Studies of photoexcited states in polyacetylene and poly(paraphenylenevinylene) by absorption detected magnetic resonance: The case of neutral photoexcitations.

Photoexcited states in {ital trans}polyacetylene ({ital t}-(CH){sub {ital x}}), and in alkoxy derivatives of poly(paraphenylenevinylene) (PPV) nondegenerate ground-state polymers, have been studied by the technique of absorption detected magnetic resonance. In addition to the stable {ital charged} photoexcitations, which are charged solitons in {ital t}-(CH){sub {ital x}} and bipolarons in PPV, we clearly identify {ital neutral} photoexcitations with {ital nonzero} spins. In {ital t}-(CH){sub {ital x}} these are spin-1/2 neutral solitons {ital S}{sup 0}({ital {bar S}} {sup 0}) with an optical transition at 1.35 eV. The neutral photoexcitations in PPV form triplet excitons which cannot dissociate into {ital S}{sup 0}{ital {bar S}} {sup 0} pairs due to the confinement caused by the nondegenerate backbone structure.