Fluorinated aluminum(III) porphyrins: Synthesis, spectroscopy, electrochemistry and photochemistry

A series of fluorinated free-base porphyrins (H2TPPFn, n = 0, 8, 12, 20, 24) and the corresponding aluminum(III) porphyrin (AlTPPFn-Ph, n = 0, 8, 12, 20, 24) derivatives have been synthesized and their spectroscopic, redox and optical properties were investigated. The absorption studies show that the spectral shapes of investigated porphyrins are sensitive to the degree of fluorination on the meso-phenyl units. Analogously, the fluorescence quantum yields and singlet-state lifetimes depend on the number of fluorine atoms, and decrease by increasing the number of fluorine atoms. The H2TPPFn and AlTPPFn-Ph (n = 8, 12, 20, 24) derivatives exhibited lower fluorescence intensities compared to the H2TPP and AlTPP, respectively. However, the AlTPPFn-Ph (n = 0, 8, 12, 20, 24) derivatives yield relatively a strong fluorescence compared to the well-known ZnTPP. As predicted, the redox potentials are shifted to the more positive side by increasing the fluorine atoms. The Lewis acidity of AlTPPFn-Ph was quantified by using the absorption and fluorescence titrations with the Lewis base N-methylimidazole (Me-Im). The titration data suggests that the Lewis acidity of the Al center rises when increasing the number of fluorine atoms on the porphyrin. Together, the high fluorescence quantum yields, high-potentials, unique optical and redox properties suggest that the investigated porphyrins could be potential sensitizers to mimic various components of artificial photosynthetic systems for the production of solar fuels.