TD-DFT Vibronic Couplings in Anthraquinones: From Basis Set and Functional Benchmarks to Applications for Industrial Dyes

Vibrationally resolved absorption spectra of a series of anthraquinoidic dyes have been obtained with a polarizable continuum model time-dependent density functional theory approach. Firstly, we assessed the impact of the atomic basis set on both the transition energies and the vibronic shapes of 1,4-NH2-anthraquinone using a large panel of Pople's basis sets, up to the 6-311++G(3df,3pd). In a second stage, an extensive functional benchmark has been performed to determine an adequate approach for the same compound. In the third step, a complete analysis of the origin of the band shape was performed for the same derivative. In the fourth stage, a set of functionals has been applied to investigate the position isomers in the dihydroxy anthraquinone series. Finally, in a last phase, the methodology has been used for three dyes of technological interest. It turns out that the chosen basis set has a relatively limited impact on the computed transition energies as well as the topology of the vibronic shape, but both are significantly influenced by the selected functional. In the present case, no single functional simultaneously provides highly accurate positions and intensities of the different bands, but omega B97XD appears to be a good compromise. This analysis allows to rationalize the difference in shapes experimentally noticed for the visible band of apparently similar anthraquinones.