We have used both action and photoelectron spectroscopy to study the response of isolated Pd(II) meso-tetra(4-sulfonatophenyl)porphyrin tetraanions ([PdTPPS](4-)) to electronic excitation over the 2.22-2.98 eV photon energy range. The action spectrum obtained by recording the wavelength-dependent intensity of charged decay products closely resembles the absorption spectrum of PdTPPS in aqueous solution (which shows pronounced Q and Soret absorption bands). The two main decay channels observed are sulfonate group loss and, predominantly, electron emission. To better understand the electron emission channel, we have also acquired photoelectron spectra at multiple detachment photon energies covering the range probed in action spectroscopy. Upon both Q and Soret band excitation, we find that electrons are emitted in three characteristic kinetic energy ranges. The corresponding detachment processes are identified as (delayed) tunneling emission from both excited singlet and triplet states (each of which is accessed by/after one-photon absorption) as well as resonant two-photon detachment. The first triplet state lifetime of isolated [PdTPPS](4-) is significantly longer than 10 μs, possibly on the 100 μs time scale. We estimate that more than 50% of the electron emission observed upon photoexcitation occurs by way of this triplet state.

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We have probed for reaction intermediates involved in the dual-gold-catalyzed activation of a conjugated 1,5-diyne substrate and its further coupling to benzene in the liquid phase. This was done by sampling the reaction mixture by electrospray ionization followed by high-resolution ion mobility mass spectrometryunder conditions allowing for the resolution of structural isomers differing in their collision cross sections by less than 0.5%. For the cationic mass corresponding to catalyst + diyne (activation stage) we resolve four isomers. At the mass corresponding to catalyst + diyne + benzene, two isomers are observed. By comparing the experimentally obtained cross sections to those inferred for model structures derived from density functional computations, we find our measurements to be consistent with the proposed solution mechanism. This constitutes the first direct observation of intermediates in dual gold catalysis and supports the previous inference that the mechanism involves cooperative interactions between two gold centers.

ABSTRACT:We have combined ion mobility mass spectrometry with quantum chemical calculations to investigate the gasphase structures of multiply negatively charged oligomers of mesotetra(4-sulfonatophenyl)metalloporphyrins comprising the divalent metal centers Zn II , Cu II , and Pd II . Sets of candidate structures were obtained by geometry optimizations based on calculations at both the semiempirical PM7 and density functional theory (DFT) levels. The corresponding theoretical cross sections were calculated with the projection approximation and also with the trajectory method. By comparing these collision cross sections with the respective experimental values we were able to assign oligomer structures up to the tetramer. In most cases the cross sections of the lowest energy isomers predicted by theory were found to agree with the measurements to within the experimental uncertainty (2%). Specifically, we find that for a given oligomer size the structures are independent of the metal center but depend strongly on the charge state. Oligomers in low charge states with a correspondingly larger number of sodium counterions tend to form stacked, cofacial structures reminiscent of H-aggregate motifs observed in solution. By contrast, in higher charge states, the stack opens to form coplanar structures. ■ INTRODUCTIONMetalloporphyrins constitute the reactive centers in a large number of biochemical systems such as hemoglobin, myoglobin, and chlorophyll. As a consequence, they are wellstudied in condensed phase. 1 It is also well-known that porphyrins and metalloporphyrins can self-assemble into larger oligomers in solution. This aggregation has attracted considerable interest, especially for porphyrins functionalized with highly polar groups such as sulfonic acids. 2−9 Much of this effort has been focused on water-soluble meso-tetra(4-sulfonatophenyl)porphyrin (TPPS). Ribóet al. 6 investigated aqueous solutions thereof spectroscopically. In studies performed at various concentrations and pH values, they found evidence for the formation of small J (edge-to-edge) and H (face-to-face) aggregates. The light scattering measurements of Micali et al. 10 have in fact suggested that even larger mesoscopicself-similar clusters of TPPS J-aggregates may form in solution. More recently, Hollingsworth et al. 9 have investigated the aggregation of TPPS in aqueous solution using UV−vis and fluorescence spectroscopy, small-angle X-ray scattering, analytical ultracentrifugation, and transmission electron microscopy. They inferred stacks of ring structures containing typically 25 monomeric units.TPPS oligomers as well as aggregates of the corresponding metalloporphyrins, MTPPS, can also be prepared and studied in gas phase, i.e., in the absence of solvent molecules. For example, multiply negatively charged aggregates of MTPPS (also containing sodium counterions) can be generated by electrospray ionization (ESI) and detected with mass spectrometry. To what extent such isolated aggregates can also provide an indirect measure of aggregatio...

We present gas-phase structures of dimers of Mn and Fe meso-tetra(4-sulfonatophenyl)porphyrin multianions with various amounts of sodium and hydrogen counterions. The structural assignments are achieved by combining mass spectrometry, ion mobility measurements, quantum chemical calculations, and trajectory method collision cross section calculations. For a common charge state, we observe significant topological variations in the dimer structures of [(MTPPS)+nX] (M=Mn, Fe; X=H, Na; n = 1-3) induced by replacing hydrogen counterions by sodium. For sodium, the dimer structures are much more compact, a finding that can be rationalized by the stronger interactions of the sodium cations with the anionic sulfonic acid groups of the porphyrins as compared to hydrogen. Graphical Abstract ᅟ.