Physicochemical Investigation of the Panchromatic Effect on β-Substituted ZnII Porphyrinates for DSSCs: The Role of the π Bridge between a Dithienylethylene Unit and the Porphyrinic Ring

Three novel dyes based on Zn-II porphyrinates combined, in beta-pyrrolic position, with the pi unit dithienylethylene (DTE) have been synthesized and investigated for application in DSSCs. The panchromatic effect due to elongation of the pi-delocalized system through a bridge between the porphyrinic ring and the DTE unit such as the 4-ethynylstyryl (1), ethynyl (2), and ethenyl (3) bonds have been investigated by computational, electrochemical, and photoelectrochemical methods. For all three dyes the pi conjugated substituents in the beta position produced the expected panchromatic effect with broadened electronic absorption spectra over a wide range of wavelengths and IPCE spectra featuring a broad plateau in the region 430-650 nm. In addition both DFT computational and electrochemical data have shown a smaller HOMO-LUMO energy gap for dye 3, when compared to dye 2 suggesting a slightly more facile conjugation between the porphyrinic core and the DTE unit through the ethenylic bond. Conversely the photoelectrochemical investigation showed improved DSSC performances from 3 to 1. These results have been rationalized by an in-depth DFT computational study of dyes 2 and 3 interacting with a cluster of 82 TiO2 units. The small energetic overlap between the LUMO and the TiO2 conduction band characterizing the more structurally distorted dye 3 would suggest low quantum yield of electron injection, while dye 2 shows a greater interaction between the LUMO of the dye and the semiconductor. Consequently the increased linearity and planarity of the structure of dye 1 seems to be the origin of its best performance in DSSC. Therefore it appears that the nature of the bridge between the DTE unit and the porphyrinic ring is quite relevant for the efficiency of these dyes for DSSC, due to distortion from the planarity and linearity of the structure of the dye and the consequent changes on the dye pi conjugation.