Conformational Dynamics of α,β,γ,δ-Tetraarylporphyrins and Their Dications

Rotational isomerism in several tetraarylporphyrin free bases and the corresponding dications has been studied by using dynamic ¹H NMR spectroscopy. The average free energy of activation for rotation of a single aryl ring for the stereoisomers of α,β,γ,δ-tetrakis(2-methoxyphenyl)porphyrin is ΔG*433 = 25.9 kcal/mol. The barrier for the corresponding dication is ΔG*m = 23.0 kcal/mol. For α, β, γ, δ-tetrakis(2, 4-dimethoxyphenyl) porphyrin and its dication, the rotational barriers are ΔG*i33 = 25.2 and ΔG*m = 21.8 kcal/mol, respectively. These results shed light on the questions of the degree of distortion of the porphyrin macrocycle in solution and the orientation of the aryl rings relative to the mean porphyrin plane.