Aromatic annular carbon allotropes: cumulenic cyclo[10]carbon and Peierls-transition-intermediate cyclo[14]carbon

Abstract

Cyclo[n]carbon allotropes built from sp-hybridized carbon atoms have aroused substantial interests, but until now even their structures (i.e., polyynic or cumulenic) and stabilities (i.e., cyclic or linear) are still under debate. Due to their high reactivity, it is rather difficult to synthesize in condensed phase, which hampers further real-space characterizations. Recently, isolated cyclo[18]carbon (C18) and cyclo[16]carbon (C16) molecules were generated on the surface by atom manipulation, which allowed to identify the polyynic structure of C18 and C16. More interestingly, for the smaller cyclo[n]carbon, questions arise: is it still stable in cyclic form? If so, what is the structure? Here, we successfully produce two aromatic cyclocarbons, that is, cyclo[10]carbon (C10) and cyclo[14]carbon (C14), via tip-induced dehalogenation and followed by retro-Bergman reaction of fully chlorinated naphthalene (C10Cl8) and anthracene (C14Cl10) molecules, on a bilayer NaCl/Au(111) surface at 4.7 kelvin. The cumulenic structure of C10 and Peierls-transition intermediate of C14, different from the polyynic structure of C18 and C16, are revealed by bond-resolved atomic force microscopy. Our results demonstrate a universal synthetic strategy to generate aromatic cyclo[n]carbons on the surface, opening an avenue for characterizing a series of annular carbon allotropes for the structure and stability.

On-Surface Synthesis of a Doubly Anti-Aromatic Carbon Allotrope: Cyclo[16]carbon

“…detected in the gas-phase 17,18 , and two examples (C6 and C8) have been trapped in solid argon and characterised by infrared spectroscopy 19,20 . Cyclo [10]carbon, cyclo [14]carbon and cyclo [18]carbon have been definitively characterised by scanning probe microscopy of individual molecules on NaCl surfaces at low temperature 9,10,21 . AFM images revealed cumulenic structures for C10 and C14 (ref.…”

“…AFM images revealed cumulenic structures for C10 and C14 (ref. 9) and a polyynic structure for C18 (refs. 10, 21).…”

Aromaticity Reversal Induced by Vibrations in Cyclo[16]carbon

“…Cyclo­[ N ]­carbons are all-carbon rings with two orthogonal π systems, usually with N electrons in each . In an analogy to annulenes, cyclocarbons with N = 4 n + 2 carbon atoms can be classed as doubly aromatic (with both π systems sustaining a diatropic ring current), while N = 4 n cyclocarbons are expected to be doubly antiaromatic. , Many cyclocarbons have been found in the gas phase, but only C 10 , C 14 , C 16 , and C 18 have been structurally characterized using scanning probe microscopy.…”

“…A recent on-surface investigation of C 16 revealed the presence of strong bond-length alternation (BLA) and confirmed that its ground state is doubly antiaromatic. 17 Here, we investigate the variation of aromaticity with geometry in the two lowest singlets (S 0 and S 1 ), the lowest triplet (T 1 ) and quintet (Q 1 ) state of C 16 . Surprisingly, we find that the total ring current in the S 1 and T 1 states can be reversed from aromatic to antiaromatic by movement along the Kekulev ibration (∼2300 cm −1 ), i.e., by changing the amount of bondlength alternation.…”

Double Aromaticity of Franck-Condon Excited States of Cyclo[16]carbon

“…Meanwhile, the generation of its homologues, namely carbon rings of other sizes, is also being vigorously explored. 41,42 Excitingly, Gao et al succeeded in preparing cyclo [16]carbon (C 16 ) very recently by taking the similar approach to that of Kaiser et al 41 For strictly planar cyclic molecules like C 18 and C 16 , the aromaticity is undoubtedly one of their most concerning properties. It is generally believed that the singlet ground-state (S 0 state) molecules follow the Hückel's rule, that is, molecular rings with [4n+2] π electrons display aromaticity, while those with [4n] π electrons are anti-aromatic.…”