Crown-annelated oligothiophenes as model compounds for molecular actuation

Article abstract of DOI:10.1021/ja026819p

Crown-annelated quater- (4T) and sexithiophenes (6T) with oligooxyethylene chains of various lengths attached at the 3-positions of the terminal thiophene rings have been synthesized. Analysis of the cation-binding properties of the macrocycles by ¹H NMR and UV-vis spectroscopy reveals the formation of a 1:1 complex with Ba²⁺, Sr²⁺, or Pb²⁺ and shows that cation complexation results in a conformational transition in the π-conjugated system. Theoretical analysis of this process by density functional methods predicts that this conformational transition results in a narrowing of the highest occupied-lowest unoccupied molecular orbital gap of the conjugated system with a decrease of the redox potentials (E⁰₁ and E⁰₂) associated with the formation of the 4T cation radical and dication. Cyclic voltammetry shows that, depending on the binding constant, the presence of metal cation produces a negative or a positive shift of E⁰₁ while E⁰₂ always shifts negatively. This unusual behavior is discussed in terms of interplay between electrostatic interactions and conformational changes associated with cation binding.

Full text of DOI:10.1021/ja026819p

Published on Web 01/09/2003
Crown-Annelated Oligothiophenes as Model Compounds for
Molecular Actuation
†
,†
†
‡
Bruno Jousselme, Philippe Blanchard,* Eric Levillain, Jacques Delaunay,
‡
§
§
‡
Magali Allain, Pascal Richomme, David Rondeau, Nuria Gallego-Planas, and
,
†
Jean Roncali*
Contribution from Groupe Syst e` mes Conjugu e´ s Lin e´ aires and Ing e´ nierie Mol e´ culaire et
Mat e´ riaux Organiques, UMR CNRS 6501, and SerVice Commun d’Analyses Spectroscopiques,
UniVersit e´ d’Angers, 2 BouleVard LaVoisier, 49045 Angers, France
Received May 7, 2002 ; E-mail: jean.roncali@univ-angers.fr
Abstract: Crown-annelated quater- (4T) and sexithiophenes (6T) with oligooxyethylene chains of various
lengths attached at the 3-positions of the terminal thiophene rings have been synthesized. Analysis of the
1
cation-binding properties of the macrocycles by H NMR and UV-vis spectroscopy reveals the formation
2+
2+
2+
of a 1:1 complex with Ba , Sr , or Pb and shows that cation complexation results in a conformational
transition in the π-conjugated system. Theoretical analysis of this process by density functional methods
predicts that this conformational transition results in a narrowing of the highest occupied-lowest unoccupied
molecular orbital gap of the conjugated system with a decrease of the redox potentials (E0 and E0
)
1 2
associated with the formation of the 4T cation radical and dication. Cyclic voltammetry shows that, depending
0
on the binding constant, the presence of metal cation produces a negative or a positive shift of E
1
while
0
E
2
always shifts negatively. This unusual behavior is discussed in terms of interplay between electrostatic
interactions and conformational changes associated with cation binding.
5
6
Introduction
co-workers, while Gaub and co-workers have investigated a
polymeric system undergoing a single-molecule optomechanical
cycle.
Motion generation at the molecular level by means of
chemical, electrochemical, or photochemical stimulation has
recently become a focus of considerable interest in the general
context of molecular machines.1-6 Stoddart and co-workers1b,2
have developed molecular shuttles in which change of the
oxidation state of a redox system imbedded in a rotaxane induces
Thiophene-based monodisperse π-conjugated oligomers have
much interest as active material in field effects transistors or
7,8
light-emitting diodes. Although these oligomers have also been
considered as molecular wires for molecular electronic devices,9
their possible use in dynamic nanosystems has been scarcely
considered so far. Bulk electrochemical actuators based on the
volume changes associated with the doping/undoping process
1c,3,4
its translocation along a linear axis. Sauvage and co-workers
have synthesized molecular machines and molecular muscles
powered by the modification of the coordination sphere of a
metal involved in rotaxanes containing tetra- and pentacoordi-
nation sites. Photochemically powered molecular machines
undergoing circular motion have been described by Feringa and
10
of conjugated polymers have been known for some time, but
(7) (a) Dimitrakopoulos, C. D.; Malenfant, P. R. L. AdV. Mater. 2002, 14, 99.
(
b) Rogers, J. A.; Bao, Z.; Dodabalapur, A.; Crone, B.; Raju, V. R.; Katz,
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2
*
Corresponding authors.
(
†
Groupe Syst e` mes Conjugu e´ s Lin e´ aires.
Ing e´ nierie Mol e´ culaire et Mat e´ riaux Organiques.
Service Commun d’Analyses Spectroscopiques.
B.; Ries, S.; Alnot, P. J. Am. Chem. Soc. 1993, 115, 8716. (g) Barbarella,
G.; Zambianchi, M.; Antolini, L.; Ostoja, P.; Maccagnani, P.; Bongini, A.;
Marseglia, E. A.; Tedesco, E.; Gigli, G.; Cingolani, R. J. Am. Chem. Soc.
1999, 121, 8920.
‡
§
(
1) (a) Drexler, K. E. Nanosystems: Molecular Machinery, Manufacturing and
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Res. 1997, 30, 393. (c) Sauvage, J.-P. Acc. Chem. Res. 1998, 31, 611. (d)
Balzani, V.; Gomez-Lopez, M.; Stoddart, J. F. 31, 405. (e) Koumura, N.;
Zijlstra, R. W.; van Delden, R. A.; Harada, N.; Feringa, B. L. Nature 1999,
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Mater. 1993, 5, 922. (b) Barbarella, G.; Favaretto, L.; Sotgiu, G.;
Zambianchi, P.; Bongini, A.; Arbizzani, C.; Mastragostino, M.; Anni, M.;
Gigli, G.; Cingolani, R. J. Am. Chem. Soc. 2000, 122, 11971. (c) Barbarella,
G.; Favaretto, L.; Sotgiu, G.; Antolini, L.; Gigli, G.; Cingolani, R.; Bongini,
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4
01, 152.
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(
(
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3
1
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10.1021/ja026819p CCC: $25.00 © 2003 American Chemical Society
J. AM. CHEM. SOC. 2003, 125, 1363-1370
9
1363

A R T I C L E S
Jousselme et al.
the molecular parent systems are still to be developed. A first
step in this direction was recently reported by Marsella et al.,
who reported a model of molecular actuator based on poly-
Scheme 1
1
1
[cyclooctatetrathiophene].
In this work we propose a different concept of molecular
actuator in which stimulation of a driving group covalently
attached at two fixed points of an oligothiophene chain is used
to produce dimensional changes in the π-conjugated system. A
particularly interesting aspect of this approach is that the
π-conjugated system simultaneously constitutes the target of
the generated movement and an optical and redox probe,
allowing its real-time monitoring by simple electrochemical and
optical techniques.
Derivatization of poly(thiophenes) by linear or macrocyclic
polyethers has been widely investigated in view of developing
1
2-15
ion sensors.
Whereas most of these systems are based on
the changes induced in the electrochemical response of the
π-conjugated system by electrostatic interactions between the
1
3
metal cation and the oxidized π-conjugated system, some
examples are known where cation binding induces geometrical
changes in the conjugated chain.1
4,15
We show here that cation complexation can be used as the
driving force to produce geometrical changes in a conjugated
oligothiophene (nT) and hence to design single-molecule
electromechanical actuators.
To this end, two series of quarter- (4T) and sexithiophenes
(6T) bearing oligooxyethylene loops of variable length have
been synthesized. The ionophoric properties of these compounds
have been analyzed in the presence of different metal cations
by H NMR and mass spectrometry, UV-vis absorption
spectroscopy, and cyclic voltammetry. These investigations
together with crystallographic and theoretical results provide a
coherent picture showing that cation binding induces large
conformational transitions in the π-conjugated chain, thus
making these new systems interesting models of molecular
actuators.
in one pot by deprotection of the thiolate functionality by cesium
hydroxyde and ring closure by reaction with an R,ω-diiodo-
oligooxyethylene chain under high dilution conditions. This
methodology utilizes the chemistry developed by Becher and
1
17
co-workers for the synthesis of tetrathiafulvalene derivatives.
Successive treatments of commercially available 3-bromo-
thiophene 8 with n-BuLi, elemental sulfur, and 3-bromopropio-
nitrile give the key compound (2-cyanoethyl)sulfanylthiophene
7
in 83% yield. Selective bromination of 7 at the 2-position of
thiophene with N-bromosuccinimide in DMF leads to the
-bromothiophene derivative 6 in 90% yield. 5,5′-Bis(tributyl-
stannyl)-2,2′-bithiophene 4 is obtained by treatment of bithiophene
with, successively, n-BuLi and tributylstannyl chloride.18 The
key compound 3,3′′′-bis[(2-cyanoethyl)sulfanyl]-2,2′:5′,2′′:
′′,2′′′-quaterthiophene 1 is then prepared by a double Stille
coupling between distannyl bithiophene 4 and bromo compound
with Pd(PPh3)4 as catalyst. Treatment of a DMF solution of
2
5
5
6
(
12) Reviews: (a) McQuade, D. T.; Pullen, A. E.; Swager, T. M. Chem. ReV.
2
000, 100, 2537. (b) Roncali, J. J. Mater. Chem. 1999, 9, 1875.
(13) (a) B a¨ uerle, P.; Scheib, S. AdV. Mater. 1993, 5, 848. (b) B a¨ uerle, P.; Scheib,
S. Acta Polym. 1995, 46, 124. (c) Rimmel, G.; B a¨ uerle, P. Synth. Met.
1
999, 102, 1323. (d) Scheib, S.; B a¨ uerle, P. J. Mater. Chem. 1999, 9, 2139.
(
e) Sannicol o` , F.; Brenna, E.; Benincori, T.; Zotti, G.; Zecchin, S.; Schiavon,
G.; Pilati, T. Chem. Mater. 1998, 10, 2167.
(
(
(
14) (a) Roncali, J.; Garreau, R.; Delabouglise, D.; Garnier, F.; Lemaire, M. J.
Chem. Soc., Chem. Commun. 1989, 679. (b) Roncali, J.; Shi, L. H.; Garnier,
F. J. Phys. Chem. 1991, 95, 8983.
Results and Discussion
15) (a) Marsella, M. J.; Swager, T. M. J. Am. Chem. Soc. 1993, 115, 12214.
Synthesis. The crown-annelated 4Ts and 6Ts have been
synthesized according to the procedure depicted in Scheme 1.
Our strategy involves the preparation of an oligomer possessing
two protected thiolate groups at the 3-positions of the terminal
thiophene rings (1).16 The target compounds are then obtained
(
b) Marsella, M. J.; Newland, R. J.; Carroll, P. J.; Swager, T. M. J. Am.
Chem. Soc. 1995, 11, 7, 9842.
16) (a) Blanchard, P.; Jousselme, B.; Fr e` re, P.; Roncali, J. J. Org. Chem. 2002,
6
7, 3961. (b) Van Hal, P. A.; Beckers, E. H. A.; Meskers, S. C. J.; Janssen,
R. A. J.; Jousselme, B.; Roncali, J. Chem. Eur. J. 2002, 8, 5415.
(17) (a) Svenstrup, N.; Rasmussen, K. M.; Hansen, T. K.; Becher, J. Synthesis
994, 809. (b) Becher, J.; Lau, J.; Leriche, P.; Mørk, P.; Svenstrup, N. J.
1
Chem. Soc., Chem. Commun. 1994, 2715. (c) Li, Z.-T.; Stein, P. C.;
Svenstrup, N.; Lund, K. H.; Becher, J. Angew. Chem., Int. Ed. Engl., 1995,
34, 2524.
(
11) (a) Marsella, M. J.; Reid, R. J., Macromolecules, 1999, 32, 5982. (b)
Marsella, M. J.; Reid, R. J.; Estassi, S.; Wang, L.-S. J. Am. Chem. Soc.
2
002, 124, 12507.
(18) Wei, Y.; Yang, Y.; Yeh, J.-M. Chem. Mater. 1996, 8, 2659.
1364 J. AM. CHEM. SOC.
9
VOL. 125, NO. 5, 2003

Crown-Annelated Oligothiophenes as Model Compounds
A R T I C L E S
1
1
Table 1. Variation of H NMR Chemical Shifts for 4TO4 (9.56 mM)
Table 2. H NMR Complexation Data for Crown-Annelated
in 1:1 CDCl3/CD3CN upon Addition of Ba(ClO4)2
Quaterthiophenesa
equiv of Ba2+ added
∆δH1a
∆δH2b
∆δH33′c
∆δH4d
compd
cation
complex
K (M )
0
-1
+
+
0
0
0
0
1
1
1
2
.2
.4
.6
.8
.0
.2
.4
.0
0.04
0.09
0.14
0.17
0.19
0.21
0.21
0.22
0.08
0.16
0.24
0.30
0.35
0.37
0.39
0.40
0.04
0.09
0.15
0.17
0.20
0.21
0.22
0.23
0.04
0.08
0.13
0.16
0.18
0.19
0.20
0.21
4TO3
Li , Na
no
no
no
1:1
1:1
1:1
no
1:1
1:1
1:1
2
+
Ba
+
+
4TO4
4TO5
Li , Na
2
+
Ba
7200 ( 600
900 ( 200
2+
Sr
Pb
2+
5
>10
+
+
+
Li , Na , Cs
2
+
5
Ba
>10
∼10
2+
4
Sr
Pb
a
b
2+
5
Shifts are relative to δH1 ) 2.83. Shifts are relative to δH2 ) 3.34.
Shifts are relative to δH33′ ) 3.25. Shifts are relative to δH4 ) 3.27.
>10
c
d
a
Complexation in CD3CN/CDCl3 + 1 equiv of metal cation.
Chart 1
quaterthiophene 1 with 2.1 equiv of CsOH‚H2O in methanol,
generates a dithiolate that is then reacted in situ with diiodo-
oligooxyethylenes 3a, 3b, and 3c under high dilution conditions
to give the target quaterthiophenes 4TO3, 4TO4, and 4TO5 in
2+
Figure 1. ESI mass spectrum of the 1:1 complex Ba /4TO4. Inset:
3
4%, 47%, and 26% yields, respectively.
experimental and calculated isotopic distributions for [M + Ba(ClO )]+.
4
As already observed, the use of the Stille reaction for the
1
9
synthesis of nTs leads to undesired homocoupling, which
results in our case in the formation of ca. 5% sexithiophene 2
during the synthesis of 1. Since this compound could not be
separated from 1, the mixture of 1 and 2 was used in the next
cyclization step, and crown-annelated sexithiophenes 6TO4 and
the occurrence of a net plateau around 1 equiv in the plot of
the normalized chemical shifts of the protons of the poly(thio)-
ether chain H1-4 versus the number of equivalents of metal
cation. Definitive evidence for 1:1 complex formation is given
by the mass spectrum of the 4TO4/Ba complex, which exhibits
the expected isotopic distribution (Figure 1).
2+
6
TO5 were finally isolated in small amounts during the synthesis
of their respective analogues 4TO4 and 4TO5.
Ionophoric Properties of Crown-Annelated Oligothio-
phenes. The cation complexing properties of 4TO3, 4TO4, and
As expected, the binding constant increases with the size of
the macrocyclic cavity. Thus, whereas 4TO3 shows no com-
plexing properties, 4TO5 exhibits the largest K values. This
result suggests that the longer oligooxyethylene loop of 4TO5
allows a better adaptation of the size and geometry of the
macrocyclic cavity to the size of the metal cation.
0
1
4
TO5 were first analyzed by H NMR (500 MHz) titration
+
+
2+
experiments with solutions of Li , Na , and Ba ions as
perchlorate salts in 1:1 CDCl3/CD3CN. Alkali cations such as
+
+
1
Li and Na have essentially no effect on the H NMR
spectrum. This absence of effect can be related to the inadequate
geometry of the macrocyclic cavity and to the presence of two
sulfur atoms in the macrocycle. In contrast, addition of sub-
Figure 2 shows the chemical shifts of the aromatic thiophenic
protons of the inner thiophenes. Whereas addition of 1 equiv
2
+
of Ba leaves the chemical shifts of the external H and Hb
a
protons practically unchanged (∆δ ) 0 and ∆δHb ) +0.03
Ha
2
+
2+
2+
2+
stoichiometric amounts of Ba , Sr , or Pb induces signif-
icant changes in the chemical shifts of various types of protons.
As shown in Table 1, the aliphatic protons of the polyether
chain (Chart 1) shift downfield by 0.20-0.35 ppm, indicating
that, as expected, complexation of Ba2 essentially occurs within
the macrocyclic cavity.
ppm for 5 equiv of Ba ), the progressive addition of small
amounts of metal cation produces a positive shift of H but a
c
negative shift of Hd.
These conflicting behaviors cannot be directly related to
cation binding, since in that case the shift of both protons should
have the same sign. This shows that cation complexation by
the macrocyclic cavity indirectly produces a modification of
the electronic environment of the protons of the inner thiophene
rings. This result thus provide a first indication that cation
binding by the macrocyclic cavity induces significant confor-
mational changes in the π-conjugated 4T chain.
To better understand the relationships between the conforma-
tion and electronic properties of the annelated nTs, a theoretical
study has been undertaken. To this end, we first analyzed the
crystallographic structure of single crystals of 4TO4 and 6TO4
+
0
Table 2 lists the values of the binding constant (K ) estimated
by treatment of the data collected from the chemical shifts of
H1, H2, H3, and H4 with the EQNMR program.20 These results
confirm that 4TOn do not complex alkali cations but form 1:1
complexes with low to moderate binding constants with alkali
earth cations. The formation of a 1:1 complex is confirmed by
(
19) Roth, G. P.; Farina, V.; Liebeskind, L. S.; Pena-cabrera, E. Tetrahedron
Lett. 1995, 36, 2191.
(20) Hynes, M. J. J. Chem. Soc., Dalton Trans. 1993, 311.
J. AM. CHEM. SOC.
9
VOL. 125, NO. 5, 2003 1365

A R T I C L E S
Jousselme et al.
Figure 2. Changes in the chemical shifts of Hc and Hd aromatic protons
(
see Chart 1) of the central thiophenes of 4TO4 in CDCl3/CD3CN versus
2
+
the number of equivalents of added Ba from a solution of 0.20 M
Ba(ClO4)2 in CD3CN.
Figure 3. Crystal structure of 4TO4 (hydrogens omitted).
Table 3. Crystal Data and Structural Results for 6TO4 and 4TO4
6
TO
4
4TO
4
chemical formula
C34H32O4S8
761.14
C26H28O4S6
596.89
formula weight (g/mol)
3
crystal size (mm )
1.23 × 0.69 × 0.06
1.29 × 0.11 × 0.09
crystal color
crystal system
space group
a (Å)
b (Å)
c (Å)
orange
yellow
monoclinic
P21
orthorhombic
Pna21
16.736(2)
21.481(4)
7.788(1)
10.2317(9)
8.1026(7)
21.592(2)
103.397(8)
1741.3(5)
2
â (deg)
3
V (Å )
2799(1)
4
Z
2
θ range (deg)
2.530
5681
3232
406
0.048
0.064
1.221
2.523
2265
865
measured data
used data [I > 3σ(I)]
no. of variables
R
Figure 4. Crystal structure of 6TO (hydrogens omitted).
4
174
0.065
0.080
1.470
thiophene rings in a syn conformation. This conformation is
stabilized by two strong S‚‚‚S intramolecular interactions
Rw
GOF (goodness-of-fit)
[d(S5-S7) ) 3.174(2) Å; d(S2-S8) ) 3.266(2) Å] shorter than
the sum of the van der Waals radii (3.7 Å). The side view shows
that the π-conjugated 6T chain adopts a quasi-planar conforma-
tion with, however, a bent geometry imposed by the insufficient
length of the polyether loop.
grown by slow evaporation of 1:1 CH3CN/CH2Cl2 (Table 3).
Despite several attempts crystals of the metal containing
macrocycles could not be obtained.
The numbering scheme and the molecular structure of the
noncentrosymmetric 4TO4 molecule are displayed in Figure 3.
The π-conjugated system presents some departure from planar-
ity; furthermore, the 4T chain contains two thiophene rings in
a syn conformation, in striking contrast with the all-anti
Theoretical calculations based on density functional methods
have been performed for dimethylsulfanyl 4T in two different
conformations with the Gaussian98 program. Becke’s three-
22
parameter gradient-corrected functional (B3lyp) with a polarized
6
-31G* basis for all atoms was used to optimize the geometry
21
conformation usually observed for nTs. The torsional angles
between the outer thiophene rings (C1, C2, C3, C4, S1) and
and to compute the electronic structure at the minima found.
The first minimun calculated conformation corresponds to
the isomer shown by the crystal structure in Figure 3, in which
the four thiophenes are in a anti-anti-syn (aas) conformation.
The second minimum conformation anti-syn-anti (asa) is
based on steric considerations associated with cation complex-
(C13, C14, C15, C16, S4) and the neighboring thiophene rings
are 33° and 22°, respectively. The torsional angle between the
middle rings (C5, C6, C7, C8, S2) and (C9, C10, C11, C12,
S3) is 18°. In the crystal structure, the shortest intermolecular
S-S distance (d ) 3.91 Å) is longer than the sum of the van
der Waals radii; nevertheless, long C-H‚‚‚S contacts (3.066 Å
(22) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M.
A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.; Stratmann,
R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin,
K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi,
R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.;
Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.;
Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz,
J. V.; Baboul, A. G.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.;
Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham,
M. A.; Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.;
Gill, P. M. W.; Johnson, B. G.; Chen, W.; Wong, M. W.; Andres, J. L.;
Head-Gordon, M.; Replogle, E. S.; Pople J. A. Gaussian 98 (Revision A.9);
Gaussian Inc.; Pittsburgh, PA, 1998.
<
d < 3.079 Å) and one short C-H‚‚‚O contact (d ) 2.54 Å)
can be found between the molecules.
The structure of 6TO4 (Figure 4) shows that the molecule is
noncentrosymmetric and that the 6T chain also contains two
(
21) (a) van Bolhuis, F.; Wynberg, H.; Havinga, E. E.; Meijer, E. W.; Staring,
E. G. J. Synth. Met., 1989, 30, 381. (b) Chaloner, P. A.; Gunatunga, S. R.;
Hitchcock P. B. Acta Crystallogr., Sect. C, 1994, 50, 1941. (c) Pelletier,
M.; Brisse, F. Acta Crystallogr., Sect. C, 1994, 50, 1942.
1366 J. AM. CHEM. SOC.
9
VOL. 125, NO. 5, 2003

Crown-Annelated Oligothiophenes as Model Compounds
A R T I C L E S
Table 5. Effect of the Complexation of Alkali Earth Metal Cations
on the Absorption Maxima for Crown-Annelated Oligothiophenes in
1:1 CH3CN/CH2Cl2
1a
4TO
a
4TO
a
2b
6TO
c
6TO d
5
4
5
4
+
+
1 equiv of Ba2
e
e
e
413
410
415
416
413
412
e
e
e
e
e
e
e
2
+
+
1 equiv of Sr
+
+
1 equiv of Pb2
381, 446
382, 444
a
b
Changes are relative to absorption maximum of 403 nm. Changes
c
are relative to absorption maximum of 442 nm. Changes are relative to
d
absorption maxima of 381 and 441 nm. Changes are relative to absorption
maxima of 388 and 439 nm. e No change.
Figure 5. Geometry optimization for dimethylsulfanyl quaterthiophene in
aas (top) and asa (bottom) conformations by the functional density method
22
(
Becke3lyp) and Gaussian 98. Bases for calculation are 6-31G* for carbons
and sulfur and 6-31G for hydrogens.
Table 4. Computed Values for the Energy Levels for Ground
State and Cation Radical of Two Conformations of
Dimethylsulfanyl Quaterthiophene
(aas)
(asa)
∆(aas) − (asa)
HOMO (eV)
E (eV)
λmax (nm)
cation radical (eV)
-5.08
3.08
402
-5.02
3.03
409
+0.06
-0.05
+7
∆
-7.91
-7.87
+0.04
Figure 6. Evolution of the UV-vis spectrum of 4TO5 in 1:1 CH2Cl2/
2+
-4
CH3CN versus the number of equivalents of Ba added from a 5 × 10
M perchlorate solution in CH3CN.
ation. In fact, as shown in Figure 5, in the absence of metal
cation, the aas conformation of the 4T chain imposes a minimal
distance of ca. 10.3 Å between the two sulfur atoms at the
3-position of the terminal thiophenes. It is noteworthy that this
distance is in excellent agreement with the 10.113 Å found in
the crystallographic structure in Figure 3. The formation of a
macrocyclic cavity with optimal geometry to fit the cation
requires a reduction of this distance to get closer to the ideal
situation where the two S atoms would be directly linked by
two methylene groups. According to our geometry optimiza-
tions, this implies a contraction of the conjugated chain with a
shortening of the S‚‚‚S distance from 10.3 to 7.5 Å, which results
in transition from an aas to an asa conformation
The data in Table 4 shows that the passage from the aas to
the asa conformation leads to a 0.06 eV increase of the HOMO
4 5
Figure 7. Electronic absorption spectra of 2 (blue), 6TO (red), and 6TO
(black).
(
highest occupied molecular orbital) level and to a comparable
emergence of a vibronic fine structure, which indicates that
cation complexation results in the rigidification of the conjugated
system. On the other hand, the 9-12 nm red shift of λmax shows
that complexation produces, as expected, a decrease of the
HOMO-LUMO gap. It is noteworthy that the magnitude of
these shifts is in good agreement with the theoretically value,
thus providing further support to a transition from an aas to an
asa conformation.
decrease of the HOMO-LUMO (lowest unoccupied molecular
orbital) gap (∆E) at the fundamental state. This decrease of ∆E
corresponds to a 7 nm bathochromic shift of the absorption
maximum. The calculations also predicts a 0.04 eV increase of
the energy of the cation radical, which should correspond to a
4
0 mV decrease of the oxidation potential associated with the
formation of the dicationic state.
Electronic Absorption Spectroscopy. In the absence of
metal cation, the electronic absorption spectra of 4TO3, 4TO4,
and 4TO5 are typical for nTs and show broad structureless
absorption bands with maxima at 403 nm (Table 5 and Figure
Comparison of the electronic absorption spectra of 6TO and
4
6TO with that of the open-chain reference compound 2 reveals
5
a small hypsochromic shif of the λmax of the main absorption
band from 442 nm for 2 and 441 and 439 nm for 6TO and
4
6
).
6TO , respectively (Figure 7). These shifts suggest, in agreement
5
The similarity of this λmax value with that of the reference
with crystallographic data, that the constraint imposed on the
open-chain compound 1 (Scheme 1) shows that fixation of the
polyether loop at both ends of the 4T chain does not produces
6T chain by the attached polyether loop induces a slight decrease
of conjugation.
major distortion in the conjugated chain. As shown in Figure
6
However, the major difference between the spectrum of the
open chain 6T 2 and those of 6TO4 and 6TO5 lies in the
, addition of 1 equiv of Ba2 to a 4TO5 solution produces the
+
J. AM. CHEM. SOC.
9
VOL. 125, NO. 5, 2003 1367

A R T I C L E S
Jousselme et al.
Figure 8. Changes in the UV-vis absorption spectra of 6TO4 (top) and
6
TO5 (bottom) in 1:1 CH2Cl2/MeCN versus the number of added equivalents
2
+
-4
of Pb (from a 4 × 10 M perchlorate solution in CH3CN ).
Figure 9. Evolution of the CV of 4TO4 (top) and 4TO5 (bottom) (1 ×
-
4
2+
-3
10
M) upon addition of Ba [added as 1 × 10 M Ba(ClO4)2] in 0.10
-1
presence of an additional absorption band around 380 nm, which
indicates the coexistence of two distinct conjugation lengths.
The 440 nm band corresponds to the quasi-planar 6T geometry,
whereas the 380 nm band can be attributed to a shorter
conjugation length resulting from the formation of a dihedral
angle between two parts of the conjugated chain by rotation
around a single bond. The maximum at 380 nm suggests that
rotation takes place in the middle of the 6T chain, dividing it
into two quasi-orthogonal 3T segments. Such an hypothesis is
consistent with (i) the absorption maximum expected for an
alkylsulfanyl-substituted 3T and (ii) the presence of a single
additional absorption band in the short-wavelength region. In
fact, a rotation leading to two different conjugated segments
M Bu4NClO4/1:1 CH3CN-CH2Cl2, scan rate 100 mV s .
Table 6. Cyclic Voltammetric Data for Crown-Annelated
Oligothiophenesa
compd
additionb
E0
1
, V
∆E0
1
, mV
E0
2
, V
∆E0
, mV
2
1
0.98
0.87
0.87
1.13
1.10
1.10
4
4
TO3
TO4
+
+
Ba2
+
-6
15
+14
-30
-25
-25
2+
Sr
2
+
+Pb
4
TO5
0.88
1.08
Ba2
Sr
+
+20
+30
-50
-30
+
+
2+
2
6
6
0.85
0.75
0.77
1.08
1.05
1.05
TO4
TO5
(e.g., two and four thiophene rings), should give rise to two
new transitions in this spectral region. The larger intensity of
the 380 nm band observed for 6TO4 compared to 6TO5 is
consistent with a stronger constraint imposed on the 6T chain
by the shorter polyether loop.
a
In 0.10 M Bu4NClO4. Scan rate 100 MV s-1, ref Ag/AgCl. b After
addition of 1 equiv of metal cation.
Addition of metal cation to solutions of 6TO4 and 6TO5
produces a decrease of the 440 nm band with a concomitant
increase of the 380 nm one. As shown in Figure 8, these changes
occur around an isosbestic point, indicating the interconversion
of two distinct species. The largest changes are observed for
In the presence of 0.10 M Bu4NClO4, the CV of 4TO4 and
4TO5 exhibits two reversible one-electron oxidation processes
with redox potentials E 1 and E 2 at 0.87-0.88 and 1.10 V,
0
0
corresponding to the successive generation of the cation radical
0
0
and dication (Figure 9 and Table 6). The E 1 and E 2 values for
the macrocyclic 4Ts are slighly less positive than those for the
open-chain reference compound 1. A similar difference is
6TO5, in agreement with the larger flexibility allowed the 6T
chain by the longer polyether loop. In both cases the strongest
effects are observed with Pb2 and are accompanied by a 5 nm
red shift of the long wavelength band (Table 5).
+
0
0
observed when the E 1 and E 2 values of 6TO4 and 6TO5 are
compared to those of compound 2. Furthermore, for both series
0
0
Cyclic Voltammetry. The cation-binding properties of the
macrocyclic nTs have been investigated by cyclic voltammetry
of macrocycles, the lowest E 1 and E 2 values are obtained for
compounds with the shortest polyether loop. Since the reference
compounds 1 and 2 are expected to adopt a fully anti
conformation in solution, these results suggest that the fixation
of the oligooxyethylene loop imposes a preferential conforma-
tion with a higher HOMO level on the nT chain.
2
+
2+
(
CV) in 1:1 CH2Cl2/CH3CN in the presence of Ba , Sr , and
2+
1
Pb . Owing to the absence of complexation indicated by H
NMR data, 4TO3 was not considered in these experiments and
alkali cations were not investigated.
1368 J. AM. CHEM. SOC.
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Crown-Annelated Oligothiophenes as Model Compounds
A R T I C L E S
Addition of metal cation has no clear effect on the CV of
Scheme 2
6
TO4 and 6TO5. Figure 9 shows the changes induced in the
2+
CV of 4TO4 and 4TO5 by addition of Ba in the electrolytic
medium. Addition of increasing amount of Ba to a 4TO4
solution produces a slight negative shift of E 1 and a larger
negative shift of E 2. Similar effects are observed for Sr with
a larger shift of E 1. For Pb a quite different behavior is
observed since E 1 now shifts toward positive potentials. It is
noteworthy that the most negative shift observed for Sr is
correlated with the smallest binding constant indicated by H
2+
0
0
2+
0
2+
0
2
+
1
2
+
NMR data, whereas the inversion of the shift sign with Pb
corresponds to a much larger binding constant (Table 2). For
0
4TO5, E 1 shifts positively with all cations while the shift of
0
E 2 remains negative but larger than for 4TO4.
In both cases, the maximum potential shift is reached after
addition of 1 equiv of cation. Although these results confirm
the formation of a 1:1 complex between the crown-annelated
2+
2+
2+
1
4
Ts and Ba , Sr , and Pb in close agreement with H NMR,
mass spectrometry, and optical data, the observed electrochemi-
cal behavior is quite unusual.
Cation complexation by redox-active crown ethers containing
reversibly oxidizable probes such as, e.g., ferrocene or tetrathia-
fulvalene (TTF) generally induces a positive shift of the
oxidation potential due to electrostatic repulsion between the
23,24
positive charges of the metal cation and the oxidized probe.
adopt an optimal geometry for cation complexation. As already
discussed, this process results in a decrease of the distance
between the two sulfide groups, which in turn induces a
transition in the 4T chain from an aas to an asa conformation
more easily oxidizable. When the binding constant is low, as
For macrocycles with two-redox states probes such as TTF, the
second oxidation step is unaffected in the presence of metal
cations. This invariance is generally attributed to the expulsion
of the cation after the first TTF oxidation.
Whereas a similar behavior has also been observed for crown
ether-derivatized polythiophenes,13 examples of redox-active
macrocycles undergoing a negative potential shift in the presence
of metal cations are scarce. Such behavior has been observed
2+
2+
for 4TO4 and Ba or Sr (Table 2), repulsive electrostatic
interactions are weak and geometrical effects predominate,
0
inducing a red shift of λmax and a negative shift of E 1. In
0
5
contrast, for 4TO5 the larger binding constant (K > 10 )
suggests a better match between the geometry of the macrocyclic
cavity and metal cation. In addition to an enhanced Coulombic
repulsion between positive charges, cation complexation results
in a decrease of the electron-releasing effect of the sulfur atoms
connected to the thiophene rings. These combined effects
25
for some TTF cryptands and for some polyether-derivatized
poly(thiophenes).14 In this latter case, spectroelectrochemical
experiments have revealed a cation-induced planarization of the
poly(thiophene) chain.1
4b
A major difference between most of the redox-active crown
ethers reported so far and the present systems lies in the large
conformational flexibility of the nT chain, which acts at the
same time as a part of the macrocyclic cavity and as redox probe.
Since the electronic properties of the nT chain strongly depend
on its geometry, the effects of cation complexation must be
discussed in the framework of an interplay between electrostatic
interactions and conformationnal changes.
0
produce a positive shift of E 1 larger than for 4TO4. Although
0
conformational changes that should produce a decrease of E 1
still occur [as demonstrated by the red shift of λmax (Table 3)],
0
the net effect is nevertheless a positive shift of E 1.
Oxidation of π-conjugated systems with a nondegenerate
ground state such as polythiophene to the cation radical state is
accompanied by a transition from an aromatic to a quinoid
2
6
On this basis, the following mechanism can be proposed to
interpret the various sets of experimental results (Scheme 2).
Metal cation complexation in the neutral state forces the
polyether chain to form a macrocyclic cavity, which tends to
structure. Consequently, once the first oxidation stage is
reached, the π-conjugated system is locked into a fixed
conformation. Thus, if metal cation complexation imposes a
preferential conformation to the neutral nT chain, this conforma-
tion is retained after the first oxidation step. Since, as suggested
by UV-vis and CV data, the presence of a metal cation in the
macrocycle imposes a conformation of higher HOMO level to
the 4T chain, it follows that even after cation expulsion
consecutive to the first oxidation step, the 4T chain retains the
asa conformation induced by the transient stay of the metal
cation in the macrocycle. Then, the second oxidation process
starts from an aas conformation of the 4T chain in the absence
(
23) Recent reviews: (a) Boulas, P. L. M.; Gomez-Kaifer, M.; Echegoyen, L.
Angew. Chem., Int. Ed. 1998, 37, 216. (b) Beer, P. D.; Gale, P. A.; Chen,
G. Z. J. Chem. Soc., Dalton. Trans. 1999, 1897. (c) Bronsted Nielsen, M.;
Lomholt, C.; Becher, J. Chem. Soc. ReV. 2000, 29, 153.
(24) (a) Hansen, T. K.; Jørgensen, T.; Stein, P. C.; Becher, J. J. Org. Chem.
1
992, 57, 6403. (b) Bryce, M. R.; Batsanov, A. S.; Finn, T.; Hansen, T.
K.; Howard, J. A. K.; Kamenjicki, M.; Lednev, I. K.; Ascher, S. A. Chem.
Commun. 2000, 295. (c) Le Derf, F.; Mazari, M.; Mercier, N.; Richomme,
P.; Levillain, E.; Becher, J.; Garin, J.; Orduna, J.; Gorgues, A.; Sall e´ , S.
Chem. Commun. 1999, 1417. (d) Le Derf, F.; Mazari, M.; Mercier, N.;
Levillain, E.; Tripp e´ , G.; Riou, A.; Richomme, P.; Becher, J.; Garin, J.;
Orduna, J.; Gallego-Planas, N.; Gorgues, A.; Sall e´ , M. Chem. Eur. J. 2001,
7
, 447.
(26) (a) Br e´ das, J. L.; Th e´ mans, B.; Fripiat, J. G.; Andr e´ , J. M.; Chance, R. R.
Phys. ReV. B 1984, 29, 6761. (b) Patil, A. O.; Heeger, A. J.; Wudl, F.
Chem. ReV. 1988, 88, 183.
(
25) Gasiorowski, R.; Jørgensen, T.; Moller, J.; Hansen, T. K.; Pietraszkiewicz,
M.; Becher, J. AdV. Mater. 1992, 4, 568.
J. AM. CHEM. SOC.
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VOL. 125, NO. 5, 2003 1369

A R T I C L E S
Jousselme et al.
of metal cation and from an asa conformation in the other case
1H NMR, optical, electrochemical, and theoretical results
provide conclusive evidence showing that cation complexation
induces a conformational transition in the oligothiophene chain.
This process can be utilized either as a means to achieve a
mechanical control of the electronic properties of the π-conju-
gated system or as a source of molecular actuation, thus
contributing to enrich the toolbox for the emerging field of
molecular machines and motors.
(Scheme 2). Theoretical calculations performed on the dimethyl-
sulfanyl 4T cation radical shows that the energy of the asa
conformation is ca. 0.04 eV higher than for the aas conformation
(
Table 4). This result nicely agrees with the 30-50 mV negative
0
shift of E 2 observed in the presence of metal cation.
Conclusion
Crown-annelated oligothiophenes have been synthesized by
the thiolate protection/deprotection method. Analysis of the
ionophoric properties of the macrocycles by H NMR, mass
Extension of this concept to other annelated oligothiophenes
containing photostimulable driving groups is now underway and
will be reported in forthcoming papers.
1
spectrometry, UV-vis spectroscopy, and cyclic voltammetry
gives consistent results which show that the compounds do not
complex alkali cations but show moderate to good binding
properties for alkali-earth cations. The cation-binding constant
increases with the length of the polyether chain, whereas shorter
polyether loops impose a stronger constraint to the conjugated
system.
Supporting Information Available: Experimental procedures
for the synthesis of all compounds and X-ray crystallographic
files for 6TO4 and 4TO4 (PDF). This information is available
free of charge via the Internet at http://pubs.acs.org.
JA026819P
1370 J. AM. CHEM. SOC.
9
VOL. 125, NO. 5, 2003