Assessment of BODIPY–Oxasmaragdyrin Dyads for Dye-Sensitized Solar Cells: Aromaticity, Photosensitization Capability, and Charge Transport

Dye-sensitized solar cells (DSSC) are presented as an alternative among renewable energies where the dye plays an important role to obtain an effective device. Our goal in this work is to examine the influence of several bridging functional groups between the BODIPY and oxasmaragdyrin systems forming dyads (D), as potential components of DSSC, on the aromatic, photophysical, and charge transport properties. A set of 11 dyads made of the oxasmaragdyrin with 2,6-dimethoxyphenyl and methylamine groups in two of their meso carbons (S) and the 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY, B) moieties that differ in the binding bridge between them has been analyzed with density functional theory (DFT). The geometries were optimized with the B3LYP/6-311G­(d,p) level of theory employing D3 Grimme′s correction, and a set of six functionals (B3LYP, BHandHLYP, CAM-B3LYP, PBE0, wB97X, TPSSh) was evaluated for reference systems in time-dependent DFT calculations. We found that TPSSh presents the best agreement with the available data of UV–vis spectra, so it was used for calculation of the electronic absorption spectra of the 11 oxasmaragdyrins and 11 dyads. When the bridge between S and B consists of one (D3), two (D5), or three acetylene units (D6), a strong absorption band in the infrared region around 1000 nm can be achieved. These bands correspond to charge-separated excited states that favor a panchromatic absorption in that region. The aromaticity index NICS(0) computed at the macrocycle center ring critical point using the GIAO/B3LYP/6-311G­(d,p) level of theory shows in all these systems an aromatic character for the oxasmaragdyrin macrocycle (from −10.7 to −12.4 ppm). We also found that all dyads present a favorable electron injection toward the semiconductor TiO₂ because the LUMO energy of the dyad is higher than the conduction band of the semiconductor (−4.3 eV) used in a solar cell. Besides, the HOMO energy of the dyads D3, D5, and D6 is lower than the redox potential (−4.8 eV) of a mediator as the I^–/I₃^– system used to recover it after circulation of electrons. Nonequilibrium Green′s function-based calculations performed for a couple of dyads, with (D6) and without (D4) a significant charge transfer band, connected to Au electrodes show that D6 was to be a better conductor, in agreement with the charge transfer results obtained from the photophysical properties. Finally, the Gibbs free energy for the formation of the dyads here investigated is calculated. All of them are shown to be exergonic reactions (ΔG_solution < 0), which suggests that these systems could be synthesized.