Cu(II)- and Mn(III)-porphyrin-derived oligomeric multianions: structures and photoelectron spectra

We present structures and photoelectron spectra of Mn(III) and Cu(II) meso-tetra(4-sulfonatophenyl)porphyrin (TPPS) multianions, as well as of homomolecular dimers and trimers thereof. The structural assignments are based on a combination of mass spectrometry, ion mobility measurements, and semiempirical as well as density functional theory (DFT) calculations. Depending on the type of central metal atom, two completely different dimer structural motifs are found. With a central Mn(III), the monomeric units are connected via sulfonic-acid-manganese bonds resulting in a tilted stack arrangement of porphyrin rings. With Cu(II) as the central atom, the sulfonic acid groups preferentially bind to the sodium counterions, resulting in a flat dimer structure with coplanar porphyrins. Photoelectron spectra were recorded for monomers, dimers, and trimers, each in a number of different negative charge states as determined by protonation degree (+nH). In some cases, e.g., [Cu(II)TPPS](4-), [(Mn(III)TPPS)2 + H](5-), and [(Mn(III)TPPS)3 + 3H](6-), we observe electron detachment energies close to zero, or even slightly negative. In all cases, we find a large repulsive Coulomb barrier. The observed trends in detachment energies can be interpreted in terms of a simple electrostatic model.