Square [5]molecular necklace formed from cucurbit[8]uril and carbazole derivative

Article abstract of DOI:10.1016/j.tetlet.2016.04.048

The formation of square 5-member molecular necklace ([5]MN) is presented by utilizing the host-guest recognition-driven self-assembly between CB[8] and carbazole derivative 1·2Cl^-, which bears two 4,4′-bipyridin-1-ium (BP) arms with a orthogonal geometry. The self-assembly and stimuli-responsive behaviors of square [5]MN are comprehensively investigated by NMR (¹H, COSY, NOSEY, and DOSY), UV-vis, DLS, and SEM methods.

Full text of DOI:10.1016/j.tetlet.2016.04.048

Tetrahedron Letters xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Square [5]molecular necklace formed from cucurbit[8]uril
and carbazole derivative
⇑
Jie Li, Yang Yu, Linzhi Luo, Yawen Li, Pinpin Wang, Liping Cao , Biao Wu
Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of the Ministry of Education, College of Chemistry and Materials Science, Northwest University, Xi’an, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
The formation of square 5-member molecular necklace ([5]MN) is presented by utilizing the host–guest
recognition-driven self-assembly between CB[8] and carbazole derivative 1ꢀ2Clꢁ, which bears two
4,40-bipyridin-1-ium (BP) arms with a orthogonal geometry. The self-assembly and stimuli-responsive
behaviors of square [5]MN are comprehensively investigated by NMR (1H, COSY, NOSEY, and DOSY),
UV–vis, DLS, and SEM methods.
Received 27 February 2016
Revised 12 April 2016
Accepted 14 April 2016
Available online xxxx
Ó 2016 Elsevier Ltd. All rights reserved.
Keywords:
Mechanically interlocked molecules
Molecular necklace
Self-assembly
Cucurbit[n]uril
Carbazole derivative
Over the past two decades, the efficient synthesis of mechani-
cally interlocked molecules such as rotaxanes, catenanes, and
knots have attracted wide attention due to not only their amazing
supramolecular architectures1–3 (e.g., olympiadane,1b pentafoil
Recently, Wu and co-workers also used DB24C8 macrocycles to
synthesize a kinetically and thermodynamically stable [5]MN.7
More interestingly, Li and Stang achieved the orthogonality of
coordination-driven self-assembly and crown ether host–guest
complexation to successfully obtain a [4]MN with a triangle geom-
etry and [7]MN with a hexagon geometry.8 Chiu and co-workers
applied four bis-crown-based molecular cages to macrocyclization
with flexible linear guests, producing up to a [5]MN.9 On another
hand, Kim and co-workers introduced cucurbit[n]uril (n = 6, 8)
macrocycles into the self-assembly of molecular necklaces using
the coordination of rotaxane-based ligands or host-stabilized
charge-transfer interactions to successfully gain cucurbit[6]uril-
based [n]MN (n = 4, 5)4,10 and cucurbit[8]uril-based [6]MN,11
respectively.
CB[8] as a member of cucurbit[n]uril family has an enough big
cavity which is capable of simultaneously encapsulating two iden-
tical or different guest molecules to form a stable 1:2 ternary com-
plex.12 For example, it can encapsulate an electron-rich guest and
an electron-deficient guest to form the stable 1:1:1 complex,
which is driven by the charge-transfer interaction between the
knot,1c
a
star of David catenane,1d Solomon links,1e and
Borromean rings1f) but also their potential functions, such as
molecular machine2 (e.g., molecular switches,2a molecular
motors,2b molecular sensors,2c molecular catalyst,2d and
)
molecular synthesizer2e and smart materials3 (e.g., memory
devices3a and molecular muscles3b). In these various structures,
[n]molecular necklaces ([n]MN) defined by Kim the first time, as
topologic isomers of catenanes, are a class of supramolecular
structures in which three or more side rings as molecular ‘beads’
are threaded onto a central ring as molecular ‘chains’.4 Sauvage
and co-workers unexpectedly observed the formation of a mixture
of [n]MN (n = 4–7) in the metal-template synthesis of multi-ring
catenanes, as identified by electrospray mass spectrometry.5 After
that, supramolecular scientists directly used molecular macrocy-
cles, such as crown ether and cucurbit[n]uril, as the ‘beads’ for
forming molecular necklaces via host–guest interactions and/or
coordination interactions. For example, Stoddart and co-workers
isolated trace crown-based [4]MN from oligocatenanes,6a and then
successfully constructed [4]MN and [5]MN with dialkylammonium
ions as host–guest interaction sites for anchoring dibenzo-24-
crown-8 (DB24C8) macrocycles via the Witting reaction.6b
p
-electron donor and acceptor molecules inside the hydrophobic
cavity of CB[8].12d Recently, Li and co-workers also found a new
1:2 binding pattern between CB[8] and two 4,40-bipyridin-1-ium
(BP) with a well-defined head-to-tail
p–p-stacking geometry in
the solid state.13a Due to its unique host–guest recognition
properties, CB[8] has been more and more employed as a linking
motif to prepare supramolecular organic frameworks,13,14
polymers,15 micelles,16 vesicles,17 microcapsule,18 hydrogels,19
⇑
Corresponding author.
E-mail address: chcaoliping@nwu.edu.cn (L. Cao).
http://dx.doi.org/10.1016/j.tetlet.2016.04.048
0040-4039/Ó 2016 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Li, J.; et al. Tetrahedron Lett. (2016), http://dx.doi.org/10.1016/j.tetlet.2016.04.048

2
J. Li et al. / Tetrahedron Letters xxx (2016) xxx–xxx
O
in the presence of excess CB[8] (2.0 equiv) (Fig. S3), which supports
O
N
N
O
N
O
O
N
O
a
b
that the stoichiometry of the complex between 1 and CB[8] is 1:1
ratio. In addition, Job’s plot using UV–vis spectroscopy displayed a
maximum absorption change at 0.5, also suggesting a 1:1 stoi-
chiometry of 1 and CB[8] (Fig. S4). Of particular note is the COSY
spectrum that showed that the chemical shifts of protons Hb and
Hc upfield shifted by 1.15 and 0.80 ppm, respectively, which are
more changes compared with the chemical shifts of Ha and Hd
N
N
N
N N N N
N
N
N
N
N
N
O
c
N
N
N
N
N
N
N
N
d
O
N
N
O
O
O
O
3
e
f
g
(Dd = 0.60 and 0.10 ppm, respectively) (Fig. S5). It is because the
N
N
protons Hb and Hc are located at the center of the cavity of CB[8]
due to the head-to-tail orientation between two staking BP units.
Furthermore, the NOESY spectrum also revealed the spatial
proximity of protons Hb and Hc as head-to-tail stacking of two BP
rings (Fig. S6).
(Hi)3C(Hh)2C
CB[8]
1
2Cl
Chart 1. Cucurbit[8]uril and structures of 1ꢀ2Clꢁ.
In order to obtain information about the constitution of the self-
assembled species formed from 1 and CB[8] in D2O, we performed
diffusion-ordered spectroscopy (DOSY) for the self-assembled spe-
cies (1ꢀCB[8])n. The 2D DOSY NMR spectrum of the self-assembled
species (1ꢀCB[8])n revealed that all the signals of 1 and CB[8] gave
comparable diffusion coefficient, which further identified the
formation of a single complex in the solution (Fig. S7). The diffu-
sion coefficients measured using nine different resonances for
(1ꢀCB[8])n averaged (1.496 0.052) ꢂ 10ꢁ10 m2 sꢁ1(Fig. 2). The D
value of (1ꢀCB[8])n is very close to the tetrameric assemble formed
from four cucurbit[7]uril derivatives21 (1.64 ꢂ 10ꢁ10 m2 sꢁ1) which
strongly suggests the formation of the square assembly (1ꢀCB[8])4.
Furthermore, if we assume that (1ꢀCB[8])4 is roughly spherical,
the hydrodynamic diameter of (1ꢀCB[8])4 calculated by the
Stokes–Einstein equation is about 2.67 nm, and consistent with
the computational model (Fig. S8).22
colloids,12e and molecular machines.20 Herein, we report a facile
route to achieve the quantitative self-assembly of square
a
[5]molecular necklace (CB[8]ꢀ1)4ꢀ8Clꢁ from four CB[8] as side rings
and four carbazole derivative 1ꢀ2Clꢁ with two 4,40-bipyridin-1-ium
(BP) arms which form the central ring via host–guest interactions,
and pH-responsive aggregation behaviors of the square [5]MN.
Our strategy is that rigid linkers with a proper angle and recog-
nition sites could lead to form cyclic geometric shapes such as tri-
angle, square, or pentagon, induced by the host–guest recognition
between CB[8] and linkers, and then assemble a structure of
molecular necklace. To achieve this idea, we designed and synthe-
sized a 3,6-disubstituted carbazole derivative 1ꢀ2Clꢁ which bears
two 4,40-bipyridin-1-ium (BP) arms with a orthogonal geometry
as guest molecules for the investigation of the self-assembly of
CB[8]-based [n]MN complex (Chart 1). Compound 1ꢀ2Clꢁ was
synthesized by the reaction of corresponding amines with N-(2,4-
dinitrophenyl) 4,40-bipyridin-1-ium salts by the Zincke reaction
(Supporting information). As shown in Scheme 1, two 4,40-bipyri-
din-1-ium arms (blue sticks) of compound 1ꢀ2Clꢁ were located
on 3- and 6-sites of the carbazole skeleton that offer two recogni-
tion sites with an around 90° angle. Spontaneously, the geometric
constraint and the head-to-tail orientation between two staking BP
units determined to form a [4+4] square macrocycle as the central
ring of [5]MN (Scheme 1).
Next, the self-assembly process of square [5]MN (1ꢀCB[8])4 was
studied by UV–vis spectroscopy. A dramatic red-shift from 383 nm
to 415 nm of the main absorption peak of the carbazole ring was
detected when 0 – 1 equiv of CB[8] was added to a solution of 1
(Fig. 3a, A–C) accompanying a color change from light yellow to
orange. We guess the reason of the color change could be ascribed
to the charge-transfer interactions between two BP rings inside the
cavity of CB[8] to finally give a central square ring, which caused
the elongated conjugation of four carbazole derivative 1 (Fig. S8,
MMFF model). Unfortunately, only 1+1 complex of CB[8] and 1
was observed by electrospray ionization mass spectrometry
(ESI-MS) because this sophisticated assembly is unstable in the
condition of ESI (Fig. S9). Taken together, the 1H, COSY, NOESY,
and DOSY NMR, strongly suggest that the complex of 1 and
CB[8] is most likely a single discrete cyclic [5]molecular necklace
(1ꢀCB[8])4 with a square geometry.
Initially, we investigated the self-assembly behavior of 1 with
CB[8] in D2O using NMR technique. Figure 1a shows the 1H NMR
spectrum recorded for 1 alone in D2O at room temperature. Adding
0.5 equiv of CB[8] to the guest solution caused the signals of 1 to
became more complicated (Fig. 1b). Figure 1b displays distinct
resonances for free 1 and the complex of 1 and CB[8], which
establishes that 1 undergoes slow exchange on the 1H NMR time
scale. Interestingly, when 1.0 equiv of CB[8] was added to a solu-
tion of carbazole derivative 1 (0.5 mM) in D2O, the signals of free
1 disappeared completely and one new set of signals to come
out, indicating the presence of a single thermodynamically stable
complex (Fig. 1c). Moreover, no obvious changes could be observed
The response of self-assembly system toward external stimuli,
such as temperature, concentration, pH, or some compounds, is
very important for potential applications. To know the thermody-
namic properties of the square [5]MN (1ꢀCB[8])4, we first studied
the response toward changes in temperature and concentration.
~90o
Square
[5]MN
Scheme 1. Schematic representation of self-assembly process between cucurbit[n]uril and carbazole derivative 1ꢀ2Cl.
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J. Li et al. / Tetrahedron Letters xxx (2016) xxx–xxx
3
Figure 1. 1H NMR spectra recorded (400 MHz, 298 K, D2O) for: (a) 1 (0.5 mM), (b) 1
and CB[8] (0.5 equiv), (c) 1 and CB[8] (1.0 equiv), (d) addition of DCl (100 equiv)
into (c), and (e) addition of NaOD (100 equiv) into (d). Here primed (0) denote
protons binding with CB[8].
Variable temperature 1H NMR experiments over the 298–348 K
range do not show any obvious changes indicative of changes in
self-assembly state (Fig. S10). Similarly, the 1H NMR spectra
recorded for [5]MN (1ꢀCB[8])4 do not change over the experimen-
tally accessible range of concentration ([CB[8]] = 0.1 mM to
2 mM) which suggest that the [5]MN (1ꢀCB[8])4 is of high thermo-
dynamic stability (Fig. S11).
Next, we investigated the pH-responsiveness of the square [5]
MN (1ꢀCB[8])4. Interestingly, when excess hydrochloric acid was
added into the solution of [5]MN (1ꢀCB[8])4, the signals for the pro-
tons on the aromatic ring of 1 were shifted downfield and broad-
ened under the acidic conditions due to protonation of the
terminal pyridine (Fig. 1d). These changes indicated the [5]MN
may dissociate and change into the formation of 1:1 complex of
CB[8] and protonated 1. At the same time, a relative decrease in
intensity as well as a slight red-shift of the absorption peak was
detected, accompanying that the color of the solution was changed
to red (Fig. 3a, D). Furthermore, it is significant to note that the
original [5]MN structure could be restored by the addition of NaOH
under neutral solution. The characteristic peak for the proton Hb on
the pyridine ring of [5]MN (1ꢀCB[8])4 appeared again (Fig. 1e). The
absorption peak and the color of the solution were changed back
(Fig. 3a, E). The reversible process can be modulated repeatedly
several times (Figs. 3b and S12).
Figure 3. (a) UV–vis absorption spectra of (a) 1 (A, black), 1 + 0.5 equiv of CB[8] (B,
red), 1 + 1 equiv of CB[8] (C, blue), 1 + 1 equiv of CB[8] + 100 equiv of HCl (D, pink),
and 1 + 1 equiv of CB[8] + 100 equiv of HCl + 100 equiv of NaOH (E, green). Inset:
color change of the solutions. (b) Reversibility of the protonating and deprotonating
process for (1ꢀCB[8])4 (The data at 410 nm gave a absorption by adding HCl and
NaOH, repetitively.).
solution of compound 1 showed a peak centered around 28.4 nm
in number distributions. Consistent with the result of DLS, com-
pound 1 aggregated together to form plate-like structures as
shown in Figure 5a. After adding 1 equiv of CB[8], the DLS result
exhibited a dramatic shift of the size distribution up to 122.4 nm,
which was quite different from, compared to that obtained before
the addition of CB[8] (Fig. 4). Figure 5b showed that upon addition
Dynamic light scattering (DLS) and scanning electron micro-
scopy (SEM) were utilized to investigate the effect of pH on the size
and morphology of the aggregation of [5]MN (1ꢀCB[8])4 in the solu-
tion and in the solid state, respectively. As shown in Figure 4, the
Figure 4. DLS results of
1 (j), 1 + 1 equiv of CB[8] (.), 1 + 1 equiv of CB[8]
+ 100 equiv of HCl (N), and 1 + 1 equiv of CB[8] + 100 equiv of HCl + 100 equiv of
NaOH (d) in water ([1] = [CB[8]] = 0.1 mM).
Figure 2. DOSY spectra recorded (600 MHz, D2O, 298 K) for (1ꢀCB[8])4 (0.5 mM).
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J. Li et al. / Tetrahedron Letters xxx (2016) xxx–xxx
Figure 5. SEM images of (a) 1; (b) 1/CB[8] (1:1); (c) 1/CB[8]/HCl (1:1:100); (d) 1/CB[8]/HCl/NaOH (1:1:100:100). ([1] = [CB[8]] = 0.5 mM).
of CB[8] to give square [5]MN (1ꢀCB[8])4 the solid state structure
transforms into well dispersed aggregates. Interestingly, the
plate-like structures were observed again when 100 equiv of HCl
was added due to the protonation of pyridine arms of compound
1 (Fig. 5c). Finally, addition of 100 equiv of NaOH results in the
formation of square [5]MN (1ꢀCB[8])4 which forms dispersed
aggregates again (Fig. 5d). Results of both DLS and SEM showed
the circulated changes by adding HCl and NaOH as the same as that
of UV–vis experiment. Notably, the aggregates of square [5]MN
(1ꢀCB[8])4 were quite stable at variable concentrations (0.01 mM
to 2.00 mM) and temperatures (up to 75 °C) in the solution, and
no precipitation was observed after storing for 6 months at room
temperature (Figs. S13 and S14).
In summary, we have presented a novel approach to the self-
assembly of square [5]molecular necklace (1ꢀCB[8])4 from four CB
[8] as side rings and four carbazole derivatives 1ꢀ2Clꢁ using the
host–guest recognition between CB[8] and 4,40-bipyridin-1-ium
units. This strategy, which successively uses highly efficient
host–guest recognition-driven self-assembly, greatly facilitates
the construction of complex supramolecules from macrocycles
and their guest molecules. We find that the cucurbit[8]uril-based
square [5]MN can be switched between dispersed aggregates and
plate-like aggregates triggered by pH. This work further establishes
CB[8] container as outstanding linkers of supramolecular
structures for application in advanced materials as well as
stimuli-responsive molecular machines.
with this article can be found, in the online version, at http://
dx.doi.org/10.1016/j.tetlet.2016.04.048.
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Please cite this article in press as: Li, J.; et al. Tetrahedron Lett. (2016), http://dx.doi.org/10.1016/j.tetlet.2016.04.048