Phenazine-based photosensitizers for singlet oxygen generation

Photosensitizers with the ability to produce singlet oxygen (O-1(2)) under light irradiation have attracted much attention because of their fascinating applications as represented by photodynamic therapy. A family of heteroanthracenes have been extensively studied as photosensitizers because of their high quantum yields of O-1(2) generation as well as tetrapyrroles. However, the previous research studies lack insight into phenazine-based photosensitizers despite their considerable potential. Here we develop new phenazine derivatives and investigate their photophysical properties and photosensitizing abilities to produce O-1(2). The photoabsorption of phenazine is enhanced in the long wavelength region by the introduction of two hydroxyl groups and conjugation with two thiophene rings, and further by modification with a nitro group, which extends the photoabsorption to the near-infrared region but interferes with the photosensitized O-1(2) generation due to the fast nonradiative decay of excited states. A superior photosensitizing ability to produce O-1(2) is imparted to the new phenazines by modification with formyl groups, according to El-Sayed's rule. The phenazine-based photosensitizers with improved photoabsorption and O-1(2) quantum yields promise future practical applications, and the obtained findings will contribute to the establishment of synthetic guidelines for new photosensitizers with the excellent ability to produce O-1(2).