Room-Temperature Phosphorescence of a Supercooled Liquid: Kinetic Stabilization by Desymmetrization

Metal-free single component liquid, developed by desymmetrizing C2-symmetrical diketone, exhibits room-temperature phosphorescence (RTP) and phosphorescent thermochromism in air. The unsymmetrical flexible core with as many as eight conformers kinetically stabilizes the supercooled liquid state. The RTP is unique the liquid that benefits from external heavy atom effect and suppresses oxygen diffusion, while produces an emissive conformer.</p

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

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

We report metal-free organic 1,2-diketones that exhibit fast and highly efficient room-temperature phosphorescence (RTP) with high color purity under various conditions, including solutions. RTP quantum yields reached 38.2% in solution under Ar, 54% in a polymer matrix in air, and 50% in a crystalline solids in air. Moreover, the narrowband RTP consistently dominated the steady-state emission, regardless of the molecular environment. A detailed investigation of the emission mechanism using ultrafast spectroscopy, single-crystal X-ray structure analysis, and theoretical calculations revealed picoseconds intersystem crossing followed by RTP from a planar conformation. Thus, we attribute the high efficiency RTP across diverse molecular environments to an inherent ~5000-s–1 phosphorescence rate constant comparable to that of platinum porphyrin complexes. The planar conformation reflected a design principle for fast and narrowband RTP. This strategy complements the streamlined persistent RTP approach and enables the development of organic phosphors with emissions independent of environmental conditions, thereby offering alternatives to precious-metal based phosphors