Indigo—A New Tribological Substance Class for Non-Toxic and Ecological Gliding Surfaces on Ice, Snow, and Water

Abstract

The biogenic substance E-indigo can form supramolecular, hydrophobic structures using self-organization. These structures show a low coefficient of friction as a gliding layer against polar surfaces. The formation of primary particles with platelet morphology based on hydrogen-bonded E-indigo molecules is ideal to produce the gliding layer. Structures with excellent gliding properties can be achieved by means of directed friction and high pressure, as well as through tempering. The resulting hard, thin gliding layer of E-indigo does not easily absorb dirt and, thus, prevents a rapid increase in friction. Field tests on snow, with cross-country skis, have shown promising results in comparison to fluorinated and non-fluorinated waxes. Based on quantitative structure–activity relationship (QSAR) data for E-indigo, and its isomers and tautomers, it has been demonstrated that both the application and abrasion of the thin indigo layers are harmless to health, and are ecologically benign and, therefore, sustainable.

Photochromic derivatives of indigo: historical overview of development, challenges and applications

“…This exceptional photostability of indigo and its derivatives has attracted significant scientific interest and, therefore, has been investigated comprehensively. To understand the photochemical and photophysical properties of indigo, a great number of theoretical and experimental studies [25][26][27][28][29][30][31][32][33][34][35] have been performed so far, which allowed to characterize excited state lifetimes, radiative and non-radiative relaxation pathways of the indigo chromophore [26,27,[30][31][32][33][34][35] as well as to estimate ionization potentials and electronic structures [28,29], singlet oxygen generation capacity [31], QSAR properties [25], and others.…”