Increasing the anti-Stokes shift in TTA upconversion with photosensitizers showing red-shifted spin-allowed charge transfer absorption but a non-compromised triplet state energy level.

Excitation into the spin-allowed S0 → 1CT absorption band of a spin-orbit charge transfer intersystem crossing (SOCT-ISC) triplet photosensitizer (PS) was used as a new strategy to increase the anti-Stokes shift in triplet-triplet annihilation (TTA) upconversion, with the advantages of red-shifted absorption but the non-compromised triplet state energy level of the triplet PS.