Facile synthesis and separation of: E / Z isomers of aromatic-substituted tetraphenylethylene for investigating their fluorescent properties via single crystal analysis

Article abstract of DOI:10.1039/c9tc00446g

The intermolecular interactions and molecular packing form of fluorescent molecules have a huge impact on their optical properties, especially for AIE molecules. As a class of typical AIE molecules, tetraphenylethene (TPE) and its derivatives have prominent optical properties; nevertheless, separation of mixtures of E/Z isomers is a great challenge. Herein, a series of aromatic-substituted TPE derivatives were synthesized and used to separate mixtures of E/Z isomers by common column chromatography with high yields, as confirmed by single crystal analysis, mass spectrometry and NMR spectroscopy. The structure-property relationships of these molecules were systematically investigated by a combination of spectroscopic methods, theoretical calculations and single crystal data analysis. E/Z isomers exhibit many different fluorescent properties, such as AIE and mechanochromic behavior. Moreover, the position of N on the substituted pyridine ring also has an effect on the molecule stack pattern and the fluorescent properties. Collectively, our findings could not only improve the fundamental understanding of the cis/trans isomerization and photophysical properties of TPE derivatives but also provide a good strategy for designing different substituted groups that can produce various functions and have more potential applications.

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Facile synthesis and separation of E/Z isomerization of aromatic‐
substituted tetraphenylethylene for investigating their fluorescent
properties via single crystal analysis
Received 00th January 20xx,
Accepted 00th January 20xx
Zhixiang Lu, Shaoxiong Yang, Xiaolan Liu, Yu Qin, Shuhan Lu,Yanxiong Liu, Ruidun Zhao, Liyan
Zheng*and Hongbin Zhang *
DOI: 10.1039/x0xx00000x
www.rsc.org/
The intermolecular interactions and molecular packing form of fluorescent molecules have huge impact on their optical
properties, especially for AIE molecules. As a class of typical AIE molecule, tetraphenylethene (TPE) and its derivatives have
prominent optical properties; nevertheless, separation of the mixtures of E/Z isomers is a great challenge. Herein, a series
of aromatic ‐substituted TPE derivatives have been synthesized and used to separate the mixtures of E/Z isomers by common
column chromatography with high yields, as confirmed by single crystal, mass spectrometry and NMR spectroscopy. The
structure‐property relationships of these molecules have been systematically investigated by the combination of
spectroscopic methods, theoretical calculations and single crystal data analysis. The E/Z isomers exhibited many different
fluorescent properties, such as AIE and mechanochromic behavior. Moreover, the position of N on the substituted pyridine
ring also has an effect on the molecule stack pattern and the fluorescent properties. Collectively, our findings could not only
improve fundamental understanding of the cis/trans isomerization and photophysical properties of TPE derivatives but also
provide a good strategy for designing different substituted groups that can produce various functions and have more
potential applications.
1. Introduction
their isomerization are thus not only useful for fundamental
studies but also offering a unique opportunity to explore their
Stereochemistry plays a crucial role on the functional materials practical applications.
and life sciences in modern organic chemistry
[1-3]
.
The
The intermolecular interactions and packing model were also
stereochemistry of double bonds holds great implications for affected by their stereochemistry, since they have huge impact on
drug discovery and dye development [4-6]. Isomers that have the their optical properties, especially for AIE molecules [22-26]. It has
same molecular formula but different geometrical positions in been revealed that the optical properties can be modulated by
space can show entirely different properties [7-9]. Drugs with cis varying the molecular interactions, such as π‐π stacking, H‐/J‐
and trans isomers have been reported to differ in their aggregation or hydrogen bonding. Another intriguing property of
pharmacodynamic and pharmacokinetic properties [10-11]. For the responsive optical materials was mechanochromic, that is,
example, trans‐tamoxifen is
a potent estrogen receptor the emission colors or intensity of materials can change when
antagonist, which is used for the treatment of breast cancer, they suffer from appropriate external force stimulus such as
however, cis‐tamoxifen is a full estrogen agonist [12]. The double grinding, shearing, pressing, and crushing, which lead to the
bonds are also built‐in elements of fluorescent dyes whose cis and changes in intermolecular interactions. In general, mechanical
trans isomers display different optical properties [13-15]. By forces can change the pristine packing of molecules, including
controlling the cis/trans isomerization under special stimulation, phase transitions from the crystalline state to the amorphous
a variety of applications can be achieved [16-18]. As the most state, or transition from one crystal stage to another, and the
classical smart molecules with light responsive cis/trans change of intermolecular interactions or molecular structures will
isomerization, azobenzene and derivatives have been induce mechanochromic appearance [27-30]
demonstrated for widespread applications such as light‐driven
.
Tetraphenylethene (TPE) and its derivatives are a class of
molecular logic gates, data recording and storage, and chemical fluorescent molecules with significant AIE effect, properties with
wiring [19-21]. For these attractive properties and important efficient light emission in the aggregate state, which have
applications, the development of synthesis and separation attracted tremendous researchers to conduct extensive design
techniques to obtain pure isomers for elucidating the and application studies [31-35]. Thus far, multiple functional group‐
characteristic properties of cis and trans geometric isomers and substituted TPE derivatives have been applied to bioimaging,
biosensing, optoelectronic devices and therapeutic applications
[36-40]. Furthermore, the design and synthesis based on TPE and its
derivatives with mechanochromic property have become other a
hot research field in recent years [41-43]. Rajneesh Misra group
reported that TPE substituted pennanthroimidazoles was used as
a highly effective mechanochromic material, and further to study
the positional effect of the TPE unit by altering the position of the
phenyl unit on the imidazole [44]. Nowadays, TPE and its
derivatives are usually synthesized by the McMurry coupling
Key Laboratory of Medicinal Chemistry for Natural Resource (Yunnan University),
Ministry of Education, Functional Molecules Analysis and Biotransformation key
laboratory of Universities in Yunnan Province, School of Chemical Science and
Technology, Yunnan University, Kunming, Yunnan 650091, China
E‐mail: zhengliyan@ynu.edu.cn (L. Zheng), zhanghb@ynu.edu.cn (H. Zhang)
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
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reaction. However, the product is the mixture of cis and trans
Here, a series of TPE derivatives with aromatic‐substituted
isomers due to the unsymmetrical coupling reaction [45]. Owing to small‐groups were designed and synthesized by introducing high
these difficulties, most articles researched the mixtures of cis and polar substituents (pyridine, thiazole, oxazole) and smaller polar
trans isomers, and these mixtures have compromising effect on substituents
（benzene, naphthalene, thiophene）, respectively.
Moreover, the compound which was introduced with high polar
[46-47]
the functions and performance in various applications
.
However, there are only a few pure cis/trans isomers have been
separated and utilized for research. Tang et al. have reported a
number of alkoxy‐substituted TPE derivatives and demonstrated
their morphology‐dependent multicolor emissions by different
external stimuli in a solid state [48]. Separating the E/Z isomers of
TPE is a huge challenge. After the successful separation, the
confirmation of cis/trans geometry is another challenging task.
They have synthesized TPE‐UPy and separated the isomers with
high yield by column chromatography, and the structures of (Z) ‐
TPE‐UPy and (E) ‐ TPE‐UPy are confirmed by 2D COSY and NOESY
NMR spectra [49]. Wu et al. synthesized and purified the E/Z
isomers of oxetane‐substituted TPE derivatives by column
chromatography. Their geometries are also confirmed by NOESY
and COSY NMR spectroscopy [50]. Single crystal investigation is the
most powerful and convincing tool to reveal the structure and
packing model of the molecule. However, most of them did not
obtain the single crystal structure of the pure E/Z isomers, which
led to difficulties in further research for clarifying their structure‐
property relationships.
Since the AIE effect and stimuli‐responsive property have
significantly changed on account that intermolecular interactions
and cis/trans isomerization (E/Z isomerization) have different
arrangement in space, the AIE phenomenon and stimuli‐
responsive property of E/Z isomerization are obviously different
due to the intermolecular interaction. In addition, the substituted
groups have great influence on the intermolecular interaction,
resulting in a large change in the optical properties. The E/Z
isomerization and different substituted groups are very important
to optical properties study of fluorescent materials, and also
provide a new strategy for expanding their research. Therefore, it
is particularly important and urgent to synthesize and separate
the E/Z isomers with a facile method to acquire the accurate
structure of pure E/Z isomers, which can be beneficial to
investigate structure‐property relationships to design functional
molecules for various applications on‐demand.
Fig. 1 Crystal structures of TPE‐2by‐3‐E (A) and TPE‐2by‐3‐Z (B).
NOESY‐NMR of TPE‐2by‐3‐E (C) and TPE‐2by‐3‐Z (D) (enlarged).
Single crystal fluorescence pictures of TPE‐2by‐3‐E (E) and TPE‐2by‐
3‐Z (F) under the fluorescent inverted microscope.
Scheme
1.
Synthetic
route
to aromatic‐
substituted tetraphenylethylene.
2 | J. Name., 2012, 00, 1‐3
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substituents can be more efficiently separated and purified to
obtain the pure E/Z isomers by macro chromatography. It is
gratifying to note that each pure E/Z isomers can obtain a single
crystal structure, which is confirmed by single crystal
measurement, 2D COSY and NOESY NMR spectroscopy. The
structure‐property relationships of these molecules have been
systematically investigated with the combination of spectroscopic
methods, theoretical calculations and single crystal data analysis.
These results revealed that both E/Z configurations and the
position of N on the substituted pyridine ring have an important
effect on the molecular packing form, fluorescent property of AIE
and mechanochromism activity.
for correction. According to this assignment and the explicit
correlations in the NOESY spectrum (Fig. S61), the protons with δ
= 8.51 and 8.50 ppm are attributed to H1, H2 and H15, H16,
respectively, because of their strong correlation with H3, H4 and
H17, H18. These protons signal with δ = 7.31 and 7.30 ppm are
attributed to H3, H4 and H17, respectively, because of their
strong correlation with H5 and H19. The resonance at δ = 6.60 and
6.61 ppm are assigned to H6, H7, H20, and H21 because of their
strong correlation with H5 and H19. The resonance at δ = 6.85 and
6.86 ppm belong to H8, H9, H22, and H23, δ = 6.96 and 6.95 ppm
are attributed to H10, H11, H24, and H25, δ = 7.12 and 7.14 ppm
are ascribed to H12, H13, H14, and H26, H27, H28, respectively.
As shown in Fig. 1C and Fig. S61, the cross‐peaks in the blue circles
indicate obvious correlations between H10/H11/H24/H25 and
H8/H9/H22/H23, which indicates that ring A and ring C are close
2. Results and discussion
2.1 Design, synthesis and characterization of all compounds.
Although most of the synthetic TPE derivatives by McMurry
coupling are highly efficient and versatile, the reaction products
are usually the mixture of E/Z isomerization. Based on the in space, and means they are on the same side of the double
similarity in the molecular structure and the polarities of the
steric conformation, TPE that links the high polar groups to the
periphery may produce sufficient quadratic properties in the
structure and polarity which contribute to achieving the
macroscopic separation of their stereoisomers. To this point, we
designed the structure of the functionalized TPE molecule and
achieved its preparation via the synthetic route as shown in
bond. However, there are no correlations for those hydrogens in
COSY NMR spectroscopy (Fig. S62−S63). In the meanꢀme, we
cannot find clear correlations between H10/H11/H24/H25 and
H8/H9/H22/H23 in Fig. 1D; because ring A/C are far from ring B/D
in Z isomers (Fig. S64−S66). Consequently, the NOESY and COSY
NMR spectroscopy confirmed the geometrical structures of the
Scheme 1.
A
mixture of 1,2‐bis(4‐hydroxyphenyl)‐1,2‐
diphenylethylene (TPE‐2OH) E/Z stereoisomers were prepared by
McMurry coupling. In the next step, E/Z isomerization were
synthesized by the one‐pot method with nucleophilic substitution
reaction between hydroxyl and bromine. We have found that
when high polar groups such as pyridine, thiazole and oxazole
were introduced, two dots were observed on silica gel plate and
the products could be easily separated by common column
chromatography. Interestingly, high mass spectrometry showed
that the molecular weights of the two new compounds were the
same as the one of target molecule, which obviously proves the
presence of E/Z isomerization (Fig, S6,9,12,15,18,21,24,27 and
30). However, when the low polar benzene, thiazole, or
naphthalene were introduced to the periphery of TPE, only one
dot was observed on silica gel plate, indicating the E/Z
isomerization mixture could not be separated by common column
chromatography.
The E/Z isomers of the two new compounds cannot be
distinguished according to 1H NMR spectroscopy, 13C NMR
spectroscopy and FTIR spectroscopy of TPE‐2by‐1, TPE‐2by‐2,
TPE‐2by‐3, TPE‐2TZ and TPE‐2EZ (Fig. S4‐33 and Fig. S43).
Fortunately, all single crystals of the E/Z isomers are able to be
obtained by the solvent diffusion method. The monomer (10 mg)
was separated and purified by column chromatography and was
dissolved in dichloromethane (0.3 mL), and then the mixture of
petroleum ether and ethyl acetate (3:1 = 2.4 mL) were slowly
added, and the corresponding single crystal could be obtained
after 2‐3 days of static storage. The detailed single crystal data
were shown in Table 1, and the specific single crystal structures
were as displayed in Fig. 1A, B and Fig. S44, which is a direct,
accurate and powerful confirmation of the E/Z isomers.
Considering the lack of the structure of TPE derivatives, and E/Z
isomers were checked based on single crystal structure
measurement, and the obtained single crystal structure of our
TPE derivatives E/Z isomers can benefit for the investigation of
structure‐property relationships. The geometry structures of E/Z
isomers were further confirmed by 2D COSY and NOESY NMR
spectroscopy, which testified these structures shown in
supplementary data.
Fig. 2 Fluorescent spectra of TPE‐2by‐3‐E (A) and TPE‐2by‐3‐Z (B)
(1 × 10−5 M, λex = 340 nm) in DMF and water mixtures with
different water fractions (below: and plots of fluorescence
intensity versus water fractions). (C) AIE enhancement multiple
Taking TPE‐2by‐3‐E isomer as an example, the resonance at δ =
5.00 ppm corresponds to the H5 and H19 protons as a reference histogram of all Z/E isomers.
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Table 1. Selected crystallographic data for compounds.
TPE‐2by‐1‐E
TPE‐2by‐1‐Z
C38 H30 N2 O2
546.64
TPE‐2by‐2‐E
C38 H30 N2 O2
546.64
TPE‐2by‐2‐Z
C38 H30 N2 O2
546.64
TPE‐2by‐3‐E
C38 H30 N2 O2
546.64
TPE‐2by‐3‐Z
C38 H30 N2 O2
546.64
TPE‐2TZ‐E
C36 H32 N2 O2 S2
588.76
TPE‐2TZ‐Z
C36 H32 N2 O2 S2
588.76
TPE‐2EZ‐E
C38 H34 N2 O4
582.67
TPE‐2EZ‐Z
C38 H34 N2 O4
582.67
Formula
C38 H30 N2 O2
fw [g∙mol−1
]
546.64
crystal color colourless
crystal system Triclinic
space group P ‐1
colourless
Monoclinic
C2/c
colourless
colourless
Monoclinic
C 2/c
colourless
Monoclinic
P 21/c
colourless
Monoclinic
P1 21/n 1
22.5985(5)
5.88283(9)
23.3075(5)
108.792(2)
2933.40(11)
4
colourless
Triclinic
colourless
Monoclinic
P1 21/n 1
18.3912(4)
16.5233(3)
10.1810(2)
97.1241(19)
3069.95(11)
4
colourless
Monoclinic
P2(1)/n
colourless
Triclinic
Monoclinic
P1 21/n 1
9.72023(14)
9.20409(15)
32.4449(4)
97.3047(13)
2879.15(7)
4
P ‐1
P ‐1
a [Å]
b [Å]
c [Å]
β [deg]
V [Å3]
Z
9.750(2)
11.263(3)
13.618(3)
92.030(3)
1478.0(6)
2
17.97(3)
17.704(19)
10.131(11)
18.53(3)
14.386(7)
11.778(11)
19.177(14)
98.760(9)
3212(4)
11.174(3)
12.359(4)
13.771(4)
80.595(4)
1777.7(9)
2
12.0607(18)
10.4171(15)
26.268(4)
101.731(2)
3231.4(8)
4
10.741(5)
12.302(6)
13.583(6)
112.723(6)
1599.5(13)
2
10.134(15)
18.39(3)
116.463(12)
117.784(13)
2940(6)
2997(8)
8
8
4
ρ
calcd [g/cm3] 1.228
1.212
0.075
293(2) K
1.261
1.235
1.197
1.238
1.319
1.274
1.198
1.210
μ [mm−1
T [K]
]
0.076
0.610
0.076
0.076
0.599
0.410
1.844
0.078
0.078
293(2)
298(2) K
293(2) K
298(2) K
298 K
293(2) K
298 K
298(2) K
298(2) K
1.674‐ 25.149
0.0630/ 0.1527
θ
min−θmax [deg] 2.092‐ 25.000 2.375‐24.996
0.959‐ 74.192
2.394‐25.148
0.0460/0.1124
2.036‐24.995
0.0544/0.1096
0.965‐74.200
0.0494/ 0.1562
1.543‐ 25.149
0.0758‐ 0.2010
0.974‐ 74.070
0.0713/0.2241
1.745‐24.997
0.0575/ 0.1328
R/wR [I >2 σ(1)
]
0.0511/ 0.1308 0.0511/ 0.1557
0.0564/0.1487
are a series of typical AIE molecules. As can be seen from Fig. 2
and Fig. S84, all compounds exhibit good solubility in N, N‐
Dimethylformamide (DMF), but they are insoluble in water due to
the hydrophobic aromatic rings. Fluorescence spectra of the
luminogens in DMF and DMF/H2O mixtures were measured. TPE‐
2by‐3‐E in pure DMF and DMF/H2O mixtures with water fractions
(fw) lower than 80 % have no emission, which is as a result of the
active intermolecular rotations of the dissolved luminogens in
these mixtures. The fluorescence intensity starts to increase
when (fw) is larger than 80 %, at which the compound begins to
aggregate. When fw was up to 99 %, its fluorescence intensity is
345‐fold higher than that in pure DMF (Fig. 2A) at 500 nm, thus
certifying its AIE behaviour. Other molecules show similar AIE
behaviour compared to TPE‐2by‐3‐E. The fluorescence
enhancement of TPE‐2by‐3‐Z at fw =99 % was 972 times stronger
than that in pure DMF (Fig. 2B). As shown in Fig. S84 and Fig. 2C,
there are two interesting phenomena that are worth noting
according to the enhancement multiplier of these E/Z
isomerization. Firstly, Z isomerization has more significant AIE
E/Z isomers of TPE‐2by‐3 molecule. This result is consistent with
the single crystal structure. The geometry structures of other
molecules (TPE‐2by‐1‐E, TPE‐2by‐1‐Z, TPE‐2by‐2‐E, TPE‐2by‐2‐Z,
TPE‐2TZ‐E, TPE‐2TZ‐E, TPE‐2EZ‐E, TPE‐2EZ‐Z) were confirmed by
the NOESY and COSY NMR spectroscopy, which were similar with
TPE‐2by‐3‐E and TPE‐2by‐3‐Z, mainly from the correlation of
hydrogen on the TPE benzene ring. The detailed information
about their NOESY and COSY NMR spectroscopy data were shown
as in Fig. S45−S82. These results of all E/Z isomers are consistent
with the single crystal structures (Fig. S44). Crystallographic data
for compounds as shown in Table 1. All the single crystal data
were submitted to the Cambridge Crystallographic Data Centre
(CCDC), and each compound has its corresponding number,
1833475 (TPE‐2by‐1‐E), 1833476(TPE‐2by‐1‐Z), 1833477(TPE‐
2by‐2‐E),
1833478(TPE‐2by‐2‐Z),1833479(TPE‐2by‐3‐E),
1833480(TPE‐2by‐3‐Z), 1833481(TPE‐2TZ‐E), 1833482(TPE‐2TZ‐ enhancement than the
E
isomerization under the same
conditions. Secondly, there is a significant difference in AIE effect
among the molecular whose position of N atom is different in
pyridine substituents. For the pyridine group of E isomerization,
the AIE enhancement fold was increased with an order of ortho‐
Z), 1833483(TPE‐2EZ‐E) and 1833484(TPE‐2EZ‐Z). All single
crystals displayed strong blue fluorescence through the
fluorescent inverted microscope, as displayed in Fig. 1E/F and Fig.
S83.
position, meta‐position and para‐position, and the
Z
isomerization is also following the same tendency. These results
indicated both E/Z isomerization and the position of N atom in
pyridine have important effect on the AIE behaviour of these TPE
derivatives.
2.2 Photophysical properties in solution.
After the structure confirmation, the AIE properties of all
compounds were investigated to verify that these TPE derivatives
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2.3 Mechanochromism property.
Fig. 3 Fluorescent images of TPE‐2by‐3‐E (A) and TPE‐2by‐3‐Z (B) in
different treatments: 1. Pristine; 2. Grinding at one side; 3. Grinding
all; 4. Fumed by DCM.
The controllable discoloration behaviour of organic materials,
including some TPE derivatives have attracted much attention
due to their potential applications in sensors, memories, logic
gate units, security inks, etc [30-38]. However, the fluorescent color
of TPE‐2OH did not change after grinding, suggesting this
molecule didn’t display mechanochromism property.
Interestingly, after introducing substituted group into TPE‐2OH,
all the compounds showed a characteristic color variation upon
grinding, suggesting the importance of introducing substituted
group for mechanochromism. In other words, introducing
substituent is an efficient and facile strategy for developing
mechanochromism materials. To evaluate the mechanochromism
behaviour of all compounds, their solid‐state emissions were
investigated. As shown in Fig. 3, the as‐prepared solid powder of
TPE‐2by‐3‐E has an intense blue fluorescence in pristine state,
and the fluorescence turns to bright green after grinding, and
then the fluorescence quickly turns back to its original blue
fluorescence after being fumed by DCM vapor. The change of
fluorescent color can be distinctly distinguished by naked eye
under UV light. Other TPE derivatives demonstrated the similar
phenomenon with TPE ‐2by‐3‐E when they were subjected to
grinding stimulation and fumed by DCM vapor.
The mechanocharomic properties of these aromatic‐
substituted tetraphenylethylene were studied by emission and
absorption spectrum in detail. The emission spectra of all
compounds under external grinding were shown in Fig. 4 A/B and
Fig. S85, and the solid UV absorption of all compounds under
external grinding were shown in Fig. 4 C/D and Fig. S87. In
addition, the corresponding spectroscopic data were summarized
in Table 2 and Table S1‐7. The TPE‐2by‐3‐E isomers at the pristine
state emits at 449 nm, which display red‐shifted emission peak
upon grinding at 489 nm. It can return back to the original state
after being fumed by DCM vapor. At the same time, the
fluorescence red‐shifted of TPE‐2by‐3‐Z isomers solid from 440
nm to 496 nm. Under the same conditions, we discovered that
the red‐shifted of TPE‐2by‐3‐Z (Δλ 56 nm) was greater than that
of TPE‐2by‐3‐E (Δλ 40 nm) after grinding. The fluorescence
spectra of other compounds in the pristine state and after
grinding are shown in Fig. S86, and the red‐shifted emission (Δλ)
is presented in Table S1‐7. These results revealed that the solid
powder samples of Z isomers exhibit a larger red‐shifted in
emission comparing with the one of E isomers, and that the red‐
shifted of para‐position N atm in pyridine substituents are larger
than those of meta ‐position and ortho ‐position of E and Z
isomers.
Fig. 4 Fluorescent spectra of TPE‐2by‐3‐E (A) and TPE‐2by‐3‐Z (B):
pristine, ground, and fumed by DCM. Solid UV absorption of TPE‐2by‐
3‐E (C) and TPE‐2by‐3‐Z (D): pristine, ground, and fumed by DCM.
Reversible switching of emission of TPE‐2by‐3‐E (E) and TPE‐2by‐3‐Z
(F) by repeated grinding‐ fuming by DCM.
Table 2. Spectroscopic data for TPE‐2by‐3‐E and TPE‐2by‐3‐Z.
Name
treatment
Em
(nm)
Δλ
(nm)
Φf
(%)
τ
(ns)
kr/108
(s‐1
k
nr/108
(s‐1
ΔE（
KJ/mol）
)
)
pristine
grind
fumed by DCM
449
489
451
27.4
44.2
26.8
3.07
4.29
3.07
0.89
1.03
0.87
2.36
1.30
2.31
40
56
TPE‐
2by‐3‐E
380.75
382.19
pristine
grind
fumed by DCM
440
496
441
33
45.5
32.8
3.14
4.35
3.14
1.04
1.05
1.03
2.15
1.25
2.16
Obvious change in the UV/vis absorption band was observed
before and after grinding the powder as shown in Fig. 4C/D and
Fig. S87. The UV/vis absorption band changes of all compounds
are almost the same. UV/vis absorption also has a significant red‐
shifted after grinding, and strong new absorption peaks are
generated at 270 nm and 330 nm. These absorption peaks could
be ascribed to n→π* transition. The red‐shift of the absorption
TPE‐
2by‐3‐Z
and fluorescence spectra after grinding may originate from the
increased molecular interaction with decreasing the optical
bandgap of the molecules in the ground sample. As shown in Fig.
4E/F, the blue‐green emission color change is reversible and
recyclable for several cycles when TPE‐2by‐3‐E and TPE‐2by‐3‐Z
repeat grinding‐ fuming by DCM process.
To better explain the different spectroscopic properties
between the luminogens, we further analysed their structure by
powder X‐ray diffraction (XRD) measurement. The XRD patterns
revealed that as‐prepared TPE‐2by‐3‐E isomers powder features
a series of sharp diffraction peaks. Upon grinding, the crystalline
structure almost completely converts to amorphous phase as
indicated from the XRD patterns, as can be seen in Fig. 5A. The
change in crystal form of TPE‐2by‐3‐Z in Fig. 5B is also a transition
from crystalline to amorphous. Detailed XRD changes of other
molecules are shown in Fig. S90. There is a significant difference
between the XRD of the E and Z isomers, but both of them are
transformed into amorphous after grinding. The ground sample
will promptly re‐crystallize when the sample was fumed by DCM
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vapor treatment as can be seen in the regain of intensity and
peaks in XRD patterns. Clearly, the reversible transition between
the ordered crystalline state and amorphous state is vital to the
switchable emission color and intensity of the luminophor. Similar
properties were observed in other TPE derivatives.
In order to understand more details about the characteristic
optical
properties
of
all
aromatic‐substituted
tetraphenylethylene solid, fluorescence quantum yield (Ф) and
fluorescence lifetime (τ) of these compounds were measured. The
fluorescence quantum yield and fluorescence lifetime of TPE‐2by‐
3‐E in crystal state are 27.4 % and 3.07 ns, TPE‐2by‐3‐Z are 33 %
and 3.14 ns, respectively. The fluorescence radiative rate
constant (kr) and non‐radiative rate constant (knr) were estimated
by combining the quantum yield [Ф= kr / kr + knr)] and fluorescence
lifetime results [τ = (kr + knr) −1]. According to the formula, the
fluorescence radiative rate constant is calculated to be 0.89 × 108
s−1 and non‐radiative rate constant is calculated to be 2.36× 108
s−1 of TPE‐2by‐3‐E isomers in the pristine state. However, the
fluorescence radiative rate constant of TPE‐2by‐3‐Z isomers is
1.04× 108 s−1, which is higher than that of TPE‐2by‐3‐E isomers.
The non‐radiative rate constant of TPE‐2by‐3‐Z isomers is 2.15×
108 s−1, which is lower than that of TPE‐2by‐3‐E isomers. This
suggests that molecular packing of the Z isomers may effectively
inhibits the excited energy loss through a non‐radiative decay
process.
After being subjected to grinding stimuli, the fluorescence
quantum yield, fluorescence lifetime of TPE‐2by‐3‐E and TPE‐2by‐
3‐Z are all increased, and the non‐radiative rate constant is
reduced, as shown in Table 2. The variations of other molecule
properties are the same as that of TPE‐2by‐3‐E, and all the data
are summarized in Table S1‐S7 and Fig. S88. The decrease of knr is
remarkable while the change of kr is not obvious after grinding,
indicating that the decreasing non‐radiative decay process
effectively suppresses energy loss and benefit for the fluorescent
emission. In the pristine state, the fluorescence quantum yield,
fluorescence lifetime of all Z isomers are larger than that of E
isomers, and the non‐radiation rate constant are smaller. In view
of the TPE derivatives of the pyridine substituents in the E
isomers, the fluorescence quantum yields, fluorescence lifetimes
of the ortho, para, and meta positions are gradually increasing,
and the fluorescence non‐radiative rate constant is gradually
reducing, and the same tendency for the Z isomers is observed.
The fluorescence non‐radiative rate constants of TPE‐2by‐3
decrease more prominently, indicating that the non‐radiative
delay process is inhibited, as shown in Fig. S89.
Fig. 5 XRD patterns of TPE‐2by‐3‐E (A), TPE‐2by‐3‐Z (B) pristine,
ground, and fumed by DCM.
HOMO and LUMO of the E and Z isomers are almost the same,
suggesting the substituent groups have no much effect on the
energy levels and electron distribution of these molecules. Here,
we employ the TPE‐2by‐3‐E and TPE‐2by‐3‐Z as representatives.
For TPE‐2by‐3‐E and TPE‐2by‐3‐Z, the electron intensity of the
LUMO is mainly distributed over the benzene ring of TPE, whereas
the HOMO is distributed in the double bond. The LUMO‐ HOMO
gap for all E/Z isomers as shown in Fig. 6A/B and Fig. S91. The
energy gap of the Z isomers is comparable to the one of E isomers,
such as, TPE‐2by‐3‐E is 380.57 KJ/ mol, TPE‐2by‐3‐Z is 382.19 KJ/
mol in the Table 2. Other detailed data comparisons were shown
in supporting information table S1‐7. In addition, for pyridine‐
substituted TPE, and the energy gaps of E and Z isomers of the
ortho, meta and para positions are gradually increasing.
2.5 Single crystal structure analysis.
Since the importance of molecule packing forming on the AIE
and mechanochromism behaviour, we analysed their single
crystal structures to investigate structure‐property relationships.
As illustrated in Fig. 1A/B and Fig. S44, all molecules adopt a highly
twisted conformation. TPE‐2by‐1‐E belongs to triclinic system in
p‐1 space group while TPE‐2by‐1‐Z is monoclinic system in C 2/c
space group. TPE‐2by‐2‐E and TPE‐2by‐2‐Z are monoclinic system
in P1 21/n 1, C 2/c space group, respectively. TPE‐2by‐3‐E and
TPE‐2by‐3‐Z belong to monoclinic system in P 21/c, P1 21/n 1
space group, respectively. TPE‐2TZ‐E is triclinic system in p‐1
space group, but TPE‐2TZ‐Z belongs to monoclinic system in P1
21/n 1 space group, TPE‐2EZ‐E belongs to monoclinic system in
P2(1)/n space group, and TPE‐2EZ‐Z is triclinic system in p‐1 space
group. Their packing structures of single crystal are shown in Fig.
6C/D and Fig. S92. Van de Waals forces, numerous intermolecular
C−H · · · π (phenyl ring), C−H · · · H−C and C−H · · · N (hydrogen
bond) contacts, and even local π−π interactions are observed in
the crystals. The length of C−H · · · N hydrogen bond of these TPE
derivatives were summarized in Fig. 7. The C−H · · · N interacꢀon
distance of these TPE derivatives are between 2.5 and 3.4 Å. The
result indicated all of them are smaller than 3.4 Å, revealing the
presence of C−H · · · N hydrogen bond. The corresponding C−H · ·
· N hydrogen bond interaction distance of TPE‐2by‐3‐E is 3.0990
Å, which is a much weaker than TPE‐2by‐3‐Z (2.5852 Å), indicating
that the C−H· · · N interacꢀon in TPE‐2by‐3‐Z is stronger
2.4 Theoretical study.
To obtain a better understanding of the optical properties of all
E/Z isomerization, we carried out theoretical calculations on their
energy levels by the density functional theory (DET) method at
B3LYP/6‐31G(d) basis in the Gaussian 09. The simulation results
including optimized molecular configuration, energy levels and
electron distribution of the high occupied molecular orbital
(HOMO) and lowest unoccupied molecular orbital(LUMO) of TPE‐
2by‐3‐E and TPE‐2by‐3‐Z are shown in Fig. 6A/B. And the
calculation of other E/Z isomers are displayed in Fig. S91.
According to our theoretical calculation results, the calculated
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and obtained single crystals of all cis/trans isomers. The
availability of pure stereoisomers and single crystal data enable
us to clearly reveal the relationship between chemical structure
and photophysical properties. Interestingly, the Z isomerization
have more significant AIE enhancement than the E isomerization
under the same conditions. Moreover, there is a significant
difference in AIE effect among the isomers with different position
of N atom in pyridine substituents. In the solid state, the Z isomers
show bathochromic emission with a higher quantum yield and
long lifetime compared with the E isomers. After the solid powder
was grounded, both the fluorescence quantum yield and
fluorescence lifetime increased, and the non‐radiative rate
constant decreased compared to the one of original state
powder. The E/Z isomers display different mechanocharomic
behaviour as they exhibit reversible and recyclable blue‐green
emission color switch for several circles gets through repeated
grinding‐ fuming by DCM process. Thanks to the unique
mechanocharomic property of TPE, it will have great potential
applications in stimuli‐responsive materials. Single crystal data
analysis revealed that all E/Z isomers have strong hydrogen bond
and the C−H· · · N interaction, which of the Z isomers is stronger
than the one of E
Fig. 6 Energy levels of LUMO and HOMO, energy gaps, and
electron cloud distributions of TPE‐2by‐3‐E (A) and TPE‐2by‐3‐Z
(B). Packing structure of single crystal about TPE‐2by‐3‐E (C) and
TPE‐2by‐3‐Z (D).
isomers. The C−H · · · N hydrogen bond interacꢀon played an
important role in the fluorescent properties, including AIE
behaviour and mechanocharomic performance. Finally, these
consequences promote us to understand the relationship
than TPE‐2by‐3‐E. Impressively, C−H· · · N interaction of the Z
isomers is stronger than the E isomers as shown in Fig. 7C. In the
meantime, C−H · · · π (phenyl ring) is discovered between the H
atom on the labelled phenyl ring and the center of the labelled
phenyl, and the distances are also summarized in Fig. 7D. The C−H
· · · π interaction distance of these TPE derivatives are among 3.0
to 3.6 Å, indicating the presence of C−H · · · π interaction in them.
However, the difference in C−H · · · π interaction distance was
much smaller than C−H· · · N interaction among E/Z isomers. For
example, the C−H · · · π interaction distance of TPE‐2by‐3‐E and
TPE‐2by‐3‐Z are 3.0541 Å and 3.0087Å, respectively.
According to the above structure analyses, the relationship
between the optical properties of the TPE derivatives E/Z isomers
and their structure can be established. The difference in optical
properties of the E/Z isomers molecules is mainly ascribed to the
C−H · · · N hydrogen bond interacꢀon between molecules. The Z
isomers have stronger hydrogen bonding force than E isomers, in
which the intramolecular rotations are greatly hampered
inducing the excited energy loss through a non‐radiative decay
process and resulting in Z isomers have more significant AIE
effect, stronger fluorescence lifetime, higher fluorescence
quantum yield, and larger mechanocharomic induced wavelength
shift than isomers. Typically, the presence of the strongest C−H · ·
· N interaction in TPE‐2by‐3‐Z among these TPE derivatives
endowed them with the most outstanding AIE and
mechanocharomic property. As previous mentioned, Z isomers
exhibit a greater red‐shifted in emission than the one of the E
isomers. Based on the crystal analysis, the C−H · · · π interaction
of Z isomers is comparative to the one of E isomers while C−H· · ·
N interaction is stronger, resulting in Z isomers exhibit a greater
red‐shifted in emission than the one of the E isomers. The
synergetic effect between the intermolecular interactions (C−H· ·
· N interaction) and molecular packing (C−H · · · π interaction)
induced the red‐shifted of para‐position N atom in pyridine
substituents is larger than those of meta ‐position and ortho ‐
position of E and Z isomers.
3. Conclusions
In this work, we have successfully synthesized a series of
aromatic‐substituted tetraphenylethylene derivatives through
reasonable structure design, and separated their pure
isomerization by the common column chromatography technique
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changjiang scholars and innovative research team in university
(IRT 17R94)
▁
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Facile synthesis and separation of E/Z isomerization of aromatic-substituted
tetraphenylethylene for investigating their fluorescent properties via single crystal
analysis.