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

Visible to near-infrared transient spectra were measured for thin films of three substituted polyacetylenes with a time-resolution less than or equal to 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 (tau=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 pi 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 pi-conjugation length (lambda(c)), a soliton size (xi), 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/lambda(c)=0.4, indicating that an overall size of the soliton-antisoliton pair approximatedly given by 2 xi+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. (C) 1996 American Institute of Physics.