MOLECULAR ENGINEERING OF SMALL BAND-GAP POLYMERS - HETEROAROMATIC BICYCLIC POLYMERS

A brief account is given of the various routes currently pursued to en gineer novel electrically conducting polymers. Using these routes, the results of an investigation of the electronic structure and conductio n properties of the nitrogen and oxygen containing analogues of polyth ieno[3,4-C]thiophene (PTTP), i.e. polypyrrolo[3,4-C]pyrrole (PPPY) and polyfurano[3,4-C]furan (PFFU), respectively, obtained on the basis of ab initio Hartree-Fock crystal orbital method, are reported. The ener gy band structures of all three polymers were found to be similar, bot h the highest occupied and the lowest unoccupied bands being of pi cha racter. The calculated band gap values were found to decrease in the o rder PTTP < PFFU <PPPY. PTTP and PPPY are predicted to be the best can didates for forming highly conducting materials on doping with electro n donors (n-doping) and electron accepters (p-doping), respectively. O n the basis of their band alignments, the copolymer pairs PTTP-PPPY, P TTP-PFFU and PPPY-PFFU are found to belong to the category of type II staggered superlattices. The electronic density of states of these thr ee pairs of quasi-one-dimensional copolymers were calculated using the negative factor counting technique within the tight binding approxima tion. The trends in the electronic structure and conduction properties of the pairs are discussed.