Synthesis, DFT Analysis, and Dyeing Performance of 4,4’-Dihyroxy-3-Substituted Azobenzene Disperse Dyes with Comparative Evaluation of Aqueous and DMF-Based Application Systems

A series of 4,4′-dihydroxy-3-substituted azobenzene disperse dyes (2a–f) were synthesized, fully characterized (FT-IR, 1H/13C NMR, CHN elemental analysis) and evaluated by DFT (B3LYP/6-31G(d, p)) to link electronic structure with dyeing performance. TD-DFT reproduces experimental λmax with deviations ≤ 12 nm, supporting the computational model. Compound 2e (3-CHO) shows the highest electrophilicity (ω = 4.6441 eV) and dipole moment (µ = 4.9366 D), while dye 2d (3-COOH) delivered the highest color strength (K/S = 21.64 in 5% DMF–95% H2O), corresponding to a ≈ 46.3% increase relative to the aqueous + dispersant system (K/S = 14.79). Exhaustion (%E) and fastness data (wash, perspiration, scorch, light) indicate generally commercial-grade fixation for the series, with light fastness uniformly high (7–8). We show that modest DMF addition (5% v/v) improves color uptake for some substituents but can slightly reduce certain fastness metrics; therefore DMF is used here as a mechanistic probe (improves solubility/partitioning under controlled lab conditions) rather than a recommended scale-up solvent. The combined experimental, DFT analysis identifies structure property rules to guide design of disperse dyes with improved color strength and predictable application behaviour.