Sequential Photoinduced Energy and Electron Transfer Directed Improved Performance of the Supramolecular Solar Cell of a Zinc Porphyrin-Zinc Phthalocyanine Conjugate Modified TiO2 Surface

Improved performance of a photosynthetic antenna-reaction center mimicking supramolecular solar cell is demonstrated. Toward this, porphyrin-phthalocyanine conjugates connected by amide linkers, as wide-band capturing solar energy harvesting materials, have been newly synthesized and characterized. Efficient singlet-singlet energy transfer from the zinc or free-base porphyrin to phthalocyanine is evidenced from steady-state emission and transient absorption studies in nonpolar and polar solvents. Further, the dyad was immobilized via axial coordination of zinc porphyrin of the dyad on semiconducting TiO2 surface modified with axial coordinating ligand functionality, phenylimidazole. Photoelectrochemical studies revealed improved performance of this cell compared to either zinc porphyrin or zinc phthalocyanine only modified electrodes under similar experimental conditions. Transient absorption studies performed on the dyad immobilized on glass/TiO2 surface suggested that upon excitation of the axially coordinated zinc porphyrin of the dyad singlet-singlet energy transfer to zinc phthalocyanine occurs within 0.2 ps instead of a competing charge injection reaction from the singlet excited zinc porphyrin to TiO2. Further, sequential photoinduced electron transfer from the newly formed singlet excited zinc phthalocyanine to zinc porphyrin producing ZnP center dot--ZnPc center dot+ with a 2 ps time constant and followed by electron injection from the ZnP center dot- to TiO2 within 30 ps has been proposed as a mechanism of photocurrent generation in the biomimetic supramolecular photocell.