To derive an appropriate perspective of the problem, a total of seven molecules/ions were examined in this study (Scheme 2). Unsubstituted tetraphenylporphyrin (H2[TPP]) provides a suitable baseline for evaluating all the calculated substituent effects. We also considered three additional tetraarylporphyrins in both their neutral and protonated states; each of these has paraaminophenyl groups at three of the meso positions, with the remaining meso position occupied analogues of these diformate complexes are well-precedented experimentally and have been referred to as porphyrin diacids in the literature, a useful term that we will use in our discussion below. The structures were all optimized with the OLYP functional with Grimme's D3(0) dispersion corrections and all electron STO-TZ2P basis sets. TDDFT calculations were carried out with the range-separated CAMY-B3LYP functional, which is the Yukawa form of CAM- Central diprotonation of all three of the tris(4-aminophenyl)porphyrins leads to a significantly reduced HOMO-LUMO gap. The primary cause is a lowering of the LUMO energy levels due to increased admixture of aminophenyl character relative to the free bases, but the exact details of the orbital energy shifts differ among the three protonated species studied. In the case of TAPP diacid, meso-aryl character mixing leads to both destabilization of the a2u-type HOMO and stabilization of the two LUMOs, resulting in a very much shrunken HOMO-LUMO gap. Protonation of the porphyrin core and of the pyridyl nitrogen of H2[TA3PyP] results in a dramatic stabilization of the LUMO and smaller stabilizations of the HOMO and LUMO+1, translating to a HOMO-LUMO gap very close to that calculated for TAPP diacid. Note that the LUMO of triprotonated H2[TA3PyP] exhibits C20-Cipso(Py) pbonding, consistent with the aforementioned Type D resonance form; a similar but weaker p-bonding interaction is also observed for the HOMO. These enhanced p-bonding interactions lead to both a shortening of the Cmeso-Cipso bonds involving the conjugating meso-aryl groups and a planarization of the Ca-Cmeso-Cipso-C2/6(aryl) dihedrals. Thus, the OLYP-D3 optimized Cmeso-Cipso(Py) distances are 1.484 and 1.447 Å in the unprotonated and triprotonated forms of H2[TA3PyP], respectively, while the aforementioned dihedrals around this bond are approximately 55 and 30°, respectively. Such structural changes have also been inferred from a detailed 1 H NMR study of the two species. Qualitatively similar structural changes are also observed for the aminophenyl groups upon protonation of the central porphyrin nitrogens.