ABSTRACT
Polymers that have unsaturated double bonds separated alternately by saturated single bonds in the backbone are called conjugated polymers. Chemical structures of several important conjugated polymers are shown in Fig. 1. The unsaturated bonds are shorter than the saturated bonds because there are 7r bonds in addition to the o• bonds corresponding to the single bonds. Bond alternation of the short and long bonds in the polymer backbone causes a gap (called the Peierls gap) in the electron density of states through the electron-lattice coupling, an important case of which is Peierls distortion [1]. All states below the gap are occupied and form the "Tr band" (in physics, it is called the valence band); the states above the gap are empty and form the "7r* band" (called the conduction band). For example, in polyacetylene they would first link two CH-radicals to a (CH)2 pair with a double bond between the two CH groups that consists of a 0-and a IT bond. There will be a bonding IT and an antibonding 7r* orbital. Forming a macromolecular chain with these (CH)2 pairs, the 7r and 7r* orbitals will split to give bands. The top of the IT band is called the highest occupied molecular orbital (HOMO) and the bottom of the 'n-* band is called the lowest unoccupied molecular orbital (LUMO). Semiconducting properties of the polymers are examined by the band gap energy between the ir and 7F* band. The 7r-7r* band gaps in the conjugated polymers are well in the region of known inorganic semiconductors (GaN, 2.9 eV; GaAs, 1.43 eV; Si, 1.14 eV; Ge, 0.67 eV).
