A comparison of shuttling mechanisms in two constitutionally isomeric bistable rotaxane-based sunlight-powered nanomotors

Article abstract of DOI:10.1071/CH06019

To find out how best to optimize shuttling of the macrocycle in a particular class of photochemically driven molecular abacus, which has the molecular structure of BR-I⁶⁺ in its Mark I prototype (Ashton et al., Chem. Eur. J. 2000, 6, 3558), we

Full text of DOI:10.1071/CH06019

RESEARCH FRONT
Full Paper
CSIRO PUBLISHING
www.publish.csiro.au/journals/ajc
Aust. J. Chem. 2006, 59, 193–206
A Comparison of Shuttling Mechanisms in Two Constitutionally
Isomeric Bistable Rotaxane-Based Sunlight-Powered Nanomotors
A
A,C
A
A
Vincenzo Balzani, Miguel Clemente-León, Alberto Credi, Monica Semeraro,
A,E
B,D
B
B
Margherita Venturi, Hsian-Rong Tseng,
and J. Fraser Stoddart
Sabine Wenger, Sourav Saha,
B,E
A
Dipartimento di Chimica ‘G. Ciamician’, Università di Bologna, 40126 Bologna, Italy.
The California NanoSystems Institute and the Department of Chemistry and Biochemistry,
University of California, Los Angeles CA 90095-1569, USA.
Current address: Instituto de Ciencia Molecular, Universidad de Valencia, 46100 Burjassot, Spain.
Current address: Department of Molecular and Medical Pharmacology and The Crump Institute
for Molecular Imaging, University of California, Los Angeles CA 90095, USA.
B
C
D
E
Corresponding authors. Email: margherita.venturi@unibo.it; stoddart@chem.ucla.edu
To find out how best to optimize shuttling of the macrocycle in a particular class of photochemically driven
molecular abacus, which has the molecular structure of BR-I6 in its Mark I prototype (Ashton et al., Chem.
Eur. J. 2000, 6, 3558), we have synthesized and characterized a Mark II version of this kind of two-station rotaxane
comprised of six molecular modules, namely (a) a bisparaphenylene[34]crown-10 electron donor macrocycle M
+
2+
and its dumbbell-shaped component which contains (b) a Ru(ii)-polypyridine photoactive unit P as one of its
ꢀ
2+
ꢀ
stoppers, (c) a p-terphenyl-type ring system as a rigid spacer S, (d) 4,4 -bipyridinium (A1 ) and (e) 3,3 -dimethyl-
ꢀ 2+
,4 -bipyridinium (A2 ) electron acceptor units that can play the role of stations for the macrocycle M, and
4
6+
(
f) a tetraarylmethane group T as the second stopper. This Mark II version is identical with BR-I in the Mark I
series that works as a sunlight-powered nanomotor (Balzani et al., Proc. Natl. Acad. Sci. USA 2006, 103, 1178),
except for the swapping of the two stations A12 and A2 along the dumbbell-shaped component, i.e. the Mark I
+
2+
and II bistable rotaxanes are constitutionally isomeric. We have found the closer the juxtaposition of the electron
2
+
2+
6+
transfer photosensitizer P to the better (A1 ) of the two electron acceptors, namely the situation in BR-II
6
+
compared with that in BR-I results in an increase in the rate — and hence the efficiency — of the photoinduced
6
+
electron-transfer step. The rate of the back electron transfer, however, also increases. As a consequence, BR-II
6
+
performs better than BR-I in the fuel-assisted system, but much worse when it is powered by visible light (e.g.
sunlight) alone. By contrast, when shuttling is electrochemically driven, the only difference between the two bistable
rotaxanes in the Mark I and Mark II series is that the macrocycle M moves in opposite directions.
Manuscript received: 15 January 2006.
Final version: 23 February 2006.
Introduction
as those present in nature, is a prohibitive task at this time.
Hence, chemists have turned their attention to the construc-
tion of much simpler systems based on bi- and multi-stable
In the past few years synthetic prowess, which has always
been the most distinguishing feature of chemists alongside
other scientists, and device-driven ingenuity, which has
evolvedfromchemists’preoccupationwithreactivity, mecha-
nism, and function, have led to the construction of a variety of
[
13–23]
catenanes and rotaxanes
in which the relative mechan-
ical movements of their components, caused by chemical,
electrochemical, and photochemical inputs, can be detected
and controlled.
[
1–6]
molecular-leveldevicesandmachines.
Muchoftheinspi-
ration for the burgeoning of this field of functional molecular
nanotechnology comes from the remarkable progress in
molecular biology which has begun of late to reveal the
secrets of the natural molecular-level devices and machines
In recent times, we have examined[24,25] exhaustively
the electrochemical and photophysical properties of a
bistable rotaxane BR-I6 composed of six molecular
components — namely a bisparaphenylene[34]crown-10
(BPP34C10) electron-donating macrocycle M and a
dumbbell-shaped component which contains (a) a Ru(ii)-
+
[
7,8]
that constitute the material base of life.
Despite some out-
however, the bottom-up de novo
construction of artificial devices and machines, as complex
standing achievements,[
9–12]
polypyridine photoactive unit P2 as one of its stoppers,
+
©
CSIRO 2006
10.1071/CH06019
0004-9425/06/030193

RESEARCH FRONT
1
94
V. Balzani et al.
Photosensitizer
and stopper
Electron
acceptor 2
Electron
acceptor 1
Rigid spacer
Stopper
ꢀ
Electron donor
macrocycle
N
ꢀ
O
O
N
A22
ꢀ
O
S
ꢀ
N
O
O
N
N
N
N
N
O
RuII
BR-I6ꢀ
ꢀ
O
N
O
N
A12ꢀ
O
O
P2
؉
ꢀ
O
O
M
N
ꢀ
N
T
A22
ꢀ
S
ꢀ
N
N
N
N
N
N
RuII
N
D-I6ꢀ
ꢀ
N
O
A12
ꢀ
O
O
O
P2
؉
O
ꢀ
N
O
O
O
ꢀ
N
O
T
O
A12
O
ꢀ
S
+
N
N
N
N
N
N
O
RuII
M
BR-II6ꢀ
+
N
A22
ꢀ
N
O
ꢀ
O
P2؉
N
ꢀ
A12
ꢀ
N
T
S
ꢀ
N
N
N
N
N
N
RuII
N
D-II6ꢀ
ꢀ
N
A22
ꢀ
O
P2؉
O
OH
T
S
N
N
N
N
N
RuII
N
PS2ꢀ
122ꢀ
P2؉
6
+
6+
6+
6+
Fig. 1. Structural formulas of the rotaxanes BR-I and BR-II , the corresponding dumbbell-shaped precursors D-I and D-II
,
2+
and the model photosensitizing compound PS which have been investigated in this comparative study.
(
b) a p-terphenyl-type ring system as a rigid spacer S,
linear nanomotor and operates with quantum efficiency of
about 12%.
ꢀ
2+
ꢀ
and (c) 4,4 -bipyridinium (A1 ) and 3,3 -dimethyl-
4
ꢀ
2+
,4 -bipyridinium (A2 ) units as two electron-accepting
We became intrigued to find out what would be the conse-
quences for this autonomous artificial nanomotor powered by
sunlight if we were to swap the two stations in the dumbbell-
shaped component and produce a Mark II version, namely
units, that play the role of recognition sites or ‘stations’ for
the macrocycle M, and (d) a tetraarylmethane group T as the
second stopper. This Mark I version of a photochemically
driven molecular-level abacus behaves[ as an autonomous
25]
6+
BR-II which is essentially a constitutional isomer of the

RESEARCH FRONT
Shuttling Mechanisms in Two Rotaxane-Based Sunlight-Powered Nanomotors
195
ꢀ
N
6PF6ꢁ
ꢀ
N
D-II•6PF6
ꢀ
N
N
N
N
N
N
RuII
N
ꢀ
N
O
O
N
Br
ꢀ
N
PF6ꢁ
3PF6ꢁ
2
ꢀ
N
N
N
N
N
N
RuII
N
ꢀ
N
1•2PF6
2•3PF6
O
O
West side
East side
Scheme 1. Retrosynthetic analysis for the synthesis of the dumbbell-shaped compound D-II·6PF6, which is formed by a covalent
attachment of two reactive intermediates — namely the ‘west side’ 1·2PF6 and the ‘east side’ 2·3PF6.
Mark I version. And so now, the Mark II version BR-II6
(
+
stopper at the other end of the abacus from the Ru(ii) com-
plex. The photoactive bistable rotaxane BR-II·6PF6, and the
corresponding dumbbell-shaped precursor D-II·6PF6, were
constructed in a modular fashion, starting with the synthe-
ses of the active components and completing the sequence
of events by bringing them together to make the target
compounds. In order to simplify the synthetic route, the
dumbbell-shaped precursor D-II·6PF6 was dissected into two
halves — as illustrated in the retrosynthetic analysis shown
Fig. 1) has been synthesized and characterized. Our mis-
sion in this full paper is to compare the electrochemical and
photochemical (under steady-state and pulsed irradiation)
properties of these two versions (Mark I and II) in order
to gain a better understanding of the role played by the var-
ious factors that determine the performance of this kind of
molecular machine.
[24]
in Scheme 1 — the ‘west side’ 1·2PF6, which constitutes
Results and Discussions
2
+
a light-harvesting moiety, composed of a [Ru(bpy)3] com-
In order to have a set of data that is as meaningful as possi-
ble, we have investigated the properties of both of the bistable
plex, tethered to a rigid p-terphenylene-type spacer con-
taining a reactive benzyl bromide functionality, whereas the
6
+
6+
,
rotaxaneBR-II anditsdumbbell-shapedprecursor D-II
and we have compared the results obtained in this Mark II
[24]
‘
east side’ 2·3PF6 constitutes
the mechanical shuttling
moiety containing a tetraarylmethane stopper adjacent to a
[24,25]
series with those previously reported
in the Mark I
ꢀ ꢀ ꢀ
,3 -dimethyl-4,4 -bipyridiniumlinkedtoa4,4 -bipyridinium
3
6+
series for rotaxane BR-I , its dumbbell-shaped precursor
D-I , and compound PS as a model for the photosensi-
tizer P . The structural formulas of all these compounds are
unit by means of a trismethylene spacer. In the first instance,
the ‘east side’containing two π-electron deficient recognition
units and the ‘west side’ containing the Ru(ii)-complex were
synthesized separately. The linking of these two sides cova-
lently afforded the dumbbell-shaped compound D-II·6PF6.
6
+
2+
2
+
displayed in Fig. 1.
[
26]
Synthesis and Structural Characterization
Slippage
of the π-donating macrocycle BPP34C10 onto
the D-II6 yielded the bistable [2]rotaxane BR-II·6PF6.
+
Based on our considerable experience[24,25] gained during the
preparation of the photochemically driven molecular-level
abacus in the Mark I series, we have designed a bistable
Constructing the ‘East Side’
2+
[
2]rotaxane BR-II·6PF6, comprising (a) a [Ru(bpy)3] type
The synthesis of the compound 2·3PF6 — the ‘east side’—
[
24,29]
complex to act as the light-harvesting entity, that also serves
is outlined in Scheme 2.Treatment of the tosylate 3
with
ꢀ
ꢀ
ꢀ
ꢀ
as a stopper, (b) a 4,4 -bipyridinium unit and its 3,3 -dimethyl
derivative to be the two π-electron accepting units, (c) a
BPP34C10 macrocycle to serve as the π-electron donating
ring component, (d) a p-terphenylene-type rigid spacer to
separate the photoactive unit from the mechanical switching
elements, and (e) a tetraarylmethane unit to act as a second
an excess amount of 3,3 -dimethyl-4,4 -bipyridine in reflux-
ing MeCN in the presence of anhydrous LiBr afforded the
monoquaternary salt 4·PF6, following counterion exchange
(NH4PF6/H2O/Me2CO), in 81% yield. Alkylation of this
compound with an excess of 1,3-dibromopropane, after coun-
terion exchange (NH4PF6/H2O), provided the diquaternary

RESEARCH FRONT
1
96
V. Balzani et al.
N
TsO
PF6ꢁ
O
ꢀ
N
(
a)
O
O
O
3
4•PF6
(
b)
N
ꢀ
N
(c)
Br
ꢀ
ꢀ
PF6ꢁ
N
N
2PF6ꢁ
3
ꢀ
ꢀ
N
N
O
O
O
O
2•3PF6
5•2PF6
ꢀ
ꢀ
Scheme 2. Synthesis of 2·3PF6. Reagents and conditions: (a) (1) 3,3 -dimethyl-4,4 -bipyridine/LiBr/MeCN, reflux, 3 days;
(
2) NH4PF6/H2O/Me2CO, RT, 12 h. (b) (1) 1,3-dibromopropane/MeCN, reflux, 4 days; (2) NH4PF6/H2O/Me2CO, RT, 12 h. (c)
ꢀ
(
1) 4,4 -bipyridine/LiBr/MeCN, reflux, 7 days; (2) NH4PF6/H2O, RT, 12 h.
(a)
(b)
Br
(HO)2B
Br
OTBDMS
OTBDMS
OH
6
7
8
OH
(
c)
Br
(e)
(d)
N
N
N
N
Br
Br
1
1
10
9
(f)
OH
Br
2PF6ꢁ
2
PF6ꢁ
N
N
N
N
N
(g)
N
N
N
N
RuII
N
RuII
N
N
1
2
•2PF6
1•2PF6
n
◦
◦
Scheme 3. Synthesis of 1·2PF6. Reagents and conditions: (a) (1) Bu Li/THF, −78 C, 1 h; (2) B(OMe)3, −78 C to RT, 16 h; (3) 5%
HCl, 30 min. (b) (1) 1,4-dibromo-2,3,5,6-tetramethylbenzene/2 M Na2CO3/Pd(PPh3)4/PhMe/EtOH, reflux, 3 days; (2) TBAF/THF, RT,
◦
1
2 h. (c) HBr/AcOH (45% w/v), reflux, 12 h. (d) dmbpy/LDA/THF, −78 C to RT, 12 h. (e) (1) 7/2 M Na2CO3/Pd(PPh3)4/PhMe/EtOH,
ꢀ
reflux, 3 days; (2) TBAF/THF, RT, 12 h. (f) (1) [Ru(4,4 -dmbpy)2Cl2]/EtOH/H2O, reflux, 24 h; (2) NH4PF6/H2O, RT, 12 h. (g) (1) HBr/
AcOH (45% w/v), reflux, 4 h; (2) NH4PF6/H2O, RT, 12 h.
Constructing the ‘West Side’
salt 5·2PF6 in 55% yield. Reaction between 5·2PF6 and
ꢀ
4
,4 -bipyridine in MeCN under refluxing conditions gener-
The synthesis of the ruthenium complex 1·2PF — the
6
ated, after counterion exchange (NH4PF6/H2O), the trisqua-
‘west side’ — is outlined in Scheme 3. The protected
ternary salt 2·3PF6 in 67% yield.
4-bromobenzyl alcohol 6 was converted into the boronic acid

RESEARCH FRONT
Shuttling Mechanisms in Two Rotaxane-Based Sunlight-Powered Nanomotors
197
N
Br
ꢀ
ꢀ
2
PF6ꢁ
N
3PF6ꢁ
ꢀ
N
N
N
N
N
N
RuII
N
ꢀ
1•2PF6
(a)
2•3PF6
N
O
O
ꢀ
N
ꢀ
N
PF6ꢁ
6
D-II•6PF6
ꢀ
N
N
N
N
N
N
RuII
N
(b)
ꢀ
N
O
O
O
O
O
ꢀ
N
O
O
ꢀ
O
N
O
PF6ꢁ
6
O
ꢀ
N
O
N
N
N
N
N
O
RuII
N
ꢀ
N
BR-II•6PF6
O
O
Scheme 4. Synthesis of BR-II·6PF6. Reagents and conditions: (a) (1) MeCN, reflux, 6 days; (2) NH4PF6/H2O, RT, 12 h. (b) BPP34C10/MeCN,
◦
5
0 C, 4 days; (2) NH4PF6/H2O, RT, 12 h.
n
derivative 7 in 79% yield by treatment with Bu Li, followed
The benzyl alcohol functionality was then converted to the
corresponding benzyl bromide 1·2PF6 in 90% yield by treat-
ing 12·2PF6 with HBr in AcOH (45% w/v), followed by
counterion exchange (NH4PF6/H2O).
◦
0
by trimethyl borate at −78 C.A Pd -catalyzed cross coupling
of the boronic acid 7 with 1,4-dibromo-2,3,5,6- tetramethyl-
benzene, followed by desilylation with tetrabuylammonium
fluoride (TBAF), afforded the benzyl alcohol 8 in 75% over-
all yield. The alcohol 8 was then transformed into the benzyl
bromide 9 in 89% yield by treatment with HBr in AcOH
Attaching the ‘East Side’to the ‘West Side’
A mixture of 1·2PF and 2·3PF in MeCN was reacted
6
6
ꢀ
ꢀ
(
45% w/v). Monolithiation of 4,4 -dimethyl-2,2 -bipyridine
under refluxing condition to obtain the dumbbell compound
withequimolaramountoflithiumdiisopropylamide(LDA)in
D-II·6PF in 57% yield, after counterion exchange (NH PF /
6
4
6
tetrahydrofuran(THF), followedbyadditionofthebromide9,
H O), Scheme 4.
2
0
generated the bipyridine derivative 10 in 66% yield. A Pd -
Generating the Bistable [2]Rotaxane by Slippage
catalyzed cross coupling of the bromides 10 and 6, followed
by desilylation withTBAF, produced the benzyl alcohol 11 in
Heating of the dumbbell compound D-II·6PF with six
6
◦
6
6
2% yield. The Ru(ii) complex 12·2PF6 was synthesized in
equivalents of BPP34C10 in MeCN at 50 C for 4 days
ꢀ
ꢀ
[24]
6% yield by exchanging a 4,4 -dimethyl-2,2 -bipyridine lig-
afforded (Scheme 4) the bistable [2]rotaxane BR-II·6PF ,
6
ꢀ
ꢀ
and from [Ru(4,4 -dimethyl-2,2 -bipyridine)2]Cl2 complex
with 11 under refluxing conditions in a EtOH/H2O mixture,
followed by counterion exchange (NH4PF6/H2O/Me2CO).
after column chromatography (SiO : MeOH/2 M NH Cl/
2
4
MeNO 7/2/1) and counterion exchange (NH PF /H O), in
2
4
6
2
52% yield.

RESEARCH FRONT
1
98
V. Balzani et al.
The rotaxane BR-II·6PF6, its corresponding dumbbell
D-II·6PF6, and the intermediate components have all been
BR-II6ꢀ
1
13
characterized by H and C NMR spectroscopy, absorp-
tion and luminescence spectroscopy, mass spectrometry, and
elemental analyses, as well as by cyclic voltammetry.
10 ꢂA
Electrochemical Behavior
All electrochemical experiments were carried out in argon-
purged MeCN solution at room temperature. Fig. 2 shows the
cyclic voltammetric curves recorded for the bistable rotax-
D-II6
ꢀ
6
+
6+
ane BR-II
and its dumbbell-shaped precursor D-II .
The results obtained are summarized in Table 1, where,
for comparison purposes, the data relating to the bistable
6
+
6+
rotaxane BR-I , its dumbbell-shaped precursor D-I , the
BPP34C10 macrocycle M and some model compounds of
the two bipyridinium stations, are also displayed. The data
6
+
6+
recorded in Table 1 show that BR-II and D-II exhibit,
respectively, ten and nine redox waves, that can be assigned
in a straightforward manner. In particular, as far as the two
electron-accepting stations are concerned, we should note the
ꢀ
1.5
ꢀ1.0
ꢀ0.5
0
ꢁ0.5
ꢁ1.0
ꢁ1.5
E (V vs SCE)
following. (a) For both D-II6 and D-I , the first reduc-
+
6+
2
+
tion potential can be assigned to the A1 station and the
Fig. 2. Cyclic voltammetric curves (MeCN, room temperature, scan
−
1
6+
6+
second one to the A22+ one. (b) For both the Mark I and
rate 200 mV s ) for BR-II and D-II in the potential range −1.5
and +1.6V versus SCE.
II variants, the first reduction wave is displaced to more
negative values in going from the dumbbell to the bistable
rotaxane; this behavior confirms that the stable translational
isomer is the one in which the BPP34C10 macrocycle M
encircles the A1 station. (c) The second reduction wave
is also displaced to more negative potentials in both vari-
ants on passing from the dumbbell to the bistable rotaxane;
this result indicates that the BPP34C10 macrocycle M encir-
with the consequent displacement of the BPP34C10 macro-
2+
+
2+
cycle M onto A2 . Reoxidation of A1 to A1 allows the
2
+
2+
macrocycle M to return to the A1 station. These results
showthattheelectrochemicallydrivenshuttlingtakesplacein
6+
[24]
6+
BR-II exactly as it was reported previously for BR-I .
The only difference between the two bistable rotaxanes is that
2
+
2+
+
cles the A2 station after A1 has been reduced to A1 .
d) The small influence of the p-terphenyl unit in stabilizing
the donor–acceptor interaction between BPP34C10 macro-
the BPP34C10 macrocycle M moves in opposite directions,
(
2+
6+
namely towards the sensitizer P in BR-I and towards the
6+
stopper T in BR-II
.
2
+
cycle M and theA2 station, whose reduction shifts more to
6+
6+
negative potentials in going from BR-I to BR-II com-
pared with the corresponding dumbbell-shaped components,
is evident.
Photochemical and Photophysical Behavior
It is well known that Ru(ii) complexes of polypyridine lig-
[
27]
In conclusion, the electrochemical results show that, in the
andsexhibit outstandingexcitedstateandredoxproperties
6+
ground state of the bistable rotaxane BR-II , the BPP34C10
and that their photoinduced electron-transfer processes with
2
+
[28]
macrocycle M encircles the A1 station and that a one-
bipyridinium-type compounds are fully reversible.
have shown
We
2
+
[24]
electron reduction causes the deactivation of theA1 station
previously that excitation with visible light
Table 1. Electrochemical dataA
Compound
Stopper
Crown ether
Ru
Viologen unit(s)
bpy ligands
6
+
B
C,D
E
E
E
BR-II
+1.64
+1.41
+1.38
+1.23
+1.13
+1.14
+1.14
+1.15
−0.43
−0.35
−0.44
−0.36
−0.77
−0.69
−0.73
−0.68
−0.83
−0.83
−0.83
−0.83
−1.04
−1.00
−1.00
−0.96
−1.44
−1.44
−1.43
−1.43
−1.6
−1.9
D-II6
+
+
+1.60
B
−1.6
E
−2.0
6
B
C,D
E
BR-I
+1.70
−1.62
−1.59
6
+
B
E
D-I
BPP34C10
+1.60
C
C
+
1.36
2
+
PS
1
+1.15
−1.43
−1.63
−1.87
ꢀ
ꢀ
,1 -Dibenzyl-4,4 -bipyridinium
dication
,1 -Dibenzyl-3,3 -dimethyl-
−0.36
−0.78
ꢀ
ꢀ
1
4
−0.74
−0.94
ꢀ
,4 -bipyridinium dication
A
B
D
Halfwave potential values, V versus SCE; reversible and monoelectronic processes unless otherwise noted; see Experimental for details.
C
Irreversible process; potential value estimated from DPV peaks. Poorly reversible process; potential value estimated from DPV peaks.
Two-electron process. E The observation of this process is hampered by adsorption phenomena.

RESEARCH FRONT
Shuttling Mechanisms in Two Rotaxane-Based Sunlight-Powered Nanomotors
199
6
+
10.0
of the bistable rotaxane BR-I and the dumbbell precur-
6
+
sor D-I does indeed cause an electron transfer from the
stopper to the A1 station. It transpires that this mech-
anism also operates in the case of both the bistable rotaxane
BR-II and the dumbbell precursor D-II . Also, in both
cases, the photoinduced electron transfer is followed by back
electron transfer.
80
40
0
2+
2+
P
8.0
6.0
4.0
2.0
0
6
+
6+
The spectroscopic and photophysical data obtained for
600
700
800
6
+
6+
BR-II and D-II are collected in Table 2 where the data
l (nm)
[24,25]
6+ 6+
previously for rotaxane BR-I , D-I , and
reported
the model compound PS are also shown for compari-
2
+
6+
son purposes. The absorption spectra of BR-II (Fig. 3)
6
+
and D-II are commensurate with what would be predicted
on the basis of the contributions from their chromophoric
units. It should be noted that the weak charge transfer
band (λmax ca. 450 nm, εmax ca. 500 L mol cm
arising from the interaction between A1
300
400
l (nm)
500
600
−
1
−1 [29]
)
2
+
Fig. 3. Absorption and (inset) luminescence (MeCN, room tempera-
and M can-
ture, full line; butyronitrile rigid matrix, 77 K, dashed line) spectra of a
not be observed because it is hidden by the much more
intense metal-to-ligand charge transfer (MLCT) band
solution of BR-II6 . Excitation was performed at 450 nm.
+
[
27]
−
1
−1
2+
(
λmax = 458 nm with εmax ca. 13400 L mol cm for 3
)
the rate of the quenching process and therefore the efficiency
of the forward electron transfer. This effect can hardly be
attributed to the very small increase in the exoergonicity of
2+
associated with the ruthenium-based component P . Upon
excitation at 450 nm, both BR-II6 (Fig. 3) and D-II
+
6+
[
24]
exhibit the well known emission band
of component P . Both the lifetime and emission inten-
sity are quenched (Table 2) when compared with the values
found for the model compound PS . Under the exper-
with λmax = 618 nm
6+
the electron transfer process — in both the BR compounds,
2
+
2+
theA1 station is reduced at a potential 0.01V less negative
6+
than in the D compounds (Table 1). It seems more likely
that the increase in the electron transfer rate is related to
2
+
−
6
−4
−1
imental conditions employed (5 × 10
to 10 mol L
2+
2+
the decrease in the distance between P and A1 , that, in
solutions), the occurrence of dynamic quenching can be
ruled out since the luminescence lifetime does not show
any concentration dependence. The quenching constants ket
and yields of the electron transfer quenching process, Φet,
have then been obtained (Table 3) from Eqns (1) and (2),
where τ0 is the luminescence lifetime of the reference
6+
6+
6+
BR-II and D-II , is about 1 nm shorter than in BR-I
6+
and D-I . It should also be noted that for the dumbbell-
shaped compound the electron transfer efficiency increases
by a factor of 1.5, whereas for the bistable rotaxane it
increases by a factor of approximately 4. This behavior sug-
gests that the BPP34C10 electron-donating macrocycle M
2
+
compound PS
2+
that surroundsA1 favors the electronic interaction between
2+
2+
.
P
and A1
ket = 1/τ − 1/τ0
Φet = ketτ
No quenching is observed in a rigid matrix at 77 K, as
expected for a moderately exoergonic electron transfer pro-
cess because of the much larger solvent reorganizational
energy. Comparison with the data reported
shows that the quenching of the luminescent excited state of
component P is much more efficient for BR-II and D-
II than it is for BR-I and D-I .This observation means
(1)
(2)
Continuous Irradiation in the Presence of Triethanolamine
[
30–32]
Itiswellknown
thatthephotoreductionofbipyridinium
ions (Q2 ) by light excitation of ruthenium polypyridine
+
2
+
compounds (e.g. [Ru(bpy)3] , Eqn 3) is followed by a
very fast back electron-transfer reaction (Eqn 4). As a con-
sequence, transient formation of the Q species can only
been observed in flash photolysis experiments. However,
when the photoreaction is carried out in the presence of
triethanolamine (TEOA), which is able to scavenge the oxi-
[24,25]
previously
+
2
+
6+
6
+
6+
6+
2+
that the swapping of the positions of the two stations A1
and A2 along the dumbbell-shaped component increases
2
+
3+
dized [Ru(bpy)3] -type species (Eqn 5) without appreciable
Table 2. Absorption and luminescence dataA
Compound
Absorption
LuminescenceB
Luminescence 77 KB,C
λmax [nm] τ [µs]
−
−1
λmax [nm]
ε [L mol 1 cm
]
λmax [nm]
Φ
τ [ns]
6
+
BR-II
458
458
458
458
458
14200
14500
14800
15300
13400
620
620
618
618
618
0.028
0.042
0.057
0.053
0.065
440
640
770
730
880
591
591
592
592
592
4.8
4.6
5.0
5.0
5.0
6
+
+
D-II
6
BR-I
6
+
D-I
2
+
PS
AData obtained in degassed MeCN solution at room temperature, unless otherwise noted. BExcitation at 450 nm. CButyronitrile rigid matrix.

RESEARCH FRONT
2
00
V. Balzani et al.
BR-II6 or D-II . Exactly the same behavior was
+
6+
Table 3. Electron transfer dataA
[
24]
6+
6+
observed
previously for BR-I and D-I .
−
−1
]
Compound
ket [s 1]
Φet
τbet [µs]
kbet [s
6
+
6
6
Flash Photolysis Experiments
BR-II
1.1 × 10
4.3 × 10
1.6 × 10
2.3 × 10
0.50
0.27
0.13
0.18
<0.2
<0.2
>5 × 10
>5 × 10
1.4 × 10
7.0 × 10
D-II6
+
+
5
5
5
6
[24,25]
We have reported
previously that, in the case of model
6
B
5 B
5 B
BR-I
D-I
7.1
1.4
compound PS2 , where photoinduced electron transfer can-
not take place because it does not contain any electron
acceptor unit, the observed spectral changes were consistent
+
6
+
B
A
Subscripts: et: forward electron transfer (Fig. 5, step 2). bet: back
electron transfer (Fig. 5, steps 5 and 6). Obtained by nanosecond laser
flash photolysis experiments at 299 K, see ref. [25].
B
2+
with the formation of the excited state of the P unit. An
isosbestic point between the absorption spectra of the ground
and excited states of the P2 unit in this model compound was
found at λiso = 398 nm and was then used to measure the elec-
tron transfer rate constants (see below). Upon flash excitation
+
2
+
quenching of the *[Ru(bpy)3] -type excited states, the
reduced Q species can accumulate
+
of BR-I6 in deaerated MeCN solutions, transient absorp-
+
∗
Ru(bpy)3] + Q2+ → [Ru(bpy)3] + Q+
[
Ru(bpy)3]3 + TEOA → [Ru(bpy) ] + Products (5)
2+
3+
[
(3)
(4)
[
25]
tion spectral changes were also observed.
spectrum recorded 6 µs after light excitation displayed the
features of the monoreduced 4,4 -bipyridinium unitA1 , and
The transient
Ru(bpy)3]3 + Q → [Ru(bpy) ] + Q
+
+
2+
2+
3
+
2+
ꢀ
+
[
3
some residual bleaching in the spectral region of the ground
6
+
6+
When photoexcitation of BR-II or D-II (ca. 5 ×
2+
state absorption of P . There was no evidence for the for-
−
5
−1
1
0
mol L ) was performed with a continuous 436-nm
ꢀ
ꢀ
mation of the monoreduced 3,3 -dimethyl-4,4 -bipyridinium
−
1
light in a degassed MeCN solution containing 0.05 mol L
+
6+
unit, A2 . The same results were obtained for D-I . For
TEOA, strong spectral changes (see e.g. Fig. 4) were
observed. Comparison with the known spectra of model
compounds[ shows clearly that the photochemical reaction
causes a permanent reduction of A1 to A1 . After 30 min
of irradiation, 100% of the A1 units were monoreduced.
Continuingirradiationfor30additionalminutesdidnotcause
any further spectral change, showing that the A1 species
are stable and that the A2 units are not reduced under the
6+
6+
both BR-I and D-I , the formation and decay of the
+
A1 unit was monitored at λ = 398 nm and the rate con-
iso
24]
2+
2+
+
stants for the forward (*P to A1 ) and the back (A1 to
2
+
+
3+
P
) were measured (Table 3) at five different temperatures.
2
+
The quantum yields of the photoinduced electron transfer
processes obtained from transient absorption data were in
agreement, within experimental error, with those found from
luminescence measurements.
+
2
+
[
33]
+
experimental conditions used — the spectrum
of A2 is
6+
When the same experiments were performed on BR-II
substantially different from that[ of A1 . If dioxygen was
34]
+
6+
and D-II , the results obtained were quite different. For both
allowed to enter the irradiated solution, the complete disap-
pearance of the characterisitic bands of A1 was observed,
with recovery of the original spectroscopic properties of
compounds there was no spectral change at λiso = 398 nm.
+
Since we know from the experiments performed in the pres-
2+
ence ofTEOA that the quenching of the *P excited state by
2+
6+
6+
A1 in BR-II and D-II is indeed caused by an electron
+
transfer process with formation of A1 (see above), we must
f
+
3+
concludethatA1 cannotaccumulateunlesstheoxidized P
species is rapidly scavenged.This observation means that, for
e
1
1
0
.5
.0
.5
0
d
c
b
a
BR-II6 and D-II , the rate of the back electron transfer
+
6+
+
3+
from A1 to P is faster than the forward reactions, and so
is contrary to what happens
D-I
In the previous paper
[
25]
6+
in the case of BR-I and
6+
.
[
25]
we used phenothiazine as elec-
f
e
d
c
b
a
6+
tron relay. With the present BR-II shuttle we could not
do that because, in order to compete with the very fast back
electron transfer process, a too high concentration of pheno-
thiazine should have been used. Under such conditions direct
reductive quenching of the *P2 excited state
prevented the forward electron transfer.
+
[35]
would have
3
00
400
500
600
l (nm)
700
800
Shuttling of the Macrocycle
The bistable rotaxane BR-I6 was synthesized with the
aim
+
6
+
Fig. 4. Spectral changes observed upon photoexcitation of BR-II
(
4.7 × 10− mol L ) with 436-nm light in the presence of 0.05 mol L
5
−1
−1
[24,25]
of obtaining an autonomous nanomotor pow-
ered solely by visible light. Subsequently, we synthesized
BR-II to gain a better understanding of the role played by
the structure of the dumbbell with the purpose of optimiz-
ing the performance of these sunlight-powered nanomotors.
The suggested mechanism, illustrated in the left part of Fig. 5
triethanolamine in a degassed MeCN solution at room temperature. Irra-
diation times are (a)–(f) 0, 1, 3, 5, 10, and 30 min. Prolonging irradiation
for 30 more minutes did not cause further spectral changes. Upon expo-
sure of the irradiated solution to air for 2 min, the absorption spectrum
represented with a dotted line was obtained. Similar spectral changes
6+
were observed in the case of D-II6
+
.

RESEARCH FRONT
Shuttling Mechanisms in Two Rotaxane-Based Sunlight-Powered Nanomotors
201
for BR-II6 , is based on the following four phases (see also
Fig. 6)
+
M (step 2), with the consequent ‘deactivation’of this sta-
tion; such a photoinduced electron-transfer process has
to compete with the intrinsic decay of *P2 (step 3).
+
(
a) Destabilization of the stable co-conformation: Light exci-
2
+
tation of the photoactive unit P (step 1) is followed by
(b) Macrocycle displacement: After reduction (‘deactiva-
2
+
2+
+
the transfer of an electron from the *P excited state to
tion’) of the A1 station to A1 , the macrocycle moves
2
+
2+
the A1 station, which is encircled by the macrocycle
by Brownian motion to A2 (step 4), a step that has
Fig. 5. Mechanisms of the photochemically driven ring shuttling in BR-II6 . Left, intramolecular mechanism. Right, mechanism assisted by two
+
low-energy fuels, namely triethanolamine (TEOA) and dioxygen. For more details, refer to the text.
6
+
6+
Fig. 6. Schematic energy level diagrams for the processes taking place in BR-II (left-hand side) and BR-I (right-hand side) after excitation
6+
with visible light. The process indicated with a dashed arrow for BR-II does not occur in practice. For more details, refer to the text.

RESEARCH FRONT
2
02
V. Balzani et al.
+
to compete with back electron-transfer from A1 to the
(Fig. 6). Since the rate of the displacement of the macro-
3
+
4
−1
6+
[25]
oxidized photoactive unit P (step 5).
c) Electronic reset: A back electron-transfer process from
cycle is very slow (1.3 × 10 s for BR-I at 299 K),
(
the macrocycle displacement efficiency η , which is 0.09 for
rd
6+
+
3+
6+
the ‘free’reduced stationA1 to P (step 6) restores the
BR-I at 299 K, is negligible for BR-II
.
2
+
electron acceptor power of the A1 station.
(
d) Nuclear reset: As a consequence of the electronic reset,
the macrocycle M moves back again by Brownian motion
Conclusions
Whereaselectrochemicallythereisnothingtochoosebetween
2
+
2+
from A2 to A1 (step 7).
the behavior of the Mark I and II versions — namely the
constitutional isomers BR-I6 and BR-II , respectively —
+
6+
The quantum yield of the shuttling process (Φ , number
sh
of shuttling events divided by number of absorbed photons)
is given by Eqn (6)
photochemically they behave quite differently. The closer the
2+
electron transfer photosensitizer P to the better electron
acceptorA12 in the Mark II compared with the Mark I shut-
tle, the faster are the rates of both the photoinduced electron
transfer and the back electron transfer steps.The consequence
is that, while the Mark II version performs best when it is
fuel-assisted, it is nothing like as efficient as the Mark I sys-
tem when it is powered by visible light (sunlight) on its own.
In the electrochemically driven situation, the only difference
between the two bistable rotaxanes is that the macrocycle
+
Φ
sh
= Φet × η × ηer × ηnr
(6)
rd
where Φet is the quantum yield of the photoinduced electron
transfer process that causes the destabilization of the initial
co-conformation, η the efficiency of ring displacement, ηer
rd
the efficiency of the electronic reset, and ηnr the efficiency
of the nuclear reset. Since the efficiency of the nuclear reset,
ηnr, is equal to 1 (k7 does not compete with any other step),
Eqn (6) reduces to
2
+
movesinoppositedirections, i.e. towardsthesensitizer P in
the Mark I rotaxane and towards the stopper T in the Mark II
rotaxane.
Φ
sh
= Φet × η × ηer
(7)
rd
Experimental
In this paper, we have demonstrated that shuttling of the
macrocycle M can be achieved easily by alternate electro-
Materials and Techniques
2
+
chemical reduction/reoxidation of A1 , as well as by a
photochemical mechanism involving the assistance of two
low energy fuels. In the latter case (right part of Fig. 5),
All reactions were carried out under N atmospheres. Chemicals were
purchased from Aldrich and used as received. 2-[4-[4-ethylphenyl-
2
bis(4-tert-butylphenyl)methyl]phenoxy]ethoxyethanol 4-methyl ben-
[
24,29]
ꢀ
ꢀ
[36]
2+
zenesulfonate
(3), 3,3 -dimethyl-4,4 -bipyridine,
4-bromo-
after the photoinduced electron transfer process from *P to
[
37]
benzyl tert-butyldimethylsilyl ether
(6), 1,4-dibromo-2,3,5,6-tetra-
2
+
A1 , the sacrificial reductantTEOA donates irreversibly one
[38]
ꢀ
ꢀ
[39]
methylbenzene,
[Ru(4,4 -dimethyl-2,2 -bipyridine)2Cl2·2H2O],
[40]
3+
electron to the oxidized P unit (step 8) before back elec-
and BPP34C10 were synthesized
according to literature proce-
+
ꢀ
ꢀ
tron transfer (step 5) can occur and, as theA1 unit is formed,
dures. The 4,4 -dimethyl-2,2 -bipyridine, tert-butyldimethylsilyl, and
p-toluenesulfonyl groups are identified by the abbreviations dmbpy,
TBDMS, and Ts, respectively. Yields refer to chromatographically
pure products. Thin-layer chromatography (TLC) was carried out on
aluminum sheets coated with silica gel 60 (Merck 5554). Column
chromatography was performed on silica gel 60 (Merck, 40–60 nm).
Melting points were determined on an Electrothermal 9200 melting
point apparatus. Microanalysis were performed by the University of
Sheffield Microanalytical Laboratories or by the University of North
London Microanalytical Services. Liquid secondary ion mass spectra
2
+
the macrocycle M moves to A2 (step 4). Upon addition of
+
a sacrificial oxidant (e.g. O2) to the solution, the A1 unit
is back oxidized to A12 (step 9) and the macrocycle moves
+
back to the original position (step 7).
Under the experimental conditions used, the scavenging
of P3 by TEOA and the back oxidation of A1 to A12+
+
+
by dioxygen are 100% efficient, so that the efficiency of the
shuttling process depends only on the competition between
(
LSIMS) were recorded on a VG ZabSpec mass spectrometer equipped
2
+
the transfer of an electron from the *P excited state to the
with a Cs ion source using m-nitrobenzyl alcohol containing a trace of
NaOAc. For accurate mass measurements using high-resolution LSIMS
2
+
2+
A1 station and the intrinsic decay of *P (step 3), that is
(
HRLSIMS), the instrument was operated at a resolution of ca. 6000 by
Φ
= Φet
(8)
narrow-range voltage scanning along with polyethylene glycol or CsI as
reference compounds. Electrospray ionization mass spectra (ESIMS)
were measured on a VG ProSpec triple focussing mass spectrometer
using MeCN as mobile phase. Electron impact (EI) MS were recorded
at 70 eV on a VG ProSpec mass spectrometer. H and C NMR and
spectra were recorded on either a BrukerAC300 or a BrukerAMX400 or
a Bruker DRX500 spectrometer. The following abbreviations are used
for the signal multiplicities or characteristics: s singlet, d doublet, dd
double doublet, t triplet, m multiplet, q quartet, and br broad.
sh
The data obtained (Table 3) show that such a fuel-assisted
process is more efficient in the case of rotaxane BR-II
than it is in the case of BR-I . This result was indeed
expected because the swapping of the two stations decreases
the distance between the two reaction partners in the Mark II
version.
6+
1
13
6
+
The swapping of the two stations, however, increases, not
only the rate of the forward, but also that of the back electron
transfer process. Indeed we have found that, in the case of
N-[2-[4-[4-Ethylphenyl-bis(4-tert-butylphenyl)methyl]-
phenoxy]ethoxy]ethyl] 3 ,3 -Di-methyl-4,4 -bipyridinium
Hexafluorophosphate 4·PF6
ꢀ
ꢀ
ꢀ
6
+
+
BR-II , no transient formation of A1 can be observed in
flash photolysis experiments, indicating that the rate of the
back-electron transfer reaction must be faster than that of
A
mixture of 2-[4-[4-ethylphenyl-bis(4-tert-butylphenyl)methyl]
[24,29]
phenoxy]-ethoxyethanol 4-methyl benzene-sulfonate
(3) (1.64 g,
.28 mmol), 3,3 -dimethyl-4,4 -bipyridine (2.34 g, 12.72 mmol), and
anhydrous LiBr (70 mg) in dry MeCN (14 mL) was heated under reflux
ꢀ
ꢀ
[34]
2
6
−1
the forward electron transfer process by about 5 × 10 s

RESEARCH FRONT
Shuttling Mechanisms in Two Rotaxane-Based Sunlight-Powered Nanomotors
203
for 3 days. The solvent was evaporated to dryness and the crude prod-
uct was purified by column chromatography (SiO2/the first eluent was
(75 mL). After purging the mixture with N2 for 15 min, the boronic acid
7 (2 g, 7.52 mmol), dissolved in EtOH (8 mL), was added. The reac-
tion mixture was heated under reflux for 3 days, during which time it
turned black. The mixture was cooled down to room temperature before
being diluted with EtOAc (80 mL) and H2O (80 mL). The aqueous layer
was discarded and the organic layer was washed with H2O, brine and
dried (MgSO4). Evaporation of solvent in vacuo gave a residue from
which an excess of 1,4-dibromo-2,3,5,6-tetramethylbenzene was recov-
ered by crystallization from MeOH. After removal of solvent from the
mother liquor, the residue was subjected to desilylation by treating with
TBAF (1 M solution in THF, 25 mL, 25 mmol) in THF (75 mL) at room
temperature for 12 h. The solvent was removed under reduced pressure
and the crude product was purified by column chromatography (SiO2:
ꢀ
ꢀ
MeOH to remove an excess of 3,3 -dimethyl-4,4 -bipyridine and the
second eluent was MeOH/MeNO2/2 M NH4Cl 8/1.9/0.1). The isolated
residue was dissolved in H2O/Me2CO and a saturated aqueous solution
of NH4PF6 was added. After the evaporation of the Me2CO, the pre-
◦
cipitated solid was filtered off, washed with H2O, and dried (70 C/0.1
◦
Torr) to yield 4·PF6 (1.62 g, 81%). δH (300 MHz, CD3CN, 25 C) 1.18
(
t, J 8 Hz, 3H), 1.27 (s, 18H), 2.03 (s, 3H), 2.08 (s, 3H), 2.58 (q, J 8 Hz,
2
(
6
H), 3.79–3.83 (m, 2H), 3.99–4.07 (m, 4H), 4.68 (t, J 4.7 Hz, 2H), 6.76
d, J 8.9 Hz, 2H), 7.09–7.14 (m, 10H), 7.27 (d, J 8.5 Hz, 4H), 7.69 (d, J
.2 Hz, 2H), 8.60 (d, J 6.2 Hz, 1H), 8.67 (s, 1H).
ꢀ
◦
2 2
CH Cl ) to yield the alcohol 8 (1.8 g, 75%), as a white solid. mp 112 C.
N-(3-Bromopropyl)-N -[2-[4-[4-ethylphenyl-bis(4-tert-
ꢀ
ꢀ
◦
H 3
δ (300 MHz, CDCl , 25 C) 1.80 (s, 1H), 1.96 (s, 6H), 2.45 (s, 6H),
butylphenyl)methyl]phenoxy]ethoxy]-ethyl] 3 ,3 -Dimethyl-
ꢀ
ꢀ
ꢀ
4
,4 -bipyridinium Bis(hexafluorophosphate) 5·2PF6
4.77 (s, 2H), 7.07, 7.09, 7.41, 7.44 ppm (AA BB , 4H). δ (75.5 MHz,
C
◦
CDCl3, 25 C) 19.2, 21.2, 65.3, 127.2, 128.4, 129.4, 133.5, 133.8, 139.2,
A solution of 4·PF6 (1.01 g, 1.15 mmol) and 1,3-dibromopropane
1
40.9, 141.6. (Found: C 64.08, H 6.12. Calc. for C17H19BrO: C 63.96,
(
2.32 g, 11.5 mmol) in dry MeCN (15 mL) was heated under reflux for 4
H 6.00%.)
days. The solvent was removed under reduced pressure and the residue
was purified by column chromatography (SiO2: MeOH/MeNO2/2 M
NH4Cl 8/1.9/0.1) to afford, after counterion exchange (H2O/Me2CO,
a saturated aqueous solution of NH4PF6), 5·2PF6 (720 mg, 55%) as a
4
-(4-Bromo-2,3,5,6-tetramethylphenyl)benzyl Bromide 9
A solution of the alcohol 8 (2.3 g, 7.2 mmol) in 40 mL HBr/AcOH (45%,
w/v) was heated under reflux for 12 h before being cooled down to
room temperature and poured into ice-water (200 mL). The mixture was
neutralized with 5% NaHCO and extracted several times with CH Cl .
◦
white-gray solid. δH (300 MHz, CD3COCD3, 25 C) 1.19 (t, J 8 Hz, 3H),
1.28 (s, 18H), 2.38 (s, 3H), 2.40 (s, 3H), 2.60 (q, J 7.5 Hz, 2H), 2.70–
2.82 (m, 2H), 3.65 (t, J 6.5 Hz, 2H), 3.91 (t, J 4.3 Hz, 2H), 4.13 (t, J
4.3 Hz, 2H), 4.23 (t, J 4.6 Hz, 2H), 4.98–5.09 (m, 4H), 6.83 (d, J 8.8 Hz,
2H), 7.07–7.13 (m, 10H), 7.30 (d, J 8.5 Hz, 4H), 8.17 (d, J 6.2 Hz, 1H),
8.24 (d, J 6.2 Hz, 1H), 9.19 (t, J 6.8 Hz, 2H), 9.30 (d, J 7 Hz, 2H).
3
2
2
The combined organic layers were washed with H O, brine and dried
2
(MgSO ). Removal of the solvent afforded the crude product, which
4
was purified by column chromatography (SiO : hexanes/CH Cl 100/4)
2
2
2
◦
to afford the bromide 9 (2.4 g, 89%) as a white solid. mp 116 C. δH
300 MHz, CDCl3, 25 C) 1.95 (s, 6H), 2.45 (s, 6H), 4.56 (s, 2H), 7.05,
.07, 7.43, 7.46 (AA BB , 4H). δC (75.5 MHz, CDCl3, 25 C) 19.1, 21.1,
3.4, 128.5, 129.2, 129.7, 133.3, 133.9, 136.2, 140.5, 142.4. m/z (EIMS,
70 eV) 382 (M , 46), 338 (13), 301 (100), 223 (22), 207 (65), 192 (46),
ꢀ
ꢀ
N-[3-(4,4 bipyridinium-1-yl)propyl]-N -[2-[4-[4-ethylphenyl-bis-
◦
(
7
3
ꢀ
ꢀ
(
4
4-tert-butylphenyl)methyl]-phenoxy]ethoxy]ethyl] 3 ,3 -Dimethyl-
ꢀ
ꢀ
◦
ꢀ
,4 -bipyridinium Tris(hexafluorophosphate) 2·3PF6
+
ꢀ
A solution of 5·2PF6 (710 mg, 0.62 mmol) and 4,4 -bipyridine (970 mg,
1
4
78 (27). (Found: C 53.53, H 4.78. Calc. for C17H18Br2: C 53.43, H
.75%.)
6
.2 mmol) in dry MeCN (14 mL) was heated under reflux for 7 days.
After removal of the solvent, the residue was purified by column
chromatography (SiO2: MeOH/2 M NH4Cl/MeNO2 7/2/1) to afford,
4
-[2-[4-[4-Bromo-2,3,5,6-tetramethylphenyl]phenyl]ethyl]-
after counterion exchange (50% aqueous solution of NH4PF6), 2·3PF6
ꢀ
ꢀ
◦
4 -methyl-2,2 -bipyridine 10
(
570 mg, 67%) as a light brown solid. δH (300 MHz, CD3CN, 25 C)
1
.18 (t, J 8 Hz, 3H), 1.27 (s, 18H), 2.20 (s, 3H), 2.23 (s, 3H), 2.59 (q,
A solution of dmbpy (1.0 g, 5.43 mmol) in dry THF (45 mL) was added
J 7.5 Hz, 2H), 2.65–2.83 (m, 2H), 3.82 (t, J 4.3 Hz, 2H), 4.06–4.11 (m,
4
7
◦
dropwise (15 min) at −78 C to a stirred solution of a freshly prepared
H), 4.67–4.78 (m, 4H), 6.82 (d, J 8.8 Hz, 2H), 7.12–7.17 (m, 10H),
.30 (d, J 8.6 Hz, 4H), 7.77–7.95 (m, 4H), 8.40 (d, J 6.8 Hz, 2H), 8.70
n
lithium di-isopropyl amine (LDA) and Bu Li (1.6 M in hexane, 3.5 mL,
5
.6 mmol) in dry THF (10 mL), which turned into a dark brown-red
(
d, J 6.2 Hz, 2H), 8.77 (br s, 2H), 8.85 (d, J 6.9 Hz, 4H).
◦
solution.Themixturewasallowedtowarmupto0 Cbeforebeingstirred
at this temperature for another 1 h. A solution of the bromide 9 (2.3 g,
4
-(tert-Butyldimethylsilyloxymethy)phenylboronic Acid 7
6
.0 mmol) in dry THF (15 mL) was added at once, turning the reaction
A solution of 4-bromobenzyl tert-butyldimethylsilyl ether[37] (6)
mixture light yellow brown. After 12 h of stirring at room temperature,
the orange solution was quenched with MeOH (2 mL) and concentrated
in vacuo. The residue was dissolved in CH2Cl2 and washed with water
and brine, and dried (MgSO4). Removal of the solvent gave the crude
product, which was subjected to flash column chromatography (SiO2:
CH2Cl2/MeOH/NH4OH 100/1/0.2) to yield the bipyridine derivative 10
◦
(
12.0 g, 40 mmol) in anhydrous THF (80 mL) was cooled to −78 C
n
under nitrogen atmosphere and a solution of Bu Li (1.6 M in hexanes,
2
7.5 mL, 44.0 mmol) was added slowly. After stirring the mixture at
◦
−
78 C for 1 h, B(OMe)3 (8.3 g, 80 mmol) was added slowly, while
◦
maintaining the temperature below −65 C and then stirring was con-
◦
◦ ◦
(1.8 g, 66%) as a white solid. mp 172 C. δH (300 MHz, CDCl3, 25 C)
tinued for another 1 h at −78 C. The mixture was allowed to warm up
ꢀ
ꢀ
to room temperature and stirred for additional 16 h. Hydrochloric acid
1.93 (s, 6H), 2.44 (s, 6H), 3.05 (s, 4H), 6.95, 6.98, 7.19, 7.21 (AA BB ,
(
5%, 40 mL) was added to the ice-cooled stirred mixture which was
4H), 7.09 (dd, J 5, 2 Hz, 1H), 7.14 (d, J 4 Hz, 1H), 8.24 (d, J 6 Hz, 2H),
◦
then extracted several times with Et2O. The combined organic layers
were washed with water and brine, and dried over MgSO4. Filtration
and evaporation of the solvent gave the crude product, which was puri-
fied by column chromatography (SiO2: hexanes/EtOAc 3/1) to yield the
boronic acid 7 (8.4 g, 79%) as a white solid. mp 144 C. δH (300 MHz,
CDCl3, 25 C) 0.12 (s, 6H), 0.97 (s, 9H), 4.84 (s, 2H), 7.44, 7.47, 8.19,
8.53 (d, J 5 Hz, 1H), 8.55 (d, J 5 Hz, 1H). δC (75.5 MHz, CDCl3, 25 C)
19.1, 21.2, 36.6, 37.5, 121.4, 122.0, 124.1, 127.7, 128.3, 128.6, 129.3,
133.6, 133.7, 139.1, 140.0, 148.2, 149.0, 151.6, 156.0, 156.3. m/z (EIMS
+
70 eV) 485 (9) [M + H] , 405 (9), 184 (34), 147 (64), 91 (77). (Found:
◦
C 71.65, H 6.05, N 5.70. Calc. for C29H29BrN2: C 71.75, H 6.02, N
5.77%.)
◦
ꢀ
ꢀ
◦
8
6
.22 ppm (AA BB , 4H). δC (75.5 MHz, CDCl3, 25 C) −5.2, 18.4, 26.0,
5.0, 125.5, 128.8, 135.7, 146.2. (Found: C 58.88, H 8.65. Calc. for
4-[2-[4-[4-Hydroxymethylphenyl]-2,3,5,6-tetramethylphenyl]ethyl]-
ꢀ
ꢀ
C13H23BO3Si: C 58.65, H 8.71%.)
4 -methyl-2,2 -bipyridine 11
A mixture of bromide 10 (1.4 g, 2.9 mmol), the boronic acid 7 (0.91 g,
4
-(4-Bromo-2,3,5,6-tetramethylphenyl)benzyl Alcohol 8
3
.4 mmol), Pd(PPh3)4 (100 mg, 3 mol%), PhMe (20 mL), aqueous 2 M
An aqueous solution of Na2CO3 (2 M solution, 10 mL, 20 mmol) and
the catalyst Pd(PPh3)4 (0.46 g, 5 mol%) were added to a solution of
Na2CO3 solution (3.5 mL, 7.0 mmol), and EtOH (2 mL) was heated
under reflux for 2 days. The reaction mixture was then cooled, H2O
(10 mL) was added, and the PhMe layer was separated. The aqueous
[
38]
1
,4-dibromo-2,3,5,6-tetrametyl benzene
(11 g, 37.6 mmol) in PhMe

RESEARCH FRONT
2
04
V. Balzani et al.
◦
layer was extracted with EtOAc (2 × 20 mL) and the combined organic
extracts were dried (MgSO4), filtered, and the solvent was removed
under reduced pressure. The residue was treated with TBAF (1 M solu-
tion in THF, 15 mL) at room temperature for 12 h. Solvent was removed
in vacuo and the crude product was purified by flash column chro-
matography (SiO2: CH2Cl2/MeOH/NH4OH, 100/2/0.35) to afford 11
(241 mg, 57%). mp 220–223 C (decomp.). δH (500 MHz, CD3COCD3,
◦
25 C) 1.22 (t, J 8 Hz, 3H), 1.32 (s, 18H), 1.82 (s, 6H), 1.90 (s, 6H),
2.40 (s, 3H), 2.42 (s, 3H), 2.54 (s, 3H), 2.56 (s, 3H), 2.57–2.58 (m, 9H),
2.63 (q, J 8 Hz, 2H), 3.10–3.21 (m, 4H), 3.22–3.29 (m, 2H), 3.95 (m,
2H), 4.17 (m, 2H), 4.28 (m, 2H), 5.11 (t, J 5 Hz, 2H), 5.17 (t, J 8 Hz,
2H), 5.26 (t, J 8 Hz, 2H), 6.32 (s, 2H), 6.87 (d, J 9 Hz, 2H), 7.01 (d, J
8 Hz, 2H), 7.10–7.17 (m, 10H), 7.28 (d, J 8 Hz, 2H), 7.29–7.34 (m, 6H),
7.34–7.40 (m, 6H), 7.77 (d, J 6 Hz, 2H), 7.81–7.87 (m, 6H), 8.21 (d,
J 6 Hz, 1H), 8.26 (d, J 6 Hz, 1H), 8.65–8.69 (m, 6H), 8.87 (d, J 7 Hz,
2H), 8.91 (d, J 7 Hz, 2H), 9.20 (d, J 7 Hz, 1H), 9.22 (d, J 7 Hz, 1H),
9.28 (s, 1H), 9.31 (s, 1H), 9.45 (d, J 7 Hz, 2H), 9.64 (d, J 7 Hz, 2H). δC
◦
◦
(
0.92 g, 62%) as a white solid. mp 194 C. δH (300 MHz, CDCl3, 25 C)
1
4
1
.63 (s, 1H), 1.92 (s, 6H), 1.93 (s, 6H), 2.44 (s, 3H), 3.07 (br s, 4H),
.77 (d, J 6 Hz, 2H), 7.07, 7.20, 7.21, 7.24 (AA BB , 4H), 8.23 (br s,
ꢀ
ꢀ
H), 8.27 (br s, 1H), 8.54 (d, J 4 Hz, 1H), 8.56 (d, J 4 Hz, 1H). δC
◦
(
75.5 MHz, CDCl3, 25 C) 18.0, 18.1, 21.2, 36.7, 37.5, 65.3, 121.4,
◦
1
1
1
22.1, 124.1, 124.7, 127.1, 128.4, 129.5, 129.7, 131.8, 132.0, 138.8,
39.0, 140.6, 140.8, 141.1, 142.2, 148.1, 148.2, 149.0, 151.8, 156.0,
56.2. m/z (LSIMS) 513.3 [M + H] . (Found: C 84.27, H 7.16, N 5.34.
(125 MHz, CD3COCD3, 25 C) 14.8, 16.2, 17.2, 17.3, 20.1, 30.6, 32.6,
33.8, 35.7, 36.6, 58.4, 58.6, 61.6, 63.0, 64.6, 67.0, 68.8, 69.4, 113.1,
124.1, 124.4, 124.8, 126.7, 127.0, 127.5, 127.6, 127.7, 127.9, 128.3,
128.6, 129.1, 129.4, 130.4, 130.5, 130.6, 131.1, 131.4, 131.8, 137.3,
+
Calc. for C36H36N2O: C 84.34, H 7.08%.)
1
38.0, 138.4, 139.5, 139.9, 140.3, 141.1, 141.4, 142.7, 143.3, 144.2,
ꢀ
ꢀ
[
2
Ru(4,4 -dmbpy)2{4-methyl-4 -(2-(4-(4-hydroxymethylphenyl)-
144.5, 144.6, 146.0, 146.1, 146.3, 146.5, 148.2, 149.9, 150.4, 150.6,
ꢀ
+
,3,5,6-tetramethyl phenyl)-phenyl-ethyl)-2,2 -bipyridine}]
152.0, 152.3, 153.1, 156.6, 156.7, 156.8. m/z (FAB) 2622 [M − PF6] ,
+
+
+
Bis(hexafluorophosphate) 12·2PF6
A mixture of [Ru(4,4 -dmbpy)2Cl2]
2474 [M − 2PF6] , 2329 [M − 3PF6] , 2184 [M − 4PF6] .
ꢀ
[39]
(0.99 g, 1.83 mmol) and alco-
hol 11 (0.85 g, 1.66 mmol) in a 3/1 EtOH/H2O mixture (60 mL) was
heated under reflux for 24 h. After removal of the solvent, the residue
was purified by flash column chromatography (SiO2: MeOH/2 M
NH4Cl/MeNO2, 7/2/1). Fractions containing the product were collected
and concentrated in vacuo. The residue was treated with an excess of a
Bistable [2]Rotaxane BR-II·6PF6
A solution of dumbbell-shaped compound D-II·6PF6 (177 mg,
0
.06 mmol) and bis-p-phenylene[34]crown-10 (206 mg, 0.38 mmol) in
◦
dry MeCN (2 mL) was stirred at 50 C for 4 days. The solvent was
removed in vacuo and the residue was purified by flash column chro-
matography (SiO2: MeOH/2 M NH4Cl/MeNO2 7/2/1) to give, after
counterion exchange (50% aqueous solution NH4PF6), the bistable
5
0% aqueous NH4PF6 solution. The red solid was filtered off, washed
◦
with H2O, Et2O, and dried in vacuo (70 C/0.1Torr) to afford the Ru
◦
complex 12·2PF6 (1.4 g, 66%). mp 210 C (decomp.). δH (300 MHz,
◦
[
(
2]rotaxane compound BR-II·6PF6 (109 mg, 52%). mp 187–191 C
◦
CD3COCD3, 25 C) 1.78 (s, 6H), 1.89 (s, 6H), 2.51–2.56 (m, 15H),
◦
decomp.). δH (500 MHz, CD3CN, 25 C) 1.15 (t, J 8 Hz, 3H), 1.24 (s,
ꢀ
ꢀ
3
4
1
1
1
1
.14 (m, 2H), 3.25 (m, 2H), 4.72 (s, 2H), 6.96, 6.99, 7.21, 7.24 (AA BB ,
18H), 1.74 (s, 6H), 1.87 (s, 6H), 2.22 (s, 3H), 2.27 (s, 3H), 2.43 (s, 3H),
2.47 (s, 3H), 2.48–2.50 (m, 9H), 2.56 (q, J 8 Hz, 2H), 2.89 (m, 2H), 3.04–
3.15 (m, 4H), 3.55–3.59 (m, 6H), 3.67–3.80 (m, 20H), 4.03–4.05 (m,
2H), 4.05–4.07 (m, 2H), 4.67–4.69 (m, 4H), 4.77–4.80 (m, 2H), 4.83–
4.87 (m, 2H), 5.97 (s, 2H), 6.12 (s, 8H), 6.80 (d, J 9 Hz, 2H), 6.90 (d, J
8 Hz, 2H), 7.08–7.18 (m, 18H), 7.27–7.36 (m, 8H), 7.48 (d, J 6 Hz, 2H),
7.49–7.51 (m, 4H), 7.76 (d, J 6 Hz, 2H), 7.81 (d, J 6 Hz, 2H), 7.88–7.93
ꢀ
ꢀ
H), 7.09, 7.12, 7.46, 7.49 (AA BB , 4H), 7.38 (m, 6H), 7.75 (d, J 6 Hz,
H), 7.82 (m, 5H), 8.65 (m, 6H). δC (75.5 MHz, CD3COCD3, 25 C)
◦
8.3, 21.1, 36.7, 37.6, 64.5, 125.4, 125.8, 127.5, 128.9, 129.3, 129.4,
29.6, 129.9, 130.0, 132.1, 132.3, 132.6, 132.7, 133.0, 139.3, 141.4,
41.5, 141.6, 141.8, 142.0, 150.8, 151.3, 151.5, 154.0, 157.6, 157.7.
+
+
+
m/z (LSIMS) 1294 [M + Na] , 1127 [M − PF6] , 981 [M − 2PF6] .
(
Found: C 56.75, H 4.75, N 6.46. Calc. for C60H60F12N6OP2Ru: C
(
m, 6H), 8.25–8.30 (m, 6H), 8.70 (d, J 7 Hz, 1H), 8.76–8.82 (m, 3H),
5
6.65, H 4.75, N 6.61%.)
+
8
2
.86–8.90 (m, 4H), 9.04 (d, J 7 Hz, 2H). m/z (FAB) 3012 [M − 2PF6] ,
+
+
2+
.
867 [M − 3PF6] , 2721 [M − 4PF6] , 1506 [M − 2PF6]
ꢀ
ꢀ
[
2
Ru(4,4 -dmbpy)2{4-methyl-4 -(2-(4-(4-bromomethylphenyl)-
ꢀ
,3,5,6-tetramethyl phenyl)-phenyl-ethyl)-2,2 -bipyridine}]
Electrochemical Measurements
Bis(hexafluorophosphate) 1·2PF6
A solution of 12·2PF6 (1.0 g, 0.79 mmol) in HBr/AcOH (12 mL, 45%
w/v) was heated under reflux for 4 h. After cooling, an excess of 50%
aqueous NH4PF6 solution was added to precipitate the product, which
Electrochemical experiments were carried out in argon-purged MeCN
solution at room temperature with an EcoChemie Autolab 30 multi-
purpose instrument interfaced to a personal computer. In the cyclic
voltammetry (CV) and differential pulse voltammetry (DPV) experi-
was then filtered off, washed with H2O, Et2O, and dried in vacuo
2
◦
ments, the working electrode was a glassy carbon electrode (0.08 cm ,
(
70 C/0.1 Torr) to give the Ru-complex 1·2PF6 (0.94 g, 90%), as an
◦
◦
Amel); its surface was routinely polished with a 0.05 µm alumina-
water slurry on a felt surface, immediately before use. In all cases,
the counter electrode was a Pt wire and the reference was a saturated
calomel electrode (SCE) separated with a fine glass frit. The concentra-
orange solid. mp 197 C (decomp.). δH (300 MHz, CD3COCD3, 25 C)
.79 (s, 6H), 1.89 (s, 6H), 2.51–2.56 (m, 15H), 3.14 (m, 2H), 3.25
1
(
ꢀ ꢀ
m, 2H), 4.76 (s, 2H), 6.97, 6.99, 7.22, 7.24 (AA BB , 4H), 7.15, 7.17,
ꢀ
ꢀ
7
7
2
1
1
.58, 7.60 (AA BB , 4H), 7.38 (m, 6H), 7.75 (d, J 6 Hz, 1H), 7.82–
.84 (m, 5H), 8.66 (m, 6H). δC (75.5 MHz, CD3COCD3, 25 C) 18.2,
1.1, 34.4, 36.7, 125.5, 125.9, 128.9, 129.3, 129.5, 129.6, 130.1, 130.2,
30.6, 132.2, 132.3, 132.6, 132.7, 132.8, 137.5, 139.3, 141.4, 141.9,
43.8, 150.9, 151.4, 151.6, 154.1, 157.7, 157.8. m/z (LSIMS) 1189
−
4
−1
−1
◦
tion of the compounds examined was 5.0 × 10 mol L ; 0.05 mol L
tetraethylammonium hexafluorophosphate was added as the supporting
electrolyte. Cyclic voltammograms were obtained as sweep rates of 20,
−
1
5
0, 200, 500, and 1000 mV s ; DPV experiments were performed with
−
1
+
+
a scan rate of 20 mV s and a pulse height of 75 mV, and a duraton of
0 ms. In order to resolve overlapping peaks, DPV experiments with a
scan rate of 4 mV s and a pulse height of 10 mV were also carried out.
Ferrocene was present as an internal standard. The experimental error
was estimated to be ± 5 mV.
[
M − PF6] , 1045 [M − 2PF6] . (Found C 54.04, H 4.37, N 6.26. Calc.
4
for C60H60BrF12N6P2Ru: C 53.98, H 4.45, N 6.29%.)
−
1
Dumbbell Compound D-II·6PF6
A mixture of 1·2PF6 (316 mg, 0.25 mmol) and 2·3PF6 (209 mg,
0
.15 mmol) in dry MeCN (15 mL) was heated under reflux in an
Photophysical and Photochemical Experiments
Ar atmosphere for 4 days. After cooling, the solvent was evaporated
and the residue was purified by fresh column chromatography (SiO2:
MeOH/2 M NH4Cl/MeNO2, 7/2/1). The fractions containing the prod-
uct were combined and concentrated in vacuo, and the residue was then
treated with an excess of a 50% aqueous NH4PF6 solution.The red solid
Measurements were carried out at 298 K on MeCN (Merck Uvasol)
−
6
−4
−1
solutions with concentrations ranging from 5 × 10 to 10 mol L
.
UV-Vis absorption spectra were recorded on air-equilibrated solutions
with a Perkin Elmer λ40 spectrophotometer. Uncorrected lumines-
cence spectra were obtained with a Perkin Elmer LS-50 or Edinburgh
Instruments FLS920 spectrofluorimeter, equipped with a Hamamatsu
was filtered off, washed with water and diethyl ether and dried in vacuo
◦
(
60 C/0.1Torr) to afford the dumbbell-shaped compound D-II·6PF6

RESEARCH FRONT
Shuttling Mechanisms in Two Rotaxane-Based Sunlight-Powered Nanomotors
R928 phototube, on solutions degassed with at least four freeze-pump-
205
[5] A. H. Flood, R. J. A. Ramirez, W. Q. Deng, R. P. Muller,
W. A. Goddard, J. F. Stoddart, Aust. J. Chem. 2004, 57, 301.
doi:10.1071/CH03307
−
9
thaw cycles and sealed under vacuum (8 × 10 bar). Luminescence
quantum yields were determined by the optically dilute method using
2
+
[41]
[
Ru(bpy)3] in air-equilibrated water (Φ= 0.028) as a standard.
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ronitrile (Fluka) rigid matrix. Photochemical experiments were carried
out by irradiation of degassed solutions (see above) at 436 nm with a Hg
medium pressure lamp (Helios Italquartz, 150W); the exciting wave-
length was isolated by means of an interference filter. Luminescence
lifetimes were measured by time-correlated single-photon counting
with an Edinburgh Instruments FLS920 equipment. The exciting light
(
λ = 300 nm) was produced by a gas arc lamp (model nF900, deuterium
filled), delivering pulses of ∼1 ns (fwmh), and the detector was a cooled
Hamamatsu R928 photomultiplier. Nanosecond transient absorption
experiments were performed by exciting the sample at λ = 532 nm
(
obtained by frequency doubling) with 10 ns (fwmh) pulses of a Con-
tinuum Surelite I-10 Nd:Yag laser (10 Hz repetition rate) and using a
pulsed 150W Xe lamp, perpendicular to the laser beam, as a probing
light. The Xe lamp was equipped with an Applied Photophysics power
supply (Model 40) and pulsing unit (Model 410, 2 ms pulses). A shutter
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(
Oriel 71445), placed between the lamp and the sample, was opened
for 100 ms to prevent phototube fatigue and photodecomposition. Suit-
able pre- and post-cutoff and bandpass filters were also used to avoid
photodecomposition and interferences from scattered light. The light
was collected in a PTI monochromator (model 01–001), detected by a
Hamamatsu R928 tube, and recorded on aTektronixTDS380 (400 MHz)
digital oscilloscope connected to a PC. Synchronous timing of the sys-
tem was achieved by means of a built-in-house digital logic circuit. The
absorption transient decays were plotted as ꢀA = log(I0/It) versus time,
where I0 and It were the probing the light intensities before the laser
pulse and after delay t, respectively. Each decay was obtained by aver-
aging ten pulses. Transient absorption spectra were obtained from the
decays measured at various wavelengths, by sampling the absorbance
changes at constant delay time.
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[
[
[
[
20] M. C. Jimenez, C. Dietrich-Buchecker, J.-P. Sauvage,
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Acknowledgments
A part of this research was supported by a National Sci-
enceFoundation(NSF)NanoscaleInterdisciplinaryResearch
Team fund (NIRT ECS-0103559), an Office of Naval
Research grant (contract number N00014-00-1-0216), an
NSF Equipment Grant (CHE-9974928), an NIH Shared
Instrument Grant (S10RR1S9S2), and the Center for Cell
Mimetic Space Exploration (CMISE) — a NASA University
Research, Engineering, and Technology Institute (URETI)
award (number NCC2-1364). The European Union is grate-
fully acknowledged for support under the auspices of the
Molecular-Level Devices and Machines RTN Network, the
Biomach STREP project, and a Marie Curie Individual Fel-
lowship to M.C.-L. The Italian authors complain that they
cannot thank any Italian agency for financial support.
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