Maximizing Property Tuning of Phosphorus Corrole Photocatalysts through a Trifluoromethylation Approach

An eight-member series of CF₃-substituted difluorophosphorus corroles was prepared for establishing a structure-activity profile of these high-potential photosensitizers. It consisted of preparing all four possible isomers of the monosubstituted corrole and complexes with 2-, 3-, 4-, and 5-CF₃ groups on the macrocycle's periphery. The synthetic pathway to these CF₃-substituted derivatives, beginning with (tpfc)PF₂, involves two different initial routes: (i) direct electrophilic CF₃ incorporation using FSO₂CF₂CO₂Me and copper iodide, or (ii) bromination to achieve the 2,3,8,17,18-pentabrominated compound using excess bromine in methanol. Crystallographic investigations revealed that distortion of the original planar macrocycle is evident even in the monosubstituted case and that it becomes truly severe for the penta-CF₃-substituted derivative 5. There is a shift in redox potentials of about 193 mV per -CF₃ group, which decreases to only 120 mV for the fifth one in 5. Differences in the electronic spectra suggest that the Gouterman four orbital model decreases in relevance upon gradual -CF₃ substitution, a conclusion that was corroborated by DFT calculations. The very significant energy lowering of the frontier orbitals suggested that photoexcitation should lead to a highly oxidizing photocatalyst. This hypothesis was proven true by finding that the most synthetically accessible CF₃-substituted derivative is an excellent catalyst for the photoinduced conversion of bromide to bromine (phenol, toluene, and benzene assay).