The development of pure organic photosensitizers remains challenging due to low intersystem crossing (ISC) efficiency and the instability of triplet excitons. Herein, a series of organic phosphorescent molecules are prepared as the photosensitizers. The extensive conjugation of fused-ring cores enhances visible-light absorption, while heteroatom-rich structures promote ISC process, effectively generating triplet excitons. 2,3,5,6,9,10-Hexabutoxy-8-phenyldithieno-triphenyleno-pyridine (TPy) exhibits high ISC efficiency and can sensitize Fe-catalyst efficiently for photoreduction CO₂-to-CO. A novel strategy of doping phosphorescent molecules into the core of amphiphilic nanoparticles (NPs) formed by polystyrene-block-poly(ethylene glycol) (PS-b-PEG) was further developed. PS, the core of the NPs, can stabilize the triplet excitons generated by TPy due to its rigid polymer backbone, resulting in enhanced photocatalytic efficiency. Meanwhile, PEG serves as the corona of the NPs, improving stability and dispersibility. Notably, this self-assembled strategy increases the sensitization efficiency of TPy (TON_CAT = 2041, 51.02 µmol CO) by nearly 50%, and even after three catalytic cycles, 93.5% of the original sensitization ability is retained. This result opens a new avenue for developing novel molecular organic photosensitizers and stabilizing their excited-states for efficient application in photoredox reactions.