Orientation and conformation of two [6]carbohelicenes in stretched polystyrene and a thermoresponsive polyaspartate.

Abstract

Two functionalized [6]carbohelicenes, one of which was also available in its two enantiomeric pure forms, were oriented in stretched polystyrene in CDCl3 , and in a recently introduced chiral thermoresponsive lyotropic polyaspartate (poly(benzyl)0.5 (phenethyl)0.5 -L-aspartate) in C2 D2 Cl4 . From the resulting 1 H,13 C residual dipolar couplings, the helical pitch of a methylated [6]carbohelicene was determined and found to be in agreement with theoretical predictions and existing crystal structures (d(C2,C2') ≈ 4.3 Å). For a second [6]carbohelicene with para-methoxyphenyl substituents, a clear conformational preference of the substituents was observed. The orientational properties of the two helicene enantiomers in the chiral polyaspartate are very similar, but both drastically change around 306 K. We suggest this behavior is due to an unusual phase transition in the liquid crystal.