Broken Mirrors: Multiple Circular Polarization and Inversion in the Ground and Photoexcited States of Mirror-Symmetric Helical Poly(di-iso-alkylsilane)s in Achiral Molecular Solvents

This paper comprehensively reports experimental proof of parity violation in the ground and photoexcited states of three mirror-symmetric Si–Si bond polymers in homogeneous solutions of achiral molecules under non-stirring conditions by analyzing 370 chiroptical datasets relating to multiple second-order helix–helix transitions in the circular dichroism (CD) of poly(di-i-butylsilane) (iBS), poly(di-i-pentylsilane) (iPS), and poly(di-i-hexylsilane) (iHS) in achiral alkanols and p-dioxane-h8/-d8. Particularly large (–)-CD of gabs = −3.1 × 10−2 at 290 nm was found for iBS in i-pentanol at 25 °C. Notably, iPS in n-propanol at −5 °C generated (–)-CD with gabs = −0.48 × 10−2 at 300 nm, but (+)-circularly polarized luminescence (CPL) with glum = +0.84 × 10−2 at 326 nm. In contrast, iHS in n-octanol at 0 °C showed only very weak (–)-CD of gabs ~−0.03 × 10−2 at 310 nm. The H/D isotopes of p-dioxane-h8/-d8 weakly affected the helix–helix transition characteristics of iBS. (–)-Sign vibrational CD signals assigned to the handed symmetric and asymmetric bending modes of the CH3 and CH2 groups of the solvents and other achiral molecules were observed. We assumed (i) three 1H nuclear-spin-1/2 induced handed motions of CH3 rotors at i-alkyl side chains and achiral alkanols, and (ii) helical main-chain Si atoms (δ+) coordinated by handed lone pairs at oxygen (δ−) in gauche-containing n- and i-alkanols induced by the CH3 rotors. A possible origin of biomolecular handedness is proposed based on the first observation of far-UV CD and UV spectra of zwitterionic glycine bearing H3N+ rotor in neutral H2O.