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Abstract
Strapped or “basket-handle” porphyrins have been investigated previously as hemoglobin mimics and catalysts. The facial selectivity of their interactions with axial ligands is a sensitive test for noncovalent bonding. Here the binding of pyridyl ligands to zinc porphyrins with thioester-linked alkyl straps is investigated in solution by NMR spectroscopy and UV–vis titration, and in the solid state by X-ray crystallography. We expected that coordination of the axial ligand would occur on the less hindered face of the porphyrin, away from the strap. Surprisingly, attractive interactions between the strap and the ligand direct axial coordination to the strapped face of the porphyrin, except when the strap is short and tight. The strapped porphyrins were incorporated into π-conjugated cyclic porphyrin hexamers using template-directed synthesis. The strap and the sulfur substituents are located either inside or outside the porphyrin nanoring, depending on the length of the strap. Six-porphyrin nanorings with outwardly pointing sulfur anchors were prepared for exploring quantum interference effects in single-molecule charge transport.
Highlights
IntroductionResearch in the field of molecular electronics was initially focused on the miniaturization of electronic components, such as transistors, to probe the limits of Moore’s law.[1,2] it was soon realized that, since molecular devices follow the laws of quantum physics, fundamentally new properties and functions may be achieved in these systems.[3,4] Recently much effort has been devoted to the exploration of quantum interference (QI) effects in single-molecule electronics.[5]
Besides the early strategy based on the preparation of a strapped bis-dipyrromethane derivative,[21,23] the main synthetic routes to porphyrins strapped across the 5,15-meso positions are (i) preforming the strap as a bridged dialdehyde, which is reacted with a dipyrromethane to form the strapped porphyrin directly,[26,27,33,50] or (ii) preparing a nonstrapped porphyrin with tailored functional groups on 5,15-meso positions and reacting it with a bifunctional bridge to add the strap.[25,30,32]
We present the design and synthesis of new porphyrins with linear hydrocarbon straps of different lengths linked to a 5,15
Summary
Research in the field of molecular electronics was initially focused on the miniaturization of electronic components, such as transistors, to probe the limits of Moore’s law.[1,2] it was soon realized that, since molecular devices follow the laws of quantum physics, fundamentally new properties and functions may be achieved in these systems.[3,4] Recently much effort has been devoted to the exploration of quantum interference (QI) effects in single-molecule electronics.[5]. There are several possible manifestations of QI, and most experimental studies focus on destructive interference in crossconjugated molecular wires.[6−8] Another form of QI arises from charge transport through two or more spatially separated paths through the same single-molecule junction. If the charge transport is in-phase and coherent constructive interference will lead to enhanced conductance.[5] For example, a symmetric conjugated cyclic molecule diametrically bridged to gold electrodes in a scanning tunneling microscope (STM)
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