Confinement effect of photogenerated soliton-antisoliton pair on the ultrafast relaxation in a substituted polyacetylene

Femtosecond pump-probe experiments were performed on poly[o-(trimethylsilyl)phenylacetylene] thin films to investigate the effect of confinement of a photogenerated soliton-antisoliton pair on its ultrafast relaxation dynamics. The decay kinetics of transient photoinduced absorption due to the photoexcitations of the polymer were found to behave a erf (t^-n) with n=0.65±0.05, implying a geminate recombination process of the soliton-antisoliton pair on a quasi-one-dimensional chain. The distance between the soliton and antisoliton could be estimated as about six repeat unit lengths, suggesting a fairly strong confinement of the pair. It is consistent with the size of a certain regular structure indicated to exist in this material by X-ray diffraction measurements. Creation of confined pairs of likely charged solitons due to interchain photoexcitation is also discussed in terms of the following energetically favored reaction between likely charged polarons on the same chain: P^±+P^±→S ^±+S^±