Polymorphism and phase transformation tuned luminescence and mechanistic insights in nonconventional luminophores

Abstract

Nonconventional luminophores (NCLs) are attracting significant attention for their unique properties and applications. However, the lack of a comprehensive mechanistic understanding impedes their further development. Particularly, a recurring assumption that impurities are responsible for the luminescence has hindered progress. To elucidate the emission mechanism, we report tunable intrinsic emission from highly purified gemini aliphatic quaternary ammonium salts (GAQASs), leveraging their polymorphism. We demonstrate that polymorphism-dependent luminescence arises from distinct molecular packings and consequent varied clustering states. Specifically, denser ion clustering enhances charge transfer and recombination, heavy atom effects and conformational rigidity, thereby accelerating radiative triplet decay and intersystem crossing, while suppressing nonradiative triplet decay, ultimately leading to enhanced phosphorescence. Furthermore, GAQAS crystals undergo irreversible phase transformations upon heating, which partially disrupt intermolecular interactions, thus allowing for tunable emission. This polymorphism and phase transformation regulated luminescence in GAQASs strongly suggests that intrinsic factors, rather than impurities, are responsible for the observed emission, and are consistent with the clustering-triggered emission mechanism. Our findings establish a direct link between molecular packing, electronic structure and luminescent properties in NCLs. This study advances the mechanistic understanding of NCL luminescence, demonstrating an effective strategy for tunable emission via polymorphism and phase transformation.