An investigation has been undertaken of the photophysics of six thiophene-based polymers. This includes the measurement of fluorescence quantum yields, quantum yields for triplet formation, and determination of fluorescence and triplet lifetimes in benzene solution at room temperature. From the above-mentioned data, the overall set of rates for the deactivation processes (fluorescence, intersystem crossing, and internal conversion), has been evaluated. Mechanisms of nonradiative decay are discussed, and it is shown that both intersystem crossing and internal conversion are important in the decay of the lowest singlet excited state of isolated polythiophenes molecules in solution. Comparison of spectroscopic and photophysical properties of the polymers with analogous oligothiophenes shows that with the polymer, the S(1)curved right arrowS(0) internal conversion deactivation pathway plays a major role, in contrast to the behavior found with the oligomers where this internal conversion deactivation channel is essentially negligible. (C) 2003 American Institute of Physics.