We present the first helicene carbon nanoohop that integrates a [6]helicene into [7]cycloparaphenylene. The [6]helicene endows the helicene carbon nanohoop with chiroptical properties and configurational stability typical for higher helicenes, while the radially conjugated seven para-phenylenes largely determine the optoelectronic properties. The structure of the helicene carbon nanoohop was unambiguously characterized by NMR, MS and X-ray analysis that revealed that it possesses a topology of a Möbius strip in the solid state and in solution. The chirality transfers from the [6]helicene to the para-phenylenes and leads to a pronounced circular dichroism and bright circularly polarized luminescence, which is affected by the structural topology of the nanohoop.
The two sulfonyl-bridged Geländer
helices 1a and 2a are obtained by oxidation
of the corresponding
sulfide bridged precursors 1b and 2b. Both
Geländer structures are fully characterized by NMR, high-resolution
mass spectrometry, and optical spectroscopies. X-ray diffraction with
a single crystal of 2a provides its solid-state structure.
Both Geländer helices 1a and 2a are
separated into enantiomers, and their racemizations are monitored
by circular dichroism. For 1a, consisting of two equally
sized macrocycles, a substantial increase in the enantiomerization
barrier is observed upon going from the sulfide to the sulfone, and
only a subtle rise is detected for the constitutional isomer 2a with two macrocycles of different size during the same
transformation. This results not only in 1a with the
highest configurational stability in the series of hitherto investigated
Geländer structures but also challenges the so far hypothesized
correlations between bridging structures and the Gibbs free energy
of enantiomerization. The simulation of the enantiomerization process
in the macrocyclic subunits suggests the proximity of the endotopic
hydrogens as parameter responsible for the heights of the enantiomerization
barrier.
The bowl-shaped, 3-fold interlinked porphyrin dimer 2 was obtained in respectable yields during macrocyclization attempts. Its bicyclic structure, consisting of a macrocycle made of a pair of acetylene interlinked tetraphenylporphyrins which are additionally linked by a C−C bond interlinking two pyrrole subunits, has been confirmed spectroscopically (2D-NMR, UV/vis, HR-MALDI-ToF MS). Late-stage functionalization provided the structural analogue 1 with acetylprotected terminal thiol anchor groups enabling single molecule transport investigations in a mechanically controlled break junction experiment. The formation of single-molecule junctions was observed, displaying large variations in the observed conductance values pointing at a rich diversity in the molecular junctions.
We present the first helicene carbon nanoohop that integrates a [6]helicene into [7]cycloparaphenylene. The [6]helicene endows the helicene carbon nanohoop with chiroptical properties and configurational stability typical for higher helicenes, while the radially conjugated seven para-phenylenes largely determine the optoelectronic properties. The structure of the helicene carbon nanoohop was unambiguously characterized by NMR, MS and X-ray analysis that revealed that it possesses a topology of a Möbius strip in the solid state and in solution. The chirality transfers from the [6]helicene to the para-phenylenes and leads to a pronounced circular dichroism and bright circularly polarized luminescence, which is affected by the structural topology of the nanohoop.
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