Coordination-driven self-organization of switchable [2]rotaxane

Article abstract of DOI:10.1016/j.tet.2009.09.051

A bistable porphyrin-containing [2]rotaxane is synthesized with a shuttling benzylic-amide macrocycle mechanically locked onto the thread subunit by formations of H-bonds with two potential stations. This macrocycle comprises two pyridine groups, which wo

Full text of DOI:10.1016/j.tet.2009.09.051

Tetrahedron 65 (2009) 9081–9085
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Coordination-driven self-organization of switchable [2]rotaxane
y
y
*
Feng-Yuan Ji , Liang-Liang Zhu , Dong Zhang, Zhao-Fei Chen, He Tian
Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East China University of Science & Technology, Shanghai 200237, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 10 July 2009
Received in revised form 4 September 2009
Accepted 11 September 2009
Available online 15 September 2009
A bistable porphyrin-containing [2]rotaxane is synthesized with a shuttling benzylic-amide macrocycle
mechanically locked onto the thread subunit by formations of H-bonds with two potential stations. This
macrocycle comprises two pyridine groups, which would be easily coordinated with zinc porphyrin. The
Zn(II) coordination of porphyrin moiety on the thread subunit, immediately followed by the coordination
with pyridine groups on the macrocycle, leads to an intermolecular axle-macrocycle-type nanostructure.
Moreover, the self-assembly way shows great difference from the two states of the rotaxane monomer:
The coordination-driven self-organization of the trans-state E2 leads to a network structure, whereas the
cis-state Z2 gives birth to an irregular assembly.
Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction
moiety is coordinated with zinc(II) ion, it can interact with the
pyridine groups of another rotaxane monomer via intermolecular
The design and construction of new supramolecular architec-
tures so as to develop multifunctional and innovative material is
essential to the nanotechnology.1 The research interest of supra-
molecular systems, such as rotaxane and pseudorotaxane, has been
changing from investigation of the single molecule in solution to the
study of regular ensemble on surface or interphase.2 Thus far, some
different prototypes of rotaxane system have been reported to
construct ordered arrays, such as cross-linked,3 dendritic4 and
main-chain-type polymerization.5 Utilization of functional and
switchable rotaxane monomer6 as building blocks to construct or-
dered molecular frameworks, to obtain sophisticated integrations
with large scale, multi-respondent and coordinative properties,
however, will still be a long-term challenge.7
metal coordination, leading to a charming self-organized rotaxane
nanostructures. For the switchable property of this [2]rotaxane
monomer, the self-assembly ways and the morphologies of
the assembly show great differences from the two tunable states
of the [2]rotaxane monomer.
2. Experimental part
The rotaxane Z2 consists of a benzylic amide macrocycle and
a thread with two potential H bonding stationsdfumaramide
group and succinic amide group. The macrocycle is locked by two
bulky stoppers, free base porphyrin and fluorene entities (see
Scheme 1). This molecular shuttle Z2 was prepared in 25% yield
from thread Z1 and could be converted into E2 by heating, which
could also be turned back to Z2 by irradiation at 254 nm. The
macrocycle contains two pyridine groups, which can undergo
coordination with many metal ions. After addition of Zn2þ to the
solution of rotaxane Z2/E2, the central hydrogen atoms in por-
phyrin moiety of the rotaxane would be replaced by zinc. Thus the
zinc porphyrin moiety of one rotaxane monomer would be axially
connected to a pyridyl unit of another monomer. There are two
symmetrical pyridyl on the two sides of the macrocycle, hence the
Benzylic-amide macrocycle based [2]rotaxane8 is considered as
an ideal candidate for developing the above-mentioned supramo-
lecular nanostructures, because the thread subunit and the mac-
rocycle are both able to be modified into a variety of functional
moieties. A porphyrin containing fluorescent [2]rotaxane based on
a benzylic-amide macrocycle was synthesized recently and its
shuttling motion was thermo- and photo-controllable.8b In spite of
the optical property of the porphyrin moiety itself, the coordination
of porphyrin with zinc(II) ion or together with other ligands also
attracts scientists’ attention. In this paper, we describe a novel
[2]rotaxane with a porphyrin moiety on the thread and two pyri-
dine groups on the benzylic-amide macro-ring. Once the porphyrin
self-assembled supramolecular structure extends to
network.
a large
2.1. Materials and characterizations
All reagents and solvents were used as supplied, unless stated
otherwise. CH2Cl2 was distilled over CaH2. DMF was dried over
molecular sieves, triethylamine were distilled from KOH, and THF
* Corresponding author. Tel.: þ86 21 64252756; fax: þ86 21 64252288.
E-mail address: tianhe@ecust.edu.cn (H. Tian).
F.-Y. Ji and L.-L. Zhu contributed equally to this work.
y
0040-4020/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2009.09.051

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F.-Y. Ji et al. / Tetrahedron 65 (2009) 9081–9085
p
i
h
O
NH
H
H
NH
N
N
c H
O
NjH f
O
i
H
N
a
N
N
e
NH
N
N
6
6
N
H
g
HN
O
O
HN
N
O
d
O
6
O
NH
b
ii
n
l
k
m
Z1
E1
6
o
iii
A
N
N
O
HN
O
O
NH
O
D
B
NH
C
HN
O
i
O
NH
NH
O
N
N
NH
NH
O
NH
NH
NH
NH
NH
N
H
HN
6
N
O
N
HN
O
O
6
ii
O
NH
HN
NH
O
HN
O
O
O
6
E2
N
6
N
Z2
O
NH
H
O
NH
H
N
N
O
H
H
N
N
N
N
Zn
N
N
Zn
6
6
N
H
N
N
O
O
N
N
O
O
6
O
NH
Z1-Zn
E1-Zn
6
Scheme 1. Synthesis of bistable rotaxane Z2/E2. (i) 400 K, 51/49 for E1/Z1 and 43/57 for E2/Z2 in 1,1,2,2-tetra-chloride ethylene; (ii) 254 nm, 25/75 for E1/Z1 and 18/82 for E2/Z2 in
CH2Cl2; (iii) para-xylylene diamine, 3,5-pyridinedicarbonyl dichloride, Et3N/CHCl3, 0 ꢀC, 25%.
was distilled over sodium. MALDI-TOF mass spectra were recorded
on a Bru¨ker Biflex IV spectrometer. Flash column chromatography
was performed on silica gel. 1H NMR and 13C NMR were measured
on a Bru¨ker AV-400, or AV-500 spectrometer with tetramethylsilane
(TMS) as internal standard at 298 K. Elemental analysis was per-
formed on a vario EL III instrument. Absorption spectra were done
on a Varian Cary 500 UV/Vis spectrophotometer (1 cm quartz cell
used). Fluorescent spectra were recorded on a Varian Cary Eclipse
fluorescence spectrophotometer at 25 ꢀC. The photo-irradiationwas
carried on a CHF-XM 500-W high-pressure mercury lamp with
a filter for 254 nm in a sealed Ar-saturated 1 cm quartz cell. The
scanning electron microscopy (SEM) was tested on JEOL (JSM-
6360LV) electron microscope.
2H, HB), 8.25 (m, 2H, Hk), 8.15 (m, 6H, ortho triphenyl Hn), 8.10 (d,
2H, 4-amino-phenyl, Hl), 7.79 (m, 3H, fluorene, Hp), 7.78–7.70 (m,
9H, meta/para triphenyl, Ho), 7.70–7.50 (m, 4H, fluorene, Hp), 7.23
(S, 8H, HC), 5.95–6.12 (d, 2H, Ha, Hb), 4.50–4.68 (br, 8H, HD), 3.20
(br, 2H, Hc), 2.95 (br, 2H, Hd), 2.00–2.40 (br, 4H, He, Hf), 1.90–0.50
(m, 46H, alkyl chain), ꢁ2.84 (s, 2H, pyrrole NH). MS: 1877.1 [MþH]þ.
Elemental analysis calcd for Z2 (C119H122N14O8) (H2O)26: C 60.97, H
7.42, N 8.37; found: C 60.90, H 7.28, N 8.15.
2.3. Synthesis of rotaxane E2
Rotaxane E2 was obtained by the thermal isomerization of
rotaxane Z2. The procedure was carried out as follows: Z2 (15 mg,
0.008 mmol) was heated to 400 K in 1,1,2,2-tetra-chloride ethylene
for 24 h. After cooling, E2 and the unchanged Z2 were separated
and collected by chromatography (silica gel, first CH2Cl2, then
MeOH/CH2Cl2¼1/40), E2 was obtained in a yield of 43%. Mp 180–
182 ꢀC. 1H NMR (400 MHz, CDCl3): 10.13 (s, 1H, Hg), 10.01 (s, 1H,
Hh), 9.28 (d, 4H, HA), 8.95 (d, 2H, HB), 8.75–8.90 (m, 8H, pyrrole,
Hm), 8.18–8.24 (m, 6H, ortho triphenyl Hn), 8.17 (m, 2H, Hk), 8.15 (d,
2H, 4-amino-phenyl, Hl), 7.98 (m, 3H, fluorene, Hp), 7.78–7.70 (m,
9H, meta/para triphenyl, Ho), 7.60–7.45 (m, 4H, fluorene, Hp), 7.10
(S, 8H, HC), 6.05–6.15 (d, 2H, Ha, Hb), 4.38–4.45 (br, 8H, HD), 3.20
(br, 2H, Hc), 3.06 (br, 2H, Hd), 2.60 (br, 2H, He), 2.49 (br, 2H, Hf),
1.95–0.50 (m, 46H, alkyl chain), ꢁ2.84 (s, 2H, pyrrole NH). MS:
1899.1 [MþNa]þ. Elemental analysis calcd for E2 (C119H122N14O8)
(H2O)3: C 74.06, H 6.64, N 10.16; found: C 73.99, H 6.78, N 9.82.
2.2. Synthesis of rotaxane Z2
The thread Z18b (80 mg, 0.060 mmol) and Et3N (61.6 mg,
1.440 mmol) in anhydrous CHCl3 (100 mL) were stirred vigorously
whilst solutions of para-xylylene diamine (97.92 mg, 0.720 mmol)
in anhydrous CHCl3 (40 mL) and 3,5-pyridinedicarbonyl dichloride
(155.52 mg, 0.720 mmol) in anhydrous CHCl3 (40 mL) were simul-
taneously added over a period of 2 h using motor-driven syringe
pumps. After a further 2 h the resulting suspension was filtered and
the solvent removed under reduced pressure. The resulting solid
was subjected to column chromatography (silica gel) to yield un-
consumed thread and [2]rotaxane Z2 (yield¼25%). Mp 175–177 ꢀC.
1H NMR (400 MHz, CDCl3/CD3OD¼10/1): 11.80 (s, 1H, Hg), 9.70 (s,
1H, Hh), 9.24 (d, 4H, HA), 8.83–8.77 (m, 8H, pyrrole Hm), 8.73 (d,

F.-Y. Ji et al. / Tetrahedron 65 (2009) 9081–9085
9083
2.4. Photoisomerization procedures of rotaxane E2
Photoisomerization procedure of rotaxane E2 was carried out as
follows: Rotaxane E2 was dissolved in CH2Cl2 (20 mL) in a quartz
vessel. The solution was directly irradiated at 254 nm using a multi-
lamp photo-reactor for 4 h. After the photostationary state was
reached the reaction mixture was concentrated under reduced
pressure to afford the crude product, then re-dissolved in CH2Cl2
and washed with satd Na2CO3 prior to purification by column
chromatography. 18% percent of E2 were converted to Z2.
2.5. Z2-Zn-Complex
The rotaxane Z2 (10 mg, 0.0053 mmol) in anhydrous CHCl3
were stirred vigorously whilst the zinc acetate (3.88 mg,
0.060 mmol) were added. This mixture was stirred overnight at
room temperature. The resulting solution was washed with brine
(2ꢂ25 mL), water, the organic layer was dried over MgSO4 and the
liquid evaporated to dryness under reduced pressure to obtain
the pure compound in quantitative yield. Mp 192–194 ꢀC. 1H NMR
(400 MHz, CDCl3/CD3OD¼10/1): 11.58 (s, 1H, Hg), 9.52 (s, 1H, Hh),
8.80–8.90 (m, 8H, pyrrole Hm), 8.34 (m, 2H, Hk), 8.18–8.22 (m, 6H,
ortho triphenyl Hn), 8.18 (d, 2H, 4-amino-phenyl, Hl), 7.95 (s, 2H,
HB), 7.75–7.80 (m, 9H, meta/para triphenyl, Ho), 7.42–7.63 (m,
7H, fluorene, Hp), 7.06 (S, 8H, HC), 5.82–5.95 (d, 2H, Ha, Hb), 4.25–
4.48 (br, 8H, HD), 3.13–3.65(m, 4H, Hc, Hd, He, Hf), 2.84 (br, 4H, HA),
1.95–0.50(m, 46H, alkyl chain). Elemental analysis calcd for Z2-Zn-
Complex (C119H120N14O8Zn) (H2O)31: C 57.23, H 7.29, N 7.85; found:
C 57.17, H 7.26, N 7.56.
2.6. E2-Zn-Complex
This compound was prepared as described above. Mp 196–
197 ꢀC. 1H NMR (400 MHz, CDCl3): 9.60 (s, 1H, Hg), 9.59 (s, 1H, Hh),
8.90–8.83 (m, 8H, pyrrole Hm), 8.40 (m, 2H, Hk), 8.15–8.32 (m, 6H,
ortho triphenyl Hn), 8.10 (d, 2H, 4-amino-phenyl, Hl), 7.88 (s, 2H,
HB), 7.70–7.60 (m, 9H, meta/para triphenyl, Ho), 7.60–7.30 (m, 7H,
fluorene, Hp), 6.83 (S, 8H, HC), 5.82–5.56 (d, 2H, Ha, Hb), 4.35–4.02
(br, 8H, HD), 3.01–2.82 (m, Hc, Hd, He, Hf), 2.70 (br, 4H, HA), 2.05–
0.50 (m, 46H, alkyl chain). Elemental analysis calcd for E2-Zn-
Complex (C119H120N14O8Zn) (H2O)47: C 51.31, H 7.68, N 7.04; found:
C 51.16, H 7.46, N 6.99.
3. Results and discussion
The detailed structure of Z2 and E2 were confirmed by 1H NMR
spectra. The 1H NMR spectra of E1/Z1, E2/Z2 and their coordinated
species in CDCl3 are shown in Figure 1. The assignment of these
proton signals is based on our previous report and other relevant
literatures about benzylic-amide macrocycle based [2]rotaxane.8,9
The chemical shifts of Ha and Hb protons of fumaramide group in
rotaxane E2 were shielded and revealed upfield shifts of up to ꢁ1.12
and ꢁ1.00 ppm, respectively, compared with those of the thread E1,
whereas Hg and Hh displayed downfield shifts of up to 0.48 and
0.31 ppm. These proton signals change was originated from ben-
zylic-amide macrocycle fixing on the thread subunit. On the other
hand, the He and Hf protons of succinic amide group resonated at
almost identical shifts in both compounds (see Fig. 1a and b), in-
dicating the macrocycle located in fumaramide group in rotaxane
E2. The 1H NMR variation situation in rotaxane Z2 was just reversed.
The chemical shifts of Ha, Hb protons didn’t show too much dif-
ference between rotaxane Z2 and thread Z1, but He and Hf protons
were shielded by xylylene rings and revealed upfield shifts of up to
ꢁ0.67 and ꢁ0.51 ppm, respectively, in rotaxane Z2 (see Fig.1d and e).
These spectral changes confirmed that the macrocycle would
shuttle from the fumaramide station to the succinic amide moiety,
Figure 1. 1H NMR (400 MHz, a–c in CDCl3, d–f in CDCl3/CD3OD¼10/1) of (a) thread E1,
(b) rotaxane E2, (c) E2-Zn-Complex, (d) thread Z1, (e) rotaxane Z2, (f) Z2-Zn-Complex.
accompanied by the conformational change of the interlocked
supramolecule from rotaxane E2 to Z2.9 After replacing hydrogen
atoms of porphyrin moiety by zinc ion, the 1H NMR signals of E2-Zn-
Complex and Z2-Zn-Complex became broad and the spectra
showed great difference compared with their corresponding
monomers (see Fig. 1c and f). All the protons of the benzylic amide
macrocycle, especially the protons HA on the pyridine groups,
shifted to the upfield significantly because of the great shielding
effects of zinc porphyrin, which the macrocycle ring sticks to. These

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F.-Y. Ji et al. / Tetrahedron 65 (2009) 9081–9085
changes confirmed that the stopper zinc porphyrin of one rotaxane
monomer was axially connected to pyridyl unit of another.10
We have also investigated into the optical properties of E2-Zn-
Complex and Z2-Zn-Complex, compared with their reference
threads E1-zn/Z1-zn. The absorption spectra of E1-zn/Z1-zn dis-
play a typical intense Soret bands at 424 nm and Q bands at 551 nm.
However, the maximum absorption of E2-Zn-Complex and Z2-Zn-
Complex shows a red shift compared with their references, re-
spectively. The phenomena are coincident with the ones described
in related reports, i.e., on axial coordination of pyridyl residues, the
absorption bands of zinc porphyrin display a red shift up to some
nanometers (Fig. 2a and b).11
(a)
100
80
60
40
20
0
E1-Zn
E2-Zn-Complex
0.030
a
E1-Zn
0.6
0.5
0.4
0.3
0.2
0.1
0.0
-0.1
0.025
0.020
0.015
0.010
0.005
0.000
-0.005
-0.010
550
600
650
700
E2-Zn-Complex
Wavelength/nm
(b)
100
90
80
70
60
50
40
30
20
10
0
540 550 560 570 580
Wavelength/nm
530
Z1-Zn
Z2-Zn-Complex
380
390
400
410
420
430
440
450
460
Wavelength/nm
0.030
0.025
0.020
0.015
0.010
0.005
0.000
-0.005
b
0.6
0.4
0.2
0.0
550
600
650
700
Z1-Zn
Z2-Zn-Complex
Wavelength/nm
Figure 3. (a) Fluorescence emission spectra (lexc¼427 nm, c¼3ꢂ10ꢁ5 M) of E2-Zn-
Complex and reference E1-Zn in CH2Cl2. (b) Fluorescence emission spectra
(
lexc¼427 nm, c¼3ꢂ10ꢁ5 M) of Z2-Zn-Complex and reference Z1-Zn in CH2Cl2.
530 540 550 560 570 580
Wavelength/nm
Interior morphologies of metalizedrotaxane E2and Z2assemblies
were studied in terms of scanning electric microscopy (SEM). The
leaner rotaxane monomer E2 could be self-aggregated to some ex-
tend, formingrodstructuresobservedinFigure 4A. Ontheotherhand,
annular structures were formed by the self-aggregation of curly
monomer Z2 (see Fig. 4C). This information reveals the different
conformations of the two isomers of the rotaxane will lead to obvi-
ously self-aggregated structures, which may be caused by
380
390
400
410
420
430
440
450
460
Wavelength/nm
intermolecular H–H bonding and the
p–p stacking of the chromo-
phores at the two sides of the thread subunit. The coordination-
driven self-organizationprocess of the rotaxane monomers has taken
place after mixing zinc(II) ion with the rotaxane in a 1:1 M ratio,
which corresponds to the building of the axle-macrocycle-type
nanostructures. Figure 4B shows the SEM image of the self-organi-
zation of rotaxane E2 (E2-Zn-Complex) obtained after the metal
coordination occurred. A regular formed network can be clearly ob-
served. The average size of single annular cavity is about 120 nm in
diameter, while the width of the edge of the hole is estimated to be
around 20 nm, observed from SEM. However, the metalized rotaxane
Z2 was organized in a different way as that did in the situation of E2.
Figure 4D shows the SEMimagesof rotaxaneZ2afterthemetalization
happening (Z2-Zn-Complex), an irregular assembly rather than the
networks. The self-organized morphology is an enlarged result,
which also reflect the greatly influence of the conformational struc-
tures of rotaxane monomers on the integrative morphology of the
nanostructure resulted by the axle-macrocycle-type coordination
procedure. Anyway, the linear E2 is prone to form slight networks
while the curly structural Z2 leads to block structures.
Figure 2. (a) UV–vis spectra of E2-Zn-Complex and reference E1-Zn in CH2Cl2
(c¼3ꢂ10ꢁ5 M). (b) UV–vis spectra of Z2-Zn-Complex and reference Z1-Zn in CH2Cl2
(c¼3ꢂ10ꢁ5 M).The insets show the corresponding Q-band absorptions.
The emission of CH2Cl2 solution of metalized rotaxane E2/Z2
assemblies and their references E1-zinc/Z1-zinc (isosbestic point
of metalized rotaxane assembly and reference threads) are shown
in Figure 3. It is interesting that the fluorescence emission of
metalized E2/Z2 assemblies also red shifted as compared with E1-
zinc/Z1-zinc, because of the formation of self-assemblies from the
rotaxane monomers. And it is notable that the assembled rotaxane
Z2 displays much stronger fluorescence than Z1-zinc (see Fig. 3b).
It is suggested that Z1-zinc’s fluorescence is weakened due to its
intermolecular
p–p stacking of porphyrin zinc(II) moieties. How-
ever, the stacking of porphyrin parts were eliminated, when
p–p
the porphyrin zinc(II) segments were mainly axially coordinated to
pyridyl residues by the complexation effect, so that the fluores-
cence became strong.

F.-Y. Ji et al. / Tetrahedron 65 (2009) 9081–9085
9085
Figure 4. Scanning electron micrograph of the rotaxane E2 before (A) and after (B) metalization by adding zinc acetate; Z2 before (C) and after (D) metalization by adding zinc
acetate.
4. Conclusions
References and notes
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Acknowledgements
This work was supported by NSFC/ China (50673025,
20603009), National Basic Research 973 Program (2006CB806200)
and Scientific Committee of Shanghai.
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Experimental procedure and characterization for reference
compounds E1-Zn and Z1-Zn, 1H NMR spectra of E1-Zn and Z1-Zn,
MALDI-TOF MS spectra of rotaxane E2 and Z2 are available free of
charge via the online version. Supplementary data associated with
this article can be found in online version at doi:10.1016/
j.tet.2009.09.051.