Towards in silico design of organic photomechanical materials*

Organic photomechanical crystals transform light into mechanical work, producing work densities that are potentially orders of magnitudes higher than those achieved by piezoelectrics or elastomer materials.  However, the complexities of the organic solid state make it difficult to anticipate how the crystal structure will transform upon photomechanical reaction, or, in many cases, to even characterize the transformation experimentally. We have recently developed a computational approach for predicting these crystalline transformations entirely in silico.  It enables us to bridge between the structure of the molecular photochrome and the solid-state photomechanical response, providing an atomistic mechanism for the crystal transformation.  Our approach combines crystal structure prediction, topochemical concepts, solid-state density functional theory, and corrections to overcome the limitations of widely-used density functionals.  Using this approach, we have studied a variety of [4+4] photodimerization reactions in anthracene reactions and ring-opening/closing in diarylethenes, from which we have have demonstrated the large ideal photomechanical work densities and highly anisotropic nature of the crystalline response.  More importantly, we have extracted a series of design principles regarding how molecular structure and crystal packing impact the photomechanical response that can guide the development of improved photomechanical materials.