INFLUENCE OF DOPING ON THE 3RD-ORDER OPTICAL NONLINEARITY IN POLY(BITHIOPHENE) - FIRST OBSERVATION OF CHARGE-TRANSFER-INDUCED SCATTERING

An in-situ study of the third-order optical nonlinearity of poly(bithi ophene), PET, during electrochemical doping is conducted using subpico second degenerate four wave mixing (DEWM). Upon oxidation we observe a gradual increase of the optical nonlinearity in the very beginning of each steady-state charging cycle. At 602 nm the second hyperpolarizab ility of the generated free radical cations is found to be approximate ly one order of magnitude higher than that of the neutral polymer. Tim e-resolved experiments reveal that the dynamics of the photoexcited sp ecies in the slightly charged polymer is not significantly different f rom those in the neutral polymer. Similar results are obtained upon re duction. Further charging of the samples causes a dramatic reduction o f the optical nonlinearity. This also has a remarkable influence on th e time-resolved characteristics. The observed changes can be accounted for by assuming opposite signs of the effective incoherent chi((3)) f or noninteracting species (the neutral polymer segments and the free r adical-cations), on one hand, and intermolecularly interacting species (pi-dimerized radical-cations), on the other hand. The relaxation of the photogenerated excited states is faster in the charged than in the neutral or slightly charged state. This is interpreted in terms of ne w decay channels for excitons resulting from intermolecular interactio ns in the charged state. For the first time, charge-transfer-induced s cattering effects are observed in a conjugated polymer. They appear at potentials where most of the material properties (e.g., conductivity and spin density) change dramatically. This scattering is interpreted in terms of a phase transition, which takes place in the polymer and w hich is the origin of hysteresis effect generally observed upon chargi ng of conducting polymers.