b-Pyrrolic Substituted Porphyrins in Dye Sensitized Solar Cells: Synthetic Benefits, Properties and Future Perspectives

Concept
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).
Motivations and Objectives
Porphyrins are one of the most widely investigated classes of molecules in chemistry due to their chemical stability, catalytic activities, interesting optical and photophysical properties. Today, the best performance in a porphyrin-sensitized solar cell has been reached by a well-engineered meso disubstituted push-pull ZnII-porphyrinate. Nevertheless such class of porphyrins can be obtained only by multistep and uneconomical synthetic routes. On the contrary b-substituted ZnII-porphyrinates are of great interest because they can easily be obtained with remarkable yields using facile synthetic procedures. In addition they are featured by a significant steric hindrance which guarantees a decrease of p-staking aggregation on TiO2 photoanode resulting highly beneficial to DSSC performances.
Results and Discussion
Our investigation has shown that b-substituted ZnII-porphyrinates are capable of serving as effective dyes in DSSCs, showing performances comparable to the most commonly used meso substituted push-pull ZnII-porphyrinates. An in-depth EIS (Electrochemical impedance spectroscopic) investigation has proven that the origin of the high efficiency, performed by DSSC devices consisting of b-substituted ZnII-porphyrinates, appears to be ascribed to a superior passivation of the TiO2 surface against the charge recombination which involves I3- species of the electrolyte. This superior screening effect can be related to the higher steric hindrance of the tetraarylporphyrinic architecture of such macrocyclic systems. Furthermore, pyrrolic functionalization of the porphyrinic core appears to be beneficial in tuning the properties of these dyes. In fact, they are strongly influenced by the introduction of p-delocalized systems which exert strong steric and electronic effects on the porphyrinic ring resulting in a dramatic alteration of their optical, electrochemical and spectroelectrochemical properties. Increasing the elongation of the p-chain in b-position, by adding thienyl units, the electronic absorption spectra of ZnII-porphyrinates cover a wide range of wavelengths producing a panchromatic effect in their IPCE spectra. The information extracted from our studies are helpful focusing on the development and synthesis of novel and more efficient β-substituted porphyrinic dyes which appear to be remarkable, viable and efficient alternatives to the more synthetically demanding porphyrinic dyes which exhibit the push-pull meso geometry.