First-Principles Analysis of Protonation-Induced Electronic Effects in Tetrakis(p-aminophenyl)porphyrin (TAPP)

The optical response of pH-sensitive porphyrins depends critically on the protonation site, not merely on the total charge accumulated. Using first-principles linear-response theory, we systematically map all symmetry-distinct protonation states of tetrakis(p-aminophenyl)porphyrin (TAPP), from neutral to fully protonated (+6), revealing that peripheral and core protonation produce fundamentally different photophysical outcomes. Peripheral protonation progressively attenuates Q-band intensity while leaving the macrocyclic core electronically intact, whereas core protonation drives conformer convergence, pronounced Q-band red-shifting, intensification, and merging of split-transition upon diprotonation. Q-band oscillator strength thus evolves nonmonotonically across protonation topologies─enhanced by core protonation through orbital mixing and suppressed by peripheral protonation through donor decoupling. At intermediate mixed protonation states (+4/+5), remaining unprotonated aminophenyl groups act as donors in aminophenyl-to-macrocycle charge transfer (CT) excitations, identified through charge-density difference (CDD) analysis. Core protonation additionally widens the singlet-triplet gap (ΔE_ST) relative to peripheral protonation, with implications for intersystem crossing (ISC) and triplet-state engineering. Together, these results establish protonation topology as a molecular design parameter for controlling absorption, CT character, and excited-state properties in pH-responsive porphyrin systems.