Ionic Helical Polymers with Expanded π-Conjugation System Derived from Through-Space Interaction in Piperazinium Ring and Their Spontaneous Dynamic Conformational Changes

Abstract

Reactions of N-(2,4-dinitrophenyl)-4-arylpyridinium chlorides (aryl = H and 4-pyridiyl (Py)) with R-(−)- or S-(+)-2-methylpiperazines caused ring-opening of the pyridinium ring and yielded ionic polymers that consisted of 5-(2-methylpiperazinium)-3-aryl-penta-2,4-dienylideneammonium chloride units, namely, polymer(R-M;H), polymer(S-Me;H), polymer(R-Me;Py), and polymer(S-Me;Py). Reactions of achiral polymer(Me;H) with R-(−)- or S-(+)-1,1′-binaphthyl-2,2′-diyl sodium phosphates caused an anion exchange that yielded polymers with chiral phosphate anions. CD measurements revealed that polymer(R-Me;H) and polymer(S-Me;H) as well as polymer(R-Me;Py) and polymer(S-Me;Py) formed opposite helical conformations; therefore, it is clear that the helical conformation can be controlled by varying the substituent on the piperazinium ring. In addition, CD measurements of achiral polymer(Me;H) in the presence of R-(−)- or S-(+)-1,1′-binaphthyl-2,2′-diyl sodium phosphates revealed that the chiral anions incorporated by the anion exchange reaction in solution induced helical conformations in the polymer backbone. The helical conformations disappeared as the 2-methylpiperazinium rings in the polymers converted from the boat to the chair form via a half-chair intermediate, and this conformational change was accompanied by a decrease in the amount of π-conjugation length. These spontaneous, dynamic changes in a polymer’s helical conformation occurred without using external stimuli. These changes affected the viscosity and the electrochemical properties of the polymer solutions; the ηsp/c value of the polymers in methanol decreased with time, and electrochemical oxidation peaks of the polymers shifted to higher potentials when the polymer solution was left standing in air.