STUDY OF CIS-TRANS ISOMERIZATION OF POLYACETYLENE INDUCED BY DOPING

In the framework of the molecular dynamics modified-neglect-of-diatomi c-overlap approximation we have studied the cis-trans isomerization pr ocess of polyacetylene (PA) induced by doping. We have considered the following four different models: 1H, the addition of a single atom H t o a positive ionized PA chain; 2H, the addition of two H atoms to the neutral PA chain-, 1I, the addition of an I atom to a positive ionized PA chain; and 2I, the addition of two I atoms to the neutral PA chain . In the cases 1H and 1I we have found only partial isomerization of t he chain. In the cases 2H and 2I we have found complete isomerization. These results are in agreement with the experimental data which show that complete isomerization can be achieved only for doping levels gre ater than 7%. The isomerization mechanism involves first a transformat ion from the cis-transoid to the trans-cisoid (TC) form, followed by t he transition from TC to the trans-transoid form. The subtraction of s ome charge from the double bonds, due to the dopant presence, is the e ssential process to induce the isomerization of the cis-rich PA film.