Introduction
Porphyrins are defined as a group of heterocyclic macrocycle organic compounds, composed of four modified pyrrole subunits interconnected at their carbon atoms via methine (=CH–) bridges. Research on porphyrins has started with the studies in which the isolation of porphyrins from natural compounds were reported and then continue with studies that showed the synthesis of porphyrins in the laboratory conditions. This is due to the variety of fascinating physicochemical properties of this macrocyclic aromatic compound. For example, porphyrin derivatives exhibit strong electronic absorption in the visible region. Owing to their large extinction coefficient in the visible region, porphyrin derivatives have become key components for light-harvesting antenna complexes in natural photosynthetic systems. Furthermore, porphyrin derivatives are engaged in the charge separation and electron transfer processes of photosynthesis. Until today, many novel porphyrin derivates have been synthesized with the growing number of synthesis methods and derivatization reactions.
Figure 1. The molecular structure of porphyrins
Phthalocyanines (PCs) are large p-planar compounds composed of four isoindole groups that are linked by nitrogen atoms. Similar to porphyrins, phthalocyanines and related compounds form complexes with the majority of elements in the periodic system. PCs have been firmly established as blue and green dyestuffs because of their intense color and resistance to photo-bleaching. Moreover, the most important features of PCs are their typical absorption spectrum, which is characterized by a sharp, intense Q-band and a broader, less intense B band.
Figure 2. The molecular structure of phthalocyanines
Applications
The application of porphyrin and phthalocyanine is mainly divided into the following aspects.
- Photodynamic therapy
Photodynamic therapy (PDT) consists of three essential components: photosensitizer, light and oxygen, is an interesting concept that destroys diseased tissue via light-driven reaction. Porphyrins and expanded porphyrins are one class of molecules under intense investigation due to their photosensitizing ability for PDT application. The strong absorption of light by a water-soluble nontoxic photosensitizing molecule in the therapeutic window resulting in maximum penetration of light into the tissues coupled with high singlet oxygen production will conceptualize an ideal photosensitizer. Among metallophthalocyanines, aluminium sulfonated phthalocyanine has its maximum absorption band in the long wavelength region and is established as a photosensitizer in PDT.
Figure 3. Application in photodynamic therapy
- Solar photovoltaic cells
Porphyrins and phthalocyanines have presented an attractive array of optical and chemical properties. Typically, such compounds can be used as components of solar cells, including organic molecular solar cells, polymer cells, and dye-sensitized solar cells. For example, A zinc phthalocyanine with tyrosine substituents (ZnPcTyr), modified for efficient far-red/near-IR performance in dye-sensitized nanostructured TiO2 solar cells. Incorporation of tyrosine groups makes the dye ethanol-soluble and decreases considerably surface aggregation of the sensitizer due to steric effects and as a result improves the solar cell performance[1].
Figure 4. Application in solar photovoltaic cells
- Catalytic
A series of metallo-porphyrins and metallo-phthalocyanines shows an increased catalytic activity in different chemical/photochemical processes. In all these processes, the porphyrins are used either in organic solutions or supported on different inorganic supports in order to increase the catalytic activity and their stability (photostability) (due to their strong interaction between the support and the complex)[2]. The porphyrin μ-oxo-dimers are recognized as the best catalysts in the olefins chemical epoxidation reactions.
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References
- He J, Benkö G, Korodi F, et al. Modified phthalocyanines for efficient near-IR sensitization of nanostructured TiO2 Electrode[J]. Journal of the American Chemical Society, 2002, 124: 4922–4932.
- Neagu M, Constantin C, Tampa M, et al. Toxicological and efficacy assessment of post-transition metal (Indium) phthalocyanine for photodynamic therapy in neuroblastoma[J]. Oncotarget, 2016, 7:69718-69732.
