Visible to near-infrared femtosecond dynamics of photoexcited gap states in substituted polyacetylenes

Abstract

Visible to near-infrared transient spectra were measured for thin films of three substituted polyacetylenes with a time-resolution ≤300 fs. A hot self-trapped exciton (STE) and an oppositely charged, spatially confined soliton–antisoliton pair were temporally and spectrally resolved in detail, which reveals a formation process of the localized excitations with geometrical relaxation taking place within a subpicosecond time scale. The hot STE showing an exponential decay (τ=115–135 fs) has a spectral peak in the energy region of 1.4–1.5 eV. The transition energies from the hot STE both to continuum state and to a biexciton state are discussed with referring to strength of the Coulomb interactions between the conjugated π electrons. The soliton–antisoliton pair which decays with a power-law behavior has a dual-peaks spectrum below the band gap energy and the two peak energies vary depending on the polymers. A π-conjugation length (λc), a soliton size (ξ), and a distance (d) between the soliton and antisoliton were evaluated based on the experimental results for each polymer. The distance was found to be nearly proportional to the conjugation length with a ratio of d/λc=0.4, indicating that an overall size of the soliton–antisoliton pair approximatedly given by 2ξ+d is limited just within a segmented conjugation chain. The decay kinetics of photoexcitations in both degenerate and nondegenerate systems has been discussed together using an adiabatic potential surface in a configuration space.