Assemblies Mediated by External Metal Centers

One of the essential impetuses to cyclic dye architectures is the development of artificial model systems of the light-harvesting complexes in purple bacteria, where cychc arrays of chromophores provide the fundamental structural feature for their functionahty [2], On the other hand, cavities of variable sizes and shapes are accessible upon the formation of macrocycles. The assemblies of this kind, thus, possess also potentials in substrate binding, molecular recognition, matter transportation and even supramolecular catalysis. 

Based on the concept of Drain and Lehn, structmally more reliable palladium and platimun-mediated square assemblies of porphyrins such as 3 could be prepared by Stang and coworkers [35-37]. The reaction of linear or right-angle ditopic porphyrin ligands with dppp chelated Pd(II) or Pt(II) complexes at room temperature afforded the respective metallosquares in essentially quantitative yields. The square structures were unambiguously confirmed by H and NMR, elemental analysis, and in some cases even by electrospray and FAB mass spectrometry. Because of the high bond strength 

The undesired fluorescence quenching could be overcome in perylene bisimide-based squares 7, which show fluorescence quantum yields of almost unity in chloroform [46,47]. The square assemblies were quantitatively accessible by mixing an equimolar amount of perylene bispyridyl imide Ugands and corresponding metal corner units in dichloromethane at room temperature. Because the pyridyl moieties are located at the imide positions of the 

The same perylene bisimide scaffold was used to construct multichro-mophoric squares 8, which contain 16 additional antenna dye imits tethered to the tetrameric perylene-based square (Fig. 2) [48- 50]. Such metaUo-supramolecular architectures are reminiscent of the cyclic dye assemblies of light harvesting bacteria, where energy transfer from outer antenna dyes to the central reaction center is essential for efficient capture of solar light. In- 

In a similar fashion, four-fold ferrocenyl-functionalized perylene bipyridyl imide dyes were assembled to square array 8c [50]. The assemblies contain in total 20 redox active imits, including 4 perylene bisimide and 16 ferrocene moieties. Due to the twisted perylene backbone, the ferrocenyl units are divided into two groups from the viewpoint of their spatial location referred to the square cavity (cf. model in Fig. 2). Indeed, the redox behavior of the ferrocenyl units depends on their spatial arrangement as established by cyclic voltammetric investigation [50]. 

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