Absorption and fluorescence spectral data and triplet energy content, as well as quantum yields of fluorescence, of triplet state formation, and of sensitized generation of singlet molecular oxygen, O₂(¹∆g), of 38 porphycene derivatives are reported. Long-wavelength absorption and the efficiency of fluorescence and O₂(¹∆g) sensitization qualify most of these porphycenes as potential photodynamic therapy agents (so-called second-generation sensitizers), unless steric or electronic effects by substitution perturb the parent aromatic 18 π system.