Protonated N-Confused Porphyrin Dimer: Formation, Structure, and Guest Binding

The protonation of 3,3'-bis (meso-tetratolyl-2-aza-21-carbaporphyrin) with various acids was studied. The stepwise formation of mono-, di-, and tetracationic species was shown on the basis of UV-vis-near-IR and low-temperature H-1 NMR. Upon going from di- to tetraprotonated form, the bis(porphyrinoid) skeleton changes its conformation from cisoid to bent-transoid, which was found by single-crystal X-ray analyses, 2D NMR, and density functional theory (DFT) calculations. The formation of cation-anion complexes was established in both the solid state and solution. The substitution of anions was studied by spectrophotometric and H-1 NMR titrations. A pronounced decrease of the HOMO-LUMO gap in the tetraprotonated species was shown by cyclovoltametry and time-dependent DFT calculations.