Evaluation of Global Conformation of Polydialkylsilane Using Correlation between Persistence Length and Excitonic Absorption

An empirical relationship between persistence length ( q ) and molar absorptivity (ε) of poly(dialkylsilane)s (PSis) was developed. Here the q value was estimated by a touched-bead wormlike-chain model with the relationship between the intrinsic viscosity and the molecular weight, which were obtained from a size exclusion chromatography equipped with a viscometer. The resulting ε– q plot showed an apparent saturation curve, which could be formulated as ε = 10 5 /(2.16 + 21.3 q –1.23 ) with a high correlation coefficient, r = 0.99. Furthermore, using this empirical ε– q relational equation, changes in the q value of the PSis among thermochromism with (i) abrupt transition, (ii) gradual transition, and (iii) nontransition were estimated. (i) Abrupt type of thermochromism: poly( n -decylmethylsilane) exhibited an UV absorption band at 308 nm above transition temperature ( T c ). This band drastically decreased in the relatively narrow range of transition temperature (ca. −35 to −50 °C), accompanied by an appearance of a new UV absorption band at λ max: 334 nm. In this case, the q value above T c at 308 nm and below T c at λ max: 334 nm was nearly identical, at ca. 1.5 nm. (ii) Gradual type of thermochromism: poly( n -hexyl -n -propylsilane) showed a gradual bathochromic shift from λ max: 320 to 350 nm in the relatively wide range of trandition temperature between −20 and −55 °C, involving conformation change from 7 3 helix to all-trans. Here the q value for the all-trans conformation with 5 nm increased approximately twice than that for the 7 3 helix conformation. (iii) Nontransition type: poly( n -hexylmethylsilane) showed a bathochromic shift of ca. 11 nm in the ranges of observation temperature (+25 to −80 °C) without an apparent transition. Here the q values were gradually and contiuously increased from ca. 1.2 to 1.4 nm. These results allowed to estimate the q value of PSis under various conditions, leading to a novel approach of investigation of the conformation-dependent chromic behavior of PSis, such as thermochromism, solvatochromism, piezochromism, and electrochromism.