Effect of Temperature and Chain Length on the Bimodal Emission Properties of Single Polyfluorene Copolymer Molecules

• Published in: The journal of physical chemistry. B 2006-09, Vol.110 (38), p.18898-18903
• Main Authors: Grey, John K, Kim, Doo Young, Donley, Carrie L, Miller, William L, Kim, Ji Seon, Silva, Carlos, Friend, Richard H, Barbara, Paul F
• Format: Article
• Language: English
• ISSN: 1520-6106; 1520-5207
• DOI: 10.1021/jp057361r

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 ∼2.35 eV (“blue”) and ∼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 ∼20 K, G(E max) collapses into a single broad distribution centered at ∼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.