A conducting poly(cyclophane) and its poly([2]-catenane) [3]

Full text of DOI:10.1021/ja000970m

9300
J. Am. Chem. Soc. 2000, 122, 9300-9301
provided by the thiophene moieties contributes to this larger
A Conducting Poly(cyclophane) and Its
Poly([2]-catenane)
binding constant.
The self-assembly of [2]-catenane 4 follows from the cycliza-
tion of the trication derived from 3a that is bound in the
macrocycle of 3. As expected, the tetracationic cyclophane portion
of 4 quenches the fluorescence of the crown ether component.
The deep-green [2]-catenane complex exhibits a charge-transfer
absorption at λ ) 626 nm (ꢀ ) 1230 M-1 cm-1), which is red-
shifted relative to the paraquat:3 complex indicating greater
intimacy between donor and acceptor in the [2]-catenane.
The crystal structure obtained for 46 (Scheme 1) supports an
interlocked π-stacked geometry. The bipyridinium aromatic units
are situated in electronically unique environments as illustrated
by the inner bipyridinium aromatic positioned within the tetra-
cationic cyclophane cavity and the outer bipyridinium aromatic
on the outer periphery of the cavity. The centroid-centroid
distances between aromatic rings in the complex are ∼3.4 Å,
indicative of strong π-π stacking interactions between aromatic
units.7 The [2]-catenane complex exhibits hydrogen bonding
between the R- and â-bipyridinium aromatic protons and crown
ether oxygen atoms as well as edge-to-face interactions between
aromatic subunits. The distance between the central oxygen atom
attached to the crown ether linkage and the R-bipyridinium protons
is 2.51 Å. The edge-to-face distance between the inside hydro-
quinone ring protons and the p-xylyl spacers is 2.93 Å. The torsion
angle between the thiophene-phenylene planes is 21.6°, orienting
the thiophene ring 3,4-ethylenedioxy bridge within the receptor
cavity. Surprisingly, the twist allows for a stabilizing interaction
between the 3-position ether oxygen atom of the thiophene ring
and â-bipyridinium protons, with a distance of 2.86 Å. Within
the unit cell, complementary π-stacking between molecules
persists with centroid-centroid distances of 3.47 Å.
Davide L. Simone and Timothy M. Swager*
Department of Chemistry and Center for Materials Science
and Engineering, Massachusetts Institute of Technology
77 Massachusetts AVenue, Cambridge, Massachusetts 02139
ReceiVed March 20, 2000
There has been great interest devoted to the development of
multicomponent arrays that undergo inter- and intramolecular elec-
tron transfer via an exergonic gradient. Dyads, triads, and higher-
order arrays have been synthesized in attempts to mimic and better
understand the photosynthetic reaction center1 and toward the
realization of molecular-based storage devices.2 Catenane and
rotaxane architectures consisting of electron-rich and electron-
poor aromatic units are ideal substrates for investigation because
they exhibit charge-transfer characteristics that facilitate electron
migration.3 Recently, a transition metal-based [2]-catenane has
been reported that exhibits migration via an intramolecular gradi-
ent;1a however, research into conductive poly([2]-catenanes) has
gone undeveloped.4 Herein, we report the synthesis and electro-
chemical study of an electrically conducting poly([2]-catenane).
We have focused our investigations on macrocycle 3 and its
[2]-catenane 4 (Scheme 1). The choice of this system is largely
based on previous work of Stoddart who showed the base
BPP34C10 cyclophane unit (compound 2 without iodo-groups)
to bind dialkylated-4,4′-bipyridiniums producing rotaxanes and
catenanes.3b,5 We prepared macrocycle 3 in two steps from 1.
The cyclization of bistosylate 1 with a diiodohydroquinone
derivative forms the strategically functionalized diiodo crown
ether, 2. Coupling of 2 with (3,4-ethylenedioxy)-thiophene groups
affords the highly fluorescent, electropolymerizable 3.
To probe 3’s ability to bind electron-deficient guests, we
undertook Stern-Volmer fluorescence quenching experiments
with (N,N′-dimethyl-4,4′-bipyridinium) bis-hexafluorophosphate,
paraquat, as the quenching analyte. The addition of paraquat to 3
in CH3CN resulted in the formation of a deep-green colored
solution on account of a charge-transfer absorption band at λ )
589 nm (ꢀ ) 204 M-1cm-1). The association stability constant
(Ka) was determined in CH3CN by monitoring the diminished
emission intensity at λ ) 392 nm (λex ) 360 nm) with increasing
analyte concentration. The linear Stern-Volmer behavior of the
1:1 complex gave a rather large Ka value of 2930 ( 30 M-1 (∆Go
≈ -4.8 kcal/mol), indicative of the highly electron-donating
nature of the thiophene-phenylene-thiophene aromatic scaffold.
For comparison BPP34C10 displays a Ka of 730 M-1 in acetone.5
It seems likely that the extended structure and electron-donation
The electrochemical polymerization of macrocyclic monomer
3 and [2]-catenane 4 proceeds via two propagating sites centered
at the 5-position of the 3,4-ethylenedioxythiophene functionality.
As the voltage is cycled to a positive potential of 0.35 V vs Fc/
Fc+, the oxidation of monomer 3 (Ep,m) occurs, indicating
formation of radical-cationic species that can then propagate via
radical combination forming dimers, trimers, etc. Further oxidative
cycling leads to polymer formation as evident by an oxidation
peak growing in at a lower positive potential. This lowering of
the oxidative peak (Ep,p) for poly(3) versus monomer 3 is
indicative of the former’s ability to stabilize cations via delocal-
ization. The acyclic dimethoxyphenylene derivative of 3 has been
synthesized by Reynolds and exhibits a Ep,m ) 0.46 V.8 The lower
Ep,m for 3 can be attributed to the stabilizing π-donation imparted
by the neutral hydroquinone aromatic to the oxidized thiophene-
phenylene-thiophene backbone.3b The electropolymerizations are
performed such that the resulting polymer is deposited on a 2
µm interdigitated electrode9 allowing in situ conductivity mea-
surements.10 The cyclic voltammogram and conductivity profile
for poly(3) in Figure 1A displays a peak oxidation for the polymer,
Ep,p ) 0 V and a maximum conductivity of 11 S/cm at 0.2 V vs
Fc/Fc+.11
(1) (a) Hu, Y.-Z.; van Loyen, D.; Schwarz, O.; Bossmann, S.; Durr, H.;
Huch, V.; Veith, M. J. Am. Chem. Soc. 1998, 120, 5822. (b) Linke, M.;
Chambron, J.-C.; Heitz, V.; Sauvage, J.-P. J. Am. Chem. Soc. 1997, 119, 11329
and refernces therein. (c) Sun, L.; von Gersdorff, J.; Niethammer, D.; Tian,
P.; Kurreck, H. Angew. Chem., Int. Ed. Eng. 1994, 33, 2318. (d) Harriman,
A.; Magda, D. J.; Sessler, J. L. J. Phys. Chem. 1991, 95, 1530. (e) Rodriguez,
J.; Kirmaier, C.; Johnson, M. R.; Friesner, R. A.; Holten, D.; Sessler, J. L. J.
Am. Chem. Soc. 1991, 113, 1652.
(2) (a) Denti, G.; Campagna, S.; Serroni, S.; Ciano, M.; Balzani, V. J. Am.
Chem. Soc. 1992, 114, 2944. (b) Hopfield, J. J.; Onuchic, J. N.; Beratran, D.
N. J. Phys. Chem. 1989, 93, 6350.
(3) (a) Benniston, A. C.; Harriman, A.; Lynch, V. M. J. Am. Chem. Soc.
1995, 117, 5275. (b) Anelli, P. L.; Ashton, P. R.; Ballardini, R.; Balzani, V.;
Delgado, M.; Gandolfi, M. T.; Goodnow, T. T.; Kaifer, A. E.; Philp, D.;
Pietraszkiewicz, M.; Prodi, L.; Reddington, M. V.; Slawin, A. M. Z.; Spencer,
N.; Stoddart, J. F.; Vicent, C.; Williams, D. J. J. Am. Chem. Soc. 1992, 114,
193.
The Ep,m of monomer 4 occurs at 0.43 V vs Fc/Fc+, indicating
that it is slightly more difficult to oxidize than 3. This is expected
due to the repulsive nature of the resulting radical-cation stacked
above the inner-dicationic portion of the bipyridinium cyclophane.
The Ep,p for poly(4), 0.07 V vs Fc/Fc+, is also shifted to a less
(6) Details of the crystal structure are given in the Supporting Information.
(7) Hunter, C. A.; Sanders, J. K. M. J. Am. Chem. Soc. 1990, 112, 5525.
(8) Irvin, J. A.; Reynolds, J. R. Polymer 1998, 39, 2339.
(9) Electrodes fabricated with the following dimensions: 2 µm interdigit
spacing, 99 gaps, 0.2005 cm electrode length, 0.04 cm electrode width.
(10) (a) Kittlesen, G. P.; White, H. S.; Wrighton, M. S. J. Am. Chem. Soc.
1984, 106, 7389. (b) Ofer, D.; Crooks, R. M.; Wrighton, M. S. J. Am. Chem.
Soc. 1990, 112, 7869.
(4) (a) For a review of interlocked macromolecules see: Raymo, F. M.;
Stoddart, J. F. Chem. ReV. 1999, 99, 1643. (b) Conducting poly(pseudoro-
taxanes) have been reported: Marsella, M. J.; Carroll, P. J.; Swager, T. M. J.
Am. Chem. Soc. 1994, 116, 9347.
(5) Allwood, B. L.; Spencer, N.; Shahriari-Zavareh, H.; Stoddart, J. F.;
Williams, D. J. J. Chem. Soc., Chem. Commun. 1987, 1064.
10.1021/ja000970m CCC: $19.00 © 2000 American Chemical Society
Published on Web 09/06/2000

Communications to the Editor
J. Am. Chem. Soc., Vol. 122, No. 38, 2000 9301
Scheme 1
bipyridinium at Ep,r2 ) -0.93 V. A final two-electron reduction,
Ep,r3 ) -1.32 V, yields a neutral cyclophane. The greater electron
affinity (first reduced) for the outer-bipyridinium than for the
inner-bipyridinium leads to the formation of an electrochemical
energy gradient within the polymer repeat unit.
The conductivity profile for poly(4) rapidly leads to a maximum
value of 0.2 S/cm at 0.12 V vs Fc/Fc+, which decays quickly
thereafter. In contrast, the profile for poly(3) (Figure 1A), shows
that oxidation of the polymer backbone occurs over a broad range
of potentials without decay. The sharp parabolic nature for the
conductivity profile of poly(4), as well as the calculated +1 charge
per repeat unit reflects a more localized electronic state with
characteristics similar to a redox conductor (i.e., each repeat unit
acts as a discrete entity) but with a σmax much larger than a typical
redox conductor.12 The optimum rate of electron transfer in the
material is realized when the ratio of neutral and oxidized repeat
units is 1:1. The tetracationic portion of poly(4) does not show
any detectable redox conductivity upon reduction of the bipyri-
dinium units.
Spectroelectrochemical investigations of poly(3) and poly(4)
films revealed similar absorption characteristics. In their neutral
(insulating) form, the λmax for poly(3) was 527 nm (2.35 eV) and
poly(4) 542 nm (2.29 eV). When oxidatively doped, both dis-
played a longer wavelength band appearing between 767 and 796
nm (1.62-1.56 eV), indicative of new states formed within the
band gap upon reaching a conductive state. Further lower energy
absorptions occur at higher oxidation potentials leading to an ab-
sorption at >1100 nm (>1.13 eV) owing to the formation of free
carriers.8 The films differ in that poly(4) requires higher oxidation
potentials to reach its conductive state than does poly(3).
Receptor 3’s penchant for associating with electron-deficient
aromatics has also prompted us to investigate the assembly of
rotaxane architectures with other conductive acceptor polymers.
Further studies into the electronic properties of poly(4) are now
underway.
Figure 1. Cyclic voltammogram (s) and conductivity profile (- - -)
-
for (A) poly(3) (0.1 M TBA ClO4 in CH3CN and (B) poly(4) (0.1 M
-
TBA ClO4-/0.1 M TBA PF6 (7:3) in CH3CN), (50 mV/sec for the cv,
10 mV/sec and 40 mV offset for the conductivity profile).
positive potential than its monomer (Figure 1B). The similarity
between Ep,p for poly(4) and poly(3) is not surprising considering
the small difference in oxidation potential of their respective
monomers.
The three-peak reduction potentials observed for the tetraca-
tionic cyclophane portion of poly(4) are identical to those for
monomer 4. This behavior suggests that the neutral polymer
backbone has the same electronic influence on the tetracation as
the thiophene-phenylene-thiophene aromatic in 4. The multiple
reduction peaks are indicative of the energetic inequality between
the inner- and outer-bipyridinium aromatics. This non-equivalence
has been observed by Stoddart and Kaifer with the BPP34C10
derived [2]-catenane.3b The first one electron reduction of poly-
(4), Ep,r1 ) -0.77 V, occurs at the more electropositive outer-
bipyridinium, followed by a one-electron reduction of the inner-
Acknowledgment. This work was supported in part by the MRSEC
program of the National Science Foundation under award number DMR
98-08941, the Center for Materials Science and Engineering at MIT and
The Office of Naval Research.
Supporting Information Available: Spectral data, cyclic voltam-
metry data, characterization for all synthesized compounds, and crystal-
lographic data for 4 (PDF). This material is available free of charge via
the Internet at http://pubs.acs.org.
(11) Non-uniform film coverage on the interdigitated electrodes required
a correction factor in the form of a poly(3-methylthiophene) standard to be
applied to the conductivity values calculated (see Kingsborough, R. P.; Swager,
T. M. AdV. Mater. 1998, 10, 1100). Detailed calculations are reported in the
Supporting Information.
JA000970M
(12) Zhu, S. S.; Kingsborough, R. P.; Swager, T. M. J. Mater. Chem. 1999,
9, 2123.