TD-DFT Evaluation of Electronic Spectra of Coumarin and Anthraquinone Dyes: Effects of Functionals, Basis Sets, and Solvation Conditions.

The electronic absorption spectra of 20 organic dyes (7 coumarins, 10 anthraquinones, and 3 other similar molecules) were investigated using TD-DFT with BLYP, B3LYP, and B3PW91 functionals and four basis sets: 6-31++G-(d,p), 6-311++G-(2df,p), aug-cc-pVDZ, and aug-cc-pVTZ. Gas-phase, implicit, and hybrid solvation models were tested. The hybrid model, guided by electrostatic potential maps, yielded the lowest mean absolute errors with respect to experimental absorption maxima, underscoring the role of specific electrostatic solute-solvent interactions. Overall, the solvent effects were most pronounced with B3LYP and B3PW91. Replacing the LYP correlation functional with PW91 led to shifts of up to 6 nm, while modifying the exchange term from B to B3 caused deviations of at least 33 nm. Basis set performance was consistent across molecules, showing approximately linear trends. Within this protocol, electrostatic-potential-guided microsolvation provides an efficient way to capture key solute-solvent interactions without resorting to more demanding QM/MM approaches.