A series of pi -conjugated rodlike polymers, poly(2,5-dialkyl p-phenyleneethynylene)s, PPE's, were synthesized with alkyl groups of n-butyl (4a), n-octyl (4b), n-tetradecyl (4c), 2-ethylhexyl (4d), and 2-cyclohexylethyl (4e). Polymers with linear alkyl side groups (4a, 4b, and 4c) formed gels in toluene solutions, but polymers with branched (4d) or bulky (4e) side groups did not. Copolymers also did not form gels. Gels of PPE-4b and 4c show liquid crystalline structures at high concentrations. In the DSC cooling scans of toluene solutions, the primary processes of gelation are identified as distinct exotherms. In the case of PPE-4c, the primary exotherm was followed by secondary exotherms. Comparison of DSC scans of nascent and aged gels led to the conclusion that molecular arrangement in the aged gel continued beyond the initial process to result in more ordered microstructures. The fluorescence emission spectra of freshly prepared PPE-4b solutions showed two peaks at 425 and 450 nn over a wide range of concentrations, from 5 x 10(-4) to 10 g/dL. Upon standing at 25 degreesC, the emission spectra of a 10 wt PPE-4b solution did not change in the first 8 h. But a new peak at 500 nm emerged after 16 h. The location of the new broad peak is the same as that of solid-state emission due to aggregates/excimers. The intensity of the new peak then grew with time and reached a plateau value after 2 days, about the time needed for gelation by visual inspection.