Enabling C–C bond-forming strategies for tailored derivatisation of heptazine frameworks

Abstract

Interest in melon-related graphitic carbon nitride (g-CN) materials has recently increased significantly due to their appealing properties as metal-free photocatalysts. Heptazine derivatives are now used in a variety of applications, including photocatalysis, organic light-emitting diodes (OLEDs), sensors, gas separation and lithium-ion batteries. However, the chemistry and functionalisation of these materials have been limited due to the difficulty of controlling the reactivity of the heptazine core. This study presents a comprehensive analysis of previously inaccessible C–C coupling reactions, including the Suzuki, Sonogashira, Stille, Kumada and Negishi reactions, which have been successfully applied to dialkylamine-substituted heptazines. These advances establish a robust platform for the derivatisation of a broader range of heptazines. Particular emphasis is placed on the Stille reaction, demonstrating the influence of aryl electronic effects on yields. The electron-withdrawing nature of the heptazine core was further confirmed by evaluating subsequent post-functionalisation (nitration, bromination, and a second Stille coupling) on selected substituted heptazines. Electrochemical (CV) and optical (UV-Vis absorption/emission) studies demonstrate a clear dependence of the electronic properties on the substituent. Overall, these findings deepen our understanding of the relationships between the structure and properties of heptazine materials, providing guidance for their targeted molecular design.