Towards the understanding of the chromatic behaviour of triphenylmethane derivatives

The absorption spectra of a large panel of substituted arylcarbonium dyes have been simulated with the PCM-TD-DFT theoretical scheme. For most compounds, there are at least two allowed excited-states showing a strong molar absorption coefficient in the UV/Vis region. Both involve a charge transfer from the para-NMe2 to the central carbonium. It turns out that B3LYP, combined with the 6-31G(d,p) basis set for the ground-state optimization, and with the 6-311G(d,p) basis set for the TD-PBE0 calculations, yields reliable auxochromic shifts when the solvation effects are included in the model. Indeed, a statistical treatment of the raw TD-DFT results allows a prediction of the λmax with a small average standard deviation: 26 nm. The accuracy required for the design of new molecules is globally reached: the mean average error is only 19 nm, in spite of the charge-transfer nature of the excitations. Moreover, linear correlations have been established between: (i) the ring-N average bond length and the experimental λmax(1-2), (ii) vibrational frequency of the chromophore and the λmax(2).