Tetrathiafulvalene radical cation dimerization in a bistable tripodal [4]rotaxane

Article abstract of DOI:10.1002/chem.200800191

The template-directed synthesis of a bistable tripodal [4]rotaxane, which has cyclobis(paraquat-p-phenylene) (CBPQT⁴⁺) as the π-electron-de-ficient rings, and tetrathiafulvalene (TTF) and 1,5-dioxynaphthalene units as the pairs of π-electron-rich recognition sites located on all three legs of the tripodal dumbbell, is described. The chemical and electrochemical oxidation of the [4]rotaxane and its tripo dal dumbbell have allowed us to unravel an unprecedented TTF⁺ radical cation dimerization. In fact, two types of TTF dimers, namely, the radical cation dimer [TTF⁺]₂ and the mixed-valence one [(TTF) ₂]⁺, have been ob served at room temperature for the tripodal dumbbell, whereas, in the case of the [4]rotaxane, only the radical cation dimer [TTF⁺]₂ is formed. This anomaly can be explained if it is accepted that most of the neutral TTF units in the [4]rotaxane are encircled by CBPQT⁴⁺ rings, which renders the formation of the mixed-valence dimer [(TTF)₂]⁺ highly unfavorable.

Full text of DOI:10.1002/chem.200800191

FULL PAPER
DOI: 10.1002/chem.200800191
Tetrathiafulvalene Radical Cation Dimerization in a Bistable Tripodal
[4]Rotaxane
Ivan Aprahamian,[a] John-Carl Olsen,[b] Ali Trabolsi,[b] and J. Fraser Stoddart*[b]
Abstract: The template-directed syn-
thesis of a bistable tripodal [4]rotaxane,
which has cyclobis(paraquat-p-phenyl-
ene) (CBPQT4+) as the p-electron-de-
ficient rings, and tetrathiafulvalene
(TTF) and 1,5-dioxynaphthalene units
as the pairs of p-electron-rich recogni-
tion sites located on all three legs of
the tripodal dumbbell, is described.
The chemical and electrochemical oxi-
dation of the [4]rotaxane and its tripo-
dal dumbbell have allowed us to unrav-
served at room temperature for the tri-
podal dumbbell, whereas, in the case of
the [4]rotaxane, only the radical cation
+
C
el an unprecedented TTF
radical
cation dimerization. In fact, two types
of TTF dimers, namely, the radical
dimer [TTF +]2 is formed. This anom-
C
cation dimer [TTF +]2 and the mixed-
aly can be explained if it is accepted
that most of the neutral TTF units in
the [4]rotaxane are encircled by
CBPQT4+ rings, which renders the for-
C
valence one [(TTF)2] +, have been ob-
C
Keywords: bistability click
·
mation of the mixed-valence dimer
chemistry · dimerization · rotax-
anes · template synthesis
+
C
[(TTF)2] highly unfavorable.
Introduction
one because mechanically interlocked compounds of this
type are destined to be incorporated into the next genera-
tion of molecular machines.[7,8] Herein, we report the tem-
plate-directed synthesis[9] of a bistable, tripodal [4]rotaxane,
1·12PF6, which incorporates cyclobis(paraquat-p-phenyl-
ene)[10] (CBPQT4+) as the p-electron-deficient rings, and tet-
rathiafulvalene (TTF) and 1,5-dioxynaphthalene (DNP)
units located on all three legs of the tripodal dumbbell 2 as
the pairs of p-electron-rich recognition sites.[6]
The past few years have witnessed[1] a surge in the use of
the CuI-catalyzed Huisgen[2] 1,3-dipolar cycloadditions[3] be-
tween alkynes and azides in the synthesis of mechanically
interlocked compounds.[4] The convergent “click chemistry”
approach[5] is an attractive one because it affords catenanes
and [n]rotaxanes in high yields.[2] The power of this synthetic
approach lies in the fact that we can now prepare higher
order [n]rotaxanes,[4c,d] and “unstable” [2]rotaxanes[4a] that
were previously unattainable. Moreover, the introduction of
1,2,3-triazole rings into the dumbbell-shaped component of
bistable [2]rotaxanes,[6] does not alter the kinetics or ther-
modynamics of the switching process.[4g] This fact is a crucial
These two compounds were prepared with the aim of in-
vestigating the effect of bringing three bistable [2]rotaxanes
and their three dumbbell components into close contact
with each other. These covalently linked entities can be
looked upon as models at the molecular level of the supra-
molecular counterparts that exist in 1) Langmuir monolay-
ers,[11] 2) self-assembled monolayers,[12] and 3) molecular-
switch tunnel junctions[13] in memory devices, all composed
of some kind of bistable [2]rotaxane or its dumbbell precur-
sor. In the context of these condensed phases, the impor-
tance of the investigations reported herein is evident in so
far as the chemical and electrochemical properties reveal
[a] Dr. I. Aprahamian
The California NanoSystems Institute
and Department of Chemistry and Biochemistry
University of California, Los Angeles
405 Hilgard Avenue, Los Angeles, CA 90095 (USA)
[b] J.-C. Olsen, Dr. A. Trabolsi, Prof. J. F. Stoddart
Department of Chemistry, Northwestern University
2145 Sheridan Road, Evanston, IL 60208 (USA)
Fax : (+1)847-491-1009
evidence of emergent phenomena,[14] namely, the observa-
tion that in both 112+ and 2 the TTF radical cation dimer-
+
C
izes.[15] Whereas, in the case of 112+, only the radical cation
E-mail: stoddart@northwestern.edu
+
C
dimer (TTF
) can be detected, in the case of 2, both
2
+
Supporting information for this article is available on the WWW
under http://www.chemeurj.org/ or from the author. It contains syn-
thetic schemes; 1H NMR spectra of 1·12PF6; and spectroelectrochem-
istry, cyclic voltammetry, and titration data.
+
C
C
(TTF )2 and the mixed-valence dimer [(TTF)2] have been
identified.
Chem. Eur. J. 2008, 14, 3889 – 3895
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3889

Results and Discussion
1) the propargyl ether 8 (3.3 equiv; see the Supporting Infor-
mation) carrying a 2,6-diisopropylphenyl stopper and 2) a
catalytic amount of CuSO4·5H2O with ascorbic acid as an in
situ reductant. Chromatographic purification (SiO2: 1% w/v
NH4PF6/Me2CO) of the crude product from the reaction
mixture afforded 1·12PF6 as a green solid in a yield of 40%.
Electrospray ionization mass spectroscopy (ESI-MS) re-
vealed peaks at m/z 1951.7, 1427.7, 1113.2, 903.4, and 753.5,
which correspond to the loss of three, four, five, six, and
The tetrahydropyranyl (THP)-protected acyclic polyether 3
(see the Supporting Information), which incorporates both
TTF and DNP units,[4g] allows the synthesis of rotaxanes
with different stoppers to be performed by using click
chemistry. Compound 3 was tosylated (tosylchloride (TsCl)/
4-dimethylaminopyridine (DMAP)/Et3N/CH2Cl2) to give 4
in a yield of 85% (see the Supporting Information). Alkyl-
À
ation (Cs2CO3/DMF) of 1,1,1-tris(4-hydroxyphenyl)ethane
seven PF6 counterions, respectively. Following the same
(5) with 4, followed by deprotection (HCl/MeOH/CH2Cl2)
afforded the tripodal triol 6 in a yield of 52% (Scheme 1a;
for further information see the Supporting Information).
procedure in the absence of CBPQT·4PF6, provided the
model tripodal dumbbell 2 (65%) as a yellow oil.
The H NMR spectrum (Figure S1a in the Supporting In-
1
Tosyl
A
formation) of 1·12PF6 in CD3CN recorded at RT is compli-
cated by 1) the presence of multiple isomers stemming from
the cis/trans isomerism of the TTF units and 2) the dynamic
processes associated with the CBPQT4+ ring.[10b] Heating
the sample to 350 K yields a much simpler spectrum (Fig-
NaN3 in DMF at 80oC for one day gave the triazide 7
(54%). When 7 was mixed with CBPQT·4PF6 (3.1 equiv) in
DMF, an intense green color was generated immediately, in-
dicating the formation of [7ꢀ3CBPQT]·12PF6. This [4]pseu-
dorotaxane was stirred for two days in the presence of
ure S1bin the Supporting Information) in which the charac-
teristic signals of bistable [2]rotaxanes can be observed.
COSY Experiments, conducted at both RT and high temper-
ature, indicate the presence in solution of predominantly
A
which the CBPQT4+ ring encircles the TTF unit.[6c] This
conclusion is based on the absence of any high-field DNP
resonances.
The mechanical switching of 112+ and the electrochemical
response of 2 were studied by using cyclic voltammetry
(CV), differential pulse voltammetry (DPV), and UV/Vis
spectroelectrochemistry (SEC). Chemical oxidation was also
used to monitor the electronic transitions in the near-IR
(NIR) region.
In typical SEC measurements[6] of bistable [2]rotaxanes,
the band centered around 840 nm in the ground state, which
originates from the charge-transfer (CT) interaction be-
tween the TTF and CBPQT4+ units,[10b,6b] starts bleaching as
the voltage is raised and new absorption bands emerge at
+
C
lmax =445 and 595 nm that correspond to the TTF radical
cation. This behavior is not evident in the case of 1·12PF6.
When the voltage is raised, the band at 840 nm, instead of
bleaching, undergoes a hypsochromic shift (lmax =820 nm;
Figure 1). Moreover, four absorption bands, centered at
lmax =595, 527, 445, and 405 nm, start to grow in their rela-
tive intensities. A similar kind of behavior is observed for 2
(Figure S2 in the Supporting Information), except that a
new absorption band centered at lmax =790 nm started to
emerge as the potential was raised, that is, a process is hap-
pening that is independent of the CBPQT4+ rings.
The additional bands (lmax =820/790, 527, and 405 nm)
are, in fact, characteristic[15e,f] of radical cation dimer
+
(TTF )2 formation (Scheme 1b) in both 1·12PF6 and 2.[16]
C
Overall, the spectra reflect the presence of a mixture of the
Scheme 1. a) Template-directed synthesis of the bistable [4]rotaxane
1·12PF6 and the synthesis of its tripodal dumbbell precursor 2. b) A sche-
matic representation of the products resulting from oxidation (Ox=
+
C
TTF radical cation monomer (lmax =595 and 445 nm) and
+
[17]
C
the (TTF )2 dimer in solution at room temperature. This
mixture is very stable in air and shows no appreciable spec-
tral decay over a period of one day at RT. When the applied
potential is increased above +0.80 V, the absorption bands
chemical and/or by SEC) of 1·12PF6 and 2 in argon-purged MeCN/
+
C
CH2Cl2 (1:1) at 298 K. The different TTF radical cation dimers are indi-
cated by using different highlights, light pink for the radical cation dimer
+
+
C
C
(TTF )2 and light green for the mixed-valence dimer [(TTF)2]
.
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Chem. Eur. J. 2008, 14, 3889 – 3895

Tetrathiafulvalene Radical Cation Dimerization
FULL PAPER
another type of dimer,[15e] that is, the mixed-valence dimer
[(TTF)2] +, which is also stable over a long period of time at
C
+
C
RT, between the TTF radical cation and a neutral TTF
unit (Scheme 1). When 1·12PF6 was titrated with Fe
A
(Figure S3 in the Supporting Information) only the absorp-
+
C
tion bands of the TTF radical cation monomer and dimer
(TTF +)2 were observed. This observation can be attributed
C
to the fact that most of the TTF units are encircled by
CBPQT4+ rings in 112+. Only a small fraction of TTF units
in 112+ are “free”[19] and so the chances of forming the
+
mixed-valence dimer [(TTF)2] are low.[20] This situation is
C
consistent with the results obtained by NMR spectroscopy,
which indicates that most of the TTF units are encircled by
CBPQT4+ rings.
The electrochemical behavior of 1·12PF6, as suggested by
CV measurements (Figure 3), is similar to that reported pre-
viously for TTF/DNP bistable [2]rotaxanes,[6] which indi-
cates that the dimerization of TTF does not affect the
switching process. The only difference is in the first oxida-
tion peak for the TTF unit. It is usually observed[4g,7c] near
+0.5 V for free TTF, and now is shifted to around
+0.3 V[16c] as a result of the formation of the radical cation
dimers.
Figure 1. The changes in the UV/Vis spectrum during the SEC measure-
ments conducted on 1·12PF6. The applied potential changes from E=0
to 0.9 V. All data were recorded at 0.25 mVsÀ1 in argon-purged MeCN/
CH2Cl2 (1:1) at 298 K. The concentrations of the sample and supporting
electrolyte were 0.5 mm and 0.1m, respectively. The UV/Vis spectra re-
corded at different applied potentials (E) are differentiated by the use of
four different colors, namely, black, green, red, and blue.
+
C
of the TTF radical cation on its own, and as the dimer,
begin to bleach and a new peak (lmax =380 nm) emerges
(Figure 1 and Figure S2 in the Supporting Information) for
the TTF2+ dication. In 1·12PF6, a small absorption peak at
530 nm, associated with the CT band between DNP and the
CBPQT4+ ring, can also be detected. When the applied po-
tential is switched off, the spectrum gradually changes back
to its original form, in keeping with the fully reversible
nature of the electrochemical reaction.
The chemical oxidation of 2 with Fe(ClO4)3 led to the ob-
G
servation (Figure 2) of a new broad absorption band (lmax
=
1870 nm) in the NIR region.[18] This band is characteristic of
Figure 3. CV of 1·12PF6. The cyclic voltammogram was recorded at
200 mVsÀ1 in argon-purged MeCN/CH2Cl2 (1:1) at 298 K. The concentra-
tions of the sample and the supporting electrolyte were 0.5 mm and 0.1m,
respectively. The peak at around +0.3 V is ascribed to the oxidation of
+
C
free TTF to the TTF radical cation monomer and dimers. A schematic
representation of the radical cation dimer (TTF +)2 is shown.
C
DPV shows (Figure 4) the small peak in the CV trace
more clearly as a peak with a maximum at +0.30 V. The
first substantial TTF oxidation peak is shifted to higher po-
tential and overlaps (Figure 3) with the second one. In DPV,
the peak maxima for the first and second oxidations were
located (Figure 4) at +0.66 and +0.73 V,[21] respectively. In
Figure 2. The change in the UV/Vis-NIR spectrum during the titration of
the cathodic scan, two well-separated CV peaks are ob-
+
2 with Fe(ClO4)3. All data were recorded in argon-purged MeCN/CH2Cl2
A
C
served that correspond to the reduction of the TTF radical
(1:1) at 298 K. The concentrations of the sample and oxidant were 5.1
and 30.0 mm, respectively. The UV/Vis-NIR spectra recorded at different
cation and the TTF2+ dication. The positions of these peaks
are consistent with those for free TTF units, which indicates
that the CBPQT4+ ring remains on the DNP unit during the
equivalents of Fe
A
colors, namely, black, green, red, brown, and blue.
Chem. Eur. J. 2008, 14, 3889 – 3895
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
3891

J. F. Stoddart et al.
Experimental Section
General: All reagents and starting materials were purchased from Al-
drich and used without further purification. CBPQT·4PF6,[10c] 5-(2-(2-(tet-
rahydro-2H-pyran-2-yloxy)ethoxy)ethoxynaphthalene-1-ol (S1),[6c] and
the monotosylated tetrathiafulvalene derivative (S3)[6d] were prepared ac-
cording to literature procedures. Thin-layer chromatography (TLC) was
performed on silica gel 60 F254 (E. Merck). Column chromatography
was carried out on silica gel 60F (Merck 9385, 0.040–0.063 mm). Deuter-
ated solvents (Cambridge Isotope Laboratories) for NMR spectroscopic
analyses were used as received. NMR spectra were recorded on Bruker
Avance 500 and 600 spectrometers, with working frequencies of 500.13
1
and 600.13 MHz for H nuclei, and 125.70 and 150.90 MHz for 13C nuclei,
respectively. Chemical shifts are quoted in ppm relative to tetramethylsi-
lane with the residual solvent peak as a reference standard. Low-resolu-
tion ESI mass spectra were measured on an IonSpec FT-ICR mass spec-
trometer. GC-MS spectra were recorded on a Shimadzu GCMS-QP2010S
Spectrometer. High-resolution mass spectra were measured either on an
Applied Biosystems Voyager DE-PRO MALDI TOF mass spectrometer
(HR-TOF), or on a Finnigan LCQ ion-trap mass spectrometer (HR-
ESI).
Figure 4. DPV results of 1·12PF6. The data were recorded in argon-
purged MeCN/CH2Cl2 (1:1) at 298 K. The peak at around +0.3 V is ascri-
+
C
bed to the oxidation of free TTF to the TTF radical cation monomer
Electrochemical and SEC experiments were carried out at room temper-
ature in argon-purged solutions in MeCN/CH2Cl2 (1:1) with a Princeton
Applied Research 263 A Multipurpose instrument interfaced to a PC.
CV experiments were performed by using a glassy carbon working elec-
trode (0.018 cm2, Cypress Systems). Its surface was polished routinely
with 0.05 mm alumina-water slurry on a felt surface immediately before
use. The counter electrode was a Pt coil and the reference electrode was
a standard calomel electrode (SCE). The concentration of the sample
and supporting electrolyte (tetrabutylammonium hexafluorophosphate
(TBA·PF6)) were 0.5 mm and 0.1m, respectively. The scan rate was set to
200 mVsÀ1. In the DPV experiment, the pulse height, pulse width, step
height, and step time were set to 50 mV, 50 ms, 5 mV, 500 ms, respective-
ly. The peak top potentials for the overlapped peaks were determined by
using the curve-fitting operation of the IGOR Pro software (Version
5.04B, Wavemetrix).
and dimers. A schematic representation of the radical cation dimer
(TTF +)2 is shown.
C
+
C
process. The CV peak of the TTF radical cation is broad-
+
C
ened as a result of the presence of a mixture of the TTF
radical cation as both a monomer and a dimer. The CV and
DPV measurements carried out on 2 reveal (Figures S4 and
S5 in the Supporting Information) the formation of the radi-
+
C
cal cation monomer TTF and dimer with a peak maximum
of +0.30 V.[22]
Spectroelectrochemical experiments were carried out in a custom-built
optically transparent thin-layer electrochemical (OTTLE) cell with an
optical path of 1 mm by using a Pt grid as working electrode, a Pt wire as
counter electrode and a Ag wire pseudoreference electrode. Experimen-
tal errors: potential values, Æ10 mV; absorption maxima, Æ2 nm. The
scan rate was set to 0.25 mVsÀ1 and the UV/Vis spectra were recorded
every 2 min.
Conclusion
The existence in solution of only the radical cation dimer
(TTF +)2 of tetrathiafulvalene in the bistable tripodal [4]ro-
C
+
C
taxane, and of the mixed-valence dimer [(TTF)2] as well
The near-IR measurements were carried on a Shimadzu UV/Vis-NIR
scanning spectrophotometer by using a cell with an optical path of 1 mm.
The argon-purged solutions (MeCN/CH2Cl2 (1:1)) of the [4]rotaxane
1·12PF6, and dumbbell-shaped component 2 (0.5 and 5.1 mm, respective-
as (TTF +)2 in its tripodal dumbbell precursor, are not insig-
C
nificant observations, given the fact that neither dimer has
so far been observed[6] in analogous bistable [2]rotaxanes or
their precursor dumbbell compounds in solution. Note that,
although they are rare,[23] both types of radical cation dimers
occur at room temperature in solution in the case of other
tethered molecules.[16] Our observations reported herein,
coupled with those already present in the literature, beg the
very important question from the point of view of fabricat-
ing devices with molecular monolayers (and multilayers) of
bistable [2]rotaxanes containing tetrathiafulvalene units.
What is happening in these condensed phases? Could it be
that the radical cation and/or mixed-valence dimers of TTF
are present when these condensed phases are oxidized? It is
certainly a scenario not beyond the realms of possibility and
will have to be considered as a serious possibility when in-
vestigating devices fabricated from bistable [2]rotaxanes
containing tetrathiafulvalene units.
ly) were titrated with Fe
ACHTREUNG
were recorded after each addition of Fe
AHCTREUNG
Synthesis of 3: A solution of S1 (62 mg, 0.18 mmol; see the Supporting
Information),[6c] S2 (100 mg, 0.17 mmol; see the Supporting Informa-
tion),[6d] K2CO3 (92 mg, 0.68 mmol), and [18]crown-6 (5 mg) in dry
MeCN (30 mL) was heated under reflux for 16 h. After cooling to RT,
the reaction mixture was filtered and the residue was washed with MeCN
(5 mL). The combined organic solution was concentrated in vacuo to
obtain the crude THP-protected compound as a yellow oil, which was ex-
tracted with CH2Cl2 (3”20 mL) and dried (MgSO4). After removal of
the solvent, the residue was purified by column chromatography (SiO2:
CH2Cl2/EtOH 99:1) to give 3 as a yellow oil (113 mg, 88%). 1H NMR
(500 MHz, CD2Cl2): d=7.86 (d, J=7.5 Hz, 2H), 7.36 (t, J=7.8 Hz, 2H),
6.87 (d, J=7.5 Hz, 2H), 6.24, 6.22, 6.20, 6.19 (4”s, 2H; TTF), 4.61 (t, J=
2.7 Hz, 1H), 4.29–4.24 (m, 8H), 3.97–3.55 (m, 20H), 2.23 (t, J=2.7 Hz,
1H), 1.75–0.88 ppm (m, 7H); 13C NMR (125 MHz, CD2Cl2): d=154.8,
154.7, 135.1, 135.0, 134.9, 134.8, 127.1, 127.0, 125.6, 125.5, 116.8, 116.7,
116.7, 116.6, 114.8, 114.7, 111.0, 110.7, 106.1, 106.0, 99.3, 72.8, 71.3, 71.2,
70.6, 70.2, 70.1, 69.9 (”2), 69.8, 68.6, 68.5 (”2), 68.4 (”2), 67.1, 62.5, 62.0,
31.0, 25.9, 19.9 ppm;[24] HRMS (HR-TOF): m/z calcd for C35H46O10S4:
754.1974; found: 754.1967.
3892
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Tetrathiafulvalene Radical Cation Dimerization
FULL PAPER
Synthesis of 4: A solution of TsCl (37 mg, 0.20 mmol) in CH2Cl2 (2 mL)
was added dropwise with stirring to a solution of 3 (100 mg, 0.13 mmol),
Et3N (0.05 mL, 0.39 mmol), and DMAP (5 mg) in CH2Cl2 (20 mL) at
08C. The reaction mixture was then stirred for 16 h at RT. After removal
of the solvent, the residue was purified by column chromatography
(SiO2: CH2Cl2/EtOH 99:1) to give the tosylate 4 as a yellow oil (100 mg,
85%). 1H NMR (500 MHz, CD2Cl2): d=7.91 (dd, J=8.4, 4.7 Hz, 2H),
7.36 (d, J=8.4 Hz, 2H), 7.44–7.38 (m, 4H), 6.91 (d, J=7.7 Hz, 2H), 6.24
(s, 2H), 4.67 (t, J=3.5 Hz, 1H), 4.33 (m, 6H), 4.25 (brs, 2H), 4.16 (m,
2H), 4.00 (m, 4H), 3.86–3.46 (m, 15H), 2.43 (s, 3H), 1.79–1.48 ppm (m,
7H); 13C NMR (125 MHz, CD2Cl2): d=154.8, 154.7, 145.5, 135.1, 135.0,
134.9, 134.8, 133.3, 130.3, 128.3, 127.1 (”2), 125.6, 125.5, 116.8, 116.7
(”2), 116.6, 114.8, 114.7, 110.9 (”2), 110.7, 106.1, 106.0, 99.3, 71.3, 71.2,
71.0, 70.2, 70.1, 69.9, 69.8, 69.7, 69.0, 68.6 (”2), 68.5 (”3), 68.4, 67.1, 62.5,
31.0, 25.9, 21.8, 19.9 ppm;[24] HRMS (HR-TOF): m/z calcd for
C42H52O12S5: 908.2062; found: 908.2056.
106.0 (”2), 71.2, 70.9, 70.6, 70.1, 70.0 (”2), 69.8, 69.7, 68.5 (”2), 68.4,
68.3, 67.7, 51.2, 50.9 ppm;[24] MS (ESI): m/z calcd for C110H123N9O24S12
:
2337.53; found: 2337.37.
Synthesis of 8: A solution of 2,6-diisopropylphenol (0.48 mL, 2.7 mmol),
propargyl bromide (1.2 mL, 8.1 mmol), and K2CO3 (1.1 g, 8.1 mmol) in
dry DMF (10 mL) was heated at 808C for 1 d. After removal of the sol-
vent, the residue was extracted with CH2Cl2 (3”20 mL) and dried
(MgSO4). After removal of the solvent, the residue was purified by
column chromatography (SiO2: hexane/EtOH 97:3) to give 8 as yellow
oil (50 mg, 86%). 1H NMR (500 MHz, CDCl3): d=7.12 (brs, 3H), 3.40,
(m, 2H), 2.57 (s, 2H), 1.25 ppm (d, J=6.8 Hz, 9H); 13C NMR (125 MHz,
CD2Cl2): d=153.0, 142.2, 125.5, 124.4, 79.5, 75.4, 62.2, 26.9. 24.4 ppm; MS
(GC-MS): m/z (%): 216 , (20%) [M+], 173 (100), 159 (65), 135 (95), 107
(95), 91 (55), 43 (65).
Synthesis of 1·12PF6: The triazide 7 (50 mg, 0.021 mmol), CBPQT·4PF6
(74 mg, 0.066 mmol), and the propargyl ether 8 (15 mg, 0.069 mmol) were
dissolved in DMF (0.5 mL) at RT to afford a deep green solution. Stock
solutions of CuSO4·5H2O in DMF (20 mL, 0.006mm) and ascorbic acid in
DMF (20 mL, 0.012mm) were added. The solution was stirred at RT for
48 h. The crude product, obtained after the removal of the solvent, was
purified by column chromatography (SiO2: Me2CO followed by a 1%
w/v NH4PF6 solution in Me2CO). The green compound present in this
salt solution was concentrated to a small volume and the pure product
was precipitated from this concentrate by adding an excess of cold water.
The bistable tripodal [4]rotaxane 1·12PF6 was isolated as a green solid
Synthesis of 6: A solution of 4 (2.04 g, 2.24 mmol), compound 5 (0.21 mg,
6.7 mmol), and CsCO3 (4.38 g, 13.44 mmol) in dry DMF (50 mL) was
heated at 808C for 1 d. After cooling to RT, the reaction mixture was fil-
tered and the residue was washed with MeCN (20 mL). The combined or-
ganic solution was concentrated in vacuo to obtain the crude THP-pro-
tected compound as a yellow oil, which was redissolved in MeOH/
CH2Cl2 (1:1, 100 mL). A concentrated aqueous solution of HCl (0.5 mL)
was added and the reaction mixture was stirred at RT for 1 h. A 1n
aqueous solution of NaOH (100 mL) was added to the reaction mixture,
which was extracted with CH2Cl2 (3”100 mL) and dried (MgSO4). After
removal of the solvent, the residue was purified by column chromatogra-
phy (SiO2: CH2Cl2/EtOH 99:1) to give the triol 6 as a yellow oil (879 mg,
52%). 1H NMR (500 MHz, CD2Cl2): d=7.84 (t, J=8.0 Hz, 6H), 7.36 (t,
J=8.0 Hz, 6H), 6.97 (d, J=8.0 Hz, 6H), 6.86 (d, J=7.9 Hz, 6H), 6.78 (d,
J=7.9 Hz, 6H), 6.24, 6.22, 6.20, 6.19 (4”s, 6H; TTF), 4.27 (m, 24H), 4.06
(m, 6H), 3.95 (m, 12H), 3.78–3.58 (m, 42H), 2.07 ppm (s, 3H); 13C NMR
(125 MHz, CD2Cl2): d=156.7, 154.2, 154.1, 152.4, 143.1, 134.5 (”2), 134.4
(”2), 129.4, 126.5 (”2), 125.1, 125.0, 116.2, 116.1 (”2), 116.0, 114.3, 114.1,
113.5, 105.6, 72.6, 70.7, 70.5, 69.6, 69.5 (”2), 69.3 (”2), 68.0 (brs), 67.9,
1
(52 mg, 40%). M.p. 1408C (dec); H NMR (500 MHz, CD3CN): d=9.31–
9.11 (m, 24H), 8.53–8.32 (m, 6H), 8.20–8.00 (m, 18H), 7.09–7.54 (m,
24H), 7.48–7.40 (m, 4H), 7.22 (m, 2H), 7.14–7.10 (m, 6H), 6.91–6.71 (m,
8H), 6.64 (brs, 5H), 6.50–6.40 (m, 3H), 6.33 (s, 1H), 6.28–6.19 (m, 4H),
6.14–6.03 (m, 4H), 5.99–5.93 (m, 4H), 5.82–5.80 (m, 4H), 5.72–5.68 (m,
4H), 4.95–4.85 (m, 6H), 4.56 (brs, 4H), 4.56–4.21 (m, 14H), 4.21–3.79
(m, 60), 3.45 (m, 6H), 3.15 (4H), 2.22 (s, 3H), 1.17–1.13 ppm (m, 36);
MS (ESI): m/z: 1950 [MÀ3PF6]3+, 1427 [MÀ4PF6]4+, 1113 [MÀ5PF6]5+
,
903 [MÀ6PF6]6+, 753 [MÀ7PF6]7+; HRMS (HR-ESI): m/z calcd for
C263H279F54N21O27P9S12: 1950.4843; found: 1950.4875 [MÀ3PF6]3+
.
67.8, 67.3, 61.5, 50.9, 29.9 ppm;[24] MS (ESI): m/z calcd for C110H126O27S12
:
Synthesis of 2: The triazide 7 (50 mg, 0.021 mmol) and the propargyl
ether 8 (15 mg, 0.069 mmol) were dissolved in DMF (1.0 mL) at RT.
Stock solutions of CuSO4·5H2O in DMF (100 mL, 0.006m) and ascorbic
acid in DMF (100 mL, 0.012m) were added. The solution was stirred at
RT for 2 d. After removal of the solvent, the residue was extracted with
CH2Cl2 (3”10 mL) and dried (MgSO4). After removal of the solvent, the
residue was purified by column chromatography (SiO2: CH2Cl2/EtOH
98:2) to give 2 as a yellow oil (40.6 mg, 65%). 1H NMR (600 MHz,
CD2Cl2): d=7.88 (d, J=8.5 Hz, 2H), 7.84 (s, 1H), 7.81 (d, J=8.5 Hz,
2H), 7.38 (m, 6H), 7.14 (s, 9H), 7.00 (d, J=8.0 Hz, 6H), 6.89 (d, J=
7.0 Hz, 6H), 6.83 (d, J=7.0 Hz, 6H), 6.25, 6.24, 6.23, 6.21 (4”s, 6H;
TTF), 4.86 (s, 6H), 4.65 (t, J=4.8 Hz, 6H), 4.29 (m, 21H), 4.15–4.06 (m,
12H), 3.99 (m, 13H), 3.81–3.61 (m, 32H), 3.38 (m, 6H), 2.11 ppm (s,
3H); 13C NMR (150 MHz, CD2Cl2): d=157.2, 154.7, 154.6, 153.3, 144.6,
142.3, 135.0, 134.9, 129.9, 127.0, 126.9, 125.6, 125.5, 125.2, 124.4, 124.1,
116.7, 116.6 (”2), 116.5, 114.9, 114.5, 113.9, 110.7, 106.0 (”2), 71.2, 71.0,
70.2, 70.1, 70.0, 69.9, 69.8, 68.5, 68.4, 68.3, 68.2 (”2), 67.8, 50.8, 30.0, 26.9,
24.1 ppm; MS (MALDI-TOF): m/z calcd for C153H181N9O27S12: 2985.9872;
found: 2985.9567.
2262.51; found: 2262.35.
Synthesis of S3: A solution of TsCl (118 mg, 0.60 mmol) in CH2Cl2
(2 mL) was added dropwise with stirring to a solution of 6 (234 mg,
0.10 mmol), Et3N (0.13 mL, 0.90 mmol), and DMAP (10 mg) in CH2Cl2
(20 mL) at 08C. The reaction mixture was then stirred for 16 h at RT.
After removal of the solvent, the residue was purified by column chroma-
tography (SiO2: CH2Cl2/EtOH 99:1) to give the tritosylate S3 as a yellow
oil (231 mg, 85%). 1H NMR (500 MHz, CD2Cl2): d=7.88 (d, J=8.5 Hz,
3H), 7.82 (d, J=8.5 Hz, 3H), 7.77 (d, J=8.0 Hz, 6H), 7.40 (m, 6H), 7.29
(d, J=8.0 Hz, 6H), 7.01 (d, J=8.5 Hz, 6H), 6.88 (d, J=7.5 Hz, 3H),
6.85–6.81 (m, 9H), 6.23, 6.22, 6.21, 6.19 (4”s, 6H; TTF), 4.28 (m, 16H),
4.20 (m, 12H), 4.08 (m, 6H), 3.97 (m, 6H), 3.91 (m, 6H), 3.78 (m, 18H),
3.67–3.61 (m, 20H), 2.10 ppm (s, 3H); 13C NMR (125 MHz, CD2Cl2): d=
157.1, 154.7, 154.6, 145.4, 142.3, 135.0, 134.9 (”3), 133.2, 130.2, 129.9,
129.8, 128.1, 127.0, 126.9, 125.6, 125.5, 116.7, 116.6 (”2), 116.5, 114.8,
114.6, 114.1, 113.9, 110.7, 106.0 (”2), 71.2, 70.9, 70.1 (”2), 70.0, 69.9, 69.8
(”2), 69.2, 68.3, 68.2, 50.9, 30.9, 21.7 ppm;[24] MS (ESI): m/z calcd for
C
131H144O33S15: 2724.5400; found: 2724.5120.
Synthesis of 7: A solution of tritosylate S3 (230 mg, 0.08 mmol) and NaN3
(82 mg, 1.2 mmol) in dry DMF (20 mL) was heated at 808C for 2 d. After
removal of the solvent, the residue was dissolved in CH2Cl2 (50 mL) and
then washed with a saturated aqueous solution of NH4Cl (2”30 mL), fol-
lowed by a saturated aqueous solution of K2CO3 (30 mL) and then finally
dried (MgSO4). The crude product, obtained after the removal of the sol-
vent, was purified by column chromatography (SiO2: CH2Cl2/EtOH 99:1)
Acknowledgements
1
to give the triazide 7 as a yellow oil (124 mg, 63%). H NMR (500 MHz,
CD2Cl2): d=7.85 (d, J=8.4 Hz, 6H), 7.37 (t, J=7.7 Hz, 6H), 6.99 (d, J=
8.0 Hz, 6H), 6.89 (d, J=8.0 Hz, 6H), 6.80 (d, J=8.4 Hz, 6H), 6.22, 6.21,
6.20, 6.19 (4”s, 6H; TTF), 4.27 (m, 24H), 4.08 (m, 6H), 3.99 (m, 12H),
3.81–3.61 (m, 36H), 3.43 (m, 6H), 2.09 ppm (s, 3H); 13C NMR (125 MHz,
CD2Cl2): d=157.1, 154.7, 154.6, 142.3, 135.0 (”2), 134.9 (”2), 129.9, 127.0
(”2), 125.6, 116.7, 116.6 (”2), 116.5, 114.8 (”2), 114.7, 113.9, 110.8, 110.7,
This work was supported by the Microelectronics Advanced Research
Corporation (MARCO) and its Focus Center of Functional Engineered
NanoArchitectonics (FENA) and Materials, Structures, and Devices
(MSD), the MolApps Program funded by the Defense Advanced Re-
search Projects Agency (DARPA) and the Center for Nanoscale Innova-
tive Defense (CNID).
Chem. Eur. J. 2008, 14, 3889 – 3895
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
3893

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[17] It is important to note that neither TTF radical cation dimers have
+
C
been observed in previously studied bistable [2]rotaxanes, or their
dumbbell components in solution.
3894
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Chem. Eur. J. 2008, 14, 3889 – 3895

Tetrathiafulvalene Radical Cation Dimerization
FULL PAPER
[18] The absorptions bands of the (TTF +)2 dimer of 2 are also observed
c) R. Ballardini, V. Balzani, A. Di Fabio, M. T. Gandolfi, J. Becher,
J. Lau, M. B. Nielsen, J. F. Stoddart, New J. Chem. 2001, 25, 293–
298.
C
(lmax =820, 527) in Figure 2, which indicates that both species exist
in solution at RT. The absorption bands of the TTF radical cation
+
C
monomer were observed (lmax =595, and 445) when the appropriate
equivalents of oxidant were added to 2. The appearance of the radi-
[22] Moreover, both spectra show a minor oxidation peak at +0.5 V that
could be attributed to the further oxidation of the various radical
+
C
cal cation monomer absorptions was accompanied by the bleaching
cationic TTF species present in solution.
+
+
C
C
of the mixed-valence dimer [(TTF)2] absorption band.
[23] The fact that TTF radical cations do dimerize in the tripodal [4]ro-
[19] These free TTF units originate from the translational isomer
taxane 112+, even though the molecule incorporates 12 positive
charges, indicates that even an accumulation of so much charge—
perhaps because it is shielded internally by counterions and sol-
A
[20] Another important point is the fact that the free TTF units will be
oxidized before the encircled ones (see text and ref. [7b]), a situa-
tion that will lower the chances even more of getting mixed-valence
dimers.
[21] a) M. Asakawa, P. R. Ashton, V. Balzani, A. Credi, G. Mattersteig,
O. A. Matthews, M. Montalti, N. Spencer, J. F. Stoddart, M. Venturi,
Chem. Eur. J. 1997, 3, 1992–1996; b) V. Balzani, A. Credi, G. Mat-
tersteig, O. A. Matthews, F. M. Raymo, J. F. Stoddart, M. Venturi,
A. J. P. White, D. J. Williams, J. Org. Chem. 2000, 65, 1924–1936;
vent—does not prevent, in an appreciable amount, communication
+
C
occurring between TTF radical cations. Clearly what has been
demonstrated to occur intramolecularly to 112+ could easily be hap-
pening in condensed phases of tetracationic bistable [2]rotaxanes.
[24] Some of the signals in the 13C NMR spectrum are doubled up be-
cause of the cis/trans isomerism.
Received: January 5, 2008
Published online: March 20, 2008
Chem. Eur. J. 2008, 14, 3889 – 3895
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
3895