Selective　photoreduction　of　CO2　into　a　given　product　is　a　great　challenge　but　desirable.　Inspired　by　natural　photosynthesis　occurring　in　hierarchical　networks　over　non-precious　molecular　metal　catalysts,　we　demonstrate　an　integration　of　single　Ni　sites　into　the　hexagonal　pores　of　polyimide　covalent　organic　frameworks　(PI-COFs)　for　selective　photoreduction　of　CO2　to　CO.　The　single　Ni　sites　in　the　hexagonal　pores　of　the　COFs　serve　as　active　sites　for　CO2　activation　and　conversion,　while　the　PI-COFs　not　only　act　as　a　photosensitizer　to　generate　charge　carriers　but　also　exert　a　promoting　effect　on　the　selectivity.　The　optimized　PI-COF　with　a　triazine　ring　exhibits　excellent　activity　and　selectivity.　A　possible　intra-　and　inter-molecular　charge-transfer　mechanism　was　proposed,　in　which　the　photogenerated　electrons　in　PI-COFs　are　efficiently　separated　from　the　central　ring　to　the　diimide　linkage,　and　then　transferred　to　the　single　Ni　active　sites,　as　evidenced　by　theoretical　calculations.　©　The　Royal　Society　of　Chemistry.