Light-melting Diketone Crystal with ON-OFF-ON Luminescence Evolution

Crystal melting by light irradiation, that is, the phenomenon of photoinduced crystal-melt transition (PCMT), can dramatically change material properties with high spatiotemporal resolution. However, both the process and mechanism of PCMT remain unknown owing to a lack of suitable probes and the poor diversity of compounds exhibiting PCMT. Here, we report the first instance of PCMT with luminescence evolution. We show that a heteroaromatic 1,2-diketone crystal exhibits ON-OFF-ON luminescence changes under continuous ultraviolet irradiation, which can be ascribed to the sequential PCMT processes of crystal loosening and conformational isomerization before macroscopic melting. Single-crystal X-ray structural analysis, thermal analysis, and theoretical calculations of three diketones revealed that weak intermolecular interactions and the presence of a disordered layer in the crystal are essential for PCMT. Our results provide fundamental insights into the melting process of molecular crystals and should aid the molecular design of PCMT-active materials that are based on emissive compounds, as opposed to the classical molecular designing based on nonemissive photochromic scaffolds such as azobenzenes.