The formation and recombination kinetics of positively charged poly(phenylene vinylene) chains in pulse-irradiated dilute solutions

In this paper, we describe how we derive the mobility of positive charges (holes) along isolated conjugated polymer chains from the results of pulse-radiolysis time-resolved microwave conductivity (PR-TRMC) experiments on dilute polymer solutions. The method is illustrated with results for oxygen-saturated, benzene solutions of the well-known poly(phenylene vinylene) derivative, MEH-PPV with an average chain length of 800 monomer units. Nanosecond pulsed irradiation results initially in ionization of the solvent with the formation of excess electrons and benzene radical cations, Bz(+). The former rapidly (<1 ns) undergo attachment to O-2 to form O-2(-) with a rate constant of 1.5 x 10(11) M-1 s(-1). The Bz(+) ions diffuse through the solvent and react with the (lower ionization potential) polymer chains via electron abstraction forming holes on the polymer backbone. This is accompanied by a large increase in the conductivity of the solution after the pulse, demonstrating that the mobility of holes on the polymer chains is very much larger than the mobility, via molecular diffusion, of Bz(+) ions in the solvent. To describe the after-pulse growth in conductivity, a large effective reaction radius, R-eff, of ca. 400 Angstrom is required for the diffusion-controlled reaction of Bz(+) with MEH-PPV chains. This requires taking into account the time-dependent term in the rate coefficient. The value of R-eff is compared with theoretical predictions for the reaction of a small entity with a 1-D linear or a 3-D cubic array of 800 reactive moieties. The conclusion that individual polymer chains in solution must have a very open structure with a large persistence length is in agreement with the results of previous static and dynamic light-scattering studies. The conductivity eventually decays because of the recombination of the positively charged polymer chains with negative counterions (O-2(-)) with a rate coefficient of 1.2 x 10(11) M-1 s(-1), which is somewhat slower than predicted by the Debije equation. No evidence could be found for first-order trapping of the mobile positive charge on the polymer chains prior to recombination. The pseudo-one-dimensional mobility of holes along the polymer backbone derived from the absolute magnitude of the conductivity transients is 0.46 cm 2 V-1 s(-1).