The Aggregation Regularity Effect of Multiarylpyrroles on Their Near-Infrared Aggregation-Enhanced Emission Property

Article abstract of DOI:10.1002/chem.202002525

Increasing the quantum yield of near-infrared (NIR) emissive dyes is critical for biological applications because these fluorescent dyes generally show decreased emission efficiency under aqueous conditions. In this work, we designed and synthesized several multiarylpyrrole (MAP) derivatives, in which a furanylidene (FE) group at the 3-position of the pyrrole forms donor-π-acceptor molecules, MAP-FE, with a NIR emissive wavelength and aggregation-enhanced emission (AEE) features. Different alkyl chains of MAP-FEs linked to phenyl groups at the 2,5-position of the pyrrole ring resulted in different emissive wavelengths and quantum yields in aggregated states, such as powders or single crystals. Powder XRD data and single crystal analysis elucidated that the different lengths of alkyl chains had a significant impact on the regularity of MAP-FEs when they were forced to aggregate or precipitate, which affected the intermolecular interaction and the restriction degree of the rotating parts, which are essential components. Therefore, an increasing number of NIR dyes could be developed by this design strategy to produce efficient NIR dyes with AEE. Moreover, this method can provide general guidance for other related fields, such as organic solar cells and organic light-emitting materials, because they are all applied in the aggregated state.

Full text of DOI:10.1002/chem.202002525

Accepted Article
Accepted Article
Title: The Aggregation Regularity Effect of Multiarylpyrroles on Their
Near-Infrared Aggregation-Enhanced Emission Property
Authors: Jiamin Qu, Fei Ren, Jianbing Shi, Bin Tong, Zhengxu Cai,
and Yuping Dong
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.202002525
Link to VoR: https://doi.org/10.1002/chem.202002525
01/2020

10.1002/chem.202002525
Chemistry - A European Journal
FULL PAPER
The Aggregation Regularity Effect of Multiarylpyrroles on Their
Near-Infrared Aggregation-Enhanced Emission Property
Jiamin Qu, Fei Ren, Jianbing Shi,* Bin Tong, Zhengxu Cai and Yuping Dong*
J. Qu, F. Ren, Dr. J. Shi, Prof. B. Tong, Dr. Z. Cai, Prof. Y. Dong
Beijing Key Laboratory of Construction Tailorable Advanced Functional Materialsand Green Applications, School of Materials Science and Engineering
Beijing Institute of Technology
5 South Zhongguancun Str., Haidian District, Beijing 100081, China
E-mail: bing@bit.edu.cn (J. Shi); chdongyp@bit.edu.cn (Y. Dong)
Supporting information for this article is given via a link at the end of the document.
Abstract: Increasing the quantum yield of near-infrared (NIR)
emissive dyes is critical for biological applications because these
fluorescent dyes generally show decreased emission efficiency under
aqueous conditions. In this work, we designed and synthesized
several multiarylpyrrole (MAP) derivatives, in which a furanylidene
(FE) group at the 3-position of the pyrrole forms donor-π-acceptor
defined as aggregation-induced emission (AIE). In recent years,
the discovery of AIE has stimulated the design and preparation of
more AIE molecular systems to solve the application problems of
traditional fluorescent dyes.7-10 In addition to the AIE effect, some
fluorescent molecules exhibit weak or moderate emission in the
single-molecule state, but their emission becomes stronger by the
formation of aggregates, showing an aggregation-enhanced
emission (AEE) feature.11-12 AIE/AEE molecules emitting in the
NIR region demonstrate “turn on” characteristics in the
aggregated state, so the AIE effect promotes the application of
NIR dyes in biological fields.13-16
molecules, MAP-FE, with
a
NIR emissive wavelength and
aggregation-enhanced emission (AEE) features. Different alkyl chains
of MAP-FEs linked to phenyl groups at the 2,5-position of the pyrrole
ring resulted in different emissive wavelengths and quantum yields in
aggregated states, such as powders or single crystals. Powder XRD
data and single crystal analysis elucidated that the different lengths of
alkyl chains had a significant impact on the regularity of MAP-FEs
when they were forced to aggregate or precipitate, which affected the
intermolecular interaction and the restriction degree of the rotating
parts, which are essential components. Therefore, an increasing
number of NIR dyes could be developed by this design strategy to
produce efficient NIR dyes with AEE. Moreover, this method can
provide general guidance for other related fields, such as organic
solar cells and organic light-emitting materials, because they are all
applied in the aggregated state.
At present, through the unremitting efforts of scientists, a large
number of NIR AIE/AEE dyes have been developed using
electron donor-acceptor (D-A) or donor- п -acceptor (D-π-A)
strategies.17-20 For example, Tang and his coworkers used
triphenylamine as a donor and a pyridine salt derivative as an
acceptor to synthesize three NIR molecules with targeting effects
for synergistic photodynamic therapy, achieving the 1+1+1> 3
effect.21 Ding’s group reported a series of AIE dyes in the NIR
region using 2-(4-pyridin-4-ylphenyl) acetonitrile as a receptor
and triphenylamine and benzene as donors and then added
tetraphenylethylene (TPE) units in the donor part to increase the
degree of molecular distortion to reduce intermolecular
interactions and to increase reactive oxygen species (ROS)
Introduction
production.22-23 Ajayaghosh et al. designed
a series of
diketopyrrolopyrrole-based NIR absorption or emission
optoelectronic functional materials by D-A strategy.24-25 Zhu et al.
reported minireviews using a quinoline-malononitrile core as a
building block for diversity-oriented synthesis of AIE luminogens
(AIEgens).26 Our group is also devoted to designing AIEgens with
long wavelength emission and a high quantum yield in the
aggregated state based on multiarylpyrroles (MAPs) as the
backbone.27-30 Among them, AIEgens, with triphenylpyrrole as the
conjugate skeleton, triphenylamine as the strong donor, 1-ethyl-
3,3-dimethyl-3H-indolium as the strong acceptor and an anchor
for the biological target, were synthesized, and the emission
wavelength (λem) reached 724 nm.30 Moreover, AIEgens can be
used for real-time bioimaging of mitochondria under a wash-free
Near-infrared (NIR) fluorescent dyes have attracted significant
attention due to their wide use in many fields, such as biological
imaging and tumor treatment, which originates from their deep
tissue penetration, reduced background autofluorescence and
minimal damage to biological samples.1-5 However, traditional
NIR fluorescent molecules are usually composed of π-conjugated
large planar structures, which can emit strong fluorescence in
dilute solution but are weakly emissive or even nonfluorescent in
the aggregated state. This phenomenon is usually called
aggregation-caused quenching (ACQ), which limits their
biological applications because most NIR dyes are hydrophobic
and prone to aggregation in living organisms. In 2001, Tang’s
group discovered an intriguing phenomenon: a silole compound,
staining
procedure.
Recently,
(3-cyano-4,5-dimethyl-2,5-
1-methyl-1,2,3,4,5-pentaphenylsilole, behaved like
a
weak
dihydrofuran-2-ylidene) propanedinitrile (FE) was introduced into
MAPs as a strong acceptor, and a series of fused-ring groups,
such as naphthalene, anthracene, and pyrene, were introduced
at the 2,5-position of the pyrrole derivatives to extend the
emitter when dissolved in a good solvent. In contrast, with the
addition of poor solvents, aggregates are formed, and the
fluorescence intensity gradually increases.6 This situation is
1
This article is protected by copyright. All rights reserved.

10.1002/chem.202002525
Chemistry - A European Journal
FULL PAPER
conjugated skeleton, as shown in Scheme 1. Compounds
MAP1~6-FE showed AEE properties with NIR emission.31
However, the introduction of a fused ring group plays a dual role
on their emission. One role is an improvement in the quantum
yield (QY, ΦF), and the other role is unfavorable for emission due
to the π-π stacking interaction of large planar structures in solids.
Therefore, the ΦF values of these compounds in solids are
generally low, and the maximum ΦF value is 6.56% for maximum
emission centered at 679 nm.
organic solvents, such as tetrahydrofuran (THF) and dimethyl
sulfoxide (DMSO).
Photophysical Properties in Solution and in Solid. The UV-vis
absorption spectra of MAP-FEs were collected in THF solution
and are shown in Figure 1a. All the compounds, MAP7~12-FE,
R
O
CN
CN
O
CN
CN
R
CN
CN
With this issue in mind, we further explore how to increase ΦF
in the solid or aggregated state and maximize the AIE advantage
of NIR materials.32-35 Liu et al. optimized the AIE property of NIR
molecules by adjusting the length of the alkoxy chain and the
position of the substituents. These strategies have been
successfully used for image-guided tumor resection.36 Tang et al.
reported that intermolecular rearrangement by increasing the
chain length was accelerated to increase the recovery rate from
amorphous to crystalline.37 Li et al. explored the effects of
molecular packing and different morphologies on emission
behavior.38-41 Tang and Zhao synthesized a series of pyridinium-
functionalized TPE salts that had different alkyl chains. The length
of the chain had little influence on the optical properties in solution
due to the similar electronic structure, but different
hydrophobicities in aqueous media affected the aggregate
morphology, leading to diverse emission behaviors.42 Simple
linear alkyl chains play a key role in molecular interactions and
packing.43 Hence, in this work, we introduced alkyl chains of
different lengths to the phenyl group at the 2,5-position of pyrrole
rings, as shown in Scheme 1, to investigate their NIR emission in
the solid state. The FE group was also chosen as an electron
acceptor due to its strong electron-withdrawing ability, with 1,2,5-
triphenylpyrrole as a donor with a twisting conformation due to its
AIE property. Thus, a typical D-π-A structure of MAP-FEs was
constructed, and all the obtained compounds exhibited AEE
properties, with the maximum emission wavelength up to the NIR
region. It is worth mentioning that the synthesized compounds
had higher QYs in the solid state in the NIR range. More
importantly, different alkyl chain lengths resulted in different
regular packing, which affected their QY in the aggregated state,
as shown through techniques such as powder X-ray diffraction
(XRD) and single crystals.
C2H5OOC
N
C2H5OOC
N
R
*
R
R =
*
*
R =
*
MAP7-FE
MAP8-FE
MAP1-FE
MAP2-FE
*
MAP3-FE
*
*
*
MAP9-FE
MAP10-FE
MAP11-FE
*
*
*
MAP4-FE
MAP6-FE
MAP5-FE
MAP12-FE
*
In this work
Our previous work in Ref. [31]
Scheme 1. Structures of MAP-FEs used in our previous work and in this work.
demonstrated similar absorption bands and had strong absorption
in the visible region from 400 to 600 nm. Even the aggregates of
MAP-FEs in the mixed solution of THF as a good solvent and H2O
as a poor solvent with a 90% water fraction (fw) also showed
similar absorption behavior, as illustrated in Figure 1b. These
results indicated that the length of the alkyl side chain of the
phenyl ring linked at the 2,5-position of pyrrole had little effect on
the UV-vis absorption spectra of MAP-FEs in solution.
MAP7-FE
MAP8-FE
MAP9-FE
MAP10-FE
MAP11-FE
MAP12-FE
MAP7-FE
MAP8-FE
MAP9-FE
MAP10-FE
MAP11-FE
MAP12-FE
0.4
0.3
0.2
0.1
0.0
0.4
0.3
0.2
0.1
0.0
(b)
(a)
300
400
500
600
700
300
400
500
600
700
Results and Discussion
Wavelength (nm)
Wavelength (nm)
Design and Synthesis. According to our previous work, the
phenyl ring at the 2,5-position of pyrrole can increase the
conjugation degree of the dye skeleton,44 the FE group is a strong
electron-withdrawing group, and p-benzoate ethyl ester group at
the 1-position of the pyrrole can be easily post-functionalized into
other functional or biological target group when necessary.31
Therefore, the aim of introducing alkyl chains on the phenyl ring
located at the 2,5-position of pyrrole is mainly to explore their
effect on the molecular packing regularity in the aggregated state,
particularly in the solid state. The detailed synthetic routes for
MAP7~12-FE are shown in Scheme S1 of the Supporting
Information (SI), and the chemical structures of all the compounds,
including the intermediates and the target dyes, were confirmed
by nuclear magnetic resonance (NMR) spectroscopy and mass
spectroscopy (MS), the results of which are also shown in Figures
S1~S52. In addition, MAP7~12-FE have better solubility in polar
Figure 1. (a) Absorption spectra of MAP-FEs in THF. (b) Absorption spectra of
MAP-FEs in the mixture of THF/H2O with a 90% water fraction. [MAP-FEs] = 1.0
× 10-5 mol/L.
Next, the normalized photoluminescence (PL) spectra of MAP-
FEs in THF and in solid were obtained and are shown in Figure 2.
MAP7~12-FE showed the same emission behaviors in THF, such
as maximum peak wavelengths (λem) at 622 nm and a full width
at half maximum (FWHM) of 100 nm (Figure 2a). However, they
showed different λem values in powder as an aggregated state
ranging from 636 to 683 nm, as shown in Figure 2b. The FWHM
of MAP7~12-FE narrowed to 64, 83, 81, 66, 85, and 87 nm,
respectively. Moreover, the λem increased in sequence with the
increasing length of alkyl chains, except MAP7-FE. The unusual
λem of MAP7-FE will be discussed in the following single crystal
2
This article is protected by copyright. All rights reserved.

10.1002/chem.202002525
Chemistry - A European Journal
FULL PAPER
section. All these results indicated that the length of the alkyl chain
of the phenyl ring linked at the 2,5-position of pyrrole had an
obvious effect on the PL property of MAP-FEs in the solid state,
which clarified their aggregation behavior as the target dyes.
increased for the solid of MAP-FEs that were compactly
aggregated. The value of knr was greatly decreased, while that of
kr showed almost no obvious change; thus, the NIR emission
intensities of all the MAP-FEs in the solid state greatly increased.
That is, the mechanism of AEE typically belongs to the restricted
intramolecular rotation (RIR) effect, which is affiliated with
restricted intramolecular motion (RIM). However, it is worth noting
that the six NIR dyes exhibited significant differences among their
ΦFs collected from their powder samples. The values of ΦF were
19.38%, 25.82%, 15.00%, 32.86%, 14.62% and 7.07% for MAP7-
FE to MAP12-FE in the solid state, respectively. Considering the
same skeleton structure of these NIR dyes, the solid-state
emission efficiency is much different and must be related to the
length of the alkyl chain. The longer alkyl chain resulted in the
looser compactness in solid. So the RIR degree of MAP12-FE is
less than others and the quantum yield in solid powder is lowest.
To sufficiently confirm that the length of the alkyl chain has a
greater effect on the emission behavior of MAP-FEs in the
aggregated state, such as solid powders, an immobilized single-
molecule state of all the MAP-FEs was prepared by dispersing the
dye molecules into polymethyl methacrylate (PMMA) films, which
can lock the rotation of the phenyl rings of pyrroles. The QYs of
the MAP-FEs in PMMA films exhibited little difference, as shown
in Table 1, which in turn, verified that the length of the alkyl chain
can affect the compaction degree in the aggregated state of these
MAP-FEs when they were forced to aggregate. In addition,
density functional theory (DFT) was used to calculate the energy
levels of the MAP-FEs in the single-molecule state by Gaussian
09 (Figure S54). The results of the DFT calculation demonstrated
that the orbital energy level of the highest occupied molecular
orbital (HOMO) and lowest unoccupied molecular orbital (LUMO)
hardly changed with the length of the alkyl chain, which was
consistent with the experimental photophysical results of these
NIR dyes in solution and in PMMA films. In other words, the length
of the alkyl chain on the phenyl ring has a profound impact on
their NIR emissive properties of MAP-FEs in the aggregated state,
such as powders or crystals. All the above results indicated that
the AEE feature authentically originated the reduced nonradiative
energy loss due to RIR and that the regularity/compactness of
NIR dyes had an important effect on the AEE efficiency.
(b)
MAP7-FE
MAP8-FE
MAP9-FE
MAP10-FE
MAP11-FE
MAP12-FE
(a)
MAP7-FE
MAP8-FE
MAP9-FE
MAP10-FE
MAP11-FE
MAP12-FE
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
550
600
650
700
750
800
850
550
600
650
700
750
800
850
Wavelength (nm)
Wavelength (nm)
Figure 2. (a) Normalized PL spectra of MAP-FEs in THF. (b) Normalized PL
spectra of MAP-FEs in powder. Excited wavelength (λex): 510 nm for THF
solution and 500 nm for powder state; [MAP-FEs] = 1.0 × 10-5 mol/L.
The AEE Characteristics with NIR Emission. As mentioned
above, both the λem and FWHM were different among these MAP-
FEs in the solid state. Therefore, their aggregated PL spectra
were obtained by changing the poor solvent of the water fraction
fw, and the results are shown in Figure S53. In a pure THF solution
of 1.0 × 10-5 mol/L, the MAP-FEs showed a certain emissive
intensity, which means that all these dyes could emit in dilute
solution. Then, the fluorescence became weak when a small
amount of water was added into the THF solution, which showed
a typical solvent effect of fluorescent dyes in polar solvents.
However, further increasing fw resulted in a fluorescence increase
after fw up to 60% or 70%, and the PL intensity reached a
maximum at fw = 90%, showing AEE characteristics.
Further, other photophysical data, including ΦF and the
fluorescence lifetime (τ) in solution and in the solid state, are
summarized in Table 1. Moreover, the radiative transition rate
constant (kr) and nonradiative transition rate constant (knr) could
be obtained from ΦF and τ according to the equations ΦF
=
kr/(kr+knr) and τ = 1/( kr+knr), respectively.45 There is almost no
difference in the QYs and lifetimes of the MAP-FEs in THF. The
value of knr is approximately two orders of magnitude higher than
that of kr, which leads to a low ΦF due to the freely rotating phenyl
groups as rotors in these NIR dyes, which largely dissipate the
radiation energy via a thermal/kinetic energy loss. Both ΦF and τ
Next, the effect of the length of the alkyl chain on the phenyl
rings of the MAP-FEs on their NIR emissive efficiency was
investigated by powder X-ray diffraction (PXRD). The pristine
samples that were obtained by removing the solvent from the
Table 1. Summary of the photophysical data for the MAP-FEs in solution, in the solid state and in PMMA films.
Solution[a]
τ[c]
Solid
τ[c]
PMMA film[d]
λem
[b]
[b]
[b]
Compounds
λex
λem
ΦF
kr
knr
λex
λem
ΦF
kr
knr
λex
ΦF
(nm)
(nm)
(%)
(ns)
(×108 s-1)
(×108 s-1)
(nm)
(nm)
(%)
(ns)
(×108 s-1)
(×108 s-1)
(nm)
(nm)
(%)
MAP7-FE
MAP8-FE
MAP9-FE
MAP10-FE
MAP11-FE
MAP12-FE
510
516
510
511
510
505
621
621
622
623
623
623
1.50
1.49
1.72
1.56
1.61
1.64
0.24
0.48
0.25
0.24
0.47
0.16
0.6250
0.3104
0.6880
0.6500
0.3426
1.0250
41.04
20.52
39.31
41.02
20.93
61.48
490
515
502
504
503
500
668
645
647
663
672
682
19.38
25.82
15.00
32.86
14.62
7.07
3.51
3.52
2.88
5.30
1.94
3.17
0.5521
0.7335
0.5208
0.6200
0.7536
0.2303
2.297
2.107
2.951
1.267
4.401
2.924
470
515
470
475
510
510
626
632
625
623
618
613
27.93
34.18
30.91
36.45
37.11
37.89
[a]
[b]
Notes: Measured in THF at [MAP-FEs] = 1.0 × 10-5 mol/L. The excitation wavelength (λex) for obtaining the absolute fluorescence quantum yield (ΦF) was
determined by integrating a sphere. [c] An average lifetime. [d] The PMMA film is made from PMMA solution containing 0.5 wt% MAP-FEs and then dried in vacuum.
3
This article is protected by copyright. All rights reserved.

10.1002/chem.202002525
Chemistry - A European Journal
FULL PAPER
rotary evaporation apparatus during the purifying process and by
embedding dyes into PMMA were used for PXRD
characterization. As shown in Figure 3a, the PXRD pattern of
MAP10-FE showed strong and sharp diffraction peaks, which
means that it had good regularity when forcing dye molecules to
precipitate from solution. This result can well explain why the
abovementioned QY of MAP10-FE in powder (32.86%) was
higher than others. The more compact the state is, the more
restricted it is. The good regularity of MAP10-FE is well self-
assembled when precipitating, which is very favorable to AEE.
Therefore, on the other hand, the length of the alkyl chain on the
rotating part had significant effects on the AEE behaviors if the
dyes were arranged spontaneously. Different alkyl chain lengths
cause different spontaneous arrangement degrees. The relatively
good QY of MAP8-FE (25.82%) also corresponded to a relatively
good regularity since relatively sharp peaks appeared. When
0.5wt% of the MAP-FEs was doped into a pure PMMA solution
and then prepared for thin films, all the sharp peaks disappeared,
as shown in Figure 3b, because the molecules of the MAP-FEs
were frozen into PMMA, making it difficult for them to effectively
Lastly, single crystals of the MAP-FEs were obtained by slowly
evaporating a mixture of dichloromethane and petroleum ether.
The molecular structures of MAP9-FE and MAP10-FE in single
crystals, as an example for a related discussion, are shown in
Figure 4. The twist angles between the pyrrole ring and other ring
substituents at the 1, 2, 3, and 5-positions are illustrated in
Figures 4a and 4e. The relatively large twisting angles between
the pyrrole ring and the phenyl ring at the 1,2,5-position of pyrrole
ensured the higher emission efficiency in the solid due to reducing
the π-π stacking interaction that caused ACQ. The very small
twisting angles between the pyrrole ring and the FE group at the
3-position of pyrrole through the vinylidene group spacer
benefited the electron transfer among D-π-A when excited, which
was favorable to narrow the energy gap and to red-shift the
emission wavelength. Single crystals of MAP7-FE, MAP8-FE,
MAP11-FE, and MAP12-FE showed similar twisting angles, as
shown in Figure S55. It is also worth noting that the twisting angle
of the FE group of MAP7-FE and the pyrrole ring was only 2.5o
(Figure S55a), which was a probable cause for its irregular λem
(see Figure 2b) compared with the other NIR dyes. In addition,
the C-Hꞏꞏꞏπ interaction of MAP9-FE and MAP10-FE was found to
have close distances of 2.686 Å and 2.788 Å, respectively, as
shown in Figures 4b and 4f, which can effectively restrict
intramolecular motion and then block the nonradiative energy
decay channel, thus promoting the AEE efficiency of solid MAP-
FEs in the NIR region. Moreover, there are also hydrogen bonding
interactions, such as the C-HꞏꞏꞏO and C-HꞏꞏꞏN interactions, in
MAP9-FE and MAP10-FE, as shown in Figures 5c, 5d, and 5g-5i,
and all these intermolecular interactions had a positive effect on
their AEE efficiency. Similar interactions of C-Hꞏꞏꞏπ and hydrogen
bonding were observed in other length of alkyl chains of MAP7-
FE, MAP8-FE, MAP11-FE and MAP12-FE. The only difference
was that MAP10-FE, which has an n-butyl on the phenyl ring
located at the 2,5-position of pyrrole, had more intermolecular
interactions than others, which is beneficial to closer packing
when the molecules are constrainedly arranged from the single
molecular state to the solid state. This is a critical cause of the
better emission efficiency of MAP10-FE in the solid state.
Therefore, according to the single crystal analysis, designing NIR
emissive dyes in the aggregated state should fortify multiple
intermolecular interactions but avoid π-π stacking through
introduction of some alkyl chains in the rotating segments.
(b)
(a)
MAP12-FE
MAP11-FE
PMMA+MAP12-FE
PMMA+MAP11-FE
PMMA+MAP10-FE
PMMA+MAP9-FE
MAP10-FE
MAP9-FE
PMMA+MAP8-FE
PMMA+MAP7-FE
MAP8-FE
MAP7-FE
10
20
2
30
40 10
20
30
40
o
o
)

(
)
2
(
Figure 3. (a) XRD data of the MAP-FEs in pristine powder. (b) XRD data of
MAP-FEs in homodisperse PMMA films.
self-arrange. This result is highly consistent with their QYs listed
in Table 1; little difference was observed for frozen molecules.
This strategy for regulating the length of the side chains on the
rotating parts provides an ideal and feasible way to design highly
efficient NIR molecules with narrow FWHMs in the aggregated
state.
Thermal analysis. The difference in the single crystals of the
Figure 4. Molecular structure of (a) MAP9-FE (CDCC:1936113) and (e) MAP10-FE (CDCC:1987059). C-Hꞏꞏꞏπ interactions between the adjacent molecules in the
single crystal of (b) MAP9-FE and (f) MAP10-FE. C-HꞏꞏꞏO interactions between the adjacent molecules of (c) MAP9-FE and (g and h) MAP10-FE. C-HꞏꞏꞏN
interactions between the adjacent molecules of (d) MAP9-FE and (i) MAP10-FE.
4
This article is protected by copyright. All rights reserved.

10.1002/chem.202002525
Chemistry - A European Journal
FULL PAPER
was purchased from Xiya Reagent. K2CO3, CH3COONH4 and chloroform-
MAP-FEs motivated us to further explore the effects of alkyl
chains on their thermodynamic properties, such as the melting
point, thermal decomposition temperature, and exothermic heat.
Therefore, thermogravimetric analysis (TGA) and differential
scanning calorimetry (DSC) were carried out, and the
corresponding graphs are shown in Figures 5 and S56. The
temperature to reach a 5% weight loss for all the dyes was higher
than 340 °C, as shown in Figure 5a, which showed good thermal
stability. However, the DSC curves in heating or cooling showed
different thermal behaviors, as shown in Figures 5b and S56. The
melting point showed the same value in the first heating and the
second heating. The values of the melting points were 292, 275,
243, 217, 179 and 170 °C for MAP7~12-FE, respectively.
Additionally, there was a certain proportionality between the
melting point and the length of alkyl chains, as described in Figure
S57; the melting point of MAP-FEs gradually decreased with the
extension of the alkyl chains. A different compact degree resulted
in different melting points due to different alkyl chains.
d
were purchased from Innochem. 4-methylphenylboronic acid, 4-
ethylphenylboronic acid, 4-propylphenylboronic acid, 4-butylphenylboronic
acid, 4-pentylphenylboronic acid and 4-hexyl phenylboric acid were all
purchased from Sukailu.
1H and 13C NMR spectra were recorded on a Bruker AV 400 spectrometer.
Mass spectra were measured by using a Finnigan Biflex III mass
spectrometer. UV/Vis absorption spectra were recorded on a TU-1901
UV/Vis spectrophotometer. Photoluminescence spectra were recorded on
a Hitachi F-7000 spectrophotometer. PL quantum yields were measured
by using an integrating sphere on a NanoLog FL3-2iHR fluorescence
spectrometer (Horiba Jobin Yvon), and PL time-resolved decays were
measured with a DeltaFlex ultrafast lifetime spectrofluorometer (Horiba
Jobin Yvon). Single-crystal data were collected on a Bruker-AXS SMART
APEX 2 CCD diffractometer. TG data were measured on a TGA/DSC3+
thermal analyzer. DSC data were measured on a DSC3+ thermal analyzer.
The XRD patterns were measured by an X'Pert Pro MRD X-ray single
crystal diffractometer.
Synthesis of compounds MAP7~12-FE
(b)
(a)
The synthesis method of compounds ethyl 4-(2,5-dibromo-1H-pyrrol-1-
MAP7-FE
100
MAP12-FE
MAP8-FE
MAP9-FE
MAP10-FE
MAP11-FE
MAP12-FE
yl)benzoate
and
2-(3-cyano-4,5,5-trimethylfuran-2(5H)-
ylidene)malononitrile has been reported.31 The synthesis steps of
intermediates MAP7~12 and MAP7~12-CHO are detailed in Supporting
Information(SI). Specific synthesis methods of target compounds
MAP7~12-FE are given below.
80
60
40
20
0
MAP11-FE
MAP10-FE
MAP9-FE
MAP8-FE
MAP7-FE
Synthesis of MAP-FEs: MAP-CHOs (0.50 mmol), 2-(3-cyano-4,5,5-
trimethylfuran-2(5H)-ylidene)malononitrile (0.1905 g, 0.55 mmol) and
CH3COONH4 (0.0425 g, 0.55 mmol) were dissolved in 10 mL THF/C2H5OH
(4/1) mixed solvents and then stirred at room temperature for 24 h. The
solvent was evaporated under reduced pressure and the residue was
100
150
200
250
300
200
400
600
800
Temperature (oC)
Temperature (oC)
purified
by
silica
gel
column
chromatography
using
a
Figure 5. (a) TGA graphs of MAP-FEs. (b) DSC curves for the secondary
heating of MAP-FEs. The heating rate is 10 K/min under N2.
dichloromethane/petroleum ether mixture (1/3, Vd/Vp) as the eluent to give
desired compound MAP-FEs with a certain yield.
MAP7-FE: The yield gives 75.9%. 1H NMR (400 MHz, CDCl3): δ = 7.90 (d,
J = 8.4 Hz, 2H), 7.65 (d, J = 15.6 Hz, 1H), 7.12 (d, J = 7.6 Hz, 2H), 7.07-
6.93 (m, 8H), 6.86 (s, 1H), 6.74 (d, J = 15.6 Hz, 1H), 4.35 (q, J = 7.2 Hz,
Conclusion
2H), 2.33 (d, J = 15.6 Hz, 6H), 1.63 (s, 6H), 1.37 (t, J = 7.2 Hz, 3H). 13
C
In summary, six NIR MAP-FEs with AEE properties were
synthesized, and their photophysical properties were observed in
different molecular states, especially in solid state. Introducing
alkyl chains with different lengths on the benzene ring at the 2,5-
position of the pyrrole ring had important effects on the NIR
emissive efficiency of the MAP-FEs. The results, as confirmed by
PXRD and single crystal analysis, indicated that the length of the
alkyl chain mainly affected the regularity of the aggregated state,
which further led to more intermolecular interactions for the
suitable length and restrained the nonradiative energy loss and
thus changed their ΦF. Additionally, the smaller dihedral angle
between the electron acceptor FE group and the electron donor
pyrrole ring facilitated the red shift of the emission wavelength. All
these results provide theoretical and experimental support for
designing and developing other excellent NIR organic dyes with
AEE characteristics.
NMR (100 MHz, CDCl3): δ = 176.20, 174.98, 165.58, 143.05, 142.84,
141.18, 139.33, 138.71, 138.01, 130.52, 130.28, 129.98, 129.42, 129.15,
128.72, 128.33, 127.83, 125.88, 121.37, 112.46, 111.66, 111.10, 110.95,
106.71, 96.88, 95.09, 61.42, 55.24, 26.55, 21.37, 21.21, 14.26. HR-MS
(APCI, m/z) Calcd for C39H32N4O3 [M+H]+: 605.2547, found: 605.2568,
error 3.37 ppm.
MAP8-FE: The yield gives 44.9%. 1H NMR (400 MHz, CDCl3): δ = 7.91 (d,
J = 8.8 Hz, 2H), 7.65 (d, J = 16.0 Hz, 1H), 7.15 (d, J = 8.0 Hz, 2H), 7.12-
6.92 (m, 8H), 6.86 (s, 1H), 6.74 (d, J = 16.0 Hz, 1H), 4.36 (q, J = 7.2 Hz,
2H), 2.74-2.53 (m, 4H), 1.62 (s, 6H), 1.37 (t, J = 7.2 Hz, 3H), 1.27 (q, J =
7.6 Hz, 6H). 13C NMR (100 MHz, CDCl3): δ = 176.25, 175.03, 165.64,
145.50, 144.25, 143.14, 142.98, 141.23, 138.76, 130.61, 130.29, 130.00,
128.77, 128.37, 128.16, 128.06, 127.94, 126.11, 121.45, 112.50, 111.70,
111.11, 110.96, 106.81, 96.92, 95.03, 61.43, 55.15, 28.55, 28.50, 26.53,
15.19, 15.01, 14.28. HR-MS (APCI, m/z) Calcd for C41H36N4O3 [M+H]+:
633.2860, found: 633.2850, error -1.65 ppm.
MAP9-FE: The yield gives 62.4%. 1H NMR (400 MHz, CDCl3): δ = 7.90 (d,
J = 8.4 Hz, 2H), 7.63 (d, J = 16.0 Hz, 1H), 7.13 (d, J = 8.4 Hz, 2H), 7.10-
6.90 (m, 8H), 6.86 (s, 1H), 6.74 (d, J = 16.0 Hz, 1H), 4.36 (q, J = 7.2 Hz,
2H), 2.65-2.48 (m, 4H), 1.69-1.56 (m, 10H), 1.37 (t, J = 7.2 Hz, 3H), 1.01-
0.84 (m, 6H). 13C NMR (100 MHz, CDCl3): δ = 176.20, 174.97, 165.62,
144.05, 143.12, 142.93, 142.78, 141.21, 138.75, 130.53, 130.24, 129.99,
128.74, 128.68, 128.52, 128.32, 128.06, 126.16, 121.44, 112.47, 111.66,
111.06, 110.94, 106.77, 96.85, 95.13, 61.41, 55.25, 37.68, 26.53, 24.42,
Experimental Section
Materials and equipment
The synthesis process is shown in Scheme S1, all compounds were used
without further purification. Pd(PPh3)4 was purchased from J&K, POCl3
5
This article is protected by copyright. All rights reserved.

10.1002/chem.202002525
Chemistry - A European Journal
FULL PAPER
24.25, 24.12, 14.27, 13.81, 13.71. HR-MS (APCI, m/z) Calcd for
C43H40N4O3 [M+H]+: 661.3173, found: 661.3194, error 3.11 ppm.
[7]
Y.-L. Wang, C. Fan, B. Xin, J.-P. Zhang, T. Luo, Z.-Q. Chen, Q.-Y. Zhou,
Q. Yu, X.-N. Li, Z.-L. Huang, C. Li, M.-Q. Zhu, B. Z. Tang, Mater. Chem.
Front. 2018, 2, 1554-1562
.
MAP10-FE: The yield gives 50.4%. 1H NMR (400 MHz, CDCl3): δ = 7.92
(d, J = 8.4 Hz, 2H), 7.65 (d, J = 16.0 Hz, 1H), 7.16 (d, J = 8.0 Hz, 2H), 7.11-
6.95 (m, 8H), 6.88 (s, 1H), 6.76 (d, J = 15.6 Hz, 1H), 4.38 (q, J = 7.2 Hz,
2H), 2.68-2.54 (m, 4H), 1.64 (s, 6H), 1.62-1.56 (m, 4H), 1.42-1.31 (m, 7H),
0.99-0.92 (m, 6H). 13C NMR (100 MHz, CDCl3): δ = 176.23, 175.02, 165.63,
144.28, 143.18, 143.01, 142.98, 141.21, 138.76, 130.55, 130.26, 129.98,
128.70, 128.47, 128.33, 128.01, 126.10, 121.45, 112.51, 111.70, 111.09,
110.94, 106.76, 96.89, 95.08, 61.42, 55.17, 35.36, 35.32, 33.32, 33.12,
26.54, 22.39, 22.34, 14.27, 13.94. HR-MS (APCI, m/z) Calcd for
C45H44N4O3 [M]+: 688.3408, found: 688.3398, error -1.47 ppm.
[8]
[9]
X. Cai, B. Liu, Angew. Chem. Int. Ed. 2020, DOI:
10.1002/anie.202000845.
Y. Zhang, H. Xu, W. Xu, C. Zhang, J. Shi, B. Tong, Z. Cai, Y. Dong, Sci.
Chi. Chem. 2019, 62, 1393-1397.
[10] R. Thirumalai, R. D. Mukhopadhyay, V. K. Praveen, A. Ajayaghosh, Sci.
Rep. 2015, 5, 9842.
[11] X. Yang, Q. Wang, P. Hu, C. Xu, W. Guo, Z. Wang, Z. Mao, Z. Yang, C.
Liu, G. Shi, L. Chen, B. Xu, Z. Chi, Mater. Chem. Front. 2020, 4, 941-
949.
[12] J. Yang, Z. Chi, W. Zhu, B. Z. Tang, Z. Li, Sci. Chi. Chem. 2019, 62,
1090-1098.
[13] J. S. Ni, P. Zhang, T. Jiang, Y. Chen, H. Su, D. Wang, Z. Q. Yu, R. T. K.
Kwok, Z. Zhao, J. W. Y. Lam, B. Z. Tang, Adv. Mater. 2018, 30,
e1805220.
MAP11-FE: The yield gives 46.0%. 1H NMR (400 MHz, CDCl3): δ = 7.92
(d, J = 8.4 Hz, 2H), 7.64 (d, J = 15.6 Hz, 1H), 7.16 (d, J = 8.0 Hz, 2H), 7.13-
6.92 (m, 8H), 6.88 (s, 1H), 6.77 (d, J = 16.0 Hz, 1H), 4.38 (q, J = 7.2 Hz,
2H), 2.69-2.52 (m, 4H), 1.64 (s, 6H), 1.67-1.56 (m, 10H), 1.46-1.20 (m,
11H), 1.00-0.82 (m, 6H). 13C NMR (100 MHz, CDCl3): δ = 176.21, 175.00,
165.62, 144.30, 143.16, 143.04, 142.95, 141.21, 138.76, 130.55, 130.25,
129.98, 128.70, 128.47, 128.33, 128.01, 126.11, 121.44, 112.50, 111.68,
111.08, 110.96, 106.75, 96.87, 95.13, 61.41, 55.20, 35.61, 31.51, 31.43,
30.84, 30.63, 26.55, 22.51, 22.48, 14.27, 14.08, 14.05. HR-MS (APCI, m/z)
Calcd for C47H48N4O3 [M+H]+: 717.3799, found: 717.3792, error -1.07 ppm.
[14] M. Gao, F. Yu, C. Lv, J. Choo, L. Chen, Chem. Soc. Rev. 2017, 46, 2237-
2271.
[15] X. Ge, M. Gao, B. Situ, W. Feng, B. He, X. He, S. Li, Z. Ou, Y. Zhong, Y.
Lin, X. Ye, X. Hu, B. Z. Tang, L. Zheng, Mater. Chem. Front. 2020, 4,
957-964.
[16] T. Xue, X. Jia, J. Wang, J. Xiang, W. Wang, J. Du, Y. He, Chem. Eur. J.
2019, 25, 9634-9638.
[17] Q. Zhao, J. Z. Sun, J. Mater. Chem. C. 2016, 4, 10588-10609.
[18] J. Merz, M. Dietz, Y. Vonhausen, F. Wöber, A. Friedrich, D. Sieh, I.
Krummenacher, H. Braunschweig, M. Moos, M. Holzapfel, C. Lambert,
T. B. Marder, Chem. Eur. J. 2019, 26, 438-453.
MAP12-FE: The yield gives 47.5%. 1H NMR (400 MHz, CDCl3): δ = 7.89
(d, J = 8.4 Hz, 2H), 7.62 (d, J = 15.6 Hz, 1H), 7.13 (d, J = 7.6 Hz, 2H), 7.09-
6.89 (m, 8H), 6.86 (s, 1H), 6.74 (d, J = 16.0 Hz, 1H), 4.35 (q, J = 7.2 Hz,
2H), 2.68-2.47 (m, 4H), 1.63-1.54 (m, 10H), 1.37 (t, J = 7.2 Hz, 3H), 1.31-
1.25 (m, 12H), 0.92-0.83 (m, 6H). 13C NMR (100 MHz, CDCl3): δ = 176.20,
174.98, 165.62, 144.30, 143.13, 143.05, 142.92, 141.22, 138.77, 130.54,
130.24, 129.98, 128.70, 128.46, 128.33, 128.01, 126.11, 121.43, 112.48,
111.66, 111.06, 110.97, 106.76, 96.85, 95.16, 61.41, 55.23, 35.64, 31.65,
31.60, 31.10, 30.89, 28.94, 28.85, 26.55, 22.59, 14.26, 14.06. HR-MS
(APCI, m/z) Calcd for C49H52N4O3 [M-H]+: 743.3967, found: 743.3962,
error -0.57 ppm.
[19] D. G. Congrave, B. H. Drummond, P. J. Conaghan, H. Francis, S. T. E.
Jones, C. P. Grey, N. C. Greenham, D. Credgington, H. Bronstein, J. Am.
Chem. Soc. 2019, 141, 18390-18394.
[20] S. Cherumukkil, S. Ghosh, V. K. Praveen, A. Ajayaghosh, Chem. Sci.
2017, 8, 5644-5649.
[21] W. Xu, M. M. S. Lee, J. J. Nie, Z. Zhang, R. T. K. Kwok, J. W. Y. Lam, F.
J. Xu, D. Wang, B. Z. Tang, Angew. Chem. Int. Ed. 2020, DOI:
10.1002/anie.202000740.
[22] C. Chen, X. Ni, S. Jia, Y. Liang, X. Wu, D. Kong, D. Ding, Adv. Mater.
2019, 31, e1904914.
[23] C. Chen, X. Ni, H. W. Tian, Q. Liu, D. S. Guo, D. Ding, Angew. Chem.
Int. Ed. 2020, DOI: 10.1002/anie.201916430.
Acknowledgements
[24] S. Ghosh, R. Raveendran, A. Saeki, S. Seki, M. Namboothiry, A.
Ajayaghosh, ACS Appl. Mater. Interfaces 2019, 11, 1088-1095.
[25] S. Ghosh, S. Shankar, D. S. Philips, A. Ajayaghosh, Mater. Today Chem.
2020, 16, 100242.
This work was financially supported by the National Natural
Science Foundation of China (Nos. 21875019, 51673024,
21975020 and 51803009), the National Key Research and
Development Program of China (No. 2018YFA0901800) and
Beijing Institute of Technology Research Fund Program for Young
Scholars.
[26] Z. Guo, C. Yan, W.-H. Zhu, Angew. Chem. Int. Ed. 2020, DOI:
10.1002/anie.201913249.
[27] Y. Lei, W. Dai, Z. Liu, S. Guo, Z. Cai, J. Shi, X. Zheng, J. Zhi, B. Tong
and Y. Dong, Mater. Chem. Front. 2019, 3, 284-291.
[28] Y. Li, Y. Lei, L. Dong, L. Zhang, J. Zhi, J. Shi, B. Tong, Y. Dong, Chem.
Eur. J. 2019, 25, 573-581.
Keywords: near-infrared emission • aggregation-enhanced
emission • multiarylpyrrole • alkyl chain length • restricted
intramolecular rotation
[29] P. Liu, W. Li, S. Guo, D. Xu, M. Wang, J. Shi, Z. Cai, B. Tong, Y. Dong,
ACS Appl. Mater. Interfaces 2018, 10, 23667-23673.
[30] F. Ren, P. Liu, Y. Gao, J. Shi, B. Tong, Z. Cai, Y. Dong, Mater. Chem.
Front. 2019, 3, 57-63.
[31] F. Ren, J. Shi, B. Tong, Z. Cai, Y. Dong, Mater. Chem. Front. 2019, 3,
2072-2076.
[1]
Z. Zheng, D. Li, Z. Liu, H. Q. Peng, H. H. Y. Sung, R. T. K. Kwok, I. D.
Williams, J. W. Y. Lam, J. Qian, B. Z. Tang, Adv. Mater. 2019, 31,
1904799.
[32] J. Yang, Z. Ren, B. Chen, M. Fang, Z. Zhao, B. Z. Tang, Q. Peng, Z. Li,
J. Mater. Chem. C. 2017, 5, 9242-9246.
[2]
[3]
[4]
B. Braun, I. Wehl, D. K. Kolmel, U. Schepers, S. Brase, Chem. Eur. J.
2019, 25, 7998-8002.
[33] Q. Wang, B. Xia, J. Xu, X. Niu, J. Cai, Q. Shen, W. Wang, W. Huang, Q.
Fan, Mater. Chem. Front. 2019, 3, 650-655.
W. Qin, N. Alifu, Y. Cai, J. W. Y. Lam, X. He, H. Su, P. Zhang, J. Qian,
B. Z. Tang, Chem. Commun. 2019, 55, 5615-5618.
[34] J.-Y. Zhu, C.-X. Li, P.-Z. Chen, Z. Ma, B. Zou, L.-Y. Niu, G. Cui, Q.-Z.
Yang, Mater. Chem. Front. 2020, 4, 176-181.
Z. Zheng, H. Liu, S. Zhai, H. Zhang, G. Shan, R. T. K. Kwok, C. Ma, H.
H. Y. Sung, I. D. Williams, J. W. Y. Lam, K. S. Wong, X. Hu, B. Z. Tang,
Chem. Sci. 2020, 11, 2494-2503.
[35] J. Zhu, J. Zou, Z. Zhang, J. Zhang, Y. Sun, X. Dong, Q. Zhang, Mater.
Chem. Front. 2019, 3, 1523-1531.
[36] J. Liu, C. Chen, S. Ji, Q. Liu, D. Ding, D. Zhao, B. Liu, Chem. Sci. 2017,
8, 2782-2789.
[5]
[6]
M. Dai, H. Lee, Y. J. Yang, M. Santra, C. W. Song, Y. W. Jun, Y. J. Reo,
W. J. Kim, K. H. Ahn, Chem. Eur. J. 2020, DOI:
10.1002/chem.202001163.
[37] Y. Dong, J. Zhang, A. Li, J. Gong, B. He, S. Xu, J. Yin, S. H. Liu, B. Z.
Tang, J. Mater. Chem. C. 2020, 8, 894-899.
J. Luo, Z. Xie, J. W. Lam, L. Cheng, H. Chen, C. Qiu, H. S. Kwok, X.
Zhan, Y. Liu, D. Zhu, B. Z. Tang, Chem. Commun. 2001, 1740-1741.
[38] C. Wang, Z. Li, Mater. Chem. Front. 2017, 1, 2174-2194.
6
This article is protected by copyright. All rights reserved.

10.1002/chem.202002525
Chemistry - A European Journal
FULL PAPER
[39] C. Wang, Y. Yu, Z. Chai, F. He, C. Wu, Y. Gong, M. Han, Q. Li, Z. Li,
Mater. Chem. Front. 2019, 3, 32-38.
[40] J. Wang, Z. Chai, J. Wang, C. Wang, M. Han, Q. Liao, A. Huang, P. Lin,
C. Li, Q. Li, Z. Li, Angew. Chem. Int. Ed. 2019, 58, 17297-17302.
[41] Q. Li, Z. Li, Acc. Chem. Res. 2020, 53, 962-973.
[42] N. Li, Y. Y. Liu, Y. Li, J. B. Zhuang, R. R. Cui, Q. Gong, N. Zhao, B. Z.
Tang, ACS Appl. Mater. Interfaces 2018, 10, 24249-24257.
[43] Z. Qiu, Z. Yang, W.-C. Chen, L. Xing, S. Hu, S. Ji, Q. Yang, N. Cai, X.
Ouyang, Y. Huo, J. Mater. Chem. C. 2020, 8, 4139-4147.
[44] Y. Lei, Q. Liu, L. Dong, Z. Cai, J. Shi, J. Zhi, B. Tong, Y. Dong, Chem.
Eur. J. 2018, 24, 14269-14274.
[45] J. Shi, Y. Wu, S. Sun, B. Tong, J. Zhi, Y. Dong, J. Polym. Sci. Part A:
Polym. Chem. 2013, 51, 229-240.
7
This article is protected by copyright. All rights reserved.

10.1002/chem.202002525
Chemistry - A European Journal
FULL PAPER
Entry for the Table of Contents
Bright NIR Dyes
10
30
40
202 o
( )
A series of multiarylpyrrole derivatives with near-infrared emission and aggregation-enhanced emission features were designed to
increase the quantum yield of dyes in solid, which powder XRD data and single crystal analysis elucidated that the different lengths of
alkyl chains had a significant impact on the regularity of NIR dyes when they were forced to aggregate or precipitate.
8
This article is protected by copyright. All rights reserved.