Host-guest complexation induced emission: A pillar[6]arene-based complex with intense fluorescence in dilute solution

Article abstract of DOI:10.1039/c4cc01560f

A host-guest inclusion complex was constructed from a water-soluble pillar[6]arene and a tetraphenylethene derivative in water and it exhibited strong fluorescence in dilute solution.

Full text of DOI:10.1039/c4cc01560f

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Host−guest complexation induced emission: a pillar[6]arene-based
complex with intense fluorescence in dilute solution
Pi Wang, Xuzhou Yan and Feihe Huang*
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/c0xx00000x
achieve this goal, two prerequisites are usually demanded: one is
that the available TPE substances exhibit no fluorescence in
50 dilute solution, and the other is that a host with suitable binding
sites and cavity can capture the TPE molecules.
A host−guest inclusion complex was constructed from a
water-soluble pillar[6]arene and tetraphenylethene
derivative in water and it exhibited strong fluorescence in
a
10 dilute solution.
Pillararenes4,5 are a new generation of macrocyclic hosts for
supramolecular chemistry after crown ethers, cyclodextrins,
calixarenes, and cucurbiturils.6,7 Their repeating units are
55 connected by methylene bridges at the para-positions, forming a
special rigid pillar-like architecture. The unique structures and
easy functionalization of pillararenes have endowed them with
outstanding abilities to selectively bind different kinds of guests
and provided a useful platform for the construction of various
60 interesting supramolecular systems.4,5
Many organic luminophores exhibit high fluorescence efficiency
in dilute solution but suffer from an aggregation-caused
quenching (ACQ)1 effect in the condensed phase. The
concentration-quenching effect was caused by the formation of
15 sandwich-shaped excimers and exciplexes aided by the collisional
interactions between the aromatic molecules in the excited and
ground states. To temper the ACQ effect, many efforts have been
made to study the aggregation effect on the optical properties of
organic dyes.
20
In 2001, a phenomenon of aggregation-induced emission
(AIE)2 was observed by Tang et al in some propeller-like
molecules,
such
as
tetraphenylethene
(TPE)3
and
hexaphenylsilole. These luminogens are nonemissive in good
solvents, but become highly luminescent in the aggregated state.
25 Restriction of intramolecular rotation (RIR) in the aggregates has
been identified as a main cause for the AIE effect. In solution, the
active intramolecular rotation of the phenyl rings of AIE
molecules consumes energy. However, in the aggregated state,
such motion is restricted, which blocks the nonradiative pathway
30 and activates radiative decay. Besides, the emission intensities of
AIE luminogens increased via RIR methodologies, such as
increasing the solvent viscosity, lowering the temperature,
applying high pressure, etc.2 The essence of RIR mechanism is
conformational planarity and structural rigidity of AIE molecules.
35 Many new AIE luminogens have been developed based on the
RIR mechanism, and applied as functional materials, especially
as chemosensors, bioprobes, and solid-state emitters.2
AIE is exactly opposite to the ACQ effect. In comparison to
the AIE effect which permits the use of concentrated solutions of
40 luminogens and their aggregate suspensions in aqueous media for
sensing applications, the traditional sensors work based on the
ACQ effect of dilute solutions of conventional luminophores.
How to make AIE molecules more emissive in dilute solution so
that AIE molecules can act as chemosensors not only in the
45 aggregated state but also in dilute solution? This is a challenging
mission for chemists. A supramolecular approach based on host−
guest chemistry can be a unique one to resolve this problem. To
Scheme 1. Compounds used in this study and the cartoon representation
of the formation of the luminescent supramolecular inclusion complex
and its application in the detection of paraquat.
65
On the basis of the above analyses, the utilization of the
recognition of water-soluble pillar[6]arene WP68 to TPE
derivatives can be a good method to investigate host−guest
complexation induced emission (Scheme 1). Herein, we report a
simple strategy, based on the host−guest complexation between
70 WP6 and a TPE derivative 1, to tune the emission behaviour of 1
in dilute aqueous solution. The spontaneously formed
[2]pseudorotaxane units efficiently restricted the intramolecular
rotation and the non-radiative relaxation channel, thereby
resulting in the strong emission of 1 in dilute solution.
75 Furthermore, the complex between WP6 and 1 served as a
fluorescence “turn-off” probe to detect paraquat because of the
competitive binding between WP6 and two guest molecules
(Scheme 1).
WP6 was prepared according to our previous work.8a
80 Compound 1 was prepared according to a previously reported
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method9 and characterized by NMR and electrospray ionization
naked eye using a simple UV-lamp.
mass spectrometry (Figs. S1−S3, ESI†). Similarly, model
compound 2 was also prepared.
View Article Online
DOI: 10.1039/C4CC01560F
1
Firstly, H NMR spectroscopy was carried out to investigate
5
the host−guest complexation between WP6 and 1 (Fig. S9, ESI†).
The proton NMR spectra of 1, WP6, and an equimolar mixture of
WP6 and 1 showed that this complexation system is fast
exchange on the proton NMR time scale. Significant chemical
shift changes were observed for some protons on 1 and WP6
10 after complexation (Fig. S9, spectra a−c, ESI†). Upfield shifts
were observed for H1−H4 and H9 of 1. Downfield shifts of the
aromatic protons Ha of WP6 were observed. From the 2D
NOESY spectrum of a mixture of 10.0 mM WP6 and 1 in D2O,
correlations were observed between protons H1−H4 and H9 of 1
15 and proton Ha on WP6, suggesting that alkyl group of 1 was
threaded into the cavity of WP6. Therefore, we concluded the
formation of the inclusion complex between WP6 and 1 in water
(Fig. S10, ESI†). The proton NMR spectrum of an equimolar
(10.0 mM) D2O solution of WP6 with 2 was also investigated
20 and similar complexation-induced chemical shift changes were
observed. The ability of 2 to form a 1:1 complex with WP6 was
60
Fig. 1 Fluorescence spectral changes of 1 (2.00 µM) upon addition of
WP6 (0.00−16.0 equiv) in water (λex = 330 nm, λem = 490 nm; slits, 5
nm/5 nm). The inset photographs shows the corresponding fluorescence
changes (left: 2.00 µM 1; right: 2.00 µM 1 and 32.0 µM WP6) upon
65 excitation at 365 nm using a UV lamp at 298 K.
1
assessed by H NMR titration of 2 into WP6 in water and the
association constant (Ka) of WP6⊃2 was calculated to be (3.22 ±
0.26) × 103 M−1 in water using a nonlinear curve-fitting analysis
25 (Figs. S13−S15, ESI†).
Encouraged by the above-mentioned results, we envisioned
that 1 might exhibit the strong emission by the addition of WP6,
because the intramolecular rotation of the phenyl rings of 1 will
be inhibited by the formation of [2]pseudorotaxane units. The
30 fluorescence properties of 2.00 µM 1 in the absence or presence
of WP6 in water were investigated (Fig. 1). The intramolecular
rotation of phenyl rings of 1 may induce the efficient nonradiative
annihilation process and thus 1 is nearly nonemissive in water.
However, upon the addition of WP6, the rotation of phenyl rings
35 of 1 is restricted. Therefore, the fluorescence intensity increased
remarkably. The change of the emission intensities nearly became
constant when 32.0 µM WP6 were added, and an approximate
20-fold fluorescence enhancement was observed. In addition,
such WP6-induced large fluorescence change of 1 was clearly
40 perceived by naked eye. When 1 was excited at 365 nm using a
UV lamp in the presence of 32.0 µM WP6, a strong cyan
fluorescence appeared, further supporting the proposed
mechanism.
Fig. 2 The quench of the fluorescence of a solution of WP6 (32.0 µM)
and 1 (2.00 µM) upon the titration with paraquat (0.00−160 µM) in water
(λex = 330 nm, λem = 490 nm; slits, 5 nm/5 nm). The inset photographs
70 shows the corresponding fluorescence changes (left: 2.00 µM 1 and 32.0
µM WP6; right: 2.00 µM 1, 32.0 µM WP6, and 160 µM paraquat) upon
excitation at 365 nm using a UV lamp at 298 K.
On account of the much higher binding constant of
45 WP6⊃paraquat (1.02 × 108 M−1
)
than that of WP6⊃2, we
8a
further investigate the fluorescence sensing of the inclusion
complex between WP6 and 1 for paraquat. Fluorescence titration
of the inclusion complex with paraquat was carried out at room
temperature in water. As shown in Fig. 2, the significant
50 quenching of the fluorescence intensity was found upon the
gradual addition of paraquat. It means that in the presence of
paraquat, compound 1 slipped out of the cavity of WP6 and WP6
was rethreaded by paraquat. The underlying optical change was
Fig. 3 TEM images: (a) TEM image of 1; (b) enlarged TEM image of (a);
75 (c) cartoon representation of the structure of micelles formed by 1; (d)
TEM image of the inclusion complex between WP6 and 1; (e) enlarged
TEM image of (d); (f) cartoon representation of the formation of the
superstructure from 1 and WP6.
After establishing the new host−guest recognition motif in
80 water, we further investigate the self-assembly behaviors of 1 and
the complex between WP6 and 1. Compound 1 is an amphiphilic
molecule which contains the tetraphenylethene unit as the
hydrophobic part and four n-butyltrimethyl ammonium bromide
units as the hydrophilic part. An aqueous solution of 1 was
85 prepared with a concentration of 1.00 × 10−4 M. According to
transmission electron microscopy (TEM), spherical micelles were
ascribed to the formation of
55 WP6⊃paraquat. Therefore, compound
a
more stable complex
became nearly
1
nonemissive again, which is a typical phenomenon of AIE in
dilute solution. Moreover, the “turn-off” fluorescence change
produced by the addition of paraquat was easily visualized by the
2
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70
75
5
10 carboxylate groups contribute to the generation of wheel-like
structures and account for the stability of the resulting
architectures (Fig. 3f).
80
In summary, we prepared a host–guest complex between
water-soluble pillar[6]arene WP6 and tetraphenylethene
15 derivative 1. The intramolecular rotation of the phenyl rings of 1
were hampered upon the addition of WP6, so the complex emits
strong fluorescence in dilute solution. This host–guest complex
was used as a novel detecting material to probe paraquat because
of the competitive host−guest complexation. In addition, the
20 complex between WP6 and 1 self-assembled into interesting
wheel-like structures in water, which were different from the
small spherical micelles formed by the amphiphilic molecule 1
alone. We expect that this design strategy of host–guest
complexation induced emission will provide a sophisticated
25 pathway for guiding the future design of supramolecular
functional materials.
85
5
90
95
This work was supported by National Basic Research
Program (2013CB834502) and the National Natural Science
30 Foundation of China (21125417).
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State Key Laboratory of Chemical Engineering, Department of Chemistry,
Zhejiang University, Hangzhou 310027, P. R. China; Fax: +86-571-
8795-3189; Tel: +86-571-8795-3189; E-mail: fhuang@zju.edu.cn
† Electronic Supplementary Information (ESI) available: Synthetic
procedures, characterizations, association constant determination, and
other materials. See DOI: 10.1039/c0xx00000x.
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DOI: 10.1039/C4CC01560F
Toc Graphic：
5
Text:
A host−guest inclusion complex was constructed from a water-
soluble pillar[6]arene and a tetraphenylethene derivative in water
and it exhibited strong fluorescence in dilute solution.
Furthermore, such complex acted as a “turn-off” fluorescent
10 sensor for paraquat via the competitive host−guest complexation.
4
|
Journal Name, [year], [vol], 00–00
This journal is © The Royal Society of Chemistry [year]