Tetracyclo(2,7-carbazole)s: Diatropicity and Paratropicity of Inner Regions of Nanohoops

Abstract

Three N-substituted tetracyclo(2,7-carbazole)s were synthesized to investigate the inner regions of nanohoops. One compound has a 5,5-dimethylnonane bridge between two neighboring anti-carbazoles, which can be used as covalently bonded "methane probes". These probes near the ring center are strongly shielded by local ring currents and exhibit a singlet at δ = -2.70 ppm in (1)H NMR. To visualize local and macrocyclic ring currents separately, we drew nucleus-independent chemical shift contour maps of tetracyclo(9-methyl-2,7-carbazole) and [n]cycloparaphenylenes (CPPs). Local ring currents make the interior diatropic, and paratropic regions exist only outside the ring. Macrocyclic ring currents in [5]CPP to [7]CPP generate deshielding cones, which are typical of antiaromatic [4n]annulenes.

[4]Cyclo‐N‐ethyl‐2,7‐carbazole: Synthesis, Structural, Electronic and Charge Transport Properties

“…[27] It suggests that the carbazole units are tilted so that the bridges idep oints towards the inside of the ring (as also observedb ys ingle-crystal X-ray crystallography,s ee below). [27] It suggests that the carbazole units are tilted so that the bridges idep oints towards the inside of the ring (as also observedb ys ingle-crystal X-ray crystallography,s ee below).…”

“…Thus, the reductivee limination step, which is unfavoured because of the strain energy,c an be assisted by increasing both the temperature (by using o-dichlorobenzene as previously reportedb yI sobe and co-workersf or cyclonaphthylene nanohoops [31] )a nd reactiont ime. [27] This is an interesting feature for the future incorporation of nanorings in electronic devices. [27] This is an interesting feature for the future incorporation of nanorings in electronic devices.…”

“…[20,[23][24][25] In the case of carbazole, the effect of the nitrogen bridge is strong. [27] These molecules, constituted of four carbazole units substituted on the nitrogen atoms by either am ethyl or ap henyl group, wered esigned to investigate the inner region of nanorings by 1 HNMR studies and their electronic properties, which despite being recorded,w ere not discussed. However,i nn anorings, these effects have not been described to date.…”

[4]Cyclofluorene: Unexpected Influence of Alkyl Chain Length

“…Yamago et al . have reported an analogue effect but with a tetracarbazole ring bearing a covalent alkyl bridge purposely designed to pass across the center of the ring. In our case, it appears that one of the two ethyl chains of each fluorene unit are positioned towards the center of the ring, exposing its methyl group to the strong shielding effect of the ring current, as displayed by the X‐ray crystal structure detailed below.…”

Pentagon‐Embedded Cycloarylenes with Cylindrical Shapes

“…Theconstruction of conjugated p-systems on unique hexagonal networks of sp 2 -hybridized carbon atoms has been demonstrated, producing nanocarbon models with discrete structures. [12][13][14] Synthetically,afew examples of pentagon-embedded nanohoops have been realized in forms of heteroaromatic curved p-systems, [15][16][17][18][19] but the incorporation of pentagons into the curved conjugated systems of cylinder-shaped molecules with sp 2 -carbon networks has not been achieved. [3,4] In contrast, odd-numbered polygonal rings, such as pentagons and heptagons,a re much less explored in nanocarbon structures.Although pentagons and/or heptagons were initially perceived as structural defects that distort the nanocarbon shapes, [5][6][7][8][9] recent theoretical investigations indicate that odd-numbered rings could also give rise to flat or curved nanocarbon sheets through anomalous periodic arrangements.F or instance,p entagon/heptagon-containing cylinders of sp 2 -carbon networks,s uch as pentaheptite and haeckelite (Figure 1), [10,11] have been proposed, and their unique conjugated systems with enhanced metallic character have been extensively discussed in the context of novel tubular nanocarbon materials.Avariety of cylinder-shaped nanocarbon structures are currently being exploited to dramatically expand the theoretical models of cylindershaped nanocarbons with odd-numbered rings.…”