LINEAR OPTICAL-PROPERTIES OF A SERIES OF LINEAR POLYENES - IMPLICATIONS FOR POLYACETYLENE

A homologous series of polyacetylene oligomers having up to 13 double bonds has been prepared, thus allowing, for the first time, an examination of optical properties of polyenes having enough conjugation length to suggest extrapolation to the polymer. Two linear optical properties, UV-Vis absorption spectra in solid and solution form and Raman scattering spectra in solid form, have been measured. Both the lowest energy electronic absorption peak and the frequency of the Raman bands become linear in 1/n for n > 7. Assuming the validity of the 1/n extrapolation, the results suggest that the familiar forms of polyacetylene have effective conjugation lengths that average approximately 30 double bonds. Raman data are discussed in terms of the competing models for dispersion of the Raman spectra in polyacetylene. Our results strongly suggest that conjugation length disorder is the origin of the dispersion