Aggregation of a Crown Ether Decorated Zinc

Article pubs.acs.org/JPCA

Aggregation of a Crown Ether Decorated Zinc−Phthalocyanine by Collision-Induced Desolvation of Electrospray Droplets Ina D. Kellner,† Uwe Hahn,‡,⊥ Maximilian Dürr,∥ Tomás Torres,‡,§ Ivana Ivanović-Burmazović,∥ 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 ∥ Bioinorganic Chemistry, Department of Chemistry and Pharmacy, University of Erlangen-Nürnberg, Egerlandstraße 1, 91058 Erlangen, Germany S Supporting Information *

ABSTRACT: The aggregation of phthalocyanines is well-known in solution but has never before been studied in the gas phase. We investigated the tetra-[18]crown-6 ether functionalized zinc−phthalocyanine (ZnPcTetCr, M) with electrospray ionization mass spectrometry (ESI-MS) in the absence of coordinating metal cations. Apart from the molecular ion M+•, singly and multiply charged aggregates Mnz(+•) were observed, bound together by electrostatic interactions, without alkali metal cations inside the crown ethers. Collision-induced dissociation (CID) experiments indicate that these clusters consist of stacked neutral M and radical cations M+•. After the oxidation of individual molecules at the electrospray needle, the aggregation occurs during desolvation of the charged droplets created in the source. Complete evaporation of the solvent and detection of the aggregates was found to require an additional acceleration of the droplets in the transfer region of the instrument, the resulting collisions with neutral gas assisting the desolvation process.

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INTRODUCTION The penchant of phthalocyanines (Pcs) for aggregating in solution is well-known and can be controlled by the choice of solvent.1−4 Calculations have shown that the Pcs preferentially stack with a small staggering angle,5 resulting from π−π interaction between the Pc cores. The angle maximizes the attractive interaction of the π-electrons of one Pc with the σ-framework of the other Pc and minimizes repulsion between the π-electrons of both aromatic structures.5−7 With the aim of creating more welldefined structures, crown ether substituted Pcs were synthesized.8−10 The addition of metal cations to solutions of these Pcs results in the formation of cofacial dimers and oligomers, in which the metal ions are coordinated by two crown ether moieties, forcing the Pc cores into a rigidly eclipsed conformation.2−4,11−14 Though the aggregation of Pcs in solution is documented quite well, their behavior in the gas phase has, to the best of our knowledge, not been studied yet. Herein, we examine the aggregation behavior of the tetra-[18]crown-6 ether functionalized zinc-phthalocyanine (ZnPcTetCr, M, Scheme 1) in the absence of coordinating metal cations with electrospray ionization mass spectrometry (ESI-MS).

Scheme 1

Small amounts (0.2−0.5 mg) of ZnPcTetCr were dissolved in dichloromethane (DCM) and then diluted with toluene (Tol) to give a 4:1 (volume:volume) mixture. The solvent ratio was chosen to achieve stable spraying conditions in the ESI source. The final concentration of ZnPcTetCr was in the range (1−5) × 10−5 mol L−1. All solvents used were of HPLC grade purity.

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EXPERIMENTAL SECTION ZnPcTetCr was synthesized by following reported procedures2,11 and its characterization is in good agreement with the literature.15 © 2015 American Chemical Society

Received: September 9, 2015 Revised: October 22, 2015 Published: October 26, 2015 11454

DOI: 10.1021/acs.jpca.5b08790 J. Phys. Chem. A 2015, 119, 11454−11460

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The Journal of Physical Chemistry A

Figure 1. (a) ESI mass spectrum of ZnPcTetCr in DCM/Tol, acquired with the QTOF1 instrument at an ISCID energy of 200 eV. (b) and (c) show enlargements of the high mass end of the spectrum at ISCID energies of 150 and 200 eV, respectively.

used there. Between the needle and the glass capillary a voltage of −4.5 kV was applied. The transport region of the QTOF employs two funnel ion guides to collect the ions, offering softer conditions than the skimmer region of the QIT.23−25 To deliberately make the conditions harsher, a voltage (0−200 V) can be applied between the two funnels to accelerate the ions and promote in-source collision-induced dissociation (ISCID). For MS/MS experiments N2 was used as collision gas. Prior to every experiment, the instrument was calibrated using a commercially available electrospray calibrant solution (Fluka, Sigma-Aldrich, Steinheim, Germany), diluted with acetonitrile (1:10, v:v). It allows calibration in a mass range up to m/z 2700 in both ion modes. QTOF2. The QTOF2 operates a longer flight tube than QTOF1, but they are otherwise almost identical. Here, a flow rate of 180 μL h−1 was used and the dry gas temperature was set to 200 °C. A voltage of −3.8 kV was applied between the needle and the glass capillary. The instrument was calibrated prior to every experiment via direct infusion of the Agilent ESI-TOF low concentration tuning mixture, which provides several peaks of singly charged ions up to m/z 2700 in both ion modes.

The ESI-MS experiments were performed with three different mass spectrometers, a quadrupole ion trap (QIT) (esquire6000, Bruker Daltonics, Bremen, Germany), a quadrupole time-of-flight instrument (QTOF) (micrOTOF QII, Bruker, herein referred to as QTOF1) and a QTOF with higher resolution (maXis 4G, Bruker, QTOF2). Schematics of the instruments can be found in the Supporting Information for this article. Recent studies on related macrocyclic compounds employing the QIT16−19 and QTOF120,21 are published elsewhere. QIT. 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 entrance to the mass spectrometer (glass capillary) was set to −4.0 kV. 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) by 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).22 The isolation width and fragmentation amplitude was adjusted in each case to ensure complete dissociation of the whole isotope pattern of the precursor. QTOF1. Due to its slightly different source geometry, a flow rate of 180 μL h−1 and a dry gas temperature of 180 °C were

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RESULTS AND DISCUSSION Electrochemical reactions at the spray needle are an inherent part of the electrospray process. The voltage applied between 11455


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The Journal of Physical Chemistry A the spray needle and the entrance to the mass spectrometer (−3.8 to −4.5 kV) results in a very high electric field at the thin tip of the spray needle and electrophoretic charge separation in the sample solution. Radical cations are produced by electrochemical oxidation of the analyte (or the solvent) at the metallic surface of the spray needle and are carried off in the charged droplets generated at the needle tip, whereas the electrons produced in the same reaction are withdrawn to ground.26−29 The selective oxidation of metalated porphyrins and phthalocyanines to their respective radical cations in ESI can be achieved in competition with the more common protonation and/or sodiation.30 In aprotic solvents, the protonation can easily be prevented.31 In the case of ZnPcTetCr, however, the suppression of sodium cation coordination is the more difficult challenge. As the molecule contains four crown ether units, which are known for their high alkali metal ion affinity,32−37 even minute sodium contamination of the solvents results in complexation of the crown moiety. When ZnPcTetCr is dissolved in a DCM/Tol mixture, both protonation and sodiation can be sufficiently suppressed, leaving oxidation of the Pc as the only viable ionization mechanism. Figure 1 shows the resulting mass spectrum, acquired with the QTOF1 instrument. Apart from the molecular ion M+• (m/z 1512), which constitutes the base peak of the mass spectrum, various singly and multiply charged oligomers of ZnPcTetCr are observed (M2−4+•, M2−72(+•), M4−103(+•), M7−134(+•), M11−185(+•), M19−226(+•)), though these are rather low in intensity (Table S1 of the Supporting Information). The notation Mnz(+•) was chosen to indicate that the clusters consist of z radical cations M+• and n−z neutral M (see below). With the quadrupole ion trap none of the larger oligomers could be observed, only the molecular ion M+• (m/z 1512) and additionally, though in very low intensity, the doubly charged monomer M2+ (m/z 756) were detected (Figure S1 of the Supporting Information). To exclude contributions of protonated species (MmHnn+), the experiment was repeated at higher resolution on the QTOF2 instrument. An enlargement of the dimer region is shown in Figure 2 (for the full mass spectrum see Supporting Information, Figure S2). The observed isotope pattern of the dimer M2+• (m/z 3029) corresponds well with its calculated pattern, showing no contribution from a protonated species M2H+ (m/z 3030). The higher resolution also allows the unambiguous assignment of multiply charged species, e.g., M42(+•) and M63(+•) in Figure 2. None of the signals with sufficient resolution and intensity shows a significant deviation from the calculated isotope patterns for purely molecular aggregates Mnz(+•); i.e., there is no evidence of protonation. To gain more insight into the composition of the aggregates, collision-induced dissociation (CID) mass spectra were acquired from the clusters M43(+•) (m/z 2020), M32(+•) (m/z 2272), and M53(+•) (m/z 2524). Fragmentation of other aggregates was not possible, as these were either too low in intensity (M74(+•), m/z 2651) or their mass-to-charge ratio exceeded the instrumental limit to precursor isolation (m/z ≥ 3000). Figure 3 shows the CID mass spectra of M43(+•) (m/z 2020) at three different collision energies as an example. At 50 eV collision energy (Figure 3a), the precursor is almost completely dissociated into the singly charged M+• (m/z 1512) and doubly charged M32(+•) (m/z 2272) (Coulomb explosion). Increasing the collision energy to 70 eV results in the subsequent Coulomb explosion of the product ion M32(+•) into M+• and M2+• (m/z 3029) (Figure 3b). The last fragmentation step is the neutral loss of M from M2+• to

Figure 2. Enlargement of the measured isotope pattern of M2+•, M42(+•), and M63(+•), acquired with the QTOF2 instrument at an ISCID energy of 200 eV (top). The combined calculated patterns (bottom) were generated using the software mMass.38,39

M+• at a collision energy of 100 eV (Figure 3c). CID of M32(+•) and M53(+•) shows the same fragmentation behavior, i.e., loss of M+• and M (Supporting Information, Figure S3). Though other aggregates could not be isolated and subjected to CID, their fragmentation behavior is not expected to deviate much from those shown above. One conceivable deviation could be the separation into two equally large fragments rather than stepwise loss of M+• or M, especially for the larger aggregates. The fragmentation behavior indicates that all of the observed aggregates consist of a combination of neutral M and radical cations M+•. Contributions by doubly charged, even-electron ions M2+ are expected to be minimal in comparison, as these were only detected with very low intensity with the QIT and not at all with either QTOF instrument. As mentioned above, the radical cations of ZnPcTetCr (M+•) are generated by oxidation in the high electric field at the tip of the spray needle. At this stage of the electrospray process, the aggregates probably do not exist yet and only isolated molecules are ionized. In solution, the degree of aggregation of crown ether substituted Pcs depends on the solvent,2−4 the central metal atom of the Pc,40 and the available cations that can form host−guest complexes with the crown ether units.2−4,11,12 At concentrations similar or higher than in our experiments (10−5 mol L−1), ZnPcs have been reported to form dimers, but no larger oligomers have been observed in solution experiments.2−4,11,12,40 The aggregates observed in the ESI mass spectra are therefore most likely not preformed in the sample solution. Instead, they could be generated during the initial desolvation of the charged droplets created by the spray. When the solvent evaporates, the concentration of M+• and neutral M in the shrinking droplets inevitably rises, promoting 11456


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of the instrument. The acceleration of the ions causes collisions similar to the use of a skimmer and promotes in-source collisioninduced dissociation (ISCID). The application of ISCID energy has been shown to have remarkable influence on the observed ions.21 In the case of ZnPcTetCr, the intensity of M+•, and the size of the aggregates Mnz(+•) seem to increase with the applied voltage (Figure 1b,c at ISCID energies of 150 and 200 eV, respectively, and Figures S2 and S4 of the Supporting Information). In addition to the clusters already observed at 150 eV ISCID energy (Figure 1b), at 200 eV the aggregates M19 6(+•) , M16 5(+•) , M13 4(+•) , M103(+•) , M17 5(+•) , M72(+•) , M185(+•), M226(+•), and M4+• are detected in the mass range m/z 4797−6060 (Figure 1c). This dependence on the transfer conditions indicates that the desolvation of the aggregates is not yet complete in this region of the mass spectrometer. The ions still reside in droplets with sizes beyond instrumental detection. Due to its much higher vapor pressure, the dichloromethane used in the sample solution should evaporate first, whereas the toluene concentration in the shrinking droplets increases. Evaporation of the last toluene molecules from the charged aggregates occurs because of collisions with the neutral gas present in the transfer region (N2, neutral solvent and analyte molecules).26,43 In the QIT instrument, acceleration toward the skimmer and the high temperature of the dry gas result in many collisions transferring enough energy to complete the desolvation and allowing the observation of M+•. In the QTOF instruments, an additional acceleration voltage has to be applied to ensure complete desolvation. With increasing massto-charge ratio of the cluster, a higher ISCID energy is required for this process. According to the calculations of Daub et al. the velocity of the charged aggregate determines whether a collision with the neutral gas leads to ejection of solvent molecules from the nanodroplets.43 The velocity gain of an ion in a static electric field is inversely proportional to the square root of its mass-to-charge ratio. The clusters with higher m/z values are accelerated less and are therefore not completely desolvated at lower ISCID energy. They can only be observed at increased ISCID energy (see above). Once the last solvent molecule has evaporated, however, additional collisions induce fragmentation of the clusters. Closely comparing the observed aggregate sizes at the ISCID energies of 150 eV (Figure 1b) and 200 eV (Figure 1c), several of the clusters with lower mass-to-charge ratios are completely fragmented at 200 eV ISCID energy (M43(+•), M74(+•), M115(+•), M125(+•), M145(+•)). The other clusters in this mass range are clearly observed in less abundance (M32(+•), M53(+•), M94(+•)). These aggregates are already completely desolvated at 150 eV ISCID energy. The increase to 200 eV ISCID energy only results in higher energy collisions and eventually fragmentation. Their dissociation contributes to the rising intensities of the monomer M+• and the smaller, lower charged clusters, M4+• (m/z 6059) for instance. As an alternative mechanism, the formation of the aggregates via inelastic collisions of charged radicals M+• and neutral M in the gas phase was also considered. The Coulomb repulsion between the charged molecules might be lessened and overcome, if the charge is delocalized in the stacks (see below). However, the buildup of the larger clusters, e.g., M226(+•), would require numerous completely inelastic collisions between charged and neutral molecules in the gas phase. Moreover, these collisions must be constructive in nature to allow the formation of larger entities. As the background pressure of the instrument consists for the most part of evaporated solvent molecules and nitrogen

Figure 3. CID mass spectra of M43(+•) at an ISCID energy of 125 eV and collision energies of (a) 50 eV, (b) 70 eV, and (c) 100 eV (QTOF1).

their aggregation. Uneven fission of the droplets also contributes to the enrichment of radicals M+• in the final droplets, as upon each fission event the smaller product droplets carry away about 15% of the charge, but only 2% of the mass of the parent droplet.41 In the course of the ESI process, the analyte concentration can thus increase by a factor of 106.42 The last stage of this desolvation process occurs in the transfer region interfacing the ESI spray chamber and the mass spectrometer. The conditions in this region were found to have a profound effect on the observed intensity of the molecular ion M+• and the number and size of the observed aggregates. As mentioned before, in the QIT instrument the ions are accelerated through a cloud of solvent molecules and heated nitrogen gas (300 °C) toward a skimmer. The resulting collisions account for rather harsh conditions during the transfer of the ions. Under these conditions the molecular ion M+• was easily observed, whereas in the default, soft conditions of both QTOF instruments almost no ions were generated. In the QTOF instruments, the skimmer is replaced by two ion guide funnels,23−25 which focus the ions, whereas neutral molecules are pumped off. In addition, the temperature of the dry gas is set lower (180 °C) here. These softer conditions, however, do not increase ion intensities as expected; instead, a decrease is observed. The mass spectra in Figures 1 and 2 were acquired while a voltage was applied between the two ion guide funnels in the transfer region 11457


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interactions without alkali metal ions present in the crown ether moieties. The ionization of ZnPcTetCr occurs by electrochemical oxidation in the spray needle. The clusters are formed during desolvation of the charged droplets created in the electrospray. The evaporation of the solvent molecules requires assistance by collisions with neutral gas, which can be induced by acceleration of the charged nanodroplet in the transfer region of the mass spectrometer. The fragmentation behavior of the aggregates indicates that they consist of stacked neutral M and radical cations M+•.

gas, such a high degree of constructive ion/molecule and ion/ion reactions was considered to be highly improbable. Though gas-phase clusters of aromatic compounds have been observed before, they were generated via either pulsed adiabatic expansion44,45 or laser desorption/ionization46,47 combined with collisional cooling,48 the aggregation to multiply charged clusters containing up to 22 molecules in a simple ESI experiment is unprecedented. The relatively flat structure of the crown ether substituted Pcs is certainly one reason for the effortless aggregation. The positive charge of the involved radical cations M+• should facilitate it even further. Radical cations of smaller π-systems are known to spontaneously form stable dimers with a corresponding neutral molecule (D+• + D → D2+•) in solution.49−52 The unpaired electron is evenly distributed between the two molecules, resulting in charge resonance stabilization of the radical cation dimer. This stabilization is at its maximum, when the two π-systems are in a parallel, sandwich-like conformation. Sterically demanding groups at the periphery of the π-system have been shown to hinder the dimerization.50 Theoretical calculations of singly charged polyaromatic hydrocarbon (PAH) clusters predict the formation of a single stack of molecules as long as the molecular diameter exceeds the length of the stack.53 For larger aggregates, the interaction of several smaller stacks is preferred to the continuation of a single stack. Ion mobility mass spectrometry of PAH clusters confirmed the growth of single π-stacks up to a certain size of the clusters and a structural change for larger aggregates. The exact structure of these modified π-stacks, however, remains unclear.46 As mentioned before, simple Pcs were calculated to stack with a small staggering angle to maximize attractive interactions between them.5 The aggregates of ZnPcTetCr can therefore be assumed to be built of similar stacks (Scheme 2). The smaller

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.5b08790. Schematics of the instruments, additional mass and CID spectra, tabular overview of the observed clusters, and M+• intensity dependence on ISCID energy (PDF)

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AUTHOR INFORMATION

Corresponding Author

*T. Drewello. E-mail: [email protected]. Phone: +49 9131 85 28312. Present Address ⊥

Laboratoire de Chimie des Matériaux Moléculaires, University of Strasbourg and CNRS (UMR 7509), European School for Chemistry, Polymers and Materials (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).

Scheme 2. Possible Structures of M52(+•) and M135(+•) 53

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REFERENCES

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clusters probably consist of one single stack, whereas for the larger aggregates multiple stacks become more likely. A probable structure for multiple stacks of ZnPcTetCr is the herringbone conformation shown in Scheme 2, which was predicted for large PAHs like circumcoronene.53 About half of the observed clusters can be constructed by alternately stacking M and M+• only (Table S1 of the Supporting Information). In most of the other aggregates, the number of neutral M exceeds the number of radical cations M+•. The additional neutral molecules are probably incorporated into the stacks, separating the charged molecules from each other. Charge transfer between the stacked molecules of a cluster is highly likely and should stabilize the stacks. Only in a minority of the aggregates does the number of monocations exceed the number of neutral molecules, so that two charged Pcs have to be adjacent to each other.

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CONCLUSIONS In summary, the crowned phthalocyanines ZnPcTetCr can be observed as radical cations in ESI-MS, when sprayed from apolar, sodium-free solvents. For the first time, various singly and multiply charged aggregates Mnz(+•) were detected in the gas phase. The clusters are bound together only by electrostatic 11458


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