Theoretical study of sensitizer candidates for dye-sensitized solar cells: Peripheral substituted dizinc pyrazinoporphyrazine-phthalocyanine complexes

We have carried out a theoretical study of the geometrical and electronic structures of a family of planar dimers constituted by zinc(II) pyrazinoporphyrazine and zinc(II) phthalocyanine with peripheral electron-donating and electron-withdrawing substituents R [where R =-OH (1),-C(CH₃)₃ (2),-CH₃ (3),-C₆H ₅ (4),-H (5),-CO₂H (7),-NO₂ (7), and-PO ₃H₂ (8)]. The complexes are connected by varying the bridge (B) ligand, where, in 1-9, B is-CH= and, in 10-12, B is-N=,-O-, and-S-, respectively. The-CO₂H group was included in complexes identified as 9-12. This was done because of the known properties of this group in acting as an anchor to adsorb a dye onto a semiconductor oxide. The aim of this work was to provide a useful theoretical basis for the design and screening of new potential dye candidates to be used in these devices, based on the properties of the dyes suitable for their good performance in solar cells, such as frontier molecular orbital spatial distributions; charge-separated states in the electronic transitions in the visible region of the spectrum; and importantly, the energy diagram of the frontier MOs of these dyes and the conduction band (CB) of the semiconductor, where the LUMO energy levels that are above of the CB suggest which dyes are capable of electron injection into TiO₂. In this sense, it is expected that complexes 1-5 and 9-12 should be very promising dyes to act as sensitizers. Finally, a linear correlation was found between the HOMO and LUMO energies of all of the systems and the Hammett constants, where these molecular orbitals become more stable when R is more electron-withdrawing.