Photoinduced Crystal Melting with Luminescence Evolution Based on Conformational Isomerisation

Abstract

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.

Synergetic Conformational Regulations in Ground and Excited States for Realizing Stimulus-Responsive and Wide-Tuning Room-Temperature Phosphorescence

“…Meanwhile, the twisted conformations are diverse and lead to a mixture of similar conformers. These factors inhibit the nucleation and promote the liquid–liquid transition, resulting in the formation of the supercooled liquid of DBBZ . Under external stimuli, the supercooled liquid can reversibly transform into different solid states, including the crystal state, intermediate state (mixture of crystalline and amorphous), and glass state (at low temperature).…”

“…At present, the current reported RTP materials with liquid-state RTP emissions are quite limited, while most of them show no RTP transition in the solid–liquid phase transition (Scheme S1B). Recently, Tani and coworkers reported a photoinduced melting molecule with the RTP transition (green to yellow) . However, the original green emission shows instability to UV irradiation due to the excited-state conformational change in the crystal.…”

Fast, Efficient, Narrowband Room-Temperature Phosphorescence from Metal-Free 1,2-Diketones: Rational Design and Mechanism

“…Using heteroaromatic 1,2-diketones, we previously developed a series of solid-state mechanoresponsive RTP materials, 15 solventfree liquid RTP materials, 16 and photoresponsive RTP crystals. 17 They exhibited RTP under non-rigid molecular environments, such as amorphous solids or solvent-free liquids. Considering that typical metal-free organic compounds rarely show RTP under these conditions, our results were encouraging enough to expect large kps for those diketones; however, their inherent molecular RTP properties have not been fully investigated.…”