Quinoidal bithiophene as disperse dye: Substituent effect on dyeing performance

A series of quinoidal bithiophene (QBT) dyes, namely QBT1-QBT7, which exhibit high absorption coefficients in the red spectral region, were synthesized and used as disperse dyes to examine their ability for dyeing poly(ethylene terephthalate) fabric. The dyeing properties, such as color representation, dye exhaustion, dyeing rate, build-up and fastness properties were investigated. Quantum chemical density functional theory (DFT) calculation suggested that there are much larger QBT-benzene interaction energy and QBT-QBT interaction energy due to the high quinoidal structural planarity. Substituent effect was also studied by DFT calculation. Suitable alkyl groups with controlled steric hindrance were suggested to facilitate the formation of single dye molecule and dye diffusion on the fiber surface and inside. As a result, QBT6 with two nbutyl groups as end groups gave the best overall dyeing performance. This study provides a preliminary suggestion for the rational design of new disperse dyes based on quinoidal structure.