Excited-states of BODIPY-cyanines: ultimate TD-DFT challenges?

We have investigated with first principle approaches the optical signatures of derivatives combining a BODIPY core and cyanine-like side chains. More precisely, we computed the 0-0 energies with a Time-Dependent Density Functional Theory (TD-DFT) procedure systematically including both vibrational and continuum solvent effects. However, despite its refinement, this protocol yields large deviations compared to experimental references. For this reason, we turned towards a mixed protocol where the potential energy surfaces of both the ground and the first electronically excited states are evaluated with TD-DFT whereas the vertical transition energies (both absorption and emission) are determined with the CIS(D)/SOS-CIS(D) approaches, that include a perturbative correction for the double excitations. The pros and cons of such a mixed method are discussed in the framework of these challenging dyes.