Oligothiophene-based catenanes: Synthesis and electronic properties of a novel conjugated topological structure

Article abstract of DOI:10.1002/anie.200602652

(Figure Presented) Make it a double: A double metal template strategy has been used to form a π-conjugated catenane consisting of inter-twined macrocycles with oligothienyl and phenanthroline units (see calculated structure). The optical and redox properties as well as the structural and conformational analyses give clear evidence that the two macrocycles in the catenane influence each other by through-space donor-acceptor interactions.

Full text of DOI:10.1002/anie.200602652

Angewandte
Chemie
DOI: 10.1002/anie.200602652
Noncovalent Interactions
Oligothiophene-Based Catenanes: Synthesis and Electronic Properties
of a Novel Conjugated Topological Structure**
Peter Bäuerle,* Martin Ammann, Markus Wilde, Günther Götz, Elena Mena-Osteritz,
Alexander Rang, and Christoph A. Schalley
Dedicated to Professor Klaus Müllen on the occasion of his 60th birthday
Oligo- and polythiophenes are organic semiconductors that
have important applications in organic electronics.[1] Various
molecular shapes and dimensions, such as 1D linear,[2] 2D
macrocyclic,[3] disk, and starlike,[4] as well as 3D dendritic,[5]
have been developed. In particular, a-conjugated macrocyclic
oligothiophenes have shown intriguing electronic[6] and self-
assembling properties[7] as a result of their circular structure
which makes them interesting as models for infinite, defect-
free conjugated oligothiophene chains.[8]
Molecular topologies such as rotaxanes, catenanes, and
knots have become a field of growing interest and underwent
a transition from being laboratory curiosities[9] to more-
accessible compounds.[10] Efficient synthesis procedures based
on the supramolecular preorganization of relevant building
blocks by directed metal–ligand complexation,[11] donor–
acceptor p–p interactions,[12] hydrogen bonding,[13] chemically
linked templates,[14] or routes which completely rely on self-
assembly[15] led to very complex and sophisticated molecular
structures. Interesting novel properties originating from the
topology and independent motion, rotation, and translation
of a ring[16] became the basis for applications as switchable
devices in molecular electronics and machines.[17]
We aimed to prepare novel p-conjugated topologies based
on oligothiophenes by combining their outstanding electronic
benefits with the specific properties of the topological
structure. In contrast to many already existing, quite flexible
intertwined molecules, fully conjugated building blocks
involve more rigid structural elements, which make their
preparation a major unresolved synthetic challenge until now.
Our first attempt towards conjugated catenanes based on
interlocked, conjugated terthienylphenanthroline macrocy-
cles ended with a CuI catenate (Scheme 1).[18a] Mutual steric
Scheme 1. Conjugated CuI catenate synthesized from terthienylphen-
anthrolines.
[*] Prof. Dr. P. Bäuerle, Dr. M. Ammann,[+] M. Wilde,[#] Dr. G. Götz,
Dr. E. Mena-Osteritz
Institut für Organische Chemie II und Neue Materialien
Universität Ulm
Albert-Einstein-Allee 11, 89081 Ulm (Germany)
Fax: (+49)731-502-2840
E-mail: peter.baeuerle@uni-ulm.de
Homepage: http://www.uni-ulm.de/oc2/
A. Rang, Prof. Dr. C. A. Schalley[$]
KekulØ Institut für Organische Chemie und Biochemie
Universität Bonn, Gerhard-Domagk-Strasse 1
53121 Bonn (Germany)
constraints between the relatively small macrocycles (28-
membered rings) prevented demetalation and formation of
the Cu-free catenane. Here, we report the synthesis and
characterization of an expanded catenate 11 (Scheme 3) and
the first demetalated conjugated catenane 12 consisting of
quaterthiophene (4T), phenanthroline, and diacetylene units.
The term “conjugated catenane” is used here not only
because the mechanically bound catenane wheels are fully
conjugated, but also to express that this novel topological
structure may potentially offer very interesting and novel
electronic and magnetic properties if one ring influences the
other “through space”.
[+] New address: NOVALED AG
Zellescher Weg 17, 01069 Dresden (Germany)
[$] New address: Institut für Organische Chemie und Biochemie
Freie Universität Berlin, Takustrasse 3, 14195 Berlin (Germany)
[#] New address: Uzin Utz AG
The well-known CuI-templated phenanthroline-based
catenane synthesis developed by Sauvage and co-workers
was utilized[10,11a,b] to provide a system with complete
p conjugation, since 2,9-bis(oligothienyl)phenanthrolines are
fully conjugated.[19] The synthesis of the enlarged system
(Scheme 2) started from 2,9-diiodophenanthroline (1) which
was subjected to a Neghishi-type coupling with zincated
thiophene to form 2,9-di(thien-2-yl)phenanthroline (2, 92%).
Subsequent selective iodination with N-iodosuccinimide led
Dieselstrasse 3, 89079 Ulm (Germany)
[**] This work was supported by the DFG in the frame of SFB 569 (Univ.
Ulm), SFB 624 (Univ. Bonn), and the Fonds der Chemischen
Industrie. We would like to thank Dr. Roland Werz (Institut für
Organische Chemie I, Univ. Ulm) for NMR spectroscopic mea-
surements and valuable discussions. This work was supported by
graduate student foundation grants (to M.A.) from the state of
Baden-Württemberg and from the Studienstiftung des Deutschen
Volkes (to A.R.).
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that of homologous terthienylphenanthroline (70%) and
quinquethiophenephenanthroline (7%).[19] This successful
Pt-templated macrocyclization of 6 was then applied to CuI
complex 7 after desilylation with CsF. The tetraethynylene
was treated with two equivalents of cis-[Pt(dppp)Cl2] for
three days at room temperature to give trismetalated PtII-CuI-
PtII-catenate 10 (34%). Subsequent reaction with two equiv-
alents of iodine in THF led to 1,1-reductive elimination of the
À
Pt corners and simultaneous C C bond formation to generate
CuI catenate 11 (74%), which represents the higher homo-
logue of the previously synthesized terthienylphenanthroline
catenate (Scheme 1). In this case, because of the larger ring
sizes (34-membered ring), decomplexation of the CuI center
was achieved by reaction with KCN, and “conjugated
catenane” 12 was isolated in pure form after chromatography
(44%). Evidence for the interlocked structure of 12 comes
1
from H NMR spectroscopy as well as tandem mass spec-
trometry.[20]
ESI-FTICR mass spectrometry of methanol solutions
(with 0.5% CF3CO2H) of 8 and 12 generates ions corre-
sponding to [8 + H+] and [12 + H+] at the expected m/z values
of 1331.5 and 2662.1 Da, respectively. Experimental and
calculated isotope patterns agree well, thus confirming the
elemental compositions (Figure 1a). After careful mass-
selection of [12 + H+], the protonated catenane was subjected
to collision-induced dissociation (Figure 1b). The only frag-
ment stemming from cleavage of a covalent bond within the
macrocycle backbone corresponds to loss of one complete
wheel. As observed earlier,[18a] a series of alkyl chain losses
from the butyl groups on the thiophene rings originating from
both the parent catenane and the macrocycle are also found.
Macrocycle 8 shows the same series of alkyl side-chain
fragmentations in an analogous MS/MS experiment. This
fragmentation pattern is not in line with a weak (proton-
bridged) complex of two non-intertwined wheels, because the
extensive losses of alkyl groups cannot compete with disso-
ciation of such a weak complex into two separate macrocycles.
Neither does the fragmentation pattern support a single, large
macrocycle with the elemental composition of the catenane,
since the only backbone fragment (excluding the losses of
alkyl groups) is the loss of one wheel. This behavior is typical
of catenanes,[18] in which only one covalent bond within a
macrocycle backbone needs to be broken, while a single
macrocycle would require the cleavage of two covalent bonds
and thus should lead to a number of different fragments. The
only remaining topology is consequently that of a catenane.
The geometrical structure and preferred conformation of
macrocycle 8 and catenane 12 were investigated by detailed
Scheme 2. Reaction sequence leading to crescent-shaped 2,9-bis-
(quaterthienyl)phenanthroline (6) by Neghishi-type couplings, selective
iodinations, and Sonogashira–Hagihara coupling. NIS=N-iodosuccin-
imide.
to diiodo derivative 3 (79%), which was coupled analogously
with zincated tetrabutylterthiophene to yield 2,9-bis(quater-
thien-2-yl)phenanthroline (4, 67%) and finally diiodo com-
pound 5 (94%). Sonogashira-type coupling of 5 with trime-
thylsilyl-protected acetylene gave crescent-shaped 6 (89%) in
good overall yield as the precursor for cyclization experi-
ments.
Complexation of 6 with [Cu(CH3CN)4]BF4 led to the
homoleptic bis-phenanthroline complex 7 (89%) in which the
two phenanthroline units are preorganized in a pseudotetra-
hedral manner around a CuI center. After deprotection of the
acetylene groups by cesium fluoride, complex 7 was treated
under Eglington and pseudo-high-dilution conditions with
Cu(OAc)2/CuCl/pyridine to accomplish the ring closure.
Surprisingly, macrocycle 8 was isolated in 50% yield instead
of the expected catenate (Scheme 3).[20] Apparently, closure
of the first macrocycle generates steric constraints, and thus
results in the second open-chain unit disentangling. Since all
typical procedures for double macrocyclization of 7 failed, we
applied our recently developed Pt-templated macrocycliza-
tion protocol[3c] using crescent-shaped 6 as a model. Metal-
lamacrocycle 9 was formed in 23% yield by reaction of
desilylated 6 with one equivalent of cis-[Pt(dppp)Cl2] and
10% CuI in toluene/Et3N. This yield is intermediate between
1
analyses of H and 2D NMR spectra in combination with
semiempirical calculations. One set of signals in the NMR
spectra of macrocycle 8, a low-field shift of the b protons on
the “inner” thiophene rings and a high-field shift of the closest
phenanthroline protons (H3, H3’) relative to those of open-
chained 6, point to a symmetric over-all conformation of the
macrocycle and to a distortion of the “inner” thiophene rings.
This proposal agrees well with calculations which gave a
symmetrical minimum energy conformation in which the
“inner” thiophene rings are rotated by 95–1008, thus leading
to a puckering of the phenanthroline unit by 278 from
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Scheme 3. 2,9-Bis(quaterthienyl)phenanthroline 6 leads to homoleptic CuI complex 7 which can be used as a central building block for the
synthesis of macrocycle 8 and catenane 12. dppp=propane-1,3-diylbis(diphenylphosphane).
Angew. Chem. Int. Ed. 2007, 46, 363 –368
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macrocycles, which is frequently found for conventional
catenanes,[16] is hindered up to 1008C. Calculations predict a
stable minimum energy conformation of catenane 12 consist-
ing of two puckered, but intertwined symmetric macrocycles
of 8. In comparison to CuI catenate 11, the distance between
the two phenanthroline units in 12 has increased by approx-
imately 1 Š during demetalation (Figure 2).
Catenane 12 is a unique molecular system in which the
conjugated macrocycles are mechanically kept in proximity
and may influence each other electronically “through space”.
Investigation of the optical and redox properties of 12 relative
to those of macrocycle 8 gave clear evidence for a mutual
interaction between its two rings.
The absorption spectra of both compounds showed a
broad band arising from the p–p* transition of the conjugated
system, with a maximum at nearly the same position and a
tailing at lower energy, which is typical for cyclothiophenes[6]
(Table 1, Figure 3). However, the molar extinction coefficient
is doubled for catenane 12, thus indicating an additive
contribution of both rings in the intertwined system. If both
spectra are normalized (same concentration of rings), a
difference spectrum shows additional bands for catenane 12
from which the band at lower energy (l ꢀ 530 nm) indicates
an inter-ring charge-transfer (CT) transition. This finding is
compatible with the conformation discussed above in which
the (oligo)thiophene donor units in one ring are in proximity
to the phenanthroline acceptor unit of the other ring
(Figure 2). The decrease in the fluorescence quantum yield
of catenane 12 (F = 2%) relative to that of macrocycle 8 (F =
6%) and open-chained 6 (F = 15%) is also consistent with a
quenching arising from an “intermolecular” charge-transfer
interaction in catenane 12.
Figure 1. a) ESI-FTICR mass spectrum of a solution of catenane 12 in
methanol (0.5% CF3CO2H). Insets: experimental and calculated iso-
tope patterns. b) Collision-induced dissociation (collision gas: Ar)
experiment conducted with mass-selected [12+H+].
coplanarity.[21] Additionally, STM experiments on monolayers
of 8 adsorbed on graphite display the symmetrical structure of
the conjugated macrocycle (Figure 2).
1
The same trends are found in the H NMR spectra of
catenane 12, and temperature-dependent measurements
showed that the dynamic motion of the two interlocked
Figure 2. Calculated minimum energy conformation of macrocycle 8 (a: topview, b: side view). STM image of a monolayer of 8 at the solid
(highly ordered pyrolytic graphite; HOPG)/liquid (1,2,4-trichlorobenzene) interface (c). Calculated minimum energy conformation of catenane 12
(d). The dashed lines illustrate “intermolecular” donor–acceptor interactions.
Table 1: Spectroscopic and redox data for catenane 12, macrocycle 8, and open-chained 6.
Compound lmaxabs [nm][a] e”104 [LmolÀ1 cmÀ1
]
Eg [eV][b] lmaxem [nm][a]
F
em [%][c] Eoox1 [V][d] Eoox2 [V][d] Eoox3 [V][d] Eoox4 [V][d] Eored1 [V][e]
6
8
12
409
412
413
6.1
8.6
17.0
2.34
2.32
2.30
553
555, 598
565, 602
15
6
2
0.38
0.33
0.55
0.46
0.42
0.55
0.73
0.83
0.89
0.73
0.90
1.04
À2.40[f]
À2.14[f]
À2.13[f]
[a] In CH2Cl2. Strongest emission peak shown in italics.[b] The optical band gap was determined from the tangent at the low energy side of the main
absorption band. [c] Standard 9,10-diphenylanthracene. [d] Redox potentials versus Fc/Fc+ (c=5”10À4 m for 8 and 2.5”10À4 m for 12 in CH2Cl2/0.1m
TBAPF6 at 295 K, v=100 mVsÀ1. [e] In THF/0.1m TBAPF6 at 10À3 molLÀ1. [f] Irreversible redox process, E8 at I8=0.855Ip.
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Chemie
withdrawing phenanthroline unit. This effect is also due to
inter-ring, through-space, donor–acceptor interactions, and
strongly corroborates the symmetrical and puckered confor-
mation discussed above.
In conclusion, we have prepared a unique “p-conjugated
catenane” consisting of intertwined oligothiophene–phen-
anthroline macrocycles as a novel topological structure in the
field of conjugated oligomers and polymers by a double metal
template strategy. The interesting optical and redox proper-
ties found are in line with the structural and conformational
analyses, which gave clear evidence that the two macrocycles
in catenane 12 influence each other by through-space donor–
acceptor interactions. Investigations on the Arahonov–Bohm
effect on a single molecule, for example, would be possible
with these types of molecules.[22]
Figure 3. Normalized absorption spectra of catenane 12 (solid black
line) in comparison to that of macrocycle 8 (dotted line) in dichloro-
methane and a difference spectrum comparing the two compounds
(gray line).
Received: July 4, 2006
Published online: October 31, 2006
Keywords: catenanes · conjugation · noncovalent interactions ·
.
oligothiophenes · template synthesis
Cyclic voltammetry studies showthe remarkably different
redox properties of 12, 8, and 6 (Table 1, Figure 4). The
important electrophores in our systems are the 4Tunits which
typically are oxidized reversibly to radical cations and
dications by successive one-electron transfers. The corre-
sponding potentials at which these processes occur generally
depend on electronic influences in the vicinity of the electro-
phore. In this respect, macrocycle 8 shows first and second
one-electron oxidation at potentials slightly negative of those
of open-chained 6 (Table 1) and correspond to the formation
of stable 4Tradical cations. Further oxidation leads to tri- and
tetracationic macrocycles at potentials which are more
positive than those of 6. All oxidation processes (of the 4T
units) of intertwined catenane 12 are in general shifted to
positive potentials because of the proximity of the electron-
[1] Handbook of Oligo- and Polythiophenes (Ed.: D. Fichou),
Wiley-VCh, Weinheim, 1999.
[2] a) P. Bäuerle in Electronic Materials: The Oligomer Approach
(Eds.: K. Müllen, G. Wegner), Wiley-VCh, Weinheim, 1998,
pp. 105 – 197; b) T. Izumi, S. Kobashi, K. Takimiya, Y. Aso, T.
Otsubo, J. Am. Chem. Soc. 2003, 125, 5286 – 5287.
[3] a) J. Krömer, I. Rios-Carreras, G. Fuhrmann, C. Musch, M.
Wunderlin, T. Debaerdemaeker, E. Mena-Osteritz, P. Bäuerle,
Angew. Chem. 2000, 112, 3623 – 3628; Angew. Chem. Int. Ed.
2000, 39, 3481 – 3486; b) G. Fuhrmann, P. Bäuerle, Chem.
Commun. 2003, 926 – 927.
[4] Y. Nicolas, P. Blanchard, E. Levillain, M. Allain, N. Mercier, J.
Roncali, Org. Lett. 2004, 6, 6433 – 6446.
[5] a) C. Xia, X. Fan, J. Locklin, R. C. Advincula, Org. Lett. 2002, 4,
2067 – 2070; b) C. Xia, X. Fan, J. Locklin, R. C. Advincula, A.
Gies, W. Nonidez, J. Am. Chem. Soc. 2004, 126, 8735 – 8743;
c) C.-Q. Ma, E. Mena-Osteritz, T. Debaerdemaeker, P. Bäuerle,
Angew. Chem. submitted.
[6] M. Bednarz, P. Reineker, E. Mena-Osteritz, P. Bäuerle, J. Lumin.
2004, 110, 225 – 231.
[7] a) E. Mena-Osteritz, P. Bäuerle, Adv. Mater. 2001, 13, 243 – 246;
b) E. Mena-Osteritz, Adv. Mater. 2002, 14, 609 – 616; c) E. Mena-
Osteritz, P. Bäuerle, Adv. Mater. 2006, 18, 447 – 451.
[8] a) A. J. W. Tol, Synth. Met. 1995, 74, 95 – 98; b) S. S. Zade, M.
Bendikov, J. Org. Chem. 2006, 71, 2972 – 2981; c) S. Fomine, P.
Guadarrama, J. Phys. Chem. A 2006, 110, 10098 – 10105.
[9] a) H. L. Frisch, E. Wasserman, J. Am. Chem. Soc. 1961, 83,
3789 – 3795; b) G. Schill, A. Lüttringhaus, Angew. Chem. 1964,
76, 567 – 568; Angew. Chem. Int. Ed. Engl. 1964, 3, 546 – 547;
c) G. Schill, Chem. Ber. 1967, 100, 2021 – 2037.
[10] Molecular Catenanes, Rotaxanes, and Knots (Eds.: J.-P. Sauvage,
C. O. Dietrich-Buchecker), Wiley-VCH, Weinheim, 1999.
[11] a) C. O. Dietrich-Buchecker, J.-P. Sauvage, J.-P. Kintziger, Tet-
rahedron Lett. 1983, 24, 5095 – 5098; b) J. P. Sauvage, Acc. Chem.
Res. 1990, 23, 319 – 327; c) L. Hogg, D. A. Leigh, P. J. Lusby, A.
Morelli, S. Parsons, J. K. Y. Wong, Angew. Chem. 2004, 116,
1238 – 1241; Angew. Chem. Int. Ed. 2004, 43, 1218 – 1221.
[12] D. B. Amabilino, J. F. Stoddart, Chem. Rev. 1995, 95, 2725 – 2828.
[13] a) C. A. Hunter, J. Am. Chem. Soc. 1992, 114, 5303 – 5311; b) F.
Vögtle, T. Dünnwald, T. Schmidt, Acc. Chem. Res. 1996, 29, 451 –
Figure 4. Deconvoluted cyclic voltammogram of catenane 12 (black
line) in comparison to that of macrocycle 8 (gray line) (12:
2.5”10À4 m, 8: 5”10À4 m, in CH2Cl2/0.1m TBAPF6 at 295 K,
v=100 mVsÀ1, potentials versus the ferrocene/ferrocenium (Fc/Fc+)
couple. TBA=tetrabutylammonium.
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Communications
460; c) D. G. Hamilton, J. K. M. Sanders, J. E. Davies, W. Clegg,
S. J. Teat, Chem. Commun. 1997, 897 – 898.
8 Hz, 2H), 7.75 (s, 2H), 7.36 (d, 3J = 4 Hz, 2H), 7.05 (s, 4H),
2.60–2.82 (m, 16H), 1.79 (brs), 1.69–1.43 (m, 32H), 0.89–
0.99 ppm (m, 24H); m/z calcd for [M+H]+ (C80H87N2S8):
1331.4629; found: 1331.6 [M+H]+ (MALDI-TOF-MS);
1331.4693 [M+H+] (HRMS (ESI-FTICR)); UV/Vis (CH2Cl2):
lmax = 413 nm (e = 8.9 ꢀ 104 LmolÀ1 cmÀ1). Analytical data for
catenane 12: 1H NMR (CDCl3, 400 MHz): d = 8.22 (d, 3J = 4 Hz,
4H), 8.13 (d, 3J = 8 Hz, 4H), 7.79 (d, 3J = 8 Hz, 4H), 7.48 (s, 4H),
7.21 (d, 3J = 4 Hz, 4H), 7.06 (d, 3J = 4 Hz, 4H), 7.03 (d, 3J = 4 Hz,
4H), 2.60–2.78 (m, 32H), 1.35–1.61 (m, 64H), 0.89–0.99 ppm (m,
48H); m/z calcd for [M+H]+ (C160H173N4S16): 2661.9186; found:
2661.9 [M+H]+ (MALDI-TOF-MS), 2661.9387 [M+H+]
(HRMS (ESI-FTICR)); UV/Vis (CH2Cl2): lmax = 412 nm (e =
1.7 ꢀ 105 LmolÀ1 cmÀ1).
[14] a) S. Duda, A. Godt, Chem. Eur. J. 1999, 5, 1728 – 1733; b) A.
Godt, Eur. J. Org. Chem. 2004, 1639 – 1654.
[15] M. Fujita, Acc. Chem. Res. 1999, 32, 53 – 61.
[16] A. R. Pease, J. O. Jeppesen, J. F. Stoddart, Y. Luo, C. P. Collier,
J. R. Heath, Acc. Chem. Res. 2001, 34, 433 – 444.
[17] V. Balzani, M. Venturi, A. Credi, Molecular Devices and
Machines, Wiley-VCH, Weinheim, 2003.
[18] a) M. Ammann, A. Rang, C. A. Schalley, P. Bäuerle, Eur. J. Org.
Chem. 2006, 1940 – 1948; see also: b) C. A. Schalley, J. Hoern-
schemeyer, X. Li, G. Silva, P. Weis, Int. J. Mass Spectrom. 2003,
228, 373 – 388; c) C. A. Schalley, P. Ghosh, M. Engeser, Int. J.
Mass Spectrom. 2004, 232–233, 249 – 258; d) M. Hutin, J. R.
Nitschke, C. A. Schalley, G. Bernardinelli, Chem. Eur. J. 2006,
12, 4069 – 4076.
[21] According to calculations, a fully coplanar conformation of
macrocycle 8 could be achieved by an anti arrangement of one
“inner” thiophene ring, thus leading to a nonsymmetric struc-
ture.
[19] M. Ammann, P. Bäuerle, Org. Biomol. Chem. 2005, 3, 4143 –
4152.
1
[20] Analytical data for macrocycle 8: H NMR (CDCl3, 400 MHz):
[22] J. C. Kimball, H. L. Frisch, Phys. Rev. Lett. 2004, 93, 093001.
3
3
d = 8.30 (d, J = 8 Hz, 2H), 8.25 (d, J = 4 Hz, 2H), 7.82 (d, 3J =
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