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

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.