Conformational Kinetics in Chiral Poly(diphenylacetylene)s: Dynamic P/M Memory Effect.

Abstract

Dynamic P/M (plus/minus) helical memory in chiral dissymmetric poly(diphenylacetylene)s (PDPA) is shown by using as model compound a PDPA that bears the benzamide of (L)-alanine methyl ester as pendant. Thus, for a single chiral polymer, it is possible to obtain either P or M macromolecular helical structures in a specific solvent without the presence of any chiral external stimuli. To do that, it is necessary to combine the conformational control at the pendant group with a high steric hindrance at the backbone. In this case, by thermal annealing in low polar solvents, an anti-conformer is stabilized at the pendant which commands a P helix in the PDPA. Next, solvent removal followed by addition of a polar solvent such as dimethyl sulfoxide (DMSO), results in the kinetic conformationally trapped P helix. However, in this medium, the preferred handedness and the thermodynamic macromolecular helix for poly-(L)-1 is M. This process also occurs in the opposite way. Electronic circular dichroism (ECD) and circularly polarized luminescence (CPL) studies show that the dynamic memory effect is present both in ground and excited states. These studies also show a remarkable long-term memory effect that can be stored for several months.