Self-assembled porphyrin and macrocycle derivatives: From synthesis to function
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Introduction Porphyrins are planar macrocyclic molecules containing four pyrroles connected in ring fashion through four methine carbons at their α-positions (Figure 1). These ubiquitous molecules are widely present in almost all living organisms in one form or another, the best known being chlorophyll and heme (or hemoglobin).1,2 It is self-evident that porphyrins are important participants in the energy supply system for both earth and life via photosynthesis and oxygen transport. Porphyrins are basically cyclic tetrapyrrole derivatives with highly delocalized π electrons that form a planar conjugated framework. This is an 18-π electron system that exhibits aromaticity (Figure 1a). The highly conjugated π-system is the origin of the strong color of these compounds as well as their characteristic electronic and redox properties. Indeed, the word “porphyrin” is derived from the Greek word for “purple.” Porphyrins mostly exist in the form of derivatives of the basic molecules due to their chemical properties. On one hand, substitution can be performed at all of the peripheral eight pyrrole β-carbon atoms and the four meso-carbon centers; this would make porphyrin exist in varied forms with structural variations. On the other hand, the porphyrin ring provides a vacant site at its center, which is ideally suited for metalloporphyrin incorporation (Figure 1b). The NH protons inside the ring of porphyrins possess acidic character and can become
deprotonated, forming dianion species. These dianion species exhibit remarkable coordination characteristics toward metal ions and mostly act as a tetradentate ligand with metal ions. The common coordination number of the metal ion in a metalloporphyrin is possibly four. The electronic delocalization leads to substantial planarity of the macrocycle and a square planar environment for the metal ion in four-coordinate complexes. A coordination number (five or six) greater than four is also possible through ligation of suitable moieties either neutral or anionic. Meanwhile, the extensive electronic delocalization that occurs in the porphyrin ring leads to substantial planarity of the macrocycle and an essentially square planar environment for the metal ion in four-coordinate complexes. The bonding between a central metal ion and the porphyrin occurs via two types of primary interactions—σ-coordination of nitrogen lone pairs directed toward the central metal atom, and π-interaction of metal pπ and dπ orbitals with nitrogen pπ orbitals. This electron system results in visible light absorption of metalloporphyrin, the normal spectrum for which shows an intense B (Soret) band (strong transition from the base to the second excited state (S0 → S2)) at ∼420 nm and two weaker Q bands (transition to the first excited state (S0 → S1)) at ∼550–600 nm.3 In nature, porphyrin-based biological functional materials are mostly constructed by metalloporphyrins with protein.4,5
Ying-Bing Jiang, Angstrom Thin Film Technologies LLC; and The University of New Mexico, USA; [email protected] Zaicheng Sun,
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