Construction of a functional [2]rotaxane with multilevel fluorescence responses

Article abstract of DOI:10.1039/c1ob06151h

A rotaxane incorporating three different stations and fluorescent states (output) was prepared. The movement of the macrocycle can be easily detected by fluorescence change as an output signal and the macrocycle could be easily controlled to locate on thr

Full text of DOI:10.1039/c1ob06151h

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PAPER
Construction of a functional [2]rotaxane with multilevel fluorescence
responses†
Yingjie Zhao,a,b Yongjun Li,*a Siu-Wai Lai,c Jien Yang,a,b Chao Liu,a,b Huibiao Liu,a Chi-Ming Chec and
Yuliang Li*a
Received 12th July 2011, Accepted 9th August 2011
DOI: 10.1039/c1ob06151h
A rotaxane incorporating three different stations and fluorescent states (output) was prepared. The
movement of the macrocycle can be easily detected by fluorescence change as an output signal and the
macrocycle could be easily controlled to locate on three different stations of the thread by the tuning of
acid/base (input).
states (output) for expanding the functionality of the system
Introduction
were seldom reported. In our hypothesis, the fluorescent hydroxy-
Rotaxanes and catenanes have received a great deal of attention
substituted tetraphenylimidazole’ derivatives were chosen as a
due to their challenging construction and potential application
third station. The intramolecular H-bonding interactions result
as mechanical or electronic molecular devices for molecular
in photophysical processes based on excited-state intramolecu-
electronics and nanochemistry.1 The rotaxanes are remarkable
precursors in supramolecular chemistry, and the rapid devel-
opment of this chemistry led to smart assemblies possessing
various architectures and properties. This new area of chemistry
has promoted the understanding of the concepts of design and
strategies of self-assembly of rotaxane structures based on energy
input for supplying their work. They can be controllably and
reversibly switched between two or more states2 and moved by
their components. Various stimulihave been applied to induce such
switches, including acid/base,3 photochemical,4 electrochemical,5
solvent,6 metal binding,7 configurational changes8 and so on.
The movements of the macrocycles within the rotaxanes
are also able to result in changes in their conformations or
configurations. These changes are usually accompanied by the
alteration of physical properties of the molecular rotaxane such
as conductivity,9 circular dichroism10 and fluorescence.11 We have
reported recently the examples of a click-reaction-based synthesis
of rotaxanes12 and catenanes.13 The 1,2,3-triazole generated by
the click reaction can also be used as a molecular station.12,14
Besides, phenolate anions can also act as an anion station,15 and
the fluorescence based on the systems can be changed in response
to external acid/base stimuli. However, the rotaxane molecular
machines incorporating three different stations and fluorescent
lar proton transfer (ESIPT),16 indicating the intramolecular H-
bonding interactions can also play as a fluorescence switch. The
results showed that a multifunctional stopper can be fabricated by
associated supramolecular interactions in the three processes of
movement in the supramolecular system.
Here we describe a three-station [2]rotaxane with multilevel
fluorescence responses by acid/base tuning shuttling of the
macrocycle along the thread. The [2]rotaxane in which one
ring moves sequentially among three different binding sites
(‘stations’) of the thread is shown schematically in Scheme 1.
The three noncovalently binding states were predicted to have
different binding affinities with the macrocycle: (i) a secondary
+
dialkylammonium (–NH2 –) center, should bind strongly with the
macrocycle because of the strong hydrogen bonding interactions
between the macrocycle ether and cationic moieties. (ii) the
1,2,3-triazole ring, should bind less well than (i) through the
hydrogen bonding interaction between the amide units in the
macrocycle and the 1,2,3-triazole nitrogen atoms; (iii) a phenolate
anion, can form hydrogen bonding with the amide units which
interact much more strongly compared to (ii). In addition, the
movement of the macrocycle could influence the intermolecular
vibronic interactions of the fluorescent stopper, accompanied
by the alteration of fluorescence intensity. The rotaxane shows
different fluorescent responses when the macrocycle located on
three stations. Thus, a new approach using fluorescence to detect
the movement of the macrocycle has been developed.
aBeijing National Laboratory for Molecular Sciences (BNLMS), CAS Key
Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of
Sciences, Beijing, 100190, P.R. China. E-mail: ylli@iccas.ac.cn; Fax: (+)
86-10-82616576; Tel: (+) 86-10-62588934
bGraduate University of Chinese Academy of Sciences, Beijing, 100190, P.R.
China
Results and discussion
cDepartment of Chemistry and HKU-CAS Joint Laboratory on New
Materials, The University of Hong Kong,
[2]Rotaxane 4 and compound 5 were synthesized according to
Scheme 2. Compound 1 and 3 were synthesized in accordance with
literature procedures.12 The synthesis of compound 2 was shown
in the supporting information. A pseudorotaxane containing half-
dumbbell 1 and macrocyle 3 was formed first. Covalent capture of
† Electronic supplementary information (ESI) available: Synthesis and
characterization; 2D 1H NMR studies, absorption and fluorescence
emission spectra changes of 4 and 5; study of fluorescence lifetimes. See
DOI: 10.1039/c1ob06151h
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region of the thread and the isophthalamide group in macrocycle
3 can interact with the triazole ring due to the hydrogen bonds
between them under acidic conditions. The signals corresponding
to phenylene protons (Hf: Dd = 0.20 and Hg: Dd = 0.32 ppm) and
the macrocycle benzyl groups (HF: Dd = 0.19 and HG: 0.28 ppm)
experience a significant upfield shift as a result of the aromatic
shielding effect between them. The polyether protons (HH–HK)
shift upfield by Dd = 0.1 ppm roughly as a result of a combination
of C–H ◊ ◊ ◊ O and N+–H2 ◊ ◊ ◊ O hydrogen bonds. Additionally, a
shielding effect is observed for the proton adjacent to the 1,2,3-
triazole ring (Dd(Hh) = 0.22 ppm). Moreover, the ring amide
protons HD experience a downfield shift of Dd = 0.21 ppm in 4
with respect to 3, which is attributed to hydrogen bonding to the
1
triazole nitrogen atoms. The H NMR spectra showed that the
macrocycle 3 in rotaxane 4 was localized on the alkyl ammonium
and also had hydrogen-bond interaction with the 1,2,3-triazole
region of the dumbbell under acidic conditions (Scheme 1).
Upon addition of 1.2 equivalents of Et3N to 4, the dialkylam-
monium group was neutralized and the hydrogen bonds between
macrocycle and ammonium group were switched off (Fig. 1b
and b¢). The signals for the protons near the 1,2,3-triazole ring
Hh and Hj shifted upfield by 0.32 and 0.18 ppm which should
be attributed to the aromatic shielding effect by 3. The amide
protons HD experience a significant downfield shift (Dd = 0.4)
with respect to 3 due to hydrogen bonding with the triazole
nitrogen atoms. The signal for He also shifted upfield by 0.15 ppm,
which indicated deprotonation of the neighboring ammonium
center. In addition, the 0.11 ppm upfield shift of the triazole
ring proton Hi is characteristic of aromatic shielding effects. All
these features indicate that the macrocycle can migrate from the
dialkylammonium center to the triazole recognition site upon
addition of Et3N in acetonitrile.
Scheme 1 The shuttling movement process of the [2]rotaxane under
different stimuli (optimized by Spartan 08).
Deprotonation of the phenol groups in 4 and 5 could be
accomplished with Schwesinger’s P1 base. 1H NMR confirms that
deprotonation of the phenol group provides the third hydrogen-
bonding station for the macrocycle (Fig. 1c and c¢). The signals
of –OH disappeared when 10 eq. of Schwesinger’s P1 base was
added. The shielding of the protons in 4 shows the ring is located
near the phenolate anion. The signals for Hm and Hn shifted
upfield by 0.19 and 0.22 ppm, which should be attributed to the
aromatic shielding effect by 3. Compared to the deprotonated 4
by Et3N, the signals for the protons Hg and Hh shifted downfield
by 0.12 and 0.06 ppm in 4- (Fig. 1b¢ and c¢). These results indicate
that the macrocycle moves to the phenolate anion recognition
site due to the hydrogen bonds between them. The movement
process was monitored by 2D NMR (ESI Part 6†). The shuttling
is readily reversible: protonation of 4- with CF3COOH smoothly
regenerates 4, which returns the macrocycle back to the original
station (Scheme 1, Figure S2†).
The absorption features of the dumbbell 5 are similar to those of
rotaxane 4. When 1.2 equivalents of Et3N were added, no change
in the absorption spectra was observed. But when 10 equivalents
of P1 base were introduced, the absorption peak at 289 nm and
307 nm decreased (Figure S3†). This indicated that Et3N could not
influence the fluorescent stopper but the P1 base could accomplish
the deprotonation of the phenolate. This is in accordance with the
1H-NMR analysis.
Scheme 2 Synthesis of 4 and 5.
the threaded intermediate by a click reaction afforded [2]rotaxane
4 and dumbbell 5 in 55% and 10% yields respectively. The
MALDI-TOF spectrum of 4 gave sharp peaks at m/z: 1784.7 (see
the ESI†) which revealed the feature of an interlocked molecule.
As shown in Fig. 1 (a, a¢, d), the 1H NMR spectra of 3, 4, 5 in
d3-acetonitrile readily confirm the interlocked structure and show
that macrocycle 3 in 4 is largely localized on the alkyl ammonium
Compared with the UV-vis spectra, more significant changes
happened in the fluorescence spectra. Fig. 2 demonstrates that
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1
Fig. 1 Partial H NMR spectra (400 MHz, 298 K, CD3CN) of 3, 4 and 5. (a) 5, (a¢) 4, (b) 5+1.2 equiv Et3N, (b¢) 4+1.2 equiv Et3N, (c) 5+1.2 equiv
Et3N+10 equiv Schwesinger’s P1 base, (c¢) 4+1.2 equiv Et3N+10 equiv Schwesinger’s P1 base, (d) 3.
effect is related to the restriction of intramolecular rotation
that accompanies cooling of the solvent.18 The decrease of the
temperature and the macrocycle play the same role. They both
influence the intramolecular rotation of the fluorescent stopper
which result in the enhancement of the fluorescence. When 10
equivalents of P1 base was introduced, the fluorescence of 5 was
quenched (~80%), which is much stronger than that in the case of
4 (~40%). The deprotonation of the fluorescent stopper by P1 base
Fig. 2 Fluorescence emission spectra changes (lex = 310 nm) of 4 and 5
suppresses the ESIPT process and thus the emission was decreased.
upon the addition of 1.2 equiv Et3N and 10 equiv Schwesinger’s P1 base in
Interestingly, the emission of 4 was decreased in a smaller degree
CH3CN (2 ¥ 10-5 M).
compared with that of 5. This could be attributed to the restriction
of the intermolecular vibronic caused by the macrocycle.
the motion of the macrocycle greatly affects the fluorescence of
4. A comparison of the emission behavior of 4 and 5 upon the
addition of Et3N and P1 base in CH3CN is shown in Fig. 2. 4
Conclusion
and 5 both displays an emission maximum centred at 505 nm. The
fluorescence intensity of 4 centred at 505 nm increased by 25%
upon the addition of 1.2 equivalents of Et3N into the solution,
while there is no similar change in the emission spectra of 5 at all.
It was initially considered that when the macrocycle moved to the
triazole recognition site, the rotational motion of the fluorescent
stopper was restricted. The restriction encumbered the radiation-
less rotation which induced the increase of the fluorescence.17
In order to prove this, effect of temperature on the fluorescence
intensity was investigated for 5. Figure S4 showed the fluorescence
emission spectra from 293 to 308 K.† The fluorescence intensity
increases with the decreasing of temperature. This temperature
We have synthesized a multistable molecular rotaxane with three
successive movement processes driven by acid/base stimuli, which
are all accompanied with fluorescent responses. The macrocycle
can be easily controlled to locate on three different stations of the
thread by the different strengths of the three sets of noncovalent
bonding interactions. The fluorescent emission intensity of the
[2]rotaxane can be tuned by the movement of the macrocycle.
The approach using fluorescence to detect the movement of the
macrocycle was developed. In addition, this artificial multilevel
molecular shuttle provides a model for interconnection of different
hydrogen bonding interactions, to achieve multistable controllable
systems with tunable responses.
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A mixture of 1 (620 mg, 0.73 mmol), 2 (435 mg, 0.8 mmol), 3
(425 mg, 0.8 mmol), and CuBF4 (CH3CN)4 (250 mg, 0.8 mmol)
was stirred in dry CH2Cl2 at room temperature under nitrogen
for 24 h. After removal of the solvent, the crude product was
purified by column chromatography to afford 4 (770 mg, 55%)
and 5 (100 mg, 10%).
1
4: H NMR (400 MHz, CD3CN) d 13.24 (s, 1H), 7.99 (d, J =
7.8 Hz, 2H), 7.92 (s, 2H), 7.53 (m, 5H), 7.41 (s, 2H), 7.27–7.37 (m,
20H), 7.21–7.13 (m, 9H), 7.07–7.11 (m, 6H), 7.04 (d, J = 8.0 Hz,
1H), 6.77 (m, 2H), 6.71 (m, 3H), 6.59 (d, J = 8.0 Hz, 4H), 6.56–6.49
(t, 1H), 5.53 (s, 2H), 4.94 (s, 2H), 4.60–4.49 (m, 2H), 4.29–4.18 (m,
2H), 3.96 (s, 4H), 3.91 (s, 4H), 3.73 (s, 4H), 3.45–3.65 (m, 8H), 3.14
(s, 2H), 2.11 (m, 2H), 1.29 (s, 27H); 13C NMR (100 MHz, CDCl3)
d 166.77, 158.95, 158.50, 156.87, 155.94, 148.54, 145.00, 144.15,
143.49, 140.74, 137.09, 135.59, 135.15, 134.66, 133.07, 132.93,
132.51, 132.40, 131.81, 131.59, 131.43, 130.73, 130.48, 130.28,
129.70, 129.34, 128.85, 128.78, 128.43, 127.24, 127.03, 126.23,
124.27, 124.14, 124.00, 123.31, 123.17, 122.83, 118.20, 117.88,
115.24, 114.24, 113.08, 112.94, 90.70, 89.10, 70.74, 70.56, 67.13,
64.26, 63.22, 61.21, 53.87, 52.05, 46.05, 43.47, 34.42, 31.50, 29.81
ppm; MALDI-TOF: m/z: 1784.7 [M-PF6]+; Elemental Analysis
calcd (%) for C116H119F6N8O10P: C, 72.18; H, 6.21; N, 5.81; found:
C, 72.09; H, 6.31; N, 5.86.
5: 1H NMR (400 MHz, CD3CN) d 13.25 (s, 1H), d 7.87 (s, 1H),
7.54 (d, J = 7.8 Hz, 4H), 7.26–7.36 (m, 24H), 7.14–7.22 (m, 7H),
7.02 (m, 3H), 6.80 (d, J = 8.4 Hz, 2H), 6.67 (d, J = 6.8 Hz, 1H),
6.53 (t, J = 8.0 Hz, 1H), 5.59 (s, 2H), 5.17 (s, 2H), 4.07–3.99 (m,
2H), 3.95 (s, 2H), 3.04–2.95 (m, 2H), 2.11 (m, 2H), 1.29 (s, 27H);
13C NMR (100 MHz, CDCl3) d 159.98, 159.32, 156.78, 149.31,
145.81, 145.01, 144.85, 144.13, 142.63, 141.44, 137.92, 136.43,
135.76, 133.88, 133.73, 133.24, 132.55, 132.24, 131.59, 131.26,
131.03, 130.53, 129.66, 129.58, 129.24, 129.16, 128.05, 127.86,
127.64, 127.03, 125.95, 125.04, 124.91, 124.29, 124.19, 123.45,
120.79, 118.99, 118.71, 116.35, 113.91, 113.74, 91.44, 89.96, 66.45,
64.61, 64.00, 62.38, 54.82, 47.32, 35.21, 32.30, 30.63; MALDI-
TOF: m/z: 1249.5 [M-PF6]+; Elemental Analysis calcd (%) for
C116H119F6N8O10P: C 74.01, H 6.14, N 6.02; found: C 74.13, H
6.21, N 5.95.
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We are grateful for financial support from the National Nature
Science Foundation of China (21031006, 90922017, 20971127 and
20831160507); the National Basic Research 973 Program of China
and NSFC-DFG joint fund (TRR 61).
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