The Synergistic Effect between Triphenylpyrrole Isomers as Donors, Linking Groups, and Acceptors on the Fluorescence Properties of D–π–A Compounds in the Solid State

Article abstract of DOI:10.1002/chem.201704020

Eight donor–π–acceptor (D–π–A) compounds employing triphenylpyrrole isomers (TPP-1,2,5 and TPP-1,3,4) as donors, malononitrile (CN) and 1H-indene-1,3(2H)-dione (CO) as acceptors, pyridone (P) and benzopyran (B) as π-linking groups were synthesized. The compounds exhibited aggregation-induced emission and piezochromic properties. Compared with previously reported donors, triphenylpyrroles induced all the compounds to have more remarkable photophysical properties. The compounds containing TPP-1,2,5 and P moieties displayed stronger fluorescence intensities, shorter emission wavelengths, and more distinct piezochromic properties. However, the same phenomenon was observed in the TPP-1,3,4-containing system if B was as π-linker. Moreover, the CN acceptor endowed the compound to have a relatively strong fluorescent intensity, in which CO induced a relatively long emission wavelength. That is, the photophysical properties of D–π–A compounds can be controlled by adjusting the structure of donor, linker and acceptor.

Full text of DOI:10.1002/chem.201704020

A Journal of
Accepted Article
Title: The Synergistic Effect between Triphenylpyrrole Isomers as
Donors, Linking Groups and Acceptors on the Fluorescence
Properties of D-π-A Compounds in the Solid State
Authors: Yunxiang Lei, Yueying Lai, Lichao Dong, Guojun Shang,
Zhengxu Cai, Jianbing Shi, Junge Zhi, Pengfei Li, Xiaobo
Huang, Bin Tong, and Yuping Dong
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To be cited as: Chem. Eur. J. 10.1002/chem.201704020
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The Synergistic Effect between Triphenylpyrrole Isomers as
Donors, Linking Groups and Acceptors on the Fluorescence
Properties of D-π-A Compounds in the Solid State
Yunxiang Lei,[a] Yueyin Lai,[a] Lichao Dong,[a] Guojun Shang,[a] Zhengxu Cai,[a] Jianbing Shi,*[a] Junge
Zhi,[b] Pengfei Li,[b] Xiaobo Huang,[c] Bin Tong,[a] and Yuping Dong*[a]
Abstract: Eight D-π-A compounds employing triphenylpyrrole
better molecular planarity, which tends to cause the molecules
isomers (TPP-1,2,5 and TPP-1,3,4) as donors, malononitrile (CN) to accumulate in a face-to-face fashion in the solid state, thus
and 1H-indene-1,3(2H)-dione (CO) as acceptors, pyridone (P) and resulting in strong intermolecular π−π interaction and
benzopyran (B) as π-linking groups were synthesized. The
compounds exhibited aggregation-induced emission and
aggregation-caused quenching (ACQ). Another method is to
design a compound with a D-π-A structure, in which the
piezochromic properties. Compared with previously reported donors, intramolecular charge can be transferred from the donor to the
triphenylpyrroles induced all the compounds to have more
acceptor (ICT).8 To date, most studies focus on this strategy
remarkable photophysical properties. The compounds containing because it can circumvent the issues of ACQ. Since there is lack
TPP-1,2,5 and P moiety displayed stronger fluorescence intensities, of clear guidance and the appropriate donors, compounds with
shorter emission wavelengths, and more distinct piezochromic
D-π-A structures are rare and often have short emission
properties. However, the same phenomenon was observed in the wavelengths or weak fluorescence intensities. At present, the
TPP-1,3,4-containing system if B was as π-linker. Moreover, the CN
most commonly used electron donors are tetraphenylethylene,9-
11
acceptor endowed the compound to have relatively strong
a
triphenylamine,12-14 carbazole,15 thiazine,16 dimethylaniline,17
fluorescent intensity, wherein CO induced a relatively long emission alkoxy,18 benzene,19 naphthalene,20 anthracene,21 and so on. As
wavelength. That is, the photophysical properties of D-π-A some of these electron donors have relatively strong
compounds can be controlled through adjusting the structure of fluorescence intensities, D-π-A compounds containing these
donor, linker and acceptor.
moieties have stronger fluorescence intensities. However,
restricted by their limited electron-donating capabilities, these D-
π-A compounds have relatively short emission wavelengths. On
the other hand, another type of electron donors has strong
electron-donating capabilities but have relatively simple
structures, thus resulting in insufficient space among their
twisted configurations. Therefore, the intermolecular interaction
can not be effectively avoided when molecules are in the
aggregate state, which causes a long emission wavelength but
weak intensity. To solve this problem, many researchers
combine these two kinds of donors in one compound. For
example, a kind of D-π-A molecule consisting of triphenylamine
and tetraphenylethylene as a co-donor22 was synthesized by
Tang’s group recently. It turned out that these two donors
exactly complemented each other in terms of functions.
Therefore, it is essential to design a donor with both strong
electron-donating capability and highly twisted configuration.
More recently, we have synthesized two triphenylpyrrole
isomers (TPP-1,2,5 and TPP-1,3,4) and demonstrated that
these two compounds were not only capable donors but also
had a highly decentralized spatial structure.23 Moreover, TPP-
1,3,4 was an AIE molecule because it had a greater space
volume and more distorted molecular structure than TPP-1,2,5.
with ACQ characteristic. Based on this previous result, we
selected TPP-1,2,5 and TPP-1,3,4 to construct new D-π-A
molecules. In the meantime, CN and CO were selected as
acceptors. Since CO has a better conjugation than CN, CO-
Introduction
Since Tang,s group firstly discovered the aggregation-induced
emission (AIE) phenomena in 2001,1 compounds with
luminescent properties have attracted considerable research
interest in solid state or aggregate state because of their broad
applications in high-tech fields, such as ion detection, in vitro
and in vivo imaging, mechanosensors, and optoelectronic
conversion.2-6 Among these materials, compounds with long
emission wavelengths are of particular significance due to their
great potential for practical applications.7 Basically, there are two
fundamental ways to increase the emission wavelength of the
molecule. One method is to enhance the conjugation degree of
the molecules, which is beneficial to the electronic delocalization.
However, a higher conjugation degree of molecule means a
[a]
Y. Lei, Y. Lai, L. Dong, G. Shang, Z. Cai, J. Shi, Pro. B. Tong, Prof.
Y. Dong
Beijing Key Laboratory of Construction Tailorable Advanced
Functional Materials and Green Applications, School of Materials
Science & Engineering, Beijing Institute of Technology, 5 South
Zhongguancun Street, Beijing, 100081, China
E-mail: chdongyp@bit.edu.cn; bing@bit.edu.cn
J. Zhi, P. Li
School of Chemistry and Chemical Engineering, Beijing Institute of
Technology, 5 South Zhongguancun Street, Beijing, 100081, China
X. Huang
[b]
[c]
containing molecules tend to have
wavelength.24 In this work, the donor and acceptor are linked via
an alkyl chain containing (Pyridone derivative) or
a
long emission
P
B
College of Chemistry and Materials Engineering, Wenzhou
University, Wenzhou 325035, China
(Benzopyran derivative). P linker has a highly twisted molecular
structure, which is favorable for the target compound to have
strong fluorescence intensity.24 On the other hand, the
Supporting information for this article can be
found under http://dx.doi.org/10.1002/chem.
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conjugation of B linker is relatively high, which is advantageous
for the material to have a long emission wavelength.25 Based on
this strategy, we designed and synthesized eight D-π-A
compounds. Among them, four of them are P derivatives, and
the others are
B derivatives. All of them exhibited the
aggregation-induced emission (AIE) properties, and most of
them showed the piezochromic properties. The results of X-ray
diffraction experiments revealed that the shift of the wavelength
was attributed to the change in the molecular stacking mode,
which means that the arrangement of molecules changed from
27
order to disorder.26,
Overall, the comparison of these
compounds indicated that compounds containing P linker had
stronger luminous intensities while compounds having the B
linker had longer wavelengths. More interesting, for the P-based
compounds with the same acceptor, with the ones having the
TPP-1,2,5 as the donor exhibited more stronger fluorescence
intensity, shorter emission wavelength, and more distinct
piezochromic properties than those having the TPP-1,3,4 as the
donor. This result could be attributed to the fact that TPP-1,2,5
facilitates the orderly aggregation of molecules, thus leading to
the P linked compounds have higher fluorescence intensities. In
contrast, for B-based compounds, the same phenomenon was
observed when TPP-1,3,4 as the donor. For the B-based
compounds, TPP-1,3,4 is beneficial to avoid the interaction of
molecules in the aggregation state due to a greater space
volume and more distorted molecular structure of TPP-1,3,4.
Moreover, precisely because the molecular conjugation is high,
the B-based compounds have relatively long emission
wavelengths. Furthermore, not only the donor would affect the
fluorescence properties of the materials, but also the acceptor
would also effect. The CN linker induces the derivative
molecules to have a relatively dispersed spatial structure, which
is favorable for the compounds to have stronger fluorescence.
On the other hand, the CO linker increases the conjugation of
the molecular, thus resulting in a longer wavelength.24, 25 Hence,
we believe our work can considerably contribute to the designing
of new AIE-compounds, to the understanding of the AIE
phenomenon and mechanism, and to expanding the application
fields of AIE-based materials.
Chart 1. Molecular structures of the compounds.
the bands of CO-P-1,2,5 and -1,3,4 were more intense than
those of CN-P-1,2,5 and -1,3,4. The second band ranging from
310 to 390 nm could be ascribed to the localized π-π* transition.
The last one ranging from 260 to 310 nm, could be regarded as
the electronic absorption band of the TPP moiety.22,
28
The B-
based compounds, however, exhibited two absorption bands
because the absorption band corresponding to the localized π-
π* transition was overlapped or not obvious. The band around
340-630 nm corresponding to the ICT from donors to acceptors,
whereas another band was attributed to the absorption of the
TPP moiety (Figure 1b).23, 24 Specially, the region corresponding
to ICT in B-based compounds was broader, more intense and
had longer absorption wavelengths than those of P-based
compounds. This is because benzopyran ring with a better
flatness and higher conjugation is more conducive to the flow of
the electronic. For D-π-A molecules, when the intramolecular
charge transfer effect is too intense, it will promote the red-shift
of the wavelength and the widening of the corresponding
absorption band, further causing the overlap with the second
region that corresponds to the localized π-π* transition.11, 24
Considering the push-pull electronic structure is helpful to
enhance the ICT effect, we tested the UV-vis and fluorescence
spectra of CN-P-1,3,4 and CN-B-1,3,4 in various solvents. As
shown in Figure S1, the absorption spectra of CN-P-1,3,4 and
CN-B-1,3,4 did not exhibited evident changes in different polar
solvents. On the other hand, the FL spectra of CN-B-1,3,4
showed a certain degree of red shift from 566 to 594 nm with
increasing solvent polarity from toluene to CH3CN, showing a
Results and Discussion
Molecular structures and optical properties of target
compounds
Chart 1 illustrates the molecular structures of the eight
derivatives. The target compounds were prepared by the
Knoevenagel condensation reactions starting from raw materials
CN-P, CO-P, CN-B, CO-B, and TPP isomers in 40-75% yields.
The molecular structures of all compounds were confirmed by
NMR and MS. These compounds are soluble in common
organic solvents, such as DMSO, CHCl3, and THF, but insoluble
in alcohol or water. Figure 1 displays the UV-vis absorption and
emission spectra of these compounds in THF solution. The P-
based compounds (CN-P-1,2,5, CN-P-1,3,4, CO-P-1,2,5, CO-P-
1,3,4) had three distinct bands in the range of 260-650 nm. The
first band was in the visible area from 390 to 530 nm, which was
attributed to the intramolecular charge transfer (ICT) from the
TPP donor to CN or CO acceptor. It is noteworthy that CO has a
better planarity, which increases the conjugation of the molecule
and enhances the intramolecular charge transfer. As a result,
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strong solvatochromism effect. In contrast, the emissions of CN-
P-1,3,4 only changed 7 nm almost without bathochromic shift.
The influence of Δf on the Stokes shift (Δυ) was further explored
by the Lippert–Mataga equation in Figure 2 and Table S129, 30
.
For CN-B-1,3,4, it was found that the plot of Δυ against Δf in
these different polar solvents had a positive correlation with a
slope of 221. However, for CN-P-1,3,4, there was no clear
correlation between Δυ and Δf. This indicated that B moiety has
a more pronounced ICT effect, which further confirmed that B
moiety possesses a higher molecular polarity (The dipole
moments of CN-B-1,3,4 and CN-P-1,3,4 based on theoretical
calculation were 11.8 db and 7.6 db, respectively).
As shown in Table 1, the four P-based compounds, CN-P-
1,2,5, CN-P-1,3,4, CO-P-1,2,5, and CO-P-1,3,4, had emission
peaks in the range from 550 to 563 nm in the THF solution,
whereas the corresponding peaks of the B-based compounds
were at 580–625 nm. Evidently, B-based compounds have
longer emission wavelengths than those containing P moiety. In
particularly, the large Stokes shifts confirm that the
intramolecular charge transfer (ICT) in the excited states
between the donor groups and acceptor groups could be
stabilized in B-based compounds.
Figure 1. (a) UV-vis absorption and PL spectra of P-based compounds in THF
(1 × 10-5 mol/L). (b) UV-vis absorption and PL spectra of B-based compounds
in THF (1 × 10-5 mol/L).
R2= -0.19
10
y=9.32+0.46x
Though the fluorescence intensities of eight compounds
were very weak in THF solution, the compounds in the solid
state had the stronger fluorescence intensities, which showed a
distinct AIE feature (Table 1). The time-resolved emission decay
behaviors of the derivatives were studied, and their lifetime data
are illustrated in Table 1. The lifetimes of B-based compounds
and P-based compounds were 1.11-1.81 ns and 2.09-2.78 ns in
the original solid state, respectively. From the data, it is clearly
concluded that the fluorescence lifetimes of B-based compounds
are relatively long，this may be related to the planar molecular
configuration.31, 32 Meanwhile, B-based compounds in solid state
have the longer emission wavelengths (604-686 nm) and lower
fluorescent quantum yield (ФF = 5.4-15.2%) than those of P-
based compounds (λem = 585-611 nm and ФF = 15.0-22.1%),
respectively. This could be attributed to the larger conjugation
and the higher spatial planarity of the two connected bridges and
donor groups in B-based compounds.23, 25, 26 It is well known that
a better planarity of the molecule might dramatically promote the
interaction between solid compounds. To avoid this kind of
interaction, it can be achieved by adjusting the donor to make
the whole molecule has a relatively dispersed spatial structure.
Based on the previous introduction, we believed that could be
8
6
4
CO-B-1,3,4
CO-P-1,3,4
R2= 0.88
y=4.95+2.21x
0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35
f
Figure 2. The plot of Stokes shift (Δυ) of CO-P-1,3,4 and CO-B-1,3,4 versus
their Δƒ in solutions (Toluene, CHCl3, THF, DCM, DMF, CH3CN)
realized by TPP-1,3,4 unit as the donor. D-π-A structure
constructed by TPP-1,3,4 and B moiety has a lower conjugation,
which induces the shorter emission wavelengths of B-based
compounds. In contrast, TPP-1,2,5 has a relatively small spatial
structure and better planarity, which facilitates the orderly
aggregation of molecules and causes the longer emission
wavelength.23 For the P-based compounds, the molecular
configuration of the pyridone bridge containing an alkyl chain is
more twisted. More important, the molecule has a relatively large
space volume because it has two donors. This structure has a
fairly twisted spatial conformation, which can effectively
suppress the interaction of molecules in the aggregation state.
However, the arrangement of these large space volume
compounds are often amorphous in the aggregate state, which
is detrimental to the fluorescence properties of materials.33-35
Compared with CN group, CO group increases the conjugation
of the whole molecule, thus resulting in a longer wavelength
(Figure 3). Evidently, the results of this work will help the
researchers to design new D-π-A compounds with AIE
characteristics in the future.
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Table 1. Photophysical properties of the eight derivatives.
Compounds
Abs/THF
nm
Em/THF
nm
Stoke's shift
nm
Em/solid
nm
ФF/THF
%
ФF/solid
%
HOMO
(eV)
LUMO
(eV)
Eg

(eV)
(ns)
-6.58
-6.61
-1.29
-1.30
5.28
5.31
CN-P-1,2,5
CN-P-1,3,4
CO-P-1,2,5
CO-P-1,3,4
CN-B-1,2,5
CN-B-1,3,4
CO-B-1,2,5
CO-B-1,3,4
303,347,426
278,363,427
296,360,453
312,368,452
291,424
550
553
561
563
582
584
620
627
124
126
108
111
158
142
132
140
585
596
597
611
616
604
686
667
1.0
22.1
19.1
17.8
15.0
9.3
1.81
1.52
1.11
1.64
2.27
2.78
2.53
2.19
0.83
0.92
0.85
0.34
0.65
0.21
0.52
-6.31
-6.37
-1.12
-1.14
5.20
5.22
-6.72
-6.88
-1.80
-1.80
4.93
5.06
302,442
15.2
5.4
-6.50
-6.75
-1.58
-1.59
4.93
5.14
286,488
310,487
13.2
AIE properties of derivatives in aggregate state
1.00
0.75
0.50
0.25
0.00
As shown in Table 1, all compounds in this work had strong
and distinct fluorescence in the solid (ФF were relatively high),
however, weak or no fluorescence in the solution (ФF were very
low), which indicated that these compounds had AIE
a
CN-P-1,2,5
CN-P-1,3,4
CO-P-1,2,5
CO-P-1,3,4
characteristics.
We
systematically
investigated
their
photophysical properties in the aggregate state and in solid. The
UV-vis absorption spectra and PL spectra of two triphenylpyrrole
isomers were measured in water-THF mixtures with different
fractions of water, and their final concentrations were kept
constant at 1 x10-5 mol/L according to the previous methods.
CO-P-1,2,5 emitted a weak yellow-green fluorescence at 561
nm in pure THF, which is caused by the free rotation of the
phenyl ring and TPP ring via rotatable carbon-carbon single
bonds and results in the consumption of energy in a non-
radiative form.2, 36 When the fraction of water (fw) in the THF-H2O
mixed solvent was below 40%, the PL intensity and wavelength
of CO-P-1,2,5 were almost unchanged. With increasing of fw
from 50% to 90%, the PL intensity was enhanced about 15 times.
Simultaneously, the emission wavelength was red-shifted from
560 to 608 nm. When fw was further increased to 99%, the PL
intensity of CO-P-1,2,5 slightly reduced. However, the emission
wavelength continued to red-shift to 618 nm, which could be
attributed to the high ratio of water. The solubility of the
compound would be drastically reduced in the case of a high
ratio of water, which leads to a relatively disorder intermolecular
aggregation13, 37 (Figures 4a). The UV-vis absorption spectra of
CO-P-1,2,5 in the THF/water mixtures solvent (1 × 10-5 mol/L)
are shown in Figure S4. The absorption spectra were almost
unchanged with increasing fw from 10% to 40%. When fw was
increased to 50%, the entire absorption spectra exhibited a
dramatic decrease and red-shift. Meanwhile, the absorption
spectra of CO-P-1,2,5 contained light-scattering tails in the long
wavelength region when the fw ≥60%, indicating that CO-P-1,2,5
had clustered into nanoaggregates in the poor solvents. It was
also found that the absorption spectra of CO-P-1,2,5 in the
THF/water mixture with higher fw showed some differences in
absorption band shapes and maxima, which might be
500
550
600
650
700
750
Wavelength (nm)
1.00
0.75
0.50
0.25
0.00
b
CN-B-1,2,5
CN-B-1,3,4
CO-B-1,2,5
CO-B-1,3,4
500
550
600
650
700
750
800
850
Wavelength (nm)
Figure 3. Fluorescence spectra of P (a) and B (b) solid samples
CO-P-1,2,5 induced by water, which blocks the non-radiative
energy transfer and enhances the fluorescence emission of the
CO-P-1,2,5 molecules. This result further confirms that CO-P-
1,2,5 is AIE active. Similarly, CO-P-1,3,4 exhibited the similar
changes in fluorescence intensity and emission wavelength
when the water content was the same as that of CO-P-1,2,5, but
the fluorescence enhancement (ca. 11 times, Figure 4b) was
lower than that of CO-P-1,2,5, which was consistent with their
considered to the formation of different nanoaggregate forms of
32
molecules.29,
Dong, et al found that crystal-rich aggregation
should be obtained in the middle range of water concentration.
By increasing water contents, amorphous state should be
dominant.38, 39 On the basis of these results, it is concluded that
the fluorescence enhancement is attributed to the aggregation of
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differences in fluorescent quantum efficiencies. These results
indicated that the configurations of triphenylpyrrole isomers have
different effects on the restriction of intramolecular rotation
through changing the aggregation structure, leading to the
differences in their fluorescent intensities.
700
600
500
400
300
200
100
0
a
Water fraction
0%
10%
20%
30%
40%
50%
60%
70%
80%
For other derivatives, they also exhibited significant AIE
properties. In general, as the fraction of water in water/THF
mixture increased to about 50-60%, the fluorescence intensity
gradually enhanced and reached the maximum value at about
90% of water fraction, and then declined to a certain extent with
a red-shifting of the emission peak (Figure S3). Interestingly, the
fluorescent enhancement ratios of the compounds containing
TPP-1,2,5 donor were higher than those of compounds
containing TPP-1,3,4 donor in aggregated state when pyridone
ring was used as the π-linking group, no matter whether CN or
CO was used as an acceptor. The fluorescent enhancement
ratios of the compounds containing TPP-1,3,4 as donor were
even higher if benzopyran moiety was used as a π-linking group,
and consistent with their grind induced wavelength alteration
(Figure 4c). In the aggregation state, chromophores would be
surrounded by the hydrophobic molecules. It is likely that
intermolecular interaction should also contribute to inducing the
bathochromic shift. These comparison results are almost same
as AIE quantum efficiencies of their corresponding compounds
(Table 2), which reveals that the photophysical characteristics of
derivatives in the aggregated state are determined by the
synergistic effect of π-linking group and donor rather than by the
type of acceptor. Based on our experimental data and the
relative results in the literature, it is concluded that the
construction of D-π-A compounds with AIE properties does not
require an AIE molecule donor, but different donors have a
significantly different effect on the optical properties of the
compounds. To further study the mechanism of AIE formation,
our lab is currently working on the development of single crystals.
Theoretical calculation
90%
99%
500
600
700
800
Wavelength (nm)
16
12
8
630
b
I/Io
CO-P-1,2,5
CO-P-1,3,4
610
590
570
550
Wavelength
CO-P-1,2,5
CO-P-1,3,4
4
0
0
20
40
60
80
100
Water Fraction (%)
25
45
I/Io
Alteration
20
15
10
5
36
27
18
9
Density functional theory calculations were performed to
reveal more information about the optical properties of D-π-A
compounds at the molecular level by using a suite of Gaussian
09 program. The HOMO and LUMO energy levels of derivatives
were calculated by using the TD-DFT with CAM-B3LYP method.
Figure 5 shows the highest occupied molecular orbital (HOMO)
and the lowest unoccupied molecular orbital (LUMO) charts of
the derivatives CO-P-1,2,5, CO-P-1,3,4,CO-B-1,2,5, and CO-B-
1,3,4.
0
0
4
4
5
5
,
5
4
,
5
,
4
,
,
,
,
,
3
3
2
2
,
2
3
,
2
,
3
,
,
,
,
,
1
1
1
1
1
-
1
1
-
1
-
-
-
-
-
-
P
P
-
P
B
-
P
B
-
B
B
-
-
-
-
-
N
O
O
N
N
O
N
O
C
C
C
C
C
C
C
C
Figure 4. (a) Fluorescence spectra of CO-P-1,2,5 (1
×
10-5 mol/L) in
The calculated energies (HOMO and LUMO) and band gaps
(Eg) are listed in Table 1. Among eight D-π-A compounds,
samples with TPP-1,2,5 donor showed higher HOMO energy. As
THF/water mixtures with different water volume fractions. (b) Fluorescence
intensities and emission wavelengths of CO-P-1,2,5 and CO-P-1,3,4 versus
the fraction of water (fw) in THF/water (concentration 1 × 10-5 mol/L).(c) The
highest fluorescence intensity ratio and wavelength alteration of all the
derivatives (The order of the compounds was same as that listed in Table 1).
can be seen from Figure
5 and Figure S2, the LUMO
distributions of all the P-based compounds mainly lay on the
pyridone ring and acceptor, and the HOMO distributions of the
P-based compounds with TPP-1,2,5 donor mostly located on the
TPP unit. Moreover, the HOMO distributions of compounds with
TPP-1,3,4 donor mostly located on the entire molecule except
for TPP unit, which is very different than compounds with the
TPP-1,2,5 donor. However, the LUMO of all the B-based
compounds almost delocalized to the whole molecule. The
HOMO distributions of the compounds with the TPP-1,2,5 donor
were also predominantly located on the TPP group, but the
HOMO distributions of compounds with TPP-1,3,4 donor were
much wider, including the C=C double bond located in the center
of the molecule. Apparently, their difference in HOMO
distributions of compounds with different donors is relatively
small. According to our previous work, compared with TPP-1,3,4,
the TPP-1,2,5 molecule has a better planar configuration and
lower dipole moments.23 In combination with this theoretical
calculation result, it can be concluded that the TPP-1,2,5 group
has
a stronger electron-donating ability than TPP-1,3,4.
Meanwhile, this result also indicates that the B moiety facilitates
the flow of electron cloud from the TPP unit to the donor core in
the excited state.13
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Figure 5. Molecular orbital diagrams and energies for the LUMOs (left) and
HOMOs (right) of the compounds CO-P-1,2,5, CO-P-1,3,4, CO-B-1,2,5, and
CO-B-1,3,4 obtained from TD-DFT with CAM-B3LYP methods.
Piezochromic properties of the derivatives.
When the original samples were ground, the emission
wavelength of the eight compounds showed a red-shift with
different degrees because of their different acceptor, donor, and
π-linker. The fluorescence images of all solid materials were
captured under the 365 nm UV lamp. As shown in Figure 6a,
after ground, the color of the sample (CO-P-1,2,5) changed from
orange to bright red, which is
a pronounced change.
Correspondingly, the emission peak also red-shifted from 597 to
640 nm. When the ground sample was fumed by common
organic solvent (e.g. CH2Cl2, CHCl3, and THF), its color could be
restored to the initial state, and the wavelength shifted back to
598 nm (Figure 7a). After the recovered sample was ground
again, the luminous color and wavelength transformed
significantly. Similarly, the sample ground in second time could
also be recovered by solvent fuming. After several repeats, it
fully demonstrated that the material had optimal reversibility and
reusability (Figure 7b). It is noteworthy that the color change of
the sample under the natural light could be distinguished by the
naked eyes. As shown in Figure 6b, the color of the sample
changed from yellow to orange. For the P-based compounds,
the piezochromic properties (the alteration of wavelength and
change of color) of compounds containing TPP-1,2,5 donor were
more evident, which are unrelated to the acceptor type. The
results are consistent with the AIE properties (Table 2, Figure S5,
Figure S6). For the B-based compounds, all of them exhibited
variation in the wavelength when stimulated by external
pressure (Table 2, Figure S7), and CN-B-1,3,4 displayed the
most significant color change among these four compounds
(Figure S8). In particular, the compound with the TPP-1,3,4
donor was more favorable for the red-shift of the wavelength
(Figure 4c), which is exactly the opposite of P-based compound
To investigate the mechanism of this piezochromic
phenomenon, we carried out XRD measurements for the
compounds in various states. The results showed both the
diffraction peaks and dispersion peaks attributing to crystal state
amorphous state, respectively, in all original compounds,
indicating the coexistence of the crystal and amorphous state
(Figure 8, S9 and S10). According to Figure 8a, the solid
compound CO-P-1,2,5 in different states had different stacking
modes. The diffraction curves of the original sample indicated
many sharp and intense reflections due to being crystal-rich
state. After being ground, the corresponding diffraction peaks
were almost eliminated, suggesting the sample
Figure 6. (a) Fluorescence images of CO-P-1,2,5 solid samples taken under a
365 nm UV lamp. (b) Fluorescence images of CO-P-1,2,5 solid samples taken
under a daylight lamp.
1.00
a
original
ground
0.75
0.50
0.25
0.00
Fuming
500
550
600
650
700
750
800
850
Wavelength (nm)
650
630
610
590
Ground
b
Fuming
1
2
3
4
5
Number of Cycles
Figure 7. (a) Fluorescence spectra of CO-P-1,2,5 solid samples under various
conditions. (b) Peak emission wavelength of CO-P-1,2,5 against number of
grinding-fuming cycles
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10.1002/chem.201704020
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was in an amorphous state at this time. However, after the
ground sample was fumed with the organic solvent, the
diffraction peaks were restored to the initial state, indicating the
compound was recovered to the crystalline state. The results
clearly demonstrated that the piezochromic properties should be
attributed to the change in intermolecular arrangement pattern
from the crystalline state to the amorphous state.40-45 Comparing
the spectra before and after being ground, the absorbance
wavelength and emission wavelength of the compounds in the
amorphous state are longer than these in the crystalline state,
which further proved the analysis (Figure S11).
Table 2. Emission wavelength of derivatives solid samples under various
conditions.
λem(nm)/ФF(%)
Alteration
(nm)
Sample
Original
Ground
Fumed
CN-P-1,2,5
CN-P-1,3,4
CO-P-1,2,5
CO-P-1,3,4
CN-B-1,2,5
CN-B-1,3,4
CO-B-1,2,5
CO-B-1,3,4
585/22.1
596/19.1
597/17.8
611/15.0
616/9.3
619/13.8
617/14.6
640/9.9
628/9.1
630/5.8
637/8.2
692/5.1
681/7.5
587/20.8
595/18.3
596/17.1
613/13.9
619/9.6
602/14.3
--
34
21
43
17
14
34
6
For other P-based compounds, when CN-P-1,3,4 and CO-P-
1,3,4 compounds were in the original state, their diffraction
peaks were not very sharp, suggesting these compounds were
not in good crystalline state (Figure 8b). After grinding, the
change of intermolecular aggregation manner was not very
604/15.2
686/5.4
significant (Figure S9). As
a result, the variation in the
666/13.2
665/12.4
15
fluorescent color was not very distinct. For the B-based
compounds, when CN-B-1,3,4 and CO-B-1,3,4 compounds were
stimulated by external pressure, the change of molecular
aggregation form was relatively evident (Figure S10). However,
only the CN-B-1,3,4 compound had distinct color change. The
color change of CO-B-1,3,4 was not distinct because of its deep
red fluorescence. The original samples of CN-B-1,2,5 and CO-B-
a
Original
Ground
Fumed
1,2,5 were almost in the amorphous state, especially CO-B-1,2,5.
25
Owing to the role of nitrile,24,
CN-B-1,2,5 compound had
relatively twisted spatial structure. With external pressure, the
intermolecular arrangement pattern changed slightly (Figure
S10), which resulted in a slight variation in the wavelength and
color. For CO-B-1,2,5, the intermolecular stacking modes hardly
changed (Figure S10), correspondingly, the emission
wavelength and the fluorescence color were almost unchanged.
These results further elaborated that the donor has a significant
influence on the spatial configuration of the molecule.
10
15
20
25
30
2(deg)
CO-P-1,3,4
b
Conclusions
CO-P-1,2,5
CN-P-1,3,4
We designed and synthesized eight D-π-A compounds with
two triphenylpyrrole (TPP) isomers (TPP-1,2,5 or TPP-1,3,4) as
the donor, malononitrile (CN) or 1H-indene-1,3(2H)-dione (CO)
as the acceptor, and pyridone or benzopyran as the π-linker.
Compared with TPP-1,2,5 with ACQ characteristic, TPP-1,3,4
with AIE characteristic had greater space volume and more
twisted molecular structure. Nevertheless, all eight compounds
exhibited the aggregation-induced emission (AIE) properties and
piezochromic properties. Moreover, all the compounds displayed
longer emission wavelengths (the longest is 680 nm) and
stronger fluorescence intensities (the highest ΦF is 22.1%). More
interesting, for P-based compounds with the TPP-1,2,5 as the
donor exhibited more stronger fluorescence intensities, shorter
emission wavelengths, and more distinct piezochromic
properties, which is attributed to the fact that TPP-1,2,5
promotes the orderly intermolecular stacking. For B-based
compounds with the TPP-1,3,4 exhibited the same phenomenon
because TPP-1,3,4 suppresses the intermolecular interaction in
the aggregation state. In addition, different acceptors also can
affect the emission wavelength of the compounds. Specifically,
malononitrile as a acceptor is favorable for the derivative to have
CN-P-1,2,5
10
15
20
25
30
2(deg)
Figure 8. (a) XRD curves of CO-P-1,2,5 solid compound under different
conditions. (b) XRD curves of four P-based compounds under original
conditions
Experimental Section
Measurements and materials
All chemicals and solvents were commercially available and not further
processed.
(CN-P), 2-(1-butyl-2,6-dimethylpyridin-4(1H)-ylidene)-1H-indene-1,3(2H)-
dione(CO-P), 2-(1-butyl-2,6-2-(2-methyl-4H-chromen-4-ylidene)
2-(1-butyl-2,6-dimethylpyridin-4(1H)-ylidene)malononitrile
a
relatively strong fluorescence, whereas the 1H-indene-
1,3(2H)-dione induces a relatively long emission wavelength.
malononitrile (CN-B) and 2-(2-methyl-4H-chromen-4-ylidene)-1H-indene-
1,3(2H)-dione (CO-B) were obtained according to the synthetic methods
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10.1002/chem.201704020
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FULL PAPER
reported in the previous literature.46, 47 The structures of all products were
confirmed by NMR and Mass Spectrum (MS). 1H spectra through a
Bruker ARX400 spectrometer with CDCl3 as the solvent, and Mass
The reaction of TPP (1,2,5 or 1,3,4) and 2-(2-methyl-4H-chromen-4-
ylidene)-1H-indene-1,3(2H)-dione were carried out using the same
experimental method, and then obtain the target compound CO-B-1,2,5
and CO-B-1,3,4. The former was a deep red solid (134 mg, 40%), And
the latter was also a deep red solid (161 mg, 48%).
Spectrum were performed by using
spectroscopy. The UV-vis absorption spectra were measured on a
Persee TU-1901 UV-vis spectrophotometer. The Fluorescence spectra
a Finnigan BIFLEX III mass
were determined on
measurements were taken with
Advance). Solid-state emission quantum yields (ΦF) were collected on a
FluoroMax-4 (Horiba Jobin Yvon) fluorometer equipped contains an
integrated sphere
a
Hitachi F-7000 spectrophotometer. XRD
Bruker X-ray diffractometer (D8
CN-B-1,2,5: 1H NMR (CDCl3, 400 MHz): 8.93 (dd, J = 8Hz, 1H), 7.44-
7.77 (m, 10H), 7.12-7.26 (m, 12H), 6.83-6.89 (m, 2H), 6.51 (s, 2H).
HRMS (EI) m/z calcd. C42H27N3O, 589.22, found 590.22 [M+].
a
CN-B-1,3,4: 1H NMR (CDCl3, 400 MHz): 8.94 (d, J = 7.5Hz, 1H), 7.52-
7.78 (m, 13H), 7.24-7.55 (m, 11H), 6.99 (d, 2H). HRMS (EI) m/z calcd.
C42H27N3O, 589.22, found 590.22 [M+].
The theoretical ground-state geometry and electronic structure of
derivatives molecules were performed using the density functional theory
(DT-DFT) with CAM- B3LYP hybrid functional at the basis set level of 6-
31+G(d, p). All the theoretical calculations were optimized using
Gaussian03 package.
CO-B-1,2,5: 1H NMR (CDCl3, 400 MHz): 8.71 (s, 1H), 7.43-7.86 (m,
14H), 6.98-7.26 (m, 14H), 6.51 (s, 2H). HRMS (EI) m/z calcd. C48H31NO3,
669.23, found 670.24 [M+].
Synthesis of PD: A mixture of TPP (1,2,5 or 1,3,4) (478.8 mg, 1.2
mmol), 2-(1-butyl-2,6-dimethylpyridin-4(1H)-ylidene)malononitrile (113.5
mg, 0.5 mmol), piperidine (0.5 mL), and DMSO (10 mL) was stirred under
nitrogen for 48 h at 140 °C. After completion of the reaction, the mixture
was cooled to room temperature, and then extracted with
dichloromethane to remove DMSO. The organic layers were dried over
anhydrous Mg2SO4. After removal of solvent under reduced pressure, the
crude product was purified by column chromatography (petroleum
ether/ethyl acetate) (5:1, v/v) to get pure compounds CN-P-1,2,5 or CN-
P-1,3,4. The former is golden yellow solid (321 mg, 65%) and the latter is
a yellow solid (361 mg, 73%).
CO-B-1,3,4: 1H NMR (CDCl3, 400 MHz): 8.73 (s, 1H), 7.54-7.83 (m,
16H), 7.23-7.35 (m, 12H), 7.01 (d, 2H). HRMS (EI) m/z calcd. C48H31NO3,
669.23, found 670.24 [M+].
Acknowledgements
This work was financially supported by the National Basic
Research Program of China (973 Program; Grant
No.2013CB834704), the National Natural Scientific Foundation
of China (Grant Nos. 51673024, 51328302, 21404010).
The reaction of TPP (1,2,5 or 1,3,4) and 2-(1-butyl-2,6-dimethylpyridin-
4(1H)-ylidene)-1H-indene-1,3(2H)-dione also used above method, And
then get the target product CO-P-1,2,5 and CO-P-1,3,4. The first is a
yellow solid (364 mg, 68%), And the second is an orange solid (396 mg,
74%).
Keywords: D-π-A • Triphenylpyrrole Isomer donors •
Piezochromic Properties • Aggregation-Induced Emission (AIE)
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The
eight
D-π-A
compounds
Yunxiang Lei, Yueyin Lai, Lichao Dong,
Guojun Shang, Jianbing Shi, Junge Zhi,
Xiaobo Huang ,Bin Tong, Yuping Dong*
employing triphenylpyrrole isomers as
donors, malononitrile (CN) and 1H-
indene-1,3(2H)-dione
acceptors, pyridone
(CO)
(P)
as
and
Page No. – Page No.
benzopyran (B) as π-linking groups
exhibit aggregation-induced emission
The Synergistic Effect between
Triphenylpyrrole Isomers as Donors,
Linking Groups and Acceptors on the
Fluorescence Properties of D-π-A
Compounds in the Solid State
(AIE)
piezochromic
properties
and
evident
The
properties.
synergistic effect between donors,
linking groups and acceptors on the
fluorescence properties in the solid
state may be clearly observed.
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