Molecular Junction Photocatalysts in Graphitic Carbon Nitride: Precise Characterization of Built-in Electric Fields and Challenges in Spatial Charge Separation

Graphitic carbon nitride (g-C$_3$N$_4$) has garnered interest as a versatile photocatalytic platform owing to its tailorable electronic architecture; however, its solar-to-chemical conversion is bottlenecked by high exciton binding energies and slow charge-carrier transport. To circumvent these impediments, the construction of ``molecular junctions'' within the conjugated polymeric scaffold enables atomically precise modulation of spatial electron configurations. Unlike conventional heterostructures relying on physical contact, molecular junctions employ robust covalent bridging, facilitating molecular orbital hybridization and $\pi$-conjugation. This review summarizes recent advances in molecular-junction-functionalized g-C$_3$N$_4$ photocatalysts, focusing on built-in electric field (BIEF) induction and its effects on charge-carrier dynamics. Molecular junctions are categorized into homojunctions exploiting structural polymorphisms and heterojunctions incorporating donor–acceptor moieties or single-atom sites. Structural asymmetry engenders steep potential gradients, mitigating exciton binding and promoting unidirectional charge migration. State-of-the-art BIEF characterization techniques—including Kelvin probe force microscopy (KPFM), density functional theory (DFT), and ultrafast transient absorption spectroscopy—are systematically examined. Finally, the catalytic efficacy of these molecular junction paradigms is assessed across solar-driven applications, including photocatalytic hydrogen evolution, overall water splitting, and hydrogen peroxide synthesis. The review concludes by outlining key bottlenecks and future directions, emphasizing atomically precise synthesis and operando characterization.