Organic　conjugated　polymers　(CPs)　are　promising　photocatalysts　for　solar-to-hydrogen　conversion,　yet　their　efficiency　is　often　limited　by　large　exciton　binding　energies　and　poor　charge　separation.　In　this　study,　we　report　a　molecular　isomerization　strategy　to　control　polymer　planarity　and　thereby　enhance　exciton　dissociation　and　photocatalytic　activity.　Two　isomeric　CPs,　Pyrene-Thiophene-Benzothiazole　(Py-T-BT)　and　Pyrene-Thiophene-iso-Benzothiazole　(Py-T-isoBT),　were　designed　and　synthesized,　employing　pyrene　as　the　donor,　thiophene　as　the　pi-bridge,　and　either　benzothiadiazole　(BT)　or　its　structural　isomer　(isoBT)　as　the　acceptor.　The　introduction　of　the　isoBT　unit　induced　steric　hindrance,　resulting　in　Py-T-isoBT　exhibiting　significantly　reduced　molecular　planarity　(dihedral　angles　of　30.3　degrees　and　9.2　degrees)　compared　to　the　highly　planar　Py-T-BT　(dihedral　angle　of　4.2　degrees).　Theoretical　calculations　and　experimental　characterization　confirmed　that　the　more　planar　Py-T-BT　possessed　a　narrower　band　gap,　a　larger　transition　dipole　moment,　and　consequently　a　significantly　lower　exciton　binding　energy.　Time-resolved　spectroscopy　revealed　that　Py-T-BT　enabled　ultrafast　formation　of　charge　transfer　excitons　(tau　CT　=　0.60　ps),　in　contrast　to　Py-T-isoBT　(tau　CT　=　3.95　ps).　As　a　result,　Py-T-BT　demonstrated　a　photocatalytic　hydrogen　evolution　rate　of　7.95　mmol　g-1　h-1　with　a　3　wt%　Pt　cocatalyst　under　visible　light　irradiation　(lambda　＞=　420　nm),　approximately　20　times　higher　than　that　of　Py-T-isoBT.　This　work　highlights　the　pivotal　role　of　molecular　planarity　in　modulating　exciton　dynamics　and　presents　a　generalizable　strategy　for　designing　high-efficiency　polymer　photocatalysts　through　isomeric　engineering.