Template-Directed Synthesis of Molecular Nanorings and Cages

Abstract

This Account is about templates as construction tools: molecules for making molecules. A template organizes the reactants and provides information to promote formation of a specific product, but it is not part of the final product. We have developed many different strategies for using oligopyridines as templates for the synthesis of alkyne-linked π-conjugated metalloporphyrin oligomers. These compounds include some of the largest macrocycles ever synthesized, such as a 50-porphyrin ring with a diameter of 21 nm containing a ring of 750 C-C bonds. Metalloporphyrins are excellent models for exploring template directed synthesis, as they can be functionalized in many different positions and the central metal (typically Zn or Mg) provides a handle for coordination to templates. Classical template-directed macrocyclization reactions have a 1:1 complementarity between the template and the product. This strategy works well for preparing nanorings of 5-7 porphyrin units, but larger templates are laborious to synthesize. Rings of 8 or more porphyrin units are most easily prepared using "nonclassical" strategies, in which several small templates work together to direct the formation of a large ring. In the Vernier approach, a mismatch between the number of binding sites on the template and the building block leads to a mathematical amplification of the length scale: the number of binding sites in the product is the lowest common multiple of those in the template and the building block. For example, a 40-porphyrin ring can be prepared by coupling a linear decamer in the presence of an octadentate template. Linear Vernier templating opens up intriguing possibilities for self-replication. When several small radial oligopyridine templates bind inside a large nanoring they can form complexes with some vacant coordination sites that display correlated motion like the caterpillar tracks of a bulldozer. These caterpillar track complexes can be used in template-directed synthesis and they provide the most convenient route to 8- and 10-porphyrin rings. Russian doll complexes provide another strategy for template-directed synthesis: a number of specifically designed ligands bind to a central nanoring to form a template for constructing a larger concentric nanoring. The same oligopyridine templates that are used to prepare nanorings can also be used to synthesize three-dimensional nanotubes and nanoballs. Again, nonclassical approaches, in which several small templates work together cooperatively, are much simpler than creating a single large template with sufficient binding sites to define the whole geometry of the product. Oligopyridine ligands can also be used as shadow mask templates to control the demetalation of magnesium porphyrin nanorings, because metal centers that are not coordinated by the template can be selectively demetalated with acid. Thus, the template forms a permanent shadow on the porphyrin nanostructure that remains after the template has been removed. Shadow mask templates provide a simple route to heterometalat...

The Supramolecular Chemistry of Strained Carbon Nanohoops

“…Strained nanohoops of an entirely different design comprise meso ‐substituted porphyrins instead of the phenylene units found in CPPAs and CPPs . Anderson and co‐workers have successfully used oligopyridine templates to prepare porphyrin nanorings such as c ‐P5 (Figure ) by oxidative Glaser reactions. Since even a templated ring‐closing step is typically not suitable for the generation of very high strain energies, c ‐P5 may represent the limit of this approach.…”

Multiple‐Porphyrin Functionalized Hexabenzocoronenes

“…Hexaphenylbenzenes (HPBs) and their oxidized derivatives hexa‐ peri ‐hexabenzocoronenes (HBCs) have received tremendous attention during the past 20 years in materials chemistry, for example in molecular electronics and nonlinear optics . They have proven to be a suitable scaffold for catalytic applications, the design of supramolecular architectures, as well as a perfectly sized template for the preparation of nano‐ring and nano‐ball structures. In contrast, porphyrins and related macrocycles, which are found in nature in different appearances such as heme, chlorophyll, or bacteriochlorophyll, have attracted the attention from the scientific community ever since their discovery.…”

“…Hexaphenylbenzenes (HPBs) and their oxidized derivatives hexa-peri-hexabenzocoronenes (HBCs)h ave received tremendous attention during the past 20 years in materials chemistry, for example in moleculare lectronics and nonlinearo ptics. [1][2][3][4][5] They have provent obeasuitable scaffold for catalytic applications, [6,7] the design of supramolecular architectures, [8,9] as well as ap erfectly sized template fort he preparation of nanoring [10,11] and nano-ball [12] structures.I nc ontrast, porphyrins and related macrocycles,w hich are foundi nn ature in different appearances such as heme, chlorophyll,o rb acteriochlorophyll, have attracted the attention from the scientific community ever since their discovery.D ue to their photophysical charac-teristics, featuring high extinction coefficients in the visiblelight region, porphyrins have provent ob ea ni deal chromophoref or light-harvesting architectures. [13][14][15][16][17][18][19][20][21] Thec ombination of both buildingb locks, porphyrins and HPBs, was initiated in 2002 by the synthesis of aH PB core, surrounded by six directly meso-attached porphyrins.…”

Templating Porphyrin Anisotropy via Magnetically Aligned Carbon Nanotubes

“…[54][55][56][57][58] While a number of efforts in constructing nanostructures through the organization of porphyrins have been achieved, their organisation over multiple scales (from the nanoscale to the microscale) remains of great interest owing to their potentials in light-harvesting applications. [59][60][61][62][63] Although one-or two-dimensional supramolecular porphyrin systems are now easily prepared, the construction of three-dimensional assemblies remains a challenge. [64][65][66] While porphyrin lowdimensional oligomers have already been prepared, both by the templated [67][68][69][70][71] and non-templated approaches, [72,73] we strived to macroscopically arrange porphyrin structures through magnetically aligned multi-walled carbon-nanotubes (MWCNTs).…”