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DidodecylsexithiopheneЧA Model Compound for the Formation and Characterization of Charge Carriers in Conjugated Chains.

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[I51 a) M. E. Fajardo, V. A. Apkarian, J, Chem. Phjs. 1986, 85. 5660-5681;
ihrd. 1988, 89, 4102-4123; rbicl. 1988, 89. 4124-4136. The absorption
spectra we measured correspond to the reported excitation spectra of the
initially formed exciplexes 8. The same is true of the emission spectra ofthe
triatomic species 9; b) I. Last, T. F. George, J. Chem. P h u . 1987, 86.37873794; I. Last, T. F. George, M. E. Fayardo, V. A. Apkarian, ihid. 1987.87.
5917 - 5927.
[I61 Triplet 2 is 27.5 kcalmol-’ higher in energy than 7. singlet 2 is
15.3 kcalmol-’ higher in energy than triplet 2: R. Janoschek, Chrm. Unscrer Zeit 1991, 25. 59-66, R. Janoschek, University of Graz. private
[17] A single-electron transfer (SET) mechanism for the ring opening of 7
cannot be assumed. The energy for the irradiation IS not enough to ionize
7 (IP = 9.57 eV; K. B. Wiberg. G. B. Ellison. J. J. Wendoloski. C . R. Brundle. N . A. Kuebler. J. A m . Chrn?.So<. 1976. 98. 7179-7187).
Didodecylsexithiophene-A Model Compound
for the Formation and Characterization of Charge
Carriers in Conjugated Chains**
redox states of 1; a dimerization of the radical cation 1‘+ to
(1);+ was also observed.
Controlled electrochemical and chemical oxidation (with
iron trichloride), and reduction (with potassium) led to the
different redox states of 1, which were characterized by cyclic
voltammetry. and absorption (Table 1) and ESR spectroscopy. The cyclic voltammogram exhibits two reversible
waves in the oxidation part (EY = 0.34V and E ; =
0.54 V[81) which correspond to a one-electron transfer in
each step leading to the radical cation 1“ and the dication
l ” , respectively (Fig. 1). The first oxidation potential of 1 is
close to that ofpolythiophene (E” = 0.30 Vr9]),which shows
only one extremely broad redox wave because of the chainlength distribution in the polymer. Since the alkyl side chains
improve the solubility, the reduction of 1 could also be examined. Two further reversible one-electron transfer steps were
evident in the reduction cycle of the cyclic voltammogram
( E : = - 2.27 V and E: = - 2.40 V) providing the radical
anion 1‘- and the dianion 1‘- (Fig. 1).
By Peter Bauerle,* Uwe Segelbacher, Kai-Uwe Caudl,
Dieter Huttenlocher, and Michael Mehring
Oligothiophenes are among the best investigated model
compounds for electrically conducting polymers.’’] The excellent properties, which in some respects surpass those of
the related polymers,[’] are attained by stepwise chemical
assembly leading to compounds with well-defined structures
and controlled chain and conjugation length. The stability of
the oligothiophenes in both their neutral and oxidized forms
allows the precise characterization of the electronic structure
and the charge carrier responsible for the conductivity along
the conjugated chains.13] The characterization of oligothiophenes in solution, in which the cooperative interactions
characteristic of the solid state are eliminated, is more diffcult because of their inherent reactivity in the oxidized state
(12 I
5 ) and their poor solubility with increasing chain length
(n2 6). However, when the reactive terminal positions141are
blocked or solubilizing alkyl groups are introduced to the
o l i g ~ m e r , the
[ ~ ~precise characterization of the oligomers in
solution is possible.
In this context we have synthesized 3””-4‘-didodecyI2,2’: 5‘,2”: 5”,2”’: 5“‘,2““:5””,2”“’-sexithiophene (I), whose
alkyl substituents in contrast to the compounds in previous
studies are fixed at defined positions, and whose structure
is established unambiguously by NMR spectroscopy and
X-ray structure analysis.“. I’ Sexithiophene 1 is the first compound of this type that can be oxidized to form a stable
dication and also reduced to form a stable dianion. We report here on the preparation and characterization of the
Dr. P. Biuerle, Dr. K.-U. Gaud1
lnstitut fur Organische’Chemie und Isotopenforschung der Universitit
Pfaffenwaldring 55. D-W-7000 Stuttgart 80 (FRG)
Dipl.-Phys. U. Segelbacher. DiplLPhys. D. Huttenlocher.
Prof. M. Mehring
2. Physikalisches Institut der Universitit
Pfaffenwaldring 57. D-W-7000 Stuttgart 80 (FRG)
Thiopfienes. Part 9. This research was supported by the Deutsche
Forschungsgemeinschaft (SFB 329) and the Bundesminister fur
Forschung und Technologie (TK 0325). We thank Dr. A. Grupp and Dr.
S. Sariciftci for helpful discussions. - Part 8 [h].
9‘; VCH
m h H , W-6940 Wemheim, 1993
Fig. 1. Cyclic voltammogram of 1 recorded with a scan rate of 100 mVs- The
oxidation was measured in dichloromethdne, the reduction in T H E The potentials are referenced to ferrocene (Fc)/Fc+
The stability of all the redox states allowed their characterization by absorption spectroscopy. In comparison to the
unsubstituted sexithiophene 2 (i,,,
432 nm, E =
2.87 eV),[31the neutral dialkylsexithiophene 1 absorbs as expected at a somewhat shorter wavelength (&, = 43 6 nm,
E = 2.97 eV), due to the steric interaction of the alkyl chains
with the conjugated i~ system. This interaction is also reflected in the crystal structure of 1 by the twisting of the relevant
thiophene rings by 10.8°.[71Two bands are observed in the
absorption spectrum of cation radical I * + , each with shoulders [sh] at higher energies ( E = 0.87, 1.14[sh], 1.60,
1.81[sh] eV) (Fig. 2 top); the structure and position of these
bands are almost identical to those of 2+ 13] (Table I), indicating that the conjugated system becomes more planar during the transition from 1 to 1”. Variable-temperature measurements show that the absorption spectrum changes with
decreasing temperature; only two of the transitions
( E = 1.60 and 0.87 eV) can be attributed to 1.’. These tran-
Table 1. Physical properties of the various redox states of I in comparison to 2 (values
in parentheses from ref. 131).
I”] la1
E , [eV] [b] 2.97 (2.87) 1.60 (1.59)
E2 Lev1 [bl
0.87 (0.X4)
1.81 (1.81 [c])
1.12 (0.98 [c])
1.47 (1.36)
1.31 (1.24)
[a] Redox potentials determined by cyclic voltammetry measured vs. Fc;Fc+.
[b] Transition energies determined from absorptions spectra. [c] These transitions were
previously assigned [3] to the monocation 2’+.
S ll).OO+.2SlO
Angrn. Chem. Ini. Ed. Ennl. 1993. 32. N o . I
A Inml
0.2 -
000 1000
E lev1
Fig. 2. Top Difference absorption spectra of cation radical I ' + (-) and dication 1' ' ( . . . ) (vs. 1) generated by chemical oxidation of 1 in dichloromethane
with iron trichloride. Bottom: Difference absorption spectra of anion radical
1.- (- -) and dianion l * - ( . . ) (YS.1) obtained by reduction of 1 in THF with
elemental potassium. OD = optical density.
sitions almost disappear at lower temperatures, whereas the
bands at E = 1.81 and 1.14 eV become more pronounced
(Fig. 3). This observation can be easily explained by the re
2000 1500
E IeVl
Fig. 3. Temperature-dependent absorption spectra of the dimer equilibrium
l . + e ( l ) ; +starlinzat 244K(--j). 2 7 4 ( . . . ) and 294K (--t).
versible equilibrium between monocation 1" and a dimeric
radical cation (1);'. Stable dimers of this type, also called
pimers. have been described for other large aromatic cations
of porphyrins,['O1phthalocyanines," 'I, perylenes,['*l and viologens.['31 In every case the formation of the 71 dimer is
accompanied by a blue-shift of the absorption bandfs) of the
monocation and the simultaneous emergence of a longwavelength charge-transfer transition.['41 The 71 dimerization of cation radicals is also discussed in the context of the
formation of conducting radical ion salts.[151
At higher concentrations the dimerization of 1" to (1):'
is Favored. ESR measurements also prove that spin pairing
AnRm. ('hrvn. f n r . Ed. Eiigl. 1993, 32, No. 1
occurs as the temperature is lowered, causing the intensity of
the ESR signal of 1" to decrease. Based on the temperaturedependence of the equilibrium constant, a value for the
dimerization enthalpy, A H = - 21 kcalmol-', was found
and determined from the intensity of the ESR signals. Although known for many years, the dimerization process has
been neglected in the discussion of the properties of conducting polymers. The first report of the dimerization of a
terthiophene cation radical was published only recently. In
that case the dimerization tendency (AH = - 10 kcalmol-')
is only half the value found for (l);', correspoonding to the
size of the molecule.['61 The variable-temperature studies of
a series of oligothiophenes (trimer to pentamer) with combined in situ UV/VIS/NIR and ESR spectroscopy clearly
demonstrates the dependence of the dimerization of the
cation radicals on the chain length.["]
Further oxidation of l'+ leads to dication l ' + . whose
absorption spectrum exhibits a band with two maxima
( E = 1.47 and 1.31 eV) (Fig. 2 top). In comparison to the
absorptions of 2'+ these bands are again shifted to higher
energies. In this case, measurements at various temperatures
do not show changes in the absorption spectrum, and an
equilibrium with the dimer can thus be ruled out.
Successive chemical reductions of 1 provide monoanion
1'- and dianion 1 2 - . The 71--x* transitions of the anions are
generally shifted to lower energies in comparison to those of
the cations 1'+ and 1'' (Table 1). The absorption spectrum
of anion radical 1'- shows a band with fine structure and a
maximum at i,,, = 781 nm ( E = 1.59 eV). An absorption at
unusually low energy in the near-IR regime is found
= 1725 nm, E = 0.72 eV; Fig. 2 bottom), which is very
similar to that observed for the anion radicals of oligo-pphenylenevinylenes.['81 The ESR spectrum of 1 - proves its
paramagnetic nature. In analogy to 12+,1'- has an absorption band at &,ax = 912 nm ( E = 1.35 eV; Fig. 2 bottom).
Although dimerizations of anion radicals are known,[191
none is observed for the anionic species of 1.
ESR measurements conducted for the unequivocal identification of the paramagnetic radical ions 1'+ and 1'- also
allowed the determination of the spin density distributions.
The g-factors ( g = 2.0023 and 2.0046, respectively) are in
agreement with those of the radical ions of unsubstituted
oligothiophenes[201and doped poly(3-methylthiophene).~2' I
The g-factor of 1'- is substantially greater than that of I",
indicating that the orbitals of the sulfur atoms are significantly involved in the SOMO of 1'-. In contrast, the electronic structure of cation radical 1" more closely resembles
that of a cis-polyene system due to the limited involvement
of the sulfur atoms in the SOMO. A preliminary determination and assignment of the spin densities was possible by
simulation of the ESR spectra and accompanying calculations.[221The spin density distributions obtained are represented in Figure 4. Besides the expected localization of spin
density at the terminal Q C atoms, it is worth noting that in
both paramagnetic species the largest hyperfine coupling
constants and the highest spin densities are localized on
B carbon atoms. This is in agreement with the fact that in the
course of the oxidative polymerization of heterocycles such
as thiophenes and pyrroles even at short chain lengths incorrect linkages and cross-couplings occur due to ~,p-couplings
and p,p-couplings, respectively, which interfere and interrupt the conjugated chain. Similar charge distributions were
calculated for pyrrole o l i g ~ m e r s . [ ~ ~ ~
The enhanced stability and solubility of 1 allow the precise
characterization of the electronic structure of the different
redox states. The temperature-dependent formation of the
diamagnetic dimeric cation radical (1): in solution pro-
VCH Vu!ugs~esel!.whqfimhH, w-6940 Wernheim, 1993
05?0-0833j93/0i01-00?7 $10.1)0+ .25/0
Fig. 4 The experimentally determined spin-density distribution of cation radical I ’ + (top) and anion radical 1.- (bottom). The radii of the circles are proportional t o the spin density; unfilled circles represent negative spin densities.
191 We measured the oxidation potential of polythiophene under identical
[lo] J. Fuhrhop, P. Wasser. D. Riesner, D. Mauzerall, J. Ani. Chmi. Soc. 1972,
94. 7996-8001.
[ l l ] E. Ough, Z. Gasyna. M. J. Stillman, Inorg. Chem. 1991. 30, 2301 2310.
[I21 K. Kimura. T. ydmazaki. S. Katsuinata. J. P h p Chm?. 1971, 75, 1768 1774.
[I31 W. Geuder, S. Hiinig. A. Sachy. 7c.truhedron 1986, 42. 1665-1677.
[14] Since the absorption bands of the dimer (I):+ are blue-shifted relative to
those of I ” . we suspect that another charge-transfer absorption exists in
the near-IR region ( E < 0.6 eV), which cannot be observed because of the
solvent absorptions in this region.
1151 V. Enkelmann, B. S. Morra. C. Kr6hnke. G . Wegner. J. Heinze. Chem.
Ph1.s. 1982. 615. 303- 313.
[I61 M . G. Hill. K.R. Mann. L. L. Miller, J.-F. Penneau. J A m . C h w . Soc.
1992. 114.2728-2730.
[17] U. Segelbacher. P. Biuerle, D. Huttenlocher, A. Grupp, M. Mehring.
unpublished results.
[18] R. Schenk, H. Gregorius, K.Miillen. Adi,. Muter. 1991, 3, 492-493.
I191 R. H. Boyd, W. D. Phillips, J. Chem. P h ~ s 1965,
43, 2927- 2929.
[?O] A. Alberti. L. Favoretto, G. Seconi, J. Chcnt. Soc. Perkin. Tiuns. 2, 1990,
93 1 93s.
1211 M. SchHrli, H. Kiess, G. Harbeke, W. Berlingrr. K. W. Blazey, K. A.
Miiller. Springer Ser. Solid Stute Sci. 1988, 76, 277- 280.
[22] Since ENDOR measurements were not possible and selectively deuterated
compounds not available, the spin-density distribution and hyperfine coupling constants were calculated with an INDO program (M. Plato. E.
Trknkle. W. Lubitz, F. Lendzian, K. Mobius, Chem. P l r ~ s 1986.
107, 185
196). Assignment ofthe fitted hyperfine values was possible by comparison
to those obtained by theoretical calculations. A detailed presentation
of the ESR results will be published in due course (D. Huttenlocher.
A. Grupp, P. Bluerle, M. Mehring, unpublished results).
[23] R. J. Waltman. J. Bargon. Tc,iruhedron, 1984. 40, 3963-3970.
[24] M. Aizawa. S. Watanabe, H. Shinohara, H. Shirakawa, J. Chem. Sot..
Chem. Commiin. 1985. 264- 265.
vides a new alternative to the description of the doping behavior of polythiophenes and conducting polymers in general. Thus, the formation of corresponding dimeric species
complementary to polarons and bipolarons, can now be considered as the first mechanistic step during doping. This fact
easily explains the observation that the ESR activity of polarons in conducting
and cation radicals in
longer o l i g ~ m e r s [ is
~ ”detected
only at unexpectedly low
levels of doping, in the first case, o r not at all, in the second.
Nevertheless, the findings described here d o not necessarily
disagree with the existence of polaronic and bipolaronic
bands in highly doped conjugated polymers. In addition, the
anionic redox states of 1 are the first models for negatively
charged defects in thiophene chains. The n-doping of polythiophene is possible otherwise only under extreme condition~.[*~]
Received: August 31, 1992 [Z5547IE]
German version: Angew. Chem. 1993, 105. 125
[I] Z. Xu, D. Fichou, G. Horowitz. F. Garnier, J. Electrounai. Chem. 1989,
267.339-342; C. vanPham, A. Burckhardt, R. Shabana, D. D. Cunningham, H. B. Mark, Jr., H . Zimmer, Phosphorous Sulfir Silicon Relut. Elem.
1989, 46, 153-168; F. Martinez, R. Voelkel, D. Naegele, H. Naarmann.
Mol. Crysl. Liq. Cryst. 1989. 167, 227-232; J. Nakayama, T. Konishi. M.
Hoshino, Heterocycles 1988.27.1731.~1754: H. Nakahara, J. Nakayama,
M. Hoshino, K. Fukuda, Thin SolidFilms 1988, 160, 87-97; D. D. Cunningham, L. Laguren-Davidson, H. B. Mark, Jr., C. vanPham, H. Zimmer, J Chem. Soc. Chem. Commun. 1987, 1021 - 1023; J. Nakayama, T.
Konishi, S. Murabayashi, M. Hoshino, Heferoc~clcs1987.26. 1793 1796.
121 B. Xu, G. Horowitz, D. Fichou, F. Garnier, Adv. Muter. 1990,2, 592-594.
[3] D. Fichou, G. Horowitz, B. Xu, F. Garnier, Synlh. Met. 1990, 39. 243260.
[4] P. BPuerle, Adv. Muter. 1992, 4. 102-107.
IS] a) A. Yassar, D. Delabouglise, M. Hmyene, B. Nessak, G. Horowitz, F.
Garnier, A&. Muter. 1992, 4,490-494; b) D. Delabouglise, M. Hmyene,
G. Horowitz, A. Yassar, F. Garnier, &id. 1992, 4, 107-110; c) W. ten
Hoeve, H. Wynberg, E. E. Havinga, E. W. Meijer, J Am. Chem. Soc. 1991,
113,5887-5889;d) E. E. Havinga, I. Rotte, E. W. Meijer. W. ten Hoeve, H .
Wynberg, Synth. Met. 1991,41,473-478; e) D. M. deleeuw, ihid., in press
(Proc. Int. Conf. Synth. Met. Goteborg. 1992).
[6] P. BHuerle, F. Pfau, H. Schlupp, F. Wiirthner, K.-U. Gaud’, M. Balparda
Caro, P. Fischer, J. Chem. Soc. Perkin Trans 2, in press.
[’I] P. Bduerle, K.-U. Gaudl, F. Pfau, S. Henkel, unpublished results. In the
X-ray structures of sexithiophene I and the related precursor 3‘-dodecyl2,2:5’,2’-terthiophene both have the same torsion angle with a value of
10.8 and 11.0”. respectively. for the thiophene rings which are sterically
hindered by the alkyl side chains. In the case of I not all of the bond lengths
and angles have yet been obtained with satisfactory precision because of
the small size of the crystals examined and the resulting weak reflections.
For this reason the X-ray structure is not discussed further here.
[XI Oxidations of I were measured in dichloromethane, reductions in tetrahymol L - ’ with tetrabutylammonidrofuran a t a concentration of 5 x
um hexdfluorophoSphate (0.1 M) as the supporting electrolyte. The working electrode was a platinum disk 1 mm in diameter, the counterelectrode
a platinum wire. and the reference electrode a Ag/AgCl electrode which
was calibrated with ferrocene after each measurement.
((3 VCH
I’erlupspe.seN.srhuft mbH, W-6940 Wemheim. 1993
Reaction of Buckminsterfullerene with
ovtho-Quinodimethane: a New Access
to Stable C,, Derivatives**
By Puvel Belik, Andreus Giigel, Joclim Spickerniunn,
and Klaus Miillen*
The fullerene synthesis reported by Kriitschmer et al. in
1990,[’]the discovery of fascinating material properties of
fullerenes and their derivatives,’” and the development of
efficient processes to separate fullerene mixturesr3’have led
to a burgeoning number of publications on this class of material in the last two years. These papers are primarily concerned with the physico-chemical aspects of fullerenes and
less with their fun~tionalization.[~’
This one-sidedness has
the following causes: 1) C,, is an exceptionally stable molecule that adds most reaction partners reversibly only,r5]
2) fullerene and fullerene adducts are only slightly soluble,
3) the determination of the structure of fullerene adducts is
extremely complex because of the Iarge number of possible
products and isomers, and 4) high-performance chromatographic methods are necessary to separate the fullerene adducts. Nevertheless, there are already some papers which
report successful syntheses and isolations of fullerene derivatives.[61Here we present a new access to stable C,, derivatives.
For addition to the electron-poor “superalkene” C,, (1).
which easily adds radicals, o-q~inodirnethane[’~
seemed particularly suitable. It can be prepared in situ containing differ[*] Prof. Dr. K. Miillen, Dip1:Chem.
P. Belik. DiplL-Chem. A . Gugel.
J. Spickermann
Max-Planck-Institut fur Polymerforschung
Ackermannweg 10. D-W-6500 Mainz (FRG)
We thank DipLChem. U. Koch for preparing thestructural formulas. This
work was supported by the Bundesministerium fur Forschung und Technologie (project 13N6076)
$ 10.00+ .25/0
A n p o l . Chem lnr. Ed €ng/. 1993. 32, N o . I
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carrier, mode, compounds, chains, formation, conjugate, didodecylsexithiopheneчa, characterization, charge
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