Diprotonation-Induced Dicationic Effects Governing Structural Saddling and π-Electron Delocalization in Tetra(meso-aryl) Porphyrins

The hyperporphyrin effect in diprotonated tetra(meso-aryl) porphyrins has been investigated based on spectroscopic experiments and theoretical analysis. Diprotonation of free-base tetra(meso-aryl) porphyrins induces steric repulsion among the four central hydrogen atoms, resulting in characteristic structural saddling of the porphine macrocycle. The dicationic nature of the diprotonated porphine macrocycle stabilizes the frontier molecular orbitals, particularly the a2u-like HOMO, bringing it into energetic proximity with the HOMO and HOMO-1 of the meso-aryl substituents. This orbital energy matching facilitates enhanced electronic coupling between the central porphine macrocycle and the meso-aryl substituents, manifesting in two coupled phenomena: (i) the relatively less-stabilized HOMO of the porphine macrocycle and (ii) coplanarization of the meso-aryl substituents, accompanied by amplified structural saddling. The reduced HOMO-LUMO gap is spectroscopically evident as bathochromic shifts in both the B (Soret) and Q-bands. Concurrently, the enhanced structural saddling weakens the optical forbidden character of the S1 state, significantly increasing Q-band oscillator strengths. These findings demonstrate that the dicationic nature of diprotonated tetra(meso-aryl) porphyrins drives a synergistic coupling between electronic coupling and structural saddling, providing the essential mechanism underlying the hyperporphyrin effect in tetra(meso-aryl) porphyrins.