b-substituted porphyrinic dyes with tunable photoelectrochemical properties

The facile synthesis of b-substituted porphyrins and their photoelectrochemical properties make this class of dyes very promising for application in Dye Sensitized Solar Cells (DSSCs). Although the best performances in porphyrin-sensitized solar cell has been reached by well-engineered meso disubstituted push-pull ZnII-porphyrinates, these latter can be obtained only by multistep and uneconomical synthetic routes. On the contrary b-substituted ZnII-porphyrinates can easily be obtained with remarkable yields using facile synthetic procedures and they also show performances comparable or even better than the structurally corresponding meso substituted push-pull ZnII-porphyrinates. This is due to a significant steric hindrance which guarantees a decrease of p-staking aggregation and a superior passivation of the TiO2 surface against charge recombination with I3-, resulting highly beneficial to DSSC performances. Furthermore, the properties of porphyrinic dyes are strongly influenced by introducing various p-delocalized systems on b-pyrrolic position exerting both strong steric and electronic effects on the porphyrinic core. These induce dramatic alterations of optical, electrochemical and spectroelectrochemical properties in the porphyrinic dyes. In fact the elongation of the p-chain, in b-position, by an additional chromophore, as a dithienylethylene (DTE) unit, increases the light harvesting capacity of ZnII-porphyrinates over a wide range of wavelengths and, thus, produces a panchromatic effect in their IPCE spectra. The photophysical properties of DTE meso and b-substituted ZnII-porphyrinates were comparatively investigated in order to gain insights about the excited state dynamics relevant to TiO2 sensitization of photoelectrochemical cells. Here, the results obtained by time-resolved spectroscopy, EIS investigation and DSSC performances will be discussed and compared. The information extracted from our studies should be helpful to the synthesis of novel and more efficient β-substituted porphyrinic dyes as alternatives to the more synthetically demanding meso-substituted porphyrinic counterparts.