Poly(9,9-dioctylfluorene-alt-bithiophene) (F8T2) exhibits two distinct conformations in solution depending on the temperature and solvent. The transition from the high- to low-temperature conformation resembles the β-phase formation of poly(9,9-dioctylfluorene) (PFO), in the sense that there is an increase of the conjugation length, while involving less pronounced enthalpy and entropy changes (ΔH = −7.1 kcal/mol and ΔS = −22.9 cal/(mol K) vs ΔH = −18.0 kcal/mol and ΔS = −68.4 cal/(mol K) for PFO). The entropy loss indicates a substantial increase in the polymer order in the low-temperature conformation. Fluorescence decays of F8T2 in solvents with various viscosities, at different temperatures and collected at different emission wavelengths, are triple-exponential in character (e.g., 20, 440, and 650 ps in methylcyclohexane, at 20 °C). The two longer decay times are related to the lifetimes of the two conformations. The shorter decay time appears as either a decay or rise time, at the onset or the tail of...