Crystal Melting by Light: X-ray Crystal Structure Analysis of an Azo Crystal Showing Photoinduced Crystal-Melt Transition

Abstract

Trans-cis photoisomerization in an azo compound containing azobenzene chromophores and long alkyl chains leads to a photoinduced crystal-melt transition (PCMT). X-ray structure analysis of this crystal clarifies the characteristic coexistence of the structurally ordered chromophores through their π···π interactions and disordered alkyl chains around room temperature. These structural features reveal that the PCMT starts near the surface of the crystal and propagates into the depth, sacrificing the π···π interactions. A temporal change of the powder X-ray diffraction pattern under light irradiation and a two-component phase diagram allow qualitative analysis of the PCMT and the following reconstructive crystallization of the cis isomer as a function of product accumulation. This is the first structural characterization of a compound showing the PCMT, overcoming the low periodicity that makes X-ray crystal structure analysis difficult.

Photoinduced Phase Transitions in Rod-shaped Azobenzene with Different Alkyl Chain Length

“…Recently, photoinduced solid-liquid phase transitions by photochromic reactions has been of great interest since our first report of the photoinduced phase transitions between solid (crystal) and liquid phase at room temperature in macrocyclic azobenzene [10][11][12][13]. Similar phenomena have been reported by several groups in azobenzene derivatives [14][15][16][17] and a spiropyran [18].…”

“…Most portion of the time profile exhibits linear component. Since the photoinduced solid-liquid phase transition is considered to take place from the surface of the crystal [12], and the retardation value is proportional to the film thickness, we plotted the slope of the time profile. Figure 5 solid-liquid phase transition than that with shorter (Cn = 1-5) and longer ones.…”

“…It required more than 3 minutes of irradiation to liquefy AZ-OC5, while only 4 seconds liquefied AZ-OC6 completely. Crystal packing and dynamics of molecules are important factors for the photoinduced solid-liquid phase transition [12]. Metastable crystals may play an important role for the phase transition.…”

Photon energy storage materials with high energy densities based on diacetylene–azobenzene derivatives

“…Even without the templates, we expect that the intermolecular interactions such as H-bonding, van der Waals force, and p-p interaction can signicantly inuence the photon energy storage density. The photoswitching vs. packing density and alignment of azobenzene materials has been explored for some macrocyclic [21][22][23] and linear 24 azobenzene derivatives, particularly for liquid crystalline materials, for applications in photolithography and actuation. The collective change of the intermolecular interactions in such materials can be observed as a macroscopic solid-liquid phase transition.…”

Synthesis of star‐shaped poly(n‐butyl acrylate) oligomers with coumarin end groups and their networks for a UV‐tunable viscoelastic material

“…Some groups have reported isothermal melting and crystallization of azobenzene (Azo)-containing small molecules, relying on trans-cis photoisomerization. [1][2][3] This strategy is preferable for photoreversible adhesive applications, which enables ON-OFF attachment and detachment of surfaces coated by them without using solvent fractions. 4 Although the IRLNC strategy should, in principle, be applicable for tuning viscoelasticity of materials, only two physical states, i.e., solid and liquid ones are allowed for such small molecules.…”