Alkali metal N-heteroarenes as a new family of luminescent materials.

Organic-based luminescent materials have broad applications across illumination, display, and imaging. However, intramolecular vibrations and intermolecular interactions, such as π-π stacking, can induce non-radiative transition and lead to low efficiency and fluorescence quenching for solid-state materials, especially for small conjugated molecules. In the present study, a facile yet versatile conceptual approach to fabricate a new type of luminescent materials, i.e., alkali metal N-heteroarene salts, was developed via a metal substitution strategy. Over 30 solid-state ionic compounds have been designed and synthesized accordingly. These compounds possess rigid crystal structures through forming an ionic bonding network of metal‒N bonds and cation-π interactions, which can minimize the vibrations of individual molecules, thereby suppressing non-radiative transition. Consequently, some small conjugated molecules with negligible fluorescence, such as imidazole, indole, and azaindole, become strong emitters upon alkali metal substitution. The alkali metal substitution forms an N-heterocyclic anion owing to the Lewis acidity of the alkali metal cation, thus increasing the electron density on the conjugated system. Concurrently, the formation of N-heterocyclic anions tailors band gaps of these organic salts, enabling diverse fluorescent emissions across the visible spectrum. The applications of these organic salts in information anti-counterfeiting are also demonstrated successfully.