Synthesis of a bistable [3]rotaxane and its pH-controlled intramolecular charge-transfer behavior

Article abstract of DOI:10.1016/j.cclet.2013.04.007

A [3]rotaxane 1 involving two naphtho-21-crown-7 (N21C7) rings and a dumbbell-shaped component 4 was synthesized. The dumbbell-shape molecule 4 contains one viologen nucleus, two secondary alkyl ammonium sites and two phenyl stoppers. After threading the N21C7 ring with the thread-like ammonium guest 3, the copper(I)-catalyzed Huisgen alkyne-azide 1,3-dipolar cycloaddition (CuAAC "click" reaction), was performed to connect the pseudorotaxanes with viologen unit 2 and generate 1. Through treating the [3]rotaxane by the base and acid circularly, the two N21C7 rings can make shuttling motion along the axle. Meanwhile the distance between the electron-deficient viologen unit and the electron-rich naphthol group can be adjusted precisely along with a remarkable intramolecular charge-transfer (CT) behavior.

Full text of DOI:10.1016/j.cclet.2013.04.007

Chinese Chemical Letters 24 (2013) 563–567
Contents lists available at SciVerse ScienceDirect
Chinese Chemical Letters
journal homepage: www.elsevier.com/locate/cclet
Original article
Synthesis of a bistable [3]rotaxane and its pH-controlled intramolecular
charge-transfer behavior
*
Hui Wang, Zhi-Jun Zhang, Heng-Yi Zhang, Yu Liu
Department of Chemistry, State Key Laboratory of Elemento-Organic Chemistry, Nankai University, Tianjin 300071, China
A R T I C L E I N F O
A B S T R A C T
A [3]rotaxane 1 involving two naphtho-21-crown-7 (N21C7) rings and a dumbbell-shaped component 4
was synthesized. The dumbbell-shape molecule 4 contains one viologen nucleus, two secondary alkyl
ammonium sites and two phenyl stoppers. After threading the N21C7 ring with the thread-like
ammonium guest 3, the copper(I)-catalyzed Huisgen alkyne-azide 1,3-dipolar cycloaddition (CuAAC
‘‘click’’ reaction), was performed to connect the pseudorotaxanes with viologen unit 2 and generate 1.
Through treating the [3]rotaxane by the base and acid circularly, the two N21C7 rings can make shuttling
motion along the axle. Meanwhile the distance between the electron-deficient viologen unit and the
electron-rich naphthol group can be adjusted precisely along with a remarkable intramolecular charge-
transfer (CT) behavior.
Article history:
Received 26 February 2013
Received in revised form 28 March 2013
Accepted 29 March 2013
Available online 18 May 2013
Keywords:
Rotaxane
Crown ether
Charger transfer (CT)
Click chemistry
ß 2013 Yu Liu. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
1. Introduction
deprotonated and the positively charged viologen unit attracted
the rings to move toward the center. The distance between the
The mechanically interlocked molecules have several compo-
nents which are not covalently linked [1]. These molecules can
exhibit various functions through integrating the features of the
components and controlling them by changing the external
stimulation such as light, electrochemical potential, pH and
solvent. Rotaxanes, one of the most useful members in this family,
have been developed and used widely in construction of various
artificial molecular devices and machines such as molecular
shuttles [2], nanovalves [3], molecular muscles [4]. Among various
macrocycles, 24-crown-8 (24C8) has been widely applied in lots of
bistable rotaxanes [5] because of its unique binding properties
toward the positively charged guests in low polar solvents.
Recently, 21-crown-7 (21C7) was found to have a much stronger
binding affinity toward the secondary ammonium guests and its
size was small enough to be stoppered by phenyl group [6]. Several
rotaxanes based on this property have been reported [7]. However,
the switchable behavior of 21C7-based rotaxane has not been
investigated.
viologen unit and the naphthol group could be close enough to
exhibit a strong CT interaction.
2. Experimental
All chemicals were commercially available unless noted
otherwise. Compound 9 was prepared according to the literature
procedure [6]. NMR data were recorded on Bruker AV400
spectrometer, and chemical shifts were recorded in parts per
million (ppm). Mass spectra were recorded using Agilent 6520 Q-
TOF LC/MS (ESI).
Preparation of 1 (Scheme 1): To a solution of 2 (174.1 mg,
0.3 mmol),
1.05 mmol) in CH2Cl2 (1.0 mL) and CH3CN (1.0 mL) was added
Cu(MeCN)4PF6 (352.2 mg, 0.945 mmol) and 2,6-lutidine (11 L,
3
(238.3 mg, 0.63 mmol), N21C7 (426.8 mg,
m
0.095 mmol). The reaction mixture was stirred for 24 h at room
temperature. Then the solvent was removed under reduced
pressure, and purified by column chromatography over silica gel
(eluent: CH2Cl2/MeOH, 50:1) to afford 1 as a yellow solid (528.9 mg,
Herein, we designed and synthesized a switchable [3]rotaxane
that containing two N21C7 rings and a dumbbell-shaped molecule.
The dumbbell-shaped molecule has a viologen unit in its center. In
the original state, the N21C7 rings located on the secondary
ammonium sites which were far away from the viologen.
After treated by base, the secondary ammonium sites were
82%). 1H NMR (400 MHz, CD3CN):
d8.95 (d, 4 H, J = 6.3 Hz), 8.35 (d, 4
H, J = 6.2 Hz), 7.71 (m, 4 H), 7.61 (s, 4H), 7.52 (s, 2 H), 7.34 (m, 18 H),
4.67 (m, 4 H), 4.45 ꢀ 4.33 (m, 12 H), 4.10 (m, 4 H), 3.92 (m, 8 H), 3.59
(m, 28 H), 3.39 ꢀ 3.31 (m, 8 H), 2.72 (m, 4 H), 2.34 (m, 4 H), 1.67 (m, 4
H), 1.58 ꢀ 1.48 (m, 4 H), 1.28 (m, 4 H), 1.19 ꢀ 1.12 (m, 4 H). 13C NMR
(100 MHz, CD3CN): d 150.6, 148.1, 146.6 133.5, 130.8, 129.8, 129.7,
129.3, 127.8, 126.9, 125.2, 108.4, 71.8, 71.6, 71.1, 71.0, 70.2, 69.4,
51.3, 47.6, 29.9, 26.6, 26.3. HRMS (ESI): m/z calcd. for
C
90H123N10O143+: [M–4PF6–H]3+ 522.9745; found: 522.9757.
* Corresponding author.
E-mail address: yuliu@nankai.edu.cn (Y. Liu).
1001-8417/$ – see front matter ß 2013 Yu Liu. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
http://dx.doi.org/10.1016/j.cclet.2013.04.007

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H. Wang et al. / Chinese Chemical Letters 24 (2013) 563–567
Scheme 1. Synthesis of [3]rotaxane 1.
[(Schme_2)TD$FIG]
Scheme 2. Synthesis of viologen 2.
Preparation of 2 (Scheme 2): Compound 5 (2.91 g, 15 mmol) and
added to the solution. After stirring for 24 h at 80 8C, the solvent
was evaporated off under vacuum, the residue was dissolved with
CH2Cl2 (100 mL) and washed with water (50 mL) twice. The
organic phase was dried and evaporated off to afford 8 as a
4,40-bipyridine (780.9 mg, 5.0 mmol) were dissolved in DMF
(30 mL), stirred at 80 8C under N2 for 1d. After cooled, the
precipitate was filtered off, washed with CH3CN, then suspended in
acetone (50 mL). A saturated aqueous solution of NH4PF6 was
added until the suspension became clear. The solvent was removed
in vacuo, and water (50 mL) was added to the residue. The
resulting mixture was then filtered, washed with water, and dried
to give 2 as a white solid (2.15 g, 74%). 1H NMR (400 MHz, CD3CN):
colorless oil (1.69 g, 93%). 1H NMR (400 MHz, CDCl3):
d 7.34–7.29
(m, 2 H), 7.26–7.19 (m, 3 H), 4.42 (m, 2 H), 3.20 (m, 4 H), 1.57 (m, 2
H), 1.43 (m, 11 H), 1.38–1.32 (m, 2 H), 1.27 (m, 2 H). 13C NMR
(100 MHz, CDCl3): d 128.4, 127.6, 127.0, 79.4, 51.2, 50.5, 49.9, 46.4,
28.7, 28.4, 27.9, 27.7, 26.4, 26.3. HRMS (ESI): m/z calcd. for
d
8.95 (d, 4 H, J = 5.9 Hz), 8.42 (d, 4 H, J = 5.4 Hz), 4.76 (t, 4 H,
C
18H29N4O2+: [M+H]+ 333.2285; found: 333.2282.
J = 7.0 Hz), 2.41 ꢀ 2.32 (m, 6 H), 2.28 (m, 4 H). 13C NMR (100 MHz,
Preparation of 3 (Scheme 3): Compound 8 (1.0 g, 3.0 mmol) was
dissolved in a mixed solvent 10% conc. HCl/EtOAc (80 mL), and
then stirred overnight. After evaporated off the solvent under
CD3CN):
d
150.8, 146.4, 127.9, 82.1, 71.5, 61.5, 29.8, 15.2. HRMS
(ESI): m/z calcd. for C20H22F6N2P+: [MꢀPF6]+ 435.1419; found:
435.1413.
vacuum, the residue was suspended in acetone (50 mL).
A
Preparation of 7 [8] (Scheme 3): Compound 6 (1.15 g, 6 mmol)
was dissolved in dry DMF (18 mL), then NaH (172.8 mg, 7.2 mmol)
was added to the solution at 0 8C. After stirring for 30 min at room
temperature, 1,6-dibromohexane (4.4 g, 18.06 mmol) was added
in three portions to the solution at 0 8C. After stirring for another
18 h at room temperature, water was added to stop the reaction.
After solvent was evaporated off under vacuum, the residue was
dissolved with CH2Cl2 (100 mL) and washed with water (50 mL)
twice. Then the residue was purified by column chromatography
over silica gel [eluent: from petrol ether to petrol ether/EtOAc
(10:1)] to afford 7 as a colorless oil (1.44 g, 65%). 1H NMR
saturated aqueous solution of NH4PF6 was added until the
suspension became clear. The solvent was removed in vacuo,
and water (50 mL) was added to the residue. The resulting mixture
was then filtered, washed with water, and dried to give 3 as a
yellow solid (1.06 g, 93%). 1H NMR (400 MHz, CD3CN):
d 7.49 (m, 5
H), 4.16 (s, 2 H), 3.32 (t, 2 H, J = 6.6 Hz), 3.03 (t, 2 H, J = 7.7 Hz),
1.73–1.64 (m, 2 H), 1.63–1.55 (m, 2 H), 1.40 (m, 4 H). 13C NMR
(100 MHz, CD3CN):
28.7, 26.3, 26.1, 26.0. HRMS (ESI): m/z calcd. for C13H21N4
233.1761: [M–PF6]+; found: 233.1762.
d 131.4, 130.6, 130.3, 129.7, 52.1, 51.5, 48.4,
+
Preparation of 4 (Scheme 4): To a solution of 2 (174.1 mg,
0.3 mmol), 3 (238.3 mg, 0.63 mmol), in CH3CN (3.0 mL) was added
(400 MHz, CDCl3):
d 7.28 (m, 5 H), 4.41 (s, 2 H), 3.38 (m, 2 H), 3.15
(m, 2 H), 1.82 (m, 2 H), 1.46 (m, 13 H), 1.26 (m, 2 H).
Cu(MeCN)4PF6 (352.2 mg, 0.945 mmol) and 2,6-lutidine (11 mL,
Preparation of 8 (Scheme 3): Compound 7 (2.03 g, 5.48 mmol)
was dissolved in DMF (150 mL), NaN3 (3.57 g, 55.0 mmol) was
0.095 mmol). The reaction mixture was stirred for 24 h at room
temperature. Then the solvent was removed under reduced
[(Schme_3)TD$FIG]
Scheme 3. Synthesis of secondary ammonium guest 3.

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H. Wang et al. / Chinese Chemical Letters 24 (2013) 563–567
565
Scheme 4. Synthesis of dumbbell-shaped molecule 4.
[(Schme_5)TD$FIG]
Scheme 5. Synthesis of N21C7.
pressure, and purified by column chromatography over silica gel
(eluent: CH2Cl2/MeOH, 30:1) to afford 4 as a yellow solid (528.9 mg,
82%). 1H NMR (400 MHz, CD3CN):
d8.97 (d, 4 H, J = 6.9 Hz), 8.37 (d, 4
stopper) onto a bifunctional stopper can be an effective way to
generate a [3]rotaxane [10]. We synthesized an azide-containing
secondary ammonium guest 3 to bind with N21C7 ring. A viologen
2 with two alkyne groups was chosen to connect with the
semirotaxanes. The CuAAC ‘‘click’’ reaction has been widely used in
rotaxane synthesis because of the mild condition and high
efficiency [11]. Using the Cu(CH3CN)4PF6 as the catalyst, we can
obtain the [3]rotaxane 1 in high yield. The 1HNMR, 13C NMR and
HRMS have been used to characterize the compounds. We assigned
all the resonances by analyzing their 1D NMR and 2D NMR spectra
(see the Supporting information for 1H–1H COSY spectra).
H, J = 6.8 Hz), 7.58 (s, 2 H), 7.47 (m, 10 H), 4.83 ꢀ 4.59 (m, 4 H),
4.41 ꢀ 4.22 (m, 4 H), 4.20 ꢀ 3.96 (s, 4 H), 3.13 ꢀ 2.86 (m, 4 H), 2.76
(m, 4 H), 2.42 ꢀ 2.26 (m, 4 H), 1.88 ꢀ 1.75 (m, 4 H), 1.71 ꢀ 1.54 (m, 4
H), 1.40 ꢀ 1.23 (m, 8 H). 13C NMR (100 MHz, CD3CN):
d 149.7, 145.5,
145.0, 130.3, 129.7, 129.4, 128.8, 126.8, 121.7, 68.3, 63.0, 60.9, 51.2,
49.3, 47.5, 30.1, 29.2, 25.0, 21.0. HRMS (ESI): m/z calcd. for
C
46H62N102+: [Mꢀ4PF6ꢀ2H]2+ 377.2574; found: 377.2579.
Preparation of N21C7 (Scheme 5): While stirring vigorously
under argon atmosphere, a suspension of K2CO3 (1.38 g, 10 mmol)
and KBF4 (629.5 mg, 5 mmol) in dry CH3CN (100 mL) was heated to
reflux. To the suspension was added dropwise a solution of
compound 9 (1.97 g, 3.33 mmol) and 2,3-dihydroxynaphthalene
(533.3 mg, 3.33 mmol) in dry CH3CN (100 mL) during 12 h. The
reaction mixture was stirred under reflux for another 3 d. After
cooled, the mixture was filtered and washed with CH2Cl2 (100 mL).
The filtrate was evaporated off under vacuum. The residue was
partitioned between CH2Cl2 (100 mL) and water (100 mL), and the
aqueous phase was extracted twice by CH2Cl2 (50 mL). Then the
organic phase was dried over anhydrous Na2SO4, then concentrat-
ed. The crude product was purified by column chromatography
over silica gel (eluent: EtOAc/petrol ether from 1:5 to 2:1) to afford
N21C7 as a yellowish solid (879.8 mg, 65%). 1H NMR (400 MHz,
The structure of 1 was confirmed through the comparison of the
1H NMR spectra of 1, uncomplexed dumbbell-shaped component 4
and N21C7 (Fig. 1). The downfield shifts of Ha and Hc adjacent to
the ammonium site and the split and the shifts of the ethylidene
[(g._1)TD$FIG]
pi rotons on the crown ether ring confirmed the threading of the
CDCl3):
7.12 (s, 2 H), 4.30–4.23 (m, 4 H), 4.03–3.97 (m, 4 H), 3.85 (m, 4 H),
3.76 (m, 4 H), 3.68 (m, 8 H). 13C NMR (100 MHz, CDCl3):
149.1,
d 7.66 (dd, 2 H, J = 6.0, 3.3 Hz), 7.32 (dd, 2 H, J = 6.1, 3.2 Hz),
d
129.3, 126.3, 124.2, 108.1, 71.4, 71.2, 71.1, 70.5, 69.6, 69.0. HRMS
(ESI): m/z calcd. for C22H30NaO7+: [M+Na]+ 429.1884; found:
429.1891.
3. Results and discussion
The rotaxanes based on crown ether were commonly synthe-
sized following the ‘‘threading-followed-by-stoppering’’ protocol
[9]. Attaching two semirotaxanes (that is, pseudorotaxane with one
Fig. 1. Partial 1H NMR spectra (400 MHz, CD3CN, 5 mmol/L, 298 K) of (a) the
uncomplexed dumbbell-shaped thread 4, (b) 1, and (c) the uncomplexed host
N21C7. * = solvent signal.

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H. Wang et al. / Chinese Chemical
3
Letters 24 (2013) 563–567
1.2
0.8
0.4
1 +2.0 base
1 +2.0 base+2.0 acid
1
0.0
400
500
600
Wavelength/nm
Fig. 3. Absorption spectra of 1 (a) the original spectrum, (b) after addition of 2.0
equivalents P1–t-Bu; and (c) further addition of 2.0 equivalents TFA.
Fig. 2. Partial 1H NMR spectra (400 MHz, CD3CN, 5 mmol/L, 298 K) of 1 (a) the
original spectrum, (b) after addition of 2.0 equivalents P1–t-Bu. * = solvent signal.
[(ig._4)TD$FIG]
Fig. 4. Molecular energy minimization of 1 before (left) and after (right) the addition of base. The geometries were optimized by the molecular mechanics method with
dreiding force field.
secondary ammonium group into the 21C7 ring. The protons H5
and H6 of the viologen and HA, HB and HC of the naphthol group
were almost unchanged, which indicated that the viologen and
naphthol groups were far away from each other.
and acceptor under external stimulation, the result presented here
offers a new prototype of performing photophysical function in
mechanically interlocked molecules.
We used the 1H NMR to investigate the base-acid controlled
movement of 1 (Fig. 2). Through addition of phosphazene base N-t-
Acknowledgments
butyl-N0,N0,N00,N00,N00,N000-hexamethyl-phosphorimidic
triamide
We thank the National Basic Research Program of China(973
Program, No. 2011CB932500), and the National Natural Science
Foundation of China (Nos. 20932004 and 20972077) for financial
support.
(P1–t-Bu) to the solution of 1, it can be found that viologen
protons H5 and H6 and HA, HB and HC of the naphthol group were all
shifted upfield. This observation suggested that the electron-
deficient viologen unit and the electron-rich naphthol group were
close to each other to form a strong CT interaction. Protons Hj, Hk
and Hi near the viologen were shifted downfield, which should be
attributed to the existence of the C–Hꢁ ꢁ ꢁO interaction between the
axle and crown ether and the deshielding effect of the naphthol
group. These observations indicated that the crown ether rings
locatedaroundthe Nterminalsofviologenunit afteradditionof base.
The base–acid controlled behavior was also investigated by
UV/vis spectroscopy (Fig. 3). In the original state, the CT interaction
was weak. After addition of P1–t-Bu, a characteristic CT band
centered around 390 nm can be found in the UV/vis spectrum,
which indicated the strong CT interaction between the naphthol
and viologen groups. Then addition of trifluoroacetic acid (TFA) can
restore the original spectrum which suggested that the process
was reversible. The energy-minimized structures of 1 (before and
after the addition of base) obtained by molecular modeling were
also consistent with the proposed structures (Fig. 4).
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.cclet.2013.04.007.
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Comment
It offers a new prototype of performing photophysical function through adjusting the special distance under external stimulation.