Cycloparaphenylene

Armchair CNT[1]

Cycloparaphenylenes (CPPs) are cylindrical molecules made of para-linked benzene rings in a hoop-like structure. For this reason they are sometimes referred to as carbon nanohoops, especially because they can be viewed as a fragment of an armchair carbon nanotube (CNT). CPPs represent a challenging target to synthetic chemists due to the ring strain incurred from forcing benzene rings out of planarity.

The first CPPs were synthesized in the research group of Carolyn Bertozzi in 2008.[2] This first publication reported the synthesis of [18]-, [12]-, and [9]- CPP. Since the publication of the initial synthesis of CPPs there has been an increase of interest in this area and there are currently synthetic methods for making CPPs of various sizes.

CPPs have potential application in several areas of chemistry including as materials, in host-guest chemistry,[3] and as seeds for carbon nanotube growth. Hybrid structures containing nanohoop-type substituents have also shown some recent promise.[4]

History

The history of cycloparaphenylenes (CPPs) could be considered to have begun in 1934 when Perekh and Guha sought to make the smallest and simplest form of CPP, [2]CPP, by connecting two aromatic rings with a sulfide bridge.[5] Upon desulfonation they hoped that the two rings would bind together in a layered formation. However, their attempt failed as the compound would have been far too strained to exist under anything but extreme conditions. Following Perekh and Guha, few serious endeavors to bring together linked aromatic rings were made for many years.

First attempts at a CPP[6]

By 1993, Fritz Vogtle had laid the groundwork that would later be applied to the formation of cycloparaphenylenes, namely his review titled “On The Way to Macrocyclic Paraphenylenes”.[6] In his review he proposed routes toward the formation of CPPs using similar methods as Perekh and Guha, however he chose to increase the number of aromatic links in the phenyl chain, [6]CPP and [8]CPP to alleviate the strain problem. This attempt was successful in bringing together a cycle of phenyl rings, each bridged together by a sulfur atom; however, all desulfonation attempts were unsuccessful and the illusive CPP was not obtained. They shifted their approach toward forming a macrocycle that could be dehydrogenated providing the CPP product, this too was unsuccessful.[7]

In the year 2000, computational work had been evaluated by Chandrasekhar, et al., wherein it was found that the characteristics of 5 and 6 membered cycloparaphenylenes, [5]CPP and [6]CPP, should have significantly different properties.[8] The results suggested that [6]CPP would contain full aromatic properties, whereas [5]CPP would have greater quinoidal character.[8] This was found not to be the case upon the formation of [5]CPP in 2014, but it represented the first computational work to contribute to the cycloparapheylene area.[9][1][10][11] The Chandrasekhar team revisited the work with a higher level of computational theory resulting in the actual benzenoid character.[8] It was also computationally postulated that the extended aromatic nature would decrease with a decrease in ring size, which was later confirmed.[9][1][10][11][3]

In 2008 the first cycloparaphenylenes were constructed by Ramesh Jasti and Carolyn Bertozzi using a similar approach as Vogtle’s attempts with cyclohexane linkers.[2] However, Bertozzi and Jasti used cyclohexa-1,4-dienes which provided a rigidity that cyclohexane lacked. The first cycloparaphenylenes that were reported were: [9]CPP, [12]CPP, and [18]CPP, along with their respective characterizations.[3] Since 2008 CPPs have been made in a variety of shapes and sizes with the smallest at [5]CPP, formed at room temperature.[10][1]

Synthesis

Various methods and techniques have been developed to produce CPPs of varying sizes that then go on to have tunable and unexpected properties. The concept of CPP synthesis predates their actuality by quite some time. Of the many attempted schemes, the earliest expedition into CPP synthesis dates back to 1934 and was conducted by Parekh and Guha.[5] Their approach led to the development of pp diphenylene disulfide rather successfully. Unfortunately, heating with copper led to only partially desulfurized compounds rather than the strained target molecule [2]CPP.

70 years after this initial attempt, Jasti and Bertozzi were able to successfully synthesize [n]CPPs (n=9, 12, 18).[2] The group targeted 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 allowed the facile construction of syn-cyclohexadiene moiety. Partial borylation of this material was the necessary partner for the subsequent macrocyclization. Under Suzuki–Miyuara cross-coupling conditions, [n]CPPs (n=9,12,18) were generated, albeit unselectively and in low yields.

After the pivotal discovery in 2008, multiple groups have made great contributions to the field.[13][12][13][14] In 2009 Itami group would report the first selective synthesis of [12]CPP, and shortly thereafter Yamago synthesized the smallest [8]CPP to date in 2010.[12][13] This synthetic route also allowed for the synthesis of [8-13]CPP using a very practical scheme: [8] and [12]CPPs were selectively prepared from the reaction of 4,4′-bis(trimethylstannyl)biphenyl and 4,4′ ′-bis(trimethylstannyl)terphenyl, respectively, with Pt(cod)Cl2(cod = 1,5-cyclooctadiene) through square-shaped tetranuclear platinum intermediates.[13] A mixture of [8-13]CPPs was prepared in good combined yields by mixing biphenyl and terphenyl precursors with platinum sources.[13]

Since then, many scientists have sought out for smaller CPPs with favorable electronic properties. [7]CPP was synthesized using a similar approach to the initial synthesis, namely the use of appropriately substituted cyclohexadiene rings to alleviate strain during a key macrocyclization step and a reductive aromatization reaction to unveil the hidden aromatic structure in the cyclohexadiene moieties.[15]

Moving forward along the chronological quest for the smallest CPP, Jasti and co-workers eventually synthesized [5]CPP through a serendipitous discovery using an intramolecular boronate homocoupling technique that was originally seen as an undesired by-product of Suzuki-Miyaura cross-coupling reactions.[1] Around the same time, Itami also published the synthesis of [5]CPP.[14]

CPPs now have selective, modular, and high yielding synthetic pathways.

Properties

Structure

CPPs are circular with the benzene rings tilted slightly in or out of plane with each other around the hoop to relieve some of the strain.[15] Despite the strain inherent in these compounds, which increases as hoop size decreases, benzenoid character is exhibited by the phenyl rings in CPPs and is even seen in the smallest reported CPP, [5]CPP.[1][14][15] Hoop size does, however, have an effect on some other properties. One of the many interesting properties shown by CPPs is the size-dependence of their HOMO-LUMO gaps.[15] As the size of the CPP decreases the HOMO-LUMO gap also decreases. This property is novel because it is opposite of the trend seen in linear oligoparaphenylenes where the HOMO-LUMO gap decreases as size increases.[11][13] This also coincides with a distinct red-shift as hoop size decreases.[11][15] Strain energy is a characteristic property of CPP’s. As expected this strain increases as the size of the hoop decreases. [5]CPP is the smallest and thus most strained nanohoop yet synthesized with a calculated strain energy of 117.2 kcal/mol.[15]

Solid-state packing

[7]-[12]CPP all adopt some sort of herringbone packing in the solid state.[15] In addition to this, [5]CPP also adopts a similar herringbone-type structure that is much denser than the structures seen in [7-12]CPP. [6]CPP is the only hoop in this series that adopts a different solid state packing, stacking in columns.[15][16]

Nanohoops and fullerenes

The hoop-like shape and electronegativity of cycloparaphenylene by simple observation appears to useful tool which can circle a molecule and non-covalently hold it in place. The π-π interactions and the concave interior of the CPPs, specifically, is expected to bind well with π conjugated systems with convex surfaces. In 2011, the Yamago group found that [10]CPP is a useful compound for selective binding of C60 fullerenes.[3] This work showed that CPPs could selectively bind to fullerenes or other π-conjugated curved compounds like nanotubes.

In 2013 the Isobe Group built on Yamago’s work by producing the “molecular bearing” by capturing a C60 fullerene within a [10]CPP.[17] 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.[17]

In related work, Itami and Sinohara examined endohedral metallo-fullerenes 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.[18][19] 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.[17]

Applications

Current

The limiting factor for the current use and applications of CPPs is due almost entirely to the recent date at which they were first synthesized: the first CPPs were materialized and characterized only in 2008.[2] However, despite their prematurity, usefulness of these compounds has been reported and eventually found purpose in the field of materials science. Specifically, host-guest capabilities with fullerenes and other carbanacious species have been demonstrated which led to changes in properties of both of the original species.[3] These ‘pseudo’ carbon peapods can be recognized as an entirely new self-assembled graphitic structures.[20] Applications of nano-peapods include nanolasers, single electron transistors, spin-qubit arrays for quantum computing, nanopipettes, and data storage devices thanks to the memory effects and superconductivity of nano-peapods.[20][21][22] Furthermore, the fluorescence of [10]CPP is quenched upon complexation with C60, demonstrating its application as a potentially useful sensor.

Another useful property is the size dependent fluorescence of [n]CPPs. Tuneable fluorescence allows for advanced analytical techniques and novel detection methods.

Potential

Perhaps the most important aspect of CPPs is their potential to act as seeds for CNT growth. CPPs, at the very least, can be imagined as the shortest cross section of an [n,n]armchair CNT. Foremost of which is their use for template ‘‘bottom-up’’ SWCNT synthesis. Approaches to this application include but are not limited to: small molecule organic templates as seeds for growth using the Scholl reaction, diels alder cycloaddition, or ring-closing metathesis as a method for belt synthesis.[3][13][15][19] Graphitic belts seem to be key intermediates in seed growth of CNTs. Uniform CNT growth is becoming a necessity for materials and infrastructure. CPPs are expected to be a crucial stepping-stone along the endeavor for a synthesis of uniform CNTs.[19] Given their electronic properties that many researchers have alluded to,[11] CPPs make for appropriate materials for organic electronics. To date they remain untapped as a resource in this regard.[23][24]

Donor-acceptor functionalization

CPPs are unique in 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 energy level and a lower LUMO energy levels.[15] 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.[25]

As the synthesis of CPPs has become easier, derivative structures have begun to be synthesized as well. In 2013 the 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.[26]

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.[27]

In 2012 the Jasti Group reported the synthesis of dimers of [8]CPP linked by arene bridges.[28] This synthesis was followed two years later by the synthesis of a directly connected dimer of [10]CPP from chloro[10]CPP by the Itami group.[29]

References

  1. 1 2 3 4 5 6 Evans, Paul J.; Darzi, Evan R.; Jasti, Ramesh (May 2014). "Efficient room-temperature synthesis of a highly strained carbon nanohoop fragment of buckminsterfullerene". Nature Chemistry. 6 (5): 404–408. doi:10.1038/nchem.1888. ISSN 1755-4349.
  2. 1 2 3 4 Jasti, Ramesh; Bhattacharjee, Joydeep; Neaton, Jeffrey B.; Bertozzi, Carolyn R. (2008-12-31). "Synthesis, Characterization, and Theory of [9]-, [12]-, and [18]Cycloparaphenylene: Carbon Nanohoop Structures". Journal of the American Chemical Society. 130 (52): 17646–17647. doi:10.1021/ja807126u. ISSN 0002-7863.
  3. 1 2 3 4 5 6 Iwamoto, Takahiro; Watanabe, Yoshiki; Sadahiro, Tatsuya; Haino, Takeharu; Yamago, Shigeru (2011-08-29). "Size-Selective Encapsulation of C60 by [10]Cycloparaphenylene: Formation of the Shortest Fullerene-Peapod". Angewandte Chemie International Edition. 50 (36): 8342–8344. doi:10.1002/anie.201102302. ISSN 1521-3773.
  4. ↑ Li, Penghao; Zakharov, Lev N.; Jasti, Ramesh (2017-05-02). "A Molecular Propeller with Three Nanohoop Blades: Synthesis, Characterization, and Solid-State Packing". Angewandte Chemie International Edition. 56 (19): 5237–5241. doi:10.1002/anie.201700935. ISSN 1521-3773.
  5. 1 2 Parekh, V.C.; Guha, P.C. (1934). "Synthesis of p,p'-diphenylene disulfide". Journal of Indian Chemical Society. 11: 95–100.
  6. 1 2 Friederich, Rolf; Nieger, Martin; Vögtle, Fritz (1993-07-01). "Auf dem Weg zu makrocyclischen para-Phenylenen". Chemische Berichte. 126 (7): 1723–1732. doi:10.1002/cber.19931260732. ISSN 1099-0682.
  7. ↑ Miyahara, Yuji; Inazu, Takahiko; Yoshino, Tamotsu. "Synthesis of [1.1.1.1]paracylophane". Tetrahedron Letters. 24 (47): 5277–5280. doi:10.1016/s0040-4039(00)88416-6.
  8. 1 2 3 Jagadeesh, Mavinahalli N.; Makur, Anindita; Chandrasekhar, Jayaraman (2000-02-01). "The Interplay of Angle Strain and Aromaticity: Molecular and Electronic Structures of [0n]Paracyclophanes". Molecular modeling annual. 6 (2): 226–233. doi:10.1007/s0089400060226. ISSN 0949-183X.
  9. 1 2 Bodwell, Graham J. (May 2014). "Closing the loop". Nature Chemistry. 6 (5): 383–385. doi:10.1038/nchem.1932. ISSN 1755-4349.
  10. 1 2 3 Kayahara, Eiichi; Patel, Vijay Kumar; Yamago, Shigeru (2014-02-12). "Synthesis and Characterization of [5]Cycloparaphenylene". Journal of the American Chemical Society. 136 (6): 2284–2287. doi:10.1021/ja413214q. ISSN 0002-7863.
  11. 1 2 3 4 5 Wong, Bryan M. (2009-12-31). "Optoelectronic Properties of Carbon Nanorings: Excitonic Effects from Time-Dependent Density Functional Theory". The Journal of Physical Chemistry C. 113 (52): 21921–21927. doi:10.1021/jp9074674. ISSN 1932-7447.
  12. 1 2 Yamago, Shigeru; Watanabe, Yoshiki; Iwamoto, Takahiro (2010-01-18). "Synthesis of [8]Cycloparaphenylene from a Square-Shaped Tetranuclear Platinum Complex". Angewandte Chemie International Edition. 49 (4): 757–759. doi:10.1002/anie.200905659. ISSN 1521-3773.
  13. 1 2 3 4 5 6 Iwamoto, Takahiro; Watanabe, Yoshiki; Sakamoto, Youichi; Suzuki, Toshiyasu; Yamago, Shigeru (2011-06-01). "Selective and Random Syntheses of [n]Cycloparaphenylenes (n = 8–13) and Size Dependence of Their Electronic Properties". Journal of the American Chemical Society. 133 (21): 8354–8361. doi:10.1021/ja2020668. ISSN 0002-7863.
  14. 1 2 3 Omachi, Haruka; Matsuura, Sanae; Segawa, Yasutomo; Itami, Kenichiro (2010-12-27). "A Modular and Size-Selective Synthesis of [n]Cycloparaphenylenes: A Step toward the Selective Synthesis of [n,n] Single-Walled Carbon Nanotubes". Angewandte Chemie International Edition. 49 (52): 10202–10205. doi:10.1002/anie.201005734. ISSN 1521-3773.
  15. 1 2 3 4 5 6 7 8 9 10 Darzi, Evan R.; Jasti, Ramesh (2015-09-01). "The dynamic, size-dependent properties of [5]–[12]cycloparaphenylenes". Chemical Society Reviews. 44 (18). doi:10.1039/C5CS00143A. ISSN 1460-4744.
  16. ↑ Xia, Jianlong; Jasti, Ramesh (2012-03-05). "Synthesis, Characterization, and Crystal Structure of [6]Cycloparaphenylene". Angewandte Chemie International Edition. 51 (10): 2474–2476. doi:10.1002/anie.201108167. ISSN 1521-3773.
  17. 1 2 3 Matsuno, Taisuke; Kamata, Sho; Hitosugi, Shunpei; Isobe, Hiroyuki (2013-07-02). "Bottom-up synthesis and structures of π-lengthened tubular macrocycles". Chemical Science. 4 (8). doi:10.1039/c3sc50645b. ISSN 2041-6539.
  18. ↑ Iwamoto, Takahiro; Slanina, Zdenek; Mizorogi, Naomi; Guo, Jingdong; Akasaka, Takeshi; Nagase, Shigeru; Takaya, Hikaru; Yasuda, Nobuhiro; Kato, Tatsuhisa (2014-10-27). "Partial Charge Transfer in the Shortest Possible Metallofullerene Peapod, La@C82⊂[11]Cycloparaphenylene". Chemistry – A European Journal. 20 (44): 14403–14409. doi:10.1002/chem.201403879. ISSN 1521-3765.
  19. 1 2 3 Lewis, Simon E. (2015-04-10). "Cycloparaphenylenes and related nanohoops". Chemical Society Reviews. 44 (8): 2221–2304. doi:10.1039/c4cs00366g. ISSN 1460-4744.
  20. ↑ Service, Robert F. (2001-04-06). "Nanotube 'Peapods' Show Electrifying Promise". Science. 292 (5514): 45–45. doi:10.1126/science.292.5514.45. ISSN 0036-8075. PMID 11294210.
  21. ↑ Kwon, Young-Kyun (1999). ""Bucky Shuttle" Memory Device: Synthetic Approach and Molecular Dynamics Simulations". Physical Review Letters. 82 (7): 1470–1473. doi:10.1103/physrevlett.82.1470.
  22. ↑ Utko, Pawel; Nygård, Jesper; Monthioux, Marc; Noé, Laure (2006). "Sub-Kelvin transport spectroscopy of fullerene peapod quantum dots". Applied Physics Letters. 89: 233118.
  23. ↑ Golder, Matthew R.; Wong, Bryan M.; Jasti, Ramesh (2013-09-30). "Photophysical and theoretical investigations of the [8]cycloparaphenylene radical cation and its charge-resonance dimer". Chemical Science. 4 (11). doi:10.1039/C3SC51861B. ISSN 2041-6539.
  24. ↑ Zabula, Alexander V.; Filatov, Alexander S.; Xia, Jianlong; Jasti, Ramesh; Petrukhina, Marina A. (2013-05-03). "Tightening of the Nanobelt upon Multielectron Reduction". Angewandte Chemie International Edition. 52 (19): 5033–5036. doi:10.1002/anie.201301226. ISSN 1521-3773.
  25. ↑ Van Raden, J. M.; Darzi, E. R.; Zakharov, L. N.; Jasti, R. (2016-06-15). "Synthesis and characterization of a highly strained donor–acceptor nanohoop". Organic & Biomolecular Chemistry. 14 (24): 5721–5727. doi:10.1039/c6ob00133e. ISSN 1477-0539.
  26. ↑ Matsui, Katsuma; Segawa, Yasutomo; Namikawa, Tomotaka; Kamada, Kenji; Itami, Kenichiro (2012-11-29). "Synthesis and properties of all-benzene carbon nanocages: a junction unit of branched carbon nanotubes". Chem. Sci. 4 (1): 84–88. doi:10.1039/c2sc21322b. ISSN 2041-6539.
  27. ↑ Wong, Bryan M.; Lee, Jonathan W. (2011-11-03). "Anomalous Optoelectronic Properties of Chiral Carbon Nanorings…and One Ring to Rule Them All". The Journal of Physical Chemistry Letters. 2 (21): 2702–2706. doi:10.1021/jz2012534. ISSN 1948-7185.
  28. ↑ Xia, Jianlong; Golder, Matthew R.; Foster, Michael E.; Wong, Bryan M.; Jasti, Ramesh (2012-12-05). "Synthesis, Characterization, and Computational Studies of Cycloparaphenylene Dimers". Journal of the American Chemical Society. 134 (48): 19709–19715. doi:10.1021/ja307373r. ISSN 0002-7863.
  29. ↑ Ishii, Yuuki; Matsuura, Sanae; Segawa, Yasutomo; Itami, Kenichiro (2014-04-18). "Synthesis and Dimerization of Chloro[10]cycloparaphenylene: A Directly Connected Cycloparaphenylene Dimer". Organic Letters. 16 (8): 2174–2176. doi:10.1021/ol500643c. ISSN 1523-7060.
This article is issued from Wikipedia. The text is licensed under Creative Commons - Attribution - Sharealike. Additional terms may apply for the media files.