Estimation of the first excitation energy in diradicaloid croconate dyes having absorption in the near infra red (NIR): A DFT and SF-TDDFT study

We report in this Letter vertical singlet–singlet excitation energies obtained for a series of croconate molecules with diradical character (DRC), using DFT and SF-TDDFT methods. We observe that at the single determinant/RB3LYP level the HOMO–LUMO gaps (HLG) match the experimental absorption for molecules with high DRC. The stable broken symmetry (BS)-UDFT solutions, which are slightly lower in total energy yield overestimated HLG due to spin contamination. For molecules with medium and small DRC stable solutions are obtained at the RB3LYP level but the HLG deviates from the experimental value, the deviation being quite large in the latter case. Excitation energies calculated using single reference spin flip (SF)-TDDFT methods are in good agreement with the experiment for the molecules with medium-small DRC.