Photoinduced Reversible Solid‐to‐Liquid Transitions for Photoswitchable Materials

Abstract

Heating and cooling can induce reversible solid‐to‐liquid transitions of matter. In contrast, athermal photochemical processes can induce reversible solid‐to‐liquid transitions of some newly developed azobenzene compounds. Azobenzene is photoswitchable. UV light induces trans‐to‐cis isomerization; visible light or heat induces cis‐to‐trans isomerization. Trans and cis isomers usually have different melting points (Tm) or glass transition temperatures (Tg). If Tm or Tg of an azobenzene compound in trans and cis forms are above and below room temperature, respectively, light may induce reversible solid‐to‐liquid transitions. In this Review, we introduce azobenzene compounds that exhibit photoinduced reversible solid‐to‐liquid transitions, discuss the mechanisms and design principles, and show their potential applications in healable coatings, adhesives, transfer printing, lithography, actuators, fuels, and gas separation. Finally, we discuss remaining challenges in this field.

Photoswitchable Adhesives Using Azobenzene‐Containing Materials

“…First, how to shorten solid‐to‐liquid transitions time and improve adhesion strengths of azobenzene‐containing materials? For small azobenzene molecules, special molecular geometries, alkyl chains or oligo(ethylene glycol) chains, multiple azobenzene‐type chromophores but not amino azobenzene chromophores or pseudo‐stilbene‐type azobenzene chromophores, and substituents that break molecular symmetry may be useful which were summarized in our early publication . For azopolymers, we should take into account the lengths of spacers and alkyls, molecular weights, molecular weight distributions, and topological architectures.…”

“…And then, the adhesion strengthso ft hem increasedb yv isible light irradiation. In addition, the adhesion strengthso fs olid azobenzene compounds with 6-10 methylene spacerw ere stronger than azobenzene compounds with the shorter (1-5) and longer (11)(12)(13)(14)(15)(16)(17)(18) ones.…”

“…Some azobenzene-containing small molecules with photoinduced phase transitions can be used as adhesives. [14] In 2012, Akiyama and Yoshida studied the photoreversible solid-toliquid transitions of sugar alcohols 1-5,w hich had different chemicals tructures and different numberso fa zobenzene arms. Sugar alcohols with mono-azobenzenea rm (1)a nd diazobenzene arms (2)c ould not change the phase state by light ( Figure 1a), [13a, 15] while the isothermal solid-to-liquid transitionso f3-5 were reversibly observed under room temperature.…”

Configurational Selection in Azobenzene‐Based Supramolecular Systems Through Dual‐Stimuli Processes

“…Noticeable applications range from classical supramolecular devices (recognition, self‐assembly, transport, sensing, catalysis, etc .) to photoresponsive architectures, passing through responsive materials and polymer science, energy‐related technologies, molecular machines, and biology . The preference for azobenzene with respect to other molecular switches relays on its easy preparation and functionalization, on the neat photophysical behavior that normally allows fast E / Z isomerization and selective irradiation of the two isomers, and on the large geometrical rearrangement that accompanies the isomerization process .…”

Photoresponsive polymers with multi-azobenzene groups

“…1 Light irradiation can induce reversible trans-to-cis isomerization. 2,3 This property enables the use of light to control a variety of properties of azobenzene-containing materials, such as their polarity, 4,5 solubility, 5,6 melting points, 7-9 glass transition temperature (T g ) values, [10][11][12] viscosity, [13][14][15] wrinkles, [16][17][18] morphology, [19][20][21][22][23] mechanical properties, 24,25 etc. In the past, the photophysical and photochemical properties of azobenzene derivatives have been extensively studied.…”