Optically Controlled Solid-to-Liquid Phase Transition Materials Based on Azo Compounds

Abstract

Phase transition materials can be utilized in a variety of functionalization scenarios, benefiting from the photoinduced changes in their physicochemical properties. Among them, small molecular crystals or liquid-crystalline polymers containing azobenzene moieties that show phototunable melting points or glass transition temperatures, respectively, have proven to be significant in optically controlled functional materials. In recent years, they have been successfully used in reversible mechanical actuation, adhesion, energy storage, self-healing, athermal nanoimprinting, and other applications, which has attracted the attention of more scientists and engineers. In this review, azobenzene-containing materials with photoinduced phase transition capabilities are systematically discussed in terms of molecular design strategies, phase change mechanisms, performance control, methods of preparation of materials, and potential applications. Existing problems and future challenges are also discussed. We hope to provide some inspiration for the development of photocontrolled phase transition materials.

A Synthetic Approach for Oxadiazole‐Decorated Azobenzene Photoswitches

“…For instance, fine tuning of the structure may lead to compounds displaying light-induced phase transition, which are very useful for novel phase transition materials. [41] DFT calculation was also performed to explain behaviour of the compounds both in terms of luminescence loss in contrast to their precursors and the stability of Z isomer and the theoretical results correlated very well with the experimental work.…”

Photoinduced solid-to-liquid transition of an N-benzylideneaniline derivative towards smart glass

“…If the phase transition of substances is utilized adequately, abundant functions could be realized. Thereinto, solid–liquid transition of crystals shows promise for development, 1 but it requires a certain amount of energy to start with. Compared with metal or ionic crystals, molecular crystals formed from organic matter are more likely to undergo phase transitions due to the weak intermolecular forces and thus perform specific functions.…”

“…2 To give another example, some optically controlled solid-to-liquid PCMs based on azo compounds are used to develop photo-actuators or light shutters. 1…”

Photoinduced Solid–Liquid Phase Transition and Energy Storage Enabled by the Design of Linked Double Photoswitches

“…For example, p -substitution of azobenzene with functional groups including F, Cl, Br, I, CN, and NO 2 result in the formation of solid phase for both E and Z isomers, due to the facile crystallization of twisted Z structures. Out of a large series of arylazopyrazole derivatives p -substituted on the aryl ring with various alkoxy groups (OC n H 2 n +1 and OC n H 2 n –1 ; n = 1–12), only one compound (OC 8 H 15 ) displays a liquid phase as Z isomer (i.e., melting point below room temperature), while the rest of the compounds with shorter or longer chains are solids as both E and Z isomers. The phase of Z isomer is also impacted by the o -substituents on azobenzene core; for example, azobenzenes o -substituted with halides (F and Cl) undergo solid–liquid phase transition, while the o -MeO-substituted azobenzene exhibits solid phases for both E and Z , despite bearing an identical p -tridecanoate group.…”

“…Photoinduced phase transition of photoswitches between solid and liquid has recently emerged as a strategy that effectively increases the total energy storage density of molecular solar thermal energy storage (MOST) systems. − In particular, photoswitches including azobenzene − and azoheteroarene , derivatives that undergo large structural changes upon E – Z isomerization have been primarily investigated for drastic phase transitions. E isomers generally exhibit a planar molecular structure that forms a crystalline phase, whereas Z isomers display a twisted , or T-shaped geometry that is less prone to crystallization at room temperature and forms a liquid phase under ambient conditions.…”