Dynamic Fluoride Anion Coordinating Si-Containing π-Conjugating Polymers

Three types of poly[dialkylsilylenephenyleneethynylenephenylene]s (dialkyl: 2 for n-dodecylmethyl; 5a for di-n-hexyl; 5b for di-n-hexyl-co-dimethyl) were newly prepared by the AB- and AA-BB-type coupling reactions. These polymers afforded moderate molecular weights (M_w) in the range 7.6 × 10³-27 × 10³. As the amount of tetra-n-butylammonium fluoride (TBAF) increases in a THF solution of 2, two π-π* absorption bands at 298 and 317 nm due to the phenyleneethynylenephenylene moiety significantly decreased. Simultaneously, a new intense absorption band at 390 nm characteristic of intramolecular charge transfer (ICT) appeared in place of the original ICT absorption band at 342 nm. Also, the intensity of fluorescence band around 393 nm (excited at 342 nm) significantly decreased upon the addition of TBAF. The changes in UV-vis absorption and fluorescence spectra of the polymers were readily seen by naked eye. As for 2, a novel three-step titration curve of UV absorbance (or apparent GPC retention time) as a function of fluoride concentration was found. It was thus assumed that 2 underwent anion charge-driven coil-to-globule conformational transitions in the presence of TBAF. In addition, the complex forming abilities of the polymers with fluoride ion were greatly dependent on the nature of alkyl groups attached to the silicon atom of the polymers.