Article pubs.acs.org/JPCA

Cu(II)- and Mn(III)-Porphyrin-Derived Oligomeric Multianions: Structures and Photoelectron Spectra Ulrike Schwarz,† Matthias Vonderach,† Markus K. Armbruster,† Karin Fink,‡ Manfred M. Kappes,†,‡ and Patrick Weis*,† †

Karlsruhe Institute of Technology, Institut für Physikalische Chemie, Fritz-Haber-Weg 2, 76131 Karlsruhe, Germany Karlsruhe Institute of Technology, Institut für Nanotechnologie, Postfach 3630, 76021 Karlsruhe, Germany



ABSTRACT: We present structures and photoelectron spectra of MnIII and CuII 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 MnIII, the monomeric units are connected via sulfonic-acid−manganese bonds resulting in a tilted stack arrangement of porphyrin rings. With CuII 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., [CuIITPPS]4−, [(MnIIITPPS)2 + H]5−, and [(MnIIITPPS)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.

1. INTRODUCTION Metalloporphyrins act as reactive centers in many biological systems such as chlorophyll and hemoglobin and are therefore well-studied in the condensed phase.1 Besides their huge biological importance, they are fascinating from a purely chemical point of view: they consist of a large heteroaromatic system interacting with a central metal atom, which can be widely varied both in charge and chemical character. Furthermore, the effective charge on the porphyrin ring can also be easily altered by functionalization with polar groups such as sulfonic acid or pyridyl substituents, e.g., adding four negative or positive charge centers, respectively. Porphyrin aggregates are also of great interest, e.g., for light harvesting. In solution, porphyrins can self-assemble into oligomers.2−6 Hollingsworth et al.7 have investigated the aggregation of meso-tetra(4-sulfonatophenyl)porphyrin (TPPS) by UV−vis and fluorescence spectroscopy as well as by small-angle X-ray scattering. Gandini et al. have observed the formation of an oxygen-bridged FeIII-dimer in solutions of FeIIITPPS.5 Gas-phase studies focus largely on the reaction of metalloporphyrin ions with small molecules.8−13 In comparison to © XXXX American Chemical Society

the large number of investigations in the condensed phase, the number of gas-phase studies of porphyrin supramolecular complexes and oligomers remains rather limited. Arai et al.14 have determined the relative stability of supramolecular porphyrin-calix[4]arene complexes and Haino et al.15 have studied the aggregation of bisporphyrin dimers. In a recent study, we have investigated the structure and stability of dimeric and trimeric oligomer multianions of Mn(III) and Fe(III) porphyrin derivatives in the gas phase by ion mobility spectrometry and collision-induced dissociation. Highly charged dimeric oligomer multianions of these systems were observed to be metastable with respect to dissociation into monomers. Dissociation is, however, prevented by a large electrostatic barrier against fragmentation.16 From a chemical point of view, porphyrins closely resemble phthalocyanines, which in turn have been investigated in the gas phase somewhat more extensively. Wang et al. have determined the photoelectron spectra of tetrasulfonated copper Received: November 13, 2013 Revised: December 11, 2013

A

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correspondingly depends on their mobility. This allows us to separate species of the same mass-to-charge ratio (isomers and oligomers) which would otherwise be indistinguishable by mass spectrometry alone. Furthermore, we use the method to obtain structural information by determining the ion mobility K, which can be easily converted into the collision cross section Ω using the relation23

phthalocyanine tri- and tetraanions. For the tetraanions, surprisingly high negative electron binding energies of ca. −0.9 eV were observed (i.e., implying metastability toward electron loss from the ground state).17,18 Interestingly, these authors also observed that the multianion photoelectron spectra were closely related to those of the respective neutral phthalocyanine.19 From this they concluded that the charges localized at the peripheral −SO3− groups uniformly shift up the molecular energy levels of the phthalocyanine moiety. A simple electrostatic model was presented to explain this level shift quantitatively. In a related study, Arnold et al.20 observed that copper phthalocyanine tetrasulfonate-tetraanions (in their ground state) can decay via electron autodetachment on a time scale of several minutes, and the rate is somewhat dependent on the substitutional isomer probed. In a femtosecond pump−probe photoelectron spectroscopy study on the same system, Ehrler et al.21 found that Q-band excitation leads to a 14 order of magnitude enhancement in electron autodetachment rate. Both findings were explained in terms of electron tunneling through the repulsive Coulomb barrier (RCB), and the rate is strongly dependent on the energy difference between the initial state and the RCB maximum. In this study we focus on metal porphyrin tetraphenylsulfonates. When free of counterions (e.g., fully deprotonated/ desodiated), these contain four negative charge centers localized on the perimeter sulfonate groups which in turn surround a porphyrin ring containing a positively charged metal center. For the purposes of this study, the metal center is either a triply charged Mn(III) or a doubly charged Cu(II). Both metal porphyrin tetraphenylsulfonate molecules (M-TPPS) can form multiply negatively charged oligomer ions when electrosprayed from solution. The questions which we would like to address in this publication are 3-fold. To what extent does the charge state of the metal influence the oligomeric structures? Are the highly negatively charged oligomer ions metastable with respect to electron detachment; if so, how high is the RCB that prevents immediate decay? Can the simple electrostatic picture proposed by Wang et al17,18 for phthalocyanine monomers also be applied to porphyrin multianions and oligomers? To address these questions we combine gas-phase ion mobility spectrometry (to determine the geometric structures) with photoelectron spectroscopy (to determine electron binding energies) and density functional calculations.

K=

3q 16N

2π 1 μkBT Ω

(1)

T and N are the temperature and number density of the helium gas; q is the ion charge, and μ is the reduced mass of helium and the ion of interest. Structures can be assigned by comparing experimental cross sections with theoretical values based on quantum chemically calculated candidate structures (here: geometry optimization at the semiempirical PM7 level using MOPAC1224). The cross sections of candidate structures are calculated with the projection approximation25 (with the parameters implemented in the MOBCAL package26) before being compared to experiment. Ions leaving the drift cell are focused by a second “exit” ion funnel through a 1 mm diameter aperture into the main chamber. To minimize contamination by neutral particles, the ion beam or packet is bent by 90° at this stage and subsequently focused into a quadrupole mass filter (Extrel). This is set to transmit a specific mass-to-charge ratio. Singly charged monomers and multiply charged oligomers of a given mass-to-charge ratio both pass the mass filter, but at different times (due to their different mobilities). In order to record a photoelectron spectrum, the detachment laser (fourth (266 nm) and fifth (213 nm) harmonics of a ND:YAG, Spectra Physics, LAB 150-30) is synchronized with the respective ion packet. The photoelectrons generated are guided by a magnetic field (“magnetic bottle”) and detected by a stack of multichannel plates. Their flight time is recorded in a high-resolution time digitizer (7888-2, FAST Comtec) and converted into electron binding energy (EBE) according to Einstein’s relation EBE = hν − E kin

(2)

Photoelectron energy spectra are plotted also taking into account the appropriate Jacobi transformation. Typically the resolution (ΔEkin/Ekin) is 3% at 1 eV kinetic energy. To minimize background contributions from residual neutral molecules, photoelectron spectra were measured with and without the ion beam on alternating laser shots. Alternating measurements were subtracted from each other and the differences then summed. The structures of the different monomeric, dimeric, and trimeric species are obtained by geometry optimization at the PM7 level as implemented in the MOPAC12 package.24 For each species we investigated different candidates, i.e., different protonation/sodiation positions and different relative orientations of the monomeric subunits. Typically 20 candidate structures were investigated for each dimer and trimer. All geometries were fully optimized. All DFT calculations were carried out with the program package TURBOMOLE27 using the RI-DFT module and the B3LYP functional28,29 with the def2-SVP basis set30 for elements H, C, N, O, Na, and S and the def2-TZVP basis set30 for the transition elements Cu and Mn (to ensure a larger flexibility for the description of their d-orbitals). For all calculations we used the quadrature grid m3 and the self-

2. METHODS Mn(III) meso-tetra(4-sulfonatophenyl)porphyrin-hydrochloride [MnClC44H24N4(SO3H)4] and Cu(II) meso-etra(4sulfonatophenyl)porphyrin-sodium salt [CuC 44 H 24 N 4 (SO3Na)4] were obtained from Frontier Scientific and used without further purification. Ions were generated by electrospraying 1 mmol/L solutions in water/methanol. Details of the experimental setup have been described elsewhere.22 Briefly, the “IMS-MS-PES” instrument used combines ion mobility spectrometry (IMS) with mass spectrometry (MS) and photoelectron spectrometry (PES). It comprises an electrospray ionization source which electrostatically floats at the entrance potential of an ion mobility drift cell into which electrosprayed ion packets are injected. This drift cell is a 60 cm long tube filled with typically 2.5 mbar of helium. Prior to injection into the cell the ions are accumulated in an “entrance” ion funnel. They are then pulse injected as a 50 μs wide ion packet. Subsequently, the ions are guided through the cell by a weak electric field (typically 10 V/cm) and their drift time B

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Figure 1. Calculated structures, optimized at the PM7 level. Deprotonated sulfonates are indicated in yellow and red, whereas protonated sulfonates are shown in green and red. For clarity all hydrogens on the porphyrin and benzene rings are omitted. The sodium ions are indicated by larger gray spheres.

treatment of multianions because of the fact that the highest occupied molecular orbitals (HOMOs) in many cases show positive orbital energies. This may be a hint of the self-

consistent convergence threshold scfconv 6. The theoretical treatment of highly charged anions in the gas phase is a great challenge. In particular, care must be taken in the DFT C

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interaction error31 in the DFT treatment. As expected, we observed that using hybrid functionals reduces sensitivity to the self-interaction problem. In Hartree−Fock theory we observed only negative orbital energies for the investigated copper monomers. The structural parameters of the investigated systems [(CuTPPS)2 + 3Na]5−, [CuTPPS]4−, and [CuTPPS + Na]3− were optimized including the continuum solvation model COSMO (with default parameters)32 to avoid positive orbital energies. This was not the case for the [MnTPPS]3− system; here, the B3YLP treatment led to negative orbital energies for all occupied molecular orbitals. The influence of the COSMO model on the structural parameters of the copper monomer systems is quite small: in the center of the ring system there is only a minor change of 0.14 pm in the Cu−N bond length. Deviations become discernible only over large distances, i.e. the distances of the sulfur atoms to the copper center (958 pm) are elongated by 5 pm. The influence of the structural change on the detachment energies was estimated to be below 0.2 eV. The detachment energies for the Cu- and Mnmonomers were calculated as vertical ionization energies of the initial highly charged systems [CuTPPS]4−, [CuTPPS + Na]3−, and [MnTPPS]3−. Note that DFT calculations of detachment energies for these highly charged systems in the gas phase were carried out without using the continuum solvation model because the additional charges are otherwise artificially stabilized by the dielectric continuum. For determination of repulsive Coulomb barriers (RCB) we used the point charge model,33 i.e. we self-consistently calculated the interaction of a negative point charge with the [CuTPPS]3− and [CuTPPS + Na]2− systems for different pathways to the molecular center.34 Contour plots of the difference density were generated with gOpenMol.35

Table 1. Calculated and Experimental Cross Sections 2

calculated cross section (Å ) monomers [MnIIITPPS]3− [MnIIITPPS + H]2− [MnIIITPPS + Na]2− [CuIITPPS]4− [CuIITPPS + Na]3− dimers [(MNIIITPPS)2 + H]5− [(MnIIITPPS)2 + Na]5− [(CuIITPPS)2 + 3Na]5−

trimers [(MnTPPS)3 + 3H]6− [(MnTPPS)3 + 4H]5−

experimental cross section (Å2)

247 (DFT: 250) 247 254 249 (DFT: 251) 250 (DFT: 255)

251 248 246 256 245

402 410 380 (DFT: 389), Structure A 399 (DFT: 402), Structure B 476 (DFT: 481), Structure C

398 406 477

544 545

565 560

calculations correspond to tilted stacks of porphyrin rings16 (Figure 1). For example, the singly protonated dimer [(MnIIITPPS)2 + H]5− is held together by two manganese− sulfonic acid bonds. This is confirmed by the cross section measurement: the calculated (402 Å2) and experimental cross sections (398 Å2) agree within the experimental uncertainty. The same structural motif is also observed for the trimers (see Table 1, cf. ref 16). Because of their equal mass-to-charge ratios the doubly charged monomer, [MnIIITPPS + H]2−, the quadruply charged dimer, [(MnIIITPPS)2 + 2H]4−‑, and the 6-fold charged trimer [(MnIIITPPS)3 + 3H]6− cannot be differentiated mass spectroscopically. However, we can (partially) separate the three species according to differences in their collision cross sections upon passing the ion mobility drift cell.16 Note that while we can determine the overall structure, we are not able to experimentally determine which sulfonic acid group is protonated. CuII-TPPS. For both the quadruply charged monomer, [CuIITPPS]4−, and the triply charged monomer, [CuIITPPS + Na]3−, experiment and calculation (PM7 minimum energy structure) agree to within 3%. The values are very similar to the cross sections obtained for the corresponding Mn-monomers. For the quintuply charged CuII-dimer, [(CuIITPPS)2 + 3Na]5−, the situation is dramatically different: for this system we observe a very large cross section of 477 Å2, much larger than the value determined for the quintuply charged MnIII-dimer, [(MnIIITPPS)2 + H,Na]5− (398, 406 Å2, see Table 1). Performing a PM7 geometry optimization starting from a tilted stack with two copper−sulfonic acid bonds (analoguous to the manganese dimer) leads to a stable, local energy minimum structure (Structure A, Figure 1 and Table 1). However, at 380 Å2 its cross section is much smaller than the experimental value. Furthermore, it is not the global minimum at the PM7 level. The lowest energy structure that we found (Structure B, Figure 1 and Table 1) is 1.4 eV lower in energy, and its cross section is larger (399 Å2) but still 20% smaller than the experimental value. We therefore rule out this structure as well. Our best fitting candidate structure (Structure C, Figure 1 and Table 1) has a cross section of 476 Å2, in almost perfect agreement with experiment. It is only 0.3 eV higher in energy than Structure B, which is within the error we expect for semiempirical PM7 calculations.

3. RESULTS Both MnIII-TPPS and CuII-TPPS easily form negatively charged oligomers upon electrospray ionization. Using a high-resolution mass spectrometer (Thermo Fisher, LTQ orbitrap XL) with an electrospray ionization source comparable to that of the IMSMS-PES instrument, we clearly observed oligomers up to the pentamer for copper and up to the heptamer for manganese (see Figure 1 in ref 16). Similar but less resolved mass spectra were recorded with our IMS-MS-PES instrument. The latter machine was used to determine the collision cross sections and photoelectron spectra of several different charge states of (i) Mn(III)-TPPS- and Cu(II)-TPPS-monomers, (ii) Mn(III)TPPS- and Cu(II)-TPPS- dimers, as well as (iii) Mn(III)TPPS-trimers. 3.1. Structure Determinations. MnIII-TPPS. By comparing the cross sections of various PM7-optimized structures with experiment we can derive the general building principle for MnIII-TPPS oligomers. For the monomers, experiment and calculation agree to within the experimental uncertainty of 2− 3%. Sodiation (by titration of the MnTPPS-solution with NaOH) instead of only protonation has a negligible effect on the measured collision cross section (and thus on the structure), see Table 1. (Note that while the structural resolution of the ion mobility method does not allow for resolving the position of a single sodium atom, it would resolve larger structural rearrangements as a consequence of sodiation). The PM7 calculations predict that the dimers and trimers are held together by ionic bonds between the positively charged central manganese atoms and the negatively charged sulfonic acid groups. The lowest energy structures found in the D

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the leading edge of the spectrum. The spectrum contains an extended shoulder at 3.4 eV and a peak at 3.7 eV, respectively. Note that we could not obtain a photoelectron spectrum upon 266 nm (4.66 eV) excitation, implying that the RCB significantly exceeds 2 eV. For the singly protonated monomer dianion, [MnIIITPPS + H]2−, a photoelectron spectrum could not be obtained at either of the detachment wavelengths available (neither 213 nor 266 nm). The quadruply charged copper porphyrin monomer, [CuIITPPS]4−, has a negative ADE of −0.2 eV, i.e. it is metastable with respect to electron loss. This can be seen in the photoelectron spectra shown in Figure 2c. Furthermore, in the 266 nm spectrum, we observe only a single peak around 0.4 eV and a steep cutoff at 1.1 eV (black dashed line), whereas the 213 nm spectrum is bimodal with a second peak centered at 1.7 eV. On the basis of the cutoff in the 266 nm spectrum, we estimate the RCB to be 3.6 eV. For the triply charged, monosodiated monomer, [CuIITPPS + Na]3−, we could obtain a photoelectron spectrum only with 213 nm excitation (Figure 2b). Irradiation at 266 nm did not yield any photoelectrons. The 213 nm spectrum has a leading edge that extrapolates to 1.5 eV (Figure 2b, red dashed line), i.e. it is shifted by 1.7 eV to higher binding energies, relative to [CuIITPPS]4−. Compared to the manganese monomer of the same overall charge, [Mn III TPPS] 3−, the spectrum of [CuIITPPS + Na]3− is shifted to lower binding energies by 1.1 eV, implying that the electron is detached from the central part of the molecule (where the influence of the additional positive charge on the metal atom, MnIII versus CuII, is strongest) and not from one of the sulfonic acid groups. Dimers. Figure 3a shows the photoelectron spectra of the 5fold charged manganese dimer, [(MnIIITPPS)2 + H]5−, obtained at detachment laser wavelengths of 213 nm (5.82 eV) and 266 nm (4.66 eV). By linear extrapolation of the leading edge of the spectra (red dotted lines), we obtain an adiabatic detachment energy of 0.1 ± 0.1 eV. The spectrum recorded at the shorter wavelength shows a first peak around 0.9 eV corresponding to the vertical detachment energy and two resolvable features at 1.8 and 2.1 eV. Above 2.3 eV the photoelectron intensity drops steeply. The 266 nm spectrum shows a high-energy cutoff at 1.3 eV (black dashed line). On

To closer investigate this issue, we calculated the groundstate energies of the three isomers of the dimer [(CuTPPS)2 + 3Na]5− with the B3LYP, BP86,28,36 and TPSS37 exchange correlation functionals using the PM7-optimized structural parameters and with and without the COSMO model. We optimized further the structural parameters of all three isomers with the B3LYP functional. All DFT calculations predicted structure C as the most stable isomer, in excellent agreement with the experimental observation via the cross section (Table 2). The single-point calculations led to the following energetic Table 2. Relative Energies in Electronvolts for the Three Different Structural Isomers of [(CuTPPS)2 + 3Na]5−a structure method PM7 B3LYP B3LYP (without COSMO) B3LYP BP86 TPSS

structural parameters

A

B

C

PM7 PM7 PM7

+1.4 +2.3318 +3.0947

0 +1.1374 +0.5760

+0.3 0.0000 0.0000

optimized PM7 PM7

+0.4206 +2.2494 +2.1035

+2.2785 +1.1446 +0.9910

0.0000 0.0000 0.0000

a

All DFT calculations predict structure C as the most stable isomer, in excellent agreement with the experimental observation via the cross sections.

sequence of the isomers: C, B, A. The optimization of the structural parameters of the isomers led to a different sequence: C, A, B. So we conclude that the copper−porphyrin dimer has a completely different structure than the manganese−porphyrin dimer. In the case of copper, the monomeric units are held together by the sodium counterions interacting with the sulfonic acid groups. As a result, both porphyrin rings are essentially coplanar. 3.2. Photoelectron Spectra. Monomers. Figure 2a shows the photoelectron spectrum of the fully deprotonated monomer trianion, [MnIIITPPS]3−, recorded at a detachment energy of 5.82 eV (213 nm). We obtain an adiabatic detachment energy of 2.6 ± 0.1 eV by linear extrapolation of

Figure 2. Photoelectron spectra of the monomers: (a) [MnIIITPPS]3− (detachment wavelength 213 nm); (b) [CuIITPPS + Na]3− (detachment wavelength 213 nm); (c) and (d) [CuIITPPS]4− (266 and 213 nm, respectively). E

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the 266 nm spectrum. On the basis of the steep cutoff at 1.9 eV for 266 nm excitation, we estimate the RCB to be 2.8 eV. It is interesting to again compare the copper and manganese species of the same charge state: for the (triply charged) monomers, the copper-containing porphyrin has a detachment energy which is roughly 1 eV lower than that observed for the manganese-containing species (see Figure 2a,b). For the quintuply charged dimers, the order is reversed: [(CuIITPPS)2 + 3Na]5− has a detachment energy larger than that of [(MnIIITPPS)2 + H]5− (see Figure 3a,c). This is a consequence of the different dimer structures pertaining (see above). Trimers. The photoelectron spectra of the 6-fold charged trimer [(MnIIITPPS)3 + 3H]6− recorded at 213 and 266 nm detachment wavelength are shown in Figure 4a. In both spectra, extrapolation of their leading edges results in an adiabatic detachment energy close to zero (0 ± 0.1 eV). The 213 nm spectrum has a rather broad plateau, in the range between 0.7 and 2 eV, whereas there is a steep cutoff around 0.9−1.0 eV in the 266 nm spectrum, corresponding to a RCB of 3.6 eV. The photoelectron spectrum of the 5-fold charged trimer [(MnIIITPPS)3 + 4H]5− recorded at 266 nm has a leading edge at 0.9 ± 0.1 eV and a first peak at 1.4 eV. 3.3. DFT-Calculated Vertical Detachment Energies. The calculations of vertical detachment energies were done with the optimized structural parameters of the highly charged systems [CuTPPS]4−, [CuTPPS + Na]3−, and [MnTPPS]3−. The calculated detachment energies are in qualitative agreement with the experimental values: the B3LYP treatment predicts [CuTPPS]4− as metastable, i.e. the detachment energy is negative, and [CuTPPS + Na]3− and [MnTPSS]3− are found to be stable with positive detachment energies. The influence of the structural changes due to the optimization without COSMO for the copper monomers on the detachment energies was below 0.2 eV (see the values in brackets in Table 3). All detachment energies are calculated without COSMO. If one calculated the detachment energies including COSMO, one would find values which are 5 eV too high and positive for all investigated systems due to the artificial stabilization of the charges by the dielectric continuum.

Figure 3. Photoelectron spectra of (a) [(MnIIITPPS)2 + H]5−, (b) [(MnIIITPPS)2 + 2H]4−, and (c) [(CuIITPPS)2 + 3Na]5− obtained at detachment wavelengths of 266 and 213 nm for each multianion.

the basis of this cutoff we estimate the RCB to be quite large as well and roughly 3.4 eV (4.66 eV − 1.3 eV). As can be seen from Figure 3b, the quadruply charged dimer [(MnIIITPPS)2 + 2H]4−, has an adiabatic detachment energy that is significantly larger than that of [(MnIIITPPS)2 + H]5−. Linear extrapolation (red dotted lines) leads to a value of 1.3 ± 0.1 eV. With 266 nm excitation we observe a broad peak centered around 2.3 eV and a steep cutoff at 2.8 eV. At the higher detachment photon energy we observe a second peak at 3.1 eV. On the basis of the cutoff we estimate the RCB to be 1.9 eV. The spectrum is complicated by the fact that some 6fold charged trimers [(MnIIITPPS)3 + 3H]6− (and also some doubly charged monomer, [MnIIITPPS + H]2− (not photodetachable)), are present in the ion packets probed together with the quadruply charged dimer, [(MnIIITPPS)2 + 2H]4− (in spite of partial separation by IMS16). The low binding energy feature in Figure 3b corresponds to this trimer contamination. The 266 nm photoelectron spectrum of the quintuply charged copper dimer [(CuIITPPS)2 + 3Na]5−, Figure 3c, is unusual inasmuch as it has a weak intensity feature starting at 0.2 eV and a much higher intensity feature starting at 0.6 eV, with a first peak at 1.2 eV and a steep cutoff at 1.9 eV. At 213 nm, we observe a second peak around 2.6 eV which is absent in

Figure 4. (a) Photoelectron spectra of [(MnIIITPPS)3 + 3H]6− obtained at detachment wavelengths of 266 nm and 213 nm. (b) Photoelectron spectrum of [(MnIIITPPS)2 + 4H]5− obtained at a detachment wavelength of 266 nm. F

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assumptions are that the HOMO is largely localized in the central part of the metalloporphyrin (either on the metal atom or on the nitrogen atoms surrounding it) and that the negatively charged sulfonic acid groups merely bias the electrostatic potential experienced by the outgoing electron. This is in line with the calculated [Cu II TPPS] 4− − [CuIITPPS]3− difference density map (Figure 7). Such an “electrostatic corral” consequently leads to reduction in the detachment energy relative to the situation without perimeter (negative) charges. To first order, the detachment energy of [CuIITPPS]4− can then be related to the ionization energy (IE) of neutral [CuIITPPS] by the expression

Table 3. Experimental and Calculated Detachment Energies system 4−

[CuTPPS] /[CuTPPS]

experiment 3−

[CuTPPS + Na]3−/[CuTPPS + Na]2− 3−

[MnTPSS] /[MnTPPS]

2−

−0.2 eV +1.5 eV +2.6 eV

theorya −0.5539 eV (−0.4395 eV) +0.6555 eV (+0.8690 eV) +1.3457 eV

a

For the copper systems, the values in parentheses correspond to the structural parameters of the initial systems optimized without COSMO.

3.4. Calculated Repulsive Coulomb Barriers (RCB) for [CuTPPS]3− and [CuTPPS + Na]2−. We calculated selfconsistently the interaction of a negative point charge with both copper monomer systems at various positions along two different paths for [CuTPPS]3− and three different paths for [CuTPPS + Na]2− (Figure 5).33 All calculations were done with the optimized structural parameters of the highly charged initial systems, e.g., [CuTPPS]4− and [CuTPPS + Na]3−. For both systems, the distance of the maxima of the repulsive Coulomb barrier (RCB) in the z direction (Figure 6, path B) is located 476 pm away from the complex center. In the x direction (paths A and C), the distances of the maxima are located in the range of 850 to 950 pm (the sulfur atoms of the suflonate groups have distances of ca. 900 pm from the copper center). The height of the RCB of [CuTPPS]3− is 3.66 eV for path B and 3.87 eV for path A, in excellent agreement with the experimental value of 3.6 eV. The barriers for the [CuTPPS + Na]2− system are shifted down by an average amount of 1.2 eV because this system has one charge less than [CuTPPS]3− because of the sodium cation coordinated on one sulfonate group. Path B (along the z direction) is the most favorable, yielding an RCB height of 2.47 eV. Paths A and C show barrier heights of 2.75 and 2.52 eV, respectively. Path C is more favorable in the x direction because of the presence of a sodium-coordinated sulfonate group, and the RCB maximum is 50 pm closer to the center than in path A (950 pm).

4

DE(monomer)4 − = IE −

∑ i=1

e2 4πε0R (Cu−S)i

(3)

The sum corresponds to the overall electrostatic destabilization of the detachable electron. It runs over the four Coulomb repulsion terms between each sulfonic acid group and the outgoing electron (assumed to be localized at the central atom). e is the elementary charge, and Ri is the distance between the ith sulfonic acid group and the central atom, calculated using Cu−S separations obtained from the PM7 geometry optimization (9.54 Å in each case). The sum adds up to 6.04 eV. We have experimentally determined the adiabatic detachment energy to be −0.2 eV (see above), resulting in an ionization energy of 5.8 eV. To the best of our knowledge, the ionization energy of neutral protonated [CuIITPPS] has not been determined experimentally. However, Khandelwal et al. have determined an experimental value for neutral, unsulfonated [CuIITPP]: 6.49 eV.38 The two numbers are in reasonable agreement, taking into account that the four sulfonic acid groups will tend to stabilize the cation, thus lowering its ionization energy by several tenths of an eV relative to the neutral molecule. Furthermore, the first order expression in eq 3 does not take into account dielectric shielding of charges. The detachment energy of the monosodiated, triply charged monomer [CuIITPPS + Na]3− can be calculated analogously (see Figure 8 for illustration)

4. DISCUSSION The charge-dependent shifts in the photoelectron spectra can be explained by a simple electrostatic model17,18 The key

Figure 5. Three possible pathways along which to calculate the interaction of an elementary point charge with [CuTPPS + Na]2−. The first path (A) is along the x-axis in the plane of the ring system, path B along the z direction perpendicular to the π-system of the ring, and path C (only for [CuTPPS + Na]2−) along the x-axis in the plane of the ring system on the side with the Na−SO3 group. The dashed lines correspond to the respective position of the Coulomb barrier. G

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Figure 6. Interaction energies of a negative point charge with [CuTPPS]3− (left panel) and [CuTPPS + Na]2− (right panel) along two and three possible paths versus distance of the point charge from the complex center. The zero point of the energy scales corresponds to the energy of the free [CuTPPS]3− and [CuTPPS + Na]2− anions, respectively.

DE(monomer)4 − − DE(monomer)3 − = −

e2 4πε0R (Cu−Na) (5)

This simple electrostatic model can be used to estimate the detachment energy of the dimer as well. For the case of [(CuIITPPS)2 + 3Na]5−, eight repulsive and three attractive Coulomb terms have to be taken into account: 8

DE(dimer)5 − = IE −

∑ i=1

e2 + 4πε0R (Cu−S)i

3

∑ j=1

e2 4πε0R (Cu−Na)j (6)

For the estimation we can now use the value of 5.8 eV previously determined for the IE of neutral [CuIITPPS] from our monomer measurements (see above). Note that the situation for the dimer is slightly more complicated than for the monomer because of the fact that the electron can be detached from each of the two porphyrin rings, see Figure 8 c,d. Because the charge distribution around the two porphyrins is not equivalent, two different electrostatic (de)stabilization energies (the sums in eq 6) are obtained, depending on which porphyrin the electron is removed from. Removing an electron from the porphyrin on the right (Figure 8c) is subject to an electrostatic destabilization energy of 5.5 eV versus 4.9 eV when it is removed from the porphyrin on the left (Figure 8d). The corresponding predicted detachment energies are 0.3 and 0.9 eV, respectively, which may explain why our photoelectron spectrum (Figure 3c) shows a tail to low binding energies corresponding to a first adiabatic detachment energy of 0.2 eV and a second steeply rising edge starting at 0.6 eV. Conceivably, these features stem from electrons being detached from one and then from the other of the two porphyrin rings. In the same fashion, the electrostatic model can be applied to the manganese−porphyrin system. Taking the triply positive charge of the central atom into account, the charge-dependent shift can be calculated according to

Figure 7. Calculated difference electron density between [CuTPPS]4− and [CuTPPS]3− using a contour value of 0.001 au: ρ([CuTPPS]4−) − ρ([CuTPPS]3−); red, positive values of the difference density; blue, negative values of the difference density. 4

DE(monomer)3 − = IE −

∑ i=1

e2 e2 + 4πε0R (Cu−S)i 4πε0R (Cu−Na) (4)

The additional term corresponds to the attractive interaction between the electron (in the center of the porphyrin ring and the positive (point) charge located at the sodium atom (R(Cu−Na) = 8.83 Å). The electrostatic contributions (based on the PM7 coordinates) add up to 4.44 eV. Subtracting eq 4 from eq 3 yields a value of 1.6 eV for the difference between the detachment energies of the two charge states (which is independent of the exact value of the neutral IE). This number is in excellent agreement with the difference in measured energies of 1.7 eV.

4

DE(monomer)3 − = IE+ −

∑ i=1

H

e2 4πε0R (Mn−S)i

(7)

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Figure 8. Schematics illustrating the simple electrostatic model used to predict photodetachment thresholds. It is based on two assumptions: (i) outgoing electrons are removed from the center of the porphyrin ring and (ii) the corresponding ionization energies are shifted relative to those of the analogous neutral molecules due to Coulomb interactions with perimeter anionic sulfonic acid groups and sodium cation(s) (see text). Electrostatic shifts were calculated by summing over all Coulomb interactions between the outgoing electron and localized point charges as indicated for (a) [CuIITPPS]4−, (b) [CuIITPPS + Na]3−, (c) and (d) [(CuIITPPS)2 + 3Na]5−, and (e) and (f) [(MnIIITPPS)2 + H]5−.

Where IE+ is the ionization energy of the [MnIIITPPS]+ cation, DE(monomer)3− is the energy to remove an electron from the [MnIIITPPS]3− trianion, and R is the distance from the respective sulfonic acid function to the center of the porphyrin ring. On the basis of a PM7 model calculation, we obtain the optimized geometry shown in Figure 1. Interestingly, this is slightly curved, not planar. Nevertheless, the four sulfonate distances are basically identical (9.45 Å). With this distance, we calculate the electrostatic destabilization energy (the sum in eq 7) to be 6.14 eV, i.e. each localized negative charge shifts the electrostatic potential by ca. 1.5 eV. Using our measured detachment energy of 2.6 eV, we therefore obtain an IE+ value of 8.74 eV. Assuming Koopman’s theorem, this agrees well with the results of a DFT calculation (TURBOMOLE, B3LYPfunctional, and def2-SVP (H,C,N,O) and def2-TZVP (Mn) basis sets) for the (unsubstituted) manganese−porphyrin cation (optimized structural parameters of the neutral species with D2h point group symmetry) which predicted a HOMO energy of 9.00 eV. For the manganese dimers the situation is complicated by the fact that two of the sulfonic acid groups are in close proximity to the manganese centers and therefore cannot be treated simply as “spectators” which bias the ionization energy (see Figures 1 and 8e,f). Nevertheless, we can still use the electrostatic model to explain the detachment energy difference between the quintuply charged dimer (DE = 0.1 eV) and the quadruply charged dimer (DE = 1.3 eV). The key assumption is that only the remaining six nonproximal sulfonic acid groups (plus any hydrogen counterions) bias the ionization energy of a (neutral) Mn−porphyrin−sulfonic-acid complex (IPMnSulfon). For the quadruply and quintuply charged dimers we obtain, respectively 6

DE(dimer)4 − = IE MnSulfon −

∑ i=1

e2 + 4πε0R (Mn−S)i

2

∑ j=1

6

DE(dimer)5 − = IE MnSulfon −

∑ i=1

e2 + 4πε0R (Mn−S)i

1

∑ j=1

e2 4πε0R (Mn−H)j

(9)

As before, the first sum runs over the sulfonic acid groups (except the two bridging groups). The second sum runs over the additional protons, which we treat as positive point charges. In the lowest-energy isomer of the quadruply charged dimer [(MnIIITPPS)2 + 2H]4−, we obtain an electrostatic destabilization energy of 5.08 eV (independent of which manganese center the electron is removed from because the distribution of the protons is symmetrical, Figure 1). With our measured detachment energy of 1.3 eV (Figure 3b), we obtain an ionization energy (IEMnSulfon) of 6.4 eV. Because there is no experimental value for this ionization energy, we calculated (TURBOMOLE, DFT, B3LYP-functional, def2-SVP (H,C,N,O,S) and def2-TZVP (Mn) basis set, optimized structural parameters of the neutral complex), the total energies of both the neutral and singly positively charged complex of benzenesulfonic acid with (unsubstituted) Mn−porphyrin, respectively. This yields an energy difference of 6.79 eV, in good agreement with our IEMnSulfon value based on the electrostatic model. For the quintuply charged dimer [(MnIIITPPS)2 + H]5−, we calculate two different electrostatic destabilization energies of 6.41 and 5.88 eV because the single protonated sulfonic acid group is closer to one of the mangenese centers (Figure 8e,f). Therefore, we expect an onset in the photoelectron spectrum close to 0 eV, in line with our experimental finding (Figure 3a). The adiabatic detachment energies of the trimers, [(MnIIITPPS)3 + 3H]6− and [(MnIIITPPS)3 + 4H]5−, differ by 0.9 eV (Figure 4). To rationalize this finding within the electrostatic model, we have to take into account that the manganese atom of the central porphyrin ring directly interacts with two sulfonic acid groups (Figure 1). For reasons of charge conservation we put −0.5 charges on each of these groups, while the two sulfonic acid groups connected to the manganese atoms of the two outer porphyrin rings formally carry zero

e2 4πε0R (Mn−H)j

(8) I

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(7) Hollingsworth, J. V.; Richard, A. J.; Vicente, M. G. H.; Russo, P. S. Characterization of the Self-Assembly of meso-Tetra(4sulfonatophenyl)porphyrin (H2TPPS4−) in Aqueous Solutions. Biomacromolecules 2012, 13, 60−72. (8) Chen, O.; Groh, S.; Liechty, A.; Ridge, D. P. Binding of Nitric Oxide to Iron(II) Porphyrins: Radiative Association, Blackbody Infrared Radiative Dissociation, and Gas-Phase Association Equilibrium. J. Am. Chem. Soc. 1999, 121, 11910−11911. (9) Jellen, E. E.; Cappell, A. M.; Ryzhov, V. Effects of size of noncovalent complexes on their stability during collision-induced dissociation. Rapid Commun. Mass Spectrom. 2002, 16, 1799−1804. (10) Hayes, L. A.; Chappell, A. M.; Jellen, E. E.; Ryzhov, V. Binding of metalloporphyrins to model nitrogen bases: Collision-induced dissociation and ion−molecule reaction studies. Int. J. Mass Spectrom. 2003, 227, 111−120. (11) Angelelli, F.; Chiavarino, B.; Crestoni, M. E.; Fornarini, S. Binding of Gaseous Fe(III)-Heme Cation to Model Biological Molecules: Direct Association and Ligand Transfer Reactions. J. Am. Soc. Mass Spectrom. 2005, 16, 589−598. (12) Chiavarino, B.; Crestoni, M. E.; Fornarini, S.; Rovira, C. Unravelling the Intrinsic Features of NO Binding to Iron(II)- and Iron(III)-Hemes. Inorg. Chem. 2008, 47, 7792−7801. (13) Karpuschkin, T.; Kappes, M. M.; Hampe, O. Binding of O2 and CO to Metal Porphyrin Anions in the Gas Phase. Angew. Chem., Int. Ed. 2013, 52, 10374−10377. (14) Arai, S.; Ishihara, S.; Takeoka, S.; Ohkawa, H.; Shibue, T.; Nishide, H. Stability of porphyrin-calix[4]arene complexes analyzed by electrospray ionization mass spectrometry. Rapid Commun. Mass Spectrom. 2004, 18, 2065−2068. (15) Haino, T.; Fujii, T.; Fukazawa, Y. Guest Binding and New SelfAssembly of Bisporphyrins. J. Org. Chem. 2006, 71, 2572−2580. (16) Schwarz, U.; Vonderach, M.; Kappes, M.; Kelting, R.; Brendle, K.; Weis, P. Structural characterization of metalloporphyrin-oligomer multianions by mass spectrometry and ion mobility spectrometry Observation of metastable species. Int. J. Mass Spectrom. 2013, 339− 340, 24−33. (17) Wang, X.-B.; Wang, L.-S. Observation of negative electronbinding energy in a molecule. Nature 1999, 400, 245−248. (18) Wang, X.-B.; Ferris, K.; Wang, L.-S. Photodetachment of gaseous multiply charged anions, copper phthalocyanine tetrasulfonate tetraanion: Tuning molecular electronic energy levels by charging and negative electron binding. J. Phys. Chem. A 2000, 104, 25−33. (19) Berkowitz, J. Photoelectron spectroscopy of phthalocyanine vapors. J. Chem. Phys. 1979, 70, 2819−2828. (20) Arnold, K.; Balaban, T. S.; Blom, M. N.; Ehrler, O. T.; Gilb, S.; Hampe, O.; van Lier, J. E.; Weber, J. M.; Kappes, M. M. Electron Autodetachment from Isolated Nickel and Copper PhthalocyanineTetrasulfonate Tetraanions: Isomer Specific Rates. J. Phys. Chem. A 2003, 107, 794−803. (21) Ehrler, O. T.; Yang, J.-P.; Sugiharto, A. B.; Unterreiner, A. N.; Kappes, M. M. Excited state dynamics of metastable phthalocyaninetetrasulfonate tetra-anions probed by pump/probe photoelectron spectroscopy. J. Chem. Phys. 2007, 127, 184301. (22) Vonderach, M.; Ehrler, O. T.; Weis, P.; Kappes, M. M. Combining Ion Mobility Spectrometry, Mass Spectrometry, and Photoelectron Spectroscopy in a High-Transmission Instrument. Anal. Chem. 2011, 83, 1108−1115. (23) Mason, E. A.; McDaniel, E. W. Transport Properties of Ions in Gases; Wiley: New York, 1988. (24) Stewart, J. J. P. MOPAC2012; Stewart Computational Chemistry: Colorado Springs, CO, 2012; HTTP://OpenMOPAC.net. (25) von Helden, G.; Hsu, M.-T.; Gotts, N.; Bowers, M. T. Carbon Cluster Cations with up to 84 Atoms: Structures, Formation Mechanism, and Reactivity. J. Phys. Chem. 1993, 97, 8182−8192. (26) Shvartsburg, A. A.; Jarrold, M. F. An exact hard-spheres scattering model for the mobilities of polyatomic ions. Chem. Phys. Lett. 1996, 261, 86−91. (27) TURBOMOLE, version 6.4; TURBOMOLE GmbH: Karlsruhe, Germany, 2012; http://www.turbomole.de.

charge (as is the case for the dimers). The other sulfonic acid groups each carry one negative charge (as before). With this model of charge partitioning and for the separations pertaining to it, we obtain an electrostatic destabilization energy (sum of the Coulomb interactions) of 9.9 eV for [(MnIIITPPS)3 + 3H]6−, assuming that the electron is removed from the manganese of the central porphyrin ring. For [(MnIIITPPS)3 + 4H]5− we obtain 8.6 eV, i.e., a difference of 1.3 eV. This is in reasonable agreement with the experimentally observed difference of 0.9 eV.

5. CONCLUSION We have determined the structures and photoelectron spectra of MnIII and CuII TPPS monomer multianions, as well as of corresponding dimers and trimers. The charge state of the central atom determines the structural motif of the dimers. MnIII leads to a connection of monomeric units via sulfonicacid−manganese bonds. This results in a tilted stack arrangement of prophyrin rings. For CuII, the sulfonic acid groups preferentially bind to the sodium counterions, resulting in a dimer structure with coplanar porphyrin. The trends in detachment energies observed in photoelectron spectra recorded for different charge states are interpreted in terms of a simple electrostatic model, which is based on the assumptions that the electron is removed from the center of the porphyrin ring and that the negatively charged sulfonic acid groups are involved only indirectly via their electrostatic potential.



AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS We thank the Deutsche Forschungsgemeinschaft (DFG) for support of this work through projects B1 and C6 of the collaborative research centre SFB/TRR 88 “3MET” (Kooperative Effekte in homo- und heterometallischen Komplexen). Funding of an orbitrap mass spectrometer by DFG and Land/ KIT (Art 91b) is also gratefully acknowledged. U.S. thanks the “Stiftung der deutschen Wirtschaft” for financial support.



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K

dx.doi.org/10.1021/jp411149e | J. Phys. Chem. A XXXX, XXX, XXX−XXX

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 homomol...
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