Acid/base-controllable fluorescent molecular switches based on cryptands and basic N-heteroaromatics

Article abstract of DOI:10.1039/c7cc07469g

Two kinds of fluorescent BMP32C10-based cryptands 1 and 2 have been developed. Cryptand 1 contains a binaphthol group, while cryptand 2 bears a coumarin group in their third arms. Based on this design, novel self-assemblies constructed from cryptand 1 or 2 and basic N-heteroaromatic guests 3-6 were successfully obtained. Moreover, the threading/dethreading processes of the host-guest complexes could be well switched by the alternate addition of acid/base, and accompanied by concurrent changes in fluorescence.

Full text of DOI:10.1039/c7cc07469g

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc3N@C2n (2n 68 and 80). Synthesis of the
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Acid/base-controllable Fluorescent Molecular Switches Based on
Cryptands and Basic N-Heteroaromatics
Ming Cheng,a Jing Zhang,a Xintong Ren,a Shuwen Guo,a Tangxin Xiao,b Xiao-Yu Hu,a Juli Jiang,*a and
Leyong Wang*a,b
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Two kinds of fluorescent BMP32C10-based cryptands 1 and 2 have reported
a pseudo[2]rotaxane constructed by DB24C8
been developed. Cryptand 1 contains a binaphthol group, while derivative bearing terpyridine moiety with lanthanide ion and
cryptand 2 bears a coumarin group in their third arms respectively. fullerene-containing ammonium salt, the formation of
Based on this design, novel self-assemblies constructed between pseudo[2]rotaxane induced an efficient quenching of the
cryptands 1 or 2 and basic N-heteroaromatics guests 3-6 were lanthanide ion due to the PET effect between the fullerene
successfully obtained. Moreover, the threading/dethreading unit and terpyridine moiety.9
processes of the host-guest complexes could be well-switched by
Although many crown ether-based fluorescent responsive
the alternate addition of acid/base, and accompanied by pseudo[2]rotaxanes were reported so far, the cryptand-based
concurrent changes in fluorescence.
fluorescent responsive pseudo[2]rotaxane was really limited.
Compared with crown ethers, cryptands have three-
dimensional spatial structures, higher association constants,
and additional binding sites.10 Moreover, among cryptand-
based systems, the responsive stimuli were mostly acted on
cryptands rather than guests. Thus, it is desirable to develop
cryptand-based stimuli-responsive systems, especially those
with the responsive stimuli acted on the guest molecules.
As well known, paraquat and its derivatives were the most
Stimuli-responsive systems1 have attracted more and more
attention in recent decades due to their potential applications
in constructing complicated artificial molecular machines, such
as molecular sensors, shuttles, switches, and probes. As one of
fundamental building motifs, pseudo[2]rotaxane plays the
critical role in the construction of stimuli-responsive systems.
The external stimuli includes chemical acid/base,2
3
photochemical,1f, ion binding,4 and so on.5 The external
common
guests
for
bis(m-phenylene)-32-crown-10
1g, 10-11
(BMP32C10-based) cryptand.1c,
However,
a main
stimuli could act on either hosts or guests to dexterously
regulate the threading/dethreading process of the
pseudo[2]rotaxane. Meanwhile, the threading/dethreading
process was always accompanied by the recognizable changes
in physical properties, such as dichroism,6 conductivity,7 and
fluorescence.1a, 1e, 1f Among them, the fluorescence change can
be detected easily and remotely, and thus it was widely used
to diagnose the threading/dethreading states of
pseudo[2]rotaxane. For example, in 1998, Prodi et al. reported
a pseudo[2]rotaxane formed by dibenzo-24-crown-8 (DB24C8)
and (9-anthrylmethyl)methylammonium hexafluorophosphate
salt, the fluorescence of anthracene unit could be observed in
the form of pseudo[2]rotaxane. But with the addition of
NBu4Cl, the fluorescence of anthracene disappeared due to
the dethreading of pseudo[2]rotaxane.8 In 2011, Liu et al.
limitation for employing paraquat derivatives as guests lies in
the lack of enough way to regulate such a host-guest system,
which greatly restricted the development and application of
BMP32C10-based cryptand system.1c, 5c, 12 For example, more
recently, we have designed and synthesized a fluorescent
coumarin-bridged BMP32C10-based cryptand
2, and the
fluorescence of cryptand was quenched when it complexed
2
with a paraquat guest.12d However, such a host-guest complex
was stable, and the threading/dethreading process was
difficult to be regulated. Therefore, it is important to develop
novel guest molecules for cryptand host with external stimuli-
responsive ability.
Along this line, N-heteroaromatics guest molecules (
3, 4, 5,
and fell into our sights based on the following
6)
considerations: (i) all of the four guests keep lone electron
pairs on N-atoms, and possess a certain degree of basicity; (ii)
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Fig. 1 X-ray crystal structures of complex
1⊃
paraquat in two different view
angles. Solvent molecules were omitted for clarity. Hydrogen bond
parameters are as follows: H…O distances (Å), C…O distances (Å), C−H…O
angles (deg): (a) 2.59, 3.23, 126.63; (b) 2.73, 3.60, 157.93; (c) 2.88, 3.48,
123.21; (d) 2.76, 3.43, 130.03. Face-to-face π-stacking parameters in the
stacked structure: centroid-centroid distance (Å): (e) 3.96; (f) 3.63.
of cryptand
HR-ESI-MS (see ESI, Section 2), and further confirmed by the
single crystal X-ray diffraction of complex paraquat. As
shown in Fig.1, the complex paraquat was mainly stabilized
by (i) hydrogen bonds marked in green dashed lines; four
hydrogen bonds (a-d) were found between cryptand and
1
was fully characterized by 1H NMR, 13C NMR, and
1
⊃
1
⊃
1
Scheme 1 Chemical structures and cartoon represents of two cryptands 1, 2 and
four guests 3, 4, 5, 6.
paraquat (Fig. 1a); (ii) face-to-face π-stacking interactions
marked with yellow dashed lines as shown in Fig. 1.
Host-guest interactions between cryptands
guests 3’
6’ were investigated in detail by 1H NMR
spectroscopy (see ESI, Section 3). It was found that both
cryptand and guest 3’ in CH3CN solutions were colorless, but
the equimolar mixture of cryptand and 3’ became yellow
immediately due to the charge-transfer interactions between
the electron-rich aromatic rings of cryptand and the
1 or 2 and
guest molecules
adding acid/base alternatively. Concretely, if adding acid, the
protonated guests 3’ 6’ (Scheme 2) could enter into the
3-6 could be protonated/deprotonated by
-
-
1
electron-rich cavities of cryptands like paraquat molecule.
However, different from paraquat, when adding base, the
1
renewed guests
3
-
6
might leave away from the cryptand
were
1
cavities; (iii) the last point was that, guests
3-6
electron-deficient pyridinium rings of guest 3’. This colour
commercially available and easy to be functionalized, which
was important for the subsequent design and synthesis of
complicated molecular devices. Compared with paraquat and
its derivatives, guests 3-6 can be used as acid/base responsive
guests for BMP32C10-based cryptands. Thus, molecular self-
assemblies constructed by BMP32C10-based cryptands and
changes were also observed in the equimolar mixtures of
cryptand
cryptand
1
and guest 5’, cryptand
2
and guest 3’, as well as
2
and guest 5’, respectively.
Meanwhile, when cryptand
1
was mixed with guest 3’ in 1:1
ratio in CD3CN, significant upfield chemical shift changes
assigned to H3a, H3b of guest 3’ and H1a, H1b of cryptand were
observed ( H3a = −0.26, H3b = −0.10 for guest 3’ H1a
−0.44, H1b = −0.36 for cryptand ) (Fig. 2a, 2b, and 2c).
Similarly, obvious upfield chemical shift changes assigned to
H5a, H5b, H5c were observed in the 1:1 mixture of cryptand
and guest 5’ H5a = −0.26, H5b = −0.43, H5c = −0.27 for
guest 5’ H1a = −0.63, H1b = −0.52 for cryptand ) (Fig. 2c,
2d, and 2e). Notable chemical shift changes could also be
1
guests 3’-6’ could be controlled by external stimuli not acting
Δ
Δ
,
Δ
=
on cryptand itself but on guest molecules.
Δ
1
Herein, two kinds of fluorescent BMP32C10-based
cryptands
contains a binaphthol group, and cryptand
group in its individual third arm. Then novel self-assemblies
have been successfully constructed from cryptands or and
1
and
2
are employed as hosts, in which cryptand
1
2
12d has a coumarin
1
(Δ
Δ
Δ
,
Δ
Δ
1
1
2
protonated guests 3’-6’ (Scheme 2). Moreover, by alternatively
adding acid/base, the threading/dethreading processes of the
obtained host-guest assemblies could be well-controlled, and
accompanied by the concurrent changes in fluorescence due
to the possible PET effect. To the best of our knowledge, this is
the first example of regulating the fluorescence of BMP32C10-
based cryptands by external stimuli acting on guest molecules
so far. It will open up new dimension in constructing stimuli-
responsive self-assemblies based on cryptands
observed when cryptand
1
was mixed with guests 4’ and 6’
,
respectively (See ESI, Fig. S11, S13).
Subsequently, 2D NOESY experiment
was
performed,
which supported further the formation of host-guest complex
between cryptands or and guests (see ESI, Section 4).
For example, when cryptand and guest 3’ was mixed in 1:1
ratio in CD3CN solution, obvious NOE correlation signals
1
2
3-6
1
between H1a, H1b, H1c of cryptand
were clearly observed (Fig. S36).
1 and H3a, H3b of guest 3’
Initially, bridged by a binaphthol group, a BMP32C10-based
BMP32C10-based cryptand
recently,12d was bridged by a fluorescent coumarin unit in the
third arm. Thus, we wondered if cryptand could be used as a
2, which was synthesized by us
fluorescent cryptand
1 was obtained in moderate yield by
using commercially available starting materials according to
the reported methods (see ESI, Scheme S1).12b, 13 The structure
2
fluorescent-responsive cryptand candidate. Similar to cryptand
2 | J. Name., 2012, 00, 1-3
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Fig. 2 Partial 1H NMR spectra (500 MHz, CD3CN, 298 K): (a) guest 3’; (b)
Fig. 3 Partial 1H NMR spectra (500 MHz, CD3CN, 298 K): (a) 5.00 mM
host-guest complexation
1
1
⊃3’; (c) host 1; (d) host-guest complexation
⊃5’; (e) guest 5’.
cryptand
4.0 equiv. TFA to (a); (c) the mixture obtained after addition of 6.0 equiv.
TEA to (b); (d) 5.00 mM guest ; (e) 5.00 mM cryptand
1 + 5.00 mM guest 3; (b) the mixture obtained after addition of
1
, Job plots based on H NMR data showed that all complexes
1
3
1.
between cryptand
2 and guests 3’, 4’, 5’ or 6’ had 1:1
stoichiometries (see ESI, Fig. S18 and S19). The corresponding
assigned to H1a, H1b of cryptand
1
also returned back to its
⊃3’.
association constants Ka measured in CD3CN were listed in
Scheme 2.14a Interestingly, the Ka values between cryptand
original position, indicating the dethreading of complex
1
2
It is the first example that the threading/dethreading
processes between BMP32C10-based cryptands and guests are
successfully controlled by regulating on guest molecules.
The key point for fabricating fluorescent switches is how to
achieve the well-controlled fluorescence changes during the
threading/dethreading process.8-9, 15 It was interesting when
and four guests were almost an order of magnitude higher
than that of cryptand . Presumably, cryptand has a smaller
1
2
and more appropriate cavity which could combine with guests
more tightly.14b Another reason might be ascribed to that the
two phenolic hydroxyl groups on cryptand
leading to the charge repulsion between cryptand
guests 3’-6’. So both cryptand and were freshly synthesized
for further investigating their fluorescent-responsive property.
As above-mentioned, all of the four guests keep the lone
electron pairs on N-atoms, and hence, guests would be
1
are more acidic,
1
and the
guest
changes in fluorescent emission was observed (Fig. 4b).
Differently, when guest was added into the solution of
cryptand , almost no change in fluorescent intensity was
observed. We speculated that possible proton transfer
between hydroxyl of cryptand and nitrogen atom of guest
might lead to the result. Moreover, after adding TFA to the
3 was added to the solution of cryptand 1, obvious
1
2
3
3-6
2
3-6
protonated/deprotonated by adding acid/base alternatively.
Therefore, we firstly tried to control the threading-dethreading
1
3
processes of complexes formed between cryptands
guests 3’ 6’ by adding acid/base. As shown in Fig. 3, after
adding 4.0 equiv. of trifluoroacetic acid (TFA) into the mixture
of cryptand and guest (Fig. 3b), the chemical shift assigned
to H1a and H1b of cryptand showed obvious upfield shift.
1 or 2 and
mixture solution of cryptand
2
and guest
decreased remarkably (Fig. 4c),
3’. An
efficient PET process occurred between guest 3’ and the
coumarin unit of cryptand 2 12d
After adding TEA to remove
protons from guest 3’, the fluorescent emission intensity
recovered again, indicating the dissociation of complex 3’
3, the fluorescent
-
emission intensity of cryptand
2
resulting from the formation of threaded complex of 2⊃
1
3
1
.
Subsequently, with the addition of 6.0 equiv. of
trimethylamine (TEA) to remove protons from nitrogen atoms
2⊃ .
of pyridinium (Fig. 3c), H3a and H3b of guest
3
returned back to
the original position of free guest
3. The chemical shift
Scheme 2 Ka Values between cryptands 1 or 2 and guests 3’, 4’, 5’, and 6’
with 1:1 complexation. 1H NMR titrations were performed with a constant Fig. 4 (a) Cartoons represent of the changes in fluorescence intensity.
concentration of host (2.00 mM) and varying concentrations of guest in the Changes of the fluorescence emission spectra in CHCl3/CH3CN (1:1 v/v), (b)
range of 1.00 – 40.00 mM in CD3CN at 298 K. The standard concentration of cryptand
the reference state is fixed to 1.0 mol/dm3
1 2
(1 × 10-6 M). λex = 250 nm; (c) cryptand (1 × 10-6 M). λex = 320
nm
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684-687; d) Y. Zhou and K. Jie, Tetrahedron Lett., 2017, 58,
2217-2222.
As a result, the threading/dethreading process of 2⊃3’ could
be well-controlled by adding acid/base alternatively, and
accompanied by the concurrent changes in fluorescent
intensity.
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In summary, two kinds of BMP32C10-based fluorescent
cryptands
1
and
2
have been used as host molecules. Cryptand
is bridged
1
is bridged by a binaphthol group, while cryptand
2
by a coumarin group in its individual third arm. On the basis of
host-guest interactions, novel self-assemblies constructed
between cryptands
1 or 2 and N-heteroaromatics guests 3-6
have been developed successfully. By adding acid/base, the
threading/dethreading processes of the obtained assemblies
could be well-controlled, and accompanied by the concurrent
changes in fluorescent intensity. It is the first example of
regulating the fluorescence of BMP32C10-based cryptands by
external stimuli acting on guest molecules.
The present study provides a new choice to construct
external stimuli responsive assemblies based on cryptands by
regulating the behaviour of guest molecules rather than hosts.
It also brings more opportunities in designing more
complicated molecular sensors and other devices, benefiting
from relative easily syntheses and functionalization of guest
molecules.
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Acknowledgements
We gratefully thank the financial support of the National
Natural Science Foundation of China (Nos. 21472088,
21472089).
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Conflicts of interest
There are no conflicts of interest to declare.
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