Evaluation of Stretching Properties of [7]Thiaheterohelicene Framework Called “Molecular Spring” Using AFM Force Measurements and Electrostatic State Calculations

The stretching properties of a [7]thiaheterohelicene framework, what we call molecular spring, have not been investigated so far, despite a variety of [7]thiaheterohelicene derivatives having very interesting characteristics due to both the rigidity arising from fused benzene rings and the flexibility like a spring originating from helical structure. In this study, a novel [7]thiaheterohelicene derivative, which has a disulfide moiety for bonding to a gold-coated substrate and a carboxy group for reacting with an amino-modified probe tip at each of its end groups, was synthesized in order to elucidate the elasticity of the [7]thiaheterohelicene framework by atomic force microscopy (AFM). The AFM force measurements were carried out using two carboxy-terminated disulfide derivatives with or without a [7]thiaheterohelicene moiety, and the deviation between two kinds of force–extension curves was related to the stretching originating from the [7]thiaheterohelicene framework here. Furthermore, its elasticity was compared to that of biphenyl, which is generally known as a rigid framework, using electrostatic state calculations.