Tuning redox potentials of phosphorus(V) tetraarylporphyrins without altering their optical properties

A series of phosphorus(V) porphyrins functionalized with meso-phenyl, tolyl, or 4-methoxyphenyl groups and axial-ethoxy, trifluoroethoxy, or ethylene glycoxy groups have been synthesized and their spectroscopic, redox, and optical properties were investigated. The X-ray crystal structures revealed a significant saddling in porphyrin molecular structures due to the presence of a small P(+5) ion in the cavity. The absorption and fluorescence properties are less sensitive to the meso-functionality, with small redshifts observed when moving from phenyl to tolyl and then to 4-methoxyphenyl on the phosphorus(V) porphyrin. In contrast, the meso- and axial-functionalities significantly influenced the redox properties. Specifically, the meso-4-methoxy phenyl group lowered the oxidation potentials, while the axial trifluoroethoxy units reduced the reduction potentials. This study demonstrates that by carefully choosing the axial and meso-substitutions, it is possible to selectively tune the redox potentials with minimal impact on optical properties. Steady-state and time-resolved measurements revealed relatively high fluorescence quantum yields, with lifetimes ranging from 2 to 4 ns. Together, the high fluorescence quantum yields and tunable redox potentials suggest that the investigated porphyrins could serve as excellent photosensitizers in model compounds for artificial photosynthesis and molecular electronic and photonic devices.