Salt Cluster Attachment to Crown Ether Decorated ... - ACS Publications

Article Cite This: J. Phys. Chem. A 2018, 122, 1623−1633

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Salt Cluster Attachment to Crown Ether Decorated Phthalocyanines in the Gas Phase Ina D. Kellner,† Uwe Hahn,‡,⊥ Tomás Torres,‡,§,∥ and Thomas Drewello*,† †

Physical Chemistry I, Department of Chemistry and Pharmacy, University of Erlangen-Nürnberg, Egerlandstraße 3, 91058 Erlangen, Germany ‡ Department of Organic Chemistry, Faculty of Science, Universidad Autónoma de Madrid, Cantoblanco, 28049 Madrid, Spain § Instituto Madrileño de, Estudios Avanzados (IMDEA)-Nanociencia, c/Faraday, 9, Cantoblanco, 28049 Madrid, Spain ∥ Institute for Advanced Research in Chemical Sciences (IAdChem), Universidad Autónoma de Madrid, 28049 Madrid, Spain S Supporting Information *

ABSTRACT: Crown ether decorated phthalocyanines were designed to form rigidly eclipsed aggregates with metal ions being sandwiched between the molecules. We studied tetra[18]crown-6 ether functionalized zinc phthalocyanine (ZnPcTetCr) in the presence of excess NaCl by electrospray ionization mass spectrometry. ZnPcTetCr was found to form aggregates in the gas phase to which several neutral NaCl molecules are attached. Collision-induced dissociation experiments revealed that the ions observed in the positive- and negative-ion modes possess remarkably different structures. Their fragmentation behavior indicates that the sodium ions providing the charge of the positively charged aggregates are strongly bound inside the crown ether moieties, while the neutral salt units are less strongly attached. However, in the negatively charged ions, none of the sodium ions is embedded in the crown ether moieties, and the NaCl molecules were found to be attached as one large, weakly bound cluster.

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studied extensively since.5−13 While Pcs usually stack with a small staggering angle (cf. Chart 2a) to increase attractive

INTRODUCTION

Phthalocyanines (Pcs) and their derivatives are well-known for their aggregation in solution.1 With the aim of creating welldefined aggregates, crown ether decorated phthalocyanines (cf. Chart 1) were first synthesized in 19862−4 and have been

Chart 2. Stacking of Crown Ether Decorated Phthalocyanines (Simplified Schematic) in Solution (a) without Metal Ions, (b) with Cations, and (c) with Excess Metal Ions

Chart 1. Structure of ZnPcTetCr (M)

interaction between the aromatic cores,10,14 crowned Pcs can be prompted to assume an eclipsed conformation (Chart 2b).5−11 Addition of metal cations which are larger than the crown ether cavities leads to the stepwise formation of cofacial dimers and oligomers (Chart 2b). In these aggregates, the metal ions are shared between two crown ether units (sandwich complex), Received: October 13, 2017 Revised: January 10, 2018 Published: January 22, 2018 © 2018 American Chemical Society

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DOI: 10.1021/acs.jpca.7b10156 J. Phys. Chem. A 2018, 122, 1623−1633

Article

The Journal of Physical Chemistry A

Figure 1. ESI mass spectra of ZnPcTetCr in DCM/ACN (1:1) (a) without addition of NaCl and (b) with a 10-fold molar excess of NaCl.

(ACN) until a 1:1 (v:v) mixture was obtained. The solvent ratio was chosen to achieve stable spraying conditions in the ESI source. The final concentration of ZnPcTetCr was in the range of 1−5 × 10−5 mol L−1. For the cluster attachment experiments, NaCl was dissolved in methanol (1 g L−1) and added in a 10-fold molar excess to the ZnPcTetCr solution. The maximum content of methanol in the final solution was 3 vol %. All solvents used were of HPLC grade purity. The majority of the ESI measurements were conducted with a quadrupole ion trap (QIT) (esquire6000, Bruker Daltonics, Bremen, Germany). The sample solution was directly injected at a flow rate of 240 μL h−1, and the temperature of the nitrogen dry gas was set to 300 °C. The voltage between the needle and the capillary entrance was set to −4 kV in the positive-ion mode and +4 kV in the negative-ion mode, respectively. After passing the spray shield and a glass capillary, the ions are accelerated through a skimmer into the next vacuum stage. As solvent molecules and nitrogen gas are still present in this part of the transfer region, collisional activation of the ions may occur here. MSn experiments are performed inside the QIT using collision-induced dissociation (CID) employing He as the collision gas at a pressure of 4 × 10−4 Pa (as measured by the vacuum gauge; the real pressure within the trap is known to be higher by approximately 2 orders of magnitude).33 The isolation width and fragmentation amplitude was adjusted in each case to ensure complete dissociation of all the ions contributing to the isotopic pattern of the precursor. The relative intensities of the product ions did not change with varying amplitude. Selected experiments were performed with an ESI quadrupole time-of-flight instrument (QTOF) (micrOTOF-Q II, Bruker Daltonics, Bremen, Germany). Due to its slightly different source geometry, a flow rate of 180 μL h−1 and a dry gas temperature of 180 °C were used in this case. Between the needle and the glass capillary, a voltage of −4.5 kV (positive-ion mode) or +2.8 kV (negative-ion mode) was applied. The transport region of the QTOF employs two funnel ion guides

forcing the Pc cores into a rigidly eclipsed conformation. Increasing the salt concentration leads to separation of the sandwich complexes and a staggered conformation, as more crown ether moieties are occupied and charge repulsion between the positively charged sites controls the structure (Chart 2c).10,15 If the added cations are small enough to be embedded in the crown ether cavities, only monomers and, in some cases, noncofacial dimers are observed in solution.5,7−9 We recently studied the aggregation of the tetra-[18]crown-6 ether functionalized zinc phthalocyanine (ZnPcTetCr, M) in the gas phase with electrospray mass spectrometry (ESI-MS).16 Even in the absence of metal ions, these Pcs form singly and multiply charged aggregates consisting of neutral molecules and radical cations. In the present investigation, NaCl was added to the ZnPcTetCr solution in a 10-fold molar excess. The ESI mass spectra obtained with this solution showed aggregates of ZnPcTetCr containing neutral NaCl units in both polarities. While salt clusters have been studied extensively in the gas phase,17−29 their aggregates with larger molecules have rarely been addressed.30,31 The aggregate ions observed in the positive- and negative-ion modes were found to behave remarkably different upon collisional activation. The fragmentation behavior of the positively charged ions indicates that the sodium ions providing the charge are strongly bound inside the crown ether moieties, while the neutral salt units are less strongly attached. However, in the negatively charged ions, none of the sodium ions is bound to a crown ether, and the NaCl seems to be attached as one large, weakly bound cluster.

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EXPERIMENTAL METHODS ZnPcTetCr was synthesized following reported procedures,5,6 and its characterization is in good agreement with the literature.32 Small amounts (0.3−0.4 mg) of ZnPcTetCr were dissolved in dichloromethane (DCM) and then diluted with acetonitrile 1624


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To gain insight into the structure of the ions, collisioninduced dissociation (CID) mass spectra were obtained for most of them. Note that we cannot comment on the structure of the NaCl clusters themselves, as they are known to rearrange and fluctuate at relatively low activation energies prior to dissociation.29,45 However, as will become clear in the discussion, we can infer the location of the charge carrying ions and the clusters’ point of attachment to the ZnPcTetCr molecule from the CID mass spectra. A few representative examples of CID spectra are shown in the following, other selected fragmentation spectra can be found in the Supporting Information (Figures S6−S10). The predominant dissociation pathway of the doubly charged monomer MNa22+ (m/z 779) is the loss of a sodium ion, yielding MNa+ (m/z 1535) (Figure 2a). However, a series of doubly charged product ions is also observed, whose m/z values correspond to sequential losses of CH2O, C2H4O, and C2H4 from the crown ether unit. As these fragment ions still contain two sodium ions, their existence proves the strong bond to both Na+ ions, causing the ZnPcTetCr to dissociate rather than release the cations. The fragment ion MNa + is also accompanied by a series of second-generation product ions, corresponding to similar neutral losses from the ZnPcTetCr framework. In contrast, the CID mass spectrum of the triply charged monomer MNa33+ (m/z 527) shows only loss of Na+ resulting in MNa22+ (Figure 2b). The third sodium cation is therefore less strongly bound by ZnPcTetCr, most likely due to charge repulsion. This also explains the lack of a quadruply charged monomer MNa44+ in the mass spectra (Figure 1b). Figure 3 shows two examples for the fragmentation of ions containing neutral NaCl units. The doubly charged monomer M(NaCl)3Na22+ dissociates via the loss of neutral NaCl molecules (Figure 3a). The most abundant product ion is MNa 2 2+ . Note that we cannot discern whether the fragmentation occurs via consecutive losses of single NaCl units or via the loss of small salt clusters. In the QIT, only the precursor ion is activated, while all the fragments are cooled down by collisions with the buffer gas.46,47 Consecutive fragmentations should therefore only occur if the ion retains enough excess energy after the first dissociation. The CID mass spectrum of M(NaCl)4Na 22+ is less ambiguous (Figure 3b). The product ions M(NaCl)2Na22+ and MNa22+ are clearly obtained via the loss of one or two (NaCl)2 dimers. This fragmentation behavior has been shown to be typical for salt clusters.17,18,24,31,48 In low-energy CID small cesium-iodide clusters dissociate via consecutive losses of CsI or (CsI)2. The loss of larger clusters only occurs from larger precursor cluster ions or at high-energy CID.24 This leads us to conclude that the NaCl is weakly bound to the charged ZnPcTetCr as a small cluster, probably to the Na+ ions embedded in the crown ether moieties. The dissociation of M(NaCl)3Na22+ (Figure 3a) therefore most likely proceeds via loss of a single salt molecule and a dimer in two successive steps. The consecutive loss of NaCl and/or (NaCl)2 units is observed for all the positively charged ions containing neutral salt (cf. Figures S6−S10 of the Supporting Information). For all but the highest charged ions (M 2 (NaCl) n Na 4 4+ and M3(NaCl)nNa5+, see below), it is the first dissociation step upon activation. Figure 4 shows the CID spectrum of M2(NaCl)3Na33+ as an example. The precursor ion first loses all attached NaCl molecules, resulting in M2Na33+. The triply charged dimer then fragments via Coulomb explosion into

to collect the ions, offering softer conditions than the skimmer region of the QIT.34−36 For MS/MS experiments, N2 was used as collision gas.

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RESULTS AND DISCUSSION Positive-Ion Mode. In a control experiment, ZnPcTetCr was first sprayed without addition of NaCl to the sample solution. The resulting ESI mass spectrum is shown in Figure 1a (for enlargements, see Figure S1−S3 of the Supporting Information). Due to the high alkali metal affinity of the crown ether units,37−40 even minute sodium and potassium salt contamination of the solvents (DCM, ACN) results in various adducts. Overall, the Na+ adducts dominate the spectrum, K+ is only detected in mixed Na+/K+ adducts in less abundance. This might solely be the result of larger sodium contamination of the solvents; however, the host−guest interaction between the [18]crown-6 moiety and Na+ is also stronger compared to K+,40,41 possibly contributing to the high intensity of the Na+ adducts. Note that the base peak of the mass spectrum (m/z 1537) consists mostly of the doubly charged dimer M2Na22+, the monomer MNa+ is only observed as a slight disturbance of the isotope pattern of the dimer (cf. Figure S2a of the Supporting Information). A rather peculiar doubly charged ion is detected at m/z 768. Its m/z value and isotope pattern correspond to the formula MNa(2+)• (cf. Figure S3 of the Supporting Information). It therefore consists of two different charge carriers, the sodium ion and the radical cation of the phthalocyanine. Considering the low oxidation potential of phthalocyanines,42 MNa(2+)• is probably generated via oneelectron oxidation from the sodiated molecule in the high electric field of the spray needle.16,43,44 With the addition of a 10-fold molar excess of NaCl to the sample solution, the mass spectrum changes quite drastically (Figures 1b, S4, and S5 (Supporting Information)). Observed are monomers carrying up to three charges (MNa+, M(NaCl)0−7Na22+, M(NaCl)0−2Na33+), dimers with two to four charges (M2(NaCl)1−4Na22+, M2(NaCl)1−9Na33+, M2(NaCl)2−5Na44+), and trimers with three to five charges (M3(NaCl)3−6Na33+, M3(NaCl)4−11Na44+, M3(NaCl)6−8Na55+) (cf. Table 1). Apart from the singly charged monomer MNa+, all species exist with a varying number of attached neutral NaCl salts. Note that no quadruply charged monomer MNa44+ was observed. Table 1. Observed Adducts in Positive-Ion Mode (n.o. = Not Observed)

monomers

dimers

trimers

ions

m/z range

observed in QIT (n = ...)

M(NaCl)nNa+ M(NaCl)nNa22+ M(NaCl)nNa33+ M(NaCl)nNa44+ M2(NaCl)nNa22+ M2(NaCl)nNa33+ M2(NaCl)nNa44+ M3(NaCl)nNa33+ M3(NaCl)nNa44+ M3(NaCl)nNa55+

1535 779−984 527−566 401− 1567−1654 1052−1189 810−853 1616−1635 1232−1320 990−1037

0 0−7 0−2 n.o. 1−4 1−9 2−5 3−6 4−11 6−8

a

observed in QTOF (n = ...) n.o. 0−2 0−2 n.o. n.o. 2−6 2−5 n.o. 5−8 5−9

a

Peak with the highest intensity of the smallest cluster to peak with highest intensity of largest observed cluster. 1625


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Figure 2. CID mass spectra of (a) MNa22+ (m/z 779) and (b) MNa33+ (m/z 527).

Figure 3. CID mass spectra of (a) M(NaCl)3Na22+ (m/z 868) and (b) M(NaCl)4Na22+ (m/z 897).

Figure 4. CID mass spectrum of M2(NaCl)3Na33+ (m/z 1091).

MNa22+ and MNa+. There is no evidence for a Coulomb fission of the precursor ion itself, which would result in singly and doubly charged monomers containing neutral salt units (M(NaCl)0−3Na22+ and M(NaCl)0−3Na+). The ZnPcTetCr dimer is therefore comparatively stable, with the (NaCl)3 cluster only weakly bound to it. This result is somewhat unexpected; as the sodium ions are small enough to be

completely encapsulated by the crown ether moieties,39,49−51 they should not be able to form a sandwich complex with the crown ether unit of a second ZnPcTetCr molecule.52 Thus, we expected at least one Cl− ion to be in a bridging position between either the central zinc atoms or two Na+ containing crown ether units of the two ZnPcTetCr molecules. The ease in which all the chloride ions are split off as NaCl, however, leads 1626


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The Journal of Physical Chemistry A Scheme 1. Possible Structure of a Triply Charged Dimer M2(NaCl)nNa33+ and Its Fragmentation (CID)

Figure 5. CID mass spectrum of M3(NaCl)8Na55+ (m/z 1025), acquired with the QTOF.

Table 2. Preferred Fragmentation Pathways of the Positive Ions monomers

dimers

trimers

a

ions

CID

MNa+ MNa22+ M(NaCl)nNa22+ MNa33+ M(NaCl)nNa33+ M2(NaCl)nNa22+ M2(NaCl)nNa33+ M2(NaCl)nNa44+ M3(NaCl)nNa33+ M3(NaCl)nNa44+ M3(NaCl)nNa55+

loss of Na+ sequential loss of NaCl and/or (NaCl)2 → MNa22+ loss of Na+ sequential loss of NaCl and/or (NaCl)2 → MNa33+ sequential loss of NaCl and/or (NaCl)2 → M2Na22+ Coul.a → MNa+ n = 1 Coul.a: n ≥ 2 loss of NaCl → M2Na33+ Coul.a n = 2−4 Coul.a; n = 4−5 sequential loss of NaCl sequential loss of NaCl → M3(NaCl)1−2Na33+ Coul.a Coul.a → predominately M2(NaCl)nNa33+ and MNa+, and M(NaCl)n1Na22+ and M2(NaCl)n2Na22+b Coul.a → predominately M(NaCl)n1Na22+ and M2(NaCl)n2Na33+

Coul. = Coulomb explosion. bSee Supporting Information Figure S10.

does not undergo salt loss, but preferentially dissociates via Coulomb explosion into the doubly charged monomer and triply charged dimer. The number of attached NaCl molecules is divided among the product ions, resulting in a broad distribution of fragment ions, M(NaCl) 0−7 Na 2 2+ and M2(NaCl)1−8Na33+. To a much lesser degree, the Coulomb fission into the triply charged monomers M(NaCl)0−3Na33+ and the doubly charged dimers M2(NaCl)1−7Na22+ is also observed. The reason for the low abundance of these product ions may be the less favorable charge distribution between fragments. As discussed above, the third sodium cation is less tightly bound than the first two; the triply charged monomer is therefore less stable than its doubly charged equivalent. Secondary fragmentation of these ions likely contributes to the high abundance of MNa22+. NaCl and/or (NaCl)2 losses from the first generation product ions also influence the ion distribution. The preferred fragmentation pathways of the aggregates in the positive-ion mode are summarized in Table 2. In short, we can conclude from the CID mass spectra that (i) the charge carrying sodium ions are tightly bound inside the crown ether moieties, (ii) the ZnPcTetCr dimers and trimers are held together by π−π interactions between the aromatic cores and possibly electrostatic interactions with the embedded sodium cations, and (iii) all the neutral NaCl molecules are only weakly attached on the outside of the ZnPcTetCr aggregates in the form of small salt clusters.

us to conclude that the neutral salt molecules are attached on the outside of the ZnPcTetCr dimer, M2Na33+ (cf. Scheme 1). As mentioned before, in solution, phthalocyanines and their derivatives are known to form aggregates due to π−π interactions between the macrocyclic aromatic cores.1 We have recently shown that ZnPcTetCr is also able to form large stacks in the gas phase in the absence of metal cations.16 These clusters are held together by electrostatic interactions between neutral molecules and radical cations. In the current case, no radical ions are involved in forming the aggregates. The dimers and trimers are probably held together mainly by π−π interactions between the phthalocyanines. Even though the sodium ions are small enough to be completely embedded in the [18]crown-6-moieties and therefore cannot attach a second crown ether, they may provide some electrostatic attraction, thus contributing to the stability of the aggregates. Judging from their CID mass spectra, all the doubly and triply charged dimers and trimers are relatively stable and dissociate only after all the attached NaCl molecules are split off. Only for the highest charged aggregates, M2(NaCl)nNa44+ and M3(NaCl)nNa5+, the charge repulsion between the sodium cations is large enough to separate the ZnPcTetCr oligomers in the first step upon activation. Figure 5 shows the CID mass spectrum of M3(NaCl)8Na55+ (m/z 1025) as an example. Enlargements of the relevant mass ranges can be found in the Supporting Information (Figure S9). The precursor ion clearly 1627


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Figure 6. Negative-ion mode ESI mass spectra of ZnPcTetCr in DCM/ACN with a 10-fold excess of NaCl as observed on the QIT (a) and on the QTOF (b).

Negative-Ion Mode. When the ZnPcTetCr solution with excess NaCl is sprayed in the negative-ion mode, mainly singly charged monomers M(NaCl)0−9Cl− are observed (Figure 6a and Supporting Information Figure S11). In less abundance, doubly charged dimers M2(NaCl)4−18Cl22− are also produced. Additionally, in the softer conditions and wider mass range of the QTOF, doubly charged trimers (M3(NaCl)9−20Cl22−) and triply charged tetramers and pentamers (M4(NaCl)14−27Cl33−, M5(NaCl)19−29Cl33−) are observed (cf. Figure 6b, Table 3, and Supporting Information Figure S12).

were too small in intensity and could not be subjected to CID successfully. Representative CID mass spectra of the aggregates are shown in the following and in the Supporting Information (Figures S13−S15). The singly charged monomer MCl− dissociates via HCl loss, resulting in a deprotonated molecule [M − H]− (Figure 7a). HCl loss is only observed for this ion, and all the aggregates containing neutral NaCl show a different fragmentation behavior: upon activation, the singly charged monomers lose all the attached neutral NaCl molecules in one step, leaving MCl− as the only fragment ion. Figure 7b,c show the CID mass spectra of M(NaCl)Cl− and M(NaCl)6Cl− as an example. The same behavior is observed for all the singly charged monomers. However, the aggregates with an odd number of attached NaCl molecules show a second, less abundant fragmentation pathway; they can lose all but one NaCl, yielding M(NaCl)Cl−. The predominant dissociation is nevertheless the loss of the complete neutral cluster (see Figure S13 of the Supporting Information). The doubly charged dimers M2(NaCl)nCl22− show a similar preference for one particular fragmentation pathway. Figure 8a shows the CID mass spectrum of M2(NaCl)13Cl22− as an example. The precursor ion dissociates via Coulomb explosion into the singly charged salt cluster (NaCl)13Cl− and the counterion M2Cl−. The salt cluster can then fragment further, sequentially losing NaCl and/or (NaCl)2. Note that both clusters (NaCl)13Cl− and (NaCl)9Cl− are magic number clusters,21 i.e., they are more stable than other cluster sizes. These clusters are therefore preferentially formed in the fragmentation, while the intensities of the clusters with n = 10−12 barely exceed the noise level. The counterion of the initial Coulomb explosion, M2Cl−, also dissociates further into MCl− via loss of a neutral molecule. This fragmentation indicates that the dimer is probably bridged by the chloride ion, which is bound to both central zinc atoms (cf. Scheme 2, top).53 It is also possible that the Pcs are only held together by π−π interactions, and both the cluster and the charge carrying

Table 3. Observed Adducts in the Negative-Ion Mode (n.o. = Not Observed)

monomers dimers trimers tetramers pentamers

a

ions

m/z range

M(NaCl)nCl− M2(NaCl)nCl22− M3(NaCl)nCl22− M4(NaCl)nCl33− M5(NaCl)nCl33−

1049−2075 1667−2076 2570−2892 2328−2581 2930−3125

observed in QIT (n = ...)

observed in QTOF (n = ...)

0−9 7−15 n.o. n.o. n.o.

0−9 4−18 9−20 14−27 19−29

a

Peak with the highest intensity of the smallest cluster to peak with highest intensity of largest observed cluster.

Compared to the positive-ion mode, the ZnPcTetCr aggregates are larger (up to five molecules) in the negativeion mode and attach a much larger number of neutral NaCl units (up to 29 instead of 11 in positive-ion mode). However, the highest charge state observed in the negative-ion mode is smaller than in positive-ion mode (3− vs 5+). Note also that the overall intensity of the ions was much lower than in the positive-ion mode and long accumulation times were needed to obtain a spectrum with reasonably low signal-to-noise-ratio. Again, CID mass spectra were obtained to gain insight into the structure of the aggregates. However, a majority of the ions 1628


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Figure 7. CID mass spectra of (a) MCl−, (b) M(NaCl)Cl−, and (c) M(NaCl)6Cl− (only fragment ions of the single charged precursor are shown; for full spectrum, see Figure S13d of the Supporting Information).

Figure 8. CID mass spectra of (a) M2(NaCl)13Cl22− and (b) M3(NaCl)13Cl22−, acquired with the QTOF. The inset in (b) shows an enlargement overlaid with the simulated isotope pattern of M3Cl− in orange.

discrimination also accounts for the intensity difference to some extent, i.e., over such a large m/z range, low and high mass ions cannot be detected with the same sensitivity. All the other doubly charged dimers we were able to isolate and subject to CID show the same fragmentation behavior (see Figures S13 and S14 of the Supporting Information). An example for the dissociation of the doubly charged trimers is shown in Figure 8b. The precursor M3(NaCl)13Cl22− fragments via the loss of the complete cluster (NaCl)13Cl−. The

anions are weakly attached to the zinc atoms from the outside (cf. Scheme 2, bottom). The initial loss of the cluster (NaCl)nCl− would still generate the product ion M2Cl−; however, secondary fragmentation would result in a neutral Pc dimer and a single Cl− ion, which is too small to be detected by our instrument. Though there is no direct evidence for the occurrence of such ions, their facile secondary dissociation would explain the rather large intensity difference between the counterions of the initial Coulomb explosion. However, mass 1629


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The Journal of Physical Chemistry A Scheme 2. Possible Structures and Fragmentation Pathways (CID) of M2(NaCl)nCl22−

none are embedded in the crown ether moieties. Only very few of the aggregates retain a single NaCl unit upon activation, of which the Na+ ion could be bound to a crown ether. In most of these cases, however, a magic-number cluster is generated during the fragmentation. The NaCl therefore most likely remains with the Pc only to allow the formation of an exceptionally stable cluster fragment. If no sodium ion is embedded in the crown ethers, the only reasonable point of attachment for the charge carrying chloride ions is the central zinc atom of the phthalocyanine. The dimers M2(NaCl)nCl22− are either bridged by one chloride ion or held together only by π−π interactions. One or both anions are attached on the outside of the dimer and probably serve as an anchor for the neutral NaCl molecules. These are only weakly bound and easily lost as one cluster upon activation. The structure of the doubly charged trimers M3(NaCl)nCl22− is similarly ambiguous. The trimers are most likely the result of π−π stacking of the Pcs, with both charge carriers and the cluster attached to the outside. If the three ZnPcTetCr molecules were bridged by the two charge carrying chloride ions and the neutral salt units are attached to the outside of the trimer, we would not expect the loss of (NaCl)nCl− to occur so easily. Another possible structure would be (NaCl)nCl− attached to the outside, leaving only one Cl− to bridge two ZnPcTetCr molecules and the third to be bound by π−π interactions only. A fourth possibility would be the singly charged cluster sandwiched between two ZnPcTetCr molecules. The third ZnPcTetCr could then be attached via the second charge-carrying Cl− ion. None of these structures can be excluded based on the available data; neither can we rule out possible rearrangement of the aggregates during their flight time.

salt cluster dissociates further, but only the magic-number cluster (NaCl)9Cl− is clearly discernible. The counterion of the initial Coulomb explosion, M3Cl− (m/z 4579), is shown in the inset of Figure 8b, overlaid with its simulated isotope pattern. Note that M3Cl− can only be observed if the instrument is tuned to detect high mass ions. This, however, compromises the measurement of the low mass fragment ions, causing a drop in the intensity of (NaCl)13Cl− and the disappearance of (NaCl)9Cl−. Due to this mass discrimination effect, Figure 8b is actually a combination of two different CID mass spectra of M3(NaCl)13Cl22−. To acquire the overview spectrum, the instrument was tuned to detect lower masses; for the inset, it was tuned to detect the high mass region (see Supporting Information Figure S15b,c for the complete spectrum with these settings). Even with these adjustments, however, the intensity of M3Cl− is very low, and a long acquisition time was required to reduce the noise level sufficiently to detect it. This is probably due both to the mass discrimination effect and secondary fragmentation of the ion. MCl− as a possible secondgeneration product ion is observed in similarly low intensity (see Supporting Information Figure S15c). In addition to M3Cl− and MCl−, several other singly charged monomers M(NaCl)nCl− (with n = 1, 2, 5, 6, 13) were also observed with these settings, though also with comparably low signal-to-noise ratios. These are the result of a second, much less favored fragmentation pathway, which probably proceeds via the loss of M2Cl− (formally; the M2Cl− ion itself is not observed) and the subsequent dissociation of the remaining M(NaCl)13Cl−. Only two other doubly charged trimers could be successfully subjected to CID (see Figure S15d−g of the Supporting Information. They all follow the same fragmentation pathway as M3(NaCl)13Cl22−, i.e., the loss of a singly charged salt cluster, preferentially a magic-number cluster. From the obtained CID mass spectra, which are summarized in Table 4, we can draw several conclusions concerning the structure of the negatively charged ions. All Na+ ions contained in these aggregates are part of the neutral NaCl cluster, and

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CONCLUSIONS The positively and negatively charged aggregates of ZnPcTetCr have surprisingly little in common. In the positive ions, the charge-carrying Na+ ions are embedded in the crown ether moieties and the oligomers are mainly held together by π−π interactions, possibly aided by electrostatic attraction between occupied and empty crown ethers. In contrast, in the negatively charged ions, the crown ether units contain no sodium ions, and the dimers are at least partially connected by chloride ions bridging the central zinc atoms. Even the attached NaCl clusters behave differently. Upon activation, the positive ions fragment via NaCl/(NaCl)2 loss, while the negative ions release the entire cluster in one step. All these dissimilarities are a result of the different structures of the aggregates. However, the question remains how such diverse aggregates can be generated simply by reversing the polarity of the ESI source. Explaining the origin of the

Table 4. Preferred Fragmentation Pathways of the Negative Ions ions

CID

dimers

MCl− M(NaCl)nCl− M2(NaCl)nCl22−

trimers

M3(NaCl)nCl22−

loss of HCl loss of (NaCl)n or (NaCl)n−1 Coul.a → predominately (NaCl)nCl− and M2Cl− Coul.a → predominately (NaCl)nCl− and M3Cl−

monomers

a

Coul. = Coulomb explosion. 1630


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The Journal of Physical Chemistry A ORCID

positively charged ions is rather straightforward. Due to the excess NaCl available and the high affinity of crown ethers for alkali metal ions, we assume that the majority of ZnPcTetCr molecules contain one or more sodium ions in the initial solution. These precharged molecules are stable and easily transferred into the gas phase. The aggregates, however, do not exist in the initial solution, as the concentrations of both the Pcs and NaCl are too low to cause their clustering. The aggregates are therefore formed during the electrospray process. As the solvent evaporates from the terminal nanodroplets, the concentrations of ZnPcTetCr and NaCl increase, and the molecules eventually cluster together. Konermann et al. recently showed that pure NaCl clusters are formed via this charge-residue mechanism (CRM) from an aqueous solution.28 According to their molecular dynamics (MD) simulations, excess charge is removed from the droplets via loss of single, solvated Na+ ions in the positive-ion mode and Cl− ions in the negative-ion mode. Small clusters are only ever released from highly charged droplets in the negative-ion mode (NaCl2−). Thus, the composition of the emerging ions is mainly determined by the content of the initial droplet. The same mechanism is probably responsible for the aggregate formation in our case in the positive-ion mode. The sodium ions embedded in the crown ethers and the central zinc atom of the Pc attract the Cl− ions, while the remaining empty crown ethers provide a favorable interaction for the Na+ ions in the droplet. The precharged ZnPcTetCr molecules can therefore serve as a seed crystal for the NaCl clustering. With several points of attachment being available, the final aggregates probably consist of several smaller clusters bound to various points of the charge-carrying ZnPcTetCr. This would also explain the preferred loss of small NaCl/(NaCl)2 units upon activation. The generation of the negative ions is more difficult to explain. Despite the 10-fold excess of NaCl, none of the crown ether moieties in these aggregates contains a Na+ ion. The bond between [18]crown-6 and Na+ is comparatively strong; the bond dissociation energy (BDE) was experimentally determined by Armentrout and co-workers to be 300 kJ mol−1.40,41 It is therefore highly unlikely that sodiated ZnPcTetCr molecules release all of their cations during the electrospray process. All the observed negative ions consequently originate from the small minority of remaining neutral ZnPcTetCr, which do not even capture a Na+ ion in the final stages of droplet evaporation. This would explain the overall low intensities of the negatively charged ions and the long accumulation time required to obtain a spectrum with reasonable signal-to-noise-ratio. With no sodium cations contained in the crown ether moieties, the only reasonable point of attachment for the NaCl clusters is the central zinc atom of the Pc. Thus, only one large cluster is formed, which is easily lost upon activation.

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Ina D. Kellner: 0000-0003-1042-9671 Uwe Hahn: 0000-0003-3077-9361 Present Address ⊥

Laboratoire de Chimie des Matériaux Moléculaires, Université de Strasbourg and CNRS (UMR 7509), Ecole Européenne de Chimie, Polymères et Matériaux (ECPM), 25 rue Becquerel, 67087 Strasbourg, France. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Deutsche Forschungsgemeinschaft (DFG)−SFB 953 “Synthetic Carbon Allotropes” for financial support. We are also grateful for the financial support of the MINECO, Spain (CTQ2014-52869-P) and Comunidad de Madrid, Spain (FOTOCARBON, S2013/MIT-2841).

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.7b10156.

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