Importance of the orthogonal structure between porphyrin and aniline moieties on the pH-activatable porphyrin derivative for photodynamic therapy

The photo-induced side effects of photodynamic therapy of cancer can be suppressed using pH-activatable porphyrin derivatives, which have been developed using an aniline moiety as a pH-responsive quencher. The acid dissociation coefficient of a pH-activatable photosensitizer increased by replacing a dimethylamino with a diethylamino group. Based on this diethylamino derivative, we studied the importance of the intramolecular orthogonal structure between porphyrin and aniline moieties on the pH-activatable properties. The character of the S-1 state of the non-orthogonal derivative (Por-NEt2) was different from that of the orthogonal derivative (Por-Me-NEt2), but the derivatives possessed the ON/OFF switching functions for both fluorescence and singlet oxygen sensitization. However, the maximum quantum yields of Por-Me-NEt2 were much larger than those of Por-NEt2, and the ON/OFF switching ratio of Por-Me-NEt2 was higher than that of Por-NEt2. This is due to the suppressed rotation of the aniline moiety, indicating that the intramolecular orthogonal structure is important for the high ON/OFF switching performances.