IODINE-DOPED TRANSPOLYACETYLENE

With a view to elucidate and develop a model for the mechanism of cond uction in doped transpolyacetylene (t-PA), we have carried out an ab i nitio study of the ground and excited state of transoctatetraene in th e presence of an iodine atom. While the ground state is calculated as characterized by a slightly distorted alternating bond geometry, with iodine remaining largely neutral on top of the plane of the polyene ne ar roughly the midpoint of the chain axis, the excited state has the g eometry characterized by a solitonic distortion with the octatetraene carrying a net positive charge close to 0.7e. The bandgap [highest occ upied to lowest unoccupied molecular orbital (HOMO-LUMO) gap] is compu ted to be nearly 0.8 eV. Moreover, the excited-state energy appears ve ry insensitive to the position of iodine along the axis of the octatet raene chain. As an extension of an earlier calculation on the transpor t of charge by a charged soliton, a similar calculation is performed o n a C8H10+ ''kink'' traveling along the backbone of C12H14+, yielding very similar results. A. computer code has been developed to compute t he transport velocity of charge based on the above model and can be us ed in conjunction with the GAMESS program package. (C) 1996 John Wiley & Sons, Inc.