Facile synthesis of 1-butylamino- and 1,4-bis(butylamino)-2-alkyl-9,10-anthraquinone dyes for improved supercritical carbon dioxide dyeing

Abstract Supercritical carbon dioxide (scCO2) has recently been conquering both material sciences and process engineering owing to its intriguing properties like high diffusivity and tuneable (de)solubilizing performance. However, except for impregnation with biocidal preservatives, utilization of scCO2 for modification of wood including dyeing has been hitherto less explored. Therefore, we recently proposed a green wood dyeing approach that relies on the excellent carrier medium properties of CO2 for non-polar disperse dyes in supercritical conditions and their reversion when leaving the supercritical state. However, using a common disperse dye of the 9,10-anthraquinone family (Disperse Blue, DB-134) without the addition of co-solvent, only a moderate colour change was obtained for the interior of small wood cuboids (1 cm × 1 cm x 2 cm) even after extensive variation of scCO2 process conditions. Insufficient solubility of the dye in scCO2 was assumed to be a major hindrance for homogeneous wood dyeing. Therefore, we synthesized a variety of 1-n-butylamino- and 1,4-bis(n-butylamino)-9,10-anthraquinone derivatives carrying different alkyl moieties in 2-position, which was confirmed to improve scCO2 solubility. Instead of a previously reported seven-step synthesis affording moderate yields only (60% on average), we propose an alternate much simpler synthesis comprising of Marschalk alkylation (2-position) and a sequence of tosylation and n-alkylamination (1-position). The latter proved to pave way to a wide variety of colours and disperse dyes of improved solubility in scCO2. Moreover, all products were characterized by various techniques including liquid-state 1H and 13C NMR and X-ray crystallography. Complementing X-ray diffraction, quantum-mechanical simulations were performed to predict or confirm conformation and colour of selected compounds.