Timehyphen;resolved observation of excitation hopping between two anthryl moieties attached to both ends of polystyrene chains

摘要

Excitation hopping between two 9hyphen;anthryl moieties attached to both ends of polystyrene chains with various molecular weights has been explored by picosecond timehyphen;resolved fluorescence anisotropy measurements. The measurements were performed at 77 K in the rigid matrix of 2hyphen;methyltetrahydrofuran (MTHF) or methylcyclohexane (MCH) in order to exclude the effect of rotational diffusion of the chromophores to the fluorescence anisotropy. The hopping behavior was found to be very sensitive to such factors as molecular weight (Mn), solvent, and concentration of the samples. In MTHF (good solvent), the excitation hopping was only observed for a sample withMnsim;104; on the other hand, it occurred unequivocally for samples withMnsim;105in MCH (poor solvent), as evidenced by a clear decay of the fluorescence anisotropy,r(t). Furthermore, the excitation hopping was anintramolecularevent only at very low concentration (sim;10minus;7M based on polymer chain), and at 10minus;6M, anintermolecularexcitation hopping was observed. Such sharp dependence of the excitation hopping on the concentration was interpreted in terms of chain collapse at low temperature. Simulation ofr(t) was performed on the basis of Fouml;rster mechanism and Gaussian chain statistics.