Comparison of theoretical approaches for predicting the UV/Vis spectra of anthraquinones

We have computed the absorption spectra of a large series of anthraquinone dyes using four semi-empirical and ab initio approaches. Without fitting, the ZINDO//AM1 and CIS//HF schemes provide only poor estimates of the λmax of substituted anthraquinones, with mean absolute errors (MAE) of 80 and 180 nm, respectively, for a set of 45 dyes. These approaches are therefore unable to deliver correct estimates of the colour of anthraquinones. On the contrary, TD-DFT schemes appear to be much more robust and provide MAE under the 20 nm limit. If one uses experimental input to calibrate the theoretical values, subsequently allowing a statistical treatment, the theory–experiment discrepancies become comparable (MAE between 11 and 13 nm) for all four approaches.