Thermochromic Behavior of Polysilanes Concerning the Dependence on the Sequence Length

Poly(dihexylsilane) and poly(dibutylsilane) having a wide range of molecular weights were prepared by the photodegradation of the corresponding high-molecular-weight polymers. These polymers showed characteristic thermochromism. Thus, the absorption maximum observed for both polymers at around room temperature disappeared and generated a new band at longer wavelengths at low temperature. The transition temperatures (Tc) (at which the intensities of these two absorption bands become equal) of these polymers were apparently proportional to the inverse of the number-average molecular weight () until the polysilanes reach a certain value of molecular weight. Beyond that value, the transition temperature was no longer dependent on the molecular weight. The thermochromic transition temperature also depended on the size of the alkyl substituents. These results have been satisfactorily explained by a model of the thermochromism based on a cooperative transition between the ordered trans and gauche states for poly(dialkylsilane)s.