Investigation on the construction, photophysical properties and dyeing mechanism of 1,8-naphthalimide-based fluorescent dyes suitable for dyeing wool fibers in supercritical CO2

Technology of supercritical carbon dioxide (scCO2) dyeing has been used in dyeing synthetic and natural fibers. It is important to understand the scCO2 dyeing mechanism owing to the possibility of using this knowledge to design dyes that are suitable for scCO2 dyeing. Here, using the N-site and 4-site modification of 1,8-naphthalimide matrix, six fluorescent dyes (1, 2, 3, 4, 5, and 6) with fluorescent properties were synthesized; dyeing performance and fluorescent images indicate that all the synthesized dyes can be utilized for scCO2 dyeing wool fibers. Therefore, the dyed wool fibers possess not only conventional dyeing characteristics but also luminescent properties. Especially, the dyed wool fibers with solid free fluorescent dye 2 exhibit strong yellow-green fluorescence. Owing to the lack of proper fixation process, the fixation rate of these six fluorescent dyes on wool fibers was not relatively high. Thus, the dye 7 with an active group N-hydroxysuccinimide (NHS) was synthesized to improve the binding force between dyes and wool fibers and it can react with the ?NH2 group in wools, which results in that the fixation rate of the synthesized dye 7 reaches approximately 91% through chemical combination. Mass spectra prove that the byproduct of 7 reacting with the amino group of wool fibers is not corrosive to dyeing equipment, which realizes the eco-friendly fluorescent dyeing of wool fibers in scCO2. Our results proved that the fluorescent dyes with conjugated system can overcome the problem of fluorescence quenching in natural fibers. Meanwhile, our strategy of constructing fluorescent dyes and the proposed dyeing mechanism will be helpful to develop reactive disperse dyes that are suitable for scCO2 dyeing.