Abstract
Electronic communication between concentric macrocycles with wave functions that extend around their circumferences can lead to remarkable behavior, as illustrated by multiwalled carbon nanotubes and photosynthetic chlorophyll arrays. However, it is difficult to hold one π-conjugated molecular ring inside another. Here, we show that ring-in-ring complexes, consisting of a 6-porphyrin ring locked inside a 12-porphyrin ring, can be assembled by placing different metals in the two rings (zinc and aluminum). A bridging ligand with carboxylate and imidazole binding sites forms spokes between the two rings, resulting in a highly cooperative supramolecular self-assembly process. Excitation is transferred from the inner 6-ring to the outer 12-ring of this Russian doll complex within 40 ps. These complexes lead to a form of template-directed synthesis in which one nanoring promotes formation of a larger concentric homologous ring; here, the effective template is an eight-component noncovalent assembly. Russian doll templating provides a new approach to amplifying the size of a covalent nanostructure.
Highlights
Hierarchical ring-in-ring assemblies, such as Russian doll complexes, have intrigued supramolecular chemists for over a decade.[1−4] Russian dolls are nested assemblies of self-similar objects such that the shells form a homologous series
Russian dolls consisting of concentric π-conjugated macrocycles, or rings of chromophores, are fascinating systems in which to study the flow of electronic excitation and charge.1f,5 We and others have recently reported the synthesis of π-conjugated macrocycles consisting of covalently linked porphyrin units that exhibit ultrafast energy migration and that mimic the photophysical behavior of natural photosynthetic antenna systems.[6−10] In purple bacteria, sunlight is captured by two circular light harvesting (LH) complexes, LH1 and LH2, which are rings of chlorophyll molecules
Excitation energy is funneled from LH2 to LH1, where it is transferred to the reaction center (RC), which is nested within the LH1 ring.[11]
Summary
Hierarchical ring-in-ring assemblies, such as Russian doll complexes, have intrigued supramolecular chemists for over a decade.[1−4] Russian dolls are nested assemblies of self-similar objects such that the shells form a homologous series. Excitation energy is funneled from LH2 to LH1, where it is transferred to the reaction center (RC), which is nested within the LH1 ring.[11] Here, we demonstrate that two nanorings, consisting of 6- and 12-porphyrin units, respectively, self-assemble in the presence of an appropriately designed bridging ligand to exclusively yield a Russian doll complex reminiscent of the LH1-RC architecture (Figure 1). This nine-component self-assembly process involves four types of subunits, which can be mixed together in any order. Russian doll templating provides a way to escalate the size of these covalent nanostructures
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