Optical probes of polydiethynylsilanes

We have applied a variety of optical probe techniques for studying the ground-state properties, photoexcitations, and optical nonlinearities of the novel class π-conjugated polymers incorporating Si (or Ge), the polydiethynylsilanes (PDES): [(HCC)₂SiR₂]_x, where R = alkyl, phenyl, etc. Our measurements imply that the PDES backbone structure consists of units of four-membered rings alternately connected to CH and SiR₂ units; thus it can be regarded as an almost degenerate ground state (AB)_x with a band gap at 2 eV. The PDES are therefore the second class of conducting polymers that can support soliton excitations. Using the photomodulation spectroscopy technique, we have found in PDES photogeneration of charged solitons similar to that in trans-(CH)_x, as predicted by theory. However, due to the short average conjugation length in PDES films, the soliton energy levels are split by end-effects confinement. We have also found at 2 eV χ⁽³⁾ = 3 × 10^{- 9} esu, which decays in a record time of 135 fs, followed by a slower decay component of 750 fs, without a long tail even at higher laser intensities.