Two Vernier-Templated Routes to a 24-Porphyrin Nanoring

摘要

Ingenious new template-directed strategies make it possible to synthesize an increasingly wide range of molecular architectures, which would otherwise be inaccessible. Recently, we developed the concept of Vernier templat-ing—the use of noncommensurate combinations of templates and building blocks to direct the formation of cyclic oligomers, such that the number of binding sites in the product is the lowest common multiple of the numbers of sites in the template and the starting material. This principle allows a small template to direct the formation of a much larger macrocycle. Previously, we illustrated this idea with the synthesis of a 12-porphyrin nanoring, c-P12, from a linear zinc porphyrin tetramer, l-P4, and a hexapyridyl template, T6. Here we report the synthesis of a 24-porphyrin ring, c-P24, by two Vernier-templated routes: a) coupling the linear porphyrin octamer l-P8 in the presence of hexapyridyl template T6, and b) coupling the linear porphyrin hexamer Z-P6 in the presence of the octapyridyl template T8 (Scheme 1). The 24-porphyrin nanoring can be prepared in yields of up to 25 %. It has a diameter of 10 nm, as confirmed by scanning tunneling microscopy (STM), and a molecular weight of 26 kDa, thus placing it in the size range of a typical protein. This flexible macrocycle can be locked into a planar it-conjugated conformation by the cooperative self-assembly of a 2:24 double-strand complex with the linear diamine ligand 1,4-diazabicyclo[2.2.2]octane(DABCO).