Photoinduced Crystal Melting with Luminescence Evolution Based on Conformational Isomerisation

The phenomenon of crystal melting by light irradiation, known as photoinduced crystal-melt transition (PCMT), can dramatically change material properties with high spatiotemporal resolution. However, the diversity of compounds exhibiting PCMT is severely limited, which hampers further functionalisation of PCMT-active materials and the fundamental understandings of PCMT. Here, we report on heteroaromatic 1,2-diketones as the new class of PCMT-active compounds, whose PCMT is based on conformational isomerisation. In particular, one of the diketones demonstrates luminescence evolution prior to crystal melting. Thus, the diketone crystal exhibits dynamic multistep changes in the luminescence colour and intensity during continuous ultraviolet irradiation. This luminescence evolution can be ascribed to the sequential PCMT processes of crystal loosening and conformational isomerisation before macroscopic melting. Single-crystal X-ray structural analysis, thermal analysis, and theoretical calculations of two PCMT-active and one inactive diketones revealed weaker intermolecular interactions for the PCMT-active crystals. In particular, we observed a characteristic packing motif for the PCMT-active crystals, consisting of an ordered layer of diketone core and a disordered layer of triisopropylsilyl moieties. Our results demonstrate the integration of photofunction with PCMT, provide fundamental insights into the melting process of molecular crystals, and will diversify the molecular design of PCMT-active materials beyond classical photochromic scaffolds such as azobenzenes.