The design of the chemical, photo-physical, and catalytic properties of graphitic carbon nitride (g-CN) is highly challenging due to the harsh solid-state reaction. Here we report a template-free, large-scale synthesis of highly photoactive and stable g-CN with a unique dot-like morphology by using melamine-halogen complexes as starting precursor. The supramolecular structures are synthesized by sequential treatment of melamine with different acids. Experimental results together with theoretical calculations reveal that different halogens result in different interactions with melamine. We found that bromide reacts with the inner melamine aggregates while chloride is present mainly in the outer shell. Upon calcination, the delicate design results in porous carbon nitride with controlled morphologies and optical properties as well as improved charge separation under illumination and excellent photoactivity and stability for hydrogen production. Our results indicate that the type of halogen can significantly affect different properties in the final g-CN. This work provides new opportunities for template-free facile synthesis of highly photoactive carbon nitride and other carbon-nitrogen based materials with controllable chemical, optical, and catalytic properties for sustainable energy-related applications owing to the utilization of hydrogen-halogen based assemblies as well as the elucidation of their unique interaction.