Curved Cyclic Trimers: Orthogonal CuâCu Interaction versus

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Curved Cyclic Trimers: Orthogonal Cu−Cu Interaction versus Tetrameric Halogen Bonding Xiao-Liang Wang,†,∥ Ji Zheng,†,∥ Mian Li,† Seik Weng Ng,§ Sharon Lai-Fung Chan,⊥ and Dan Li*,†,‡ †

Department of Chemistry and Key Laboratory for Preparation and Application of Ordered Structural Materials of Guangdong Province, Shantou University, Guangdong 515063, P. R. China ‡ College of Chemistry and Materials Science, Jinan University, Guangzhou 510632, P. R. China § Chemistry Department, Faculty of Science, King Abdulaziz University, P.O. Box 80203 Jeddah 21589, Saudi Arabia ⊥ Department of Applied Biology and Chemical Technology, The Hong Kong Polytechnic University, Hung Hom, Hong Kong, P. R. China S Supporting Information *

ABSTRACT: In this work, two polymorphs and a pseudopolymorph of a trinuclear copper(I) pyrazolate complex, namely, [Cu(L-Br)]3 (L-Br = 4-(4-bromophenyl)-3,5-dimethylpyrazolyl), were isolated. One of them shows remarkable molecular shape curving, which is imposed by short intermolecular CuI− CuI interaction and parallelogram-like Br4 halogen bonding. These two sets of noncovalent interactions propagate along different directions and do not interact directly with each other. Interestingly, these cooperative interactions give rise to an undulating layer in the crystal structure of one polymorph. We used a variety of theoretical methods, such as electron density distribution (e.g., atoms-in-molecules analysis and reduceddensity-gradient mapping), molecular orbitals, charge analysis, electrostatic potentials, and Hirshfeld surface analysis, to demonstrate the nature and strength of multiple CuI−CuI and Br···Br bonding qualitatively and quantitatively. Moreover, preliminary calculations based on time-dependent density functional theory were performed to shed light on the structure−property correlation of this luminescent complex.

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INTRODUCTION Cooperative noncovalent interactions have been found to govern various interesting physical, chemical, and biological phenomena in self-assembly.1 Two or more types of weak interactions in action without crosstalk (generally known as orthogonality2) provide opportunities to access unusual supramolecular entities and may further give rise to dynamic or even adaptive behaviors, spanning from molecular to macroscopic levels.3 For example, Naumov and co-workers recently reported an interesting plastically bent molecular crystal of the planar hexachlorobenzene,4 which was achieved through manipulating halogen−halogen bonding5−9 versus π−π interaction at the molecular level. Metal−metal interaction has been regarded as an intermediate between van der Waals force and covalent bonding, although it has been historically debated.10−30 The nature of CuI−CuI interaction is less complicated by relativistic effects, compared with AgI and AuI analogues.11 After several decades’ efforts from many researchers, it has been largely considered as a type of close-shell interaction,10−20 and the spectroscopic and photophysical properties of related CuI coordination complexes can be attributed to excited-state CuI−CuI covalent bonding.21−24 © XXXX American Chemical Society

We call attention to a class of cyclic trinuclear complexes (CTCs), in the present work copper(I) pyrazolate CTCs,31−42 in studying multiple metal−metal bonding. The distinctions, compared with other copper(I) complexes,10−30 are made in the following aspects. (i) Individual pyrazolated CTC molecule is neutral, which can be further dimerized through ligandunsupported25−28 CuI−CuI interaction in solution or solid state. Therefore, the ambiguities caused by electrostatic force18,19 and bridging effects12−15,22−24 can be avoided to some extent. (ii) The CuI−CuI bonding pattern in CTCs involves multiple CuI sites (i.e., more than two metal centers), which are arranged in a topological fashion. Therefore, topological analyses based on electron density distribution, such as quantum theory of atoms in molecules (QTAIM)29,43−45 and reduced density gradient (RDG),30,46 are suitable for studying the spatial bonding pattern involving multiple CuI centers. We have been studying copper(I) pyrazolate CTCs for a decade.37−42 From a structural viewpoint, both intra- and Received: April 13, 2016 Revised: July 23, 2016

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DOI: 10.1021/acs.cgd.6b00571 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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intermolecular CuI−CuI contacts exist in these complexes; usually the former is ca. 3.2 Å, while the latter ranges from 2.9 to 3.9 Å38 (cf. Cu−Cu van der Waals radii sum of 2.8 Å). From a spectral viewpoint, there are evidence suggesting the phosphorescence of copper(I) pyrazolate CTCs is govern by intermolecular CuI−CuI covalent bonding in the excited states.35,38 In this work, peripheral Br sites are attached to the 4-phenyl moieties of a copper(I) pyrazolate CTC, aiming at introducing cooperative weak interactions in practice of crystal engineering.6

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RESULTS AND DISCUSSION Synthesis and Polymorphism. The target complex, namely, [Cu(L-Br)]3 (L-Br = 4-(4-bromophenyl)-3,5-dimethylpyrazolyl), was readily synthesized via procedures similar to reported works (see the Supporting Information for experimental section). 42 In the solvothermal synthesis and crystallization processes, the CuII reactant was reduced to CuI, which is now commonly utilized for preparing CuI complexes.47,48 Single crystals of the complex suitable for diffraction measurements can only be obtained with high yields in the presence of solvent additives such as 1,3,5-trimethylbenzene or p-xylene. Moreover, different polymorphs were obtained through varying the types and amounts of additives. Two polymorphs (labeled Br-α and Br-β) and a pseudopolymorph (i.e., with cocrystallized p-xylene, labeled Br-γ) of [Cu(L-Br)]3 were isolated. However, the rationalization for the crystallization of Br-α and Br-β is difficult due to the subtle variation (i.e., only the amounts of 1,3,5-trimethylbenzene were changed). Sometimes Br-α and Br-β appeared concomitantly, and Br-γ was not always reproduced. Crystal Structures and Hirshfeld Surface Analysis. Brα, Br-β, and Br-γ crystallized in the P21/c, P21, and P1̅ space group, respectively (see the Supporting Information for crystal data). The coordination bond lengths and angles, as well as intramolecular Cu−Cu distances, in these three (pseudo)polymorphs are similar to those of reported copper(I) pyrazolate CTCs.31−42 They all contain the [Cu3(L-Br)3]2 dimer formed through ligand-unsupported CuI−CuI interactions, but the numbers of molecules in the asymmetric units are different (Figure S3 in the Supporting Information). Notably, the intermolecular Cu−Cu distances differ significantly in Br-α, Br-β and Br-γ, being ca. 2.8, 3.0, and 3.2 Å, respectively. For Br-α at 100 K, the CuI−CuI distance is up to 2.817 Å, much shorter than previous documentations31−42 and approaching the Cu−Cu van der Waals radii sum of 2.8 Å. In fact, this is by far the shortest CuI−CuI distance in copper(I) pyrazolate CTCs. Such CuI−CuI interaction is so strong that the cyclic complex exhibits a curving shape (Figure 1a). Curvatures emerging at the molecular, supramolecular, and even biological scales are fascinating phenomena pursued by chemists who work in the interdisciplinary fields.49−52 For comparison, we have optimized the dimer (denoted as Br-opt) and monomer geometry by using density functional theory (DFT, see the Supporting Information for computational details). The optimized dimer (compare Figure 1b,c) shows a much longer intermolecular CuI−CuI distance of 3.095 Å, indicating the curved molecule existing in the crystal of Br-α is energetically strained. We will return to the topic of CuI−CuI bonding later. In examining the supramolecular interactions, we performed Hirshfeld surface analysis53,54 (see the Supporting Information for details), which provides a more comprehensive and

Figure 1. (a) Molecular shape curving in Br-α (100 K), shown in red and overlaid with DFT-optimized dimer (in green) and monomer (in blue) models. Comparison of dimer packing from (b) crystal structure and (c) DFT calculation. The CuI−CuI bonding pattern is highlighted in green triangles (intramolecular) and red rectangles (intermolecular). Atomic color codes: Cu in pink, N in blue, C in gray, Br in red, H omitted.

objective way of assessing the contacts between molecules in crystals. We defined the [Cu3(L-Br)3]2 dimer as a unit in the space partitioning as the intermolecular Cu−Cu contacts are already established. As shown in Figure 2 (see also Figure S4 in the Supporting Information), when the values of normalized contact distance (i.e., dnorm, red indicating distances shorter than the sum of van der Waals radii) are mapped onto the Hirshfeld analysis, it is clearly revealed that the peripheral Br sites exhibit versatile roles in forming the three (pseudo)polymorphs. For Br-α (100 K), the most significant Br-related supramolecular interaction is Br···Br halogen bonding (Figure 2a),5−9 which shows a head-to-head fashion with a Br···Br distance of 3.40 Å, much shorter than the sum of van der Waals radii (3.70 Å). For Br-β (100 K), there are several types of weak interactions in the dense crystal packing, in which the Br···Cu/Br···π interactions36 are notable contacts (Figure 2b). Br-γ (293 K) shows limited interactions between the dimers and crystallized p-xylene; only very weak Br···H contacts are detected for the Br sites (Figure 2c), which may be partially responsible for the poor reproducibility of Br-γ. Quantitative results of the percentage contributions to the Hirshfeld surface area from breaking down the fingerprint plots also support the above assignment on major Br-related interactions (see Table S4 in the Supporting Information). These three types of Br-related noncovalent interactions are diverse in nature and properties: halogen bonding has a possible σ-hole bonding mechanism5−9 and is thus highly directional for supramolecular engineering;6 Br···Cu/Br···π interactions may be related to π-acidity/basicity (mostly electrostatic force) and less directional; Br···H contacts are weak, nondirectional van der Waals force. An interesting structural feature is the formation of a parallelogram-like Br4 bonding pattern in Br-α (Figure 3a, see also Figure S5 in the Supporting Information). We noticed this by plotting the shape index diagrams on the Hirshfeld surface (Figure S6 in the Supporting Information). There are two sets of Br4 parallelograms lying above and below the grooves of each dimer of curved cyclic trimers, respectively, showing shape complementarity. Such a Br4 synthon55,56 is different from the commonly observed halogen bonding geometry (e.g., X2 or X3 synthons, X = halogen).5−7 Although there is only one set of Br···Br distance (3.40 Å) which is shorter than the sum of van der Waals radii (3.70 Å),55 the parallelogram-Br4 bonding B



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Figure 2. Hirshfeld surfaces (left) with dnorm (isovalue = 0.5) over the range −0.15 to 1.50 (red through white to blue), showing notable intermolecular contacts and corresponding fingerprint plots (right) for (a) Br-α (100 K), (b) Br-β-1 (100 K), and (c) Br-γ-A (293 K).

pattern in the [Cu 3 (L-Br)3 ] 2 dimer involves multiple interacting sites. For instance, the intermolecular CuI−CuI bonding involves four CuI sites contacting with each other, and meanwhile these CuI sites are also subject to intramolecular CuI−CuI bonding (Figure 4, top). It is very complicated to study the bonding nature and strength in such a many-bodylike system. We used QTAIM (aka Bader analysis)29,43−45 to analyze the electron density distribution (see the Supporting Information for computational details). For Br-α (100 K), interestingly, there is no bond path revealed within the intramolecular Cu3

seems to act as a type of cooperative noncovalent interaction. We will return to the topic of Br···Br bonding later. The [Cu3(L-Br)3]2 dimers are then extended by this Br4 synthon to construct sets of parallel undulating layers (Figure 3b). The direction of the intermolecular CuI−CuI bonding is almost orthogonal to that of the Br4 synthon. It is believed these cooperative interactions (or to say, orthogonality2) caused the generation of the energetically strained, geometrically curved cyclic trimers within Br-α. Multiple CuI−CuI Interactions: Electron Density Distribution and Molecular Orbitals. The CuI−CuI bonding C



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Figure 3. Crystal structure of Br-α (100 K). (a) Tetramer of dimers integrated with Hirshfeld surface, highlighting the Br4 tetrameric halogen bonding region marked with Br···Br distances. (b) Undulating layers extended by the Br4 halogen bonding synthon in the overall crystal structure.

Supporting Information). The existence of RCPs and CCPs was rarely noted in the studies on metal−metal bonding,29 which is a kind of potentially important property for systems involving multiple metal centers. We also used RDG analysis (see the Supporting Information for computational details)30,46 for visualizing the weak interactions between two adjacent trinuclear molecules (Figure 2, middle, see also Figure S10 in the Supporting Information). It is clearly shown that for the curved dimer in Br-α, the regions corresponding to intermolecular CuI−CuI bonding are colored in blue-green. This is in contrast to the light green color for the same regions in other cases, indicating stronger attraction for Br-α; in turn the surrounding red regions are indicators of steric hindrance and repulsion. Remarkably, the connectedness of electron density spanning the Cu3−Cu4−Cu3 pattern in the RDG mapping resembles the contour of frontier molecular orbitals (i.e., FMOs, Figure 5, bottom, see also Figure S11, Tables S9 and S10 in the Supporting Information), more specifically, the lowest unoccupied molecular orbital (LUMO) in many cases, obtained by using DFT calculations (see the Supporting Information for computational details). Table 1 summarizes the quantitative results for assessing CuI−CuI bonding under AIM and DFT regimes. Some observations are notable. (i) It is consistent that the values for Mayer bond order (MBO), electron density (ρ), and its Laplacian (∇2ρ) drop gradually upon increasing intermolecular CuI−CuI distance, expect for Br-γ-B which has no BCP found. (ii) The small values of ρ and positive values of ∇2ρ, along with the proposed |Vc|/Gc criteria (see Table S7 in the Supporting Information),29 all indicate the CuI−CuI bonding is characteristic of close-shell interaction in the ground state.29,43−45 Such a result is in stark contrast to the AIM data reported in the literature, which suggested covalent bonding in a series of dinuclear CuI complexes.29 (iii) The LUMO energy levels are sensitive to the variation of intermolecular CuI−CuI distance, with a smaller dCu−Cu value corresponding to lower-lying LUMO. This is in line with the excited-state CuI−CuI covalent bonding, which was inferred from photoluminescence studies.31−42 (iv) The relative total electronic energies of the isolated dimer models do not exhibit a consistent trend in

Figure 4. QTAIM analysis, showing electron density distribution gradient paths, bond paths and critical points, in the planes containing intramolecular (Cu3, shown in green) and intermolecular (Cu4, shown in red) CuI−CuI bonding in the dimer of Br-α (100 K).

plane, while two bond paths are observed for the intermolecular Cu4 plane (Figure 4, bottom). Among all six models studied (see Table S5, Figure S7 and S8 in the Supporting Information), Br-γ-B which has the longest intermolecular CuI−CuI distance (3.276 Å, cf. 3.215 Å in Br-γ-A) shows no bond critical points (BCPs) for CuI−CuI bonding. Here Br-γ-A and Br-γ-B refer to two types of dimers corresponding to the two crystallographically independent Cu3(L-Br)3 molecules in the asymmetric unit of Br-γ. Moreover, several sets of ring critical points (RCPs) and cage critical points (CCPs) are found which lie beyond the frames of the molecules (Figure 5, top, see also Figure S9 in the D



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Figure 5. Selected results of AIM (top), RDG (middle), and FMO (bottom) analyses based on optimized and real models of the [Cu3(L-Br)3]2 dimer. In the AIM molecular graphs, black balls are nuclear critical points (NCPs), corresponding to Cu, C, or N atoms; related bond paths are shown in orange lines; blue balls are bond critical points (BCPs), red ring critical points (RCPs), green cage critical points (CCPs). In the RDG maps (isovalue = 0.5 au), the blue-green-red coloring scale (ranging from −0.03 to 0.02 au) indicates attractive interaction through repulsive interaction (e.g., green indicates van der Waals attraction, while brown indicates weak steric hindrance). In the FMO contours, only the lowest MOs related to CuI−CuI interaction are shown.

Table 1. Multiple Quantitative Assessments on Ligand-Unsupported CuI−CuI Bonding from DFT and AIM Calculations models

dCu−Cua (Å)

MBOb

ΔEc (eV)

ELUMOd (eV)

ρe (e/Å3)

∇2ρf (e/Å3)

Br-opt Br-α (100 K) Br-α (293 K) Br-β (100 K) Br-γ-A (293 K) Br-γ-B (293 K)

3.095 2.817 2.862 3.019/3.037 3.215 3.276

0.069 0.160 0.157 0.074/0.089 0.057 0.053

0 29.61 33.06 33.93 29.58 29.48

−0.679 −0.691 −0.629 −0.626 −0.577 −0.572

0.067 0.115 0.101 0.074 0.074 none

0.602 1.398 1.229 0.771 0.723 none

Shortest intermolecular CuI−CuI distance. bMayer bond order (MBO). cTotal electronic energy relative to Br-opt. dLUMO energy level. eElectron density at BCPs. fLaplacian of electron density at BCPs. See Tables S6−S8 in the Supporting Information for all data of BCPs, RCPs, and CCPs. a

Figure 6. (a) AIM contour plot in the parallelogram-like Br4 halogen bonding region of Br-α (100 K), showing electron density distribution gradient paths, bond paths, and critical points. (b) RDG gradient isosurfaces (isovalue = 0.5 au) in the same region. The coloring scale is the same as that in Figure 5. (c) Molecular electrostatic potentials (in kcal/mol) at the isodensity surface (0.001 au), showing the contact regions for each two adjacent Br sites.

relation to CuI−CuI distances, indicating other supramolecular forces (e.g., Br-related interactions) may play a role in stabilizing the curved molecules in crystals.

Tetrameric Halogen Bonding: Electron Density Distribution and Electrostatic Potentials. The parallelogramlike Br4 halogen bonding observed in Br-α is also of interest E



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theoretically as it provides a real model involving multiple halogen sites. The exact nature of the halogen bonding is still disputedthere were studies which suggested an electrostatic nature (i.e., σ-hole model) or charge-transfer effects (i.e., electron donor−acceptor model), respectively.5−9 In a tetrameric system, the assignment and quantification of such a cooperative bonding pattern are very complicated. Recently, the many-body approach has been applied to estimate the character of interaction in a square-like Br4 halogen bonding system.56 Here, we instead examined the character of electron density distribution for the tetrameric halogen bonding in Br-α (100 K) through AIM and RDG analyses. In the AIM plot (Figure 6a), it is clear that a BCP exists between each two Br atoms except for the ones lying at the longer diagonal of the Br4 parallelogram. Similar to the situation of intramolecular CuI−CuI bonding (cf. Figure 4), there are also RCPs found in the centers of each three Br atoms. Quantitatively, the shortest Br···Br bonding (3.40 Å) exhibits much stronger van der Waals attraction than the others, but they are all close-shell interactions, indicated by the values of ρ, ∇2ρ, and |Vc|/Gc (Table S11 in the Supporting Information). The RDG isosurface (Figure 6b) also confirms the nature of van der Waals attraction for all these Br···Br bonding (note green indicates van der Waals attraction in the coloring scale). For Mayer bond order (MBO) analysis, no value larger than 0.05 can be found for the Br···Br interaction. For natural population atomic (NPA) charge analysis, after the formation of tetramer, the changes of NPA charges for Br atoms are slight (Table S12 in the Supporting Information). Both results suggest the nature of close-shell interaction for this parallelogram-like Br4 halogen bonding. On the basis of molecular electrostatic potentials (ESP, see the Supporting Information for computational details), it is found that the Br atoms involved in the shortest Br···Br bonding (Figure 6c, left) display good matching between the σhole (in red) of a Br atom and the belt of negative potential (in blue) of another Br atom. However, it is not well matched for the other two Br···Br pairs (Figure 6c, middle and right), resulting in longer Br···Br distances. The surface maxima for σhole of the Br atoms involving in the tetramer range from 9.42 to 9.71 kcal/mol, and the surface minima for the belt of negative potential range from −14.01 to −13.51 kcal/mol. Luminescence and Time-Dependent DFT Calculations. The photoluminescence spectra of Br-α and Br-β were recorded at room temperature (Figure 7a). The characteristic spectral behaviors, e.g., structureless emission band and large Stokes’ shift, were observed, similar to reported copper(I) pyrazolate CTCs.31−42 The slightly lower excitation energy for Br-α (red-shifted by ca. 10 nm) can be attributed to the contracted intermolecular CuI−CuI distance, which is usually related to the composition and energy level of LUMO and also valid here (Figure 5 and Table 1). However, the emission spectra almost overlapped for Br-α and Br-β, with the maximum at 670 nm. This is unusual because normally the emission wavelengths are also dependent on the intermolecular CuI−CuI distances for copper(I) pyrazolate CTCs.31−42 In order to shed light on the structure−luminescence correlation, we performed preliminary calculations based on time-dependent density functional theory (TDDFT, see the Supporting Information for computational details). Only the [Cu3(L-Br)3]2 dimer in Br-α (293 K) was taken into account because the crystal structure of Br-β at room temperature could not be obtained due to severe disorder. From the electron

Figure 7. (a) Solid-state excitation (dash lines) and emission (solid lines) spectra for Br-α (black lines) and Br-β (red lines) at room temperature. The emission spectra were excited with λex = 350 nm for Br-α and λex = 340 nm for Br-β. Both excitation spectra were monitored at the 670 nm emission peak. (b) Electron density difference maps (left and middle, density transferring from the parts in cyan to purple) of selected singlet−singlet transition states (marked with energy levels and oscillator strength f) for the [Cu3(L-Br)3]2 dimer in Br-α (293 K), and the molecular orbitals (right) involved in the major transitions for these singlet states.

density difference maps (Figure 7b and Table S13 in the Supporting Information), it is found that the LUMO is indeed involved in the major transitions for the first singlet−singlet transition state (S 1 ). However, S1 with an origin of intermolecular metal-to-metal charge transfer (MMCT) is not effectively populated (oscillator strength f = 0). Such a result is different from the common speculation, which is based on previous DFT or TDDFT calculations,35,38,57,58 that the emissions of copper(I) pyrazolate CTCs originate from the excited states involving intermolecular CuI−CuI bonding. In comparison, here the strongest transition is found to be S20 (f = 0.8635), which is ascribed to an origin with mixed contributions from intraligand charge transfer (ILCT), metal-to-ligand charge transfer (MLCT) and intramolecular MMCT (Figure S12 in the Supporting Information). These results suggest the phenylpyrazolyl conjugation may play an important role on the origin of luminescent property in this system. F



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ACKNOWLEDGMENTS This work was financially supported by the National Basic Research Program of China (973 Program, Nos. 2012CB821706 and 2013CB834803), the National Natural Science Foundation of China (Nos. 91222202 and 21171114).

To sum up, the present work reports several aspects of new experimental and computational observations on the copper(I) pyrazolate CTCs system. (i) An energetically strained, geometrically curved copper(I) pyrazolate CTC has been isolated in a polymorph of [Cu(LBr)]3, which exhibits by far the shortest ligand-unsupported CuI−CuI distances in this large CTC family. (ii) By performing calculations under the DFT and AIM regimes, we conclude in this system the intermolecular CuI− CuI interaction and Br···Br bonding are both close-shell interaction in nature, with a topological bonding pattern (i.e., RCPs and CCPs observed). The possibility of CuI−CuI covalent bonding may occur at the excited states, because in most cases the LUMO is found to be of a characteristic nature of intermolecular CuI−CuI bonding. (iii) A type of parallelogram-like Br4 halogen bonding has been identified, which interacts cooperatively/orthogonally with the CuI−CuI interaction to afford the interesting undulating layers in the crystal structure. Although the manybody problem may obscure the understanding on the nature of tetrameric halogen bonding, our results based on charge analysis and electrostatic potentials suggest electrostatic nature similar to the σ-hole model. (iv) Preliminary TDDFT calculations show that the first singlet−singlet transition state, which is characteristic of MMCT responsible for the formation of intermolecular CuI− CuI bonding, is not effectively populated. Instead, ligand-based π−π* transition and MLCT states are assigned to dominate the light absorption process. Such a finding suggests the origin of luminescence can be modulated, providing a pathway for tuning the luminescent property in the copper(I) pyrazolate CTC system. In all, this work provides new insight into the understanding of multiple metal−metal interaction and halogen bonding. Further studies on the influence of different halogen sites on the structures and luminescent properties are ongoing.

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ASSOCIATED CONTENT

* Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.6b00571. Experimental section and computational details (PDF) Accession Codes

CCDC 1444193−1444196 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions ∥

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X.-L.W. and J.Z. contributed equally.

Notes

The authors declare no competing financial interest. G



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