Torsional Defects and Photoinduced Charge Transfer in Ring-Containing Polymers

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Abstract
The importance of ring torsion angle for photoinduced charge generation and transfer in phenyl-ring-containing polymers such as polyaniline and polyparaphenylene sulfide is discussed. The magnitude of the ground state ring torsion angle in these materials is determined by the competition between electron delocalization, favoring planar rings, and the steric repulsion between rings, favoring rings twisted far out of the plane. Upon photoinduced charge transfer, the hole formed upon removal of an electron from the valence band becomes self-localized into a polaronic state in part through changes in ring torsion angle; the effective mass of this defect is estimated to be at least an order of magnitude greater than that of bond-alternation defects. The existence of these ring-torsional defects is borne out by the experimental observation of large effective masses, long relaxation times, and weak photoconductivity for photoexcited defects, as well as by x-ray structural evidence of the dependence of torsion angle on doping level.