Polaron pair formation, migration, and decay on photoexcited poly(phenylenevinylene) chains

Poly(2-(3,7-dimethyloctoxy)-5-methoxy-1,4-phenylenevinylene), (dMOM-PPV), undergoes a thermochromic transition on heating in benzene at approximately 35 degrees C from a red gel to a yellow solution. The latter is metastable on cooling to room temperature and eventually reverts to the gel form after several hours. The absorption and emission spectrum of the gel can be resolved into two components, one which is identical with that for the solution and which is therefore associated with single-strand segments, and a second of similar spectral shape but lower energy which is ascribed to aggregated segments. Photoexcitation of dMOM-PPV in both the gel and solution results in a transient change in the microwave conductivity as monitored using the time-resolved microwave conductivity (TRMC) technique. The change in microwave conductivity for the gel is approximately an order of magnitude larger than for the solution. No TRMC transient is found on flash photolysis of a benzene solution of a dMOM-PPV derivative with 13% nonconjugated units in the polymer backbone. The effects are attributed to formation of mobile charge carriers (polaron pairs) on a single polymer chain in the case of the solution, whereas in the gel interchain charge transfer results in the predominant formation of charge carriers located on separated chains. The product of the quantum efficiency for polaron pair formation, phi(p), and the sum of the mobilities, Sigma mu, is phi(p) Sigma mu = (2.0 +/- 0.3) x 10(-8) m(2)/Vs and (2.0 +/- 0.5) x 10(-9) m(2)/Vs for the gel and solution respectively, The decay in microwave conductivity is nonexponential and extends to hundreds of nanoseconds.