Stimuli-responsive blue fluorescent supramolecular polymers based on a pillar[5]arene tetramer

Article abstract of DOI:10.1039/c4cc03105a

A tetraphenylethene-bridged pillarene tetramer with aggregation-induced emission properties forms an A₄/B₂-type supramolecular polymer and a gel with a symmetric neutral guest linker, showing a remarkable fluorescence emission enhancement in solution and the solid state and a good responsiveness to temperature and solvent composition. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4cc03105a

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Stimuli-responsive blue fluorescent
supramolecular polymers based on a
pillar[5]arene tetramer†
Cite this: Chem. Commun., 2014,
50, 8231
Received 26th April 2014,
Accepted 4th June 2014
Nan Song, Dai-Xiong Chen, Yu-Chen Qiu, Xiao-Yue Yang, Bin Xu, Wenjing Tian
and Ying-Wei Yang*
DOI: 10.1039/c4cc03105a
www.rsc.org/chemcomm
A tetraphenylethene-bridged pillarene tetramer with aggregation-
induced emission properties forms an A4/B2-type supramolecular polymer
and a gel with a symmetric neutral guest linker, showing a remarkable
fluorescence emission enhancement in solution and the solid state and a
good responsiveness to temperature and solvent composition.
Supramolecular polymerization and depolymerisation play a crucial
role in biosystems, where relatively simple molecular precursors are
brought together or separated in a precisely specific manner through
non-covalent interactions with the stimulation of the environment.1
Inspired by the captivating process of self-assembly in biosystems,
considerable effort has been devoted to the design of artificial
functional supramolecular systems.2,3 Pillarenes (or pillar[n]arenes),4
as a new generation of supramolecular macrocyclic receptors, firstly
reported in 2008,4a consisting of hydroquinone units linked by
methylene bridges at 2,5-positions, have been the focus of much
research and have shown great potential in molecular switches/
machines,5 artificial transmembrane channels,6 metal–organic frame-
Scheme 1 Schematic illustration of the construction of fluorescent
supramolecular polymers of G2CH1 and other host–guest complexes of
G2CH2, G1CH1 and G1CH2 for comparison.
works,7 controlled drug delivery systems,5a,b,8 sensors,9 supramole- for various applications in sensors, photonics, electronics,
cular polymers,3d,10 hybrid absorbents,11 virus inhibitors,12 etc., owing biomedicine, etc.
to their unique structures and superior host–guest properties.13,14
Herein, we report the synthesis of the first tetrameric pillarene,
Compared with other major synthetic macrocycles, pillarene deriva- i.e., tetraphenylethene (TPE)-bridged pillar[5]arene (P5) tetramer, TPE-
tives have recently shown excellent binding abilities towards neutral (P5)4 (H1), and its interaction with a newly designed triazole-based
guest molecules14 in organic solvents, facilitating a new pathway for neutral linker (G2) to construct pillarene-based stimuli-responsive
supramolecular self-assembly of neutral host–guest species in organic supramolecular gels with strong blue fluorescence (Scheme 1) for
media with special physical, chemical and optical properties.
the first time. Considering the significance of supramolecular poly-
Significantly, bridged pillarenes, such as dimers10a,b and merization and depolymerization in nature,1 the on–off switchable
trimers,10c have been reported to form linear and multi-dimensional fluorescent properties of G2CH1 based on supramolecular assembly–
assemblies upon binding with guest linkers, e.g., viologens. However, disassembly via the aggregation-induced emission (AIE) mechanism
pillarene tetramers or pentamers have not yet been synthesized, may facilitate the evolution of novel fluorescent probe technology for
especially incorporating fluorogenic functionalities, which is hoped the labeling and tracking of cells, proteins and nucleic acids.
to provide a versatile platform for the construction of stimuli-
The compounds, i.e., H1, TPE-P5 monomer (H2) and TPE-
responsive supramolecular polymers/gels with specific properties (monomer)4 (H3), were synthesized according to the synthetic routes
provided in the ESI.† Briefly, tetrahydroxyl TPE was synthesized
State Key Laboratory of Supramolecular Structure and Materials,
by Cross McMurry reactions15 between two bis(4-hydroxyphenyl)-
methanones. Then H1 was synthesized by introducing four bromo-
College of Chemistry, Jilin University, 2699 Qianjin Street, Changchun, 130012,
China. E-mail: ywyang@jlu.edu.cn
butyl mono-substituted P5 units (CoP5s) to react with the four
hydroxy groups of the tetrahydroxyl TPE, catalyzed by K2CO3 and
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c4cc03105a
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KI in MeCN. Monohydroxyl TPE was synthesized by Cross McMurry
reactions of (4-hydroxyphenyl) phenyl methanone and benzophenone,
and then further reacted with CoP5 in MeCN to give H2 (Fig. S5–S12,
ESI†). Meanwhile, the monomer of CoP5, i.e., 1-(4-bromobutoxy)-4-
methoxy benzene, was also reacted with tetrahydroxyl TPE to give H3
without a host cavity as a control compound. All the new compounds
1
were fully characterized by H NMR, 13C NMR and MALDI-TOF MS
spectroscopy (Fig. S4–S12, ESI†). H1 and H2 were endowed with AIE
characteristics in the mixed solvents of H2O–THF, indicating similar
fluorescent behavior to traditional TPE derivatives and providing
diverse functionalities with potential applications (Fig. S22, ESI†).
On the other hand, we designed and synthesized an asymmetric
neutral guest G1 by the CuI-catalyzed Huisgen alkyne–azide 1,3-dipolar
cycloaddition, the so-called CuAAC ‘click’ reaction (Fig. S14–S16, ESI†).16
Before we set out to study the supramolecular self-assembly properties
of the above host–guest compounds, the inclusion complexation of the
neutral guest monomer (G1) and the model host compound (H2), with
only one pillarene cavity as the recognition motif, has been investigated
by isothermal titration calorimetry (ITC), which is a powerful method
for measuring the host–guest association constants (Ka) and thermo-
dynamic parameters (Fig. S26–S27, ESI†). Chloroform was chosen as
the solvent for investigating the interactions between G1 and H2. The
titration data can be well fitted by computer simulation using the ‘‘one
set of binding sites’’ model, further confirming the stoichiometry of the
complexation between G1 and H2 to be 1 : 1. The Ka value was
determined to be (1.46 Æ 0.05) Â 104 MÀ1, indicating a strong binding
between the pillarene cavity and the neutral guest moiety. The inclusion
complexation of G1 and H2 is primarily driven by favorable enthalpy
changes (DH1 = À57.76 Æ 4.19 kJ molÀ1), accompanied by negative
entropic changes (TDS1 = À34.07 Æ 0.12 kJ molÀ1), which manifests
that the binding process is mainly caused by C–HÁÁÁp interactions,
C–HÁÁÁN/C–HÁÁÁO hydrogen bonds and van der Waals forces.4b
Fabrication of highly efficient luminescent supramolecular
polymers based on pillarenes is in urgent need for the con-
struction of stimuli-responsive soft matter with unique optical
and sensing properties. Chloroform, as an extremely favorable
Fig. 1 Fluorescence emission spectra of (a) H1 (black line) and G2CH1 (red
line) (lex = 350 nm; lem = 481 nm; slit widths: ex. 5 nm, em. 5 nm; 25 1C,
concentration: [H1] = 100 mM, [G2] = 400 mM); (b) G2CH1 (lex = 350 nm;
lem = 481 nm; slit widths: ex. 5 nm, em. 5 nm; 25 1C, concentration: [H1] =
200 mM, [G2] = 0 mM, 53 mM, 106 mM, 159 mM, 212 mM, 260 mM, 318 mM,
373 mM, 400 mM, 426 mM, 506 mM, 586 mM, 640 mM, 693 mM, 746 mM,
800 mM); (c) G2CH2 (lex = 425 nm; slit widths: ex. 10 nm, em. 10 nm; 25 1C,
concentration: [H2] = 1 mM, [G2] = 0.0 mM, 0.2 mM, 0.3 mM, 0.4 mM, 0.5 mM,
0.6 mM, 0.7 mM, 0.8 mM, 0.9 mM, 1.0 mM, 1.1 mM, 1.2 mM, 1.3 mM, 1.5 mM, 1.7 mM,
2.0 mM; [H2] = 0 mM, [G2] = 2.0 mM); (d) G2CH3 (lex = 350 nm; lem
=
489 nm; slit widths: ex. 5 nm, em. 5 nm; 25 1C, concentration: [H3] =
200 mM, [G2] = 53 mM, 106 mM, 159 mM, 212 mM, 260 mM, 318 mM, 373 mM,
400 mM, 426 mM, 506 mM, 586 mM, 640 mM, 693 mM, 746 mM, 800 mM);
(e) fluorescence emission intensity changes of G2CH1 at lem = 481 nm;
(f) the comparison of fluorescence emission intensity changes of G2CH1
(at lem = 481 nm, red line) and G2CH3 (at lem = 481 nm, black line).
solvent for H1 and G2, was chosen as a solvent medium for the the CHCl3 solution of H2 (Fig. 1c), which possesses only one
study of H1–G2 assembly. The pillarene tetramer (H1) with four pillarene and can only form 1 : 2 host–guest complexes with G2
recognition sites connecting through one TPE core can include the instead of forming a supramolecular network, there is no obvious
linear neutral guest dimer (G2) (Fig. S13–S21, ESI†) consisting of two fluorescence enhancement. The 1 : 2 inclusion complex, i.e.,
triazole units. H1 dissolved in CHCl3 shows relatively weak fluores- G2C(H2)2, is dissolved well in CHCl3 without any internal rotations
cence; however, upon gradual addition of G2, the PL intensity restricted. Moreover, H3, i.e., TPE functionalized with four mono-
(Fig. 1a, b and e) of the mixture is continuously enhanced. This mers of P5, was also synthesized as a control compound to H1. The
phenomenon can be explained by the mechanism of AIE proposed mixture of H3 and G2 shows no fluorescence enhancement with the
by Tang et al.,17 attributing to the TPE core of H1: (a) when G2 is continuous addition of G2 (Fig. 1d and f) because they are dispersed
added to H1 solution, supramolecular assembly occurs due to the in CHCl3 individually without any host–guest complexation due to
efficient inclusion complexation of H1 and ditopic guest G2; (b) H1 the lack of host cavities. All the above experiments indicated that
molecules are linked together by this highly efficient host–guest supramolecular self-assembly occurs between H1 and G2 in CHCl3
binding where the TPE cores of H1 molecules are maximally brought and there was an obvious fluorescence enhancement accompanied
close to each other; (c) therefore, the internal rotations of the with their supramolecular polymerization attributing to the restric-
molecules have been largely restricted, thus blocking the nonradia- tion of internal rotation (RIR) of the TPE cores of H1. The high-order
tive relaxation channel and populating the radioactive decay to the complexes via supramolecular interactions provide us innovative
ground state,17 making the material emissive.
To further confirm the explanation that fluorescence enhance- fluorescence for sensing and tracking.
thoughts to simulate the process in biosystems accompanied with
ment results from the supramolecular assembly of H1 and G2,
Compared with traditional polymers, one of the important
control experiments have been performed. When G2 is added to features of supramolecular polymers is the ease of controlling
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its assembly–disassembly by adjusting those non-covalent interactions
between the building blocks, mimicking the polymerization and
depolymerization in biological systems. Supramolecular polymers
responsive to solvent composition are degradable new materials upon
changing the polarity of the mixed solvent.3d Herein, we found that
solvent composition also plays a key role in influencing the fluores-
cence behavior of G2CH1. We tested several mixed solvents to show
the influence of solvent composition on the assemblies by studying
the intensity changes of their fluorescent emissions. According to the
previous experiments, H1 and G2 can form stable supramolecular
polymers in pure CHCl3 owing to their strong binding affinity.
Therefore, we use CHCl3 as the basic solvent to prepare G2CH1
solutions where the concentration of H1 and G2 is 10 mM and 20 mM,
respectively. As shown in Fig. S29 (ESI†), when G2CH1 was prepared
in a mixed solvent of CH3CH2OH : CHCl3 (v/v = 1 : 1), its PL intensity
was enhanced. This is because H1 and G2 cannot be dissolved in pure
CH3CH2OH and the intramolecular rotations of TPE cores of H1 are
further inhibited as the viscosity of the mixture is increased. When
other mixed solvent systems, i.e., CH2Cl2 :CHCl3 (v/v = 1 : 1),
CH3COCH3 :CHCl3 (v/v = 1 : 1), and CH3CN : CHCl3 (v/v = 1 : 1), were
employed, the PL intensity exhibited a small (24.1%, I0 = PL intensity
of G2CH1 in pure CHCl3) or remarkable decrease (52.5% and 49.3%),
in a sharp contrast to the CH3CH2OH : CHCl3 (v/v = 1 : 1) situation.
These phenomena can be ascribed to the fact that in pure CHCl3, the
molecular recognition events between pillarene moieties of H1 and
guest G2 are more favored compared with those in the other three
mixed solvents.
In the process of molecular recognition and supramolecular self-
assembly, elevated temperatures always decrease the stabilities of
host–guest systems due to the accompanying more unfavorable
entropy term (TDS1) governing their complexation free energies.2b,18
We envisage that the increase of system temperature will result in
the decrease of the PL intensity since elevated temperature will
weaken the binding affinities between pillarene hosts and neutral
linear guests. The experimental results showed that upon gradually
increasing the solution temperature from 0 1C to 57 1C (b.p. of CHCl3
is 61.2 1C), the fluorescent emission intensity of the CHCl3 solution of
H1 (0.1 mM) and G2 (0.2 mM) decreased by 53.3% (calculated
through the measurement of the FL intensity at 488 nm) (Fig. 2a
and c). In contrast, the fluorescent emission intensity of the mixture of
H1 and G2 increased by 110.7% (calculated through the measure-
ment of the FL intensity at 488 nm) (Fig. 2b and d) upon lowering the
temperature from 57 1C to 0 1C. All the above experiments indicated
that the temperature-dependent fluorescent change is reversible,
which can be repeated for many cycles. These experimental data
proved that the stability of the formed supramolecular assemblies of
H1 and G2 in CHCl3 governs the fluorescent emission intensity by
affecting the degree of the RIR of TPE cores in H1.
Fig. 2 Fluorescence emission spectra of G2CH1 (a) upon elevating
the temperature and (b) lowering the temperature. (lex = 364 nm;
lem = 488 nm; slit widths: ex. 5 nm, em. 5 nm; 25 1C, concentration:
[H1] = 100 mM, [G2] = 200 mM); (c) relative emission intensity changes at
lem = 488 nm when elevating the temperature from 0 1C to 57 1C
(PL intensity decrease/% = I/I0 Â 100%); (d) relative emission intensity
changes at lem = 488 nm when lowering the temperature (PL intensity
increase/% = I/I0 Â 100%). I0 represents the emission intensity at 0 1C.
show that, at the same concentrations and/or molar ratios of host
and guest, none of the CHCl3 solutions of the H2–G2 complex, the
mixture of H3 and G2, and individual H1 or G2 forms gels,
indicating that they could not form supramolecular networks for
gelation and host–guest design is extremely important (Fig. 3a–d).
Significantly, the gels constructed from G2CH1 exhibited a strong
blue fluorescence emission with the maximum wavelength of 492 nm
as observed by solid-state fluorescence spectroscopy (Fig. 3g).
Intriguingly, the inclusion complexation of H1, with four P5
macrocycles, and G2, as a ditopic linear linker, in CHCl3 induced
gelation successfully at a concentration of 70 mM (CH1 = 70 mM,
CG2 = 140 mM, molar ratio: H1 :G2 = 1 : 2) due to the formation
of A4/B2-type supramolecular polymers to result in a large-scale
multiple complexes. This solution-gelation process has been shown
in Fig. 3e, where the solution stops flowing at high concentrations
upon turning the test tube upside down. Controlled experiments
Fig. 3 Photographs of (a) G2CH2, (b) G2CH3, (c) H1, (d) G2, and (e)
G2CH1 (upright and upside down); (f) confocal laser scanning microscope
image of the supramolecular gel constructed of G2CH1 (lex = 405 nm);
(g) solid-state fluorescence emission spectra of the supramolecular gel con-
structed of G2CH1 observed by fluorescence microscopy (lem = 488 nm);
(h) fluorescent images observed by the fluorescence microscopy (lex
=
365 nm, at magnification Â200); (i) SEM images of the gel (scale bar: 1 mm).
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Meanwhile, fluorescence microscopy (Fig. 3h) and confocal
laser scanning microscopy (CLSM) (Fig. 3f) further revealed
the gel morphology of G2CH1 in a micrometer scale and its PL
properties of strong blue emission. In addition, the scanning
electron microscopy (SEM) image of the gel also shows its
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Thermogravimetric analysis (TGA) and differential scanning
calorimetry (DSC) (Fig. S30 and S31, ESI†) are used as special
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molecular gel. Especially, TGA data show that the gel has no
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In summary, we report the design and synthesis of a new
pillarene tetramer derivative (H1) with a TPE core and four
pillarene cavities, which can bind strongly to the linear neutral
guest linker G2 with cyano sites and triazole sites to form A4/B2-type
supramolecular polymers, G2CH1, in CHCl3. Fascinatingly, the
supramolecular self-assembly of H1 and G2 results in dramatic
blue fluorescent emission enhancement due to the RIR of TPE
cores. The fluorescent intensity of G2CH1 assemblies in CHCl3
showed good temperature and solvent responsiveness. Further-
more, blue supramolecular gels can be obtained at a host concen-
tration of 70 mmol LÀ1 with a host–guest molar ratio of 1 : 2. To
the best of our knowledge, this is the first report on pillarene
derivatives with intriguing fluorescent enhancement phenomena,
which paves a new way of efficient fabrication of pillarene-based
fluorescent materials.
This research was supported by the National Natural Science
Foundation of China (21272093), the Innovation Program of
the State Key Laboratory of Supramolecular Structure, and
Materials, and the National Science Foundation for Fostering
Talents in Basic Research of the National Natural Science
Foundation of China (Grant No. J1103302).
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