What is the Difference Between Phthalocyanine and Porphyrin

The key difference between phthalocyanine and porphyrin is that phthalocyanine molecules contain four indole units or pyrrole rings linked through nitrogen atoms that are conjugated with benzene rings, whereas porphyrin molecules contain four pyrrole rings linked through methane carbon bridges.

Phthalocyanine or H2Pc is a large, aromatic, macrocyclic organic compound having the formula (C8H4N2)4H2. Porphyrin compounds are the conjugate acids of ligands that can bind with metals forming complexes.

CONTENTS

1. Overview and Key Difference
2. What is Phthalocyanine  
3. What is Porphyrin 
4. Phthalocyanine vs Porphyrin in Tabular Form
5. Summary – Phthalocyanine vs Porphyrin 

What is Phthalocyanine?

Phthalocyanine or H2Pc is a large, aromatic, macrocyclic organic compound having the formula (C8H4N2)4H2. It has theoretical and specialized interests in chemical dyes and photoelectricity. This substance has four isoindole units that are linked to each other through a ring of nitrogen atoms. This compound has two-dimensional geometry and a ring system containing 18 pi electrons. There is extensive delocalization of the pi electrons, which can cause the molecule to have useful properties which can lend themselves to applications in dyes and pigments. Moreover, the derivatives of this compound, such as metal complexes, are significant in catalysis, organic solar cells, and photodynamic therapy.

Furthermore, phthalocyanine compounds and their metal complexes are able to aggregate, and therefore, they have a low solubility in common solvents. For example, benzene has a low solubility than phthalocyanine when the same temperatures are considered. Moreover, most phthalocyanine compounds are thermally stable while they undergo sublimation without undergoing melting at high temperatures.

When considering the synthesis of phthalocyanine, it forms through the cyclotetramerization of various phthalic acid derivatives such as phthalonitrile, diiminoisoindole, phthalic anhydride, and phthalimides. In addition, we can also use the method of heating phthalic anhydride when there is enough amount of urea present to get H2Pc.

The primary use of phthalocyanine is using it as dyes and pigments. However, modifications in this molecule can be useful in tuning the absorption and emission properties of Pc to give differently colored dyes and pigments. Other derivatives are also useful in photovoltaics, photodynamic therapy, nanoparticle construction, and catalysis.

What is Porphyrin?

Porphyrin compounds are the conjugate acids of ligands that can bin with metals forming complexes. The metal ion of this complex is typically a +2 or a +3 charged cation. We can call a porphyrin compound without a metal ion in its cavity a “free base.” There are some complexes that contain iron at the metal center. We call them “heme complexes” or simply “hemes.” There are proteins consisting of iron which are known as hemoproteins. We can find this type of protein extensively in nature. Moreover, there are two major oxygen binding proteins in our blood named hemoglobin and myoglobin. They are iron porphyrins. Besides, there are various different cytochromes that we can name hemoproteins.

H₂porphyrin + [ML_n]²⁺ → M(porphyrinate)L_n−4 + 4 L + 2 H⁺, where M = metal ion and L = a ligand

What is the Difference Between Phthalocyanine and Porphyrin?

Phthalocyanine or H2Pc is a large, aromatic, macrocyclic organic compound having the formula (C8H4N2)4H2. Porphyrin compounds are the conjugate acids of ligands that can bin with metals forming complexes. The key difference between phthalocyanine and porphyrin is that phthalocyanine molecules contain four indole units or pyrrole rings linked through the nitrogen atoms that are conjugated with benzene rings, whereas porphyrin molecules contain four pyrrole rings linked through methane carbon bridges.

The below infographic presents the differences between phthalocyanine and porphyrin in tabular form for side by side comparison.

Summary – Phthalocyanine vs Porphyrin

The key difference between phthalocyanine and porphyrin is that phthalocyanine molecules contain four indole units or pyrrole rings linked through the nitrogen atoms that are conjugated with benzene rings, whereas porphyrin molecules contain four pyrrole rings linked through methane carbon bridges.

Reference:

1. Lopes, David M., et al. “Technological Applications of Porphyrins and Related Compounds: Spintronics and Micro-/Nanomotors.” IntechOpen, 10 May 2019.

Image Courtesy:

1. “Phthalocyanine” By Choij – Own work (Public Domain) via Commons Wikimedia
2. “Copper Phtalocyanine Blue” By Shangy (talk) – Own work (Public Domain) via Commons Wikimedia