DENSITY-FUNCTIONAL STUDY OF POLYTHIOPHENE DERIVATIVES

Density functional calculations within the Car-Parrinello framework ar e used to obtain self-consistently the electronic and the geometrical structures of polythiophene derivatives. The relation between the elec tronic states and quinoid vs aromatic structures is studied on a serie s of substituted polythiophenes (PTh) and polyisothianaphthenes (PITN) , Excellent agreement is found with experimental data, where these are available. Some shortcomings of semiempirical methods in describing p i bonds with heteroatoms are revealed, A unique minimal energy structu re is obtained for each of the polymers; no other local minima are fou nd, The general rule is that a polymer adapts the structure that leads to the largest possible band gap, which for PTh-like polymers is the aromatic structure and for PITN-like polymers is the quinoid structure . Substitutions do not change the basic geometry and electronic struct ure of either PTh or PITN but can alter the band gap by several tenths of an electronvolt.