Poly(heptazine　imide)　(PHI),　a　semicrystalline　version　of　carbon　nitride　photocatalyst　based　on　heptazine　units,　has　gained　significant　attention　for　solar　H2　production　benefiting　from　its　advantages　including　molecular　synthetic　versatility,　excellent　physicochemical　stability　and　suitable　energy　band　structure　to　capture　visible　photons.　Typically,　PHI　is　obtained　in　salt-melt　synthesis　in　the　presence　of　alkali　metal　chlorides.　Herein,　we　examined　the　role　of　binary　alkali　metal　bromides　(LiBr/NaBr)　with　diverse　compositions　and　melting　points　to　rationally　modulate　the　polymerization　process,　structure,　and　properties　of　PHI.　Solid　characterizations　revealed　that　semicrystalline　PHI　with　a　condensed　π-conjugated　system　and　rapid　charge　separation　rates　were　obtained　in　the　presence　of　LiBr/NaBr.　Accordingly,　the　apparent　quantum　yield　of　hydrogen　using　the　optimized　PHI　reaches　up　to　62.3%　at　420　nm.　The　density　functional　theory　calculation　shows　that　the　dehydrogenation　of　the　ethylene　glycol　has　a　lower　energy　barrier　than　the　dehydrogenation　of　the　other　alcohols　from　the　thermodynamic　point　of　view.　This　study　holds　great　promise　for　rational　modulation　of　the　structure　and　properties　of　conjugated　polymeric　materials.　©　2024　The　Authors.　Interdisciplinary　Materials　published　by　Wuhan　University　of　Technology　and　John　Wiley　&　Sons　Australia,　Ltd.