Blurring and Broadening of Reflection Spectra Due to Structural and Alignment Changes of Conjugated Chains Caused by Isomerization and Stretching in Trans-Polyacetylene

An optimized thermal isomerization condition has been established to produce trans-(CH) x films which show the reflection spectra with well-resolved vibronic structure peaking at the lowest subband. The conjugation lengths with about 100 and 150 double-bond (C=C) units are estimated to be responsible for the 1.56 eV peak of as-grown (CH) x and the 1.51 eV peak of trans-(CH) x , respectively. The 0.05 eV peak shift accounts for the extension of the trans-part by 50% of the initial length. The effect of mechanical stretching for alignment on the reflection spectra has also been studied by paying particular attention to the conjugation-length distribution. The reflection spectra obtained using light polarized parallel and perpendicularly to the stretching direction indicate that the shorter chains align more easily than the longer ones; the observed perpendicular reflection does not arise from the reflection with light polarized perpendicularly to the (CH) x chains but from the reflection with light polarized parallel to the longer ones which remain unaligned. It appears the real dimensionality of trans-(CH) x must be reconsidered by taking account of the distribution of conjugated chain lengths.