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meta-Phenylene Units as Conjugation Barriers in Phenylenevinylene Chains.

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33.68, 36.42, 38.77, 60.88, 114.00, 141.69,219.74; satisfactory C, H analysis, b) C. S . Sell, Ausr. J. Chem. 1975, 28, 1383; c) K. B. Sharpless, M. W.
Young, J. Org. Chrm. 1975, 40, 947; P. A. Grieco, S . Gilman, M.
Nishizawa. ibid. 1976, 41. 1485.
[13] For similar effects of crown ethers, see: R. S. Glass, D. R. Deardorff, K.
Henegar, Terrahedron Left. 1980, 21, 2467; T. Takeda, T. Hoshiko, T.
Mukaiyama, Chem. Lett. 1981, 797; L. Viteva, Y Stefanovsky, Tetrahedron Lrrr. 1989, 30, 4565; L. Viteva, Y Stefanovsky, C. H. R. Tsvetanov,
L. Gorrichon, J. Ph.ys. Org. Chem. 1990, 3, 205.
[14] C. L. Liotta, H. P. Harris, J. A m . Chern. SOC.1974.96,2250, and references
cited therein.
[i5] For previously invoked acyclic transition state, see, inter aka: C. H.
Heathcock. D. A. Oare, J. Org( Chem. 1985, 50, 3022; Y Yamamoto. S.
Nishii. ibid. 1988. 53. 3597, and references cited therein.
rneta-Phenylene Units as Conjugation Barriers
in Phenylenevinylene Chains
By Heike Gregorius, Martin Baumgarten. Ronald Reuter,
Nikolai Tyutyulkov, and Klaus Miillen*
Dedicated to Professor Emanuel Vogel on the occasion
of his 65 th birthday
The incorporation of meta-phenylene units into conjugated oligomers and polymers has wide-reaching effects on their
properties. Alongside the commonly observed increase in
solubility‘’] is the formation of “non-Kekule” structures,
which hinder the spin pairing in potential high-spin systems
such as l.[’] In compounds with singlet states, meta systems
such as 2 and 3 are expected at first glance to display extend-
ed n: conjugation just like the “para” analogues (for example,
4). We can, however, now show that for the neutral and ionic
(doped) derivatives of 2 and 3 the rneta-phenylene unit interrupts the conjugative interaction and induces charge localization. As a result, intervalence bands appear in the absorption spectra of the charged species at extremely high
wavelengths in the near-IR region.
The meta-distyrylbenzene 3a (= 2 b) and the homologous
oligo(meta-phenyleneviny1ene)s 3 b 3 d were each synthesized by Wittig olefination. In this process the use of 3,5-ditert-butylphenyl terminal groups to increase the solubility
(as in the preparation of the para analogues 4) was invaluable.[31A Wittig reaction with 514] and 3-methylbenzaldehyde yields the one styryl unit longer 3,5-di-tert-butyl-3‘methylstilbene, which on bromination with N-bromosuccinimide affords both the phosphonium salt 6 and the
corresponding aldehyde 8 formed in a Sommelet reaction.
Oligomers with even n (3b and 3d) arise in the reaction of
stilbene-3,3’-bis(methyltriphosphoniumbromide)[’] with the
aldehydes 7L61and 8, respectively, whereas those with uneven
n (3a and 3c) are formed from isoterephthaldialdehyde and
the phosphonium salts 5 and 6, respectively. Subsequent isomerization with iodine yields the corresponding all-trans
compounds 3 ad.[’]
5: R=CH2P+Ph3 Br-
7: R=CHO
6: R=CH2P+Ph3Er-
8: R=CHO
‘ /
The new hydrocarbons were electrochemically and chemically (with alkali metals) reduced. The electron-transfer
products were characterized by cyclic voltammetry, electron
absorption spectroscopy (Table 1, Fig. I), and ESR and
NMR spectroscopy.
a: R = H
b: R=feft-butyl.
Table 1. Absorption maxima A,,
[nm] of neutral and charged oligo(mefa-
1, (neutral)
(CHCI,. T = 300 K)
A,, (monoanion)
(THF/K+, T = 300 K)
a: n.1
b: n-2
c: n-3
d: n=4
a: n-1
b: n=Z
c: n = 3
d: n=4
e: n=5
Prof. Dr. K. Miillen. H. Gregorius, Dr. M. Baumgarten, R. Reuter
Max-Planck-Institut fur Polymerforschung
Ackermannweg 10, D-W-6500 Mainz (FRG)
Prof. Dr. N. Tyutyulkov
Acadamy of Science, Sofia (Bulgaria)
AngrM,. Chem. Int. Ed. Engl 1992. 31, No. 12
The meta-distyrylbenzene 3a has a distinctly higher redox
activity than para-distyrylbenzene 4a: whereas 4a is reduced
by alkali metals only to the dianion,”] 3a yields the tetraanion, which can be detected chemically by trapping it with
in a well-resolved ‘H NMR spectrum the
signals of the olefinic protons appear at very high field
(6 = 2.5-3.5).
The cyclic voltammetry shows that the first reduction potential of 3a-3d is almost independent of the number of
repeating units (3a: -2.41, 3b: -2.29, 3c: -2.29, 3d:
-2.34V). These values are similar to those for stilbene,
Verlags~esellschajimbH. W-6940 Weinheim, 1992
#57#-#833/92/1212-1653 $3.50+.25/0
Fig. 1. Absorption spectrum of3a'- (THFIK. T = 300 K). I
sity in arbitrary units.
relatlve inten-
whose first reduction potential is a t -2.45 V (for comparison see 4[31).1101
Even more drastic differences between nieta and para systems are observed in the absorption spectra of the reduction
products: Whereas the radical anion 4a'-/Kt has its longest
wavelength band at i
= 1085 nm, 3a'-/K' absorbs at
, 1810 nm(!). Moreover, whereas in the homologous
series of the radical anions of the para compounds 4 an
increasing bathochromic shift is observed with extension of
the chain," the position of the bands in the nzetu series 3 is
largely independent of the chain length."21 Apart from the
longest wavelength band, the spectra of the monoanions of
the meta species agree with the spectrum of the stilbene
monoanion in each case (I",,
= 496, 699 nm).
These results are readily explained by a localization of the
charge on a stilbene unit in the meta-phenylenevinylenes.
The ion 3a'- can be described by structure A, which contains
a charged and an uncharged unit. The longest wavelength
absorption of 3a'- at 1810nm is thus attributable to an
intervalence or charge-transfer band. Both the extreme
broadening of this band and its bathochromic shift of about
140 nm on lowering the temperature (200 K) are in accord
with this assignment.
A calculation of the absorption spectrum of the paramagnetic species 3 a ' - with an open-shell PPP SCFjCI procedure" 31 does not reproduce the extremely long wavelength
band and the two characteristic VIS transitions at approximately 500 and 700 nm. If, however, a localized structure A
(Scheme 1) is assumed and a decoupling (e.g., a significant
twisting about the formal single bond) of the uncharged
styryl group is taken into account, the calculation yields a
band at 1 = 1740 nm as charge-transfer transition between
the charged stilbene subunit and the uncharged styryl group.
plings [0.540 (2H), 0.267 (4 H), 0.137 mT (4H)] were recorded for the oligomers 3b'-, 3c'-, and 3 d - . The hyperfine
coupling constants and their multiplicity leave no doubt that
the spin density is localized in a central stilbene unit. Only
for the tneta-distyrylbenzene radical anion 3a'- (K+, 2methyltetrahydrofuran, - 78 "C) can an electron delocalization be detected. The contradiction between the results from
ESR and VIS/near-IR spectroscopy for 3a'- is resolved if
the different timescales of the two experiments are considered (lo-' and lo-" s - ' ) . [ ' ~ ]A rapid charge fluctuation
between A and A' gives rise to the ESR findings (Scheme 1).
The characterization of the neutral and anionic mefaphenylenevinylenes 3 is in accord with a conjugation barrier
caused by topological features : the resonance structures cannot be formulated over the whole chain, but only over each
stilbene unit. This resonance form represents a polaron in a
poly(metu-phenylenevinylene) chain (B in Scheme 2). In contrast, in poly@ara-phenylenevinylene) chains polaron structures can be formulated which contain different numbers of
quinoid arene rings (C).
Scheme 2. Structures of polarons of poly(melu-pheny1enevinyirne)s and poly(prrrrr-phenyleneviny1ene)s.
The special situation of conjugated systems with metaphenylene units documented here suggests corollaries: The
type of charge distribution in doped polymers often serves as
a criterium for the description of charge defects such as polarons and bipolar on^,^'^^ which are considered the charge
carriers when conductivity is present. Furthermore, it has
been postulated that on doping rnera-phenylene systems such
as 2, ferromagnetism can arise if the two side chains are
sufficiently long for bipolaron formation.[16]We shall therefore soon report on the behavior of high molecular weight
compounds of series 2 and 3.
Received: June 17. 1992 [Z5410IE]
German version: Angels. Cheni. 1992. 104. 1621
CAS Registry numbers:
3a. 144084-15-1: 3 a (monoanion), 144176-81-8; 3 a 4 - . 144084-39-5, 3b.
144084-16-2; 3 b (monoanion), 144176-82-9: 3c. 144084-17-3; 3c (monoanion).
144176-83-0 ; 3d. 144177-76-4 : 3 d (monoanion), 144084-18-4 ; stil bene
monoanion, 96295-17-9.
Scheme 1. Charge delocalization in 3 a ' -
The type of charge and spin-density distribution in 3'- can
also be determined by ESRiENDOR spectroscopy. Identical
ESRiENDOR spectra each with only three hyperfine couI654
VCH Vrr/agge.rell.~rhyfr
mbH, W-6940 Wririherm, 1991
[I] S. Claesson, R. Gehin, W. Kern, Makromol. Chem. 1951. 7. 46.
[2] H. Iwamui-a. .4ch. P l z w Org. Chrm. 1990, 26. 179; A. RaJCd,S. Utamapanya, J. Xu. J. An]. Chrm. SOC.1991, I/3, 9235, 9241.
[3] J. Heinze. J. Mortensen, K. Miillen. R. Schenk, J Chem. Sor. Chrm. Comt m n . 1987. 701.
$3.50+ .25/0
Angrtv. Chcm. lnr. Ed. Engl. 1992, 31, No. 12
[4] J. M. A. Baas, H. van Berkum, M. A. Hoefnagel, B. M. Webster, R e d .
Trav. Chim. PUJSBus 1969,88,1110; G. W. Herne, T. W. Evans, V. W. Buls,
C. G . Schwarzer, Ind. &ng. Chem. 1955, 47, 2311; L. A. Paquette. W. D.
Klobucar. R. A. Snow, Svnth. Commun. 1976, 6, 575.
[5] H.-H. Horhold, A. Miiller, R. Ozegowski, J. Prakl. Chc>m.1975,317, 877.
[6] M. S. Newman, L. F. Fee. J Org. Chem. 1972, 37, 4468.
[7] Selected spectroscopic data for 3b: 'H NMR (200 MHz, CDCI,, 300 K):
S = 7 . 7 5 ( s , 2H. HIO), 7.54-7.34 (m, 12H. arom. H), 7.28,7.16 (2d, 4H,
J = 16 Hz. outer olefinic H), 7.24 (s, 4H. inner olefiuic H), 1.45 (s, 36H.
/iv/-butyl); "C NMR (50MHr. CDCI,, 300K): 6 =151.6 (C3. C5),
138.6. 138.2. 137.0 (Cl, C9, C11). 137.0, 129.5, 129.4, 128.3 (C7, C8, C13,
C15). 126.3, 126.1, 125.2(CIO.C12,C14), 122.7(C4), 121.4(C2,C6), 35.4,
32.0 (terr-butyl); MS (EI, 70 eV): m/z608 ( M + ,90%), 57 (tert-butyl, 100).
[XI R. Schenk, H. Gregorius, K . Meerholz, J. Heinze. K. Miillen, J. Am. Chem.
Soc. 1991. 113, 2634.
[9] Trapping o f 3 a 4 - with dimethylsulfate (quadruple methylation): MS (EL
70 eV): ni!z 566 ( M ' , 8%), 57 (tert-butyl, 70%).
[lo] The apparatus has been described fully: A. Bohnen, H. J. Rider,
K. Miillen, Sjnth. Met. 1992,47,37. All measurements were conducted in
THF at - 10°C with tetrabutylammonium bexafluorophosphate as supporting electrolyte (c = lo-' M). The concentration of the electroactive
M. A gold disk of 1 mm diameter was used as working
substances was
electrode. and a platinum wire wound concentrically around it as counterelectrode. Silver wire served as quasi-reference, and the calibration was
= + 0.310 V vs. a standard
made with ferrocene/ferrocenium
calomel electrode).
[Ill R. Schenk, H. Gregorius, K. Miillen, Adv. Muter. 1991, 3, 492.
[12] The optical absorptions of the neutralcompounds likewiseshowed no shift
of the wavelength A,, with increasing chain length (2.max x 309 nm; see
Table 1 1 , in contrast to the para analogues 4. The calculation of the absorption spectra according to a closed-shell PPP SCFjCI procedure agrees well
with the experimental data if based on alternating bond lengths for the
double and single bonds and on a slight twist (30") about the formal single
bond (i.,,,(calcd) = 306 nm).
[I 31 H. C. Longuet-Higgins, J. A. Pople, Pror. Phjs. Soc. London Srct. A 1955,
68. 591.
(141 Connected with the charge fluctuation is a counterion migration, which
should be slowed most when contact ion pairs are present. Although the
chosen experimental conditions (see above) favor the formation of contact
ion pairs. we could not freeze out the interconversion of A and A at
- 78 "c.
1151 R. Chance. D. S. Boudreaux, J:L. Bredas, R. Silbey in Handbook o f c o n dirr,!ingPo!,,mers. Vol. 2 (Ed.: T. A. Skotheim), Marcel Dekker, New York,
1986. p. 826.
116) H. Fukutome. A. Takabashi, M. Ozaki, Chem. Phys. Left. 1987, 133, 34.
The charge distributions of the simple aromatic molecules
benzene and hexafluorobenzene are of considerable interest;
their symmetry implies that all odd electric moments vanish
and that there is just one independent quadrupole moment.
The large magnitudes and the difference in the polarity of the
quadrupole moments of benzene and hexaflu~robenzeneI~~
imply that the electrostatic interaction between these two
molecules is particularly strong. Indeed, mixing equimolar
quantities of C,H, and C,F, yields a crystalline solid with a
melting point of 24 0C.[41
From calorimetric and NMR experiments performed on
the deuterated analogue,[51three thermodynamic anomalies
are observed at 199, 247.5, and 272 K. To understand the
origin of these phase transitions, one of us has investigated
the proton dynamics of the benzene partner in a series of
quasi-elastic and inelastic neutron scattering experiments.[6.'1 A neutron and X-ray scattering study of molten
C,H,: C,F, indicated the existence of dimers in the liquid
state.[81Structural studies of the solid are limited to a singlecrystal X-ray investigation of the disordered highest temperature phase I, which is made up of columns of alternating
benzene and hexafluorobenzene molecules arranged perpendicular to the threefold axis in a rhombohedra1 lattice.[']
Single crystals of phase I do not survive cooling below the
phase transition temperature of 272 K ; thus the low-temperature structures have remained unsolved.
Initial neutron diffraction studies (A = 2.99 A) of a deuterated sample were only partially successful; because of the
lower resolution and the similar scattering lengths of D and
Structure of the Lowest Temperature Phase
of the Solid Benzene-Hexafluorobenzene Adduct**
By JeJfrey Huw Williams,* Jeremy Karl Cockcroft,
and Andrew Nicholas Fitch
We report the results of complementary high-resolution
powder diffraction studies by neutron and synchrotron radiation of the solid phases of C,H,:C,F, including the ab-initi0 structure determination of the lowest temperature
phase IV. Although in the last few years powder diffraction
has been used to solve the structures of inorganic materials
or compounds containing second-row or heavier elements,'" 21 we believe that this work represents a significant
achievement by demonstrating that moderately complicated
structures of organic compounds can be solved directly by
powder methods for those cases where single-crystal techniques are not applicable.
Dr J. H. Williams. Dr. J. K. Cockcroft
Institut Max von Laue - Paul Langevin
BP 156X. F-38042 Grenoble Cedex (France)
Dr. A. N. Fitch
Department of Chemistry. Keele University
GB-Staffordshire STS 5BG (UK)
We thank the I L L Grenoble and the SERC Daresbury Laboratory for the
provision of neutron and synchrotron-radiation beam time, respectively,
and the European Community for financial support.
A n p e w Cheni. Inf. Ed. Engl. 1992, 31, N o . 12
2000 1500lO0OC
Fig. 1. Study ofphase IV of C,H,:C6F, a) with synchrotron radiation at 30 K,
b) with neutron radiation at 1.5 K. The vertical ticks between the observed and
calculated diffraction profiles (top dotted and solid lines, respectively) and the
difference profile (bottom) indicate the reflection positions (956 and 567 for a)
and b), respectively). A square-root scale has been used for the intcnsity of a)
to show the quality of the weaker high-angle data. The residual phase I11 is
clearly visible in the difference plot.
VCH VerlagsgesellschaJi mhH, W-6940 Weinheim, 1992
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unit, meta, chains, phenylenevinylene, phenylene, barriers, conjugation
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