Triazine- and heptazine-based polymeric systems hold great promise for applications in a range of fields, namely optoelectronics, photocatalytic water splitting, gas adsorption, and sensors, and, most importantly, as multifunctional heterogeneous metal-free catalysts and catalyst support materials for several organic transformations.^1–4  Despite being described as the oldest materials in the chemical literature, they have attracted much attention after pioneering work done by Schnick and co-workers and Antonietti and co-workers. Carbon nitride (g-CN, commonly termed as g-C₃N₄)¹ is one of the familiar materials that belongs to such a class (triazine- and heptazine-based polymers).⁵  The initial investigations on carbon nitrides have been known in the literature since the 1830s. The material is a linear polymer of interconnected tri-s-triazines through a secondary nitrogen with the general formula (C₃N₃H)_n, first reported by Berzelius; the name “melon” was coined by Liebig.^6,7  Later on, seven different phases of carbon nitrides, namely α-C₃N₄, β-C₃N₄, cubic C₃N₄, pseudocubic C₃N₄, g-h-triazine, g-o-triazine and g-h-heptazine, were identified.⁸  All these allotropes were constructed with triazine (C₃N₃) and tri-s-triazine/heptazine (C₆N₇) rings as basic tectonic building blocks. However, among all the allotropes, tri-s-triazine-based graphitic carbon nitride (g-C₃N₄) was reported as a highly stable phase, which was due to its poly(tri-s-triazine) structure and the high degree of condensation (Figure 3.1).⁹  This was also supported by many theoretical studies.^10,11  g-C₃N₄ material is structurally similar to that of graphite bearing strong covalent C–N bonds in conjugation connected through tertiary nitrogen atoms, while the layers are held by van der Waals forces. The defect-rich, N-bridged polymer with a medium-band gap, an in-plane quantum confinement effect and indirect semiconductor features predominantly favour its usage in many applications, in particular, as a multifunctional heterogeneous metal-free catalyst.²  Apart from the usage of g-C₃N₄ materials in hydrogen evolution and photocatalytic dye degradation, these materials find interesting applications in Friedel–Crafts type reactions,¹²  O₂ activation for the selective oxidation of alcohols,¹³  oxidative coupling of amines¹⁴  and CO₂ activation/reduction^15–17  reactions.