Mechanochromic and acidochromic response of 4H-pyran derivatives with aggregation-induced emission properties

Article abstract of DOI:10.1016/j.dyepig.2017.02.045

Several 1- and 2-position naphthylethylene-substituted 4H-pyran derivatives comprising two different end groups were synthesized. These two pairs of positional isomers with aggregation-induced emission properties exhibit different stimuli-responsive solid-state fluorescent properties. Both the linking position of the naphthylethylene and the electron-withdrawing end group have a significant impact on the mechanochromic properties of the compounds, whereas, the acidochromic properties are only related to the latter. The transformation between amorphous and crystalline states accounts for the mechanochromic properties and is attributed to loose molecular packing with weak intermolecular interactions, as revealed by crystallographic data. The reversible acidochromic properties are mainly ascribed to protonation/deprotonation of the carbonyl group, which is confirmed by ¹H NMR experiments. Additionally, because the emission of these 4H-pyran derivatives in solid state can be tuned by external stimuli, several molecular logic gates were established by using appropriate stimuli operations as the inputs and fluorescent colors as the outputs.

Full text of DOI:10.1016/j.dyepig.2017.02.045

Accepted Manuscript
Mechanochromic and acidochromic response of 4H-pyran derivatives with
aggregation-induced emission properties
Yibin Zhou, Yanze Liu, Yang Guo, Miaochang Liu, Jiuxi Chen, Xiaobo Huang, Wenxia
Gao, Jinchang Ding, Yixiang Cheng, Huayue Wu
PII:
S0143-7208(16)31505-4
10.1016/j.dyepig.2017.02.045
DYPI 5824
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 31 December 2016
Revised Date: 23 February 2017
Accepted Date: 28 February 2017
Please cite this article as: Zhou Y, Liu Y, Guo Y, Liu M, Chen J, Huang X, Gao W, Ding J, Cheng Y,
Wu H, Mechanochromic and acidochromic response of 4H-pyran derivatives with aggregation-induced
emission properties, Dyes and Pigments (2017), doi: 10.1016/j.dyepig.2017.02.045.
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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
Mechanochromic and acidochromic response of 4H-pyran
derivatives with aggregation-induced emission properties
a
a
b
a
a
a,
Yibin Zhou , Yanze Liu , Yang Guo , Miaochang Liu , Jiuxi Chen , Xiaobo Huang *,
a
a
b
a,
Wenxia Gao , Jinchang Ding , Yixiang Cheng , Huayue Wu *
a
College of Chemistry and Materials Engineering, Wenzhou University, Wenzhou, 325035, P. R. China
b
School of Chemistry and Chemical Engineering, Nanjing University, Nanjing, 210093, P. R. China
*Corresponding authors. Tel.: +86 577 88368280; fax: +86 577 88368280
E-mail addresses: xiaobhuang@wzu.edu.cn (X. Huang); huayuewu@wzu.edu.cn (H. Wu)
ABSTRACT: Several 1- and 2-position naphthylethylene-substituted 4H-pyran derivatives
comprising two different end groups were synthesized. These two pairs of positional isomers with
aggregation-induced emission properties exhibit different stimuli-responsive solid-state
fluorescent properties. Both the linking position of the naphthylethylene and the
electron-withdrawing end group have a significant impact on the mechanochromic properties of
the compounds, whereas, the acidochromic properties are only related to the latter. The
transformation between amorphous and crystalline states accounts for the mechanochromic
properties and is attributed to loose molecular packing with weak intermolecular interactions, as
revealed by crystallographic data. The reversible acidochromic properties are mainly ascribed to
1
protonation/deprotonation of the carbonyl group, which is confirmed by H NMR experiments.
Additionally, because the emission of these 4H-pyran derivatives in solid state can be tuned by
external stimuli, several molecular logic gates were established by using appropriate stimuli
operations as the inputs and fluorescent colors as the outputs.
Keywords: Dicyanomethylene-4H-pyran derivatives, cyano(ethoxycarbonyl)methylene-4H-
pyran derivatives, aggregation-induced emission, mechanochromic properties, acidochromic
properties, molecular logic gates
1
. Introduction
In recent years, strongly fluorescent organic solid state materials that exhibit solid-state variable
fluorescent colors induced by an external stimulus have been of growing research interest because
they can be applied in sensing materials [1-9]. As strong emission in the solid state is required for
various stimuli-responsive behavior, organic molecules with aggregation-induced emission (AIE)
properties [10] have been considered very important sources for developing excellent
mechanochromic (MC), acidochromic (AC), vapochromic, and thermochromic materials in which
the external stimuli are pressure, pH, vapor, and temperature, respectively [1-3]. Additionally,
AIE-active molecules often have strongly twisted conformations that can hinder close
intermolecular stacking and intense π−π interactions. The twisted conformations are thought to
help change the molecular packing mode in response to external pressure, resulting in a
1

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crystalline−amorphous phase transformation and generating MC properties [2]. Attracted by the
fascinating application prospects, structure–property studies are necessary to gain insight into the
mechanism and influence factors of MC behaviors of AIE molecules. To date, some factors have
been demonstrated to play functional roles in modulating MC properties, such as the length,
position, and isomerization of alkyl chains [3,11-22], the introduction of heavy atoms [13-25], the
presence of different aromatic substituents [26,27], and position isomerism [28-30], etc.
Considering that MC organic materials are still in the initial stage of development, the tuning and
altering of the structures of AIE molecules are important to control access to high-contrast MC
materials.
4
H-Pyran has been proved to be a good building unit for constructing donor-π-acceptor (D-π-A)
fluorescent materials such as dicyanomethylene-4H-pyran (DCM) derivatives and
cyano(ethoxycarbonyl)methylene-4H-pyran (CEM) derivatives [31]. Recently, a series of DCM
derivatives with highly twisted conformations have been reported to exhibit AIE properties due to
the restriction of the intramolecular rotation (RIR) in the aggregated state [32-34]. Very recently,
we reported that some D-π-A-type indene-1,3-dionemethylene-4H-pyran derivatives displayed
noteworthy AIE properties and obvious alkyl-length-dependent chromic properties, such as MC
properties and solvent-induced emission changes in the solid state [35]. However, MC materials
based on DCM or CEM derivatives have rarely been reported until now. To enrich the
structure types used for MC materials and widen the application of 4H-pyran derivatives, we
designed and synthesized several D-π-A naphthylethylene-substituted DCM derivatives (5a and
5
b) and CEM derivatives (5c and 5d) (Scheme 1). Herein, the two compounds in each pair (5a
and 5b) and (5c and 5d) are positional isomers with the substitution of the naphthylethylene
changing from the 1-position to the 2-position, respectively. The structures of 5a [36] and 5b [37]
had been reported in the previous literatures, however, their possible AIE and MC, and AC
properties were not mentioned. The effect of the position of the naphthylethylene and the different
electron-withdrawing end groups on the photophysical properties of 5a–5d in the aggregated state
was investigated. Using crystallographic data, the target compounds were confirmed to have a
distorted conformation owing to the introduction of a naphthalene ring, and all of them were
AIE-active and had a weak fluorescence in tetrahydrofuran (THF) solution but enhanced emission
in THF/water mixtures. Interestingly, these two pairs of positional isomers exhibit different
solid-state stimuli-responsive fluorescent properties. The MC activities of these compounds are
related to not only the linking position of the naphthalene ring but also to the
electron-withdrawing end groups. External pressure did not change the solid-state emission colors
of 5b and 5c, whereas, 5a and 5d exhibited outstanding reversible MC properties. The reason for
this difference is discussed based on their crystallographic data. Additionally, as the 4H-pyran unit
+
may be protonated using the oxygen atom as the binding site for H , solid-state AC properties of
these compounds were expected. However, only the CEM derivatives 5c and 5d showed
reversible AC properties upon acid/base stimulation, which is only related to the
2

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electron-withdrawing end groups and is mainly ascribed to the protonation/deprotonation of the
carbonyl group. In particular, among these compounds, 5d containing 2-substituted
naphthylethylene exhibited intriguing solid-state multi-stimulus-responsive fluorescent behavior
induced by pressure, acid vapor, and solvent. Furthermore, because the emission of these
4
H-pyran derivatives in the solid state can be tuned by an external stimulus, several molecular
logic gates such as NOR and INHIBIT were established by using appropriate stimuli operations as
the inputs and fluorescent colors as the outputs.
Scheme 1. Synthetic routes to target compounds 5a–5d.
2
. Experimental part
2
.1. Measurements and materials
1
13
H and C NMR spectra were measured on a Bruker DRX 500 NMR spectrometer. C, H, and N
elemental analyses were measured on an Elementar Vario MICRO analyzer. Electrospray
ionization-mass spectrometry (ESI-MS) spectra were performed with a Finnigan DECAX-30000
LCQ Deca mass spectrometer. Fluorescence spectra were performed with a HITACHI F-7000
fluorometer. Absorption spectra were recorded on a Perkin-Elmer Lambda 25 spectrometer. X-ray
diffraction (XRD) patterns were measured on a Bruker D8 Advance X-ray diffractometer. The
measurements of the average particle sizes of the aggregates in solution were conducted on a
Zetasizer Nano ZS Laser Particle Size Analyzer. Absolute emission quantum yields (Ф ) of the
F
target compounds in solid and solution were performed with a FluoroMax-4 (Horiba Jobin Yvon)
fluorometer using an integrating sphere. Single-crystal X-ray diffraction measurements were
conducted on a Bruker-Nonius Smart Apex CCD diffractometer with graphite monochromated Mo
Kα radiation. 2,6-Dimethyl-4-pyrone (1), malononitrile (2), ethyl 2-cyanoacetate (3),
1
-naphthaldehyde, and 2-naphthaldehyde were obtained from commercial suppliers and used
directly.
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2
.2. Synthesis of compounds 5a–5d
A solution of compound 4a/4b (1.0 mmol), 1-naphthaldehyde/2-naphthaldehyde (937.2 mg, 6.0
mmol), piperidine (1.0 mL) and acetonitrile (6 mL) was heated at 80 °C under argon for 3 h (5a and
b) or 24 h (5c and 5d). After cooling down to room temperature, the mixture was poured in
5
methanol (50 mL) to precipitate out the crude product. The crude product was successively
washed with acetone and methanol five times and then dried to afford the pure target compound.
Characterization data for 2-(2,6-bis((E)-2-(naphthalen-1-yl)vinyl)-4H-pyran-4-ylidene)-
1
malononitrile (5a). Yellow solid (312.6 mg), 69.7% yield, m. p. 270−271°C. H NMR (500 MHz,
CDCl ): δ 8.53 (d, J = 15.5 Hz, 2H), 8.28 (d, J = 8.5 Hz, 2H), 7.97 (d, J = 8.5 Hz, 2H), 7.95 (d, J =
3
8
2
1
.0 Hz, 2H), 7.90 (d, J = 7.0 Hz, 2H), 7.61-7.58 (m, 6H), 6.96 (d, J = 15.5 Hz, 2H), 6.82 ppm (s,
1
3
H). C NMR (125 MHz, CDCl ): δ 158.0, 155.8, 134.7, 133.9, 131.6, 131.4, 131.0, 129.2, 127.2,
3
+
26.5, 125.7, 124.9, 122.7, 120.7, 115.0, 107.6 ppm. MS (ESI, m/z): 449.15 (M +H). Anal. Calcd
for C H N O: C, 85.69; H, 4.49; N, 6.25. Found: C, 85.39; H, 4.45; N, 6.20.
32
20
2
Characterization data for 2-(2,6-bis((E)-2-(naphthalen-2-yl)vinyl)-4H-pyran-4-ylidene)-
1
malononitrile (5b). Yellow solid (269.5 mg), 60.1% yield, m. p. 249−250°C. H NMR (500 MHz,
3
4
CDCl ): δ 8.05 (s, 2H), 7.94-7.91 (m, 4H), 7.88 (dd, J = 6.0 Hz, J = 3.0 Hz, 2H), 7.76 (d, J = 7.5
3
3
4
Hz, 2H), 7.74 (d, J = 16.0 Hz, 2H), 7.57 (dd, J = 6.0 Hz, J = 3.0 Hz, 4H), 6.91 (d, J = 16.0 Hz,
1
3
2
H), 6.75 ppm (s, 2H). C NMR (125 MHz, CDCl ): δ 158.2, 155.7, 138.1, 134.3, 133.4, 132.1,
3
1
29.8, 129.1, 128.6, 127.9, 127.6, 127.0, 123.1, 118.7, 115.1, 107.4 ppm. MS (ESI, m/z): 449.10
+
(
M +H). Anal. Calcd for C H N O: C, 85.69; H, 4.49; N, 6.25. Found: C H N O: C, 86.02; H,
3
2
20
2
32 20
2
4
.52; N, 6.21.
Characterization data for ethyl 2-(2,6-bis((E)-2-(naphthalen-1-yl)vinyl)-4H-pyran-4-
1
ylidene)-2-cyanoacetate (5c). Orange solid (357.8 mg), 72.2% yield, m. p. 272−273°C. H NMR
(
500 MHz, CDCl ): δ 8.45 (d, J = 15.5 Hz, 2H), 8.28 (d, J = 8.0 Hz, 2H), 8.19 (d, J = 2.0 Hz, 1H),
3
3
4
7
.95-7.92 (m, 4H), 7.85 (t, J = 6.5 Hz, 2H), 7.65-7.55 (m, 6H), 6.95 (dd, J = 16.0 Hz, J = 10.5
1
3
Hz, 2H), 6.87 (d, J = 2.0 Hz, 1H), 4.30 (q, J = 7.0 Hz, 2H), 1.38 ppm (t, J = 7.0 Hz, 3H).
C
NMR (125 MHz, CDCl ): δ 165.1, 157.7, 157.2, 153.1, 133.9, 133.2, 133.0, 132.1, 131.9, 131.3,
3
1
1
30.5, 130.4, 129.05, 129.04, 127.03, 127.02, 126.3, 125.73, 125.70, 124.62, 124.59, 122.9, 122.8,
+
22.0, 121.5, 118.7, 109.8, 108.3, 80.1, 60.7, 14.4 ppm. MS (ESI, m/z): 496.15 (M +H). Anal.
Calcd for C H NO : C, 82.40; H, 5.08; N, 2.83. Found: C, 82.89; H, 5.04; N, 2.81.
3
4
25
3
Characterization data for ethyl 2-(2,6-bis((E)-2-(naphthalen-2-yl)vinyl)-4H-pyran-4-
1
ylidene)-2-cyanoacetate (5d). Yellow solid (441.8 mg), 83.1% yield, m. p. 125−126°C. H NMR
(
500 MHz, CDCl ): δ 8.10 (d, J = 2.0 Hz, 1H), 7.98 (s, 2H), 7.91-7.84 (m, 6H), 7.71 (d, J = 8.5 Hz,
3
3
4
3
4
2
H), 7.63 (dd, J = 16.0 Hz, J = 5.0 Hz, 2H), 7.55-7.53 (m, 4H), 6.88 (dd, J = 18.0 Hz, J = 16.5
1
3
Hz, 2H), 6.77 (d, J = 2.0 Hz, 1H), 4.25 (q, J = 7.0 Hz, 2H), 1.35 ppm (t, J = 7.0 Hz, 3H).
C
NMR (125 MHz, CDCl ): δ 165.1, 157.8, 157.4, 152.9, 136.5, 136.2, 134.03, 134.01, 133.4, 132.4,
3
1
32.3, 129.38, 129.35, 128.92, 128.90, 128.46, 128.45, 127.8, 127.24, 127.21, 126.8, 123.1, 120.0,
4

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+
1
19.4, 118.7, 109.6, 108.1, 81.6, 60.6, 14.3 ppm. MS (ESI, m/z): 496.20 (M +H). Anal. Calcd for
C H NO : C, 82.40; H, 5.08; N, 2.83. Found: C, 82.82; H, 5.11; N, 2.85.
3
4
25
3
3
. Results and discussion
3
.1. Synthesis and features of 5a–5d
The synthetic route of 5a–5d is depicted in Scheme 1. 2-(2,6-Dimethyl-4H-pyran-
-ylidene)malononitrile (4a) [38] and ethyl 2-cyano-2-(2,6-dimethyl-4H-pyran-4-ylidene)acetate
4
(4b) [39] were obtained from the reaction of 2,6-dimethyl-4-pyrone (1) with malononitrile (2) and
ethyl 2-cyanoacetate (3), respectively, in acetic anhydride according to the literatures. The
Knoevenagel condensation of compounds 4 with 1-naphthaldehyde and 2-naphthaldehyde gave
1
compounds 5a–5d in the yields of 60-83%. The target compounds were characterized by H and
1
3
C NMR, mass spectrometry, and elemental analysis. Furthermore, the structures of 5b, 5c, and
d were confirmed by single-crystal X-ray diffraction analysis, which also indicated that these
5
target compounds were E isomeric form (see below). Compounds 5a–5d dissolved in organic
solvents such as toluene, CH Cl , CHCl , and THF but showed low solubility in water and
2
2
3
methanol.
3
.2. Photophysical properties of 5a–5d
1
.0
1.0
0.8
0.6
0.4
0.2
5
a
(
a)
(b)
5a
5
b
5
5
5
b
c
d
0
.8
.6
.4
.2
.0
5
c
5d
0
0
0
0
.0
0
4
50 500 550 600 650 700 750
3
50
400
450
500
550
Wavelength (nm)
Wavelength (nm)
−
5
Fig. 1. Normalized absorption (a) and fluorescence spectra (b) of 5a–5d in solution (THF, 1 × 10
mol/L).
Fig. 1 shows the absorption and emission spectra of 5a–5d in THF solution, and the
photophysical data are listed in Table 1. Compounds 5b and 5d display very similar absorption
spectra and have four absorption peaks in the range of 310–500 nm, whereas 5a and 5c show two
obvious absorption peaks and two shoulder peaks. Compounds 5a and 5c have similar
max
fluorescence spectra with the maximum emission wavelengths (λem ) centered at approximately
5
26 nm, whereas, 5b and 5d show maximum emission peaks at 516 and 521 nm, respectively.
Moreover, the optical energy gaps (E ) of 5a–5d obtained from the onset of the UV−vis absorption
g
edge (λonset) in THF solution were 2.505, 2.546, 2.510, and 2.551 eV, respectively. The 1-position
5

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naphthylethylene-substituted 5a and 5c have lower energy levels compared with 2-position
naphthylethylene-substituted 5b and 5d. The results indicated that the linking position of the
naphthalene ring had a more significant influence on the photophysical properties of these
compounds in THF solution compared with the electron-withdrawing end group.
a
Table 1. Photophysical data of 5a–5d in THF solution.
c
d
Compound
UV-vis
nm)
λ
em
λ
onset
E
g
(
(nm)
(nm)
495
487
494
486
(eV)
2.505
2.546
2.510
2.551
b
5
5
a
330, 421, 448 (s), 480 (s)
322, 416, 439, 470
487(s), 526
483, 516
b
5
c
326, 420, 445 (s), 476 (s)
319, 414, 438, 469
483(s), 525
479(s), 521
5
d
[
[
[
a] Standard deviation of the experimental data is shown in Supporting Information (Figs. S1-S2).
b] s = shoulder peak.
c] λonset was estimated from the onset of the UV−vis absorption edge.
[
d] E was estimated by using the equation: E (eV) = 1240/[λonset (nm)].
g
g
The UV-vis absorption spectra, fluorescence spectra, and absolute Ф values of 5a-5d in various
F
solvents, such as toluene, chloroform, dimethyl sulfoxide (DMSO), and N, N-dimethylformamid
(DMF), were further investigated (Figs. S3, S4, and Table S1, see Supporting Information). The
absorption spectra of all four compounds show no significant changes in different polar solvents,
indicating the polarity of the solvents has little effect on the ground states of these compounds.
max
The λem of 5a-5d displays 28, 5, 26, and 15 nm redshift with increasing solvent polarity from
toluene to DMF, respectively, indicating compounds 5a-5d show a certain degree of
solvatochromic properties, which should be attributed that strong polar solvents are beneficial to
the stability of the excited states of these compounds [40]. Moreover, it can be concluded that
1
-position naphthylethylene-substituted 5a and 5c exhibit larger red shift in emission spectra
compared with 2-position naphthylethylene-substituted 5b and 5d. Additionally, the Ф value in
F
weaker polarity solvent, such as toluene and chloroform, is higher than that of the other solvents
including THF, DMSO, and DMF, indicating weaker polarity solvent can result in stronger
emission.
For a better understanding of the UV-vis absorption properties of these compounds,
time-dependent density functional theory (TD-DFT) calculations were performed with ORCA
program [41], using Becke’s three-parameter set with Lee−Yang−Parr modification (B3LYP) with
the 6-311G** basis set. The calculated oscillator strength (f) and excitation energies are listed in
Table 2. By analyzing the wave functions from TD-DFT calculations, for all four compounds, the
largest contribution of the first excited state (S →S transition) and the second excited state
0
1
(
S →S transition) is mainly attributed to the transition from the HOMO-1 to LUMO and from the
0 2
HOMO to LUMO, respectively (Table 2 and Fig. 2). As shown in Table 2, the calculated
6

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excitation energies of the first excited states are well consistent in tendency with the E results
g
estimated from the onset of the absorption edge in THF solution, which validates the rationality of
the experimental results of UV-vis absorption spectra.
Fig. 2. Molecular orbital diagrams of the HOMO-1 (down), HOMO (middle), and LUMO (up) of
5
a–5d.
Table 2. TD-DFT Calculations of 5a–5d at B3LPY/6-311G** level.
a
b
Compound
Transition
Excitation energy
2.748
Main orbital transition (CIC)
HOMO-1→LUMO (0.966)
HOMO→LUMO (0.842)
HOMO-1→LUMO (0.902)
HOMO→LUMO (0.850)
HOMO-1→LUMO (0.936)
HOMO→LUMO (0.836)
HOMO-1→LUMO (0.876)
HOMO→LUMO (0.851)
f
5
a
S
0
S
0
S
0
S
0
S
0
S
0
S
0
S
0
→S
→S
→S
→S
→S
→S
→S
→S
1
2
1
2
1
2
1
2
0.258
0.507
0.241
0.657
0.263
0.579
0.260
0.731
2.903
5
b
2.837
2.987
5
c
2.798
2.950
5
d
2.869
3.014
[
a] CI expansion coefficients for the main orbital transitions.
b] Oscillator strength.
[
7

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4000
3000
2000
1000
0
(b)
0
20
40
60
80
100
Water fraction (volume %)
0.30
0.25
0.20
0.15
0.10
0.05
0.00
(c)
water vol. fractions
0
1
2
3
4
5
6
7
8
9
9
%
0%
0%
0%
0%
0%
0%
0%
0%
0%
9%
350
400
450
500
550
600
650
Wavelength (nm)
Fig. 3. (a) Fluorescence spectra of 5a in THF/water mixtures with different water volume fraction
(f ) values. Inset: photographs of solutions with a f of 0%, 60%, and 99% under 365 nm UV light.
w w
(
b) The effect of the f values on the maximum emission intensity of 5a in THF/water mixtures. (c)
w
−
5
Absorption spectra of 5a in THF/water mixtures with different f values. Concentration: 1 × 10
w
mol/L.
3
.3. AIE properties of 5a–5d
The AIE properties of compounds 5a–5d were investigated. We chose THF as a good solvent
and water as a poor solvent. The absorption and fluorescence spectra of these compounds in
THF/water mixtures with a different water volume fraction (f ) were studied. As shown in Fig. 3a,
w
the dilute pure THF solution of compound 5a exhibited a very faint green light, which was
ascribed to the nonradiative decay of the excited state owing to the free rotation of the naphthalene
ring and 4H-pyran ring through the carbon–carbon single bond [10,38,42]. The fluorescence
intensity of the solution showed a certain degree of enhancement with increasing f from 0 to 50%.
w
As water was gradually added to the THF solution of 5a, the dissolved molecules began to
aggregate and the intramolecular rotation was restricted, resulting in enhancement of the emission.
Meanwhile, the aggregation of the fluorescent molecules forced them to take a more planar
conformation and the emission wavelength of the mixture showed a redshift of 12 nm from 526 to
5
38 nm. A dramatic fluorescence enhancement and redshift of the emission wavelength behavior
were observed when f was 60%, and the fluorescence intensity reached a maximum value that
w
8

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was approximately 14 times higher than that in pure THF solution. The fluorescence intensities
first decreased and then increased as f was increased from 70 to 99%. The complex change in the
w
emission peak intensities at high water fractions for 5a has often been observed in the previous
literatures of AIE compounds [12,43-45], but the reasons were unclear. The possible reason for
this phenomenon is that at high f values the solute molecules might aggregate into bluer crystal
w
particles with stronger emissive and redder amorphous particles with weaker emissive,
respectively. Since the formation of nanoparticles was difficult to control at high f , the
w
fluorescence spectra displayed a complex change. The UV−vis absorption spectra of 5a in
THF/water mixtures are shown in Figure 3c. The absorption spectra exhibited a dramatic decrease
and leveled-off tails appeared in the visible region when f was up to 60-99%, which was ascribed
w
to light-scattering effects and suggested that a large number of aggregates were generated [46].
The results indicated that 5a possessed obvious AIE and color-tuning characteristics. Similar
results were observed for 5b-5d (Figs. S5–S7, see Supporting Information). The formation of the
aggregates in THF/water mixtures at high f values were confirmed the measurements of the
w
average particle sizes (Figs. S8-S11 and Table S2, see Supporting Information). The average
diameters of the aggregates of 5a at f = 60%, 70%, 80%, and 90% are ∼1395, ∼678, ∼384, and
w
∼
222 nm, and those of 5b, 5c, and 5d are in the range of 259-817 nm, 256-748 nm, 265-779 nm,
respectively, indicating that the aggregates of 5a-5d in the mixture at high fw are AIE
nanoparticles.
To gain a better understanding of the mechanism of AIE, we performed the solution cooling
experiments of 5a–5d in THF solution and thickening experiments in methanol/glycerol mixtures.
The fluorescence spectra of the THF and methanol/glycerol mixtures of compounds 5a–5d both
exhibited a significant enhancement with decreasing solution temperatures from 20°C to -78 °C
(
Fig. S12, see Supporting Information) and increasing amounts of glycerol (Fig. S13, see
Supporting Information), respectively. The results suggested that the solution thickening and
cooling processes could restrict the intramolecular rotation of the molecules and increase the
emissions of these compounds, suggesting that the AIE properties of the 4H-pyran derivatives
could be ascribed to the RIR process [47].
3
.4. MC properties of 5a–5d
The original solids of these compounds emitted strong yellow (5a) or orange (5b, 5c, and 5d)
fluorescence in the range of 563–589 nm (Fig. 4 and Table 3). When these original solid samples
were ground with a pestle in a mortar, the color change of the fluorescence was not obvious for 5b
and 5c, whereas a color change was clearly seen from yellow to orange for 5a and orange to red
for 5d, suggesting that 5a and 5d exhibited obvious MC properties (Fig. 5). In addition, after
grinding, the colors of the original solids of 5a and 5d under natural light changed from yellow to
orange (Fig. S14, see Supporting Information), which suggested that the grinding process changed
not only the fluorescence spectra but also the UV-vis absorption spectra. For a quantitative
9

ACCEPTED MANUSCRIPT
analysis of the fluorescent wavelength changes of 5a–5d after they were ground, the fluorescent
spectra in the solid state were performed (Fig. 6) and the corresponding emission data are listed in
max
Table 3. For 5b and 5c, the λem of their ground samples only exhibited a redshift of 1 and 3 nm,
respectively, compared to those of the corresponding original samples. Unlike 5b and 5c, a
redshift of 24 and 25 nm was observed for 5a and 5d, respectively. The results revealed that the
MC activities of these compounds were related to not only the linking position of
naphthylethylene but also to the electron-withdrawing end groups. For 5a, we investigated the
changes of its solid-state emission spectra and colors upon different external stimuli (Fig. 5).
When annealing the ground sample of 5a at a temperature (100 °C) for 30 min, or exposing it to
chloroform vapor at room temperature, the emission returned to that of the original sample,
indicating reversible MC behavior. Because of its good reversible MC properties, 5a could be used
as a simple and convenient erasable board (Fig. 5), suggesting potential application in rewritable
optical media. Similar color changes under natural light and 365 nm UV light were observed in 5d
under particular stimuli. Interestingly, crimson-emitting solids were obtained when the original
sample of 5d was dissolved in THF or chloroform and then evaporated to dryness by vacuum
rotary evaporation. The emission wavelengths of the solids exhibited a redshift of 111 and 89 nm
to 700 and 678 nm, respectively, which was similar to MC properties and was attributed to
solvent-induced solid-state emission changes caused by a dissolution–desolvation process (DDP)
[35,42,48,49]. However, ethyl acetate (EA) had little influence on the fluorescent color of the
original sample and was used as organic vapor to recover the ground sample by fuming. The
results suggested that aggregates of different morphologies were generated in the treatment of
DDP using different solvents and that solid-state 5d could be developed as a fluorescent sensor for
the detection of volatile organic molecules.
1.0
0.8
0.6
0.4
0.2
0.0
5
a
5b
5
c
5d
500
550
600
650
700
750
Wavelength (nm)
Fig. 4. Normalized fluorescence spectra of the original samples of the 5a–5d solids.
1
0

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Fig. 5. Photographs of 5a–5d solid samples taken under 365 nm UV light: (a) original sample; (b)
ground sample; (c) fumed sample (chloroform for 5a and ethyl acetate for 5d); (d) annealed
sample; (e) and (f) DDP samples obtained using THF and chloroform as a solvent, respectively; (g)
photographs of writing and erasing on an erasable board with 5a coated on filter paper under
natural light (NL) and 365 nm UV light (UV).
1
.0
0.8
.6
1.0
0.8
0.6
0.4
0.2
0.0
Original
Ground
5b
5
a
Original
Ground
Annealed
Fumed
0
0.4
0.2
0.0
500 550 600 650 700 750
450 500 550 600 650 700 750
Wavelength (nm)
Wavelength (nm)
1
.0
Original
Ground
Annealed
Fumed
DDP-chloroform
DDP-THF
1.0
0.8
0.6
0.4
0.2
0.0
5d
5
c
Original
Ground
0.8
0.6
0.4
0.2
0.0
5
00 550 600 650 700 750
500 550 600 650 700 750 800
Wavelength (nm)
Wavelength (nm)
Fig. 6. Fluorescence spectra of 5a–5d solid samples.
1
1

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Table 3. Emission wavelengths (nm) of 5a–5d in the solid state under various conditions.
a
Compound
λOriginal
563
λGround
587
∆λMC
24
3
λFumed λAnnealed λDDP-chloroform
λDDP-THF
5
5
a
563
564
-
-
-
-
-
b
b
588
591
-
5
c
572
573
1
-
-
-
-
5
d
589
614
25
567
594
700
678
[
a] ∆λMC = λGround-λOriginal
.
[b] no detection.
3
2
1
000
000
000
0
5
a
6000
5b
Original
5000
4000
3000
2000
3000
2000
1000
0
Ground
Annealed
Fumed
Original
Ground
1
000
3000
2000
1000
0
6
000
5000
000
4
3000
2000
1000
0
3000
2000
1
000
1
0
15
20
25
30
35
10
15
20
25
30
35
40
2θ/deg
2θ/deg
6
5
4
3
2
1
000
000
000
000
000
000
5000 5d
2500
5
c
Original
0
5
2
000
500
0
Ground
Annealed
Fumed
Original
5
2
000
500
0
6
5
4
3
2
1
000
000
000
000
000
000
5
2
000
500
0
5000
2500
0
Ground
DDP-chloroform
1
0
15
20
25
30
35
40
10
15
20
25
30
35
2θ/deg
2θ/deg
Fig. 7. XRD curves of 5a–5d samples.
To gain a better understanding of the MC behavior and solvent-induced solid-state emission
changes led by the DDP, XRD measurements of the solid samples of 5a–5d were conducted (Fig.
7
). As shown in Fig. 7, the diffraction curves of the original samples of 5a and 5d had intense
reflection peaks, which are the characteristic of the well-ordered microcrystalline structures,
whereas the reflection peaks of their ground samples and the DDP-chloroform sample of 5d
weakened significantly or disappeared, suggesting obvious amorphous features. The results
indicated that the original sample underwent a morphological change from a crystalline state to an
amorphous state upon external stimuli. Accordingly, the treated sample exhibited a lower ФF
(Table S3, see Supporting Information) and a bigger emission wavelength (Table 3) than those in
the original state. The diffraction peaks and the crystalline structures were recovered after being
fumed or annealed, which resulted in a blueshift in the fluorescence spectra. In contrast, the
1
2

ACCEPTED MANUSCRIPT
reflection peaks of the ground samples of 5b and 5c were still sharp and intense, indicating there
were no significant changes in morphology. These results indicated that the MC properties or
solvent-induced emission changes of 5a and 5d could be ascribed to the transformation between
amorphous and crystalline states.
We investigated the time-resolved emission decay experiments of the solid samples of 5a–5d
under various conditions (Table S3, see Supporting Information). Two relaxation pathways, a slow
channel and a fast channel, were observed for the fluorescence decay of the excited states of
5
a–5d original samples, which suggested that the time-resolved fluorescence spectra of these
compounds included independent emissions from the segments with different π-conjugated
lengths [43,45]. In the case of 5b and 5c, the weighted mean lifetime (<τ>) value of the original
sample was similar to that of the ground sample. In contrast, the <τ> values of the original and
ground sample of 5a were 1.85 and 1.16 ns, respectively. For 5d, the <τ> values were 0.74, 0.94,
and 1.09 ns for the original, ground, and DDP sample using chloroform as a solvent, respectively.
The results indicated that the time-resolved emission decay behavior of the compounds with
altered-morphology-induced emission exhibited obvious changes under different conditions,
which was in agreement with the results reported for other MC materials [43,45].
Fig. 8. Single-crystal structures and torsion angles of 5b (a), 5c (b), and 5d (c).
3
.5. Crystal structures
The single crystal X-ray crystallographic data of these compounds can be used to gain a better
1
3

ACCEPTED MANUSCRIPT
understanding of the relationship between the optical properties and the molecular conformation
and packing mode. Single crystals of 5b, 5c, and 5d suitable for X-ray structural analysis were
obtained by slow evaporation from CHCl /EA, CH Cl /CH OH, and EA/DMSO mixtures,
3
2
2
3
respectively, at room temperature (Fig. 8) [50]. The unit cell of 5b was triclinic with space group
Pī and that of 5c and 5d was monoclinic with space group P2(1)/c (Table S4, see Supporting
Information). As revealed by the crystal structure of 5b, the dihedral angles between the central
4
H-pyran ring (O1, C13, C14, C15, C16, and C17) and the naphthalene rings (C1-C10) and
o
o
(
C20-C29) were 7.2 and 3.3 , respectively. The torsion angles between the naphthalene rings and
o
o
the two ethylene units were 7.1 and 1.3 , respectively (Fig. 8 and Table S5, see Supporting
Information). In the case of 5c, the dihedral angles between the 4H-pyran ring and the naphthalene
o
o
rings were 9.0 and 1.6 , and the torsion angles between the naphthalene rings and the two
o
o
ethylene units were 2.3 and 7.6 , respectively. Interestingly, there were two independent
molecules (labeled as structure A and structure B) present in the crystal of 5d. The two molecules
had different conformations with different torsion angles. Compared with structure A, structure B
had a more twisted conformation in which the torsion angles between the ethylene unit and the
naphthalene ring (C35-C44) and the 4H-pyran ring (O4, C47, C48, C49, C50, and C51) were
o
o
2
7.1 and 15.4 , respectively. The results indicated that these compounds have a twisted
conformation, which can significantly weaken the fluorescence quenching resulted from the
formation of an intense π–π stacking effect in the aggregated state and then enhance the
fluorescence. We further performed density functional theory (DFT) calculations to gain the
lowest energy spatial conformation of these compounds (Fig. S15, see Supporting Information)
[41]. By comparison with the optimized structures, the molecular structure of 5c in the crystals has
smaller torsion angles, whereas, those of 5b and 5d have bigger torsion angles, which indicated
that intermolecular interactions could regulate the distortion degree of the molecular structures in
the aggregated state.
We investigated the molecular stacking modes of crystals 5b, 5c, and 5d. The molecules of 5b
arranged in one column were stabilized by weak π–π stacking interactions and C–H···π bonds
from adjacent molecules (Fig. 9), adopting a head-to-tail packing pattern in which the naphthalene
ring stacked with the 4H-pyran ring or another naphthalene ring in an offset face-to-face manner.
The distances between the naphthalene ring and the 4H-pyran ring in the upper and lower layers
were measured to be 3.621 and 3.800 Å, respectively, suggesting the existence of weak π-π
stacking in the crystal. Four aromatic C–H···π bonds were formed between the two naphthalene
rings (3.526 Å) and six aromatic C–H···π bonds were formed between naphthalene rings and
cyano triple bonds (3.267 and 3.354 Å). Furthermore, another C–H···π bond (2.763 Å), in which
the naphthalene ring donates a hydrogen atom and cyano group acts as the acceptor, restricted the
adjacent molecules of 5b between the two columns. The molecules of 5c packed in a herringbone
o
arrangement and the herringbone angle was 60.82 (Fig. 10). The intermolecular distances were
6
.386 Å and 11.822 Å, respectively, which were difficult to form the π–π interaction. The
1
4

ACCEPTED MANUSCRIPT
molecules of 5c in the same layer were restricted by two aromatic C–H···N bonds (2.642 Å)
between the β-H of the naphthalene moiety and the N atom in the cyano group, two aliphatic
C–H···N bonds (2.629 Å) between the H atom of the olefinic double bond and the N atom of the
cyano group, and van der Waals forces (with a distance of 2.221 Å) between the aromatic
hydrogen atoms in the naphthalene rings. In addition, aliphatic C–H···O bonds (2.519 Å) between
the H atom of the methyl chain and the O atom of the carbonyl group restricted the adjacent
molecules of 5c between the two columns of the herringbone arrangement. As mentioned above,
the multiple molecular interactions in the crystal structures of 5b and 5c not only rigidify the
conformation, which would enhance the emission in the aggregated state, but also induce tight
intermolecular packing in the crystal lattice, which might be the main reason that their crystalline
structures are difficult to be destroyed and thus they do not show MC properties after grinding.
Fig. 9. Schematic intermolecular interactions and packing motif of 5b crystal.
Fig. 10. Schematic intermolecular interactions and packing motif of crystals 5c (a, b) and 5d (c,
d).
1
5

ACCEPTED MANUSCRIPT
By comparison with 5b and 5c, the structure A and structure B in the crystal of 5d stack in two
adjacent columns in an offset face-to-face manner, respectively. For the structure A, there are
weak π–π interactions (3.867 and 3.655 Å) between the upper and lower layers. For the structure
B, weak π–π interaction (3.822 and 3.762 Å) and aromatic C–H···π bonds (2.841, 2.936, and 3.227
Å) restrict the molecules of the upper and lower layers. Additionally, one C–H···N bond (2.684 Å)
between the H atom of the naphthalene moiety and the N atom of the cyano group restricts these
two independent structures in the same layer. The offset face-to-face stacking and weak
intermolecular interactions in the crystal of 5d result in loose molecular packing. Attempts to grow
single crystals of 5a suitable for crystallographic analysis were unsuccessful, however, we can
conclude that 5a adopted a loose molecular packing mode because 5a had weak and broad
reflection peaks in the XRD measurements similar to those of 5d, which were obviously different
from the strong and sharp reflection peaks of 5b and 5c with tight intermolecular packing modes
(
Fig. 7). Herein, the loose molecular packing modes of 5a and 5d should result
from the joint action from the electron and steric effects of the dicyanomethylene unit and the
naphthylethylene unit. As a result, the loose molecular packing of 5a and 5d leads to easy
destruction of their crystalline aggregates by planarization of the molecular conformation or slip
deformation under external pressure, resulting in MC properties.
3
.6. AC properties of 5a–5d
We investigated the possible solid-state emission change of 5a–5d with response to acid vapor.
When the original solid samples of 5a–5d were exposed to trifluoroacetic acid (TFA) vapor,
fluorescence color changes were not observed for 5a and 5b (Fig. 11). However, a color change
from orange to red was clearly observed for 5c under 365 nm UV light and the corresponding
maximum emission wavelength was redshifted by 22 nm from 572 to 594 nm (Fig. S16),
suggesting that 5c exhibited obvious AC properties. Furthermore, the fluorescence spectra of the
acid-fumed sample was nearly returned to the original form when it was exposed to triethylamine
(
TEA) vapor for approximately 5 min or by standing at room temperature for approximately 8 min
owing to the volatility of TFA. The results indicated that the two-channel emissions of solid-state
c could be switched by exposing to acidic and basic vapor. Similar to that of 5c, the fluorescence
5
color of 5d also showed a change from orange (589 nm) to red (605 nm) when its original sample
was exposed to TFA vapor (Fig. 11 and Fig. S16, see Supporting Information). However, the
recovering of the emission of 5d acid-fumed sample could not be recovered by just standing at
room temperature. In particular, when the acid-fumed sample of 5d was ground with a pestle in a
mortar, its fluorescence color changed from red to deep red and restored to the original form after
exposure to TEA vapor, whereas, grinding did not further change the fluorescent color of
acid-fumed sample of 5c. As mentioned above, it can be concluded that the AC properties of 5c
and 5d were only related to the electron-withdrawing end groups in these cases, independent of
1
6

ACCEPTED MANUSCRIPT
the linking position of the naphthylethylene unit. When the words “Chem” and “material” were
written on the surface of the original solids of 5c and 5d coated on filter paper using a cotton swab
tip with acid liquor, respectively, the color changes occurred only on the written area and
the characters disappeared after fuming with TEA vapor, indicating that 5c and 5d could be
developed as rewritable fluorescent optical recording media.
Fig. 11. Photographs of 5a–5d solid samples taken under 365 nm UV light: (a) original sample; (b)
fumed sample exposed to TFA vapor; (c) fumed sample exposed to TEA vapor; (d) obtained from
the protonated sample by standing; (e) obtained from the protonated sample by grinding; (f)
photographs of writing and erasing on an erasable board with 5c and 5d coated on filter paper.
To find out the reason of the solid-state color change of 5c and 5d caused by TFA, we
1
investigated the changes of the H NMR spectra of 5d before and after the addition of TFA. The
addition of TFA resulted in downfield shift of the H and H signals in the 4H-pyran ring and the
1
2
H and H signals in the ethylene unit (Fig. S17, see Supporting Information), which could be
3
4
ascribed to the binding of a hydrogen ion to the O atom of the 4H-pyran ring. Additionally, the
signals of the hydrogen atoms of the COOCH CH group exhibited upfield shifts, indicating the
2
3
protonation of the carbonyl groups. Although protonation of the 4H-pyran ring in 5b upon the
1
addition of TFA was also evident according to its H NMR spectra in CDCl (Fig. S18, see
3
Supporting Information), 5b did not show AC properties, indicating that the protonation of the
carbonyl group played the key role in tuning the AC behavior of 5d. We further measured the
emission changes of 5d and 5b in the CHCl solution with the response to the addition of TFA
3
(
Fig. S19, see Supporting Information). It was found that the fluorescence spectra of 5d in the
1
7

ACCEPTED MANUSCRIPT
solution showed an obvious enhancement and redshift from 524 to 554 nm upon the gradual
addition of TFA, and the change in optical properties induced by TFA could be recovered by the
addition of TEA. In contrast, although the addition of TEA also led to the fluorescence
enhancement of 5b owing to the inhibition of photoinduced electron transfer caused by the
protonation of the O atom with the nonbonding electron pair in the 4H-pyran unit, there was no
obvious redshift of the emission wavelength. These results further indicated that the protonation of
the carbonyl group in 5d contributed to the redshift of the emission wavelength as a result of its
AC properties.
Considering that the acid-fumed sample of 5d showed a fluorescence color change from red to
deep red after grinding, XRD measurements of 5d were obtained after treatment with TFA,
pressure, and TEA (Fig. S20, see Supporting Information). After the original sample had been
o
fumed by TFA vapor, most of the diffraction peaks disappeared (except for one peak at 2θ = 15.2 )
because TFA was able to dissolve this compound and destroy the crystal structure. The residual
diffraction peak disappeared after the fumed sample was further ground, indicating that the
crystalline−amorphous phase transformation also contributed to the fluorescence switching
property of 5d. Similar phenomena were observed in 5c (Fig. S20, see Supporting Information),
except that TFA vapor had maximally destroyed its crystalline structure, and further grounding of
the sample did not change the fluorescence color.
3
.7. Molecular logic gates
Recently, a large number of molecular logic gates based on organic molecules have been
developed by using fluorescent sensors having different recognition sites, which show different
response patterns to different guest molecules [31,51-55]. However, the development of molecular
logic gates based on solid-state materials has rarely been reported until now [56]. Herein, based on
the different response behavior of the 4H-pyran derivatives in the solid state upon different
external stimuli, several molecular logic gates such as INHIBIT and NOR gates were established
by using appropriate operations as the inputs and fluorescent colors as the outputs. Fig. 12a shows
an INHIBIT logic gate incorporating an AND gate with a NOT gate based on compound 5a. The
INHIBIT logic gate was obtained using the grinding operation and the fuming operation
(chloroform) as the inputs. Only the grinding operation gave the red fluorescence and this output
was assigned to be ‘1’. The other operations gave yellow fluorescence, such as the absence of the
grinding operation or the presence of the fuming operation, and this output was assigned to ‘0’.
Fig. 12b shows a NOR logic gate based on compound 5d by combining a NOT gate with an OR
gate and using the grinding operation and the DDP (using chloroform as a solvent) as the inputs.
In the presence of grinding and/or the DDP, the original solids of 5d exhibit red fluorescence,
defined as output ‘0’, only the condition without two inputs (0, 0) results in orange fluorescence
(‘1’ state). For compound 5c, when utilizing TEA vapor and TFA vapor as the inputs, another
INHIBIT logic gate was obtained: red fluorescence in the presence of only TFA vapor was defined
1
8

ACCEPTED MANUSCRIPT
as output ‘1’, while orange fluorescence obtained from the other operations was assigned to output
‘0’ (Fig. 12c).
Fig. 12. Molecular logic gates obtained by using the fluorescence properties of the 4H-pyran
derivatives in solid states: (a) IMPLICATION logic gate based on 5a; (b) NOR logic gate based on
5
d; (c) IMPLICATION logic gate based on 5c.
4
. Conclusion
Several 4H-pyran derivatives with distorted conformations were synthesized to investigate the
effect of the linking position of naphthylethylene and different electron-withdrawing end groups
on their photophysical properties in the aggregated state. Interestingly, these two pairs of
positional isomers with AIE properties exhibit different stimuli-responsive fluorescent properties
in the solid state. The 1-position naphthylethylene-substituted DCM derivative 5a and 2-position
naphthylethylene-substituted CEM derivative 5d exhibited outstanding reversible MC properties,
whereas, external pressure did not change the solid-state emission colors of 5b and 5c. The results
indicate that both the linking position of naphthylethylene and the electron-withdrawing end group
have a significant impact on their MC properties. The results of XRD experiments suggest that the
MC properties of 5c and 5d should be ascribed to the transformation between an amorphous state
and a crystalline state under external stimuli. In contrast to 5b and 5c, which show tight
intermolecular packing in the crystal lattice, the molecules in the crystal of 5d adopt a loose
molecular packing mode with weak intermolecular interactions. This allows the crystalline
aggregates to be easily destroyed and results in MC properties. Additionally, compared with DCM
derivatives, the CEM derivatives 5c and 5d show reversible AC properties upon acid/base stimuli
owing to the protonation/deprotonation of the carbonyl group, which is only related to the
electron-withdrawing end groups. As
a
result, 5d exhibits intriguing solid-state
multi-stimulus-responsive fluorescent behavior induced by pressure, acid vapor, and solvent.
1
9

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Besides their potential application in rewritable optical media and fluorescent sensors, these
4
H-pyran derivatives in the solid state can also be used to establish molecular logic gates such as
INHIBIT and NOR gates owing to different responsive behavior upon different external stimuli,
using appropriate stimulus operations as the inputs and fluorescent colors as the outputs.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (Grants
2
1572165, 21474048, and 21272176) and the Zhejiang Provincial Natural Science Foundation
(Grant LY16B040005).
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Highlights:
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►
►
►
Synthesis and characterization of four AIE-active 4H-pyran derivatives.
Two pairs of positional isomers exhibit different stimuli-responsive solid-state fluorescent properties.
These compounds show mechanochromic and/or acidochromic properties.
These fluorescent sensors can be used to establish molecular logic gates.