Exploring n-π* Electronic Transitions in Graphitic Carbon Nitride: Fundamentals, Strategies, and Photocatalytic Advances.

Photocatalysis utilizes solar energy to drive redox reactions, offering a promising solution to global energy and environmental challenges. Up to now, graphitic carbon nitride (g-CN) has been widely studied due to its unique organic structural framework and favourable electronic properties. However, its photocatalytic efficiency is often constrained by a limited light absorption range (up to ~460 nm) and modest charge-carrier mobility. Recent findings suggest that n-π* electronic transitions can extend the absorption edge of g-CN to approximately 600 nm, enhancing the activity. Herein, we first outline the fundamental structural and electronic features of g-CN, highlighting the role of both n-π* electronic transitions. We then provide a comprehensive overview of various strategies for activating n-π* transitions, emphasizing the necessity of distorting the planar and symmetric tri-s-triazine units to enable this transition. The subsequent sections demonstrate how modulating n-π* transitions can broaden the light absorption spectrum and facilitate more efficient charge separation, thereby enhancing photocatalytic processes. Finally, we propose potential future directions for exploiting n-π* transitions to further advance g-CN-based photocatalysts. This review aims to fill a gap in the literature by providing a focused summary of n-π* transitions in g-CN and their significance for sustainable energy and environmental applications.