Abstract：The photosensitizer was applied photodynamic therapy (PDT), a molecule that can absorb light with a certain wavelength. And the photosensitizer can transfer excited energy to ground state oxygen. As a result, the ground state oxygen gets the energy and becomes the singlet oxygen. Currently, those applied photosensitizers are almost planar molecules containing porphyrin ring, and those planar molecules have a big delocalized π bonds. Simultaneously, there is a slight inter-system crossing and a long triplet lifetime after the planar molecules are excited by light. So those planar molecules have a high yield of singlet oxygen. However, the absorption bands of the applied photosensitizers always lie in the UV regions, which easily damage the body issue. Due to the photo-damage character is not beneficial for the therapy, so the study on photosensitizers with Vis-IR absorption bands were widely concerned. Based on the above reason, we investigate three photosensitizers (earing-porphyrin (a), trisulfo-phthalocyanine (b) and trisulfo-phthalocyanine Ni(Ⅱ) (c)) using the DFT and TD-DFT. The optimized results show that all atoms of (a) are in a plane, the radius of (a) is almost 7 Å, and the cavity radius is 5 Å. All atoms of (b) are also in a plane, the radius of trisulfo-phthalocyanine is 8 Å, and the radius of the cavity is 4 Å. But (c) is a distorted plane due to the coordinated mode of Ni(Ⅱ). Therefore, the earing-porphyrin (a) in a big cave can capture more ground-state oxygen. The orbital energies and populations show that the HOMO energy of (a) is the biggest among them. That is, the electrons of the earing-porphyrin (a) were excited to higher energy levels easily. The energy gaps (Ehomo-lumo) of three molecules are 0.072, 0.076 and 0.075 a.u. The earing-porphyrin (a) has the lowest energy gap. The orbital populations show that the atomic p orbitals constitute the big delocalized bond, and the d orbitals of Ni also join in the delocalized bond in the molecule (c). At last, the absorption spectra of three molecules were simulated by TD-DFT/B3LYP/6-311G(d, p). For three planar molecules, there are the Soret band and Q band in their spectra. The Q band lies at 450~900 nm for molecules (a) and (c) and about 400~800 nm for molecule (b). In conclusion, this paper has calculated and discussed the structural optimization, the orbital energies, and the absorption bands of three planar molecules. The investigated results will improve the discovery and development of photosensitizers with near-infrared absorption bands, and it also will pro-vide the theoretical basis for the study of the photosensitizer.
Key words：Photosensitizer; Porphyrins; Density functional; Absorption spectra
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