Self-Organization and Excited-State Dynamics of a Fluorene−Bithiophene Copolymer (F8T2) in Solution

Abstract

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 the emission band, respectively, and is assigned to conformational relaxation on the basis of its dependence on solvent viscosity and temperature.