Organic Photomechanical Materials

Abstract

Organic molecules can transform photons into Angstrom-scale motions by undergoing photochemical reactions. Ordered media, for example, liquid crystals or molecular crystals, can align these molecular-scale motions to produce motion on much larger (micron to millimeter) length scales. In this Review, we describe the basic principles that underlie organic photomechanical materials, starting with a brief survey of molecular photochromic systems that have been used as elements of photomechanical materials. We then describe various options for incorporating these active elements into a solid-state material, including dispersal in a polymer matrix, covalent attachment to a polymer chain, or self-assembly into molecular crystals. Particular emphasis is placed on ordered media, such as liquid-crystal elastomers and molecular crystals, that have been shown to produce motion on large (micron to millimeter) length scales. We also discuss other mechanisms for generating photomechanical motion that do not involve photochemical reactions, such as photothermal expansion and photoinduced charge transfer. Finally, we identify areas for future research, ranging from the study of basic phenomena in solid-state photochemistry, to molecular and host matrix design, and the optimization of photoexcitation conditions. The ultimate realization of photon-fueled micromachines will likely involve advances spanning the disciplines of chemistry, physics and engineering.

Switchable molecule-based materials for micro- and nanoscale actuating applications: Achievements and prospects

“…From a more general point of view, these Reproduced from Ref. [69], with permission of the copyright holders. molecular switches must be considered as phase change materials.…”

“…Reports on molecule-based photomechanical switches were published more than forty years ago [65][66][67][68] and several recent reviews have covered this field [69,70]. For this reason we restrict ourselves here to discussion of a few selected examples in which actuating performance and mechanical properties were reported.…”

“…Even if these phenomena remain rather case specific and difficult to quantify, the underlying mechanistic and structural aspects have recently been investigated more deeply for several different (mainly organic) compounds. The Barden group reported the photo-induced deformation of a variety of organic crystals and analyzed the deformations in terms of photo-induced structural changes [69]. Strictly speaking some of the examples are not molecular switches because the photo-induced changes are more related to a variation of the crystal packing (dimerization, etc.)…”

Photomotility of polymers

“…Photomechanical effects in crystalline materials have also been subject to recent research, including demonstrations of bending, jumping and twisting56. Photomechanical effects in polymers and crystalline materials have been subject to a number of recent reviews7891011. Although not a requirement for the realization of photoinduced deformation in polymers, considerable recent attention has focused on examinations of azobenzene-functionalized liquid crystalline polymer networks (azo-LCNs) and elastomers (azo-LCEs).…”

Mechanical property design of molecular solids

“…Understanding of the mechanical behavior of materials, a domain specialization of metallurgists and mechanical engineers hitherto, has become a topic of interest for chemists in the recent past [16][17][18][19][20][21][22][23][24][25]. This is due to the realization of the importance of the mechanical properties of molecular solids in determining/realizing the application potential of a number of exciting solids such as organic semiconductors, artificial muscles and thermosalient and mechanochromic organic solids [26][27][28][29][30][31][32][33]. In the case of APIs, which are often subjected to a wide variety of mechanical operations during tablet http formulation such as grinding, milling and crushing, detailed knowledge of the mechanical properties is essential so that they can suitably be exploited during the manufacturing in the pharmaceutical industry [20,21,[34][35][36][37][38].…”