Intensity-dependent relaxation dynamics and the nature of the excited-state species in solid-state conducting polymers

Femtosecond transient-absorption dynamics are presented fur an oligomer and a number of polymers from the poly(arylene vinylene) family for excitation densities in the range from 10(18)-10(21) cm(-3). We report careful studies of the wavelength and pump-intensity dependence of the photoinduced absorption in a model five-ring oligomer, 2-methoxy-5-(2'-ethylhexyloxy)-distyryl benzene. We show that a single photoexcited species, the intrachain exciton, is responsible for all observable spectral features at low excitation densities. At higher excitation densities, nonlinear relaxation processes are observed in both polymers and the oligomer, consistent with exciton-exciton annihilation as a nonlinear-decay mechanism.