The influence of polymer concentration on the internal motion — intramolecular pyrene excimer formation — of a low molecular weight probe in solution

Fluorescence decay measurements were used to study the kinetics of end-to-end cyclization of the molecule Py(CH₂)₅Si(Me₂)OSi(Me₂)(CH₂)₅Py (1), where Py=1 ? pyrenyl. Experiments were carried out in toluene at 22°C and in cyclohexane at 35°C, in the absence and in the presence of added polystyrene (PS). Addition of even small amounts (0.01 g/ml) of PS caused the cyclization rate of probe molecule 1 to decrease substantially in cyclohexane, whereas in toluene, a much milder retardation of cyclization rate with increasing PS was observed. These effects are interpreted in terms of a coupling between the chain dyanmics and that of the probe molecule 1, mediated by the solvent. These hydrodynamic screening interactions depend sensitively upon whether the polymer is in a good or a poor solvent.