**3. Porphyrin‐based homogeneous photocatalysts**

Increasing emphasis has been placed on photocatalysts as an environmentally friendly pro‐ cess to decompose organic pollutants in contaminated water and air. It is well documented that porphyrins and metalloporphyrins have contributed a lot to photooxidation catalysis in homogeneous media.

#### **3.1. Reaction mechanisms**

As for highly effective triplet‐state porphyrins, two possible mechanistic pathways are involved in a photocatalytic process: energy transfer and electron transfer from the triplet excited state [60, 61]. Singlet oxygen species (<sup>1</sup> O2 ) is commonly involved during the energy transfer process, whereas other active oxygen species such as a superoxide radical anion (O<sup>2</sup> •−) or hydroxyl radical (•OH) are essentially involved in the case of the electron transfer process. Time‐resolved spectroscopic methods thus provide a powerful tool to detect the transient species derived from the photocatalyst for the study of fast reaction kinetics.

Homogeneous porphyrins are well known to generate 1 O2 [40, 62–64]. For example, the triplet quantum yield of *meso*‐tetra (2,6‐dichloro‐phenyl) porphyrin (TDCPP) was reported to be 0.995, with its corresponding singlet oxygen quantum yield around 0.98 [65]. As highly recognized, the photochemically generated singlet oxygen acts as a primary oxidant in photodegrading organic pollutants and viruses in natural water. In the case of *meso*‐tetra(2,6‐ dichloro‐3‐sulfophenyl) porphyrin (TDCPPS) [66] or its iron complex (FeTDCPPS) [67] when oxidizing phenols, the main photodegradation pathway involved reaction with singlet oxy‐ gen, as suggested by the following observations: the triplet state of the porphyrins was effi‐ ciently quenched by molecular oxygen; singlet oxygen phosphorescence was detected by time‐resolved measurements.

#### **3.2. Homogeneous photocatalysis**

As the catalyst is dissolved, it is easy to get access to all active sites, resulting in high catalytic activities. For instance, water‐soluble TDCPPS and its metal complexes were successfully used in the photodegradation of 4‐chlorophenol, giving rise to the main photoproducts such as *p*‐benzoquinone, whereas 2,6‐dimethylphenol was transformed into 2,6‐dimeth‐ ylbenzoquinone [60]. The same product was obtained when 4‐chlorophenol was treated with water‐soluble FeTDCPPS [68] and sodium *meso*‐tetra (4‐sulphonatophenyl)porphyrin (NaTPPS). Photodegradation of atrazine and ametryn by *meso*‐tetra(4‐sulphonatophenyl) porphyrin (TPPS) or TDCPPS resulted in a mixture of photoproducts [69]. Further exam‐ ples are the photooxidation of 2,4,6‐trinitrotoluene (TNT) with TPPS and its iron complex (FeTPPS) to give trinitrobenzoic acid and trinitrobenzene [70]. In most cases given above, water‐soluble porphyrin derivatives are adopted, which are more suitable for practical wastewater treatment.

As another case, hydrogen production is a typical photocatalytic reaction that occurs under light irradiation. Photoinduced hydrogen production from water is regarded as an efficient and cost‐effective method for the conversion and storage of solar energy. This process is usu‐ ally accomplished by a system containing a photosensitizer, electron carrier, electron donor, and a catalyst. Chlorophyll and ferredoxin are the natural photosensitizer and electron carrier, while porphyrins often act as an artificial photosensitizer [71]. An example of water‐soluble zinc *meso*‐tetra(1‐methylpyridinium‐4‐yl)porphyrin chloride [ZnTMPyP4+]Cl4 as a photosen‐ sitizer, viologens as an electron carrier, ethylenediaminetetraacetic acid (EDTA) as an electron donor, and hydrogenase (H<sup>2</sup> ase) as a catalyst was provided by Qian et al. Lazarides et al. reported the use of the same [ZnTMPyP4+]Cl4 as a photosensitizer, but with cobaloxime com‐ plex as a catalyst [72]. Using this system, the photocatalytic activity maintained for 20 h pro‐ ducing in total about 280 TON of hydrogen. In some other studies, Co, Fe, and Rh porphyrins were shown to be active as the hydrogen evolution catalysts via photoinitiation using other sensitizers [73]. Scandola and coworkers reported the efficient photochemical hydrogen evo‐ lution from 1 M pH 7 phosphate buffer by using water‐soluble cationic cobalt (II) porphyrin as the catalyst, ascorbic acid as the electron donor, and [Ru(bpy)<sup>3</sup> ]2+ (bpy = 2,2′‐bipyridine) as the photosensitizer, in achievement of TON up to 725 [74]. Kinetic studies revealed a rapid electron transfer process from [Ru(bpy)<sup>3</sup> ]2+ to cobalt (II) porphyrin with a calculated rate con‐ stant of 2.3 × 109 M−1 s−1.
