Technology based on the use of high-performance lasers and sensors for the generation and detection of infrared light promises to be one of the key technologies of the 21st century. Realization of this goal will require the use of dyes that absorb light specifically in the near-infrared region of the spectrum, and such compounds are therefore of special interest to both researchers and commercial firms. One class of chemicals that show great potential in this area are the so-called peri-arylenes which, thanks to their excellent stability properties, are already being used in a wide range of technical applications. A research team led by LMU chemist Professor Heinz Langhals now reports the synthesis of the first peri-arylene consisting of six basic subunits, whose absorption spectrum is almost entirely confined to the near-infrared (NIR) region. In other words, the substance appears colorless to the human eye, and this makes it suitable for use as an inconspicuous labeling agent. The findings appear in the Journal of Organic Chemistry.
The peri-arylenes are made up of naphthalene molecules linked together to form a ribbon, in which the two fused aromatic rings of each naphthalene subunit are aligned in parallel. Crucially, their absorption spectra are determined by the number of subunits in the chain. The shortest member of the class, with a single naphthalene unit, absorbs light in the ultraviolet region of the spectrum. "For that reason, this substance is now being tested for use in sunscreens, but it has not yet been incorporated into any such product," Langhals reports. The next in the series – with two subunits – absorbs light in the visible range, and it is already in commercial use in several contexts, including organic solar cells.
"As more naphthalene subunits are added, the absorption spectrum shifts further towards the long-wavelength end of the spectrum," Langhals explains. "However, as the chain-length is increased, the molecules become less soluble, and that significantly limits their range of application." Langhals has now succeeded in solving this problem, and synthesized the first peri-arylene that contains six naphthalene building blocks. "The decisive step was the introduction of functional groups that we had previously developed, which markedly increase solubility," says Langhals. "By this means, we were ultimately able to isolate the precursors of the new substances, which were then successively linked together to yield the desired end-product."
The 6-member derivative absorbs essentially only in near-infrared region, which lies "below the red" and is therefore invisible to the human eye. "Fabrics dyed with the new compound appear colorless but the dyestuff can be detected with a suitable sensor," Langhals explains. One conceivable application for such a marker lies in the recycling of expensive textiles, since the compound would greatly simplify the sorting process. But in addition to the end-product of the synthesis, the shorter members of the series are also of considerable practical interest. This is because they exhibit intriguing light-induced responses, which may make them suitable for use in fluorescent solar collectors. "One other advantage of the new substances lies in their environmentally benign character: They consist entirely of organic materials that are biodegradable and fully combustible," Langhals adds.
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Heinz Langhals et al. Sexterrylenetetracarboxylic Bisimides: NIR Dyes, The Journal of Organic Chemistry (2015). DOI: 10.1021/acs.joc.5b02092