In this work eight porphyrins (p) and eight chlorins (c) are theoretically characterized [BLYP/6-31G(d)] in their singlet and triplet states. Nine of them (1p, 1c, 2p, 3p, 4p, 5p, 6c, 7c, and 8c) have already been synthesized and are in trial use in photodynamic therapy (PDT). The seven remaining were built up as chlorins analogous to porphyrins 2p-5p and porphyrins analogous to chlorins 6c-8c. The aim is to investigate the effect of the chlorin structure on the Q-band of electronic spectra at BLYP/6-31G(d) (gas phase, methanol solution) and at BHANDHLYP/6-31+G(d) (methanol solution), and on the triplet --> singlet energy emission, as these two factors determine the quality of a good photosensitizer. It is found that meso substituents lead to greater geometry distortions than beta-substituents in both porphyrins and chlorins and in both singlet and triplet states. In methanol solution, chlorin-like structures with beta substitution present significantly red-shifted Q-bands in comparison with their porphyrin analogues, so they would be better photosensitizers than porphyrins. Concerning to the triplet --> singlet energy emission calculated in methanol solution, three porphyrins (4p, 6p, and 8p) and all the studied substituted chlorins could be useful to generate active (1)O2. 4c would be the best photosensitizer, as it absorbs the largest wavelength in the therapeutic window (approximately 690 nm) and releases the amount of energy closest to the required one (1.22 eV).