Abstract：The photosensitizer was applied photodynamic therapy (PDT), which is a kind of molecule that can absorb light with 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 almost are planar molecules containing porphyrin ring, and those planar molecule have big delocalized π bond. As same time, there is the slight inter-system crossing and the long triplet lifetime after the planar molecules were excited by light. So those planar molecules have 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 benefit for the therapy, the study on photosensitizers with Vis-IR absorption bands was widely concerned. Base 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 radius of cavity is 5 Å. All atoms of (b) are also in a plane, the radius of trisulfo-phthalocyanine is 8 Å, and the radius of cavity is 4 Å. But (c) is a distort plane due to the coordinated mode of Ni. Therefore, the earing-porphyrin (a) with big cave can capture more ground-state oxygen. The orbital energy and population 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 level easily. The energy gaps (Ehomo-lumo) of three molecules all are 0.072 a.u., 0.076 a.u., and 0.075 a.u. and the earing-porphyrin (a) has the lowest energy gap. The orbital populations show that the atomic p orbitals constitute the big delocalized π bond, and the d orbitals of Ni also join in the delocalized π bond in 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 Soret band and Q band in their spectra. The Q band lies in 450 - 900 nm for molecule (a) and (c), and about 400 – 800 nm for molecule (b). In conclusion, the structural optimization, the orbital energy and the absorption bands of three planar molecules are calculated and discussed in this paper. The investigated results will improve the discovery and development of photosensitizer with near infrared absorption bands, and it also will provide the theoretical basis for the study of photosensitizer.
