Study of Intrachain Charge Transfer in a Blue Emissive Polyfluorene Random Copolymer

Photophysics of a blue light-emitting fluorescent random copolymer, consisting of arylated polydioctylfluorene (aryl-F8), polydioctylfluorene (F8), and amine comonomers in a ratio of 80:15:5 is reported. In a solution of 10^-6 M, solvatochromism in absorption and photoluminescence (PL) is observed with an increased lifetime of PL as the polarity of the solvent increases. Dual fluorescence is observed in the 10^-9 M diluted solution, comprising a structured emission from a localized state in the aryl-F8 comonomer and a broad emission peak from the charge-transfer (CT) state at a lower energy. Emission wavelength-dependent time-resolved photoluminescence studies in different polar media confirm the presence of the emissive intrachain CT state in this copolymer. Analyzing the PL decay kinetics, we calculated the formation rate of the intrachain CT state to be ∼3.0 × 10⁹ s^-1. Repopulation of the localized state from the CT state is observed in the lower polarity medium with a rate of 7 × 10⁸ s^-1, which is almost absent for the large Stokes-shifted CT emission in the higher polarity medium. Along with the fundamental understanding of the photophysics of the random copolymer, this study suggests that the emission spectrum can be tailored by the concentration of polymer and the polarity of surrounding media.