Ground-State Redox Potentials Calculations of D-π-A and D-A-π-A Organic Dyes for DSSC and Visible-Light-Driven Hydrogen Production

The prediction of ground-state redox potentials by quantum chemical methods has a prominent role in the rational design of novel organic photosensitizers both for dye-sensitized solar cells (DSSCs) and photocatalytic systems for the production of H-2. Indeed, the ground-state redox potential of the photosensitizers is one of the key parameters to identify the most promising candidates for such applications. Here, the ground-state redox potentials of 16 organic donor-pi-acceptor D-pi-A and donor-acceptor-pi-acceptor D-A-pi-A dyes having a medium to large size of the conjugated scaffold are evaluated, using the methods of the Density Functional Theory (DFT), in terms of free energy differences between their neutral and oxidized ground-state forms. These results are compared to the available experimental data and to the computed highest occupied molecular orbital energy -epsilon(HOMO) values as an approximation of ground-state redox potentials according to Koopmans' theorem. Using the MPW1K functional in combination with the 6-31+G* basis set, the strategy based on the free energy cycle, including solvent effects, reproduces with a good level of accuracy the observed values (mean absolute error (MAE) < 0.2 eV) and trend of redox potentials within related families of dyes. On the other hand, the -epsilon(HOMO) values are only able to capture the experimental trends in redox potential values.