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Microstructure of a Discotic Polymer as Revealed by Electron Diffraction and High-Resolution Imaging.

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191 3: P2,/n, a= 1242.1(2), h= 1877.2(3), c=864.1(2) pm, b= 109.95(2)',
1.94 g cm-': R=0.026 (MoKo=71.069 pm, 3356 reflecZ = 4 , pFIlcd=
tions, 3044 with I > 2 o ( l ) , anisotropic temperature factors for Fe, P, F,
C; H atoms isotropic; 299 refined parameters). 5b: P21/c, u=967.3(3),
h = 1459.2(4), c = 1851.9(5) pm, /3=90.20(3)", Z = 4 , pc,,lcd=
1.85 g cm-';
R=0.035 (MoKn=71.069 pm, 4607 reflections, 3962 with I > Z o ( l ) , anisotropic temperature factors for Fe, Mn, P, F, C ; H atoms isotropic;
409 refined parameters, empirical absorption correction with the program DIFABS from D. Stuart and N. Wulker [12]. Further details of the
crystal structure investigations can be obtained from the Fachinformationszentrum Energie, Physik, Mathematik GmbH, D-7514 EggensteinLeopoldshafen 2, on quoting the depository number CSD-53426, the
names of the authors, and the journal citation.
[lo] K. Wade, Adu. Inorg. Chem. Radiochem. 18 (1976) 1.
11 11 a) W. Hiibel, E. H. Braye, J. Inorg. Nucl. Chem. 10 (1959) 250: b) R. P.
Dodge, V. Schomaker, J . Organomet. Chem. 3 (1965) 274; A. A. Hock,
0. S. Mills, Acra CrystaNogr. 14 (1961) 139.
[I21 N. Walker, D. Stuart, Acra Crysfallogr. Sect. A39 (1983) 158.
la. n = 1 4
OR
Microstructure of a Discotic Polymer as Revealed by
Electron Diffraction and High-Resolution Imaging
R = H,C+CH&CH,
By Ingrid Gisela Voigt-Martin,* Heinz Durst,
Volker Brzezinski, Herbert Krug, Willi Kreuder, and
Helmut Ringsdorf
Fig. 1. Schematic representation of a repeating unit of the discotic polymer
la and structure of the model compound l b .
Electron microscopy has become a powerful tool to obtain atomic-level as well as molecular-level information."]
It has recently been applied to study the microstructure
and defects in smectic liquid
without resorting
to staining methods which are liable to cause artifacts.
Polymeric liquid crystals are well suited for such investigations because they are nonvolatile and their mesophases
can be frozen in below their glass transition temperatures.['] This allows detailed structural investigations because both the microstructure and defects in polymeric liquid crystals are retained in the glassy state. The smectic
layers exhibit well-defined undulations and contain single
and multiple dislocations.["
Recently, polymers with disclike mesogens as side
groups[71or in the main
have been described.
They exhibit a discotic columnar structure as elucidated by
X-ray investigations.["' For low-molecular-weight compounds such liquid-crystalline microstructures have been
known since 1977.["] These compounds contain flat, rigid
discs (e.g., triphenylene) and several flexible alkyl
chains."'] In discotic main-chain polymers[*I around ten to
one hundred of such molecular discs are linked via flexible
alkyl spacers (Fig. 1).
The polymer la[81shows an ordered, hexagonal columnar mesophase (Dho)in complete analogy to its low-molecular-weight model compound l b . The mesophase width is
even broader than for the hexakis(penty1oxy)triphenylene
lb.[I3' In contrast to l b , which crystallizes on cooling, the
Dhomicrostructure of the polymer can be frozen in below
the glass transition temperature T,=6O0C (Fig. 2).
The electron diffraction pattern (primary diffraction) indicates a hexagonal microstructure. The diffraction spots
are sharp and show both first- and second-order reflections (Fig. 3). The measured lattice spacing is d = 18.1
from which a hexagonal lattice constant of a=20.9 A
could be calculated in agreement with the values obtained
by X-ray studies (17.7 A and 20.4 A, respectively["I).
6,
[ * ] Dr. I. G. Voigt-Martin, H. Durst, V. Brzezinski, H. Krug
lnstitut fur Physikalische Chemie der Universitat
Welder-Weg 15, D-6500 Mainz (FRG)
Dr. W Kreuder, Prof. Dr. H. Ringsdorf
lnstitut fur Organische Chemie der Universitat Mainz (FRG)
Angew Chem. Int. Ed. Engl. 28 (1989) No. 3
lb
Fig. 2. Schematic representation of the Dho mesophase of the polymeric triphenylene derivative l a .
Fig. 3. Electron diffraction of the discotic polymer l a : Dho phase viewed
along the column axis.
The spots are arranged in a hexagon, but they are not
arced as in the X-ray pattern of the same polymer.['o1Two
explanations can be put forward. Firstly, the electron beam
probes a very small area of about 2 gm in diameter, within
which the orientation is apparently almost perfect. In contrast to this, X-rays probe about 1 mm2, so that an average
orientation is obtained. Secondly, the thickness of the film
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323
for electron microscopy is of the order of several hundred
angstroms only. This corresponds roughly to one hundred
molecular triphenylene discs stacked on top of one another (see Fig. 2). This, in turn, is the typical correlation
length within the columns of discotic liquid
The
thin-film technique enables almost undisturbed columns to
be observed which extend through the whole thickness of
the polymer films.
While diffraction techniques give important general information regarding molecular packing, it is difficult and
often impossible to elicit details regarding defects and correlation distances from them without making massive assumptions. The reason is that phase information is always
lost in a diffraction pattern so that the Fourier transform
contains amplitude information only. Phases can be reintroduced mathematically but this involves assumptions or
additional information. An alternative approach is to apply high-resolution direct imaging. This method allows the
observation of molecular structures. In Figure 4 the twoFig. 5. Computer-treated high-resolution electron micrograph of discotic
polymer l a (cf. Fig. 4).
late provocative ideas regarding applications in the fields
of electronics and optoelectr~nics.['~~
The submicrometer
structures in the systems applied so far are still roughly a
hundred times larger than those in the discotic polymer
la. For instance, in the field of information storage one
can envisage addressing such regularly ordered nanometer
structures. Configurations similar to scanning tunnel electron microscopy could be applied for implanting electrons
into single columns of ordered discotic polymers. Furthermore, these columns are densely stacked 7t systems surrounded by paraffinic chains, which give an insulation
against the six columns in the immediate neighborhood, so
that they might act as molecular conductors. The predominant conductivity[*"] across the thin film might be further
increased by doping with electron-acceptor molecules.[*'I
Experimental Procedure
In order to study the mesophase of polymer l a by electron microscopy, 400films were prepared by spreading a polymer solution in chloroform (30/
100 w/v) on a water surface. The dried film was sandwiched between two
carbon films and transferred onto copper grids for electron microscopy.
Orientation was obtained by annealing in a magnetic field of 1.5 T just below
the D,,-isotropic
phase transition temperature. The magnetic field direction was chosen to be in the film plane, so that the columns are oriented
perpendicular to the film, as is to be expected from the known anisotropy of
the magnetic susceptibility of a similar low-molecular-weight discotic compound 1171. Finally, the film was cooled to ambient temperature and thus the
orientation was frozen in. The production and interpretation of a high-resolution electron micrograph is not a trivial matter. It is particularly difficult in
the case of beam-sensitive samples, where special cryogenic methods combined with the low-dose technique have to be developed and applied [3]. In
order to understand the difficulties regarding interpretation of the electron
micrographs, it is necessary to explain the physics involved without detailed
reference to the mathematics, which is related elsewhere 1151.
Imaging involves a two-stage process, mathematically expressed by two Fourier transforms. The first involves the production of an electron diffraction
pattern (primary diffraction) in reciprocal space from the object in real space
(first Fourier transform). Subsequently, in a second Fourier transformation,
an image is produced from the diffraction pattern. It is important to realize
that the objective lens of the microscope produces a phase shift so that the
expression for the phases and amplitudes in reciprocal space is modified by
an oscillating microscope transfer function which depends on the defocus
value [4], as well as on manipulations in reciprocal space involving the objective aperture. In order to image objective details having certain spatial frequencies, therefore, a very specific transfer function must be chosen. In the
low-angle range which is relevant for these samples, the band width is very
narrow and only a very small range of frequencies is transferred. Furthermore, because of the rapidly oscillating transfer function, higher-order re-
A
Fig. 4. Bright-field high-resolution electron micrograph of the discotic polymer la. Middle: Dho phase viewed along the column axis. Lower half
Tilted domain.
dimensional hexagonal packing of the molecular columns
of the discotic liquid-crystalline polymer l a is shown for
the first time. The single columns of the packed triphenylene discs (cf. Fig. 2) can be resolved and their distance
(ca. 18
is in agreement with the values obtained from
electron diffraction (see Fig. 3 ) and X-ray diffraction."'' In
the same specimen other domains were observed with a
slight tilt angle between the local column axis and the
beam axis. This high-resolution direct imaging involves
special techniques and is-in contrast to inorganic materials-difficult to apply to beam-sensitive samples like this
polymer. For comparison, the computer-treated image is
shown in Figure 5.[16' In the context of investigations of
ultrathin filmst1x1these electron-microscopic images stimu-
A)
324
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Angew. Chem. Inl. Ed. Engl. 28 (1989) No. 3
flections may have suffered a phase change so that the information in the
appropriate frequency range is transferred with reversed contrast.
The second stage in image analysis involves image reconstruction, where the
electron micrograph now serves as object. Filtering of Fourier frequencies in
reciprocal space (secondary diffraction pattern) is performed generally with
the aid of a computer. The various stages of frequency filtering are very complex and must be treated with careful consideration of the physics involved.
Otherwise, artifacts can easily be introduced. The safest way to guard against
this is to compare the original image (which must be of very high quality)
with the computer-treated image, and to ensure that defects have not been
introduced which were not in the original. Details regarding these procedures will be published elsewhere [16].
Received: September 22, 1988;
supplemented: December 2, 1988 [Z 2974 IE]
German version: Angew. Chem. 101 (1989) 332
.,
CAS Registry numbers:
la, 118891-82-0; lb, 69079-52-3.
R 0 + OCo(dmgH)2py
[ I ] L. D. Marks, D. J. Smith, Nafure (London) 303 (1983) 316; J.-0. Malm,
J.-0. Bovin, A. Petford-Long, D. J. Smith, G. Schmid, N. Klein, Angew.
Chem. I00 (1988) 580; Angew. Chem. Int. Ed. Engl. 27 (1988) 555.
[2] H. Durst, 1. G. Voigt-Martin, Makromol. Chem. Rapid Commun. 7 (1986)
785; 1. G. Voigt-Martin, H. Durst, Liq.Cryst. 2 (1987) 585; I. G. VoigtMartin, H . Durst, H. Krug, Macromolecules, in press.
131 1. G. Voigt-Martin, H. Durst, Liy. Cryst. 2 (1987) 601.
[4] 1. G. Voigt-Martin, H. Durst, B. Reck, H. Ringsdorf, Macromolecules 21
(1988) 1620.
[5] 1. G. Voigt-Martin, H. Durst, Macromolecules 22 (1989) 168.
161 H. Finkelmann, Angew. Chem. 99 (1987) 840; Angew. Chem. I n f . Ed.
Engl. 26 (1987) 816: H. Finkelman, G. Rehage, Ado. Polym. Sci. 60/61
(1984) 174.
171 W. Kreuder, H. Ringsdorf, Makromol. Chem. Rapid Commun. 4 (1983)
807.
[S] Synthesis and characterization of 1 : W. Kreuder, H. Ringsdorf, P.
Tschirner, Makromol. Chem. Rapid Commun. 6 (1985) 367.
[9] G. Wenz, Makromol. Chem. Rapid Commun. 6 (1985) 577.
[lo] 0. Hermann-Schanherr, J. H. Wendorff, W. Kreuder, H. Ringsdorf, Makromol. Chem. Rapid Commun. 7 (1986) 97.
[I 11 S. Chandrasekhar, B. K. Sadashiva, K. A. Suresh, Pramanu 9 (1977) 471 ;
Chem. Abstr. 88 (1978) 3 0 5 6 6 ~ .
[I21 J. Billard, J. C . Dubois, Nguyen Huu Tinh, A. Zann, Nouv. J. Chim. 2
(1978) 535; C. Destrade, P. Foucher, H. Gasparoux, Nguyen Huu Tinh,
A. M. Levelut, J. Malthete, Mol. Crysf. Liy. Cryst. 106 (1984) 121.
[I31 A. M. Levelut, J . Phys. Lefr. 40 (1979) 81.
1141 D. Goldfarb, R. Poupko, 2. Luz, H. Zimmermann, J. Chem. Pfiys. 79
(1983) 4035.
[IS] J. Amoros, M. Amoros: Molecular Ctystals: Their Transform and Diffuse
Scattering. Wiley, New York 1983; J. Cowley: Diffraction Physics, North
Holland, Amsterdam 1986.
1161 1. G. Voigt-Martin, H. Durst, H. Krug, Mucromolecules, in press,
[17] M. Gharbia, M. Cagnon, G. Durand, J. Phys. Left. 46 (1985) 683; cf. A.
M. Levelut, J . Chem. Phys. 80 (1983) 149.
[IS] H. Ringsdorf, B. Schlarb, J. Venzmer, Angew. Chem. 100 (1988) 117; Angew. Chem. In!. Ed. Engl. 27 (1988) 113; H. Ringsdorf, G. Schmidt, J.
Schneider, Thin Solid Films 152 (1987) 207.
1191 T. Iwayanagi, T. Ueno, S. Nonogaki, H. Ito, C. G. Willson: "Muterials
and Processes for Deep U. V. Lithography", Adv. Chem. Ser.. in press.
[20] L. Y. Chiang, J. P. Stokes, C. R. Safinya, A. N. Bloch, Mot. Cryst. Liy.
Crysf. 125 (1985) 279; B. Mourey, J. N. Perbet, M. Hareng, S. Le Berre,
ibid. 84 (1982) 193.
[21] P. Davidson, A. M. Levelut, H. Strzelecki, V. Gionis, J . Phys. Lefr. 44
(1983) 823.
On the Formation of "Free Radicals" from
Alkylcobalt Complexes**
By Bernd Giese,* Jens Hartung, Jianing He,
Otfmar Hiiter, and Andreas Koch
Alkylcobaloximes of type 1 are becoming of increasing
importance in organic synthesis, since their photolysis gen[*I
[*'I
Prof. Dr. B. Giese, Dip1.-Ing. J. Hartung, Dipl.-lng. J. He,
Dip1.-lng. 0. Hiiter, Dipl.-Ing. A. Koch
Institut fur Organische Chemie der Technischen Hochschule
Petersenstrasse 22, D-6100 Darmstadt (FRG)
This work was supported by the Stiftung Volkswagenwerk.
Angew. Chem. In[. Ed. Engl. 28 (1989) No. 3
erates radicals which can be used for both carbon-carbod'] and carbon-heteroatom bond-forming reactions."c.21
Preparatively useful reactions include cyclizations""' and
intermolecular additions to olefins,[lb'dl which, depending
on the reaction conditions and the substituents, afford alkanes 2 and/or alkenes 3 (H,dmg = 2,3-butanedionedioxime).
4
H2C=CHX
>
RCH2-CHX-Co(dmgH)2py
6
5
On the basis of ESR measurements, it has been concluded that alkyl radicals generated from alkylcobaloximes
in a matrix still undergo an interaction with the Co" complex 5.I3I By carrying out reactivity and selectivity studies
on alkylcobaloximes in organic solvents, we have now
been able to show that C-C bond formation involves the
reaction of free radicals that do not differ from those generated from other radical sources.
7
8
T
9
10
11
U
12
In order to determine the reactivity, the hexenylcobaloxime 9 was photolyzed in the presence of various quantities
of CCI, at 26°C using a 300 W sun lamp. From the ratio of
the products 12 and 7 and the known rate of cyclization of
the hexenyl radical
the rate of abtraction of chlorine
from CCI, could be determined as 6.9 x lo3 M - ' s-'. This
value agrees remarkably well with the literature value for
the hexenyl radical, which was generated from the corresponding azo compound."]For intermolecular addition
reactions with the alkene 16, the reaction rates for the intermediates generated by reduction of the cyclohexyimercury salt 13f61
and by photolysis of the cyclohexylcobaloxime 14 were also identical within the limits of experimental error (Table 1). Since the mercury salt gives rise to uncomplexed radicals, these reactivity experiments lead to
the conclusion that free radicals are formed in the photolysis of cobaloximes.
These findings were supported by stereochemical studies. Thus, in the cyclization of the l-methylhexenylcobal-
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0570-0833/89/0303-0325 $ 02.50/0
325
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