TIME-RESOLVED OBSERVATION OF EXCITATION HOPPING BETWEEN 2 ANTHRYL MOIETIES ATTACHED TO BOTH ENDS OF POLYSTYRENE CHAINS

Excitation hopping between two 9-anthryl moieties attached to both ends of polystyrene chains with various molecular weights has been explored by picosecond time-resolved fluorescence anisotropy measurements. The measurements were performed at 77 K in the rigid matrix of 2-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 (M(n)), solvent, and concentration of the samples. In MTHF (good solvent), the excitation hopping was only observed for a sample with M(n) approximately 10(4); on the other hand, it occurred unequivocally for samples with M(n) approximately 10(5) in MCH (poor solvent), as evidenced by a clear decay of the fluorescence anisotropy, r(t). Furthermore, the excitation hopping was an intramolecular event only at very low concentration (approximately 10(-7) M based on polymer chain), and at 10(-6) M, an intermolecular excitation hopping was observed. Such sharp dependence of the excitation hopping on the concentration was interpreted in terms of chain collapse at low temperature. Simulation of r(t) was performed on the basis of Forster mechanism and Gaussian chain statistics.