Cu(II)- and Mn(III)-Porphyrin-Derived Oligomeric Multianions: Structures and Photoelectron Spectra

Abstract

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.

Photoelectron spectroscopy of the protoporphyrin IX dianion

“…Porphyrins have been studied in the gas phase using mass 6 and ion mobility 7,8 spectrometry, as well as infrared, 9 photodissociation 10 and photoelectron spectroscopy. [11][12][13][14] Protoporphyrin IX is a D 2h porphyrin with two propionic acid groups as substituents on the macrocycle, which can form a carboxylate monoanion or dicarboxylate dianion following the removal of one or two protons. The structure of PP 2is shown in figure 1.…”

“…11,12 More recently, Kappes and coworkers have used photoelectron spectroscopy to study manganese, copper and palladium meso-tetra (4-sulphonatophenyl) porphyrin (TPPS) polyanions and the corresponding dimers and trimers. 13,14 Electron loss was observed to occur via direct detachment, as well as delayed emission via tunnelling from the S 1 and T 1 states in [PdTPPS] 4 -. 13 To date there have been no photoelectron spectroscopy studies of PP 2 -.…”

Q and Soret Band Photoexcitation of Isolated Palladium Porphyrin Tetraanions Leads to Delayed Emission of Nonthermal Electrons over Microsecond Time Scales

“…Their unique spectral properties have also led to a large number of applications, for example, as light-harvesting molecules in solar cells. , Furthermore, water-soluble metal porphyrins play an important role as photosensitizers in photodynamic therapy . In this respect, Pd II meso -tetra­(4-sulfonatophenyl)­porphyrin (PdTPPS) has attracted considerable interest due to its long triplet state lifetime, its high singlet oxygen sensitization efficiency, and the observation of efficient radical formation upon photoexcitation (of aqueous solutions comprising [PdTPPS] 4– tetraanions). , Compared to the large number of condensed-phase studies, spectroscopic investigations of porphyrins and related molecules in the gas phase are still rather limited. − Such measurements, free of the oftentimes perturbing solvent environment, are useful to gain a more detailed understanding of the underlying fundamental photophysical properties of the (metallo)­porphyrins and their aggregates . Depletion spectroscopy, or the corresponding “action” measure of decay products with mass-to-charge ratios different from those of the photoexcited parents, has been extensively used to characterize the electronic absorption properties of large isolated ions.…”

Reaction Monitoring and Structural Characterisation of Coordination Driven Self-Assembled Systems by Ion Mobility-Mass Spectrometry

“…Another example of careful comparison between modelled and experimental cross sections is demonstrated by the structural determination of metalloporphyrin oligomeric assemblies ( Schwarz et al, 2013 ; Schwarz et al, 2014 ). Interestingly, as hydrogen counterions were exchanged for sodium counterions, topological changes of the dimer were observed using TWIMS ( Schneider et al, 2018 ).…”