Effect of temperature and chain length on the bimodal emission properties of single polyfluorene copolymer molecules

Fluorescence emission spectra were recorded for isolated polymer chains of the polyfluorene copolymer, F8BT [poly(9,9-di-n-octylfluorene-alt-benzothiadiazole)], at 298 and 20 K for two molecular weights (chain lengths). For long-chain F8BT at 298 K, the observed distribution of single-molecule emission maxima G(E-max) is bimodal, with peaks at similar to 2.35 eV (blue) and similar to 2.25 eV (red). Previously, the red forms have been assigned to polymer chains that possess intrachain contacts, which lowers the local singlet exciton energy. At similar to 20 K, G(E-max) collapses into a single broad distribution centered at similar to 2.3 eV for long-chain F8BT. However, this distribution can be further divided into a high-energy edge that is dominated by the blue form, while the remainder of the distribution is composed of the red form. Low-molecular-weight F8BT samples emit exclusively from the blue form, and no shift in peak maxima with low temperature was observed. A Franck-Condon analysis reveals a decrease in emitting state displacements between spectra measured at 298 and 20 K, suggesting that temperature-induced structural displacements are responsible for the change in the bimodal emission.