π-Dimer formation as the driving force for calix[4]arene-based molecular actuators

Article abstract of DOI:10.1021/ol8013039

(Chemical Equation Presented) Stable π-dimers are formed upon oxidation of the model units of proposed calix[4]arene-based molecular actuators in a solvent of low dielectric constant (CH₂CI₂) at room temperature. Evidence from UV-vis, EPR, and DPV are all in agreement with the π-dimer formation. In addition, π-dimer formation is dependent upon the conformational flexibility of the calix[4]arene hinge.

Full text of DOI:10.1021/ol8013039

ORGANIC
LETTERS
2008
Vol. 10, No. 16
3575-3578
π-Dimer Formation as the Driving Force
for Calix[4]arene-Based Molecular
Actuators
Changsik Song and Timothy M. Swager*
Department of Chemistry and Institute for Soldier Nanotechnologies, Massachusetts
Institute of Technology, Cambridge, Massachusetts 02139
tswager@mit.edu
Received June 14, 2008
ABSTRACT
Stable π-dimers are formed upon oxidation of the model units of proposed calix[4]arene-based molecular actuators in a solvent of low dielectric
constant (CH2Cl2) at room temperature. Evidence from UV-vis, EPR, and DPV are all in agreement with the π-dimer formation. In addition,
π-dimer formation is dependent upon the conformational flexibility of the calix[4]arene hinge.
Conducting polymer (CP) actuators can change dimensions
in response to electrochemical stimulation.1,2 They are
particularly attractive because of their light weight, low
driving voltages, and processibility. The accepted mechanism
for actuation in conducting polymers is the so-called “swell-
ing mechanism”, in which a volume change of bulk materials
is caused by uptake or release of counter-ions and ac-
companying solvent molecules upon oxidation or reduction.1
A complementary approach is to design molecular actuators,
which are materials having geometrical changes at the
molecular level to create dimensional changes. It should be
noted that unlike bulk actuators (e.g., polypyrrole), polymeric
molecular actuators seek to harness dimensional variations
resulting from changes in the backbone structure of a
polymer. We envision that if we utilize the conformational
changes of single polymer chains, we may obtain faster
responses and larger strains, which can enable the bottom-
up development of nanodevices and machines.
Our group designed a calix[4]arene-based molecular
actuator (Figure 1a), in which electroactive oligothiophenes
are connected by the calix[4]arene scaffold.3 We proposed
that each calix[4]arene moiety acts as a molecular hinge and
that new noncovalent interactions between oxidized olig-
othiophenes (i.e., π-dimer or π-stack) can drive the dimen-
sional changes. The π-dimer involves the nonconventional
multicenter bonding between cofacial radical cations, and
this interaction has drawn particular attention as a model for
interchain bipolarons as charge carriers in conducting poly-
mers.4 π-Dimer formation has been demonstrated in solution
(3) (a) Anquetil, P. A.; Yu, H. -h.; Madden, J. D.; Madden, P. G.;
Swager, T. M.; Hunter, I. W. Smart Mater. Struct. 2002, 4695, 424–434.
(b) Yu, H.-h.; Xu, B.; Swager, T. M. J. Am. Chem. Soc. 2003, 125, 1142–
1143. (c) Yu, H. -h.; Swager, T. M. IEEE J. Oceanic Eng. 2004, 29, 692–
695.
(4) Miller, L. L.; Mann, K. R. Acc. Chem. Res. 1996, 29, 417–423.
(5) For example, see: (a) Ba¨uerle, P.; Segelbacher, U.; Maier, A.;
Mehring, M. J. Am. Chem. Soc. 1993, 115, 10217–10223. (b) Graf, D. D.;
Duan, R. G.; Campbell, J. P.; Miller, L. L.; Mann, K. R. J. Am. Chem. Soc.
1997, 119, 5888–5899. (c) Sakai, T.; Satou, T.; Kaikawa, T.; Takimiya,
K.; Otsubo, T.; Aso, Y. J. Am. Chem. Soc. 2005, 127, 8082–8089. (d)
Knoblock, K. M.; Silvestri, C. J.; Collard, D. M. J. Am. Chem. Soc. 2006,
128, 13680–13681. (e) Yamazaki, D.; Nishinaga, T.; Tanino, N.; Komatsu,
K. J. Am. Chem. Soc. 2006, 128, 14470–14471.
(1) Baughman, R. H. Synth. Met. 1996, 78, 339–353
.
(2) ElectroactiVe Polymer (EAP) Actuators As Artificial Muscles: Reality,
Potential, and challenges; Bar-Cohen, Y., Ed.; SPIE Press: Bellingham,
2001
.
10.1021/ol8013039 CCC: $40.75
Published on Web 07/11/2008
2008 American Chemical Society

that support the hypothesis that π-dimer formation can indeed
be a driving force for the calix[4]arene-based molecular
actuator.
To conduct model studies of the actuating unit, we
synthesized 1-4 (Figure 1b), which contain a calix[4]arene
hinge and two oligothiophene derivatives as electroactive
segments (Scheme 1). First, we sought to examine which, if
Scheme 1. Synthesis of Model Compounds 1-5
Figure 1. (a) Schematic description of the calix[4]arene-based
molecular actuator. (b) Model compounds (1-4) used in this study
and a reference molecule (5).
and in the solid state for oligothiophene derviatives.5,6 In
order to take advantage of such interactions as a driving force
for molecular actuators, we considered that a segmented
polymer would be more suitable than a fully conjugated one
because it is able to attain larger interactions due to the spatial
confinement of the radical cation’s wave functions. More
diffuse radical cations will necessarily result in weaker orbital
interactions in these dimeric structures. It should be noted,
however, that higher degrees of spatial confinement can result
in Coulombic repulsion of like charges, and counterions are
likely to play a major role.
The ab initio calculations by Scherlis and Marzari model-
ing one actuating unit supported the concept that π-stacking
between oxidized oligothiophenes induces conformational
changes.7 They utilized 1,3-alternate calix[4]arene hinges,
which are more flexible than the cone conformation, and
found that a mixed-valence complex exhibits π-dimer
bonding interaction in the gas phase. Interestingly, Casanovas
and co-workers reported a different actuation mechanism.8
On the basis of calculations, they concluded that oxidized
oligothiophenes do not interact each other due to Coulombic
repulsion of like charges. Instead, they suggested that for
the calix[4]arene scaffold deprotonation can drive the con-
formational change.
any, of these compounds would give rise to stable radical
cations when oxidized and whether π-dimer formation would
take place. Second, the effect of the calix[4]arene’s confor-
mation (cone vs 1,3-alternate) was the subject of investigation.
We examined several oxidizing agents to produce radical
cations of 1-5 and finally chose Et3OSbCl6, a Meerwein’s
salt, as a 1-electron oxidant10 (not as an alkylating agent)
because it is relatively easy to handle and, more importantly,
it does not exhibit a strong absorbance above 300 nm in the
UV-vis spectrum. Therefore, we were able to monitor the
diminution of the neutral absorption and the concurrent
evolution of new absorptions without any interference (Figure
2a-c). Upon addition of the oxidant, all oligothiophenes
were converted to deep-blue or violet radical cations, which
were stable to moisture. However, the color was slowly lost
(returned to the neutral state) when air was bubbled through
the solution.
For the oxidation of monomeric 5 in CH2Cl2 under ambient
conditions (Figure 2c), the initial π-π* absorption (382 nm)
decreased and new peaks (645, 1079 nm) developed, which
can be attributed to the polaronic absorptions (radical
cations).11 These sub-bandgap transitions with vibronic
structures are in good accord with literature precedents.5,12
We were not able to observe the formation of dications even
It is clear that the stacking behavior of oxidized oligoth-
iophenes is greatly affected by solvation and counterion
screening.9 In addition, π-dimer formation is greatly en-
hanced if oligothiophenes are connected by a linker.5c,d
Therefore, we thought π-dimer formation was still a viable
mechanism, and we present herein a series of model studies
(6) Kingsborough, R. P.; Swager, T. M. J. Am. Chem. Soc. 1999, 121,
8825–8834
.
(7) Scherlis, D. A.; Marzari, N. J. Am. Chem. Soc. 2005, 127, 3207–
3212.
(10) Rathore, R.; Kumar, A. S.; Lindeman, S. V.; Kochi, J. K. J. Org.
Chem. 1998, 63, 5847–5856.
(8) Casanovas, J.; Zanuy, D.; Alema´n, C. Angew. Chem., Int. Ed. 2006,
45, 1103–1105.
(11) Cornil, J.; Bre´das, J. -L. AdV. Mater. 1995, 7, 295–297.
(9) Scherlis, D. A.; Marzari, N. J. Phys. Chem. B 2004, 108, 17791–
(12) Nishinaga, T.; Wakamiya, A.; Yamazaki, D.; Komatsu, K. J. Am.
17795.
Chem. Soc. 2004, 126, 3163–3174.
3576
Org. Lett., Vol. 10, No. 16, 2008

alternate 4 are very similar to those of cone 2 (Supporting
Information). The conformation of 4 is fixed because propyl
groups or larger substituents are too large to rotate through
the annulus of the calix[4]arene macrocycle at room tem-
perature.14 Therefore, it is unlikely that there is an intercon-
version of 4 to 2 and vice versa. We can conclude that the
cone and 1,3-alternate conformations of 2 and 4 are suf-
ficiently flexible to allow π-dimer formation when oxidized.
Table 1 summarizes the absorption maxima of 1-5 in their
neutral and oxidized states.
Table 1. Absorption Maximaa and Half-Wave Potentialsb of
1-5
absorption maxima (nm)
potentials (V)
1
2
neutral
polaronic
π-dimeric
E1/2
E1/2
1
2
3
4
5
384
382
409
382
382
665, >1100
655, 1084
730, >1100
658, 1094
645, 1079
c
0.33
0.33
0.23
0.33
0.42
0.57
0.83
0.71
0.82
0.76
593, 948
663, 1062
598, 955
c
a Absorptions were measured in CH2Cl2 upon addition of oxidant
Figure 2. UV-vis spectral changes of 1 (a), 2 (b), and 5 (c) in
Et3OSbCl6 at room temperature. b Half-wave potentials (all vs Fc/Fc+) were
measured in CH2Cl2 with 0.1 M TBAPF6 as a supporting electrolyte under
ambient conditions. c Not observed.
CH2Cl2 at room temperature upon the increasing addition of the
oxidant Et3OSbCl6. Spectra of neutral absorptions are displayed
by dashed lines.
π-Dimer formation was further confirmed by EPR spec-
troscopy. EPR spectra were acquired for each of the CH2Cl2
solutions (0.2 mM for 1 and 2, 0.4 mM for 5) at room
temperature. As expected, bis(radical cation) 12(•+) (Figure
3a, broken line) was EPR-active, showing a rather broad and
after adding excess amounts of Et3OSbCl6 under the above
conditions.
Remarkably distinct behavior was observed upon oxidation
of 2 (Figure 2b). Polaronic peaks, similar in shape to those
of 5•+, were dominant at the low levels of oxidation.
However, as more oxidant was added, blue-shifted absorp-
tions were evident. Such blue shifts are characteristic of
π-dimer formation.5 The same phenomenon was observed
upon oxidation of 3, the terthiophene-substituted version
(Supporting Information). It was expected that the longer
oligothiophene forms stronger dimers, probably due to the
reduced Coulombic repulsion.5c The results of the present
study are noteworthy in that a stable π-dimer is generated
at room temperature in a solvent of low dielectric constant
(CH2Cl2) from a framework as short as two thiophenes.
Interestingly, when we added the oxidant Et3OSbCl6 to 1,
only polaronic absorptions were observed in the UV-vis
spectra (Figure 2a). The only difference is that 1 has free
hydroxyl groups at the lower rim of the calix[4]arene
moiety.13 We attribute this reluctance to form a π-dimer to
the calix[4]arene’s conformational rigidity resulting from
lower-rim hydrogen bonding. However, it should be noted
here that the π-dimer formation is coupled to the “motional”
flexibility of the calix[4]arene hinge (hydrogen-bonded vs
tetraalkylated).
Figure 3.
(a) 9-GHz EPR spectra of radical cations 12(•+) (broken
line), 22(•+) (solid line), and 5•+ (dotted line) in CH2Cl2 at room
temperature. (b) Differential pulse voltammograms of 2 (solid line)
and 5 (dotted line) in CH2Cl2 (∼0.5 mM) on a Pt button electrode
with 0.1 M TBAPF6 as a supporting electrolyte.
featureless signal, which is very similar to that of radical
cation 5•+ (dotted line). Note that 12(•+) consists of two
independent radical cations. In contrast, 22(•+) (solid line) was
almost EPR-silent, which indicates that the two radical
cations are bound to form a π-dimer.
The effect of the calix[4]arene’s conformation on the dimer
formation appeared minimal. Oxidation profiles of 1,3-
The evolution of the EPR signals of oxidized 2 and 5 was
monitored as the oxidant (Et3OSbCl6 in CH2Cl2) was added
(13) There is a possibility that the hydroxyl groups of 1 were alkylated
by the Meerwein’s salt. However, oxidation of 1 with a different oxidant
(FeCl3) resulted in the same absorption specta (Figure S2, Supporting
Information).
(14) Shinkai, S. Tetrahedron 1993, 49, 8933–8968.
Org. Lett., Vol. 10, No. 16, 2008
3577

incrementally (Supporting Information). In the case of 5, the
signal increased gradually to a maximum. However, the
initially developed signal for 2 decreased as more oxidant
was added. This is in accord with what was observed by the
UV-vis spectroscopy (Figure 2b); in 2, radical cations
appeared at the initial stages of oxidation, but the π-dimer
dominated at higher oxidation levels.
The oxidation potentials of dimeric 2 and monomeric 5
were measured in CH2Cl2 solutions with 0.1 M TBAPF6 as
a supporting electrolyte under ambient conditions. In cyclic
voltammetry, both 2 and 5 showed two 1-electron oxidation
peaks, which correspond to radical cation(s) and dication(s),
respectively (Table 1). However, the first oxidation of 2 took
place at a potential lower than that of 5 (0.33 and 0.42 V,
respectively, vs Fc/Fc+), and the peak was broader. In
contrast, the second oxidation of 2 was shifted to the higher
potential. Differential pulse voltammetry (DPV, Figure 3b)
reveals the differences more clearly. The first oxidation of
the dimeric 2 occurred at a lower potential and the peak was
broader, whereas the second oxidation was at higher potential
when compared to that of the monomeric 5. These data also
are consistent with the π-dimer formation because an
overpotential is required to oxidize the π-dimer, which is a
stabilized species.
In conclusion, we have demonstrated that a stable π-dimer
is formed upon oxidation of compounds 2, 3, and 4, the
model units of the proposed molecular actuator, in a solvent
of low dielectric constant (CH2Cl2) at room temperature.
Evidence from UV-vis, EPR, and DPV are all in agreement
with the π-dimer formation. In addition, we found that
π-dimer formation is dependent upon the conformational
flexibility of the calix[4]arene hinge. We are currently
seeking to produce polymeric actuating materials based upon
these compounds.
Acknowledgment. This work was supported by the U.S.
Army through the Institute for Soldier Nanotechnologies,
under Contract with the U.S. Army Research Office.
Supporting Information Available: Experimental details
on the syntheses of 1-5, UV-vis and EPR spectra, and
cyclic voltammograms. This material is available free of
charge via the Internet at http://pubs.acs.org.
OL8013039
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Org. Lett., Vol. 10, No. 16, 2008