Predicting light absorption properties of anthocyanidins in solution: a multi-level computational approach

A multi-level computational protocol is devised to calculate the absorption spectra in ethanol solution of a series of anthocyanidins relevant for dye-sensitized solar cells. The protocol exploits the high accuracy of second-order multi-reference perturbation theory to correct the results of the more feasible TD-DFT calculations, which were performed on hundreds of configurations sampled from molecular dynamics (MD) trajectories. The latter were purposely carried out with accurate and reliable force fields, specifically parameterized against quantum mechanical data, for each of the investigated dyes. Besides yielding maximum absorption wavelengths very close to the experimental values, the present approach was also capable of predicting reliable band shapes, even accounting for the subtle differences observed along the homolog series. Finally, the atomistic description achieved by MD simulations allowed for a deep insight into the different micro-solvation patterns around each anthocyanidin and their effects on the resulting dye's properties. This work can be considered as a step toward the implementation of a computational protocol able to simulate the whole system formed by the organic dye and its heterogeneous embedding that constitutes dye-sensitized solar cells.