4.1 Structural modification red-shift the spectral absorption band of porphyrin photosensitizers
In the past few years, most studies have focused on enhancing the photophysical properties of PS through different structural modifications. Such as binding with other molecules, metallization and nanotechnology applications(Lin et al. 2020). Hilmey and co-workers synthesized a series of dithioporphyrin-based photosensitizers and evaluated a series of photodynamic properties(Hilmey et al. 2002). The results showed that the different combinations of heteroatoms in the center of the porphyrin ring resulted in the I-band absorption peaks of these compounds with longer wavelengths than those of Photofrin. And the new coordination porphyrin compounds synthesized in this study efficiently generate 1O2 under the irradiation of I-band. The red-shift of the I-band absorption peak can increase the effective penetration depth of light, which is of great significance for the clinical application of porphyrin-based photosensitizers (Fig. 6). Cheng and co-workers made a composite photosensitizer(Cheng et al. 2019) by simply mixing DNA G-quadruplex with hydrophilic porphyrin (TMPipEOPP)4+•4I. This new photosensitizer showed a new absorption band near 700 nm. More interestingly, the absorption intensity of the new photosensitizer in the Q-band is much higher than that of the free TMPipEOPP. For example, the molar absorption coefficient at 700 nm of the complex formed by TMPipEOPP with G-tetramer AS1411 is about 47,000 L·mol-1·cm-1, which is 7.4 times higher than the molar absorption coefficient of free TMPipEOPP at 650 nm. Compared with the conventional porphyrin photosensitizer, the excitation wavelength of the composite photosensitizer is red-shifted by ~50 nm (from 650 nm to 700 nm), which is favorable for light penetration. In addition, the light absorption efficiency of the composite photosensitizer was increased by ~7.4 times, which greatly improved the 1O2generation capacity and PDT effect.