Two-dimensional　graphitic　carbon　nitride　(2D　g-C3N4)　nanostructures　have　been　the　focus　of　substantial　research　interest　recently　owing　to　their　promising　photoactive　properties　for　use　in　solar-to-fuel　conversion.　However,　the　synthesis　of　2D　g-C3N4　nanomaterials　remains　a　significant　challenge.　Here　we　successfully　synthesized　2D　g-C3N4　nanosheets　via　a　supramolecular　chemistry　approach.　2D　g-C3N4　nanostructures　not　only　enhance　the　visible　light-harvesting　property　but　also　provide　more　catalytic　sites　for　improved　hydrogen　evolution　reaction　(HER).　More　importantly,　we　describe　experimental　findings　concerning　the　dynamics　of　charge　and　energy　transfer　by　using　ultrafast　spectroscopy　in　combination　with　in-situ　electron　spin　resonance　(ESR)　characterization.　The　mechanistic　investigation　reveals　that　the　introduction　of　a　benzene-substituted　melamine　motif　can　remarkably　accelerate　the　electron-hole　separation　and　suppress　the　recombination　of　photogenerated　charge　carriers.　The　carrier　separation　dynamics　is　expected　to　be　tailored　by　engineering　the　surface　and　morphology　of　g-C3N4,　which　favors　enhanced　solar　photocatalytic　HER　efficiency.