In solution, BODIPY dyes as simple as ps1 are poor triplet sensitizers ; therefore, the finding that ps1 acts as an efficient photosensitizer in DESC is surprising. To obtain mechanistic insights into DESC, we performed transient absorption spectroscopic (TAS) and computational studies (Fig. ). First, we studied the photoinduced dynamics of the (ps1)2⊂C homodimer using femtosecond (fs) TAS. Figure shows the fs absorption decays at two different wavelengths, following excitation of (ps1)2⊂C with a 500 nm laser. The increasing transient absorption at 412 nm at delay times >100 ps, accompanied by the increasing bleach at 483 nm, can be attributed to ISC from the S1 to the T1 state. Using microsecond (μs) TAS, we found the resulting triplet state to be remarkably stable, with a monoexponential lifetime of 16.5±0.5 μs under ambient conditions (Supplementary Fig. ). As expected from a triplet state, the lifetime is strongly dependent on the amount of oxygen in the solvent; decreasing the amount of O2 by bubbling N2 for 4 min and 10 min extended the lifetime of the T1 state of ps1 to 160±4 μs and 10.1±0.1 ms, respectively (Supplementary Fig. ).