Controllable aggregation-induced emission based on a tetraphenylethylene- functionalized pillar[5]arene via host-guest recognition

Article abstract of DOI:10.1039/c4cc03127j

A novel TPE-functionalized pillar[5]arene (TPEP5) was successfully synthesized, and the motion of the TPE motif was restricted via pillararene-based host-guest recognition-mediated cross-linking, resulting in the efficient "turn-on" of fluorescence emissi

Full text of DOI:10.1039/c4cc03127j

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Controllable aggregation-induced emission based
on a tetraphenylethylene-functionalized
pillar[5]arene via host–guest recognition†
Cite this: Chem. Commun., 2014,
50, 9122
Received 28th April 2014,
Accepted 17th June 2014
Jie Wu,‡a Shu Sun,‡ab Xiaoqing Feng,a Jianbing Shi,b Xiao-Yu Hu*a and
Leyong Wang*a
DOI: 10.1039/c4cc03127j
www.rsc.org/chemcomm
A novel TPE-functionalized pillar[5]arene (TPEP5) was successfully derivatives containing adenine or thymine moieties could
synthesized, and the motion of the TPE motif was restricted via be used as ‘‘turn on’’ chemosensors for selective detection of
pillararene-based host–guest recognition-mediated cross-linking, Ag+ and Hg2+ ions.5c
resulting in the efficient ‘‘turn-on’’ of fluorescence emission based
on the AIE mechanism.
Recently, non-covalent interactions such as host–guest recogni-
tion have been proved to be efficient strategies to restrict the
intramolecular motions of TPE molecules, concomitantly
Molecules with aggregation-induced emission (AIE)1 or aggregation- accompanied by the turn-on of fluorescence emission via the
induced enhanced emission (AIEE)2 characteristics have provided AIE mechanism.7 For example, Liu and co-workers integrated
a promising platform for design and creation of efficient light the concept of AIE with the specific host–guest supramolecular
emitters ranging from optical materials to sensors, owing to the recognition between K+ ions and crown ether moieties to develop
enhanced emission in their aggregate or solid-state forms.3 effective fluorometric K+ probes.7a Considering the unique struc-
Since the first AIE molecule reported by Tang et al. in 2001,1a ture and interesting host–guest chemistry of pillararenes, which
a wide variety of AIE molecules have been synthesized based can form supramolecular inclusion complexes with various kinds
on the mechanism of restriction of intramolecular motions of linear guests,8 the grafting of pillararenes onto the periphery
(RIM).1b,3b Among various AIE molecules, tetraphenylethene of TPE can provide a novel strategy for fabricating various
(TPE) and its functionalized derivatives can be readily obtained functional AIE luminogens and achieving the fluorescent detection
via facile synthetic transformations, which are non-emissive in of various types of guest compounds mediated by the pillararene-
the molecularly dissolved state, but enhanced fluorescence based host–guest interactions.9 Herein, we designed and for
emission could be achieved in both the aggregated form and the first time successfully synthesized a TPE-functionalized
the solid state.4 In contrast to the aggregation-caused quenching pillar[5]arene (TPEP5) by attaching four DMPillar[5]arene
(ACQ) effect of conventional organic luminophores, TPE-based (DMP5) groups onto the periphery of TPE (Scheme 1). It was
AIE-active materials are demonstrated to have improved efficiency found that TPEP5 dissolved in CHCl3–acetone solution with
and sensitivity as chemosensors, bio-probes, and solid-state negligible fluorescence emission, whereas, upon addition of
emitters and have already shown practical applications in these the guest molecule (G1), TPEP5 could be effectively induced to
fields.5 For example, Tang and co-workers synthesized a peptide- aggregate due to the pillararene-based host–guest recognition-
conjugated TPE derivative, which could be used as a live-cell- mediated cross-linking via the formation of the TPEP5*G1
permeable, fluorescent light up probe for real-time cell apoptosis (1 : 2 molar ratio) inclusion complex, which concomitantly resulted
imaging.6 In addition, Zhang et al. demonstrated that the TPE in the ‘‘turn-on’’ of fluorescence emission based on the AIE
mechanism. Moreover, the fluorescence ‘‘turn-off’’ was observed
upon the gradual addition of adiponitrile (G3, a competitive guest),
which was easily visualized by the naked eye. Thus, this novel
a Key Laboratory of Mesoscopic Chemistry of MOE, Center for Multimolecular
Organic Chemistry, School of Chemistry and Chemical Engineering, Nanjing
supramolecular system based on the TPEP5*G1 complex
creates unique possibilities to fabricate novel types of pillararene-
based fluorescent probes.
The TPE-functionalized pillar[5]arene (TPEP5) was prepared
by attaching four DMPillar[5]arene (DMP5) groups onto the
periphery of TPE through the alkyne–azide click reaction
(Scheme S1, ESI†).10 To the best of our knowledge, this is the
University, Nanjing 210093, China. E-mail: huxy@nju.edu.cn, lywang@nju.edu.cn;
Fax: +86 025 83317761; Tel: +86 025 83592529
b College of Materials Science and Engineering, Beijing Institute of Technology,
Beijing 100081, China
† Electronic supplementary information (ESI) available: Synthesis and character-
ization data; determination of the association constants; fluorescence spectra and
DLS study. See DOI: 10.1039/c4cc03127j
‡ Jie Wu and Shu Sun contributed equally to this work.
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and similar complexation-induced chemical shift changes were
observed (Fig. S21, ESI†). Moreover, a 2D NOESY experiment was
also performed to study the host–guest complexation between
DMP5 and G1 (Fig. S23, ESI†). NOE correlation signals were
observed between protons H1 on DMP5 and Hd, He, Hf, Hg on
0
0
G1, as well as protons H2 /H3 on DMP5 and Hd, He, Hf, Hg on G1,
which also confirmed the above threading binding mode.
Further investigation of the complex stoichiometry between
TPEP5 and G1 was carried out by Job’s plot method using
model compound DMP5, and the result indicated that a 2 : 1
stoichiometry complex was formed between DMP5 and G1
(Fig. S22, ESI†), which further confirmed our envision that
the guest molecule G1 could serve as a cross-linker to bind
with two molecules of DMP5 motifs (Fig. S31, ESI†), leading to
the aggregation of TPEP5 and form a supramolecular network
(for details, see ESI,† Fig. S34).8d In order to investigate the
binding affinity of the pillar[5]arene–G1 recognition motif, model
compounds DMP5 and G2 (1-(4-bromobutyl)-4-((4-methoxyphenoxy)-
methyl)-1H-1,2,3-triazole, analogues of G1) were applied for the
1H NMR titration experiments, where the association constant
(Ka) for the formation of the 1 : 1 DMP5*G2 complex was
calculated to be (7.30 ꢀ 0.49) ꢁ 102 Mꢂ1 (CDCl3–acetone-d6,
Fig. S26–S28, ESI†).
Considering the AIE feature of the TPE core in TPEP5 and
based on the above established novel DMP5*G1 (2 : 1 molar
ratio) supramolecular inclusion complex, we envisage that
the addition of G1 will induce the aggregation of TPEP5,
which concomitantly results in the ‘‘turn-on’’ of fluorescence
emission based on the AIE mechanism. Thus the fluorescence
properties of such host–guest recognition-induced aggregation
of TPEP5 were further investigated. In preliminary experiments,
we found that the choice of solvents also played important roles
in observing the host–guest recognition-induced AIE. Lots of
efforts were therefore made in the initial stage, searching for an
appropriate solvent system, and finally, CHCl3/acetone (1/8, v/v)
was selected as the best solvent system for such supramolecular
aggregation (for details, see Fig. S25, ESI†). It was found that
TPEP5 dissolved in CHCl3/acetone (1/8, v/v) showed negligible
fluorescence emission due to the efficient nonradiative annihila-
tion caused by the intramolecular rotation of the phenyl rings in
the TPE core of TPEP51a (Fig. 2, inset A). However, when G1 was
added into the above TPEP5 solution, the fluorescence emission
increased gradually due to the formation of a supramolecular
network and the rotation of phenyl rings in the TPE core of
TPEP5 is restricted. As shown in Fig. 2, a dramatic emission
enhancement was observed when 16.0 equiv. of G1 was added,
and this fluorescence enhancement can be easily distinguished
by the naked eye when illuminating the solution with UV light
(365 nm) as indicated in the inset of Fig. 2, which further
supports the proposed AIE mechanism. Moreover, the quantum
yield of TPEP5 with 8.0 equiv. of G1 was determined to be 12.3%,
measured by using quinine sulfate in 0.1 M H2SO4 (quantum
yield = 54.6%) as the standard (Fig. S32, ESI†).
Scheme 1 Schematic illustration of the construction of the luminescent
supramolecular aggregates based on aggregation-induced emission (AIE)
of pillar[5]arene-functionalized tetraphenylethene (TPEP5), induced by
host–guest recognition-mediated cross-linking between G1 and
pillar[5]arene moieties, and its application in the detection of adiponitrile.
first example of successful synthesis of a TPE-functionalized
pillar[5]arene (TPEP5), which could be used to fabricate functional
luminescent supramolecular aggregates induced by host–guest
recognition-mediated cross-linking between G1 and pillar[5]arene
moieties based on the AIE mechanism.
The complexation between TPEP5 and G1 was initially inves-
tigated by 1H NMR spectroscopy as shown in Fig. 1. The proton
NMR spectra of TPEP5, G1, and a mixture of TPEP5 and 4 equiv.
of G1 showed that this complexation system is a fast-exchanging
process on the proton NMR time scale. As can be seen from
Fig. 1b, after complexation the peaks of phenyl protons H4, H5,
H6, methylene and methoxyl protons H10, H11, H12 from the
pillar[5]arene, and triazole protons H1 on TPEP5 shifted down-
field slightly. The proton signals derived for Hd, He, Hf, Hg, and
Ha of G1 shifted upfield remarkably due to the shielding effect
of the electron-rich cavities of the pillar[5]arene on TPEP5.
While, no obvious change was observed for the protons Hb
and Hc on G1. The above results revealed that the pillar[5]arene
motifs on TPEP5 were fully threaded by guest G1 with the
protons Hd, He, Hf, Hg and Ha in the pillar[5]arene cavities
and other protons Hb and Hc out of the cavities. In addition,
the 1H NMR spectrum of a mixture of the model compound
DMpillar[5]arene (DMP5) and 0.5 equiv. of G1 was also investigated
According to the previous report, dinitrile compounds show
very strong binding affinities with the pillar[5]arene based on the
cooperative multiple hydrogen bond and dipole–dipole interactions.8f
Fig. 1 1H NMR spectra (CDCl3/acetone-d6 (1 : 8, v/v), 300 MHz, 298 K) of
(a) 5 mM G1; (b) 5 mM TPEP5 and 20 mM G1; (c) 5 mM TPEP5.
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Fig. 4 TEM images: (a) TEM image of the TPEP5*G1 complex; (b)
enlarged TEM image of (a). Samples were prepared by placing one drop
of the CHCl3–acetone solution of the mixtures of TPEP5 with 4 equiv. of
G1 onto a carbon-coated copper grid.
Fig. 2 Fluorescence spectral changes of TPEP5 (0.04 mM) upon gradual
addition of G1 (0–0.64 mM) in CHCl3/acetone (1/8, v/v) (lex = 330 nm).
The inset shows the photographs of the solution of TPEP5 in the (A) aggregates was about 2 mm in diameter (Fig. S33, ESI†), which
absence and (B) presence of G1 (0.64 mM) under UV light (365 nm)
illumination at 298 K.
is in good agreement with the above TEM results. Therefore,
the above results further confirmed the formation of large
sized supramolecular aggregates via host–guest recognition-
mediated cross-linking.
Hence, we envisioned that the complexation between TPEP5 and
In summary,
a novel TPE-functionalized pillar[5]arene
G1 could also be destroyed after the addition of size-fit dinitrile,
such as adiponitrile, resulting in the fluorescence ‘‘turn-off’’ of
the above supramolecular TPEP5*G1 system. To investigate the
fluorescence sensing effect of the above TPEP5*G1 system for
adiponitrile, fluorescence titration experiments were performed
by adding different concentrations of adiponitrile (G3) to the
TPEP5*G1 system in CHCl3/acetone (1/8, v/v). As shown in Fig. 3,
significant quenching of the fluorescence intensity was observed
upon the gradual addition of adiponitrile, which could also be
easily visualized by the naked eye when illuminating the solution
with UV light (365 nm). For the quenching of the fluorescence, a
possible reason is that after addition of the completive guest G3, a
more stable inclusion complex TPEP5*G3 was formed (Fig. S29
and S30, ESI†), which could not lead to the cross-linking of
TPEP5 due to the fact that G3 can bind with only one molecule of
DMP5, generating a simple 1 : 1 inclusion complex. Therefore,
TPEP5 could not be induced to aggregate and result in the
fluorescence ‘‘turn-off’’.
Furthermore, transmission electron microscopy (TEM) was
also used to provide further insight into the size and shape of
the supramolecular aggregates formed from TPEP5 and G1. As
shown in Fig. 4, spherical aggregates with a diameter of B2 mm
were observed for the supramolecular aggregates formed in
CHCl3–acetone solution (Fig. 4a and b). Moreover, the dynamic
light scattering (DLS) measurements showed that different size
distributions were observed and the mean size of the above
(TPEP5) was successfully synthesized by incorporating four
pillar[5]arene groups onto the periphery of TPE through the
alkyne–azide click reaction. The formation of the TPEP5*G1
(1 : 2 molar ratio) supramolecular inclusion complex based on
host–guest interactions led to the effective aggregation of
TPEP5, resulting in the ‘‘turn-on’’ of fluorescence emission
based on the AIE mechanism. Moreover, fluorescence ‘‘turn-off’’
could be observed upon further addition of adiponitrile due to
the competitive host–guest complexation. In addition, DLS and
TEM images confirmed the formation of large sized spherical
aggregates due to the host–guest recognition-induced cross-
linking. Therefore, this novel supramolecular system offers a
new opportunity for the fabrication of novel types of pillararene-
based AIE luminogens. Future work will focus on the design and
synthesis of highly efficient and selective pillararene-based
functional AIE materials.
We are grateful for the financial support from the National
Basic Research Program of China (2014CB846000) and the
National Natural Science Foundation of China (No. 91227106
and 21202083).
Notes and references
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