Photomechanical materials driven by photoisomerization or photodimerization

Light is an adjustable, multiparameter stimulus that can be used for fine, noncontact manipulation or as an energy supply. Recently, deformable light-controlled macroscopic materials have gained attention both from a fundamental research perspective and for various actuator applications. The main challenge in developing these materials is converting the photoinduced effects at the molecular level to macroscopic movements in the working pieces; a variety of mechanisms have been proposed for this. Both crystals and polymers containing photoreactive compounds have been intensively studied and have exhibited different advantages. Crosslinked liquid crystalline polymers have also attracted attention because they combine the advantages of macroscopically deformable polymers and crystals. In most circumstances, photodeformable materials contain photoreactive molecules that absorb light of a specific wavelength and thus undergo structural changes. This is followed by concomitant changes in their physical and chemical properties, resulting in macroscopic mechanical movements. Therefore, various photoreactions have been studied to induce macroscopic deformations using light. The purpose of this review is to highlight key examples in the design of photodeformable materials using various photoreactions and to introduce some new and evolving trends by highlighting recent research. Macroscopic deformable materials controlled by light have been the subject of growing research interest in the last few decades due to both their fundamental properties and their wide applicability. In this review, examples of photomechanical effects in crystals, polymers, and CLCPs based on photoisomerization and photodimerization are reviewed.