On-Surface Synthesis of Indenofluorene Polymers by Oxidative Five-Membered Ring Formation

Abstract

On-surface synthesis is a successful approach to the creation of carbon-based nanostructures that cannot be obtained via standard solution chemistry. In this framework, we have established a novel synthetic pathway to one-dimensional conjugated polymers composed of indenofluorene units. Our concept is based on the use of ortho-methyl groups on a poly( para-phenylene) backbone. In this situation, surface-assisted oxidative ring closure between a methyl and the neighboring aryl moiety gives rise to a five-membered ring. The atomically precise structures and electronic properties of the obtained indenofluorene polymers have been unambiguously characterized by STM, nc-AFM, and STS, supported by theoretical calculations. This unprecedented synthetic protocol can potentially be extended to other polyphenylenes and eventually graphene nanoribbons, to incorporate five-membered rings at desired positions for the fine-tuning of electronic properties.

Synthetic Applications of Oxidative Aromatic Coupling—From Biphenols to Nanographenes

“…Specifically, the molecular precursor 232 undergoes polymerization followed by a series of CDH processes to afford graphene nanoribbon 234 with a zigzag edge topology (Scheme A). This concept was further extended to dibromodimethylterphenyl 235 (Scheme B), where the polymerization/surface‐assisted oxidative ring‐closure sequence between a methyl group and the neighboring aryl moiety gives rise to indenofluorene‐based polymers 237 and 238 . This strategy could eventually be employed in the preparation of graphene nanoribbons containing five‐membered rings, because such a modification at specific positions can allow for the fine‐tuning of electronic properties.…”

“…[270] Specifically,t he molecular precursor 232 undergoes polymerization followed by as eries of CDH processes to afford graphene nanoribbon 234 with azigzag edge topology (Scheme 31 A). This concept was further extended to dibromodimethylterphenyl 235 (Scheme 31 B), [271] where the polymerization/surface-assistedo xidative ring-closure sequence between am ethyl group and the neighboring aryl moiety gives rise to indenofluorene-based polymers 237 and 238.This strategy could eventually be employed in the preparation of graphene nanoribbons containing five-membered rings, because such am odification at specific positions can allow for the fine-tuning of electronic properties.A ccording to the same strategy,u ltra-low-gap open-shell molecules,s uch as peri-tetracene (240 from 239), ab enzenoid graphene fragment with az igzag edge topology,a nd dibenzo-[a,m]dicyclohepta[bcde,nopq]rubicene (242 from 241), anonbenzenoid nonalternant structural isomer of peri-tetracene with two embedded azulene units,w ere synthesized [272] by surface-assisted CDH (Scheme 31 C). Spin-polarized DFT calculations revealed that both compounds should exhibit an open-shell singlet ground state,t hus making them promising candidates for spintronic applications.…”

On-Surface Synthesis of Antiaromatic and Open-Shell Indeno[2,1-b]fluorene Polymers and Their Lateral Fusion into Porous Ribbons

“…Starting from 4,4''-dibromo-4',6'-dimethyl-1,1':3',1''terphenyl as the monomer (1 in Scheme 1b) on Au(111) under UHV conditions, we exploit the well-known dehalogenative aryl-aryl coupling to link the repeating units in a covalent fashion, [30][31][32][33][34][35][36][37][38] and use the methyl groups to selectively form five-membered rings via oxidative ring closure. 24 We demonstrate the successful on-surface synthesis of polymers consisting of…”

“…Very recently, we described the on-surface synthesis and characterization of 1D polymers made from two of the five possible indenofluorene isomers, indeno [1,2-b]fluorene and indeno[2,1-a]fluorene. 24 However, the monomer design led to regioisomerism and failed to afford perfect structures. Further, the polymers were found to have a relatively large band gap of 2.3 eV and a closed-shell electronic configuration.…”

On‐Surface Synthesis and Characterization of Triply Fused Porphyrin–Graphene Nanoribbon Hybrids

“…[13] Ac omplementary approach to modify the electronic properties of GNRs is to "dope" them with heteroatoms,s uch as nitrogen [14] or to include fused five-membered rings in their structure. [15] In this context, an interesting yet poorly explored strategy is fusing GNRs with molecular species that contain both five-membered rings and heteroatoms in their structures.Among the building blocks that fulfil these requirements, Pors stand out as ideal candidates due to their four pyrrolic units (i.e., nitrogencontaining,f ive-membered rings), planar geometry and aromatic nature,rich redox chemistry,u nique optoelectronic features and excellent long-range charge transport abilities. [16] Interestingly,t heir electron transport ability,mainly explored in conjugated Por oligomers and polymers, [17] significantly depends on the type of inter-ring connection.…”