Covalent bonded carbon nitride (CN) has stimulated extensive attentions from photosynthesis to optoelectronics. However, the acquisition of correct numbers beyond stoichiometry and composition remains indefinable. Moreover, the electronic coupling by the substrates to the photoexcitation processes in value-added oxidation reactions is essential, but still poorly understood. Herein, we report by far the minimum active structure of CN by constructing fragments consisting of melem (M1) and its incomplete condensed form with cyanide termination (M2). Surprisingly, such configuration endowed a boosted activity of 11 times of traditional bulk CN in photocatalytic oxidation of tetracycline as a showcase application of water cleaning and sanitation. The mechanism studies disclosed that M1 and M2 were primarily responsible for light absorption and charge separation, respectively; meanwhile the electronic coupling by the O2 substrate participated the photoexcited processes thus synergistically enhanced the photocatalytic reactions.