Molecular actuator: Redox-controlled clam-like motion in a bichromophoric electron donor

Article abstract of DOI:10.1021/ol900371m

The one-electron oxidation of tetramethoxydibenzobicyclo[4.4.1]undecane (4) prompts it to undergo a clam-like electromechanical actuation into a cofacially -stacked conformer as established by (i) electrochemical analysis, (ii) by the observation of the i

Full text of DOI:10.1021/ol900371m

ORGANIC
LETTERS
2009
Vol. 11, No. 9
1939-1942
Molecular Actuator: Redox-Controlled
Clam-Like Motion in a Bichromophoric
Electron Donor
Vincent J. Chebny, Ruchi Shukla, Sergey V. Lindeman, and Rajendra Rathore*
Department of Chemistry, Marquette UniVersity, P.O. Box 1881,
Milwaukee, Wisconsin 53201
Rajendra.Rathore@marquette.edu
Received February 20, 2009
ABSTRACT
The one-electron oxidation of tetramethoxydibenzobicyclo[4.4.1]undecane (4) prompts it to undergo a clam-like electromechanical actuation
into a cofacially π-stacked conformer as established by (i) electrochemical analysis, (ii) by the observation of the intense charge-resonance
transition in the near IR region in its cation radical spectrum, and (iii) by X-ray crystallographic characterization of the isolated cation radical
salt (4+• SbCl6-).
The design and synthesis of functional organic molecules
for use as molecular sensors, switches, conductors, ferro-
magnets, and in nonlinear optics continues to grow with an
unabated pace owing to their potential applications in the
ever evolving areas of molecular electronics and nanotech-
nology.1,2 Among these are a variety of synthetic foldamers,3
molecular muscles,4 molecular switches,5 and functional
molecular rotaxanes6 that exhibit structural (or conforma-
tional) changes as well as changes in their physical properties
(such as magnetism, conductivity, and optical response) when
triggered by external stimuli such as heat, light, pH-changes,
metal ion binding, electron transfer, etc. Electro-mechanical
molecular actuators7 are another class of such molecules in
which a conformational change (or structural contraction or
expansion) is caused by a redox (electrochemical) event,
which is tantamount to the conversion of electrical energy
into mechanical energy; and such molecules are potentially
important as the building blocks for functioning molecular
muscles.4
(1) (a) Introduction to Molecular Electronics, Petty, M. C., Bryce, M. R.,
Bloor, D., Eds.; Oxford Univ. Press: New York, 1995. (b) Maiya, B. G.;
Ramasarma, T. Curr. Sci. 2001, 80, 1523
.
(2) (a) Lewis, F. D.; Letsinger, R. L.; Wasielewski, M. R. Acc. Chem.
Res. 2001, 34, 159–170. (b) Grimme, W.; Kaemmerling, H. T.; Lex, J.;
Gleiter, R.; Heinze, J.; Dietrich, M. Angew. Chem., Int. Ed. Engl. 1991,
30, 205, and references cited therein
.
(3) (a) Hill, D. J.; Mio, M. J.; Prince, R. B.; Hughes, T. S.; Moore, J. S.
Chem. ReV. 2001, 101, 3893. (b) Chebny, V. J.; Rathore, R. J. Am. Chem.
Soc. 2007, 129, 8458, and references therein.
In this report, we demonstrate that a readily available8
bichromophoric dibenzobicyclo[4.4.1]undecane derivative,
(4) (a) Marsella, M. J.; Reid, R. J. Macromolecules 1999, 32, 5982. (b)
Tseng, H.-R.; Vignon, S. A.; Stoddart, J. F. Polym. Prepr. 2003, 44, 377.
(5) (a) Molecular Switches; Feringa, B. L., Ed.; Wiley-VCH: Weinheim,
Germany, 2001. (b) Feringa, B. L. J. Org. Chem. 2007, 72, 6635. (c)
Rathore, R.; Magueres, P. L.; Lindeman, S. V.; Kochi, J. K. Angew. Chem.,
Int. Ed. 2000, 39, 809. (d) McQuade, T. D.; Pullen, A. E.; Swager, T. M.
Chem. ReV. 2000, 100, 2537, and references cited therein.
(7) (a) Baughman, R. H. Synth. Met. 1996, 78, 339. (b) Song, C.; Swager,
T. M. Org. Lett. 2008, 10, 3575, and references cited therein.
(8) Mataka, S.; Shigaki, K.; Sawada, T.; Mitoma, Y.; Taniguchi, M.;
Thiemann, T.; Ohga, K.; Egashira, N. Angew. Chem., Int. Ed. 1998, 37,
2352. Also see: Mataka, S.; Takahashi, K.; Hirota, T.; Takuma, K.;
Kobayashi, H.; Tashiro, M.; Imada, K.; Kuniyoshi, M. J. Org. Chem. 1986,
51, 4618.
(6) (a) Kaifer, A. E.; Stoddart, J. F. Nature (London) 1994, 369, 133.
(b) Grossel, M. C. Nanomater. Chem. 2007, 319. (c) Saha, S.; Stoddart,
J. F. Funct. Org. Mat. 2007, 295.
(9) Lavoisier-Gallo, T.; Rodriguez, J. Synth. Commun. 1998, 28, 2259.
10.1021/ol900371m CCC: $40.75
Published on Web 03/30/2009
 2009 American Chemical Society

which exists in rapidly interchanging noncofacial conforma-
tions and switching between the mirror image conformations
in the neutral state does not involve the cofacial conformation
as an intermediate.8 Interestingly, however, it undergoes a
clam-like molecular actuation upon 1-electron oxidation on
an electrochemical time scale, that is, eq 1.
Å, as confirmed by X-ray crystallography (Scheme 1). A
monomeric model donor, that is, 3,4-dimethyl-1,2-dimethox-
ybenzene (6) was easily obtained by a lithium aluminum-
hydride reduction of the dibromide 1 in refluxing tetrahy-
drofuran. The structures of 3-5 were established by X-ray
crystallography and by 1H/13C NMR spectroscopy (see
Supporting Information for the details).
1
Variable temperature H NMR spectroscopy of the con-
formationally mobile 4 in dichloromethane-d2 over a tem-
perature range of +20 to -80 °C showed that the interchange
between mirror image conformations can be frozen at ∼-50
°C; and the activation energy for the interchange into two
isoenergetic (noncofacial) conformers in eq 1 was estimated10
to be Ea ) 6.5 kcal mol-1 K-1 by line-shape analysis of the
signals in the variable temperature 1H NMR spectra in Figure
S1 (see Supporting Information).
The clam-like functioning of the redox-controlled me-
chanical motion in a conformationally mobile tetra-meth-
oxydibenzo-bicyclo[4.4.1]undecane, containing two redox-
active veratrole (1,2-dimethoxybenzene) moieties, is delineated
with the aid of electrochemistry, optical spectroscopy, and
by X-ray crystallography of the cofacially locked cationic
conformation, as follows.
Tetramethoxydibenzobicyclo[4.4.1]undecane (4) was syn-
thesized by adapting the literature procedures8,9 employed
for the preparation of the unsubstituted derivative (see
Scheme 1). The noncofacial arrangement of the veratrole
To establish that 4 exists exclusively in an extended
(noncofacial) conformer in solution, we examined the
electron donor-acceptor (EDA) complexes of 4 together
with the conformationally rigid, π-stacked acetal 5, in which
the veratrole moieties are arranged cofacially, and the model
monochromophoric 6 with 1,2,4,5-tetracyanobenzene (TCNB)
as an electron acceptor as follows. It is well-known that the
redox potentials (or π-basicity) of the cyclophane-like
aromatic donors are significantly lower than their extended
analogues,11 and can be readily gauged by formation of the
EDA complexes in solution according to the Mulliken
theory.12 Thus, when colorless solutions of 4 and 6 were
exposed to a colorless solution of TCNB in dichloromethane,
they instantaneously produced red-colored solutions, and they
each showed upon UV-vis spectral analyses a broad charge-
transfer (CT) absorption band centered at λmax ) 472 and
468 nm, respectively. The π-stacked cyclophane-like donor
5 is expected to have a lower redox potential than the model
electron donor 6 or bichromophoric 4 in an extended
conformation. Indeed, when a dichloromethane solution of
5 was exposed to TCNB, it immediately produced a magenta-
red solution which upon UV-vis spectral analysis revealed
that the charge-transfer absorption band was considerably
red-shifted to λmax ) 540 nm (see Figure 1A). Thus, the
identical energies of CT transitions for EDA complexes of
Scheme 1. Synthetic Scheme for the Preparation of
Bichromophoric Actuator and the Molecular Structures of 3-5
Obtained by X-Ray Crystallography
moieties in 4 was confirmed by X-ray crystallography
(Scheme 1). Interestingly, a ketalization of the ketone 3 using
neopentyl glycol afforded a conformationally rigid (model)
bichromophoric 5 in which the two veratrole moieties are
cofacially juxtaposed at a center to center distance of ∼3.03
(10) Bovey, F. A. Nuclear Magnetic Resonance Spectroscopy; Academic
Press: New York, 1969; Chapter VII.
Figure 1. (A) UV-vis absorption spectra of the EDA complexes
(11) Rathore, R.; Abdelwahed, S. H.; Guzei, I. A. J. Am. Chem. Soc.
2003, 125, 8712, and references cited therein.
of 4-6 with TCNB in CH2Cl2 at 22 °C. B/C. The Cyclic (B) and
square-wave (C) voltammograms of 4-6 (1 mM) in CH2Cl2,
containing 0.2 M n-Bu4NPF6, were measured at a V ) 200 mV s-1
at 22 °C. The oxidation potentials were referenced using ferrocene
(Eox ) 0.45 V vs SCE) as an internal standard.
(12) (a) Singer, L. A.; Cram, D. J. J. Am. Chem. Soc. 1963, 85, 1080.
(b) Taniguchi, M.; Mataka, S.; Thiemann, T.; Sawada, T.; Mimura, K.;
Mitoma, Y. Bull. Chem. Soc. Jpn. 1998, 71, 2661. (c) Rathore, R.;
Lindeman, S. V.; Kochi, J. K. J. Am. Chem. Soc. 1997, 119, 9343, and
references therein.
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Org. Lett., Vol. 11, No. 9, 2009

4 and 6 and significantly red-shifted CT transition (∼70 nm)
for the EDA complex of the rigid cyclophane-like donor 5
clearly implies that there is little or no contribution of the
π-stacked (cofacial) conformer of the bichromophoric 4 in
solution. As such, this conclusion is further supported by
the fact that the UV-vis absorption spectrum of bichro-
mophoric 4 was found to be characteristically similar (λmax
) 286 ( 1 nm) to that of the model monochromophoric 6,
whereas the cyclophane-like donor 5 showed an additional
band at 305 nm (see Figure S3 in the Supporting Informa-
tion).
when compared to the monochromophoric donor 6 (Eox
1.22 V) by ∼260 mV.13
)
As such, the through-space electronic coupling between
the cofacially stacked aryl moieties can also be seen in the
electronic spectra of their cation radicals, where characteristic
charge-resonance transitions in the near-IR region are ob-
served.13,14 Accordingly, we generated the cation radicals
of 4-6 using stable cation-radical salts [such as CRET+•,
Ered ) 1.14 V vs SCE and NAP+•, Ered ) 1.34 V vs SCE]15,16
as one-electron aromatic oxidants in dichloromethane as
follows.
For example, a treatment of the orange-red solution of
CRET+• (λmax ) 518 nm, ꢀ518 ) 7300 M-1 cm-1)15 with
substoichiometric increments of 4 led to the disappearance
of the absorption bands due to CRET+•, and a concomitant
growth of a new band at 466 nm together with a broad
featureless absorption band centered at ∼1224 nm (Figure
2A). Addition of donor 4 beyond one equivalent showed no
The cyclic and square-wave voltammograms of 4, 5, and
6 recorded in dichloromethane are compiled in Figure 1B/
C. The bichromophoric donors 4 and 5 showed two reversible
oxidation waves at the potentials of 0.96, 1.22 V and 0.87,
1.22 V vs SCE, respectively, whereas the monochromophoric
model donor 6 showed only one reversible oxidation wave
at a potential of 1.22 V vs SCE. The observation of two
well-separated one-electron oxidation waves (∆E ) 350 mV)
for cyclophane-like 5, in Figure 1B/C, is consistent with the
fact that the removal of the first electron results in a cation
radical (5+•) where the cationic charge is delocalized over
both the (preorganized) cofacial veratrole moieties, and
thereby rendering the ejection of the second electron difficult
by roughly ∼350 mV. Interestingly, however, the observation
of two similar well-separated one-electron oxidation waves
(∆E ) 260 mV) in conformationally mobile 4 suggests that
upon removal of the first electron, the extended (noncofacial)
conformer of 4+• undergoes an instantaneous transformation
into the π-stacked conformer on the electrochemical time
scale (see Scheme 2) and thereby rendering the second
Figure 2
.
(A) Spectra obtained upon the reduction of 1.5 × 10-4
M CRET+• (red) in CH2Cl2 by an incremental addition of 5.4 ×
10-3 M 4. (B) Plot of increase of absorbance of 4•+ (at 1224 nm)
and decrease of absorbance CRET+• (at 518 nm) against the
equivalent of added 4 in A. (C) Overlay of the absorption spectra
of the 4+•, 5+•, and model 6+• (λmax ) 442 nm; ꢀ442 ) 5,400 M-1
cm-1) in CH2Cl2 at 22 °C.
Scheme 2
.
Mechanism of Redox-Controlled Electromechanical
Actuation
additional spectral changes (Figure 2B). Also, Figure 2C
shows that the electronic spectra of 4+• (λmax ) 466, 1224
nm; ꢀ1220 ) 6,990 M-1 cm-1) is indistinguishable with that
of the rigid cyclophane-like 5+• (λmax ) 464, 1218 nm; ꢀ1220
) 7,170 M-1 cm-1). Moreover, the observation of a highly
characteristic charge-resonance transition at ∼1220 nm in
both 4+• and 5+•, and its singular absence in the monochro-
mophoric 6+• (Figure 2C), further suggests that 4+• adapts a
cofacial conformation similar to that of rigid 5+•.
oxidation to occur at a potential (Eox2 ≈ 1.22 V) strikingly
similar to that of 5.
To obtain definitive X-ray crystallographic evidence for
the transformation of the extended cation radical E-4+• into
the cofacially stacked C-4+• in Scheme 2, the single crystals
The observed difference (∼90 mV) in the first oxidation
potentials of conformationally mobile 4 when compared to
the rigid π-stacked 5 can be attributed to the energy required
for the transformation of the extended conformation E-4 into
the cofacially stacked C-4 conformation (see Scheme 2).
-
of the highly robust 4+• SbCl6 were obtained by a slow
diffusion of toluene into its dichloromethane solution during
(13) Compare: Rathore, R.; Chebny, V. J.; Kopatz, E. J.; Guzei, I. A.
Angew. Chem., Int. Ed. 2005, 44, 2771.
Furthermore, the driving force for the conformational
transformation E-4+•fC-4+• is largely due to the stabilization
of the cationic charge by the effective electronic coupling
between the π-stacked veratrole moieties (vide infra) and
thereby significantly lowering its first oxidation potential
(14) (a) Nelsen, S. F. Chem.-Eur. J. 2000, 6, 581. (b) Badger, B.;
Brocklehurst, B. Nature (London) 1968, 219, 263. (c) Kochi, J. K.; Rathore,
R.; Le Magueres, P. J. Org. Chem. 2000, 65, 6826
.
(15) Rathore, R.; Burns, C. L.; Deselnicu, M. I. Org. Synth. 2005, 82,
1.
(16) Rathore, R.; Burns, C. L.; Deselnicu, M. I. Org. Lett. 2001, 3, 2887
.
Org. Lett., Vol. 11, No. 9, 2009
1941

a period of 2 days at 22 °C (see Supporting Information for
a comparison with its neutral form, the structure of which
was established by X-ray crystallography, correspond to the
predominant contributions from para-quinoidal resonance
structures A/B, as judged by the significant shortening of
bonds labeled a and elongation of bonds labeled d and e,
along with only marginal shortening of the bonds labeled b
and c, (see Scheme 3 and Table 1). It is also noted that the
-
experimental details). The molecular structure of 4+• SbCl6 ,
obtained by X-ray crystallography, establishes the cofacial
arrangement of the veratrole moieties as shown in Figure 3.
Scheme 3
.
Resonance Stabilization of the Cationic Charge in
4+•
Figure 3
.
Crystal structure of 4+• SbCl6 cation radical showing
-
the cofacial arrangement of veratrole moieties (left), and a slight
twist of ∼8° around the local 2-fold symmetry axis of the 4+• (right).
alternate resonance structures C-E do not seem to contribute
to the stabilization of the cationic charge in any significant
way.
The experimental observations of the bond length changes
in 4+• were found to be in reasonable agreement with the
calculated values using DFT calculations at B3LYP/6-31G*
level (see Table 1).18
-
The crystal structure of 4+• SbCl6 also revealed that it
contained two symmetrically independent molecules of 4+•
that have essentially the same geometry. Unlike, the neutral
4, both of the 7-membered rings of the bicyclododecane
framework in the 4+• adopt a chairlike conformation17 which
allows a close cofacial arrangement of the veratrole rings
(with dihedral angles of 15.2 and 15.6° for the two
independent molecules) with an interplanar separation be-
tween the veratrole rings as close as 2.73 Åswhich is much
less than the equilibrium van-der-Waals separation of 3.4 Å
and is indicative of efficient electronic coupling between
them.
A closer look at the structural parameters of the veratrole
groups in 4+• suggests that they are identical (within the
experimental precision of ∼0.004 Å for various C-C bond
lengths) and the cationic charge is evenly distributed over
both the veratrole moieties (see Table 1). Furthermore, the
bond length changes in the cation radical 4+•, together with
In summary, we have demonstrated that conformationally
mobile 4 exists exclusively in the extended conformation in
the neutral state as judged by the spectral analysis of the
electron donor-acceptor complexes of 4 in comparison with
the cofacially stacked model compound 5 and a monochro-
mophoric donor 6 with tetracyanobenzene as an electron
acceptor. The one-electron oxidation readily transforms the
extended 4 into a clam-like folded π-stacked conformer as
confirmed by the electrochemical analyses and by the
observation of the intense charge-resonance transitions in the
near IR region in its cation radical spectrum. Moreover,
definitive evidence for the clam-like electromechanical
actuation of the extended cation radical E-4+• into the
cofacially stacked C-4+• was obtained by its isolation and
characterization by X-ray crystallography. We are actively
engaged in exploiting the usefulness of such a redox-
controlled molecular motion in readily available 4 as well
as the design and synthesis of higher homologues of these
electro-active materials.
Table 1. Experimental and Theoretical Bond Lengths of the
Neutral and Cation Radical of 4 Presented in Picometers (pm)
Acknowledgment. We thank the National Science Foun-
dation (CAREER Award) for financial support and my
colleague James R. Gardinier for valuable discussions.
B3LYP/6-31G*
4+• ∆ (4+•-4)
X-ray crystallography
Supporting Information Available: Synthetic details, 1H/
bonda
4
4
4+•
∆ (4+•-4)
1
13C NMR data, VT H NMR, and X-ray data of various
a
b
c
d
e
f
136.5 134.5
139.3 139.7
140.6 139.6
140.4 143.1
141.3 143.5
141.6 143.7
-2.0
+0.4
-1.0
+2.7
+2.2
+2.1
-
137.1 135.4
138.3 138.2
140.1 139.3
139.9 142.1
140.5 142.7
142.5 144.1
-1.7
-0.1
-0.8
+2.2
+2.2
+1.6
-
compounds. This material is available free of charge via the
Internet at http://pubs.acs.org.
OL900371M
(17) The cofacial arrangement of the veratrole moieties in 4+• induces
a slight distortion in the chairlike conformations of the 7-membered rings
in bicyclododecane framework from the ideal chair conformations.
(18) Compare: Banerjee, M.; Vyas, V. S.; Lindeman, S. V.; Rathore,
R. Chem. Commun. 2008, 1889.
σ
-
-
0.2
0.4
a Average of equivalent bonds and/or independent molecules.
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Org. Lett., Vol. 11, No. 9, 2009