Computing Handedness:  Quantized and Superposed Switch and Dynamic Memory of Helical Polysilylene

Abstract

Two new conjugating helical polymers comprising a rodlike silicon backbone and enantiopure chiral pendants, poly[(R)-3,7-dimethyloctyl-(S)-3-methylpentylsilylene] (PS-1) and its diastereomeric poly[(S)-3,7-dimethyloctyl-(S)-3-methylpentylsilylene] (PS-2), were prepared. Molecular mechanics calculations of PS-1 and PS-2 model oligomers indicated a double well potential energy curve corresponding to almost enantiomeric helices with dihedral angles of 150-160 degrees (P-motif, global minimum) and 200-210 degrees (M-motif), regardless of their tacticity. Experimentally, it was found that PS-1 in dilute isooctane revealed switchable ambidextrous helicity on application of a thermal energy bias. Although PS-1 featured three distinct switching regions, viz. "region 1, between -80 and -10 degrees C", "region 2, between -10 and +10 degrees C", and "region 3, between +10 degrees C and +80 degrees C", the switching properties were interpreted as the result of superposed P- and M-helicities, undergoing dynamic pseudo-racemization or oscillation. Oscillating helicity in region 2 was roughly estimated to be about 13 cm(-)(1). The superposed helicity in region 2 was critical since it afforded molecular recognition ability with a dynamic memory function that was highly susceptible to solvent molecular topology and volume fraction. This could lead to potential as a molecular information processor to serve as a gauge of chemical properties. On the other hand, PS-2 could not switch its preferential screw-sense in the range of -80 to +80 degrees C. This may be related to greater differences the potential energy curve between P- and M-motifs.

Nonclassical forces: Seemingly insignificant but a powerful tool to control macromolecular structures

“…In the last two decades, we have designed and synthesized a wide range of polysilanes comprising chiral and/or achiral side groups. [34][35][36][37][38][39][40][41][42][43][44][45][46][47][48] MF realized that polysilanes were gifted with unique features of exhibiting intense and sharp UV absorption, circular dichroism (CD), and photoluminescence (PL) spectra around 300-400 nm, characteristic of the r-conjugation in the helical silicon-catenated main chain. Individual rod-like helical polysilanes in solution afford a highly intense, narrow exciton absorption band along with a sharp mirror-imaged emission band.…”

A Chemical Device That Exhibits Dual Mode Motions: Dynamic Coupling of Amide Coordination Isomerism and Metal‐Centered Helicity Inversion in a Chiral Cobalt(II) Complex

“…The acidbase reaction of the corresponding cobalt complex triggered the interconversion of coordinating atoms between amide nitrogen atoms and amide oxygen atoms, giving rise to a stretching (extension/contraction) molecular motion. Since we previously demonstrated that the helicity of the Co II complex with H 2 L2 was dynamically inverted from the L cis-a form to the D cis-a form by adding achiral NO 3 À ions, [4][5][6] the employed H 2 L1-Co II complex was designed to work as a novel type of molecular machine that exhibits coupled stretching and inverting motions. Several types of helical ligands have shown extension/contraction molecular motion on metal complexation/decomplexation [2d-e, 7] and/or protonation/deprotonation, [7, 8] but the present type of kinetically labile Co II complex allows a dual molecular motion in a highly dynamic fashion, as would be required for a sophisticated supramolecular switching device.…”

Chirality responsive helical poly(phenylacetylene) bearing L‐proline pendants

“…Switching of the macromolecular helicity of dynamic helical polymers controlled by chiral stimuli is quite interesting, but is still a challenge 20–25 (For inversion of the macromolecular helicity observed in other helical polyisocyanates and polysilanes, see Ref. 22–25). We found that the poly‐1 enantioselectively underwent the inversion of helicity in an aqueous solution in the presence of a hydrophobic guest, 1,1′‐2‐binaphthol ( 3 ).…”