STUDIES OF PHOTOEXCITATIONS IN CONDUCTING POLYMERS BY THE ABSORPTION-DETECTED MAGNETIC-RESONANCE TECHNIQUE

The nature of photoexcitations in conducting polymers has been elucidated by the absorption-detected magnetic resonance (ADMR) technique. In this technique we measure changes in the photoinduced absorption (PA) spectrum due to changes in spin-dependent recombination rates induced by resonant absorption of mu-waves in magnetic fields. We have used the ADMR technique at 3 GHz to study spin states in trans-(CH)x, oriented poly(para-phenylene vinylene) (PPV), polythiophene (PT) and polydiethynylsilane (PDES). We have verified that the charged excitations are solitons (S-/+) in trans-(CH)x and PDES, and polarons (P-/+) and bipolarons (BP2-/+) in other nondegenerate ground state polymers. The long-lived neutral excitations, on the other hand, are self-trapped triplet excitons in all polymers, except for trans-(CH)x and PDES in which the triplets dissociate into neutral soliton (S0) pairs due to their backbone degenerate ground state structure.