-π-A 2,6-dimethyl-4-pyrone-cored derivatives were designed
and synthesized. Evaluation by spectroscopic methods indicated that all the
compounds exhibited aggregation-induced emission enhancement (AIEE) effect and
solvatochromism. The experimental results revealed that the restriction of
intramolecular rotation is the key element for showcasing the AIEE phenomenon.
Moreover, all of the compounds presented red-shifted mechanochromic behavior, the
extent of the red-shift was mainly dependent on the position and length of alkoxyl
group. The PXRD and DSC profiles demonstrated a transformation from crystalline to
amorphous state upon grinding. The D-A strategy introduced in this work could
facilitate the development and application of ingenious mechanochromic materials.
Introduction
Stimuli responsive fluorescent materials display promising prospects by virtue of
their wide practical applications, such as chemsensors[1], pH indicators[2], fluorescent
probes[3], biosensors[4], photoelectronic devices[5], viscosity probes[6] and
mechanochromic materials[7]. In which, mechanochromic materials have attracted
intensive attention due to their noteworthy applied value. The phenomenon termed as
mechanofluorochromism (MFC) originally referred to as piezochromism or
tribochromism reflects color change in response to external force (pressing or
grinding), is of significant importance for diverse applications[8]. However,
aggregation caused quenching effect (ACQ) is known as an unfavorable phenomenon

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that remarkably inhibits the utilization of MFC materials, for the ACQ materials only
emit fluorescence in solution instead of solid state[9-11]
.
Fortunately in 2001, Tang[12] and coworkers first introduced a new concept
referred to aggregation induced emission (AIE). These kinds of compounds emit
extremely weak fluorescence in solution but intense fluorescence as aggregates form.
In 2010, Park[13] first presented cyano distyrylbenzene derivatives that display MFC
effect based on AIE property. By virtue of the perfect combination of AIE and MFC
materials, number of MFC templates with AIE property are successively reported,
including distyrylanthrancene derivatives[14-18], silole derivatives[19], tetraphenylethene
(TPE)
derivatives[20-25]
,
cyano-substituted
diarylethene
derivatives[26-28]
,
triphenylamine derivatives[29-31]. However, owing to the lack of clear guidelines for
designing molecules possess MFC properties, the study of MFC fluorescent materials
is still at the inceptive stage, intensive study and the creation of new templates are just
around the corner.
Herein, we report three series of D-π-A 2,6-dimethyl-4-pyrone derivatives
(Scheme 1), including 2,6-bis((E)-4-alkoxystyryl)-4H-pyran-4-one (PASPs),
2,6-bis((E)-2-alkoxystyryl)-4H-pyran-4-one
(OASPs)
and
2,6-bis((E)-2-(2-alkoxynaphthalen-l-yl)vinyl)-4H-pyran-4-one (ANPs). These target
compounds based on pyrone skeleton provides new members to AIE family. The MFC
feature of these compounds would open a new pathway to fabricate “smart”
mechanofluorochromic
materials.
These
compounds
show
obvious
aggregation-induced emission enhancement (AIEE) phenomenon, their fluorescence
properties and MFC behavior were investigated using photoluminescence spectra.
Furthermore, differential scanning calorimetry (DSC) and powder wide-angle X-ray
diffraction (PXRD) were implemented to study the relationship between
mechanofluorochromism and structure.
Experimental Section
Materials and characterization
All the raw materials were obtained commercially and used without further

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1
purification. H NMR were recorded by using Bruker Avance 400 MHz spectrometer
with CDCl3 as solvent. High resolution mass spectrum were measured on Bruker
Paltonicsmicro TOF-Q II instrument. IR spectra were recorded using a Nicolet 380
FT-IR spectrometer. Photoluminescence spectra of liquid and photoluminescence
spectra of solid were obtained with Hitachi F-2500 spectrophotometer and Horiba
Jobin Yvon Fluorolog-3 spectrophotometer, respectively. Powder wide angle X-ray
diffraction (PWXD) were recorded on a Miniflex 600 Powder X-ray Diffractometer of
Rigaku, with the following instrument parameter: 35V, 15A, 5°min-1. Differential
scanning calorimetry (DSC) experiments were operated on a Perkin-Elmerat at a
heating rate of 10 ℃min-1. Annealing experiment: The pressed samples were heated at
the temperature of Tm-10 ꢀ (Tm is the isotropic melt point of each compound) for 10
min in a muffle furnace.
General procedures for the synthesis of compounds A-n, B-n and C-n
Compounds A-n, B-n and C-n were similarly synthesized according to the
literature method[32]
.
A mixture of hydroxybenzaldehyde (0.010 mol) and K2CO3 (1.66 g, 1.20 mol)
was added to N, N-dimethylformamide (20 mL). Alkyl bromide (0.012 mol) was
added subsequently. After stirring at 90 ꢀ for 8 hours, the mixture was poured into
distilled water (200 mL) and extracted with dichloromethane (3×20 mL). The
combined organic phases were washed with brine (3×50 mL), dried over Na2SO4,
concentrated to yield the crude product, which was used for the next step without
further purification.
General procedures for PASPs , OASPs and ANPs
Compounds PASPs, OASPs and ANPs were similarly synthesized according to
the following procedure.
2,6-dimethyl-4-pyrone (0.31 g, 2.50 mmol) and B-n (6.25 mmol) were dissolved
in ethyl alcohol (20 mL). Sodium ethoxide (0.43 g, 6.25 mmol) in ethyl alcohol (10
mL) was added dropwise to the above solution. After stirring at 25-40 ℃for 2 days, 5%
hydrochloric acid was employed to adjust the pH of the mixture to 7. The mixture was
then extracted with dichloromethane (3×20 mL), the organic phases were washed with

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brine (3×50 mL), dried over anhydrous Na2SO4, concentrated to afford the crude
product. It was purified by column chromatography (petroleum ether/ethyl acetate)
and recrystallized in ethyl alcohol to afford the target compound.
2,6-bis((E)-4-butoxystyryl)-4H-pyran-4-one (PASP-4): Yellow solid (0.74 g),
67% yield, m.p. 187-188 ꢀ. IR (KBr, cm-1): 3040, 2930, 2876, 1643, 1615, 1509,
1
1391, 1245, 1182, 963, 745. H NMR (CDCl3, 400 MHz) δ (ppm) : 7.54 (d, J = 8.4
Hz, 4H), 7.46 (d, J = 16.0 Hz, 2H), 6.97 (d, J = 8.8 Hz, 4H), 6.64 (d, J = 16.0 Hz, 2H),
6.24 (s, 2H), 4.05 (t, J = 12.8 Hz, 4H), 1.85-1.78 (m, 4H), 1.58-1.49 (m, 4H), 1.04 (t,
J = 14.8 Hz, 6H). 13C NMR (CDCl3, 100 MHz) δ (ppm): 180.40, 161.39, 160.62,
135.55, 129.09, 127.56, 117.48, 114.96, 113.17, 67.89, 31.24, 19.23, 13.83. HRMS
(ESI): calcd for C29H32O4: 467.2193 (M+Na)+, found 467.2195.
2,6-bis((E)-4-(octyloxy)styryl)-4H-pyran-4-one (PASP-8): Brownish yellow
solid (1.11 g), 71% yield, m.p. 161-162 ꢀ. IR (KBr, cm-1): 3031, 2932, 2855, 1644,
1
1615, 1509, 1391, 1245, 1182, 972, 718. H NMR (CDCl3, 400 MHz) δ (ppm) : 7.54
(d, J = 8.8 Hz, 4H), 7.46 (d, J = 16 Hz, 2H), 6.97 (d, J = 8.4 Hz, 4H), 6.64 (d, J = 16
Hz, 2H), 6.24 (s, 2H), 4.04 (t, J = 13.2 Hz, 4H), 1.86-1.79 (m, 4H), 1.51-1.32 (m,
20H), 0.93 (t, J = 13.6 Hz, 6H). 13C NMR (CDCl3, 100 MHz) δ (ppm): 180.39, 161.62,
160.62, 135.54, 129.09, 127.55, 117.48, 114.96, 113.19, 68.21, 31.81, 29.35, 29.23,
29.20, 26.03, 22.65, 14.09. HRMS (ESI): calcd for C37H48O4: 579.3445 (M+Na)+,
found 579.3442.
2,6-bis((E)-4-(dodecyloxy)styryl)-4H-pyran-4-one
(PASP-12):
Brownish
yellow solid (1.15 g), 69% yield, m.p. 142-144 ꢀ. IR (KBr, cm-1): 3040, 2919, 2848,
1
1647, 1613, 1510, 1401, 1245, 1182, 967, 719. H NMR (CDCl3, 400 MHz) δ (ppm):
7.54 (d, J = 8.4 Hz, 4H), 7.46 (d, J = 15.6 Hz, 2H), 6.96 (d, J = 8.4 Hz, 4H), 6.64 (d, J
= 16.0 Hz, 2H), 6.24 (s, 2H), 4.04 (t, J = 13.2 Hz, 4H), 1.86-1.79 (m, 4H), 1.49-1.29
(m, 36H), 0.92 (t, J = 13.6 Hz, 6H). 13C NMR (CDCl3, 100 MHz) δ (ppm): 180.40,
161.64, 160.62, 135.55, 129.09, 127.56, 117.49, 114.97, 113.20, 68.22, 31.92, 29.66,
29.63, 29.59, 29.57, 29.38, 29.34, 29.20, 26.02, 22.68, 14.11. HRMS (ESI): calcd for
C45H64O4: 691.4697 (M+Na)+, found 691.4691.
2,6-bis((E)-2-butoxystyryl)-4H-pyran-4-one (OASP-4): Yellow solid (0.72 g),

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65% yield, m.p. 123-124 ꢀ. IR (KBr, cm-1): 3067, 2930, 2866, 1640, 1588, 1455,
1
1391, 1245, 1126, 968, 744. H NMR (CDCl3, 400 MHz) δ (ppm): 7.83 (s, 1H), 7.79
(s, 1H), 7.56 (s, 1H), 7.54 (s, 1H), 7.36 (t, J = 8.4 Hz, 2H), 7.02-6.96 (m, 6H), 6.22 (s,
2H), 4.11 (t, J = 12.8 Hz, 4H), 1.92-1.85 (m, 4H), 1.61-1.51 (m, 4H), 0.98 (t, J = 14.8
Hz, 6H). 13C NMR (CDCl3, 100 MHz) δ (ppm): 180.62, 162.00, 157.68, 131.95,
130.77, 128.90, 124.11, 120.76, 120.65, 113.54, 112.13, 68.20, 31.35, 19.43, 13.84.
HRMS (ESI): calcd for C29H32O4: 467.2193 (M+Na)+, found 467.2195.
2,6-bis((E)-2-(octyloxy)styryl)-4H-pyran-4-one (OASP-8): Yellow solid (0.90
g), 65% yield, m.p. 86-87 ꢀ. IR (KBr, cm-1): 3067, 2921, 2848, 1645, 1590, 1463,
1
1390, 1244, 1126, 990, 970, 742. H NMR (CDCl3, 400 MHz) δ (ppm): 7.85 (s, 1H),
7.81 (s, 1H), 7.56 (s, 1H), 7.54 (s, 1H), 7.36 (t, J = 16.8 Hz, 2H), 7.02-6.93 (m, 6H),
6.27 (s, 2H), 4.09 (t, J = 13.2 Hz, 4H), 1.93-1.86 (m, 4H), 1.54-1.23 (m, 20H), 0.88 (t,
J = 13.6 Hz, 6H). 13C NMR (CDCl3, 100 MHz) δ (ppm): 180.62, 161.99, 157.67,
131.93, 130.75, 128.77, 124.15, 120.68, 120.66, 113.55, 112.16, 68.57, 31.78, 29.34,
29.29, 29.22, 26.20, 22.59, 14.07. HRMS (ESI): calcd for C37H48O4: 579.3445
(M+Na)+, found 579.3445.
2,6-bis((E)-2-(dodecyloxy)styryl)-4H-pyran-4-one (OASP-12): Yellow solid
(1.12 g), 67% yield, m.p. 68-70 ꢀ. IR (KBr, cm-1): 3040, 2921, 2839, 1641, 1593,
1463, 1381, 1244, 1126, 971, 746. 1H NMR (CDCl3, 400 MHz) δ (ppm): 7.86 (s, 1H),
7.82 (s, 1H), 7.56 (s, 1H), 7.54 (s, 1H), 7.36-7.32 (t, J = 16.8 Hz, 2H), 7.02-6.94 (m,
6H), 6.31 (s, 2H), 4.09 (t, J = 13.2 Hz, 4H), 1.93-1.86 (m, 4H), 1.54-1.22 (m, 36H),
0.91 (t, J = 14 Hz, 6H). 13C NMR (CDCl3, 100 MHz) δ (ppm): 180.59, 161.97, 157.67,
131.91, 130.74, 128.76, 124.15, 120.69, 120.66, 113.56, 112.16, 68.57, 31.92, 29.64,
29.61, 29.60, 29.57, 29.39, 29.34, 29.29, 26.19, 22.69, 14.11. HRMS (ESI): calcd for
C45H64O4: 691.4697 (M+Na)+, found 691.4698.
2,6-bis((E)-2-(hexadecyloxy)styryl)-4H-pyran-4-one (OASP-16): Yellow solid
(1.21 g), 62% yield, m.p. 52-54 ꢀ. IR (KBr, cm-1): 3059, 2922, 2840, 1640, 1592,
1464, 1381, 1245, 1127, 970, 744. 1H NMR (CDCl3, 400 MHz) δ (ppm): 7.85 (s, 1H),
7.81 (s, 1H), 7.56 (s, 1H), 7.54 (s, 1H), 7.36 (t, J = 15.6 Hz, 2H), 7.02-6.93 (m, 6H),
6.27 (s, 2H), 4.09 (t, J = 13.2 Hz, 4H), 1.93-1.86 (m, 4H), 1.33 (m, 52H), 0.92 (t, J =

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13.6 Hz, 6H). 13C NMR (CDCl3, 100 MHz) δ (ppm): 180.62, 161.98, 157.66, 131.91,
130.74, 128.76, 124.16, 120.70, 120.66, 113.57, 112.15, 68.57, 31.93, 29.69, 29.67,
29.61, 29.57, 29.37, 29.28, 26.19, 22.69, 14.11. HRMS (ESI): calcd for C53H80O4:
803.5949 (M+Na)+, found 803.5956.
2,6-bis((E)-2-(naphthalen-l-yl)vinyl)-4H-pyran-4-one (ANP): Yellow solid
(0.75 g), 75% yield, m.p. 192-194 ꢀ. IR (KBr, cm-1): 3040, 1647, 1557, 1504, 1249,
1
1168, 953. H NMR (CDCl3, 400 MHz) δ (ppm): 8.49 (s, 1H), 8.46 (s, 1H), 8.31 (s,
1H), 8.29 (s, 1H), 7.96 (d, J = 7.6 Hz, 4H), 7.90 (d, J = 7.2 Hz, 2H), 7.64-7.56 (m,
6H), 6.95 (d, J = 16 Hz, 2H), 6.40 (s, 2H). 13C NMR (CDCl3, 100 MHz) δ (ppm):
180.40, 161.19, 133.85, 132.79, 132.16, 131.37, 130.25, 129.03, 126.97, 126.30,
125.66, 124.52, 122.90, 122.22, 114.29. HRMS (ESI): calcd for C29H20O2: 423.1356
(M+Na)+, found 423.1359.
2,6-bis((E)-2-(2-butoxynaphthalen-l-yl)vinyl)-4H-pyran-4-one
(ANP-4):
Yellow solid (0.99 g), 73% yield, m.p. 186-187 ꢀ. IR (KBr, cm-1): 3049, 2928, 2866,
1
1637, 1592, 1562, 1458, 1389, 1264, 1150, 956, 743. H NMR (CDCl3, 400 MHz) δ
(ppm): 8.33-8.26 (m, 4H), 7.89-7.84 (m, 4H), 7.53 (t, J = 8 Hz, 2H), 7.44 (t, J = 7.6
Hz, 2H), 7.37 (s, 1H), 7.35 (s, 1H), 7.34 (s, 1H), 7.29 (s, 1H), 6.33 (s, 2H), 4.27 (t, J =
12.8 Hz, 4H), 1.94-1.87 (m, 4H), 1.62-1.53 (m, 4H), 0.99 (t, J = 14.4 Hz, 6H). 13C
NMR (CDCl3, 100 MHz) δ (ppm): 180.85, 162.12, 156.15, 132.89, 131.13, 129.03,
128.80, 128.72, 127.52, 125.10, 123.87, 122.78, 117.08, 113.98, 113.88, 69.13, 31.53,
19.43, 13.79. HRMS (ESI): calcd for C37H36O4: 545.2686 (M+H)+, found 545.2688.
2,6-bis((E)-2-(2-(octyloxy)naphthalen-l-yl)vinyl)-4H-pyran-4-one (ANP-8):
Yellow solid (1.15 g), 70% yield, m.p. 102-104 ꢀ. IR (KBr, cm-1): 3067, 2921, 2848,
1
1637, 1555, 1510, 1458, 1387, 1264, 1147, 958, 744. H NMR (CDCl3, 400 MHz) δ
(ppm): 8.33-8.26 (m, 4H), 7.89-7.84 (m, 4H), 7.53 (t, J = 7.2 Hz, 2H), 7.44 (t, J = 7.6
Hz, 2H), 7.36 (s, 1H), 7.34 (s, 1H), 7.32 (s, 1H), 7.29 (s, 1H), 6.32 (s, 2H), 4.26 (t, J =
13.2 Hz, 4H), 1.94-1.87 (m, 4H), 1.55-1.48 (m, 4H), 1.32-1.24 (m, 16H), 0.88 (t, J =
13.2 Hz, 6H). 13C NMR (CDCl3, 100 MHz) δ (ppm): 180.78, 162.07, 156.15, 132.88,
131.12, 129.05, 128.81, 128.76, 127.52, 125.09, 123.86, 122.81, 117.12, 114.00,
113.89, 69.50, 31.77, 29.48, 29.28, 29.23, 26.19, 22.61, 14.08. HRMS (ESI): calcd for

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C45H52O4: 679.3758 (M+Na)+, found 679.3754.
Results and discussion
Synthesis
Compounds A-n, B-n and C-n were prepared by literature method. The target
compounds were easily obtained from the condensation of pyrone with compounds
13
A-n, B-n and C-n, respectively[33]. Their structures were confirmed by 1H NMR, C
NMR, HRMS and IR. (Fig. S1, S2, S3 and S4 ESI ).
†
Photophysical properties of PASPs, OASPs and ANPs
In order to determine the AIEE effect of PASPs, OASPs and ANPs, their
fluorescence in solution was studied using THF and water as good and poor solvent,
respectively. In molecularly dissolved solution, molecules exist in a relative random
way which can result in weaker intermolecular hydrogen bonding and steric
hinderance. In this regard, the intramolecular torsion is relatively free, which
accelerate the nonradiative pathway and affords weak fluorescence[34,35]. Adding
water to the well-dissolved system can result in the formation of aggregates and
further lead to changes of their photoluminescence (PL) spectra. As shown in Fig.1
(Fig. S5, ESI
), the THF solution of OASP-4 (1×10-5 mol L-1) emitted weak blue
†
fluorescence originally. As expected, this nonradiative relaxation pathway is possibly
ascribed to the free rotation of the phenyl ring around single bond. Along with the
increase of water fraction (fw), the fluorescence intensity of OASP-4 increased
gradually and was accompanied with large bathochromic shift. The corresponding
emission peak showed nearly 126 nm red shift until reaching the maximum PL
intensity at 70% water fraction. This phenomenon may be related to the
transformation from the locally excited state (LE) to the twisted intramolecular charge
transfer state (TICT) along with the increase of solvent polarity which can be
confirmed by solvatochromic experiment (Fig. 2). It usually occurs in the molecules
containing electron donor and acceptor. Solvation effect stabilizing HOMO and
LOMO orbits simultaneously but more to LOMO, thus give rise to a narrowed band

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gap and a red-shifted emission spectrum[36,37]. Moreover, the PL intensity of OASP-4
was 18 times larger than that in pure THF due to the restriction of intramolecular
rotation (RIR). When water was added, the molecules began to aggregate in the
limited space and the free rotation was blocked to some extent, further facilitating the
radiative relaxation pathway and afford enhanced emission[38-40]. Moreover, hydrogen
bonds were considered to enhance the molecular rigidity which may also contribute to
fluorescence enhancement. However, further addition of water results in the reduction
of fluorescence intensity. When fw exceeds 70%, parts of the molecules begun to
assemble into amorphous aggregates to yield a mixture of crystalline and amorphous
particles and result in weak luminescent[41-43]. Surprisingly, further incorporation of
water led to blue-shifted PL spectra, but the reason remained unclear. All of the
PASPs, OASPs and ANPs derivatives showed similar luminescent phenomenon apart
from a few exceptions. The PL intensity might depend on the aggregate stages and
shows a complex phenomenon at different stages.
On account of their D-π-A structures, solvatochromic property was examined. As
described in Fig.2, the florescence spectra showed a gradual red shift as the solvent
changed from cyclohexane to DMF, exhibited 44 nm, 36 nm, 42 nm spectra shift for
OASP-8, PASP-8 and ANP-8, respectively. These changes are ascribed to larger
dipole moment and charge separation degree in the exited state than that in the ground
state[44]. Obvious intramolecular charge transfer (ICT) characteristics were verified in
these compounds.
Mechanochromic properties
The mechanochromic properties of OASPs, PASPs and ANPs were examined
upon grinding using a mortar and pestle. The typical PL spectra upon external
pressure are illustrated in Fig.3 (Fig. S6, ESI ) with fluorescence images embedded
†
inside. It was found that OASP-8, PASP-12 and ANP-8 exhibited blue, dark green
and yellow fluorescence, respectively. Upon mechanical stimuli, all of them displayed
obvious color changes and turned into cyan, brownish yellow and orange yellow. The

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resultant red shift is attributed to the planarization induced by grinding. In general,
AIE molecules usually adopt a twisted configuration owing to the steric hindrance
between moieties. The loose packing mode can lead to the formation of
intermolecular cavities. When the samples are pressed, the binding energy released,
accompanied by the destruction of crystal lattice. In this regard, configuration
planarization gave rise to a red-shifted fluorescence spectra[45]
.
To deeply understand the fluorescence property, all the corresponding
spectroscopic data were summarized in Table 1. As we can see, the mechanochromic
property showed a zigzag tendency along with the chain length and reached its
maximum at OASP-8 (∆λPFC = λground-λannealed = 20 nm). The shrink of ∆λPFC can be
attributed to the bulky alkyl group located in the ortho position. The intramolecular
steric hindrance impeded the typical planarization to afford inferior mechanochromic
property. However, for PASPs, the longer the chain length, the more conspicuous the
MFC to be induced by grinding. Molecules with longer alkyl chain may adopted a
more twisted backbone to provide cavity and loose stacking mode. Therefore,
mechanical force can induce a more obvious red shift. For ANPs molecules, the
presented MFC property stems from the combination of the above two forces.
PXRD and DSC experiment
To further confirm the packing mode of the original and ground form of the
present compounds, powder X-ray diffraction (PXRD) and differential scanning
calorimetry (DSC) were performed. As shown in Fig. 4 (Fig. S7, ESI ), the diffraction
†
pattern of the original OASP-4, PASP-4 and ANP-4 exhibit multiple sharp and
intense reflections which manifest regular micro-crystalline conformation. In contrast,
the XRD profile of the ground state displayed weak and broad diffractograms. This is
because of the destruction of a somewhat crystalline phase and the formation of a
disordered amorphous state[46]. These results provided convincing evidence among the
molecular packing mode and the MFC property.
The thermal properties of OASP-4, PASP-4 and ANP-4 were investigated by

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differential scanning calorimetry (DSC). As described in Fig.5 (Fig. S8, ESI ),
†
OASP-4 has one endothermic peak, while both of the PASP-4 and ANP-4 display
three endothermic peaks. In general, the maximum endothermic peak can be realized
as the isotropic melt transition (Tm) and there is no change upon grinding.
Furthermore, it is easy to find that the Tm decreases with the increase of chain length.
This phenomenon could be ascribed to the loose packing mode and weak
intermolecular interaction. However, it is unexpected that all of these compounds
display no exothermic cold crystallization peak in the ground state. The endothermic
peak in the low-temperature zone are considered to be the transformation from solid
state to liquid crystalline (TL) state. It is noted that TL of ANP-4 and PASP-4 shifted
from 106ꢀ to 113ꢀ and from 102℃ to 136℃ respectively after grinding with the
peaks became broad and flat accordingly. Moreover, the two melting endotherm peaks
lied in PASP-4 can be attributed to the recrystallization procedure in a slow heating
process. In which, the incomplete crystals with low melting temperature can
recrystallize into more perfect lamellae to present a “double melting” phenomenon[47]
To sum up, there is little relationship between MFC property and phase
transformation.
.
Conclusion
In this work, three series of D-π-A compounds OASPs, PASPs and ANPs with
AIEE property were designed and successfully synthesized. These compounds
exhibited solvatochromism: the fluorescence spectra red-shifted with the increase of
solvent polarity. Interestingly, all of these compounds exhibit red-shifted MFC
properties, in which, PASP-12 displayed the largest MFC spectra shift (∆λPFC
=
23nm). It was found that the MFC properties were distinctly position and alkyl-length
dependent. For PASPs and ANPs, the longer the alkyl chain, the more remarkable
MFC properties displayed. While for OASPs, the MFC characteristic showed an
inverted U-shaped tendency. All the ground state can be recovered completely or
partly through an annealing process. PXRD and DSC revealed that the MFC
mechanism was ascribed to the transformation among crystalline state and the

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amorphous state by grinding. We believe the work presented here can provide new
strategy for designing new MFC templates based on AIE.
Acknowledgement
We are grateful for the financial support from the National Natural Science
Foundation of China (21576194).
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Caption of figures
Scheme 1. Synthesis and structures of OASPs, PASPs and ANPs
Figure 1. PL spectra of the compounds in water/THF mixtures
(A) PASP-4; (B) OASP-4; (C) ANP (concentration, 10-5 mol L-1; excitation wavelength,
PASP-4: 320 nm; OASP-4: 330 nm; ANP: 340 nm)
The insets depict the changes in PL peak intensity and emission images of the compound in
different water fraction mixtures under 365 nm UV illumination
Figure 2. PL spectra of ANP-8, PASP-8 and OASP-8 in different solvents with varying polarities
(concentration, 10-4 mol L-1; excitation wavelength,
ANP-8: 365 nm; OASP-8: 330 nm; PASP-8: 320 nm)
The insets depict the emission images of the compounds in cyclohexane, toluene, THF, CH2Cl2,
DMF (from left to right) under 365 nm UV illumination
Figure 3. Normalized fluorescence emission spectra of ANP-8, PASP-12, OASP-8 samples
mixed with KBr upon grinding, annealing.
The insets depict the fluorescence images of ANP-8, PASP-12, OASP-8, samples taken at
365 nm UV lamp after grinding and annealing.
Figure 4. Powder wide-angle X-ray diffraction (PXRD) patterns of PASP-4, ANP-4 and
OASP-4 in original and ground states

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Figure 5. Differential Scanning Calorimetry (DSC) curves of PASP-4, ANP-4 and OASP-4 in
original and ground states
Caption of table
Table 1 Peak emission wavelength (nm) of solid-state OASPs, PASPs and ANPs derivatives
under different conditions

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Table 1 Peak emission wavelength (nm) of solid-state OASPs, PASPs and ANPs derivatives
under different conditions
Samples
OASP-4
OASP-8
OASP-12
OASP-16
PASP-4
PASP-8
PASP-12
ANP
λoriginal
466
458
455
442
490
460
472
505
517
516
λpressed
478
478
472
473
508
500
503
514
524
526
λannealed
464
458
460
465
493
481
480
504
513
512
∆λPFC
14
20
12
8
15
19
23
10
11
14
ANP-4
ANP-8

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Scheme 1. Synthesis and structures of OASPs, PASPs and ANPs

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Figure 1. PL spectra of the compounds in water/THF mixtures
(A) PASP-4; (B) OASP-4; (C) ANP (concentration, 10-5 mol L-1; excitation wavelength,
PASP-4: 320 nm; OASP-4: 330 nm; ANP: 340 nm)
The insets depict the changes in PL peak intensity and emission images of the compound in
different water fraction mixtures under 365 nm UV illumination

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Figure 2. PL spectra of ANP-8, PASP-8 and OASP-8 in different solvents with varying polarities
(concentration, 10-4 mol L-1; excitation wavelength,
ANP-8: 365 nm; OASP-8: 330 nm; PASP-8: 320 nm)
The insets depict the emission images of the compounds in cyclohexane, toluene, THF, CH2Cl2,
DMF (from left to right) under 365 nm UV illumination

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Figure 3. Normalized fluorescence emission spectra of ANP-8, PASP-12, OASP-8 samples
mixed with KBr upon grinding, annealing.
The insets depict the fluorescence images of ANP-8, PASP-12, OASP-8, samples taken at
365nm UV lamp after grinding and annealing.

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Figure 4. Powder wide-angle X-ray diffraction (PXRD) patterns of PASP-4, ANP-4 and
OASP-4 in original and ground states

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Figure 5. Differential Scanning Calorimetry (DSC) curves of PASP-4, ANP-4 and OASP-4 in
original and ground states

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Highlights
The synthesized new AIE templates exhibit obvious mechanochromic behavior.
D-π-A 2, 6-dimethyl-4-pyrone-cored derivatives present AIEE and solvatochromism.
Mechanochromic behavior was mainly depended on substituents’ position and length.