The Type D Hyperporphyrin Effect

DFT and TDDFT calculations lend credence to the importance of a Type D resonance form for triply protonated 5,10,15-tris(4-aminophenyl)-20-(4-pyridyl)porphyrin (with all central nitrogens and the pyridine nitrogen protonated), in which all four meso-aryl groups engage in conjugative interactions with the porphyrin core. The LUMO of this species is strongly stabilized relative to the HOMO and LUMO+1 as a result of such an interaction with the pyridinium group, which leads to both a narrowing of the HOMO-LUMO gap and strongly split HOMO-to-LUMO and HOMO-to-LUMO+1 transitions in the Q region. This orbital energy pattern also explains key elements of the hyperporphyrin character of the species, strong redshifts of both the HOMO-to-LUMO transition in the Q region and the HOMO-1-to-LUMO transition in the Soret region.