Behavior of Chiral Induction from Polysilsesquioxanes Bearing Chiral and Ammonium Groups to Anionic Pyrene Derivatives

The behavior of chiral induction from ladder-like polysilsesquioxanes (R-60- and S-60-PSQs) bearing chiral and ammonium side-chain groups (compositional ratio = ca. 60:40) to the anionic pyrene derivative, 1,3,6,8-pyrenetetrasulfonic acid tetrasodium salt (PTSNa4), was investigated. The circular dichroism (CD) spectra of PTSNa4/(R-60- and S-60-PSQs) mixtures in methanol exhibited reverse peaks at 230-250, 280-290, and 310-370 nm, which were assigned to PTSNa4 and indicated that the achiral PTSNa4 molecule had chirality induced from the chiral PSQs. In addition, it was found that an eximer of PTSNa4 was formed by hybridization with the chiral PSQs, i.e., the emission peaks not only due to a monomer state at 385 and 408 nm but also due to an eximer at 460-520 nm were observed in the fluorescence spectrum excited at 377 nm. This result suggests that the negatively charged PTSNa4 formed an eximer along the positively charged side-chain ammonium groups of PSQs. To investigate the influence of the structures of the pyrene derivatives on chiral induction behavior, the CD measurements of the mixtures of R-60- and S-60-PSQs in methanol with different pyrene derivative, 8-hydroxypyrene-1,3,6-trisulfonic acid trisodium salt (pyranine), were also performed. It was found that the intensities of the absorptions for the pyanine solutions were lower than those for PTSNa4 solutions. These results suggest that the point-symmetry of the pyrene derivatives influences the level of chiral induction. Therefore, the point-symmetry of pyrene derivatives is an important factor for determining the behavior of chiral induction from chiral PSQs to these compounds.