Influence of a fluorescent probe on the local relaxation times for a polystyrene chain in the fluorescence depolarization method

The influence of the fluorescent probe on the fluorescence depolarization s tudy was examined by using molecular dynamics (MD) simulation together with the fluorescence depolarization measurement. The relaxation times of the l ocal motion, T-m, for two series of polystyrene (PS) (r-PS and a-PS), which have different molecular structures in the vicinity of the anthryl group u sed as a fluorescent probe, were compared. The T-m, for r-PS, which has mor e space between the anthryl group and PS unit than a-PS, was smaller by a f actor of 2 than that for a-PS, which has one methylene group as the spacer. The steric hindrance of the anthryl group with the phenyl ring of PS is co nsidered to make the relaxation time longer. On the other hand, the activat ion energy may predominantly reflect the inherent chain mobility of the PS chain. The MD simulation was performed for the anthryl group-labeled PS cha in with various numbers of methylene groups between the probe and styrene u nit. The result explains the difference in the relaxation time between a-PS and r-PS. The T-m for the local motion of the probe-free PS segment at the chain center showed that the chain mobility of PS is slightly reduced by t he introduction of an anthryl group The effect of the direction of the tran sition moment in the fluorescent probe for the relaxation time was also exa mined.