Aggregation-induced emission or aggregation-caused quenching? Impact of covalent bridge between tetraphenylethene and naphthalimide

Article abstract of DOI:10.1016/j.cclet.2020.12.031

Understanding the physical mechanisms governing aggregation-induced-emission (AIE) and aggregation-caused-quenching plays a vital role in developing functional AIE materials. In this work, tetraphenylethene (TPE, a classical AIEgen) and naphthalimide (NI, a popular fluorophore with ACQ characteristics) were connected through non-conjugated linkages and conjugated linkages. We showed that the nonconjugated-linkage of TPE to NI fragments leads to substantial PET in molecular aggregates and ACQ. In contrast, the conjugated connection between TPE and NI moieties results in the AIE phenomenon by suppressing twisted intramolecular charge transfer. This work provides an important guideline for the rational design of AIE materials.

Full text of DOI:10.1016/j.cclet.2020.12.031

Chinese Chemical Letters 32 (2021) 1790–1794
Contents lists available at ScienceDirect
Chinese Chemical Letters
journal homepage: www.elsevier.com/locate/cclet
Communication
Aggregation-induced emission or aggregation-caused quenching?
Impact of covalent bridge between tetraphenylethene and
naphthalimide
Xiaoxie Maa,1, Weijie Chib,1, Xie Hana,c,1, Chao Wangb, Shenghua Liua, Xiaogang Liub,
,
*
Jun Yina,
*
a
Key Laboratory of Pesticide and Chemical Biology, Ministry of Education, Hubei International Scientific and Technological Cooperation Base of Pesticide and
Green Synthesis, International Joint Research Center for Intelligent Biosensing Technology and Health, College of Chemistry, Central China Normal University,
Wuhan 430079, China
b
Singapore University of Technology and Design, Singapore 487372, Singapore
School of Chemistry and Chemical Engineering, Wuhan University of Science and Technology, Wuhan 430081, China
c
A R T I C L E I N F O
A B S T R A C T
Article history:
Understanding the physical mechanisms governing aggregation-induced-emission (AIE) and aggrega-
tion-caused-quenching plays a vital role in developing functional AIE materials. In this work,
tetraphenylethene (TPE, a classical AIEgen) and naphthalimide (NI, a popular fluorophore with ACQ
characteristics) were connected through non-conjugated linkages and conjugated linkages. We showed
that the nonconjugated-linkage of TPE to NI fragments leads to substantial PET in molecular aggregates
and ACQ. In contrast, the conjugated connection between TPE and NI moieties results in the AIE
phenomenon by suppressing twisted intramolecular charge transfer. This work provides an important
guideline for the rational design of AIE materials.
Received 30 October 2020
Received in revised form 16 December 2020
Accepted 17 December 2020
Available online 3 March 2021
Keywords:
Aggregation-induced-emission
Tetraphenylethene
Naphthalimide
Conjugated-linkage
Mechanofluorochromism
© 2021 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences.
Published by Elsevier B.V. All rights reserved.
Aggregation-induced emission luminogens (AIEgens) have
been attracting increasing research interests for their broad range
of applications (i.e., such as biosensors, photodynamic therapy
agents, organic light-emitting diodes, and wave-guides) [1–10]. As
a popular AIEgen design strategy, tetraphenylethene (TPE) has
been frequently attached to other fluorophores of diverse
molecular structures and emission colors to achieve tailored
functionalities [1]. Yet, this strategy yielded varied successes,
resulting in the creations of compounds with both aggregation-
induced emission (AIE) and aggregation-caused quenching (ACQ)
characteristics [11]. To facilitate the effective development of such
AIEgens, it is critical to reveal the relationship between their
molecular structures and AIE characteristics.
TPEAn) [20]. The monomers of TPEpy and TPEAn exhibited weak
fluorescence in tetrahydrofuran (THF), but their aggregation
substantially enhanced the emission intensity. Our group also
design a series of TPE-based AIEgens of various colors spanning
across the entire visible to near-infrared spectrum, by linking TPE
to different fluorophores, such as naphthalimide (NI) and boron-
dipyrromethene (BODIPY) moieties [21]. Choi et al. reported
several NI-TPE isomeric conjugates, which were connected with
single-bond, vinyl, and acetylene linkages between NI and TPE
fragments [22]. Their results showed that the 3-substituted and 4-
substituted dyes with the single-bond linkage exhibited the AIE
phenomenon, whereas 4-substituted dyes with vinyl and acety-
lene linkages showed ACQ. Besides this study, the ACQ phenome-
non was noted in several TPE-perylenebisimide compounds [23]. It
is thus intriguing to understand both the structural factors and
photophysical mechanisms governing the distinct AIE and ACQ
phenomenon in TPE-fluorophore hybrid dyes, to better aid the
rational creations of functional AIE materials.
To the end, significant research efforts have been devoted to
design and synthesize novel TPE-based AIEgens [12–19]. Tang et al.
functionalized TPE to several planar ACQ fluorophores, such as
anthracene and pyrene, and obtained new AIEgens (TPEpy and
In this communication, we reported the synthesis, character-
izations, and theoretical investigation of the AIE/ACQ character-
istics of sixteen TPE-NI derivatives with various substituents and
substituent positions (Scheme 1 and Fig. 1). Our analysis shows
that the nature of the covalent bridges (in spanning the
* Corresponding authors.
E-mail addresses: xiaogang_liu@sutd.edu.sg (X. Liu), yinj@mail.ccnu.edu.cn
(J. Yin).
1
These authors contributed equally to this work.
https://doi.org/10.1016/j.cclet.2020.12.031
1001-8417/© 2021 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences. Published by Elsevier B.V. All rights reserved.

X. Ma, W. Chi, X. Han et al.
Chinese Chemical Letters 32 (2021) 1790–1794
(THF)/water binary solution of these compounds, we noted a
consistent drop in emission intensities due to the formation of
molecular aggregates, as the volume ratio (fw) of water increased
(Figs. S2-S5 and Table S1 in Supporting information). These results
also suggest that a phenyl substituent is not bulky enough to
effectively separate NI dimers and avoid strong intermolecular
interactions. To further decrease intermolecular interactions and
prevent ACQ, we decided to link a bulky TPE unit to NI. The UV–vis
absorption and fluorescence spectra of the mixture of TPE and 5-
NI-5 demonstrated that simple mixing of TPE and NI fluorophores
could not change the ACQ phenomenon of NI fluorophores (Fig. S29
in Supporting information). Therefore, we covalently linked TPE to
NI via two types of linkages: non-conjugated linkages and
conjugated linkages.
Scheme 1. Synthetic routes of TPE-NI derivatives.
p
-conjugation of the TPE-NI hybrid dyes) plays a decisive role in
activating AIE in the resulted compounds.
Scheme shows the synthesis route to obtain TPE-NI
1
We firstly connected TPE to NI via non-conjugated linkers. The
non-conjugated covalent linkage could be realized either via
inserting a –CH2– moiety between TPE and NI, such as in
compounds 1-NI-3, 3-NI-1, 4-NI-1, and 1-NI-1 (Fig. 1), or choose
connection sites that disrupt conjugations (such as in 2-NI-1 and 2-
NI-3 in Fig. 1). In 2-NI-1 and 2-NI-3, the bond alternation breaks at
the imide functionalization site of NI.
We subsequently synthesized these six compounds and
investigated their spectral properties (Fig. 2 and Figs. S5-S10 in
Supporting information). In these compounds, two distinct UV–vis
absorption peaks are noted, registered at ꢀ290 nm and ꢀ440 nm,
respectively (Fig. 2a). These peaks matched very well with the
reported data of TPE and NI derivatives, respectively [25,26].
Subsequent quantum chemical calculations further support these
assignments. Time-dependent density functional theory (TD-DFT)
calculations show that the first and second absorption bands are
derivatives. The involved intermediates were synthesized accord-
ing to previously reported methods with good yields. With the
corresponding Br-substituted NI moiety and NH2-substituted
aromatic moiety, the targeted compounds were prepared by a
typical Pd-catalysed Buchwald reaction. All compounds were
purified by silica gel columns, and their structures were well
characterized by 1H,13C NMR spectroscopy, and EI-MS. Fortunately,
single crystals of 1-NI-3, 3-NI-4, and 3-NI-2 were obtained by
diffusing hexane into DCM, THF, or EA solution at room
temperature, respectively (Fig. S30 in Supporting information).
The detailed synthesis procedures and characterizations are
available in the supporting information (Figs. S32–S78 in
Supporting information).
Many NI fluorophores exhibited ACQ characteristics, due to
strong intermolecular
p-p interactions [24], such as compounds 4-
NI-3, 4-NI-5, 5-NI-3, and 5-NI-5 (Fig. 1). In the tetrahydrofuran
Fig. 1. Compounds reported in this work.
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X. Ma, W. Chi, X. Han et al.
Chinese Chemical Letters 32 (2021) 1790–1794
(in the NI fragment). The electron from HOMO-1 or HOMO sitting
at the TPE fragments could transfer to HOMO-2 and quench the
fluorescence via PET (Fig. 2f) [32].
Next, we connected TPE to NI via conjugated linkages. We
hypothesized that twisted intramolecular charge transfer (TICT)
rotation could be employed in NI derivatives to enable AIE
characteristics [33–35]. During TICT rotation, the amino group at
the 4-positon of NI experiences a ꢀ90ꢁ rotation with respect to the
NI scaffold, forming a non-emissive specie [36,37]. However, upon
molecular aggregation, TICT rotation could be effectively sup-
pressed, thus recovering bright emissions from NI (provided that
intermolecular
p-p interactions are weak in molecular aggre-
gates). Moreover, the TICT rotation could be activated by replacing
the secondary amino group with either a dialkylated amino group
or an amino-phenyl group [38,39].
To verify this speculation, we designed, synthesized, and
characterized 3-NI-4, 4-NI-4, and 5-NI-4. We also successfully
obtained the crystal structure of 3-NI-4 (Table S3 in Supporting
information). Notably, as the solvent polarity increases (from
toluene to acetonitrile), the emission intensity of 3-NI-4 and 4-NI-
4
showed a large drop (Figs. S16 and S17 in Supporting
information). This drop of emission intensities is attributed to
the formation of TICT, as TICT is usually activated in polar solvents
[21]. Indeed, our viscosity dependent studies in the binary mixture
of methanol/glycerol showed that increasing viscosity enhanced
the emission intensities of 3-NI-4, corroborating the TICT
mechanism (Fig. 3a and Fig. S18 in Supporting information).
Fig. 2. (a) UV–vis absorption spectra of 1-NI-3 in various solvents; [1-NI-
3] = 10
mmol/L. (b) Electron and hole distributions of 1-N1-3 during the S1 (the
first absorption band) and S2 (the second absorption band) vertical excitation. (c)
The normalized fluorescence spectra of 1-NI-3 in various solvents; [1-NI-
3] = 10
mmol/L; lex = 420 nm. (d) The fluorescence spectra of 1-NI-3 in the binary
mixture of THF/water as a function of the volume ratio of water (fw); the inset shows
the photographs of the samples under UV radiations in a dark room; [1-NI-
3] = 10
mmol/L; lex = 420 nm. (e) A dimer structure of 1-NI-3 as extracted from its
single crystal structure. (f) Calculated frontier molecular orbitals of this dimmer in
vacuo, which suggest both intramolecular and intermolecular PET from TPE to NI
fragments in vacuo; the energy levels are not drawn in scale for clarity.
contributed by the photoexcitation of the NI and TPE fragments,
respectively (Fig. 2b) [27,28]. Moreover, the emission wavelength
from the NI fragment showed an obvious redshift as the polarity of
the solvent increases (Fig. 2c).
Next, we measured the fluorescence intensities of these
compounds in the binary THF/water mixture. Taking 1-NI-3 as
one representative example, as fw increases from 0 to 90% to induce
the formation of aggregates, the emission peak experiences a slight
red shift from 491 nm to 535 nm (Fig. 2d). This redshift is
attributed to the enhanced ICT effect in the NI fragment (as solvent
polarity increases). Notably, the corresponding quantum yield of 1-
NI-3 considerably dropped from 66.5% to only 10.4%, exhibiting the
ACQ characteristics. Other compounds in this group demonstrated
similar ACQ effects, as reflected by a significant reduction of
fluorescence intensity (Figs. S11-S15 in Supporting information).
To elucidate this ACQ mechanism, we performed DFT and TD-
DFT calculations on the dimer of 1-NI-3 at M06-2X/def2-SVP level
[29]. The initial structure is from the crystal structure of 1-NI-3
(Table S2 in Supporting information). We showed that the photo-
induced electron transfer (PET) process could be substantially
activated as the distance between TPE and NI becomes smaller in
the excited state, i.e., via the formation of aggregation. For example,
intramolecular TPE-NI distance amounts to 8.559 Å (relatively
large). In contrast, the distance between NI and another TPE unit
from a neighbouring molecule (in molecular aggregates) decreases
to only 7.663 Å (Fig. 2e). The reduced distance could substantially
activate PET from TPE to NI, thus resulting in the ACQ of 1-NI-3
[30,31]. Indeed, our TD-DFT calculation on the dimmer of 1-NI-3
showed that S1 photoexcitation is mainly from HOMO-2 to LUMO
Fig. 3. (a) The viscosity dependence of the emission intensities of 3-NI-4 in the
binary mixture of methanol and glycerol, as a function of the volume ratio of
glycerol; [3-NI-2] = 10
1 state of 3-NI-4 as a function of the amino group rotation in tetrahydrofuran. (c)
The emission spectra of 3-NI-2 in various solvents; [3-NI-2] = 10 mol/L, lex = 450
mmol/L, lex = 450 nm. (b) The potential energy surface in the
S
m
nm. (d) The viscosity dependence of the emission intensities of 3-NI-2 in the binary
mixture of methanol and glycerol, as a function of the volume ratio of glycerol; [3-
NI-2] = 5
NI-2 as
m
a
mol/L, lex = 450 nm. (e) The potential energy surface in the S1 state of 3-
function of the amino group rotation in tetrahydrofuran; (f) The
fluorescence spectra of 3-NI-2 in the binary mixture of THF/water as a function of
the volume ratio of water (fw); the inset shows the photographs of the samples
under UV radiations in a dark room; [3-NI-2] = 10 mmol/L, lex = 450 nm.
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X. Ma, W. Chi, X. Han et al.
Chinese Chemical Letters 32 (2021) 1790–1794
Fig. 4. (a) The emission spectra of 3-NI-2 (lex = 450 nm) in the crystalline (unground and annealed) and the amorphous (ground) states. (b) The photography of 3-NI-2 under
daylight and UV light (in a dark room) upon grinding and annealing. (c) The powder X-ray diffraction patterns of 3-NI-2 in the crystalline (unground and annealed) and the
amorphous (ground) states.
Finally, the potential energy surface of excited-state calculations
[38] also showed that the TICT state is energetically favorable for 3-
NI-4 in polar solvents (Fig. 3b). Similar polarity-dependent and
viscosity-dependent emission results are also obtained in 4-NI-4,
and 5-NI-4 (Figs. S19 and S20 in Supporting information). These
results affirmed the TICT fluorescence quenching mechanism. Yet,
the emission intensities of 3-NI-4, 4-NI-4, and 5-NI-4 in molecular
aggregates remained weak, presumably due to strong intermolec-
ular interactions in molecular aggregates.
Inspired by the emission characteristics of 3-NI-4, we next
connected TPE to NI via conjugated linkages for two consider-
ations: (1) TPE is much bulky and could effectively suppress
intermolecular interactions in aggregates; (2) Conjugated linkage
contacts (intermolecular interactions) along with the conforma-
tional changes in TPE is likely to cause interesting mechanochro-
mic properties.
To verify this hypothesis, we tested 3-NI-2 and found that this
compound indeed possesses favourable mechanochromic proper-
ties (Figs. 4a and b). After grinding, the peak emission wavelength
of 3-NI-2 exhibited a notable redshift from 582 nm to 605 nm. The
peak emission intensity was also enhanced, with the absolute
quantum yield changing from 4.16% in the pristine form to 14.02%
upon grinding, or intensified by ꢀ3 times. The spectral changes are
reflected by significant colour changes both under ambient light
and UV light (Fig. 4b). Interestingly, upon annealing at 180 ꢁC for
5 min, the fluorescence properties reverted to the original state.
Along with these spectral changes, the diffraction peaks in
PXRD results demonstrated that grinding effectively convert the
powder from crystalline to amorphous phase, and the crystalline
phase was recovered after annealing (Fig. 4c).
In summary, through the systematic synthesis and character-
izations of 16 compounds, we showed that the nonconjugated-
linkage of TPE to NI fragments leads to substantial PET in molecular
aggregates, endowing these compounds with aggregation-caused-
quenching (ACQ) characteristics. In contrast, the conjugated
connection of TPE to NI moieties not only electronically activates
TICT in polar solvents in the resulting compounds but also provides
steric hindrance to minimize intermolecular interactions as such
compounds form aggregates. Accordingly, the inhibition of TICT in
the molecular aggregates leads to significant AIE. This rational
modulation of ACQ and AIE via linker engineering is likely to apply
to many other fluorophores and will provide an important
guideline for developing functional materials (i.e., for photo-
thermal therapies and bioimaging applications).
of TPE preserves the
p-conjugation and could retain TICT in the
resulted compounds and introduce additional rotational modes
due to the flexible structure of TPE. Collectively, these two factors
could enable AIE characteristics based on the RIR mechanism [20].
We have thus designed and synthesized compounds 3-NI-2, 4-
NI-2, and 5-NI-2. We also obtained the crystal structure of 3-NI-2
(Table S4 in Supporting information). As expected, when solvent
polarity increases from toluene to acetonitrile, the emission
intensities of these compounds experience a rapid reduction, due
to the activation of TICT in polar solvents (Fig. 3c and Figs. S21-S23
in Supporting information). In contrast, increasing viscosity greatly
enhanced the emissions, indicating the inhibition of TICTand other
rotation modes in these compounds (Fig. 3d and Fig. S24 in
Supporting information). Potential energy surface calculations
further affirmed the presence of the TICT state in the monomers of
these compounds (Fig. 3e, Figs. S25 and S26 in Supporting
information).
In contrast, in the binary mixture of THF and water, increasing
fw is associated with a continuous and significant intensification of
fluorescence by 11 times for 3-NI-2, 42 times for 4-NI-2, and 4
times for 5-NI-2, respectively (Fig. 3f, Figs. S27 and S28 in
Supporting information). The bright aggregate emissions suggest
that TPE effectively avoid intermolecular interactions in the solid-
state. It is thus clear that the conjugated connection of TPE to a
fluorophore “kills two birds” (TICT + steric hindrance) with one
stone, and represents a promising design strategy to develop
AIEgens. In addition to the significant AIE characteristics, we
hypothesized that conjugated linkages of TPE to NI might enable
notable mechanofluorochromism. This is because the TPE moiety
is highly flexible. Changing the conformation of TPE via mechanical
grinding could greatly affect its electron-donating strength to the
NI moiety, thus shifting the emission wavelength [40]. Moreover,
mechanical grinding may further modify intermolecular inter-
actions due to transitions between the crystalline and amorphous
phases.
Declaration of competing interest
The authors declare no conflict of interest.
Acknowledgments
J. Yin acknowledge financial support from the National Natural
Science Foundation of China (Nos. 21676113, 21772054), Distin-
guished Young Scholar Program of Hubei Province (No.
2018CFA079), the 111 Project B17019, the Scholar Support Program
of CCNU (No. 0900-31101090002), and the Excellent Doctoral
Dissertation Cultivation Grant of CCNU from the colleges’ basic
research and operation grant (MOE, No. 2019YBZZ029). The study
was supported by Ministry of Education Key Laboratory for the
Synthesis and Application of Organic Functional Molecules (No.
KLSAOFM2012), Hubei University, China. And also supported by
excellent doctorial dissertation cultivation grant of CCNU from the
colleges’ basic research and operation of MOE (No. 2019YBZZ029).
X. Liu is indebted to A*STAR under its Advanced Manufacturing and
Engineering Program (No. A2083c0051). The authors would like to
Indeed, our analysis of the intermolecular contacts in the crystal
structure of 3-NI-2 revealed multiple intermolecular N-HÁ Á ÁO, C-
HÁ Á ÁO, C-HÁ Á Á
p hydrogen bonds, and p-p stacking interactions
(Fig. S31 in Supporting information). The alternation of these
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Chinese Chemical Letters 32 (2021) 1790–1794
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