Dispersive dynamics of photoexcitations in conjugated polymers measured by photomodulation spectroscopy

We study the dynamics of the long-lived photoexcitations in p-phenylene-vinylene based conjugated polymers containing acid-sensitive bipyridine subunits, and in poly(5,5’-vinylene-2,2’-bipyridylene), by following the dependence of the intensity of the photoinduced absorption bands on the laser modulation frequency. The photoinduced absorption spectra contain two broad bands, whose relative intensities strongly depend on the protonation state of the polymers and whose modulation frequency dependence is distinctly different. Whereas the higher-energy band follows the modulation frequency dependence expected for either mono- or bimolecular recombination process, the lower-energy band shows a fractional power law frequency dependence at high modulation frequencies, strongly indicating a ‘‘dispersive relaxation’’ process. The value of the fractional exponent is sample dependent and is a quantitative measure for the distribution of the relaxing centers.