On‐surface synthesis and atomic scale characterization of unprotected indenofluorene polymers

Abstract

Polycyclic hydrocarbons with nonzero radical character have attracted enormous interest as potential active media for organic electronics and spintronics. In this context, indenofluorenes are an intriguing class of formally antiaromatic, biradical materials with a radical character that depends on the connectivity of their six‐ and five‐membered rings. Synthesis of indenofluorene polymers and related compounds, first achieved in the early ‘90s with the production of ladder‐type chains, represents a major step toward incorporation of these systems into devices. However, solution‐based synthetic protocols require bulky protecting groups to stabilize the most reactive sites and, at the same time, to improve solubility and processability of such compounds. The preparation of various pristine – that is, unprotected—indenofluorene polymers has recently become possible via the on‐surface synthesis approach, where the resulting nanostructures are supported and efficiently stabilized by the underlying substrate in ultrahigh vacuum conditions. Here, an overview of these recent works is given, with a focus on synthetic challenges, structural details and electronic properties.

Carbon-based nanostructures as a versatile platform for tunable π-magnetism

“…These low HOMO-LUMO gaps in turn facilitate their spin-polarization. Examples of structures synthesized and studied under vacuum in which odd-membered rings facilitate the spin-polarization are polyindenoindene [176] or polyindenofluorene [177,178], in which their prevailing openshell configurations gain only two Clar sextets for every pair of radicals, both cases being therefore below the common 'three sextets per radical pair' threshold described in section 3.3. Nanographenes with a single pentagonal ring [52,108] and GNRs with five-membered rings at the edges [27] have been characterized.…”

Revisiting indeno[1,2-b]fluorene by steric promoted synthesis while isolating the second stable 4nπ indeno[2,1-a]fluorene

“…The presented synthesis approach has the potential for constructing heteroatom-embedded isoelectronic IFs 41 and π-conjugated IF oligomers or polymers by solution-based methods which is currently underway in our lab, considering that the studies on IF-based polymers are mostly limited to on-surface synthesis. 42…”

Two‐Dimensional Nonbenzenoid Heteroacene Crystals Synthesized via In‐Situ Embedding of Ladder Bipyrazinylenes on Au(111)

“…The most common strategy for tuning the properties of acenes is the introduction of topological defects (the structural defects in pristine graphene-based materials), which mainly include introducing nonhexanal rings into acenes to afford nonbenzenoid acenes and embedding heterocycles into acenes to form heteroacenes. The OSS of nonbenzenoid acenes mainly involves dehydrogenation and deholgenation cyclization to generate the target acenes, [34][35][36][37][38][39][40][41][42][43][44][45] which requires high annealing temperature and produces bromide residues, limiting the formation of 2D nonbenzenoid acene crystals. The OSS of heteroacenes primarily involves the intramolecular elimination reaction of precursor molecules with heterocycles by thermal annealing or tip-assisted atomic manipulation.…”