[[File:Cycloparaphenylene_3D_Model.stl|thumb|Interactive 3D model of [5]cycloparaphenylene]]
[[File:Cycloparaphenylene-3D-spacefill.png|thumb|Space-filling model of an [8]cycloparaphenylene molecule.]]
A '''cycloparaphenylene''' is a molecule that consists of several [[benzene ring]]s connected by covalent bonds in the [[arene substitution pattern|para]] positions to form a hoop- or necklace-like structure. Its [[chemical formula]] is {{chem2|[C6H4]_{''n''}|}} or {{chem|C|6''n''|H|4''n''}}  Such a molecule is usually denoted '''['''''n''''']CPP''' where ''n'' is the number of benzene rings. Its [[chemical formula]] is {{chem2|[C6H4]_{''n''}|}} or {{chem|C|6''n''|H|4''n''}}. [[File:CNT CPP.png|thumb|An "armchair" carbon nanotube and a cycloparaphenylene molecule.<ref name=evans2014/>]]

A cycloparaphenylene can be considered as the smallest possible armchair [[carbon nanotube]]. For this reason, they belong to a class of molecules known as [[Carbon nanohoop|carbon nanohoops]].<ref name=lbl/> Cycloparaphenylenes are challenging targets for [[chemical synthesis]] due to the [[ring strain]] incurred from forcing benzene rings out of planarity.

== History ==
In 1934 by [[V. C. Parekh]] and [[P. C. Guha]] described the first published attempt to synthesize a cycloparaphenylene, specifically [2]CPP.  They connected two aromatic rings with a [[sulfide bridge]], and hoped that removal of the latter would yield the desired compound. However, the attempt failed as the compound would have been far too strained to exist under anything but extreme conditions.<ref name=pare1934/>

[[File:Product S.png|thumb|First attempts at a CPP<ref name=rolf1993/>]]
By 1993, Fritz Vögtle attempted to synthesize the less-strained [6]CPP and [8]CPP by the same approach.  He produced a hoop of [[phenyl ring]]s, bridged together by a sulfur atom. However, his attempts to remove the sulfur failed too. They also synthesized a [[macrocycle]] that upon dehydrogenation would yield a CPP, but could not perform this final step.<ref name=miya1983/>.<ref name=rolf1993/> 

In the year 2000, Chandrasekhar and others concluded, by computational analysis, that [5]CPP and [6]CPP should be significantly different in their aromaticity.<ref name=mavi2000/> However, the synthesis in 2014 of [5]CPP refuted this conclusion.<ref name=evans2014/><ref name=bodw2014/><ref name=kaya2014/><ref name=wong2009/> 

In 2008 the first cycloparaphenylenes were synthesized by [[Ramesh Jasti]] during his post doctoral research in the lab of [[Carolyn Bertozzi]]. He used [[cyclohexa-1,4-diene]]s which are closer in oxidation state to the desired phenylene than the cyclohexanes used previously by Vögtle. The first cycloparaphenylenes that were reported and characterized were: [9]CPP, [12]CPP, and [18]CPP.<ref name=iwam2011/> In 2009, the [[Kenichiro Itami|Itami]] group would report the selective synthesis of [12]CPP, and shortly thereafter Yamago synthesized [8]CPP in 2010.<ref name=yama2010d/><ref name=iwam2011e/> The Jasti Group then synthesized all increasingly smaller CPPs using new methodology that allowed [7]CPP,<ref name=sisto2011/> [6]CPP,<ref name=xia2012/> and finally [5]CPP<ref name=evans2014/> to be reported in relatively quick succession.

== Properties ==

=== Structure ===
The normal configuration of each phenylene element would be planar, with the bonds in the para position pointing opposite to each other in a straight line.  Therefore, the cycloparaphenylene molecule is strained, and the strain increases as the number of units decreases. The strain energy of [5]CPP was calculated as 117.2 kcal/mol. In spite of the strain, the phenyl rings retain their aromatic character, even in the [5]CPP.<ref name=evans2014/><ref name=oma2010/> However, as the size of the CPP decreases the HOMO-LUMO gap also decreases. This trend opposite to that observed in linear [[polyparaphenylene]]s where the HOMO-LUMO gap decreases as size increases.<ref name=wong2009/><ref name=iwam2011e/> This causes a red-shift of the fluorescent emission.<ref name=wong2009/>

=== Solid-state packing ===
Cycloparaphenylenes with 7 to 12 rings all adopt a [[herringbone pattern|herringbone]]-like packing in the solid state. A similar but denser structure was observed for [5]CPP, whereas [6]CPP forms columns.<ref name=xia2012/> This columnar packing structure has been of interest due to a potentially high internal surface area. By partial fluorination, it was found that this packing geometry could be engineered.<ref name=leon2018/>

== Synthesis ==
There are three main methods used for cycloparaphenylene synthesis.

===Suzuki Coupling of Curved Oligophenylene Precursors===
In the initial synthesis, cycloparaphenylenes with ''n'' = 9, 12, and 18 have been synthesized starting from macrocycles containing 1,4-syn-dimethoxy-2,5-cyclohexadiene units as masked aromatic rings. Lithium–halogen exchange with [[p-diiodobenzene]] followed by a two-fold [[nucleophilic addition]] reaction with [[1,4-benzoquinone]] yielded a syn-cyclohexadiene moiety. [[Borylation]] of this material followed macrocyclization under Suzuki–Miyuara cross-coupling with an equivalent of the diiodide produced macrocycles in low yields which could be separated by column chromatography. These macrocycles were then reductively aromatized using sodium naphthalenide to yield [''n'']cycloparaphenylenes. Since this initial synthesis uses symmetric building blocks it is challenging to use it to make smaller CPPs. Therefore, instead of benzoquinone, benzoquinone monomethyl ketal was used to allow the use of asymmetric building blocks. This innovation allowed the selective synthesis of [12]CPP to [5]CPP.<ref name=darzi2012/>

[5]CPP is synthesized with an intramolecular boronate [[homocoupling]] technique that was originally seen as an undesired by-product of Suzuki-Miyaura cross-coupling reactions in the synthesis of [10]CPP.<ref name=evans2014/><ref name=oma2010/>
Cycloparaphenylenes now have selective, modular, and high yielding synthetic pathways.

===Reductive Elimination of Platinum Macrocycles===
A quicker route to [8-13]CPPs starts by selectively building [8]CPP and [12]CPP from the reaction of 4,4′-bis(trimethylstannyl)biphenyl and 4,4′ ′-bis(trimethylstannyl)terphenyl, respectively, with Pt(cod)Cl2 (where cod is [[1,5-cyclooctadiene]]) through square-shaped tetranuclear platinum intermediates.<ref name=iwam2011e/> A mixture of [8-13]cycloparaphenylenes can be obtained in good combined yields by mixing [[biphenyl]] and [[terphenyl]] precursors with the platinum sources.<ref name=iwam2011e/>

===Alkyne Cyclotrimerization===

A third lesser used method developed in the Tanaka group uses rhodium catalyzed alkyne cyclotrimerization for the synthesis of cycloparaphenylenes.<ref name=haya2017/>

== Potential applications ==

Potential applications of cycloparaphenylenes include [[host–guest chemistry]],<ref name=iwam2011/>  seeds for [[carbon nanotube]] growth, and hybrid [[nanostructure]]s containing nanohoop-type substituents.<ref name=li2017/> A cycloparaphenylene can be seen as minimal single-walled carbon nanotube of the armchair type.  As such, a cycloparaphenylene may be a seed for synthesis of longer nanotubes.<ref name=iwam2011/><ref name=iwam2011e/><ref name=lews2015/>  Their electronic properties may also be useful.<ref name=gold2013/><ref name=zabu2013/>

=== Fullerene binding ===
The [[π-π interaction]]s and the concave interior of the cycloparaphenylenes is expected to bind well with π conjugated systems with convex surfaces. In 2011, the Yamago group found that [10]cycloparaphenylene can selectively bind [[C60 fullerene]]s. The fluorescence of [10]CPP is quenched upon complexation with C60, which suggests its potential as a C60 sensor.<ref name=iwam2011/>

Cycloparaphenylenes have shown affinity to fullerenes and other carbonaceous,<ref name=iwam2011/> creating 'pseudo' [[carbon peapods]]. Potential applications of these structures include nanolasers, single electron transistors, spin-qubit arrays for quantum computing, nanopipettes, and data storage devices.<ref name=sci2001/><ref name=kwon1999/><ref name=utko2006/>

In 2013 the Isobe Group built on Yamago's work by producing the "molecular bearing" by capturing a [[Buckminsterfullerene|C60 fullerene]] within a [10]CPP.<ref name=matsu2013/> Yamago had the first C60 interaction with a CPP, but the Isobe group was the first that met the criteria to be called a bearing, specifically they developed a method in which the fullerene remained in the ring long enough to be observed on the NMR timescale.<ref name=matsu2013/>

In related work, [[Kenichiro Itami|Itami]] and Sinohara examined endohedral [[metallo-fullerene]]s and their intermolecular ball-in-hoop interactions. An endohedral metallo-fullerene is an electronegative fullerene containing a positively charged metal ion in its core, so it was expected to have stronger interactions with the CPP.<ref name=iwam2014/><ref name=lews2015/> This also provided a desired method to separated metallo-fullerenes, which may help the study of metallo-fullerene electronic properties. The metal containing fullerenes, when interacting with [12]CPP, had decreased solubility in toluene, which allowed for the metallo-fullerenes to be selectively precipitated out and collected.<ref name=matsu2013/>

CPPs are known to bind [[fullerene]]s,<ref name=iwam2011/> and in 2018 were used to create CPP-fullerene [[rotaxane]]s.<ref name=xu2018/>

== Related compounds ==
As the synthesis of CPPs has become easier, derivative structures have begun to be synthesized as well. In 2013 the [[Kenichiro Itami|Itami]] group reported the synthesis of a nanocage made completely of benzene rings. This compound was especially interesting because it could be viewed as a junction of a branched nanotube structure.<ref name=katsu2012/>

Other [[chiral]] derivatives of cycloparaphenylenes (which may serve as chemical templates for synthesizing chiral nanotubes) have also been characterized. Similar to the original (n,n) cycloparaphenylenes, these chiral nanorings also exhibit unusual optoelectronic properties with excitation energies growing larger as a function of size; however, the (n+3,n+1) chiral nanoring exhibits larger photoinduced transitions compared to the original (n,n) cycloparaphenylenes, resulting in more readily observable optical properties in spectroscopic experiments.<ref name=wong2011/>

In 2012 the [[Ramesh Jasti|Jasti]] Group reported the synthesis of dimers of [8]CPP linked by arene bridges.<ref name=xia2012c/> This synthesis was followed two years later by the synthesis of a directly connected dimer of [10]CPP from chloro[10]CPP by the [[Kenichiro Itami|Itami]] group.<ref name=ishii2014/>

=== Donor–acceptor functionalization ===
CPPs are unique in that their donor–acceptor properties can be adjusted with the addition or removal of each phenyl ring. In the all-carbon nano-hoop systems a reduction in width corresponds to a higher [[HOMO]] and a lower [[LUMO]]. Additional donor–acceptor selectivity was observed by the addition of an aromatic heterocycles into the larger ring. N-methylaza[n]CPP showed that a lowering of the LUMO could be enhanced by decreasing the ring size, while the HOMO energy level remained the same.<ref name=rade2016/>

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</references>

[[Category:Cyclophanes]]
[[Category:2008 in science]]