Enolate Enables Unexpected Red Luminescence from Through-Bond/Through-Space Complexation between Imide and Organic Base

Maleimides have been commonly employed for color reactions in the last few decades, as they produce a bright color when treated with organic or inorganic base. For color formation or the luminescence mechanism, their chromophores have remained unexplored. In this study, we disclosed this well-known phenomenon with commercially available maleimide derivatives based on complex formation between maleimide/succinimide and the organic base of 1,8-diazabicyclo[5.4.0]-undec-7-ene (DBU). We provided direct experimental evidence of the polymerization process when maleimide is mixed with DBU. The maleimide/DBU complex emits prominent yellow-red fluorescence at long wavelengths due to its through-bond enolate complex formation between the maleimide-based polymer and DBU. DBU can also form an enolate complex with succinimide via a through-space manner, resulting in fluorescence emission similar to that of maleimide does. Their structures were rationally characterized and explored. Additionally, external factors including solvents and the aromatic ring effect can be applied to further stabilize the complex formation, which provides insights for designing novel fluorescent materials.