Di-n-alkyl substituted polyfluorenes with alkyl chain lengths of 6, 7, 8, 9, and 10 carbon atoms (PF6, PF7, PF8, PF9, and PF10) are studied in dilute solution in MCH using optical spectroscopy. Beta-phase is formed upon cooling in solutions (similar to 7 mu g mL(-1)) of PF7, PF8, and PF9 only, which is observed as an equilibrium absorption peak at similar to 437 nm and strong changes in the emission spectra. Beta-phase is formed upon thermal cycling to low temperature in solutions (similar to 7 mu g mL(-1)) of PF7, PF8, and PF9, which is observed as an equilibrium absorption peak at similar to 437 nm and strong changes in the emission spectra. Beta phase is found to occur more favorably in PF8 than in PF7 or PF9, which is attributed to a balance between two factors. The first is the dimer a re ate formation efficiency, which is poorer for longer (more disordered) alkyl chain lengths, and the second is the Van der Waals bond energy available to overcome the steric repulsion and planarize the conjugated backbone, which is insufficient in the PF6 with a shorter alkyl chain. Beta phase formation is shown to be a result of aggregation, not a precursor to it. A tentative value of the energy required to planarize the fluorene backbone of (15.6 +/- 2.5) kJ mol(-1) monomer is suggested. Excitation spectra of PF6, PF7, PF8, and PF9 in extremely dilute (similar to 10 ng mL(-1)) solution show that beta phase can form reversibly in dilute solutions of PF7, PF8 and PF9, which is believed to be a result of chain collapse or well dispersed aggregates being present in solution from dilution of more concentrated solutions. PF7, PF8, and PF9 also form beta phase in thermally cycled solid films spin-cast from MCH. However, in the films the PF7 formed a larger fraction of beta phase than the PF9, in contrast to the case in solutions, because it is less likely that the close-packed chains in the solid state will allow the formation of planarized chains with the longer PF9 side chains.