Synthesis of radiaannulene oligomers to model the elusive carbon allotrope 6,6,12-graphyne

Abstract

Graphyne allotropes of carbon are fascinating materials, and their electronic properties are predicted to rival those of the “wonder material” graphene. One allotrope of graphyne, having rectangular symmetry rather than hexagonal, stands out as particularly attractive, namely 6,6,12-graphyne. It is currently an insurmountable challenge, however, to design and execute a synthesis of this material. Herein, we present synthesis and electronic properties of molecules that serve as model compounds. These oligomers, so-called radiaannulenes, are prepared by iterative acetylenic coupling reactions. Systematic optical and redox studies indicate the effective conjugation length of the radiaannulene oligomers is nearly met by the length of the trimer. The HOMO-LUMO gap suggested by the series of oligomers is still, however, higher than that expected for 6,6,12-graphyne from theory, which predicts two nonequivalent distorted Dirac cones (no band gap). Thus, the radiaannulene oligomers present a suitable length in one dimension of a sheet, but should be expanded in the second dimension to provide a unique representation of 6,6,12-graphyne.

Unlocking Acyclic π-Bond Rich Structure Space with Tetraethynylethylene–Tetravinylethylene Hybrids

“…10 Interest in designed carbon allotropes and associated (generally hydrocarbon-based) model molecular materials comprising sp 2 and sp carbons is rapidly expanding. 11 These efforts are focused on systems that rival the conducting, no band gap, "wonder material" graphene, 12 or exhibit specific band gaps required for devices such as semiconductors.…”

Cyclo[18]carbon: Insight into Electronic Structure, Aromaticity, and Surface Coupling

“…Nevertheless, carbon allotropes are still in focus of synthetic attempts and theoretical predictions. [9][10][11][12][13][14][15] One of the most impressive recent achievements in this field is the successful synthesis and characterization of the closed-circle polyyne containing 18 sp-hybridized carbon atoms, called cyclo [18]carbon. 16 It is notable that cyclo [18]carbon possesses the special "double" aromaticity 17 because of delocalization of two scaffolds of 18 -electrons oriented in the plane of C18 ring (18in) and perpendicular to it (18out, Figure 1) that was first mentioned by Diederich et al 18 and developed further by Fowler et al 19 .…”

“…Carbon is one of the most diverse elements of the periodic table with respect to possible allotropes. , Among them, sp 3 -hybridized diamond, multilayered sp 2 -hybridized graphite and monolayer graphene, fullerenes and nanotubes, sp-hybridized carbyne, and linear polyynes , are the most attractive materials and unique objects for practical applications and theoretical surveys. − The number of hypothetically predicted carbon allotropes reaches more than 500 different structures in accordance with the SACADA database last updated on May 2017. Nevertheless, carbon allotropes are still the focus of synthetic attempts and theoretical predictions. − One of the most impressive recent achievements in this field is the successful synthesis and characterization of the closed-circle polyyne containing 18 sp-hybridized carbon atoms, called cyclo[18]­carbon . It is notable that cyclo[18]carbon possesses the special “double” aromaticity because of delocalization of two scaffolds of 18 π-electrons oriented in the plane of the C 18 ring (18π in ) and perpendicular to it (18π out ) (Figure ) that was first mentioned by Diederich et al and developed further by Fowler et al…”

Structural, electronic and mechanical properties of all-sp2 carbon allotropes with density lower than graphene

“…By tuning the latter two variables, for example by using cycloalkanes, such as cyclopropane or cyclopropenium ions, and different metallic substrates one can in principle design a wide range of different 2D carbon allotropes. Furthermore, very recent results were reported concerning the synthesis of other 2D carbon allotropes, such as the radiaannulene oligomers by iterative acetylenic coupling reactions [73]. These oligomers have rectangular rather than the hexagonal symmetry of graphene, and their geometry is obtained by adding sp -hybridized carbon atoms into the 2 sp -carbon graphene framework.…”