Cumulene-like bridged indeno[1,2-b]fluorene π-conjugated polymers synthesized on metal surfaces

Abstract

Among the plethora of polycyclic structures that have emerged in recent years, indenofluorenes comprise a unique class of compounds due to their potential in organic electronic systems such as OLEDs,...

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

“…The reported on‐surface synthetic strategies rely on dehalogenative aryl‐aryl coupling of dibromo‐terphenyl precursors followed by the oxidative cyclization of methyl groups on the polyphenylene backbone to achieve the five‐membered rings of the IF scaffolds. Alternative approaches where the molecular precursors featured the five‐membered rings already before surface deposition required stabilization of the reactive sites—with extra hydrogen atoms 73 or dibromomethyl groups 76 . In the reported examples—except the case of cumulene‐poly‐II —the well‐established dehalogenative aryl‐aryl coupling step was thermally induced at 150 °C on Au(111).…”

“…In all examples of on‐surface synthesized IF polymers discussed so far, the synthetic pathway relied on dehalogenative aryl‐aryl coupling followed by methyl cyclization, resulting in polymer chain growth along the long axis of the molecular building blocks. An alternative on‐surface synthetic protocol based on dibromomethylene functional groups was reported to afford polymerization via cumulene bridges between five‐membered ring apexes 76 . Installation of dibromomethylene groups at the apexes of five‐membered rings of an indeno[1,2‐ b ]fluorene unit was shown to protect the reactive sites of the precursor molecule and to afford polymers that extend perpendicularly to the long axis of their building blocks.…”

“…A 6,12-bis(dibromomethylene)-6,12-dihydroindeno[1,2-b] fluorene precursor (5), obtained from an indeno[1,2-b] fluorene-6,12-dione via solution synthesis, was employed on Au(111) to grow cumulene-poly-II (Scheme 1B). 76 Although the reactive apexes of five-membered rings in these polymers are protected by the presence of connections between repeat units, this case has been included in the present review because of its interesting findings in terms of favored electronic configuration.…”

“…This led to 78% of B being incorporated in copolymers, while the remaining ones were found as isolated species. 69% of the heptacenes in the copolymers were separated by only one IF linker (i.e., in the 76 Copyright 2021, Royal Society of Chemistry targeted -B-L-B-L-B-L-sequence), but the copolymer chains had a fairly short average length, with few chains reaching 20 nm or more. DFT calculations of the electronic properties of a regular copolymer segment adsorbed on Au(111) revealed a small band gap of 0.25 eV, strongly reduced from the values of poly-II (1.20 eV) and heptacene (0.50 eV).…”

“…Depending on precursor design, this resulted in step‐ladder‐ or ladder‐type IF polymers, which extended in the direction of the long axis of their IF constituents 7,74,75 . In another study, bromoethylene functional groups located at the five‐membered ring apexes of an IF precursor afforded IF polymers that extended perpendicular to the long axis of their repeat units, with cumulene‐like bridges between them 76 . Finally, IF precursors were also used as linkers to overcome steric hindrance in aryl‐aryl homocoupling 77 .…”