Multiporphyrin system

Fisher JAN, Susumu K, Therien MJ, Yodh AG (2009) One- and two-photon absorption of highly conjugated multiporphyrin systems in the two-photon Soret transition region. J Chem Phys 130 134506... 

Over the years many different synthetic strategies have been employed to construct ET compounds or multiporphyrin systems and the available synthetic repertoire is continuously expanding110. 

The design of dendritic multiporphyrin systems [18] permits energy transfer over longer distances. The outer shell of the dendrimer shown in Fig. 5.14 is made up of eight porphyrin-zinc complexes as energy donor units. Excitation of the units of the outer shell leads to fluorescence emission of the metal-free porphyrin core as a result of energy transfer from the periphery to the energy acceptor [19]. 

In order to build synthetic analogues of the bacterial reaction centre, several approaches using multiporphyrin systems have recently been reported (Dubowchik and Hamilton, 1985, 1986 Wasielewski et al., 1982). However, the number of molecular arrays whose rigidity is sufficient to ensure control of the distance and orientation of the respective tetrapyrrolic subunits is highly limited (Dubowchik and Hamilton, 1987 Cowan etal., 1987, Sessler and Johnson, 1987). 

This book summarizes recent work on metal-mediated and hydrogen-bonded multiporphyrin assemblies which are stable enough to be characterized in solution. Assemblies coordinated only in soUds (e.g. crystals) are not included. Emphasis is given on the photophysical and molecular recognition properties of the supramolecular multiporphyrin systems. 

The aim of these multiporphyrin systems was to study the energy transfer reaction from the singlet excited state of the zinc(II) porphyrin to the free-base porphyrin within a hydrogen-bond assembly. Nevertheless, it was difficult to explain unambiguously the luminescence quenching of the zinc porphyrin. Due to the flexibility around the amino group, intracomplex diffusional quenching of the zinc porphyrin excited state by the free-base porphyrin could not be ruled out. 

Before discussing self-assembling systems, it is necessary to roughly survey some typical examples of covalently linked multiporphyrin systems because they provide prototypes of artificial multiporphyrin systems and some of them may be useful as components for further large-scale assemblies. 

The cyclic oligomers are another interesting class of synthetic multiporphyrin systems. The.se systems may not only mimic the highly concentrated porphyrin clusters found in biological structures containing closely coupled tetrapyrroles (such as the photo.synthetic reaction center and the cytochrome families of various types),- - but also show an interesting manner of molecular recognition and self-... 

Most of the large-scale multiporphyrin systems found in nature are situated within peptide frameworks, and their relative configurations and orientations are held fast with noncovalent interactions between protein residual groups and the chromophores. The advantages of the use of noncovalent interactions is twofold (1) a variety of noncovalent interactions having different characteristics is available for assembling molecules into a large-scale... 

Figure 17. Various types of self-assembling processes of multiporphyrin systems.

CoUin, J.P, A. Harriman, V. Heitz, F. Odobel, and J.P. Sauvage (1996). Transition metal-assembled multiporphyrinic systems as models of photosynthetic reaction centre. Coord. Chem. Rev. 148, 63-69. 

Lee SU, Belosludov RV, Mizuseki H, Kawazoe Y (2008) The role of aromaticity and the 7Z-conjugated framework in multiporphyrinic systems as single-molecule switches. Small 4 (7) 962-969... 

The most used approaches for the construction of multiporphyrin systems are the self-assembly methods. For example, electrostatic assembly is an easy and widely used method based on the interaction of oppositely charged porphyrins for preparation of more complex systems [1-12]. Self-assembly of porphyrins are also possible by coordination of nucleophihc groups to the central metal ion or using compounds having two nucleophihc sites to coordinate two metalloporphyrins [13-22]. These methods have been used not only for the preparation of porphyrin dimers and trimers, but also for the formation of larger supramolecular networks. Despite the apparent simphcity of the preparation methods of multiporphyrin materials by self-assembly, they generally are less stable as compared to covalently bond... 

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