Narrow gap polymers and the quinonoid-aromatic transition

Summary form only given. A large group of conducting polymers possesses a nondegenerate ground state which can be structurally described as aromatic (A) or quinonoid (Q). Due to the strong electron - nuclear coupling typical for these conjugated materials, most of them change their structures significantly upon electronic excitation or doping: ones with an A ground state move towards a Q structure and vice versa. We review the following aspects of this A - Q transition: -Design of low band gap polymers via the control of the A - Q transition through side groups, topological modifications, copolymerization of groups with A and Q ground states with or without methine linking groups; -Interpretation of the electronic structures of new ladder polymers; -Calculation and interpretation of the vibrational spectra (IR and Raman) of doped and undoped poly(p-phenylene), PPP, and thiophene derivatives, determination of the percentage of Q character in doped PPP.