Synthesis of a [2]rotaxane operated in basic environment

Article abstract of DOI:10.1039/c1ob05726j

A tight [2]rotaxane with two chromophores as stoppers is described, in which the macrocycle is able to reversibly move by tuning of base. This moving process can result in intramolecular photo-induced electron transfer (PET), changing the photo-physical p

Full text of DOI:10.1039/c1ob05726j

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Synthesis of a [2]rotaxane operated in basic environment†
Wenlong Yang,a,b Yongjun Li,*a Jianhong Zhang,a,b Yanwen Yu,a,b Taifeng Liu,a,b Huibiao Liua and
Yuliang Li*a
Received 9th May 2011, Accepted 6th June 2011
DOI: 10.1039/c1ob05726j
A tight [2]rotaxane with two chromophores as stoppers is described, in which the macrocycle is able to
reversibly move by tuning of base. This moving process can result in intramolecular photo-induced
electron transfer (PET), changing the photo-physical properties.
base was added into the system. Normally, the acid–base driven
Introduction
[2]rotaxanes based on a crown–dialkylammonium interaction
During the past decade, scientists have developed new concepts
need to be protonated for the shuttling movement, that is, the
and morphologies for synthesizing and characterizing many new
[2]rotaxanes have to be in acidic and basic environment. However,
[2]rotaxanes that work fully in a basic environment are rare.13
Herein, we synthesized a tight [2]rotaxane with two stations linked
by a short bridge, which is operated only in a basic environment
and accompanied by fluorescent responses (Fig. 1).
supermolecular systems based on molecular machines. Rotaxanes
have become typical candidates in the design of artificial molecular
machines because some of their components are able to reversibly
move between two or more stations on application of an external
stimulus.1 Various stimuli have been employed to induce such
switching by illumination2 and variation of the electrochemical
potential,3 solvent4 and pH change.5–7
Synthetic rotaxanes are often used as analogues of natural
systems, and such research has also led to the discovery of
new applications of these systems, for example in ion and
material transportation8 or energy and electronic transfer that
transform energy in both directions. The alteration of the relative
positions of the interlocked components constitutes a basic kind of
mechanical switch, which can vary the physical properties of the
molecular rotaxane such as conductivity,9 circular dichroism,10
and fluorescence.11 The use of fluorescent change as an output
signal is preferable because it is easy to detect signals and is
inexpensive. Recently, some rotaxanes that can switch between
two or more fluorescent states (output) have been constructed.12
In the traditional rotaxanes, the stations were mostly linked by
long alkyl chains, between which the macrocycle can reversibly
move upon application of an external stimulus. The stations were
far away from their adjacent stopper, and there was no steric
hindrance between the macrocycle and the corresponding stopper
when the macrocycle moved towards the station. Examples are
the [2]rotaxanes driven by acid–base,5 in which the macrocycle
moved towards and resided around the other station when acid or
Fig.
responses.
1 Schematic of the DB24C8 movement and the fluorescent
Results and discussion
The shuttle was synthesized according to Scheme 1. The already
reported secondary dialkylammonium was chosen as the template
moiety for the preparation of the [2]rotaxane R-1-a, and as one
station for DB24C8 in the targeted molecular machine R-2-a.
Another station, the cationic electron-poor aromatic ring N-
methyltriazolium, was prepared by regioselective methylation of
the [2]rotaxane R-1-a, which could interact with the DB24C8,
either by p–p stacking with the electron-rich catechol ring, by ion–
dipole interactions between the cationic charge and the oxygens
of the macrocycle, or by hydrogen bonding between the vinylic
hydrogen of the N-methyltriazolium and the oxygens of the
macrocycle.14
1
The direct comparison of the H NMR spectra (Fig. 2) of the
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, liyj@iccas.ac.cn; Fax: (+) 86-10-82616576; Tel: (+) 86-10-
62588934
thread T-2-a, the rotaxane R-2-a and the uncomplexed DB24C8
in CD3CN readily confirms the interlocked nature of R-2-a and
indicates the localization of the DB24C8. The 1H NMR spectrum
of the rotaxane R-2-a revealed well-resolved peaks (Fig. 2b),
all of which could be assigned with the help of COSY-NMR
spectroscopic measurements (Supporting Information Fig. S0†).
The peaks arising from the -OCH2CH2O- repeating unit appear
bGraduate University of Chinese Academy of Sciences, Beijing, 100190,
P.R. China
† Electronic supplementary information (ESI) available: Synthesis, 2D,
NMR studies, UV/visible. See DOI: 10.1039/c1ob05726j
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Scheme 2 Movement process of R-2-b under base-only stimulus.
Scheme 1 Synthesis of R-1-a and R-2-a.
through the method reported by Coutrot,14 but the rotaxane R-
2-b was obtained instead of R-2-c (Scheme 2), with the DB24C8
still sitting on the -NH- station (R-2-b), which is in contrast with
the normal rotaxanes with these two stations.6,14 This may be
due to the greater flexibility of the -NH- moiety (blue, R-2-b)
than the short bridge (green, R-2-b). However, when more DIEA
was added to R-2-b, the DB24C8 could move towards the N-
methyltriazolium station (R-2-c). This phenomenon was clearly
1
observed from the H NMR spectral titration of R-2-b (Fig. 3).
When 1 equiv. of DIEA was add to the solution of R-2-b, the
rotaxane partially adopted the R-2-c conformation and no evident
change was found after 24 h standing; when more DIEA was
continuously added, more rotaxane transformed from the R-2-b
to R-2-c conformation. When 50 equiv. of DIEA was added, nearly
all the rotaxane adopted the R-2-c conformation, which indicated
that the DB24C8 had moved towards the N-methyltriazolium
station almost quantitatively. This interpretation was supported by
the following (from Fig. 3): 1) the peak for the N-methyltriazolium
proton Hg is shifted downfield (Dd = 1.04 ppm) on association
Fig. 2 1H NMR spectra (400 MHz, CD3CN, 298 K) of a) T-2-a, b) R-2-a,
and c) DB24C8. The letters corresponding to the protons are shown in
Scheme 1.
1
with the DB24C8 ring; 2) H NMR signals for Hk and Hl are
in the region d = 3.2–4.2 ppm and are not individually resolved on
+
account of the association of the DB24C8 ring with the -NH2 -
+
center. In the rotaxane R-2-a, Hk and Hl adjacent to the -NH2 -
center resonate (Fig. 2b) at d = 4.77 and 4.42 ppm (dramatically
shifted downfield Dd = 0.43 and 0.22 ppm, respectively), and
no significant variation in the chemical shift of Hg was noticed,
indicating that the DB24C8 mainly resides around the ammonium
station. In comparison with the free DB24C8, the signals of
the oligo(ethylene glycol) moiety of the macrocycle experienced
upfield shift due to a combination of C–H ◊ ◊ ◊ O and N+–H ◊ ◊ ◊ O
hydrogen bonds.6 The chemical shifts of the complexed crown
ether hydrogens HC and HE of the rotaxane R-2-a are split,
indicating that they are facing the two non-symmetrical ends of
the threaded.15
Actually, the DB24C8 may probably prefer to sit over the
alkyl chain rather than over the sterically hindered spacer14,15 and
diisopropylethylamine (DIEA) is strong enough to deprotonate
the NH2+ center.16 The deprotonation of the [2]rotaxane R-2-a was
carried out in acetone with a large excess of DIEA and purified
Fig. 3 1H NMR spectra of R-2-b (1.5 mM) in CD3CN at 298 K upon
titrational addition of DIEA.
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shifted upfield (Dd = 0.91 and 0.53 ppm, respectively) as a result of
both the deprotonation of the neighbouring ammonium and the
shuttling of the macrocycle; 3) the signal for proton Hh is shifted
downfield (Dd = 0.7 ppm) when the DB24C8 ring migrates to the
N-methyltriazolium station, while Hf is shifted upfield (Dd = 0.38
ppm).
To our surprise, on removal of the excess of DIEA under
vacuum, R-2-b is obtained which is shown in the middle part of
Scheme 2, which may be due to the bulky anthracene stopper.
The bulky anthracene substituent on the N-methyltriazolium
enhanced the spatial limitation for the solvent to stabilize the
N-methyltriazolium cations, so the disassociation of the N-
methyltriazolium–PF6 ion-pair was depressed,17 and the macro-
cycle can not interact with the N-methyltriazolium in the ion-pair.
With the addition of the strong Lewis base DIEA, which can
solvate the cations,18 the N-methyltriazolium–PF6 ion-pair tended
to dissociate to free ions, and the DB24C8 could interact with the
N-methyltriazolium cation.
The absorption spectra of T-1-a, R-1-a, T-2-a and R-2-a showed
superposition features of the hydroxylsubstituted tetraphenylimi-
dazole (HPI) and anthracene moieties (Fig. 4a). After regioselec-
tive methylation of T-1-a and R-1-a, three absorption peaks arising
from the anthracene unit experienced a red shift of 2 nm for T-2-a
and R-2-a respectively, which can be ascribed to the interaction
between the electron-deficient N-methyltriazolium station and the
anthracene unit in the ground state. As shown in Fig. 4b, T-1-a
exhibited an emission at 412 nm, while R-1-a exhibited a stronger
emission around 413 nm. This enhancing effect can be attributed
to the existence of DB24C8, which inhibited the aggregation of
the rotaxane. The anthracene fluorescence of R-2-a was quenched
completely due to the strong photo-induced electron transfer
(PET) from anthracene to the N-methyltriazolium station, while
T-2-a exhibited higher fluorescence emission because of the easier
aggregation of the thread than the rotaxane.
Upon addition of 1 equivalent of DIEA to R-2-b, the emission
of the anthracene moiety increased slightly; when an excess of
base was added to it, the emission intensity was found to increase
continuously (Fig. 5). The emission exhibited a linear growth with
the increase of DIEA from 1 equiv. to 15 equiv. and tended to
remain at equilibrium until 50 equiv. of DIEA was added (inset
of Fig. 5). To the thread T-2-b, however, no evident fluorescence
emission change was found with the addition of DIEA from 1
to 50 equiv (Fig. S4†). So, the fluorescence emission changes of
R-2-b can be attributed to the movement of the DB24C8. When
the DIEA was added to the R-2-b, the DB24C8 moved towards
the N-methyltriazolium station, and the PET was restricted from
anthracene to N-methyltriazolium because of the association
between DB24C8 and N-methyltriazolium. After removal of the
excess of DIEA under vacuum, the fluorescence intensity was
recovered, indicating the DB24C8 was back to the -NH- station
(Fig. S2†). A similar phenomenon can also be observed when
the HPI unit was excited (Fig. S3†). The DB24C8 resides at the
-NH- station and the HPI unit exhibited weak emission; this was
due to the strong interaction between the HPI stopper and the
DB24C8. With increasing DIEA, the DB24C8 moved towards
the N-methyltriazolium station, the distance is longer between
the HPI stopper and the DB24C8, leading to the interaction
being weakened and the fluorescence emission of the HPI unit
increased. All the observations in the fluorescence experiments
were in accordance with the 1H NMR spectra.
Fig. 5 Fluorescence spectra of R-2-b (lexc = 370 nm, 1 ¥ 10-5 M) in CH3CN
with various equivalents of DIEA (0 to 50 equiv.).
Conclusion
In summary, we have presented a tight [2]rotaxane shuttling in a
basic environment. The macrocycle could move towards the N-
methyltriazolium station quantitatively in the presence of base,
while the shuttling process can be inverted by removal of the base.
The shuttling movement of the macrocycle was also accompanied
by fluorescent responses, which can be used as a fluorescent switch
in basic environments.
Fig. 4 Absorption and fluorencent specta of compound T-1-a, T-2-a,
R-1-a and R-2-a (1 ¥ 10-5 M, 298 K) in CH3CN.
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the aqueous layer was extracted with CH2Cl2 (¥3). The organic
layers were combined, dried over Na2SO4, and concentrated to
obtain quantitatively ◦the rotaxane R-2-a (123 mg) as a yellow
Experimental
Synthesis of compounds 5, R-1-a, R-2-a, R-2-b
1
Compound 5: A solution of the compound 2 (0.418 g, 1
mmol) and 3 (0.16 g, 1 mmol) in toluene (50 mL) was heated
under reflux overnight by using a Dean–Stark apparatus. The
solvent was removed under reduced pressure after the reaction was
cooled to room temperature. The residue was dissolved in THF
(50 mL), then NaBH4 (0.4 g, 10.5 mmol) was added cautiously
at 0 ◦C. The mixture was stirred at room temperature for a
further 4 h. Water was added to quench the excess NaBH4. The
solvent was evaporated off, and the residue was extracted with
CH2Cl2. The combined organic layers were dried over Na2SO4.
After concentrating in vacuo, the crude product compound 4
(300 mg, 0.53 mmol) was dissolved in acetone and a few drops
of trifluoroacetic acid were added. After 0.5 h, the solvent was
removed under vacuum. The residue was dissolved in a mixture of
acetone and water. Then an aqueous solution of NH4PF6 (122 mg,
0.75 mmol) was added. The mixture was stirred for 1 h and then the
acetone was evaporated off. The aqueous solution was extracted
with CH2Cl2 several times. The collected organic layers were dried
over Na2SO4, filtered, and concentrated in vacuo to yield 5 as a
yellow solid (600 mg, 85%). Mp = 115–116 ◦C. 1H NMR (CDCl3,
400 MHz): d = 7.53 (m, 2 H), 7.46 (m, 2 H), 7.16–7.38 (m, 10 H),
6.95–6.99 (m, 4 H), 6.65 (m, 1 H), 6.45 (m, 1 H), 4.78 (s, 2 H),
3.81 (s, 2 H), 3.67 ppm (s, 2 H); 13C NMR (CDCl3, 100 MHz)
d = 158.49, 157.03, 144.22, 136.5, 132.89, 132.22, 131.96, 131.62,
131.19, 130.69, 129.69, 128.75, 127.56, 122.7, 119.22, 117.89,
115.47, 114.98, 111.17, 78.06, 76.15, 55.77 ppm; MS (MALDI-
TOF): m/z 562.3; elemental analysis (%) calcd for C38H31N3O2 : C
81.26, H 5.56, N 7.48; found: C 81.59, H 5.53, N 7.51.
solid. Mp = 128–129 C. H NMR (CD3CN, 400 MHz, 298 K):
d = 8.85 (s, 1 H), 8.36 (d, 2 H, J = 9.14 Hz), 8.24 (m, 3 H), 7.74 (t,
2 H, J = 6.81, 7.62 Hz), 7.65 (t, 2 H, J = 7.84, 6.81 Hz), 7.52–7.55
(m, 4 H), 7.49 (m, 2 H), 7.38 (d, 1 H, J = 8.33 Hz), 7.27–7.32
(m, 7 H), 7.19 (m, 1 H), 7.09 (d, 2 H, J = 8.65 Hz), 7.03 (m, 1
H), 6.78–6.84 (m, 10 H), 6.57 (m, 1 H), 6.54 (d, 2 H, J = 8.62
Hz), 6.44 (m, 1 H), 4.95 (s, 2 H), 4.75 (m, 2 H), 4.42 (m, 2 H),
4.16 (s, 3 H), 4.05–4.09 (m, 4 H), 3.93–3.96 (m 4 H), 3.65–3.68
(m, 8 H), 3.51–3.55 (m, 4 H), 3.31–3.35 ppm (m, 4 H); 13C NMR
(CD3CN, 100 MHz, 298 K) d = 159.20, 158.30, 149.52, 148.28,
148.15, 145.67, 140.53, 139.54, 132.64, 132.36, 132.18, 132.15,
131.21, 130.49, 130.17, 129.97, 129.58, 129.35, 129.07, 127.20,
126.68, 115.44, 115.25, 113.83, 113.63, 71.46, 71.29, 70.92, 70.74,
69.85, 69.52, 68.88, 58.77, 52.96, 52.19, 50.98, 39.60 ppm; MS
(MALDI–TOF): m/z: 1257.6 [M - H]+; elemental analysis (%)
calcd for C78H78N6F12O10P2: C 60.46, H 5.07, N 5.42; found: C
60.69, H 5.04, N 5.44.
Rotaxane R-2-b: To a solution of the rotaxane R-2-a (1 equiv)
in acetone was added a large excess of DIEA (100 equiv) and the
mixture was stirred for 1 h. After evaporation, and in order to
remove the diisopropylethylammonium hexafluorophosphate, the
crude mixture was diluted with CH2Cl2 and water was added. The
aqueous layer was extracted with CH2Cl2 (¥3) then the organic
layers were combined, dried over Na2SO4 and concentrated. Et2O
was added to dissolve the excess of DIEA then removed to obtain
1
the rotaxane R-2-b. H NMR (CD3CN, 400 MHz, 298 K): d =
8.85 (s, 1 H), 8.36 (d, 2 H, J = 9.0 Hz), 8.23 (m, 3 H), 7.74 (t, 2
H, J = 6.64, 8.88 Hz), 7.65 (t, 2 H, J = 8.0, 7.0 Hz), 7.52–7.55 (m,
3 H), 7.49 (m, 2 H), 7.38 (m, 2 H), 7.27–7.32 (m, 7 H), 7.19 (m,
1 H), 7.08 (d, 2 H, J = 8.4 Hz), 7.03 (m, 1 H), 6.78–6.84 (m, 10
H), 6.59 (m, 1 H), 6.55 (d, 2 H, J = 8.4 Hz), 6.44 (m, 1 H), 4.96
(s, 2 H), 4.77 (m, 2 H), 4.42 (m, 2 H), 4.17 (s, 3 H), 4.06–4.09 (m,
4 H), 3.93–3.97 (m 4 H), 3.65-3.68 (m, 8 H), 3.51–3.54 (m, 4 H),
3.33–3.36 ppm (m, 4 H).
Rotaxane R-1-a: A mixture of compound 5 (354 mg, 0.50
mmol), compound 6 (117 mg, 0.50 mmol), macrocycle DB24C8
(211 mg, 0.48 mmol), and [Cu(MeCN)4]PF6 (175 mg, 0.47 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 (SiO2: CH2Cl2/MeOH 60 : 1)
◦
1
to afford rotaxane R-1-a (845 mg, 68%). Mp = 139–140 C. H
NMR (CD3CN, 400 MHz, 298 K): d = 8.56 (s, 1 H), 8.36 (m, 2
H), 8.06 (m, 2 H), 7.52 (m, 2 H), 7.19–7.46 (m, 17 H), 7.15 (m, 1
H), 7.01 (m, 1 H), 6.98 (m, 2 H), 6.88 (m, 4 H), 6.72 (m, 4 H), 6.68
(m, 2 H), 6.42 (m, 1 H), 6.27 (m, 1 H), 4.92 (s, 2 H), 4.61 (s, 2 H),
4.20 (s, 2 H), 4.08 (m, 4 H), 3.99 (m, 4 H), 3.59 (m, 8 H), 3.33 (m,
4 H), 3.28 ppm (m, 4 H); 13C NMR (CD3CN, 100 MHz, 298 K)
d = 160.22, 159.75, 150.04, 148.83, 146.26, 144.78, 138.93, 136.22,
135.87, 134.58, 133.23, 132.94, 132.61, 132.5, 132.13, 131.85,
131.72, 131.35, 131.02, 130.81, 130.64, 130.43, 130.19, 129.92,
128.91, 127.82, 127.44, 126.96, 126.61, 124.94, 124.87, 124.75,
122.83, 116.56, 115.78, 114.43, 114.12, 71.94, 71.39, 69.46, 62.51,
53.67, 52.59, 47.49 ppm; MS (MALDI-TOF): m/z: 1243.5 [M]+,
1275.5 [M+O2]+; elemental analysis (%) calcd for C77H75N6F6O10P :
C 66.56, H 5.44, N 6.05; found: C 66.78, H 5.42, N 6.09.
Rotaxane R-2-a: Rotaxane R-1-a (100 mg, 0.08 mmol) was
dissolved in iodomethane (2 mL) and the mixture was stirred for
24 h at 40 ◦C. Then iodomethane was evaporated and the solid was
washed with Et2O to give an orange solid. Then, to a suspension
of the previous solid in H2O (10 mL) were added NH4PF6
(16.3 mg, 0.1 mmol) and CH2Cl2 (15 mL). Then the resulted
bilayer solution was vigorously stirred for 1h. After separation,
Acknowledgements
We are grateful for financial support from the National Natural
Science Foundation of China (21031006, 20831160507, 20721061,
20971127), NSFC-DFG joint fund (TRR 61) and the National
Basic Research 973 Program of China.
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6026 | Org. Biomol. Chem., 2011, 9, 6022–6026
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