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Tuning strained alkyne reactivity via organic synthesis has evolved into a burgeoning field of study largely focused on cyclooctyne, wherein physical organic chemistry helps guide rational molecular design to produce molecules with intriguing properties. Concurrent research in the field of carbon nanomaterials has produced new types of strained alkyne macrocycles, such as cycloparaphenyleneacetylenes, that possess uniquely curved aromatic π systems but hover on the edge of stability. In 2018, we introduced a strained alkyne scaffold that marries the synthetic accessibility and stability of cyclooctyne with the curved π system of carbon nanomaterials. These molecules are strained alkyne-containing cycloparaphenylenes (or [n+1]CPPs), which have been shown to possess size-dependent reactivity as well as the classic characteristics of the unfunctionalized parent CPP, such as a tunable HOMO–LUMO gap and bright fluorescence for large sizes. Herein, we elaborate further on this scaffold, introducing two modifications to the original design and fully characterizing the kinetics of the strain-promoted azide–alkyne cycloaddition (SPAAC) for each [n+1]CPP with a model azide. Additionally, we explain how electronic (the incorporation of fluorine atoms) and strain (a meta linkage which heightens local strain at the alkyne) modulations affect SPAAC reactivity via the distortion–interaction computational model. Altogether, these results indicate that through a modular synthesis and rational chemical design, we have developed a new family of tunable and inherently fluorescent strained alkyne carbon nanomaterials.
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Pinwheel-like Curved Aromatics from the Cyclotrimerization of Strained Alkyne Cycloparaphenylenes
Curved aromatic hydrocarbons often display better solubility and more desirable electronic properties in comparison to their flat counterparts. Macrocyclic curved aromatics possess these traits as well as shape-persistent pores ideal for host-guest interactions. A quintessential macrocyclic curved aromatic molecule is the cycloparaphenylene, or [n]CPP. Our group has developed a new class of these carbon nanohoops, called [n+1]CPPs, that incorporate a strained alkyne (“+1”) into the carbon backbone. We have previously shown the [n+1]CPPs to be a promising new class of strain-promoted azide-alkyne cycloaddition click reagents. Herein, we show that the [n+1]CPPs can also be converted into pinwheel-like multi-pore large molecules via a straightforward and high yielding metal-mediated alkyne cyclotrimerization reaction. We provide insight into suitable metals for this transformation, the photophysics of these trimeric molecules, as well as their strain profiles and crystal packing.
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- Award ID(s):
- 2102567
- PAR ID:
- 10565031
- Publisher / Repository:
- ChemRxiv
- Date Published:
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract A new class of macrocyclic angle‐strained alkynes whose size and reactivity can be precisely tuned by modular organic synthesis is disclosed. Detailed analysis of the size‐dependent structural and electronic properties provides evidence for considerable distortion of the alkyne units incorporated into the cycloparaphenylene (CPP)‐derived macrocycles. The remarkable increase of the alkyne reactivity with decreasing macrocycle size in [2+2]cycloaddition–retrocyclization was investigated by joint experimental and theoretical studies and the thermodynamic and kinetic parameters that govern this reaction were unraveled. Additionally, even the largest, least strained macrocycle in this series was found to undergo strain‐promoted azide–alkyne cycloaddition (SPAAC) efficiently under mild conditions, thereby paving the way to the application of alkyne‐containing CPPs as fluorescent “clickable” macrocyclic architectures.more » « less
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Abstract Mechanically interlocked molecules (MIMs) represent an exciting yet underexplored area of research in the context of carbon nanoscience. Recently, work from our group and others has shown that small carbon nanotube fragments—[n]cycloparaphenylenes ([n]CPPs) and related nanohoop macrocycles—may be integrated into mechanically interlocked architectures by leveraging supramolecular interactions, covalent tethers, or metal‐ion templates. Still, available synthetic methods are typically difficult and low yielding, and general methods that allow for the creation of a wide variety of these structures are limited. Here we report an efficient route to interlocked nanohoop structures via the active template Cu‐catalyzed azide‐alkyne cycloaddition (AT−CuAAC) reaction. With the appropriate choice of substituents, a macrocyclic precursor to 2,2′‐bipyridyl embedded [9]CPP (bipy[9]CPP) participates in the AT−CuAAC reaction to provide [2]rotaxanes in near‐quantitative yield, which can then be converted into the fully π‐conjugated catenane structures. Through this approach, two nanohoop[2]catenanes are synthesized which consist of a bipy[9]CPP catenated with either Tz[10]CPP or Tz[12]CPP (whereTzdenotes a 1,2,3‐triazole moiety replacing one phenylene ring in the [n]CPP backbone).more » « less
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Strain has a unique and sometimes unpredictable impact on the properties and reactivity of molecules. To thoroughly describe strain in molecules, a computational tool that relates strain energy to reactivity by localizing and quantifying strain was developed. Strain energy is calculated local to every coordinate in the molecule and areas of higher strain are shown experimentally to be more reactive. Not only does this tool directly compare strain energy in parts of the same molecule, but it also computes total strain to give a full picture of molecular strain energy. It is freely available to the public on GitHub under the name StrainViz and much of the workflow is automated to simplify use for non-experts. Unique insight into the reactivity of curved aromatic molecules and strained alkyne bioorthogonal reagents is described within.more » « less
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Abstract Cycloparaphenylenes (CPPs) are the smallest possible armchair carbon nanotubes, the properties of which strongly depend on their ring size. They can be further tuned by either peripheral functionalization or by replacing phenylene rings for other aromatic units. Here we show how four novel donor–acceptor chromophores were obtained by incorporating fluorenone or 2‐(9H‐fluoren‐9‐ylidene)malononitrile into the loops of two differently sized CPPs. Synthetically, we managed to perform late‐stage functionalization of the fluorenone‐based rings by high‐yielding Knoevenagel condensations. The structures were confirmed by X‐ray crystallographic analyses, which revealed that replacing a phenylene for a fused‐ring‐system acceptor introduces additional strain. The donor–acceptor characters of the CPPs were supported by absorption and fluorescence spectroscopic studies, electrochemical studies (displaying the CPPs as multi‐redox systems undergoing reversible or quasi‐reversible redox events), as well as by computations. The oligophenylene parts were found to comprise the electron donor units of the macrocycles and the fluorenone parts the acceptor units.more » « less
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Posted Content:
- https://doi.org/10.26434/chemrxiv-2024-lljcg
