Controlled Encapsulation and Release of an Organic Guest in the Cavity of α,α′,δ,δ′-Tetramethylcucurbit[6]uril

Article abstract of DOI:10.1002/ejoc.201801652

Binding behaviors of TMeQ[6] and Q[7] with 1-(4-carboxybenzyl)-4-[2-(4-pyridyl)-vinyl]-pyridinium chloride (guest 1) were investigated by various experimental techniques. The results indicate that TMeQ[6] can perform the encapsulation and release of guest

Full text of DOI:10.1002/ejoc.201801652

A Journal of
Accepted Article
Title: Controlled encapsulation and release of an organic guest in the
cavity of α,α’,δ,δ’-tetramethylcucurbit[6]uril
Authors: Kun Zhang, Wenqi Sun, Ruilian Lin, Xin Xiao, Bing Bian, Zhu
Tao, and Jingxin Liu
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.201801652
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10.1002/ejoc.201801652
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DOI: 10.1002/ ((please add manuscript number))
Article type: Full Paper
Controlled encapsulation and release of an organic guest in the cavity of α,α’,δ,δ’-
tetramethylcucurbit[6]uril
Kun Zhang, Wenqi Sun, Ruilian Lin, Xin Xiao,* Bing Bian, Zhu Tao, and Jingxin Liu*
((Optional Dedication))
K. Zhang, R. Lin, Prof. W. Sun, Prof. J. Liu
College of Chemistry and Chemical Engineering, Anhui University of Technology, Maanshan
243002, China.
E-mail: jxliu411@ahut.edu.cn
Prof. X. Xin, Prof. Z. Tao
Key Laboratory of Macrocyclic and Supramolecular Chemistry of Guizhou Province,
Guizhou University, Guiyang 550025, China.
E-mail: gyhxxiaoxin@163.com
Prof. B. Bian
College of Chemical and Environmental Engineering, Shandong University of Science and
Technology, Qingdao, 266590, China.
Keywords: cucurbit[n]uril, supramolecular self-assembly, molecular switch
Binding behaviors of TMeQ[6] and Q[7] with 1-(4-carboxybenzyl)-4-[2-(4-pyridyl)-vinyl]-
pyridinium chloride (guest 1) were investigated by various experimental techniques. The
results indicate that TMeQ[6] can perform the encapsulation and release of guest 1, which
was controlled by UV irradiation and heating through reversible trans-cis isomerisation of
guest 1. In contrast, no reversible trans-cis isomerisation of guest 1 is observed when included
in Q[7].
1. Introduction
In the last few decades, molecular encapsulation and controlled release have attracted
growing interest due to their potential applications in various fields such as stimuli-responsive
materials, chemical reaction catalysis, nutrient preservation, environmental protection,
fragrance release, and drug delivery.[1-10] A variety of chemical architectures, such as carbon
nanotubes[11], microporous inorganic materials[12,13], polymers[14], and supramolecular
macrocyclic hosts[15-18], have been employed as carriers for small molecules. Among them,
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European Journal of Organic Chemistry
macrocyclic hosts are particularly appealing for encapsulating target guests. Not only are
supramolecular interactions in these systems predictable, but intermolecular interactions are
reversible and highly tunable, which facilitates to encapsulate and release key molecules in a
controlled way.
In comparison with other macrocyclic hosts, including crown ethers, cyclodextrins,
calix[n]arenes and pillar[n]arenes, the cucurbit[n]uril family of hosts (Q[n], n = 5-8, 10)[19-23]
particularly interesting due to their facile synthetic preparation, rigid hydrophobic cavities and
hydrophilic carbonyl portals. They can thus selectively bind with hydrophobic and positively
charged molecules through host−guest interactions (including hydrophobic contact and ion–
dipole interaction). In the past studies, much efforts have been made in the development of
Q[n]-based containers that can perform encapsulation and release of guest molecules by
external stimuli, e.g., pH[24], light[25], temperature[26], redox potential[21,27], and metal
coordination[28]. As the first example, back in 1996, Kim and coworkers reported a molecular
container assembly capable of controlling the binding and release of THF by complexation
and decomplexation of sodium ions at the portals of the Q[6][28]. After this, much/some
literatures involving Q[n]-based molecular containers with similar capabilities have also been
reported by Kim, Kaifer, Scherman, and other groups.[24-28] However, it remains challenging
to design and create a stimuli-responsive molecular container based on Q[n].
Our efforts in this vein has been focused on the use of Q[n] derivatives. We previously
discovered that the oval cavity of symmetrical α,α’,δ,δ’-tetramethyl-cucurbit[6]uril
(TMeQ[6]) is complementary in size and shape to the aromatic ring of organic molecules such
as pyridine and benzene derivatives.[29-31] According to structural features of the TMeQ[6], we
prepared a derivative of 4,4'-vinylenedipyridine, 1-(4-carboxybenzyl)-4-[2-(4-pyridyl)-vinyl]-
pyridinium chloride (guest 1, Figure 1) with trans- and cis- isomers, which
photoisomerisation process may allow control of guest encapsulation and release. We
reasoned that trans-1 can be encapsulated into the TMeQ[6] cavity, while cis-1 couldn’t be
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accommodated in the TMeQ[6] cavity because of size effect. In the present study, we
investigated the binding of trans- and cis-1 to TMeQ[6] by NMR spectroscopy, MALDI-TOF
MS, and UV-vis and fluorescence spectroscopy. For comparison purposes, the binding of
trans- and cis-1 to Q[7] were also investigated.
2. Results and Discussion
The binding of TMeQ[6] with trans-1 was first probed by examining 1H NMR chemical
shifts of trans-1 upon addition of TMeQ[6]. After the addition of 1.14 equiv. of TMeQ[6], the
side pyridyl protons and ethylene protons of the trans-1 are shifted upfield while the rest of
protons are shifted downfield (Figure 2B). It should be noted here that 2D COSY spectra
were employed to assist the assignments of the trans-1 protons. This observation indicates
that the pyridylethylene group of the trans-1 is located in the cavity of TMeQ[6], while the
rest of the trans-1 resides outside of the portal of TMeQ[6], generating a 1:1 inclusion
complex of trans-1@TMeQ[6].
As expected, irradiation with UV light (365 nm) of the aqueous solution of trans-
1@TMeQ[6] for 10 h causes isomerization of the guest and release of cis-1 from TMeQ[6].
As evidenced by 1H NMR spectroscopy (Figure 2C), irradiation of the aqueous solution of
trans-1@TMeQ[6] causes a diminution of the signals of included trans-1, with a concurrent
appearance of the signals of free cis-1. Indeed, the cis-1 does not form inclusion complex with
TMeQ[6] because of size effect. In other words, the TMeQ[6] cavity is not large enough to
accommodate the guest 1 in its cis form. Interestingly, when the solution of cis-1 and
TMeQ[6] was heated at 90 °C for 10 h, the NMR signals of free cis-1 returned back to those
of included trans-1 (Figure 2D). Obviously, heating causes the isomerization of guest 1 from
cis to trans form. As a result, the trans-1 was encapsulated into the cavity of TMeQ[6] once
again (Scheme 1). Based on above experimental data, we conclude that the encapsulation and
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release of guest 1 can be reversibly controlled by external stimuli (light irradiation and
heating), and TMeQ[6] can be used as stimuli-responsive molecular container.
The binding of TMeQ[6] with trans-1 was further detected by UV-vis (Figure 3) and
fluorescence (Figure S4) spectroscopic titration experiments. The decreasing absorbance and
red shift effect was observed in 320 nm. According to the absorption spectrophotometric data,
the binding interaction of TMeQ[6] with trans-1 was fitted to 1:1 binding model, and its
binding constant was found to be 3.80×108 M−1. Additionally, the formation of this 1:1
complex was further confirmed by MALDI-TOF MS (Figure S5).
For comparison purposes, the binding interaction of trans-1 with Q[7] in aqueous
solution was also studied by multiple techniques. As shown in Figure 4B, the complexation-
induced chemical shifts of trans-1 with Q[7] are completely different from those observed
with TMeQ[6]; the ethylene protons and both pyridyl protons of the trans-1 are shifted
upfield while the rest of protons are shifted downfield. It is easily to conclude that Q[7] can
envelop the trans-1 to form inclusion complex trans-1@Q[7]. When the aqueous solution of
trans-1@Q[7] was irradiated by UV light of 365 nm for 3 h, we observed one new set of
signals of included cis-1 (Figure 4C). The assignments of the trans-1 and cis-1 protons are
supported by 2D COSY spectra. Obviously, some of the trans-1, about 40 percent according
NMR integral calculation, have been converted to cis-1 isomer and the cis-1 remains included
in Q[7]. However, cis-1@Q[7] is quite stable; there is no any changes in its 1H NMR
spectrum even heating the solution of trans-1@Q[7] and cis-1@Q[7] at 90 °C for 3 h
(Scheme 2). To further explore the binding strength of the host-guest interactions between
Q[7] and trans-1 or cis-1, the binding energy of trans-1 to Q[7] (553.61950850 KJ/mol ) and
cis-1 to Q[7] (621.16319983 KJ/mol) was also obtained, Table S1, indicating the stronger
binding of cis-1 to Q[7] than trans-1. Kim, Isaacs and other groups previously reported
similar structures, in which the cis isomer can isomerize back to the trans isomer by
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heating.[32] In the present study, it is probably that the strong binding interaction between the
cis-1 and Q[7] stop the isomerization.
UV-vis (Figure 5) and fluorescence (Figure S8) titration experiments were also
performed to detect the binding interaction of trans-1 with Q[7]. As can be seen in Figure 5,
the titration curve of Q[7] with trans-1 indicates a 1:1 binding model, and the binding
constant of trans-1 with Q[7] was found to be 4.35×106 M−1, which is smaller than that of
trans-1 with TMeQ[6]. Similary, MALDI-TOF MS further confirmed the formation of the 1:1
complex of trans-1 with Q[7] (Figure S9).
To gain a better understanding of the binding interactions of TMeQ[6] and Q[7] with
guest 1, we carried out ab initio calculations on trans-1@TMeQ[6], trans-1@Q[7] and cis-
1@Q[7] in the gas phase.[33] The results of molecular energy minimization of all inclusion
complexes are shown in Figure 6. Obviously, the direct binding interactions between host and
guest include the ion−dipole interactions and the van der Waals interactions.
3. Conclusion
In conclusion, we confirmed that the molecular container TMeQ[6] can perform the
encapsulation and release of guest 1 (in trans- and cis- form) by interchanging light
irradiation and heating. Trans-1 encapsulation into TMeQ[6] is attributed to the strong host–
guest interactions while cis-1 release from TMeQ[6] is attributed to the size effect. When
encapsulated in Q[7] cavity, only 40 percent of trans-1 can isomerize to the cis isomer by
irradiation, and the encapsulated cis-1 could not convert back to trans-1 even after heating
long enough. Our study provides an insight into designing drug delivery and molecular
machines.
4. Experimental Section
Materials and methods: The starting materials and solvents for syntheses were commercially
available and used as supplied without further purification. TMeQ[6] and Q[7] were prepared
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10.1002/ejoc.201801652
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by the method of literature.[34,35] All the NMR data were performed on a JEOL JNM-
ECZ400S 400 MHz spectrometer at 293.15K. MALDI-TOF mass spectra were recorded on a
Bruker BIFLEX III ultra-high resolution Fourier transform ion cyclotron resonance (FT-ICR)
mass spectrometer. Absorption spectra of the host−guest complexes were recorded on an
Aglient 8453 spectrophotometer at room temperature. Fluorescence spectroscopic
experiments were performed on a Varian RF-540 fluorescence spectrophotometer.
Synthesis of guest 1: 4,4’-Vinylenedipyridine (0.5 g, 0.0027 mol) and 4-
(Chloromethyl)benzoic acid (0.94 g, 0.0055 mol) was dissolved in Etanol (40 ml). In the oil
bath heated to 70 °C refluxing reaction 12 h, a large number of pink precipitate, cooling to
room temperature and then filtered, rinsed with acetone 3 times,dried in vacuum to give guest
1(0.61 g, 42%). 1H NMR (400 MHz, ) δ 8.73 (d, J = 6.6 Hz, 2H), 8.59 (d, J = 6.1 Hz, 2H),
8.07 (d, J = 6.5 Hz, 2H), 8.01 (d, J = 6.3 Hz, 2H), 7.79 (d, J = 7.9 Hz, 2H), 7.75 – 7.63 (m,
2H), 7.35 (d, J = 8.0 Hz, 2H), 5.68 (s, 2H).
UV spectra and fluorescence measurement: All UV spectra were recorded with an Agilent
8453 spectrophotomete (Hewlett Packard, U.S.A) at room temperature. All fluorescence
spectra were recorded with a VARIAN Cary Eclipse (Varian Inc, U.S.A) at room temperature.
The solutions of Q[7] (1.0×10-4 mol/L), TMeQ[6] (1.0×10-3 mol/L), were prepared by directly
dissolving host in double-distilled water. The solutions of guest 1 (1.0×10-3 mol/L) were
prepared for both absorption spectra and fluorescence spectra determination. Samples of these
solutions were combined to give host : guest ratios of 0, 0.2:1, 0.4:1, 0.6:1,… 3:1
respectively. The data were analyzed by ORIGIN 8.0 and photoshop software.
1H NMR measurements: To analyze the host-guest complexation of Q[7], TMeQ[6] and guest
1, All 1H NMR spectra, including those for the titration experiments, were recorded at 20 ℃
on a JEOL JNM-ECZ400S 400 MHz. D2O was used as a field-frequency lock and the
observed chemical shifts are reported in parts per million (ppm) relative to that for the internal
standard (TMS at 0.0 ppm). The ratio of guest 1 to Q[7], TMeQ[6] was calculated by the ratio
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10.1002/ejoc.201801652
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of their integral areas for special peaks. The concentrations of the guest 1 are 1.0×10-4 mol/L
in the 1H NMR experiments.
MALDI-TOF mass spectrometer:MALDI-TOF mass spectra were taken on a Bruker BIFLEX
III ultra-high resolution Fourier transform ion cyclotron resonance (FT-ICR) mass
spectrometer with a-cyano-4-hydroxycinnamic acid as matrix. The solution concentration is
1.0×10-4 mol/L (guest 1: Q[7] /TMeQ[6]= 1:1).
Computations: The geometries of TMeQ[6] and Q[7] were first optimized with the Hartree-
Fock (HF) method using 3-21G* basis sets. The geometry optimized structures of inclusion
complexes trans-1@TMeQ[6], trans-1@Q[7] and cis-1@Q[7] were optimized using density
functional theory (DFT) at the B3LYP/6-31G* level. All the calculations were performed
using a suite of Gaussian 03 program.[33]
Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements
The authors are grateful to the National Natural Science Foundation of China (No. 21861011,
21371004), Natural Science Foundation of Anhui Province of China (1808085MB43), the
Creative Research Groups of Guizhou Provincial Education Department (2017-028), the
Innovation Program for High-level Talents of Guizhou Province (No. 2016-5657) and the
Science Technology Fund of Guizhou Province (No. 2016-1030, 2018-5781) for the generous
support of this work.
Received: ((will be filled in by the editorial staff))
Revised: ((will be filled in by the editorial staff))
Published online: ((will be filled in by the editorial staff))
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Figures and Figure Captions:
Figure 1 Structure the TMeQ[6] and the guest 1 (trans-1 and cis-1) used in this work.
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Figure 2 1H NMR spectra of trans-1 (4.0 mmol/L) in the absence (A) and presence of 1.14
equiv of TMeQ[6] (B) in D2O at 20 °C; (C) cis-1@TMeQ[6] after irradiation of solution (B)
for 10 h; (D) trans-1@TMeQ[6] after heating of solution (C) at 90 °C for 10 h; (E) trans-1
1
and cis-1; (F) H NMR spectrum of TMeQ[6] (4.0 mmol/L) in D2O at 20 °C.
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Figure 3 UV-vis spectra of trans-1 with increasing concentrations of TMeQ[6]. Inset shows
absorbance vs NTMeQ[6]/Ntrans-1 recorded at 320 nm.
Figure 4 (a) 1H NMR spectra of trans-1 (4.0 mmol/L) in the absence (A) and presence of 1.73
(B) equiv of Q[7] in D2O at 20 °C; (C) cis-1@Q[7] and trans-1@Q[7] after irradiation of
solution (B) for 3 h.; (D) trans-1 and cis-1; (E) 1H NMR spectrum of Q[7] (4.0 mmol/L) in
D2O at 20 °C.
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Figure 5 UV spectra of trans-1 with increasing concentrations of Q[7] and A vs. NQ[7]/Ntrans-1
curve.
Figure 6 Energy minimized structures of trans-1@TMeQ[6] (a), trans-1@Q[7] (b) and cis-
1@Q[7] (c) in the gas phase by a HF/3-21G** calculation.
Scheme 1 Schematic representation of the controlled encapsulation and release of the guest 1
in TMeQ[6] cavity.
Scheme 2 Schematie representation of the trans-1 isomerize to cis-1 in Q[7] cavity.
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European Journal of Organic Chemistry
By interchanging light irradiation and heating, the molecular container TMeQ[6] can perform
the encapsulation and release of an organic guest, which is attributed to the reversible trans-
cis isomerisation of the guest.
cucurbit[n]uril, supramolecular self-assembly, molecular switch
Kun Zhang, Wenqi Sun, Ruilian Lin, Xin Xiao,* Bing Bian, Zhu Tao, and Jingxin Liu*
Controlled encapsulation and release of an organic guest in the cavity of α,α’,δ,δ’-
tetramethylcucurbit[6]uril
15
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