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Structural and binding features of cofacial bis-porphyrins with calixarene spacers: pac-man porphyrins that can chew.

Based on the efficient combination of calixarene spacers and acetylenic porphyrin derivatives, a new generation of cofacial bis-porphyrins has been synthesized. The first crystal structure of a cofacial bis-porphyrin-calixarene conjugate is reported. Their unique architectural features, analogous to those of pac-man-type bis-porphyrins, allow these calixarene-porphyrin conjugates to adapt their shape to the size of bidentate guests, such as diazabicyclo[2.2.2]octane (dabco) and 1,4-pyrazine. The predefined, cofacial arrangement of the porphyrin moieties observed in the solid state and in solution results in extremely high affinities (in the range of 10(9) M(-1)) for these guests. The 1,3-alternate calixarene conformations afford "open-mouth" pac-man structures whose ability to bite on nitrogen bidentates depends on their functionalization. A cone conformer provides a much more flexible structure that exhibits the highest affinity for dabco and pyrazine.