Synthesis, design, and characterization of N,N'-tethered cis-indigos
Date
2021-08-26
Authors
Hong, Hyejin
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Abstract
This dissertation explores the structural modification of indigo into its rare cis form by substitution of a short organic bridge between the two indole nitrogen atoms. The synthesized N,N′-tethered cis-indigoids are assessed for their physicochemical properties in order to understand the effect of the organic tether on cis-indigoid systems.
Three literature compounds containing “simple” tether structures are synthesized and subjected to complete characterization. Among them, alkyl group based cis-indigos, 2.5 and 2.6 exhibit similar absorption wavelengths as the parent indigo while oxalyl-tethered indigo 2.7 shows a large hypsochromic shift due to the strong electron-withdrawing nature of the oxalyl group. The electronics of the tether also affected the HOMO and LUMO energy levels; the oxalyl tether lowered both energy levels in comparison to the alkyl tethers. However, the indigoid co-planarity was strongly affected by the ring size from the tether rather than electronics.
A new tethered cis-indigo structure type was discovered in reactions involving quinones. This new tether type consists of a 2,2′-dihydroindigo unit connected to the cis-indigo backbone through a central C–C bond of the former. Incorporation of the quinone moiety was observed in 3.2 where the final structure is comprised of two molecules of indigo and one naphthoquinone, while structures of 3.3 and 3.4 contained two indigoid units only. Investigation by CV revealed the strength of the quinone altered the early reaction intermediates. These dimeric cis-indigos show a small hypsochromic shift in the absorption compared to the parent indigo and relatively planar cis-indigoid backbone. 3.2 demonstrates rich redox behaviours. 3.3 and 3.4 display dynamic behaviours observed by solution VT 1H NMR spectroscopy.
Oxalyl-tethered cis-Nindigo 4.6a and cis-indigo monoimine 4.7a were synthesized and compared with the indigoid counterpart 2.7 to examine the influence of the arylimine groups. Absorption wavelengths of the imine group containing species depended strongly on the electronics of the tether, yet the number of imine groups affected redox potentials. Protonation of 4.6a (to give H+oxalyl Nindigo 4.6aH+) causes a bathochromic shift in the absorption and allows for much easier reduction. The acidity (pKa) of 4.6aH+ is estimated between 3.6 – 4.46 in DMSO. A protonated cis-Nindigo derivative 4.9aH+ was obtained via reduction of the oxalyl group and is compared with 4.6aH+.
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Keywords
cis-indigo, cis-Nindigo, NMR, CV, UV-Vis, X-ray crystallography, quinone, indigo, indigoid, Nindigo