Self-Assembly of Porphyrinic Materials on Surfaces
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Porphyrins are tetrapyrrole macrocycles found in nature in systems such as hemoglobin, myoglobin, various cytochromes (C, P450, etc.), chlorophylls, methyl-Co-M reductase in methanogenic bacteria, and a plethora of other biological systems. They function as transporters of small molecules such as dioxygen, mediators of electron transport, oxidation catalysts, light-harvesting and energy transport conduits, and catalysts for the reduction of a methyl thioether, respectively. Although porphyrins can bind virtually every metal in the periodic table (and a few nonmetals), in biological systems these metals are usually Mg, Fe, Co, and Ni—where the redox chemistry of the latter three provides the function. The limited flexibility and restricted aperture of the square planar ligand modulate both the redox potential and the coordination geometry of bound metals, which in turn modulate the redox and photophysical properties of the macrocycle. The remarkable stability of the macrocycle (it is found in shale oils as a vanadium complex) and the wide distribution in nature have indicated to some researchers that porphyrins may be prebiotic.
Because of their biological relevance and diverse functions, a large number of porphyrin derivatives have been synthesized in the laboratory in the last decades using the methods of Adler et al. and Lindsey and, more recently, a solventless “green” synthesis. These methods have allowed commercial applications of porphyrin derivatives that include oxidation catalysts, blood substitutes, therapeutics, sensors/actuators, photonic materials, dyes, and various forms of “toners” in photolithography. In the past decade, there has been substantial effort to create and characterize the properties of multiporphyrinic systems for materials applications. Two related macrocycles—phthalocyanines and porphyrazines—have also been the subject of much research for similar applications, but multichromophoric systems of these dyes are less well developed at present. Although all three types of macrocycles have related photophysical properties, the porphyrins have been the primary focus of research because of synthetic accessibility and greater solubility in organic solvents.
In the context of nanotechnology and materials chemistry, the aforementioned properties of porphyrinoids can be exploited for a variety of applications that include:  molecular electronics;  components of nanoscaled sensors, actuators, and photonics;  both structural and chemically active elements in molecule-constructed molecular sieves;  catalysts; and  part of biomolecular encapsulating systems. For many of these applications, the precise geometric alignment of the chromophores in the materials is essential for function. The problem, then, is how to make nanomaterials of organic compounds wherein a few to a few hundred molecules are brought together in a predefined or predictable geometry—as opposed to aggregates, colloids, and gels (all of which have important applications in nanotechnology as well). The remarkable progress in organic chemistry has provided many elegant molecular systems with more than one porphyrin linked by covalent bonds. These include acetylene, phenylacetylene, bridged, fused, and other linkages. For many of these molecular systems, the interspatial relationships of the macrocycle are well defined. However, the significant limitation of these molecular systems is that the product yield drops precipitously as the number of chromophores increases. There are a variety of polymeric systems bearing porphyrins, or with porphyrins as part of the polymeric chain where the dispersity varies from system to system, but the yields are generally better than for the above molecular system. Both the molecular and polymeric systems have provided a wealth of information of the nature of energy and electron transport mediated by the porphyrins and the various linkers, as well as the means to gate these processes. Several functional materials have also been reported, vide supra. The focus of this review is on self-assembled and self-organized porphyrinic systems on surfaces.