Article pubs.acs.org/cm
Temperature-Mediated Polymorphism in Molecular Crystals: The Impact on Crystal Packing and Charge Transport Loah A. Stevens,† Katelyn P. Goetz,† Alexandr Fonari,‡ Ying Shu,§ Rachel M. Williamson,∥ Jean-Luc Brédas,‡,⊥ Veaceslav Coropceanu,*,‡ Oana D. Jurchescu,*,† and Gavin E. Collis*,§ †
Department of Physics, Wake Forest University, Winston-Salem, North Carolina 27109, United States School of Chemistry and Biochemistry and Center for Organic Photonics and Electronics, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, United States § CSIRO Manufacturing Flagship, Private Bag 10, Clayton South MDC, Victoria 3169, Australia ∥ MX Beamlines, Australian Synchrotron, 800 Blackburn Road, Clayton, Victoria 3168, Australia ⊥ Solar and Photovoltaics Engineering Research Center, Division of Physical Sciences and Engineering, King Abdullah University of Science and Technology, Thuwal 23955-6900, Kingdom of Saudi Arabia ‡
S Supporting Information *
ABSTRACT: We report a novel synthesis to ultra high purity 7,14-bis((trimethylsilyl)ethynyl)dibenzo[b,def ]-chrysene (TMS-DBC) and the use of this material in the growth of single crystals by solution and vapor deposition techniques. We observe that the substrate temperature has a dramatic impact on the crystal growth, producing two distinct polymorphs of TMS-DBC; low temperature (LT) fine red needles and high temperature (HT) large yellow platelets. Single crystal X-ray crystallography confirms packing structures where the LT crystals form a 1D slipped-stack structure, while the HT crystals adopt a 2D brickwork motif. These polymorphs also represent a rare example where both are extremely stable and do not interconvert to the other crystal structure upon solvent or thermal annealing. Single crystal organic field-effect transistors of the LT and HT crystals show that the HT 2D brickwork motif produces hole mobilities as high as 2.1 cm2 V−1 s−1, while the mobility of the 1D structure is significantly lower, at 0.028 cm2 V−1 s−1. Electronic-structure calculations indicate that the superior charge transport in the brickwork polymorph in comparison to the slipped-stack polymorph is due to the presence of an increased dimensionality of the charge migration pathways.
1. INTRODUCTION The use of small-molecule organic semiconductors in organic field-effect transistors (OFETs) has increased significantly over the past decade. Small molecules are easy to synthesize and purify, readily accessible in large quantities, can be deposited by solution and evaporation techniques, and have produced the highest mobilities reported to date.1−11 The design of new materials with even higher mobilities and long-term device stability for practical applications is a continuing challenge.5,12,13 Crystal packing in small molecule semiconductors plays an extremely important role in the electrical performance of devices based on these materials. However, other factors that are less well understood and not easily measured also contribute, making designing new materials with desirable properties an empirical process.14,15 Progress in the molecular design of semiconductor materials by crystal engineering using in-silico methods is still in its infancy in the field of organic electronics.16 In recent years, a large number of publications on structure−property relationships of small molecules in organic transistors have provided a © 2015 American Chemical Society
valuable insight into the complexity and correlations associated with material structure, molecular packing, processing conditions, and charge transport properties.17−20 These relationships are best studied using single crystal organic field effect transistors (SCOFETs).21−29 Single-crystal devices offer highly ordered systems with minimal grain boundary defects, single polymorphs, a continuous crystal lattice, and access to materials containing a minimum level of impurities. Not surprisingly, mobility data acquired from single crystals can be substantially higher than those from thin films; for example pentacene SCOFET have produced mobilities as high as 35 cm2 V−1 s−1,30 while thin-film OFET mobilities of 5 cm2 V−1 s−1 have been reported.4 SCOFETs offer a unique platform to study the impact of polymorphism on charge transport, where the effect of subtle changes in molecular packing can be investigated without the complexity of the Received: September 17, 2014 Revised: December 15, 2014 Published: January 2, 2015 112
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acetylide, followed by reduction with acidic tin oxide afforded, after chromatography and recrystallization, high purity TMSDBC 1 (HPLC >99.3%) that agrees with the literature NMR data for TMS-DBC 1 obtained previously from the commercial DBC dye source.18 Although the minor isomer 6 presumably also undergoes addition of TMS-acetylide, the five-membered ring prevents cycloaromatization from occurring, and therefore the minor isomer is easily removed during purification. TMSDBC 1 is thermally stable up to 315 °C (see SI Figure S1) and shown to have a high hole mobility (1.17 cm2 V−1 s−1) when fabricated into thin film bottom gate/top contact OFETs by vacuum deposition.18 2.1. Polymorphism, Crystal Growth Methods, and Single Crystal X-ray Crystallography. Polymorphism of small molecule organic semiconductors has previously been reported for systems such as pentacene,35−37 5,11-substituted (hetero)pentacenes,38−40 oligothiophenes,41 tetrathiafulvalene,42 and diketopyrrolopyrroles.43 Polymorphism has also been reported in bulk films of pentacene and oligiothiophene derivatives, where the crystal packing of the first few monolayers can be significantly different from the molecular packing found in the bulk.35,44 To date, polymorphs derived from the same molecular formula have produced different crystal unit-cell structures that show minor variations in crystal packing. These arise from small changes to the crystal intermolecular contact distances and the degree of π−π overlap. Polymorph formation is typically dictated by the choice of solvent employed in crystal growth or film deposition, postsolvent and thermal annealing of thin films, and more recently, by the mechanical forces created during the processing of materials into films (e.g., shearing and spin coating).6,7 Some linear acenes are known to have many polymorphic structures that are metastable under processing conditions or that can be easily interconverted.36−39,45 In any of these cases, it is well established that crystal polymorphs contribute differently to charge transport within these crystal architectures, and understanding and controlling these crystal transitions is critical in maximizing device performance and long-term device stability. Here, we find that two polymorphs can form in TMS-DBC 1, which strongly differ in their electronic properties, and are not interconvertible under the conditions tested in this study. We have grown crystals by both solution and gas-phase methods, and our results show that the resulting polymorphism type is dictated by the substrate temperature at which crystal growth occurs and is independent of the growth method. We refer to the two structures as the low temperature (LT) and the high temperature (HT) polymorphs, respectively (Figure 1). Physical Vapor Transport (PVT)21,27,46 of TMS-DBC (sublimation temperature 240 °C) results in the formation of both polymorphs in the crystallization tube. A temperature gradient is present in the PVT tube, and we observe that TMS-DBC crystal formation occurs in the temperature regions from ∼25 °C up to 170 °C (see SI Figure S2 for a detailed description of the growth tube). Visual examination of the different regions of the PVT tube clearly indicates the formation of two differently colored and shaped crystals. In regions where the tube temperature is ∼25−65 °C (the end of the tube), we observed fine small red needles we refer to as the TMS-DBC red LT polymorph, while from 130 °C to 170 °C, we observed large yellow platelets, which we refer to as the TMS-DBC yellow HT polymorph. The temperature region from 65 °C to 130 °C is a transition region containing both red and yellow crystals. The
microstructure, which is always present in thin-film devices. Such information is critical in material design and crystal engineering. SCOFET data also allow easier comparison with the results of electronic-structure calculations, to evaluate the intrinsic charge-transport processes in these materials. Polyaromatic hydrocarbon (PAH) materials with a nonlinear molecular structure have recently received much attention as, unlike their linear oligoacene counterparts, they have been shown to be much less reactive with other molecular entities, such as fullerenes,31,32 and have produced high mobility in OFETs.17,18,33 Here, we describe the synthesis and characterization of a high-purity PAH, 7,14-bis((trimethylsilyl)ethynyl)dibenzo[b,def ]-chrysene (TMS-DBC) 1. The results reveal the first example of a single compound that exhibits two distinct crystal structures (with 1D and 2D packing, respectively) and very high crystal structure stability. The crystal packing is dictated by the substrate temperature from which the crystals are grown, and the extremely stable nature of the two polymorphs does not allow interconversion between the two different crystal lattice structures once formed within the processing parameters investigated in this study. We evaluated the intrinsic electronic properties of the two polymorphs using SCOFETs and performed electronic-structure calculations in order to understand the differences observed in the chargetransport properties.
2. RESULTS AND DISCUSSION High purity TMS-DBC 1 was accessed from a new synthetic route shown in Scheme 1 (see the Supporting Information (SI) Scheme 1. (a) Pd(PPh3)4, Toluene, 2M Na2CO3, Reflux; (b) CF3SO3H, CH3SO3H, 140 °C; and (c) (i) −78°C, TMSAcetylene, n-BuLi, (ii) SnCl2·2H2O/Aqueous 1M HCl
for full synthetic details). Suzuki coupling of boron ester 2 with bis-triflate 334 gave the diester product 4 in high yield. Subjection of diester 4 to extremely acidic conditions (TFSA/ MSA at 140 °C) facilitated intramolecular Friedel−Crafts acylation to give two isomeric products, the 6,6-diketone 5 and 5,6-diketone 6 as an inseparable mixture. 1H NMR spectroscopy confirmed the formation of two isomers with the major compound being the desired dibenzochyrsene[b,def ]dione (DBC) 5. Reaction of this isomeric mixture with TMS113
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Figure 1. (a) and (d) Crystal color, size, and shape of the LT red and HT yellow polymorphs of TMS-DBC. (b) and (c) Side and top views of the crystal packing in the red LT polymorph. (e) and (f) Side and top views of the crystal packing in the yellow HT polymorph. The directions corresponding to the largest calculated electronic couplings are indicated with arrows.
LT polymorph (SI Figure S3b) the acetylene bond lies ∼2.36° out of the aromatic plane, while in the HT polymorph it is further distorted at ∼3.04° (SI Figure S4b). Finally, the most obvious difference is the size and shape of the crystals. The HT polymorph is obtained as large platelets, whereas the LT polymorph occurs as small needles. Crystal growth by solution methods was also investigated using drop cast (DC) and solvent-assisted crystallization (SAC) techniques.47 Crystals were grown from solutions of TMS-DBC 1 in xylenes, acetonitrile, dimethyl sulfoxide, chlorobenzene, or mixtures of these solvents on device substrates. Interestingly, we observe that in all cases, when the crystallization occurs at or near room temperature, only red crystals are obtained. When the substrate temperature is raised over 65 °C, a mixture of red LT and yellow HT polymorphs is observed. Unfortunately, rapid solvent evaporation precludes crystal growth in the temperature range where the pure HT polymorph is expected. Our observations on both PVT and solution-grown crystals indicate that the crystallization temperature plays a key role in determining which polymorph is formed. Combined experimental and theoretical work is currently in progress to elucidate the mechanism for this polymorphism formation and to determine a complete parameter space (e.g., pressure, solvent type, cooling rate, and combinations of the above) that yield each of the structures. With the crystal structures of the two polymorphs resolved, interconversion between the polymorphs was investigated. Attempts to transition the red LT polymorph to the yellow HT polymorph by thermal annealing were unsuccessful. This can also be observed from the feature-less DSC spectrum around the temperature where the HT polymorph is expected to form (SI Figure S1). In addition, extended heating (2 days) above 65 °C has also not resulted in polymorphism interconversion. Likewise, room temperature solvent annealing for several hours of the red LT and yellow HT polymorph also proved to induce no change on the crystal structure. Polymorphism interconversion by solvent annealing at higher temperatures was precluded by rapid evaporation of the solvent. We were also not able to test PVT crystal growth by resublimation of the crystals obtained in one run because of the low yield of our crystal growth. We did note, however, that solution-grown LT polymorph can be converted in a mixture of HT and LT crystals upon subliming it using PVT.
two observed polymorphs of TMS-DBC 1 crystallize in significantly different triclinic cells with very different degrees of cofacial packing. Unit cell parameters for the LT and HT polymorphs are detailed in Table 1 and graphically represented in SI Figures S3a and S4a, respectively. It is clear from the crystal structures that the c-axis of the unit cell of the yellow HT polymorph is significantly longer than that of the LT polymorph, with the interactions between the alkyl groups giving rise to a less closely packed structure and a greater gap between neighboring π−π stacks. Table 1. Single-Crystal X-ray Crystallographic Data of TMSDBC 1 Polymorphs Measured at 100 K a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) space group
red LT polymorph
yellow HT polymorph
6.4070 (13) 8.8220 (18) 12.726 (3) 72.82 (3) 76.84 (3) 83.84 (3) 668.6 (2) triclinic, P-1
6.1670 (12) 6.5710 (13) 18.077 (4) 92.65 (3) 90.94 (3) 113.17 (3) 672.3 (2) triclinic, P-1
Close examination of the red crystals by single-crystal X-ray diffraction indicates these to be identical to the crystal previously grown at room temperature from solution methods.18 The red LT polymorph exhibits a 1D slipped stacked packing (Figure 1b), where the intrastack distance between aromatic carbon−carbon atoms (CC) is short at ∼3.353 Å, with a high degree of spatial intermolecular π−π overlap (Figure 1c) between adjacent PAH units (i.e., 6 close CC contacts per 3 DBC units).18 The crystal packing of the newly discovered HT polymorph exhibits a 2D brickwork motif with a slightly larger unit-cell volume compared to the LT polymorph (Figure 1e). Even closer CC contacts of ∼3.336 Å are observed; however, this is compensated by a slightly lower degree of π−π spatial overlap (Figure 1f) between adjacent PAH rings (i.e., 4 close CC contacts per 3 DBC units), presumably due to steric interactions of nearby TMScapping groups. The steric congestion brought by the TMSgroups is confirmed by both the LT and HT polymorphs showing distortion of the TMS−acetylene linkages, with these bonds not being fully coplanar with the DBC PAH core. In the 114
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channel length L = 100 μm, and channel width W = 108 μm yielded a mobility μ = 2.1 cm2V−1s−1. Variation of mobility data for SCOFETs were observed. Such differences are typical and originate from incomplete crystal lamination, anisotropy effects, or morphology defects in the crystal studied,38 as shown in the histogram presented in SI Figure S5a. The red LT polymorph crystals gave devices with significantly lower hole mobilities on the order of 10−5−10−3 cm2 V−1 s−1, the best reaching 10−2 cm2 V−1 s−1 (see SI Figure S5b for the mobility distribution). The device presented in Figure 2c for example has a mobility of μ = 0.028 cm2 V−1 s−1. These values are significantly different from the ones measured in the HT polymorph, where the highest mobility was in the order 1 cm2 V−1 s−1, with the majority of the samples showing mobilities of 10−1 cm2 V−1 s−1 (SI Figure S5a). Note that the crystals were not aligned during device fabrication; however, by measuring a large number of devices (>50) for each polymorph, we have accessed all crystallographic directions, and we can conclude that the HT polymorphs yield better hole transport. The inferior electrical performance of the LT polymorph is partially due to the fact that the majority of these crystals were obtained by crystallization from solution, which typically produces lower quality crystals (increased surface roughness, lower purity) compared to the PVT, which yielded most of our HT polymorph crystals. This is also supported by the higher value of the subthreshold slope measured in the LT crystals (e.g., 6.67 V/dec for the device in Figure 2c, compared to 1.04 V/dec for the transistor in Figure 2b). Nevertheless, these effects cannot fully explain the large differences observed in the device mobilities, and thus other factors will be discussed below. 2.3. Electronic Structure. In order to get a better understanding of the intrinsic charge-transport properties of the crystalline polymorphs, we investigated their electronic couplings (transfer integrals, t) and related electronic band structures using density functional theory (DFT) (see Figures 1b,e and 3). In the LT polymorph, the largest transfer integrals
In contrast to other aromatic and heteroacene systems such as TIPS-PEN45 and TES-ADT derivatives,38,39 where temperature-induced transitions resulted in phase-interconversion, the type of temperature-driven polymorphism discovered in TMSDBC appears to be distinctly different and rare, owing to its reluctance to interconvert between polymorphs. Temperatureinduced formation of the 1D or 2D polymorphs is, to the best of our knowledge, the first example where a significant and irreversible change in crystal packing has been observed for organic molecular crystals. From a device perspective, the formation of a stable polymorph that can be determined by regulation of the substrate temperature is highly advantageous for the uniformity, reproducibility, long-term stability, and operation of devices. 2.2. SCOFET Measurements. Crystals grown by the three different methods were evaluated in bottom-gate, bottomcontacts SCOFETs (Figure 2a). The substrates used consisted
Figure 2. (a) Schematic representation of the SCOFET device structure. (b) Transfer curve for a crystal exhibiting the HT polymorph. (c) Transfer curve for a crystal exhibiting the LT polymorph.
of highly doped silicon for the gate electrode, 200 or 300 nm thermally grown silicon dioxide for the gate electrode, and an array of gold source and drain contacts. The PVT single crystals were laminated by hand onto these substrates. The solutiongrown crystals were allowed to crystallize directly onto the prefabricated testbeds for SAC and DC crystals. Figure 2b,c represent typical current−voltage characteristics for the HT and LT polymorphs, respectively. On the right axis, in blue, we show the drain current (ID) as a function of the applied gate-to-source voltage (VGS) for a constant source-drain voltage (VDS = −40 V). On the left axis, in black, we show the square-root value of the ID, and the slope of this curve was used in the determination of the field-effect mobility in the saturation regime (SI Eq. S1). The device in Figure 2b, with
Figure 3. DFT-B3LYP band structure for the relaxed geometry of LT (left) and HT (right). The points of high symmetry in the first Brillouin zone are Γ = (0, 0, 0), X = (0.5, 0, 0), Y = (0, 0.5, 0), Z = (0, 0, 0.5), V = (0.5, 0.5, 0), U = (0.5, 0, 0.5), T = (0, 0.5, 0.5) all in crystallographic coordinates. The zero of energy corresponds to the top of the valence band.
were found along the 1D slipped stack (Figure 1b), with the corresponding values for holes and electrons estimated as 76 and 5.5 meV. In the framework of a one-dimensional tightbinding model, the bandwidths correspond to four times the transfer integrals, therefore such transfer integrals would result in valence and conduction bandwidths of 0.3 and 0.02 eV, 115
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nonlocal coupling is also not likely.52,53 If the nonlocal coupling is treated as a perturbation (band-like transport regime), then the calculations based on the 1D Boltzmann theory (see SI) indicate that the intrinsic hole mobility can be as large as 6 and 7 cm2 V−1 s−1 in the LT and HT polymorphs, respectively. We note, however, that the applicability of the Boltzmann theory is strictly justified only when σ/⟨t⟩ ≪1, therefore, transport models that can be applied for intermediate values of σ/⟨t⟩ ∼1 need yet to be developed to obtain more accurate values of the charge carrier mobilities in these and similar systems. Overall, on the basis of the results of the electronic-structure calculations, the LT and HT polymorphs are expected to exhibit very good intrinsic charge-transport properties for holes with one-dimensional and two-dimensional character, respectively. Thus, the significant difference observed between the SCOFET mobilities in the LT and HT polymorphs is believed to be related to the lesser effect played by trap states and defects on charge transport in systems with higher dimensionality.
respectively. These values are in good agreement with those obtained directly from the periodic boundary conditions calculations (0.35 and 0.05 eV, respectively; see Figure 3). As a result of a relatively large hole electronic coupling, the effective hole mass along the stacking direction is small, 0.85 m0 (where m0 is the electron mass in vacuum). The other components of the effective mass tensor are large (>3m0), which suggests 1D character of the hole transport. The smallest electron effective mass is calculated to be 4.04 m0 (SI Table S2). In the HT polymorph, as a result of a 2D brickwork packing motif, there are two large transfer integrals (see Figure 1e), t1 and t2 (53 and 50 meV for holes, and 52 and 66 meV for electrons). The valence and conduction bandwidths estimated from band-structure calculations are about 0.40 and 0.44 eV. For this polymorph, two small effective masses are found along the stacking directions: 1.03 m0 and 2.54 m0 for holes and 1.34 m0 and 3.0 m0 for electrons, which is indicative of an intrinsic 2D transport. For the sake of comparison, we note that the two smallest components of the hole effective mass in rubrene are 0.78 m0 and 1.95 m0, respectively.48 In addition to electronic interactions, the charge-transport properties also depend on electron−phonon interactions. In a previous work, we have shown that the local electron-phonon interaction (the modulation by vibrations of the site energy) in TMS-DBC like systems29 leads to polaron binding energy for holes of about 80 meV. This value is smaller than the half-width of the valence bands in both LT and HT polymorphs implying that formation of small molecular polarons in these systems is not favorable. It is also important to note that the main contribution to the polaron binding energy results from the interaction with high-frequency molecular vibrations whose excited levels are thermally inaccessible even at room temperature, therefore it is expected that the main role of local electron−phonon interactions is to moderately reduce (renormalize) the electronic couplings.14 There is currently a growing consensus that a second electron-vibration mechanism referred to as the nonlocal coupling mechanism plays a dominant role in organic semiconductors. This mechanism is due to the dependence of the transfer integrals on the distances between adjacent molecules and their relative orientations.14 The overall strength of the nonlocal electron−phonon coupling at a given temperature is given by the variance of the transfer integral due to thermal fluctuation, σ (see SI).49−51 A combination of molecular dynamics simulations and quantum-chemical calculations was used to compute the probability distribution of the transfer integrals. From this distribution, we then estimated σ and ⟨t⟩ (Table 2, SI Table S3), here ⟨···⟩ represents the average over geometrical configurations resulting from thermal motion. For the largest hole coupling, the calculations show that the ratio σ/⟨t ⟩ is 0.3 for the HT polymorph and 0.4 for the LT polymorph. This is smaller than unity in both polymorphs implying that formation of self-localized polarons due to
3. CONCLUSIONS We have described a novel synthesis to ultrahigh purity TMSDBC 1, the growth of polymorph single crystals by solution and evaporation conditions, the evaluation of these crystals in SCOFET devices, and the elucidation of the differences in the intrinsic charge transport of these polymorphs by undertaking a theoretical analysis of their electronic structure. Two polymorphs (LT and HT) of TMS-DBC can be formed with 1D and 2D packing motifs, respectively, which can be generated by controlling the temperature at which crystal growth occurs. The polymorphs are visually markedly different, as the LT crystals occur as fine red needles, while the HT crystals are large yellow platelets. These polymorphs represent a rare example of a situation where both polymorphs are stable and do not interconvert to the other crystal structure upon solvent or thermal annealing. SCOFET studies indicate that the HT polymorph, with a 2D brickwork crystal packing motif, exhibits hole mobilities as high as 2.1 cm2 V−1 s−1, while the hole mobility of the LT polymorph with 1D slipped stacking packing is considerably lower, 0.028 cm2 V−1 s−1. The electronic-structure calculations point to a single large hole electronic coupling in the LT polymorph and two large hole electronic couplings in the HT polymorph. The higher mobility of the HT polymorph is likely related to the better crystal quality and the availability of alternative pathways for charge carriers encountering defects or trap states. The current work highlights further opportunities for functionalized PAH semiconductors systems for use in organic electronics applications, as well as the need for further investigations of conduction in single crystals with different dimensionalities of the crystal packing.
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S Supporting Information *
Detailed synthetic experimental procedures; full characterization data of TMS-DBC; differential scanning calorimetry (DSC) data; single crystal growth conditions; single crystal Xray data; and SCOFET device data. This material is available free of charge via the Internet at http://pubs.acs.org.
Table 2. Calculated Average Values (⟨th⟩) and Standard Deviation (σ) of the Transfer Integrals for Holes
HT t1 HT t2 LT
⟨th⟩, meV
σh, meV
σh/⟨th⟩
−77.3 −41.4 −86.8
26.0 32.5 33.2
0.3 0.8 0.4
ASSOCIATED CONTENT
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. 116
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[email protected]. *E-mail:
[email protected].
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Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Flexible Electronics Theme and is part of the CSIRO Future Manufacturing Flagship. We acknowledge financial support from the CSIRO Office of the Chief Executive program for Y.S. and G.C. Work at WFU was supported by the National Science Foundation, under Grant ECCS 1254757 and GRFP DGE-0907738. The work at Georgia Tech was supported in part by the National Science Foundation under Award No. DMR-1105147. Data for X-ray structure determination were collected on the MX2 beamline at the Australian Synchrotron, Victoria, Australia.
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