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.