Positive charge carriers on isolated chains of MEH-PPV with broken conjugation: optical absorption and mobility

Abstract

Pulse radiolysis of oxygen-saturated dilute solutions of poly[2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylene vinylene]s in benzene showed charge-transfer from the solvent radical cation to the polymer, yielding positive charge carriers (radical-cations or "holes") on the isolated chains. The charge-transfer reactions are diffusion controlled with (time-dependent) rate coefficients that are determined by the conformation adopted by the polymer chains. The radical cation of the fully conjugated polymer exhibited an absorption maximum at 1.32 eV. The absorption maximum shifted to higher energies with decreasing conjugated fraction, in a way consistent with thermal equilibration of the positive charge over the conjugated segments of different length. The mobility of the positive charge along the fully conjugated MEH-PPV chains was determined to be 0.42 cm² V^-1 s^-1. The introduction of conjugated breaks decreased the charge mobility strongly, which is attributed to the disruption of the p conjugation and/or the conformational disorder caused by the flexibility of the chains at the sites of conjugation breaks.