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

Abstract

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 = 105/(2.16 + 21.3q–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 (Tc). 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 λmax: 334 nm. In this case, the q value above Tc at 308 nm and below Tc at λmax: 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 λmax: 320 to 350 nm in the relatively wide range of trandition temperature between −20 and −55 °C, involving conformation change from 73 helix to all-trans. Here the q value for the all-trans conformation with 5 nm increased approximately twice than that for the 73 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.