Low-Symmetry Ω-Shaped Zinc Phthalocyanine Sensitizers having Panchromatic Light-Harvesting Property for Dye-Sensitized Solar Cells.

Two low-symmetry phthalocyanines (Pcs) substituted with thiophene units at the non-peripheral (alpha) and peripheral (beta) positions were synthesized and their optical, electronic-structure, and electrochemical properties were investigated. The substitution of thiophene units at the alpha 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 Omega-shaped ZnPcs (alpha PcS1 and alpha PcS2) bearing a pi-conjugated side chain with an adsorption site at an alpha position of the Pc macrocycle were synthesized as sensitizers for dye-sensitized solar cells (DSSCs). The absorption spectra of alpha PcS1 and alpha PcS2 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 pi MOs. Compound alpha PcS1 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.