**3.1 General property of phosphorus(V) porphyrin**

General procedure of synthesis method of phosphorus(V) porphyrin is a reflux of free base porphyrin with phosphoryl chloride in dry pyridine [51]. The photochemical property of phosphorus(V) porphyrin can be improved by the substitution of the *meso*- or *β*-positions and the axial ligand (**Figure 2**) [42–53]. An example of phosphorus(V) porphyrin, diethoxyP(V)tetrakis(4-methoxyphenyl) porphyrin chloride, is shown in **Figure 3**. The calculation with density functional theory (DFT) at ωB97X-D/6-31G\* level shows the distorted structure of phosphorus(V) porphyrin. Their distorted structures have been reported from the results of X-ray crystal analysis [54]. Phosphorus(V) porphyrins introduced in this chapter are listed in **Table 1**. Because phosphorus(V) porphyrin is a cationic

**Figure 3.** *Optimized structure of* **Por10** *by the DFT calculation at* ω*B97X-D/6-31G\* level.*

porphyrin, its water solubility is relatively large. Furthermore, hydrophilic substitution markedly increases the water solubility [55]. One of the most important characteristics of phosphorus(V) porphyrin is relatively small *E*red value due to the positive charge of the central phosphorus atom, resulting in the strong oxidative activity in the photoexcited state. This character is very important as electron transfer-supported photosensitizer for PDT. Furthermore, in general, phosphorus(V) porphyrin has relatively large quantum yield of photosensitized 1 O2 generation in an aqueous solution (ΦΔ is more than 0.5, **Table 1**) due to the effective intersystem crossing [42–47]. In the presence of enough oxygen molecules, phosphorus(V) porphyrin can oxidize biomolecule through <sup>1</sup> O2 generation, a traditional PDT mechanism.
