Conformational and Tautomeric Effects on Sulfonated Azo Dyes: A Prelude to Photodegradation

In this work, we present a systematic investigation of the ground- and excited states of six sulfonated azo dyes with broad applications in the textile industry: Allura Red, Amaranth, Carmoisine, Ponceau, Red 2G, and Sunset Yellow. Ground-state properties were evaluated using density functional theory (DFT) at the ωB97X-D4/ma-def2-SVP level of theory. To identify the most suitable functional for excited-state calculations and to assess vertical excitation energies to low-lying excited states, we tested a series of hybrid, meta-hybrid, and range-separated hybrid functionals from Jacob’s ladder using the Tamm-Dancoff approximation (TDA) and the ma-def2-SVP basis set and compared to results from RI-CC2 and STEOM-DLPNO-CCSD approaches. Excited-state properties were then further examined using TDA at the CAM-B3LYP//ωB97X-D4/ma-def2-SVP level of theory. Additionally, the quantum theory of atoms in molecules (QTAIM) was applied to explore density-related properties in both ground and excited states. We found, perhaps not surprisingly, that excitation weakens the N−−N bond in both the enol and keto forms of the dyes, but with variations depending on the molecule, conformer, and electronic state. This bond weakening, as reflected by changes in electron density descriptors, suggests a possible route for photodegradation in aquatic environments.