The change in shape inducible in some photo-reversible molecules using light can effect powerful changes to a variety of properties of a host material. The most ubiquitous natural molecule for reversible shape change is the rhodopsin–retinal protein system that enables vision, and this is perhaps the quintessential reversible photo-switch. Perhaps the best artificial mimic of this strong photo-switching effect however, for reversibility, speed, and simplicity of incorporation, is azobenzene. This review focuses on the study and application of reversible changes in shape that can be achieved with various systems incorporating azobenzene. This photo-mechanical effect can be defined as the reversible change in shape inducible in some molecules by the adsorption of light, which results in a significant macroscopic mechanical deformation of the host material. Thus, it does not include simple thermal expansion effects, nor does it include reversible but non-mechanical photo-switching or photo-chemistry, nor any of the wide range of optical and electro-optical switching effects for which good reviews exist elsewhere.

We demonstrate the first supramolecular cocrystallisation strategy to generate crystalline azobenzene materials with a range of photo-mechanical and thermochemical properties: from those that exhibit isomerisation without any change in crystal shape to those that undergo a crystal-to-crystal cis–trans isomerisation accompanied by large scale bending. The latter permitted the use of variable temperature single crystal X-ray diffraction for the first in situ monitoring of structural changes behind the cis–trans isomerisation in the solid-state, which revealed a topotactic process mediated by an amorphous phase. While the design of photo-mechanical azobenzene solids has so far focused on polymer matrices and gels, the herein presented cocrystallisation approach represents the first methodology for generating new photo-mechanical azobenzene crystals from a limited number of photo-active building blocks, opening a route to potential bio-mimetic and light-harvesting materials based on crystalline solids.

Azobenzene‐containing polymers offer tremendous advantages and opportunities over other stimuli‐responsive materials to interface with biology. Azobenzene's fast, reversible, and innocuous cis–trans geometrical isomerization can be leveraged into dramatic intra‐ and inter‐molecular changes when incorporated in polymeric materials. Azobenzene use has grown from a colorant, through to optical storage materials, and most recently in a variety of biologically themed applications. This review highlights the broad impact this photo‐switch has had in recent years and offers a snapshot of the research landscape at the interface between photochemistry and biology. From photo‐reversible micelles and peptides to controlled drug release and sensing, the versatility of azobenzene makes it a favored photo‐switch found in many emerging applications. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 3058–3070