Porphyrins are the collective term for macrocyclic compounds and their derivatives, composed of four pyrrole rings linked on the α position through four methine groups. The central nitrogens form a stable complex with various metals such as iron, magnesium and cobalt. These metal complexes play important roles in biochemical processes. Metal porphyrin derivatives such as hemoglobin, vitamin B₁₂, chlorophyll and cytochromes exist in living organisms. From the point of their specific optical and redox characteristics, their applications to solar cells or catalysts for chemical reactions have been studied.