**1. Introduction**

Various (metallo)tetrapyrrole compounds, for example, porphyrins (P), porphyrazines (Pz) and phthalocyanines (Pc) (**Figure 1**), are representatives of the huge class of π‐conjugated

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**Figure 1.** Molecular structures of some tetrapyrrole macrocycles. Reprinted (adapted) with permission from Liao et al. [20]. Copyright 2005 American Chemical Society.

(aromatic) organic heterocycles [1−5]. They can be found as cofactors in numerous enzymes: as hemes in various cytochromes, catalases, peroxidases, etc.; as chlorophyll and pheophytin in photosynthetic proteins and as corrin and corphin in other proteins [1−3, 5]. The metal‐ loporphyrins have numerous biological functions such as: (i) O2 transport and storage, (ii) oxidative metabolism, (iii) gas sensing, (iv) antibactericides/microbicides, (v) collection and transport of light energy, (vi) conversion of solar energy to chemical energy, (vii) electron transfer and (viii) NO scavenging and a significant number of other functions [1−3, 5−10]. Numerous technological applications of porphyrins include: catalysis [1, 2, 4, 11, 12], molecu‐ lar photonic devices [4, 13, 14], medicine [1, 2, 4, 15], artificial photosynthesis [16, 17], sensitiz‐ ers for dye‐sensitized solar cells [18] and sensor devices [19].

The size, shape, electronic properties and binding ability of porphyrins can be broadly tuned by replacing one or more pyrrole nitrogens with other elements [21−24]. This type of the por‐ phyrin core modification is a highly promising approach for tuning the various properties of porphyrin species. It brings to life the following questions:

(i) What structures will core‐modified porphyrins adopt? (ii) How will atomic charges and other electronic properties (frontier orbital energies, HOMO/LUMO and optical gaps, ioniza‐ tion potentials, electron affinities, etc.) in core‐modified porphyrins differ from regular tetra‐ pyrroles? How can we tune these properties? (iii) What novel properties will core‐modified porphyrins possess?

In recent years, there has been increasing interest in porphyrin core modification with the chalcogens (O, S, Se), which resulted in numerous experimental and computational works in this extremely promising area. Core modification of tetrapyrroles by P has been of long‐ lasting interest as well. Of course, it would not be possible to cover all the studies on core modification of porphyrins in this review. Thus, this chapter will cover the most significant and interesting works devoted to the core modification of porphyrins and derivatives with the principal focus on *completely core‐modified* compounds. The important works on *partially core‐modified* compounds will be considered as well.
