Control of rocking mobility of rotaxanes by size change of stimulus-responsive ring components

Article abstract of DOI:10.1246/cl.2007.810

A rotaxane having a ring molecule composed of a metaphenylene unit which swings as a pendulum and a dianthrylethane unit which undergoes isomerization in response to external stimuli was synthesized. The rates of the rocking motion were switched reversibly and changed substantially in response to photo and thermal stimuli by changing the size of the ring component. Copyright

Full text of DOI:10.1246/cl.2007.810

810
Chemistry Letters Vol.36, No.6 (2007)
Control of Rocking Mobility of Rotaxanes by Size Change
of Stimulus-responsive Ring Components
Keiji Hirose,ꢀ Kazuaki Ishibashi, Yoshinobu Shiba, Yasuko Doi, and Yoshito Tobe
Division of Frontier Materials Science, Graduate School of Engineering Science, Osaka University,
1-3 Machikaneyama, Toyonaka 560-8531
(Received April 9, 2007; CL-070372; E-mail: hirose@chem.es.osaka-u.ac.jp)
A rotaxane having a ring molecule composed of a meta-
Rotaxanes 1 and 2 that we designed are shown in Scheme 1.
phenylene unit which swings as a pendulum and a dianthryl-
ethane unit which undergoes isomerization in response to exter-
nal stimuli was synthesized. The rates of the rocking motion
were switched reversibly and changed substantially in response
to photo and thermal stimuli by changing the size of the ring
component.
We planed to control the rates of rocking motion (oscillation) of
the metaphenylene unit, by changing the size of the ring compo-
nent.12 We expected that the barrier to the quasi-rotation of the
phenylene moiety would change between open (1) and closed
(2) states of the rotaxanes. In order to induce appropriate steric
barrier, a methoxy group was attached at the flanked position
of the metaphenylene unit. Since, it is well known that dianthryl-
ethane derivatives undergo reversible photodimerization and
thermal reversion reaction quantitatively, the size of the ring
molecules can be changed by employing this protocol.13,14 For
the preparative reason, dibenzyl ammonium ion was used as
the axle component. Finally, 3,5-bis(triisopropylsilyl)phenyl
group was employed as a stopper component because it is bulky
enough to prevent the dethreading of the axle in the open form.
The synthetic route of rotaxanes 1 is shown in Scheme 2. By
irradiation of crown ethers 315 in benzene with a high-pressure
mercury lamp, closed form 4 was formed by photodimerization
of the anthracene moieties. After solvent exchange into a mix-
ture of dichloromethane and acetonitrile (10:1), pseudorotaxanes
6 were formed by association of 4 with secondary ammonium
salt 5 at ꢁ10 ꢂC. The acylation end-capping reaction16 of 6
with anhydride 7 afforded the corresponding rotaxane 2 which
then reverted to the respective open type rotaxane 1 during the
work-up and isolation procedures. The yield of rotaxane 1 from
3 (4 steps) was 46%.
Rotaxanes1 are thought to be prime candidates for the
construction of artificial molecular machines2 and molecular
electronic devices,3 since the ring and dumbbell components
of rotaxanes are capable of exchanging the position of one
component relative to that of the other by their motions such
as shuttling4 (linear motion), circumrotation5 (rotary motion),
and rocking6 (pendular motion). These motions can in principle
be controlled by external impetus such as on chemical,7 electri-
cal,8 or photochemical9 stimuli. The exchanges by shuttling
motions have been studied most extensively aiming at the appli-
cation to molecular electronic devices. In contrast to control
of relative position by the shuttling motion, control of rocking
mobility has not been studied.10 If a ring component of a
rotaxane has large dipole moment, and its direction or oscillation
frequency is controlled, the rotaxane can be applied to switching
devices in a molecular scale. Indeed, it is reported that efficiency
of photoinduced electron transfer is differentiated appreciably
by the direction of dipole moment of helical peptides immobi-
lized on substrate.11 In this connection, we report here the
syntheses of rocking-rate controllable rotaxanes 1 and 2 and
effective switching of the rates of the rocking motion by 20 times
(1 vs 2).
The photochemical ring closure and the thermal reversion
of the anthracene units took place reversibly between the open
rotaxane 1 and its closed form 2. Upon irradiation of a solution
of 1 in CD3CN in an ice-water bath with a high-pressure mercury
1
lamp, the H NMR signal of 1 assigned to the ethylene protons
of the dianthrylethane unit disappeared, while a characteristic
Scheme 1. Switching of rocking rates (k) of rotaxanes 1 and 2
based on the change of the size of the ring component.
Scheme 2. Synthesis of rotaxane 1.
Copyright Ó 2007 The Chemical Society of Japan

Chemistry Letters Vol.36, No.6 (2007)
811
This work was partly supported by the Izumi Science and
Technology Foundation and a Grant-in-Aid for Scientific
Research from the Ministry of Education, Culture, Sports,
Science and Technology of Japan.
References and Notes
1
2
3
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´
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Table 1. Rates of rocking and kinetic parameters of 1 and 2
4
ꢀHz
/kJ molꢁ1
ꢀSz
/J Kꢁ1 molꢁ1
Tc/K
k203K/sꢁ1
1
2
nda
241
nda
31 ꢄ 1:2
b
nda
ꢁ65 ꢄ 5:0
>470b
23
5
6
7
aNot determined. Estimated from the Gutowski’s equation
assuming that the ꢀꢀ value of Hf signals of 1 is same as that
of 2.
´
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signal of the cyclobutane protons of 2 appeared as shown in
Supporting Information,17 indicating that the ring closure
proceeded efficiently. The spectrum of 2 reverted to that of 1
when the NMR solution of 2 was stood at room-temperature
overnight, implying that the thermal reversion 2 to 1 proceeded
quantitatively. The rate of the thermal reversion of 2 was deter-
mined following the change of the UV–vis absorption spectra.
The rate constant of 2 at 30 ꢂC was determined to be
49:7 ꢃ 10ꢁ5 sꢁ1 (the half life: 23 min).
8
9
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mined by the line-shape analysis of their VT-NMR spectra in
1
THF-d8. Figure 1 shows partial experimental H NMR spectra
of Hf on the axle component of 2 between 283 and 203 K and
the simulated spectra assuming the rate constants shown. The
rates of oscillations of 2 were estimated on the basis of the
line-shape analysis of the VT-NMR spectra. The kinetic param-
eters were determined from the Eyring plot as listed in Table 1.
In the case of 1, the room-temperature spectrum indicates
that the rocking motion of the phenylene unit is rapid on
the NMR time scale and the spectrum did not change when
the solution was cooled to 165 K. Therefore, the minimum rate
of oscillation (k203) of 1 was estimated to be >470 sꢁ1. Table 1
summarizes the kinetic parameters and the oscillation frequen-
cies at 203 K of rotaxanes 1 and 2.
10 For mobility control, see: a) A. S. Lane, D. A. Leigh, A.
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12 For
a ring contraction of a rotaxane induced by
As shown in Table 1, the rate of oscillation of open 1
(k203 > 470 sꢁ1) is at least 20 times larger than that of closed
2 (k203 ¼ 23 sꢁ1). These results demonstrate clearly that the rate
of oscillation was substantially changed by the external stimuli.
In conclusion, we synthesized rotaxane having a dianthryl-
ethane moiety in the ring unit of which ring size was changed re-
versibly and quantitatively by photochemical cycloaddition and
thermal reversion. Substantial difference between the oscillation
rates of open and closed rotaxanes was observed, demonstrating
that the rocking motion was controlled effectively by the exter-
nal stimuli.
coordination of a transition metal, see: I. Yoon, M. Narita,
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15 Preparation of 3 is given in Supporting Information.
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17 Supportiong Information is available electronically on the
CSJ-Journal web site, http://www.csj.jp/journals/chem-lett/.
Published on the web (Advance View) May 26, 2007; doi:10.1246/cl.2007.810