Stimulating changes in the elastic modulus of polymer materials by molecular photochromism

Abstract

Photonastic materials are those that deform or change shape in a pre-determined or non-random fashion when exposed to light. We report here the elastic modulus of a photonastic polymer containing the photoreversible photochrome [Ru(bpy)₂(pySO-NB)]²⁺, where bpy is 2,2′-bipyridine, and pySO-NB is a chelating ligand containing a norbornene monomer attached through an amide group (Scheme 1), both pre- and post-irradiation. Nanoindentation results show that the polymer exhibits an increase in the elastic modulus (E) upon irradiation. We propose a bilayer cantilever model to explain the macroscopic deformation. Our analysis reveals that the concentration of the ruthenium is an important parameter through the optical density of the film, the optical penetration depth and the force generated from irradiation.