Synthesis of a zigzag carbon nanobelt

Abstract

The structure-selective precise synthesis of carbon nanotubes (CNTs) has been long sought in materials science. The aromatic molecules corresponding to segment structures of CNTs, i.e. carbon nanobelts (CNBs), have been of interest as templates for CNT growth. Although two of three types, armchair and chiral CNBs, have been synthesized recently, zigzag CNBs remain elusive. Herein we report the synthesis and isolation of a zigzag CNB. The synthesis involves an iterative Diels-Alder reaction sequence followed by reductive aromatization of oxygen-bridged moieties. As predicted by theoretical calculations, this CNB was isolated as a stable compound. The structure of the zigzag CNB was fully characterized by X-ray crystallography, and its wide energy gap with blue fluorescence properties were revealed by photophysical measurement. With synthetic strategies towards all three types of CNBs in hand, the road to the precise synthesis of CNTs can now proceed to the next stage. File list (3) download file view on ChemRxiv CheungZigzag_.pdf (9.75 MiB) download file view on ChemRxiv SI_CheungZigzag_.pdf (4.72 MiB) download file view on ChemRxiv cheung.cif (1.23 MiB)

Synthesis of Zigzag Carbon Nanobelts through Scholl Reactions

“…Because carbon nanobelts are double-stranded macrocycles having two conjugated and non-coincident edges, as uccessful synthesis of carbon nanobelts needs to achieve both macrocyclization and ladderization, the latter of which is ap rocess to form ad ouble stranded structure with the connectivity similar to aladder. In the synthesis of [4n]benzo-[6n]cyclacene (n = 2, 3) by Itami [20] and Chi, [21] macrocyclization and ladderization are achieved simultaneously through repetitive Diels-Alder cycloadditions.T he synthesis of nanobelts 1-3 herein relies on an alternative strategy,w here macrocyclization is followed by ladderization through regioselective Scholl reactions.The synthetic precursors of 1-3 are hybrid cyclic arylene oligomers 4-6,r espectively.F igure 3 shows the structures of 4-6 illustrating two key concepts of designing.F irst, unlike the nanoring precursors for the reported synthesis of armchair and chiral carbon nanobelts, [9] 4-6 contain meta-phenylene units (highlighted in yellow), which not only provide azigzag edge in the resulting nanobelt but also alleviate the strain of nanorings [22] to facilitate macrocyclization. As ar esult, 4-6 are less strained than the corresponding cycloparaphenylenes,a nd in particular, 6 is as train-free macrocycle.S econd, the formation of 1-3 relies on selective formation of C À Cb onds to the a-positions (highlighted with red balls) in the naphthalene units in the course of Scholl reactions.S uch regioselectivity has been preliminarily demonstrated in the reported synthesis of curved polycyclica romatics.…”

Synthesis of Zigzag Carbon Nanobelts through Scholl Reactions

“…Nanobelt 3' '' ' has as lightly larger strain than 1' '' ' and 2' '' ' despite its larger ring size because it involves more [4]helicene moieties.N anobelts 1' '' ' and 2' '' ' have smaller strain than [16]cyclacene (82.8 kcal mol À1 ), [40] while 3' '' ' has larger strain than [24]cyclacene (55.2 kcal mol À1 ) [40] for the same reason. Thes trains of 1-3 are all slightly larger than that of [12]benzo [18]cyclacene (63.3 kcal mol À1 as calculated by linear regression analysis [20] ). As ar esult of containing six mphenylene units,m acrocycle 6 is presumably as train-free molecule.T his is in agreement with av ery small enthalpy change (1.0 kcal mol À1 )c alculated from the hypothetical homodesmotic ring-opening reaction shown in Scheme S2 (see Supporting Information).…”

“…[7] Thef irst armchair and chiral carbon nanobelts were synthesized by the groups of Itami [8] and Miao [9] in 2017 and 2019, respectively.The basic structure of zigzag carbon nanobelts is [n]cyclacene (Figure 1a), which was first proposed in 1954 as hypothetical molecules for theoretical study. [10] Attempts to synthesize cyclacenes were reported by the groups led by Stoddart, [11] Cory, [12] Schlüter, [13] PeÇa [14] and Wang, [15] who successfully constructed belt-shaped molecular frameworks mainly by Diels-Alder reactions,b ut failed to achieve the fully conjugated structures.T he first successful synthesis of zigzag carbon nanobelts has not been achieved until very recently on the basis of am odified design, [m]benzo-[n]cyclacene (m n), [16][17][18] which is more stable than [n]cyclacene because benzannulation increases the number of Clarsa romatic sextets.I tami and co-workers reported, in ap reprint format in May 2020 [19] and later in af ormal publication, [20] the synthesis of [12]benzo [18]cyclacene (Figure 1a)t hrough repetitive Diels-Alder cycloadditions followed by reductive aromatization;a nd af ew months later, Chi and co-workers published the synthesis of [8]benzo- [12]cyclacene (Figure 1a)u sing essentially the same strategy. [21] Herein we report new zigzag carbon nanobelts 1-3 (Figure 1b)o nt he basis of ad esign different from [n]cyclacene or [m]benzo[n]cyclacene.Carbon nanobelts 1 and 2 are apair of constitutional isomers,whose polycyclicframeworks represent two different cutouts from zigzag (16,0) SWCNT, and 3 is alarger analogue of 2 representing asidewall segment of (24,0) SWCNT.F igure 2c ompares the polycyclic frameworks of 1 and 2 with [16]cyclacene by mapping them on an unrolled honeycomb lattice of (16,0)SWCNT,s howing the alkoxylated benzenoid rings in 1 and 2 as grey hexagons and other six-membered rings as light blue hexagons.T he light blue hexagons in 1 and 2 compose the essential belt structures, while the grey hexagons are not necessary to keep the belt structure but allow expansion along the direction of the nanotube axis.…”

“…Because carbon nanobelts are double-stranded macrocycles having two conjugated and non-coincident edges, as uccessful synthesis of carbon nanobelts needs to achieve both macrocyclization and ladderization, the latter of which is ap rocess to form ad ouble stranded structure with the connectivity similar to aladder. In the synthesis of [4n]benzo-[6n]cyclacene (n = 2, 3) by Itami [20] and Chi, [21] macrocyclization and ladderization are achieved simultaneously through repetitive Diels-Alder cycloadditions.T he synthesis of nanobelts 1-3 herein relies on an alternative strategy,w here macrocyclization is followed by ladderization through regioselective Scholl reactions.The synthetic precursors of 1-3 are hybrid cyclic arylene oligomers 4-6,r espectively.F igure 3 shows the structures of 4-6 illustrating two key concepts of designing.F irst, unlike the nanoring precursors for the reported synthesis of armchair and chiral carbon nanobelts, [9] 4-6 contain meta-phenylene units (highlighted in yellow), which not only provide azigzag edge in the resulting nanobelt but also alleviate the strain of nanorings [22] to facilitate macrocyclization. As ar esult, 4-6 are less strained than the corresponding cycloparaphenylenes,a nd in particular, 6 is as train-free macrocycle.S econd, the formation of 1-3 relies on selective formation of C À Cb onds to the a-positions (highlighted with red balls) in the naphthalene units in the course of Scholl reactions.S uch regioselectivity has been preliminarily demonstrated in the reported synthesis of…”

Towards the Highly Efficient Synthesis and Selective Methylation of C(sp3)‐Bridged [6]Cycloparaphenylenes from Fluoren[3]arenes

“…[3][4][5][6][7] Thechemical synthesis of aromatic belts has been al ong sought-after target. [8] Up to date,some successful examples for the synthetic segments of arm-chaired, [9] chiral [10,11] and zig-zag [12][13][14] carbon nanotubes have been reported. Nonetheless,the design and construction of aromatic belts are still ahot topic and represent achallenging synthetic target.…”