Molecular hosts porphyrin

Figure 57. A porphyrin pentamer (101) made of a molecular box based on coordination of the Zn(ll) complex of porphyrin 95 to fac-Re(CO)3CI complex metal fragments, which hosts porphyrin 61.

Macrocyclic receptors made up of two, four or six zinc porphyrins covalently connected have been used as hosts for di- and tetrapyridyl porphyrins, and the association constants are in the range 105-106 M-1, reflecting the cooperative multipoint interactions (84-86). These host-guest complexes have well-defined structures, like Lindsey s wheel and spoke architecture (70, Fig. 27a), and have been used to study energy and electron transfer between the chromophores. A similar host-guest complex (71, Fig. 27b) was reported by Slone and Hupp (87), but in this case the host was itself a supramolecular structure. Four 5,15-dipyridyl zinc porphyrins coordinated to four rhenium complexes form the walls of a macrocyclic molecular square. This host binds meso-tetrapyridyl and 5,15-dipyridyl porphyrins with association constants of 4 x 107 M-1 and 3 x 106 M-1 respectively. 

Metalated container molecules can be viewed as a class of compounds that have one or more active metal coordination sites anchored within or next to a molecular cavity (Fig. 2). A range of host systems is capable of forming such structures. The majority of these compounds represent macrocyclic molecules and steri-cally demanding tripod ligands, as for instance calixarenes (42), cyclodextrins (43,44), and trispyrazolylborates (45-48), respectively. In the following, selected types of metalated container molecules and their properties are briefly discussed and where appropriate the foundation papers from relevant earlier work are included. Porphyrin-based hosts and coordination cages with encapsulated metal complexes have been reviewed previously (49-53) and, therefore, only the most recent examples will be described. Thereafter, our work in this field is reported. 

The cucurbit [n]uril family (CB[n]) of molecular containers possess remarkable binding affinities and selectivities (Ka values up to 1012M-1, Krei values up to 106) which renders them useful as a component of molecular machines, sensors, and biomimetic systems (123-125). Recently, Wagner and coworkers have reported (126) that CB[10] - with its spacious 870A3 cavity - is capable of acting as a host for free base and metalated tetra (Af-methylpyridinium)porphyrins 19a-d (Fig. 17). Despite the large ellipsoidal deformation of CB[10] upon complexation, the complexed porphyrins retain their fundamental UV/VIS, fluorescence, and electrochemical properties. The CB[ 10] porphyrin... 

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