Ladder Phenylenes Synthesized on Au(111) Surface via Selective [2+2] Cycloaddition

Abstract

Ladder phenylenes (LPs) composed of alternating fused benzene and cyclobutadiene rings have been synthesized in solution with a maximum length no longer than five units. Longer polymeric LPs have not been obtained so far because of their poor stability and insolubility. Here, we report the synthesis of linear LP chains on the Au(111) surface via dehalogenative [2+2] cycloaddition, in which the steric hindrance of the methyl groups in the 1,2,4,5-tetrabromo-3,6-dimethylbenzene precursor improves the chemoselectivity as well as the orientation orderliness. By combining scanning tunneling microscopy and noncontact atomic force microscopy, we determined the atomic structure and the electronic properties of the LP chains on the metallic substrate and NaCl/Au(111). The tunneling spectroscopy measurements revealed the charged state of chains on the NaCl layer, and this finding is supported by density functional theory calculations, which predict an indirect bandgap and antiferromagnetism in the polymeric LP chains.

On‐Surface Synthesis of [3]Radialenes via [1+1+1] Cycloaddition

“…In the past decade, on‐surface reactions have provided a unique pathway for the atomically precise synthesis of novel nanostructures, [11–25] including graphene nanoribbons, [14, 15] cyclo[18]carbon, [16] and biphenylene sheets [17] . Recently, several cycloaddition reactions have been achieved on surfaces, [26–37] such as azide–alkyne cycloaddition, [26, 27] Diels–Alder reaction, [28–30] the [2+2+2] cyclotrimerization of alkynes, [31] the dehalogenative [2+2] [32] and [2+2+2] cycloaddition of ortho ‐dihalogenated arenes, [33] and the [3+2] cycloaddition of azomethine ylide and cyano, [34] which provide various carbocycles and heterocycles. More recently, Li et al.…”

“…In the past decade, on-surface reactions have provided a unique pathway for the atomically precise synthesis of novel nanostructures, [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25] including graphene nanoribbons, [14,15] cyclo [18]carbon, [16] and biphenylene sheets. [17] Recently, several cycloaddition reactions have been achieved on surfaces, [26][27][28][29][30][31][32][33][34][35][36][37] such as azide-alkyne cycloaddition, [26,27] Diels-Alder reaction, [28][29][30] the [2+2+2] cyclotrimerization of alkynes, [31] the dehalogenative [2+2] [32] and [2+2+2] cycloaddition of ortho-dihalogenated arenes, [33] and the [3+2] cycloaddition of azomethine ylide and cyano, [34] which provide various carbocycles and heterocycles. More recently, Li et al reported the one-step [1+1+1+1] cycloaddition of phenylacetylenes to achieve the synthesis of tetraphenyl- [4]radialenes on Cu(100); [38] this is the only example of the synthesis of radialenes on a surface to date.…”

“…For surface-confined reactions, the steric hindrance of the precursor substituents plays an important role in modulating the adsorption conformation and molecular arrangement configuration, controlling the assembly structure and reaction pathways, and even determining the final products. [32,[39][40][41][42][43][44] Herein, we report the first synthesis of aza [3]radialenes on a Ag(111) surface via the highly selective [1+1+1] cycloaddition of isocyanides, in which the steric hindrance of the chlorine substituents guides the assembly of isocyanides and controls the selectivity and orientational order of the products (Scheme 1). The atomic structure of aza [3]radialenes was determined by a combination of scanning tunneling microscopy (STM), non-contact atomic force microscopy (nc-AFM), and time-of-flight secondary ion mass spectrometry (ToF-SIMS).…”

Nanoconfined synthesis of conjugated ladder polymers

“…12). 147 The electronic properties of LP were examined by scanning tunnelling spectroscopy (STS) measurements after transferring the LP chains from the metal surface to NaCl islands via tip manipulation. LP was found to be positively charged, with singly occupied (SOMO) and associated unoccupied (SUMO) molecular orbitals identified by d I /d V maps and confirmed by density functional theory (DFT) calculations.…”

Pseudo-atomic orbital behavior in graphene nanoribbons with four-membered rings

“…Nanoribbons with four-membered rings, however, have so far remained elusive due to challenges in their chemical synthesis (38). Although [2 + 2] cycloaddition has been shown to generate four-membered rings via on-surface synthesis, it has so far only been used to couple smaller acenes and triphenylenes (31,(39)(40)(41)(42). The CBD units in these systems exhibit relatively little antiaromaticity due to their fusion pattern (i.e., because the CBD bonds have more single bond character in the dominant resonance structures that maximize the number of Clar sextets), resulting in a large bandgap (31,36,43).…”