Adjusting　the　electronic　structure　of　graphitic　carbon　nitride　(g-C3N4)　photocatalyst　through　pi-pi　conjugation　is　an　effective　method　to　achieve　efficient　photogenerated　carrier　separation.　One　key　challenge　of　pi-pi　conjugation　control　is　to　tune　the　degree　of　such　conjugation　without　destroying　the　g-C3N4　structure.　Herein　we　report　a　conceptual　design　that　achieves　a　coplanar　heterojunction　by　enhancing　the　pi-pi　conjugation　via　the　doping　of　crystalline　g-C3N4　using　a　conjugated　double　bond　ring　molecule,　1,3,5-benzenetriol,　during　calcination　process.　The　selection　of　the　dopant　enables　the　facile　creation　of　a　unique　coplanar　heterojunction　which　not　only　retains　the　pristine　network　structure　of　g-C3N4,　but　remarkably　promotes　separation　and　transfer　of　photogenerated　carriers　through　the　enhanced　pi-conjugated　endogenous　electric　field.　As　a　result,　the　new　g-C3N4　photocatalyst　efficiently　photocatalytically　produces　hydrogen　from　water　under　visible　light　irradiation　with　a　high　H-2　production　rate　up　to　94.94　mu　mol/h,　and　a　notable　external　quantum　efficiency　of　16.4%　at　420　nm.　(C)　2021　Published　by　Elsevier　B.V.　on　behalf　of　Chinese　Chemical　Society　and　Institute　of　Materia　Medica,　Chinese　Academy　of　Medical　Sciences.