Four years ago chemists developed a Lego-like method for making giant molecular aromatics. Now the same team has broken all records by creating a huge, nearly 13 nm long, twisted nanoribbon with 53 linearly fused rings and over 320 conjugated atoms. The molecule also exhibits exciting optoelectronic properties.1

“Creating huge flavors is a very difficult task,” explains Aurelio Mateo-Alonso of Polymat in Spain, who led the study. “Large aromatic structures pile up in piles, making them insoluble and near-insoluble materials that cannot be purified and characterized using conventional techniques,” he adds. Twisted aromatics comprise sterically overcrowded entities that force them into curled arrangements, making them less prone to stacking and generally more soluble than their planar counterparts.

The team already held the record for the longest molecular nanotape with a 2018 design that contained 30 linearly fused rings.2At that time, the previous record doubled – a 16-ring nanotape that was practically insoluble and difficult to characterize. “Then we used an iterative synthesis approach in which pyrene- and nitrogen-doped anthracene building blocks were combined,” explains Mateo-Alonso. However, they had to completely redesign the process to achieve longer systems. “Rather than gradually adding small pieces, we decided to prepare larger units and combine them later,” he adds. To improve solubility, they added pendant alkylsilyl groups and introduced coronene junctions that enable twisting. ‘Combined [with] Side groups and multiple twists along the nanoribbon backbone have increased the solubility to a level we cannot imagine, ”says Mateo-Alonso.

The team also conducted computer studies to better understand the properties of twisted nanoribbons. “The calculation of its electronic properties was not very easy given its large dimensions,” explains Manuel Melle-Franco from Ciceco, Portugal, who carried out the calculation work. To our delight, the theoretical properties correlate very well with the experimental ones. ‘

Chunyan Chi, an expert on π-conjugated systems at the National University of Singapore, says: “The breakthrough is the synthesis of a new family of molecular nanoribbons that exceed both the greatest and the greatest lengths [number of] aromatic kernels [previously] reported ‘. She also appreciates the synthetic strategy, noting that “the twisted aromatic backbone together with the presence of carefully selected solubilizing groups resulted in excellent solubility in common organic solvents at room temperature”. Among other things, this enabled simple purification by column chromatography and broad characterization of the new nanoribbons using common techniques such as NMR, UV-Vis and fluorescence spectroscopy.

Max von Delius, an expert on supramolecular organic systems at the University of Ulm, explains that the production of these compounds is interesting “from a fundamental perspective”. In contrast to graphene, nanoribbons have an adjustable band gap, which is very attractive for applications in optoelectronics, photonics and photovoltaics. “It is very rare that we see gray solutions [with] intensive absorption of visible light over a wide spectral range, ”says von Delius. ‘[These molecules] can very well find applications in photo detectors, ”he adds.

Von Delius also highlighted the value of the Lego-like modular synthesis. The side chains could be tailored, which further improves the processability of the molecules in solution. In addition, he believes that the introduction of entities such as sulfur atoms could bind the nanoribbons to noble metal electrodes and enable the manufacture of single-molecule transistors. “The strategic use of nitrogen atoms … to ensure the stability of the products on the test bench is also impressive,” he adds.

Mateo-Alonso explains that their twisted nanoribbons have a red fluorescence in addition to light absorption. “That was really surprising, light emission is an unusual effect in giant aromatics,” he says. “These results suggest possible applications in light emitting diodes and lasers.” The group is already exploring new record-breaking molecules and other applications for their giant aromatics.

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