Helix magic. Thermo-driven chiroptical switching and screw-sense inversion of flexible rod helical polysilylenes

An ideal optically active helical chromophoric polymer comprising a flexible rodlike silicon main chain and enantiopure alkyl side chains, poly{(S)-3,7-dimethyloctyl-3-methylbutylene}, underwent a thermo-driven helix-helix transition at -20 degrees C in isooctane. The transition characteristics. including transition temperature, transition width, the population of right- and left-handed helical motifs, global shape, and screw-pitch, were to be characterized quantitatively by spectroscopically analyzing circular dichroism (CD) and UV absorption characteristics. This is based on the unique property of the rodlike polymer in which the CD band completely matches the corresponding UV band profile at all temperatures. Moreover, fine controlling the contents and chirality of an additional chiral silylene unit incorporated in the copolymers allows free manipulation of the transition temperature in the range from -64 to +79 degrees C. Molecular mechanics calculation showed remarkable differences in the potential energy curve of the main chain torsion angle between flexible and rigid rodlike polysilylenes. These results and knowledge gained should assist in designing and controlling new types of helix-helix transition polymers directed to diverse screw-sense related properties and applications.