Polymeric　carbon　nitride　(PCN),　in　either　triazine　or　heptazine　form,　has　been　regarded　as　a　promising　metal-free,　environmentally　benign,　and　sustainable　photocatalyst　for　solar　hydrogen　production.　However,　PCN　in　most　cases　only　exhibits　moderate　activity　owing　to　its　inherent　properties,　such　as　rapid　charge　carrier　recombination.　Herein　we　present　a　triazine–heptazine　copolymer　synthesized　by　simple　post-calcination　of　PCN　in　eutectic　salts,　that　is,　NaCl/KCl,　to　modulate　the　polymerization　process　and　optimize　the　structure.　The　construction　of　an　internal　triazine–heptazine　donor–acceptor　(D-A)　heterostructure　was　affirmed　to　significantly　accelerate　interface　charge　transfer　(CT)　and　thus　boost　the　photocatalytic　activity　(AQY=60　%　at　420　nm).　This　study　highlights　the　construction　of　intermolecular　D-A　copolymers　in　NaCl/KCl　molten　salts　with　higher　melting　points　but　in　the　absence　of　lithium　to　modulate　the　chemical　structure　and　properties　of　PCN.　©　2018　Wiley-VCH　Verlag　GmbH　&　Co.　KGaA,　Weinheim