**2. Porphyrins: structures and optoelectronic properties**

In nature, porphyrin‐related molecules are important photosynthetic pigments that per‐ form the light‐harvesting and charge/energy transfer functions in biological photosynthesis [45–48]. The role of porphyrins in photocatalysis is mainly related to their optical feature. As shown from the basic porphyrin ring (**Figure 1**), porphyrins are tetrapyrrole derivatives which are composed of four pyrrole subunits interconnected via ─CH═ bridges. The inner 16‐membered ring with 18 π electrons constitutes its electronic "heart," which is responsible for the optical spectra. Many authors have investigated its optoelectronic properties because of simplicity. Again, molecular engineering is easily attainable by various chemical modifica‐ tions to this basic ring, leading to proper tuning of the optoelectronic properties [49–51].

**Figure 1.** An illustration of a representative tetraarylporphyrin.

efficiency of these catalysts correlating with band engineering [20, 21], texture modification [16, 22], or configuration organization [23, 24] always involve complicated fabrication pro‐ cesses. All these limit their practical affordable applications. Still, much effort is needed to find other photoactive materials as alternatives for facile preparation and economical applications. During the last decades, increasing attention has been paid to the field of semiconducting organic materials for optoelectronic applications [25–27]. One of the most important advan‐ tages concerning these organic materials is that their molecular structure and functions can be easily modulated via molecular design and tailoring. Additionally, integration of them into lightweight, large‐area devices can be simply realized through solution processing at low cost. In addition, organic semiconductors, also referred to as π‐conjugated molecules, are characterized by a delocalized π‐electron system that makes them ideal building blocks for the fabrication of advanced functional nanomaterials and nanodevices [28–30]. As a typical representative of π‐conjugated molecules, porphyrins are of particular interest due to some key aspects, such as their excellent light‐harvesting property, p‐type semiconducting behav‐ ior, ease of chemical modification, good supramolecular assembly, and film‐forming features by means of either solution‐based or thermal‐based techniques [31–33]. Coupled with their chemical stability and flexibility, the use of porphyrin in optoelectronics has become a fast‐ growing research focus, and great development has been made in the field of organic solar cells (OSCs) [34, 35], organic field‐effect transistors (OFETs) [36, 37], organic light‐emitting

120 Phthalocyanines and Some Current Applications

diodes (OLEDs) [38, 39], even in flexible organic semiconductor devices.

achieve absorption of visible light is not covered in this chapter.

**2. Porphyrins: structures and optoelectronic properties**

As a photocatalyst, porphyrins were first used in homogenous photocatalysis [40]. The prob‐ lem with it is the limited stability of porphyrin molecules and the recovery of them for suc‐ cessive use. Fortunately, this could be circumvented by mobilizing porphyrin molecules on solid supports or assembling them into robust nanostructures [41, 42]. Recently, more efforts have been made on the development of a semiconductor‐based photoelectrochemical (PEC) water splitting device [43, 44], and thus organic photoelectrodes have aroused special atten‐ tion. Relating progresses are dealt with in detail in separate sections. Before that we have a brief introduction of the relation between porphyrin molecular structure and optoelectronic properties. The use of molecular porphyrin as modification of inorganic semiconductors to

In nature, porphyrin‐related molecules are important photosynthetic pigments that per‐ form the light‐harvesting and charge/energy transfer functions in biological photosynthesis [45–48]. The role of porphyrins in photocatalysis is mainly related to their optical feature. As shown from the basic porphyrin ring (**Figure 1**), porphyrins are tetrapyrrole derivatives which are composed of four pyrrole subunits interconnected via ─CH═ bridges. The inner 16‐membered ring with 18 π electrons constitutes its electronic "heart," which is responsible for the optical spectra. Many authors have investigated its optoelectronic properties because of simplicity. Again, molecular engineering is easily attainable by various chemical modifica‐ tions to this basic ring, leading to proper tuning of the optoelectronic properties [49–51].

First, central substituent of porphyrin ring has a major effect on the optical spectra. Depending on the atom or group that occupies the center, porphyrins can be basically divided into free‐ base type (two hydrogens in the center) and metal‐type [52, 53], or the so‐called metallopor‐ phyrin that is formed by exchange of the two protons in freebase porphyrin by a metal ion. Considerable varieties in the optoelectronic properties just arise from such center difference. Particularly, freebase porphyrin has a four‐banded visible spectrum notably different from the two‐banded spectrum exhibited by metal complex [54]. This spectral difference is attrib‐ uted to the fact that the two freebase hydrogens in the center greatly reduce the symmetry from square to rectangular. In the case of metalloporphyrin [55], the change of metal in some cases can strongly influence absorption spectra. It is now known that the central metal per‐ turbs the absorption spectra mainly through the interaction of the metal electrons with those of the ring, and sometimes the coordination type can also affect the spectra.

In addition to central substituents, peripheral substituents at various locations around the ring, including four *meso* and eight β‐positions, can also impart different properties to a greater or lesser extent to the molecule [56–58]. Xie et al. have introduced various numbers of triphenylamine and trimethoxyphenyl groups to the *meso*‐positions as electron donors, in an attempt to systematically tune the highest occupied molecular orbital‐lowest unoccupied molecular orbital (HOMO‐LUMO) energy levels [59]. As a photocatalyst, HOMO‐LUMO bandgap determines the absorption wavelength for light‐harvesting efficiency, and the suit‐ able HOMO and LUMO levels ensure an efficient electron injection and dye regeneration process. With regard to porphyrins, the modulation of the HOMO‐LUMO levels, along with the corresponding optoelectronic properties, can be simply realized through proper choice of an anchoring group to the ring. In another work, Sharma and coworkers reviewed the importance of various anchoring groups linked to either *meso* or β‐positions in improving the light collection efficiency of dye‐sensitized solar cells (DSSCs) [58]. As the most widely used anchoring group, the position of carboxylic acid (COOH) was found to vary the performance of solar cells. Increased photocurrent was generated when the position of COOH changed from the *para* position to the *meta* position. Also in some cases, porphyrin is functionalized with donor and acceptor moieties. Upon photoexcitation, the generated exciton diffuses to the donor‐acceptor interface, affording enhanced charge transfer character. Meanwhile, the enlarged electron conjugation leads to a narrowing of the optical bandgap, giving rise to broad light‐absorbing dye.
