Low‐Symmetry Ω‐Shaped Zinc Phthalocyanine Sensitizers with Panchromatic Light‐Harvesting Properties for Dye‐Sensitized Solar Cells

Two low-symmetry phthalocyanines (Pcs) substituted with thiophene units at the non-peripheral (α) and peripheral (β) positions were synthesized and their optical, electronic-structure, and electrochemical properties were investigated. The substitution of thiophene units at the α positions of the phthalocyanine skeleton resulted in a red shift of the Q band and significantly modified the molecular-orbital electronic distributions just below the HOMO and just above the LUMO, with distortion of the typical Gouterman four-orbital arrangement of MOs. Two amphiphilic Ω-shaped ZnPcs (αPcS1 and αPcS2) bearing a π-conjugated side chain with an adsorption site at an α position of the Pc macrocycle were synthesized as sensitizers for dye-sensitized solar cells (DSSCs). The absorption spectra of αPcS1 and αPcS2 showed red shifted Q bands and a broad band from 350 to 550 nm assignable to the intramolecular charge-transfer transition from the ZnPc core to the side chains. Time-dependent DFT calculations provided a clear interpretation of the effect of the thiophene conjugation on the typical phthalocyanine core π MOs. Compound αPcS1 was used as a light-harvesting dye on a TiO2 electrode for a DSSC, which showed a panchromatic response in the range 400–800 nm with a power conversion efficiency of 5.5 % under one-sun conditions.