Helical Conformations of Conducting Polymers
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HELICAL CONFORMATIONS OF CONDUCTING POLYMERS MIKLOS KERTESZ AND CHANGXING CUI Department of Chemistry, Georgetown University, 20057.
Washington
DC
ABSTRACT Helical conformations of polymers occur due to a variety of factors, some of which are electronic in origin. We briefly review the situations in which there is an electronic driving force towards a helix including a case that might be called a helical Peierls distortion. Energetics and geometries of helical conformations of polythiophene and polypyrrole are also analyzed. EXPERIMENTAL EVIDENCES OF HELICAL CONDUCTING POLYMERS There is ample evidence for the existence of helical conformations for conducting polymers in their undoped and doped states [1-8]. For instance in the solid state x-ray diffraction shows that doped poly(3-methylthiophene) (PMeT) is in a coil conformation. Small angle neutron-scattering experiments on poly(3-butylthiophene ) indicate that it has a coil or helical conformation in solution, and upon doping it becomes rodlike. The existence of such a coil structure has been confirmed recently for doped polypyrrole and polythiophene by scanning tunneling microscopy [3,4]. For polyacetylene helical conformation has also been found by x-ray diffraction. Theoretical calculations show that helical structures of various conducting polymers are energetically close and even sometime more stable than planar conformations [5-8]. As shown below polyketone (PK) and polyisocyanide (PIC) which have small band gaps also have helical conformations [9]. WHAT DRIVES A POLYMER TOWARDS A HELICAL CONFORMATION? We briefly summarize the factors influencing the formation of helical conformations for polymers. Hydrogen bonding in biopolymers often drive the formation of a helix [10]. Lone-pair repulsions are responsible for helical conformation for a group of inorganic and organic polymers such as polyethylene, polymeric sulfur and poly(oxymethylene) [8]. Crystal packing plays an important role in the helical conformation of HgS. Side group repulsions in some polymers such as (CF 2 )x are responsible for their spiral conformation [11]. Structural crowding is very important in the helical conformation of polythiophene and polypyrrole [12]. Other helical polymers have been considered in R. Hoffmann's group [13]. In this work we show that in the case of polyketone and polyisocyanide there is a new type of driving force towards a helix, which is similar to Peierls-like distortion. METHODS We have employed a solid state version of the Modified Neglect of Diatomic Overlap (MNDO) band theory [14] and one of its recent version: MNDO-AMl [15] in the bond distance and bond Mat. Res. Soc. Symp. Proc. Vol. 173. @1990 Materials Research Society
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/ H=O
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p= helical radius
0 =helical angle
i
h = basic translation \H / PK
Fig.
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one atom)
P
1. Helical structures of PK and PIC.
oA repeat unit (maybe more than
Fig. 2.
Parameters to describe a helix.
angle calculations for PK, PIC, polythiophene (PT), polypyrrole (PPy). The MNDO approach produces reasonable geomet
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