A new green macrocyclic sensitizer for photoelectrochemical devices

The absorption properties of p-extended macrocycles such as porphyrins and phthalocyanines have been intensively applied to the light -harvesting process related to organic photovoltaics, with particular focus on polymer cells and Dye-Sensitized Solar Cells1. In the last few years a large amount of new porphyrins2 have been synthesized and studied and amongst them an interesting push-pull structure, having an impressive efficiency value of 12%, thus comparable to that of N719, has been obtained and published3. Even if phthalocyanines have obtained lower values in terms of efficiency, having the most efficient ones recently exceeded 6%4, their outstanding stability and high molar extinction coefficients (e> 105) make them very attractive sensitizers with possible applications in Graetzel cells and justify the intensive study carried on these molecules. Our goal was to investigate the performances of macrocyclic-like compounds having a structure that could potentially combine the best features of both the molecules described above. Benzoporphyrins, never investigated as chromophores in DSSC devices, show an intense absorption in the visible and rear-IR regions of the solar spectrum which can be modulated by an accurate tailoring of their structure. These structures can possibly be substituted on the free meso and b-positions, allowing the formation of push-pull systems that can affect the frontier orbitals of the systems to promote and favour the electron injection into the TiO2 surface. Moreover, the insertion of bulky electron-donating groups on the periphery of the macrocycle could suppress or at least diminish those aggregation phenomena which can lower down the efficiency of the correspondent solar cells. In the present contribution, the multistep synthesis of triphenyltetrabenzoporphyrin TP1 and its optical, chemical and electrochemical characterizations will be presented. Theoretical calculations of both frontier molecular orbitals and simulated absorption spectra with the preliminary measurements of the related DSSCs will be also discussed.