Intersystem crossing (ISC) of triplet photosensitizers is a vital process for fundamental photochemistry and photodynamic therapy (PDT). Herein, we report the co-existence of efficient ISC and long triplet excited lifetime in a heavy atomfree bodipy helicene molecule. Via theoretical computation and time-resolved EPR spectroscopy, we confirmed that the ISC of the bodipy results from its twisted molecular structure and reduced symmetry. The twisted bodipy shows intense long wavelength absorption (epsilon = 1.76 x 10(5)M(-1) cm(-1) at 630 nm), satisfactory triplet quantum yield (Phi(T)= 52 %), and long-lived triplet state (T-T = 492 mu s), leading to unprecedented performance as a triplet photosensitizer for PDT. Moreover, nanoparticles constructed with such helical bodipy show efficient PDT-mediated antitumor immunity amplification with an ultra-low dose (0.25 mu g kg(-1)), which is several hundred times lower than that of the existing PDT reagents.