Phthalocyanine dyes are nitrogenous macrocyclic compounds. Phthalocyanine ring is a big conjugated system containing eighteen π electrons and the electron density is very homogeneous, which makes it hard for four benzene rings in the molecular to distort. Therefore, phthalocyanine dyes have the advantages of good stability, high heat resistance and excellent acid resistance. Moreover, the phthalocyanine ring contains holes with a radius of 0.135nm, which can complex with many metals and transition metal elements such as cobalt, iron, copper, nickel and zinc to form phthalocyanine metal compounds. Most phthalocyanine dyes are synthetic. Porphyrin dyes have a similar structure to phthalocyanine dyes, but porphyrin dyes are a kind of substances widely existing in nature. Porphyrin is a general term of a macrocyclic compound with substituent groups on porphin ring. Porphin ring is a planar conjugated structure formed by twenty carbon atoms and four nitrogen atoms. The two hydrogen atoms and nitrogen atoms of porphyrin molecule could be replaced by metal ion to form corresponding porphyrin metal compounds. Due to the existence of the conjugated π system, the energy level difference between the HOMO and LUMO of porphyrins is reduced, so that porphyrin dyes can absorb and emit visible light, and the quantum yield is high.

Figure 1. Example of phthalocyanine dyes and porphyrin dyes molecular structure.

Applications:

• Optical field: Due to their special structure and excellent optical properties, phthalocyanine dyes and porphyrin dyes have been widely used in optical field as excellent photosensitizers. Moreover, when connected with other electrically or optically active groups, phthalocyanine dyes and porphyrin dyes can show many unique optical properties, which are widely used in electrostatic copiers, solar cells, electroluminescent diodes and the others.

Figure 2. Porphyrin dyes used in solar cells.

• Electronics field: Phthalocyanine dyes and porphyrin dyes have excellent electrical properties, and have been widely used in the electronics field. For example, adding phthalocyanine dyes in lithium-ion batteries and nickel hydrogen batteries, the charging and discharging property of the battery can be improved effectively and the battery life can be increased.
• Semiconductor field: Based on their special molecular structure, phthalocyanine dyes and porphyrin dyes can be converted into semiconductor materials by doping or polymerization with some materials. For example, highly modified phthalocyanine dyes have been used in organic thin film field effect transistors to study disk-like organic semiconductor materials.
• Sensor field: When phthalocyanine dyes and porphyrin dyes come into contact with some gases or liquids, the conductivity and catalytic activity may change, which can be used to design sensors. In addition, phthalocyanine dyes also can be applied as humidity sensors for the detection of atmospheric humidity.
• Photodynamic therapy field: In photodynamic therapy, the ability of photosensitizers to selectively stay on tumor cells is the key to killing malignant tumor cells. Phthalocyanine dyes and porphyrin dyes have the advantages of high purity, suitable hydrophilicity, good physiological activity and suitable optical parameters, which is the ideal choice.

Classification:

According to the solubility, phthalocyanine dyes and porphyrin dyes can be divided into lipid soluble phthalocyanine dyes and porphyrin dyes and water-soluble phthalocyanine dyes and porphyrin dyes.

• Lipid soluble phthalocyanine dyes and porphyrin dyes: Those compounds are soluble in organic solvents, such as chloroform, dichloromethane, ethyl acetate, benzene and the others.
• Water-soluble Phthalocyanine dyes and porphyrin dyes: These compounds are soluble in water, methanol, ethanol, acetone and the other hydrophilic organic solvents.

References:

1. Obraztsov I, Kutner W, D’Souza F. Evolution of Molecular Design of Porphyrin Chromophores for Photovoltaic Materials of Superior Light‐to‐Electricity Conversion Efficiency[J]. Solar RRL, 2017.