Porphyrins molecular boxes

The molecular box 14 contains two types of chromophores, zinc-porphyrin and free-base porphyrin. The behaviors of the monomeric models of these units, Zn and Fb, have been summarized in Sect. 4.1. As expected for supramolecular species, the absorption spectrum of the box is a good superposition of those of the molecular components (Fig. 22). The energy level diagram for the molecular box, obtained as a combination of those of the Fb and Zn models (Fig. 23), shows a significant driving force for energy transfer from the Zn-porphyrin to the free base units. 

Selective excitation of the free-base chromophore in the molecular box can be carried out at 640 nm, yielding the typical fluorescence of this unit. When excitation is carried out at 569 nm, on the other hand, light is predominantly absorbed by the Zn-porphyrin chromophores. Very weak fluorescence is obtained from this unit, however, whereas intense free-base fluorescence is again observed, clearly indicating the occurrence of singlet energy transfer in the molecular box. The energy transfer process can be directly monitored by... 

Fig. 32 Schematic representation of the molecular box 14 (phenyl groups of the pillar porphyrin are omitted for clarity), and of its antenna effect...

In contrast to the vast assortment of molecular squares now in existence, relatively few examples of molecular boxes or cubes were synthesized, unless one views squares having tall edges (for example, porphyrinic squares) as open-ended flexible boxes. As defined here molecular boxes are right-angle-containing complexes, where the metal coordination units are arranged in three dimensions, instead of two. 

Therefore, the shielding field of the peripheral porphyrins influences even the central part of the template porphyrin. H NMR studies also show that the phenyl substituents are fixed in space they are perpendicular both to the Ru(II) porphyrin mean planes and the central, assembling, free-base porphyrin, forming the rims of a molecular box. In fact, the pentamer really plays the role of a molecular box when the central porphyrin is complexed with Zn(II) and is able to selectively recognize S-bonded Ru(II) complexes of Me.SO. 

Figure 55. A cyclic trimer (97) of the Zn(ll) complex of porphyrin 51, assembled at from-Pt(PEt j) complex metal fragments. The /f-substituents of the porphyrin rings have been omitted for clarity. The resulting molecular box binds guest 98 with a high affinity constant.

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.

Figure 60. A porphyrin trimer (103) made of a covalent molecular box incorporating Zn(ll) porphyrin subunits hosting a third, free-base porphyrin.

The bis-porphyrin supramolecular box 26 has also been assembled using a molecular recognition process involving hydrogen bonding between pendant-arm... 

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