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 oligome r 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 th e 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 sp ectral features at low excitation densities. At higher excitation dens ities, nonlinear relaxation processes are observed in both polymers an d the oligomer, consistent with exciton-exciton annihilation as a nonl inear-decay mechanism.