## 8 Porphyniroids and Titanium(IV) Oxide-Based Hybrid Materials and Their Photocatalytic Activity  
Porphyrinoids are macrocyclic compounds that can be used as catalysts for the photodegradation of chemical pollutants (Figure 11). Representatives of this group are porphyrins (Pp), porphyrazines (Pz), and phthalocyanines (Pc). Porphyrins consist of four pyrrole rings linked by _meso_-carbon bridges, whereas in porphyrazines, the _meso_ carbons are replaced by nitrogens. Phthalocyanines have additional benzene rings annulated to the porphyrazine core [149]. Multitude of active sites allows for broad functionalization of porphyrinoids with different functional groups, further enhancing their properties in the desired direction. Moreover, macrocyclic porphyrinoids have an active center that can coordinate metal ions. Insertion of different metals in the coordination center of a macrocycle can give rise to different properties of the molecule, allowing us to achieve desired features. For example, porphyrins that coordinate metal ions belonging to block d of the periodic table are often used as catalysts in oxidation reactions of organic compounds [150]. Porphyrinoids find use in many fields of science. Most notably, they are often used as photosensizers in photodynamic therapy [151; 152; 153; 154; 155]. A great amount of research effort is focused on finding applications of porphyrins and other macrocyclic compounds in oncology [156], as contrast agents, fluorescent dyes [157; 158], drug/gene carriers [159], and as parts of photovoltaic cells [160]. As we have previously mentioned, TiO\({}_{2}\) absorbs mainly UV light, which is not ideal for its broad application in photocatalysis. Ideally, the material should also be photocatalytically active in visible light. Porphyrazines and phthalocyanines have well-developed absorption maxima in the 600-800 nm range (Q-band) of the UV-Vis spectrum, which corresponds mostly to the red light. In general, absorption peaks within the Q band range for porphyrins are less developed, and hypsochromically shifted in relation to Pzs and Pcs. They appear within a spectrum range specific for green and near-red light. Coming from these qualities, porphyrinoids can be used as photosensizers for TiO\({}_{2}\) to achieve photocatalytic activity in visible light. However, for this purpose, they need to be appropriately functionalized with functional groups that promote covalent binding to the surface of TiO\({}_{2}\) such as carboxylic or pyridyl groups [161]. According to the literature study, the effective energy transfer in the case of photosensitizer deposition through surface adsorption might not be efficient and the photosensitizer molecules need to be covalently bound to the surface of TiO\({}_{2}\) to sensitize the material toward visible light, at least when photodegradation in aqueous solution is considered [162]. In one of the reviewed studies, polycrystalline TiO\({}_{2}\) was impregnated with _tert_-butylphenyl functionalized metal-free porphyrins and metalloporphyrins (i.e., copper, iron, and manganese) in order to achieve complexes with enhanced photoactivity [163]. The potential of synthesized TiO\({}_{2}\)-porphyrin systems was examined through photodegradation of 4-nitrophenol (4-NP) in UV light. 4-NP is an intermediate compound in the synthesis of paracetamol, which is a common analgesic. Results showed that TiO\({}_{2}\) samples impregnated with CuPp and FePp had higher photoactivity than anatase TiO\({}_{2}\). However, MnPp-TiO\({}_{2}\) showed reduced photoactivity in the degradation of 4-NP. The results of the photocatalytic tests were presented by the authors in the form of quantum efficiency, which is the ratio of the number of mol of the reacted 4-NP to the number of mol of the photons emitted by the UV lamp. CuPp-TiO\({}_{2}\) revealed the highest quantum efficiency (1.71-1.93 depending on the concentration of the photosensitizer), followed by FePp-TiO\({}_{2}\) (0.84-0.97) and demetallated TiO\({}_{2}\)-H\({}_{2}\)Pp (0.90-0.95).