An alternative strategy to polymorph recognition at work: The em

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An alternative strategy to polymorph recognition at work: The emblematic case of Coronene. Tommaso Salzillo, Andrea Giunchi, Matteo Masino, Natalia Bedoya-Martínez, Raffaele G. Della Valle, Aldo Brillante, Alberto Girlando, and Elisabetta Venuti Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00934 • Publication Date (Web): 24 Jul 2018 Downloaded from http://pubs.acs.org on August 1, 2018

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Crystal Growth & Design

An alternative strategy to polymorph recognition at work: The emblematic case of Coronene Tommaso Salzillo,*[a]† Andrea Giunchi,[a] Matteo Masino,[b] Natalia Bedoya-Martinez,[c] Raffaele G. Della Valle,[a] Aldo Brillante,[a] Alberto Girlando,[b] and Elisabetta Venuti*[a] [a]

Department of Industrial Chemistry “Toso Montanari” and INSTM−UdR Bologna, University of Bologna, Viale del Risorgimento, 4 – 40136 Bologna (Italy). [b] Dipartimento di Scienze Chimiche, della Vita e della Sostenibilità Ambientale (SCVSA) and INSTM-UdR Parma, Università di Parma, Parco Area delle Scienze, IT-43124 Parma (Italy). [c] Institute of Solid State Physics, NAWI Graz, Graz University of Technology, Petersgasse 16, 8010 Graz (Austria).

Supporting Information Placeholder ABSTRACT: We show that the development of highly accurate DFT calculations coupled to low-frequency Raman spectroscopy constitutes a valid method for polymorph characterization alternative/complementary to X-ray. The method is applied here to the temperature induced first order phase transition of coronene, known since a long time, but remained structurally uncharacterized due to crystal breaking during the process. The astonishing fidelity of the Raman calculated spectra to the experiments allows us to unambiguously identify the low temperature phase with the β-coronene polymorph, recently reported as new, and obtained in the presence of a magnetic field. We also suggest that additional measurements are needed to confirm that the magnetic field can actually drive the growth of a β-polymorph surviving indefinitely at ambient temperature.

Polymorphism in organic molecular solids, with its effects on the resulting physical properties, is currently object of intensive research due to the booming relevance of these materials in the field of organic electronics, that adds to a well established interest of the pharma industry.1,2 Although many progresses have been made, investigation of this phenomenon is made difficult by the crystallization process, whose outcomes, sometimes erratic or scarcely reproducible, may appear as the result of serendipity or magic.3 From this point of view, the recently reported possibility of guiding the growth of a new coronene polymorph through the use of a magnetic field has raised great expectations.4 The identification of polymorphs through X-ray analysis may present problems, especially when small energy differences cause different phases to befall in the same batch, or even in the same crystallite, or when it is not possible to obtain single crystals suitable for X-ray diffraction experiments. Years ago, some of us showed how low-frequency Raman spectroscopy can be of fundamental assistance in these cases.5 At the time, however, X-ray analysis was in any case necessary to associate the Raman spectrum with a given polymorph. Here, instead, we show that through the development of highly accurate DFT calculations6 we were able to reproduce the low-frequency Raman patterns (frequencies and relative intensities) of different polymorphs, just starting from a given or supposed crystal structure. Thus, the phase identification by low-frequency Raman spectroscopy can be made without

the collection of X-ray data on the spectroscopically studied samples. The method is applied to the clarification of the ambient pressure polymorphism of coronene, demonstrating that the newly discovered β-polymorph coincides with the structure thermodynamically stable below 160 K. Coronene (Scheme 1) belongs to the class of polycyclic aromatic hydrocarbons (PAH), and with its derivatives has drawn much attention as a model system, being regarded as the small building block of graphene.7 Found in minerals or produced by anthropic actions, coronene is widespread in the environment and its phase diagram has been investigated especially at high pressures and temperatures, to determine the conditions in which it is formed and remains stable in nature.8,9 Accordingly, phase transitions and polymorph structural data have been reported as a function of pressure starting from the phase stable at ambient conditions.8–11 Less information is available on the ambient pressure, low temperature behavior: Various spectroscopic analyses evidence a phase transition around 160-140 K,12–14 but the low temperature structure has remained uncharacterized due to crystal damage at the transition.

Scheme 1

As already mentioned, there has been a recent report4,15 on an unforeseen polymorph grown above room T in the presence of a magnetic field. Such a polymorph has been named β, as opposed to the known γ form stable at ambient conditions, after the kind of herringbone packing in either structure.4,16 In addition, powders XRD measurements have probed the partial transformation of the usual γ polymorph into β at ≈ 150 K,4 thus suggesting the β polymorph correspond to the low temperature phase reported in the past. Unfortunately, the single crystal XRD data needed to demonstrate that β-coronene can actually also be obtained (and maintained) at high temperature as a result of the application of the magnetic field was not provided. In fact, and rather puzzlingly, the only single crystal structure reported for β-coronene on the Cambridge Structural Database (CSD) has been measured at 80 K, that is below the reported transition temperature.

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Aiming to provide a powerful tool for coronene polymorph identification, we exploit the synergy between corrected highly accurate Density Functional Theory (DFT) calculations including dispersion (or van der Waals i.e. vdW) corrections and lattice phonon Raman characterization.6 In the Raman experiment, a γcoronene single crystal has been subjected to slow cooling to minimize the disruptive effects of the observed extended fragmentation, and the lattice phonons have been measured as a function of the temperature to trace out the transition and record the spectrum of the new phase. DFT methods using the Perdew-BurkeErnzerhof (PBE) exchange and correlation functional, in combination with the projector-augmented wave (PAW) approach have been employed17–20, with two different corrections to account for dispersive interactions and calculate vibrations: i) the many-body dispersion approach MBD-vdW21 and ii) the computationally much cheaper pair-wise D3-BJ by Grimme et al.22 They both have provided theoretical spectra so faithful to experiments to allow for the unambiguous identification of the β-polymorph, at low temperature. Experimental details on Raman spectra and a comprehensive account of the computational methods and outcomes are reported in the Supplementary Information (SI), Spectroscopic and Computational Methods. Despite the remarkable differences in packing, the γ21 and β4 polymorphs share the same monoclinic P21/n crystal symmetry, and both pack with Z=2 molecules per unit cell. The D6h symmetry of the isolated coronene molecule is not preserved in the crystal, but deviations from planarity are minimal in either structure. In fact, they are larger for γ-coronene, with an average distance from the molecular plane of 0.0135 Å (calculated D3-BJ 0.0145 Å), than for β-coronene, with a value ≈ 0.007 Å (calculated D3-BJ 0.007 Å). At 0 K with D3-BJ, the energy of β is computed to be 0.7 kcal/mol lower than γ’s, which is within the accuracy of the method for the energy differences23 (0.05 kcal/mol). The result thus agrees with the experimental evidence indicating that this is the phase stable at low temperature. In Figure 1 we report the sequence of the Raman spectra collected by cooling down a γ-coronene single crystal (top spectrum) from room temperature to 79 K. Only a selected number of spectra are shown in the Figure for the sake of clarity. In the low energy interval (normally below 150 cm-1) the vibrations observed for an organic molecular crystal correspond to modes where the molecules as a whole oscillate around their equilibrium positions in the crystal lattice, hence lattice phonons. The restoring forces and torques of these motions are the weak ones of the vdW intermolecular interactions. That, linked to the high inertia moments of the whole molecular body, is responsible for the low energy of the modes, neatly separable from the intramolecular vibrations of the isolated molecule. Only in the case of large, flexible molecules these motions cannot be separated from the low frequency internal modes. Accordingly, the low energy interval is where the crystal vibrational spectra of the polymorphs differ.24 In Figure 1, it is possible to observe in the temperature range 298 to 163 K the invariance of the spectra, which correspond to those of stable γ-coronene form. In a series of experiments performed on different crystals most of them underwent shattering just below this temperature, but the fragments originating in the process were large enough to allow being singled out and measured. The spectrum collected at 158 K has changed abruptly, displaying a lattice phonon pattern where none of the spectral features observed in the higher temperature range are left, and which resembles that recorded at much lower resolution reported by Ohno et al.12 at a single low temperature value. This marks the occurrence of a (first order) phase transition, with the transformation to a new polymorph which has involved the entire selected fragment. As can be noticed in the images of Figure 1, the phase

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transition is not accompanied by any change of color of the sample, a detail which will be discussed below. On heating the sample up, the process is reversible but displays a large hysteresis, as evidenced by the persistence at 253 K of the low temperature phase bands, which eventually shift to lower wavenumbers as a result of the thermal expansion of the lattice. The spectrum of γcoronene is fully recovered at 258 K. The intensity ratios for the peaks of the latter spectrum differ from those of the starting material, likely as a result of either sample movements in the cryostat chamber during the fragmentation, or variable orientations of the crystal domains formed during the thermal cycle. In all the crystals subjected to cooling, the shattering marked the occurrence of the transition, at a temperature T depending on the sample history and dimensions. The highest T recorded on cooling (around 160 K) sets the experimental upper thermodynamic boundary for the existence of the low temperature phase. Note, however, that the application of the mechanical constraint of a glass slide keeping the sample set in place prevented the occurrence of the transition, as demonstrated by the persistence of γ-coronene Raman spectrum at low T (Figure S2). The hysteresis for the backward transition on increasing temperature has also been observed for all samples. However, regardless of the heating rate, the low temperature phase eventually undergoes the transformation and is never recovered at ambient conditions. Figure 1 In Figure 2 the experimental Raman spectra of coronene crystals at 298 K and 158 K below 120 cm-1 are compared to the D3BJ spectra for the γ and β polymorphs, respectively, calculated at the experimentally determined structures4,21, as indicated in the SI. In the chosen range for both polymorphs only six frequencies (the lowest) were obtained, drawn in the Figure as Lorentzian bands with FWHMs chosen to conform to the experimental features. Deconvolution has been performed on the experimental bands, checking that the fitted peaks were in agreement with the values yielded by the analysis of the Raman polarized spectra of oriented crystals of Figure S3. Even at first sight, the almost perfect match between theoretical and experimental spectra enables us to recognize the low temperature phase as polymorph β, while the equally good agreement for polymorph γ actually validates the method. It is important to outline that the high fidelity of the theoretical spectra to the experiments, not just in reproducing the phonon frequencies but also the relative intensities of the bands, is in fact remarkable.

Figure 2 Based on the knowledge of the crystal structures and with the aid of the calculations, the interpretation of the Raman spectra in the selected energy range is straightforward. This has been detailed in the SI, Raman Spectra Analysis, where we demonstrate that the first calculated (and experimental) six modes of both polymorphs do correspond to the pure librations of rigid bodies and therefore to the lattice phonon vibrations characteristic of each crystal structure. The SI also provides the animations for such modes yielded by the calculations. In summary, the low temperature phase transition transforming γ-coronene into a new form has been fully investigated by Raman microscopy in the low frequency region, and found to occur around 160 K, in agreement with what reported in the past.12 By


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Crystal Growth & Design means of DFT-vdW corrected calculations of the vibrational properties of coronene crystals, it has been possible to ascertain that such a form corresponds to the β structure recently characterized by XRD measurements.4 It is necessary to stress out that this is an important result, in which the high quality of the calculations exploits the full potential of the spectroscopic technique in the challenging task of identifying solid phases. Until now, lattice phonons calculations have been used to implement the spectroscopic information about phases whose identity was known. Reversely, here we have been able to make a phase assignment relying only on the precise reproduction, through DFT calculations, of the Raman spectral pattern, thus yielding polymorph recognition independently of X-ray structural determination. In fact, the structure of various possible polymorphs of a substance can be possibly predicted directly in-silico.25 As for the experimental findings, we may conclude that, once triggered, the transformation goes to completion, as there is no evidence of the co-existence of the two polymorphs, which was instead observed on cooling γ-coronene powder.4 Puzzlingly, the change of colour which should characterize the β polymorph was never detected in our experiments, even though it has been reported to be quite striking and related to the strongly modified optical properties of this material. On the other hand, available electronic absorption measurements of the low-temperature phase do not evidence dramatic changes with respect to the ambient temperature one.14 The β structure is claimed to be obtained between 328 and 298 K via the assistance of a magnetic field, and subsequently to be stabilized at room temperature indefinitely. This raises a question, as the magnetic field might well drive the kinetics of the polymorph formation, as suggested in Ref 4, thus allowing the preferential growth of a metastable structure, but certainly cannot change its thermodynamics. While confirming the enantiotropic relationship between γ and β polymorphs, our temperature cycling experiments prove that β always reverts back into γ well below room T, despite the hysteresis. In these conditions, the lack in the literature of a β structure measured above the detected transition temperature is quite unfortunate and may pose the problem of a lack of evidence/reproducibility. Notwithstanding the interest aroused by controlling polymorphism with the additional degree of freedom of the magnetic field, also other factors, such as impurities, may account for the significant property changes observed in some coronene crystals grown at ambient conditions. We then believe that further investigations are needed to confirm the effect of magnetic field on the crystal growth process of coronene.

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT We thank CINECA Supercomputing Center for providing computer time through the ISCRA scheme (project C HP10CA2TWT). A.Giunchi acknowledges funding by the exchange program “Erasmus+ Mobilità per tirocinio 2017/2018” (Convenzione n. 2017-1-IT02-KA103-035637). This work has been partly financially supported by the Austrian Climate and Energy Fund (KLIEN) and the Austrian Research Promotion agency (FFG) through the project “ThermOLED” [FFG No. 848905].

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ASSOCIATED CONTENT (10)

Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Crystal growth method; spectroscopic and computational methods; γ-coronene at low T under mechanical constraint; polarized Raman spectra for β and γ polymorphs; Raman Spectra Analysis. (PDF) Animations for the lattice phonons modes provided by the calculations for γ and β polymorphs. (AVI format)

AUTHOR INFORMATION

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Corresponding Author *[email protected] *[email protected]

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Present Addresses †Present address: Institut de Ciéncia de Materials de Barcelona (CSIC), Campus de la UAB, 08193 Bellaterra (Spain).

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Crystal Growth & Design For Table of Contents Only

An alternative strategy to polymorph recognition at work: The emblematic case of Coronene Tommaso Salzillo,*[a]† Andrea Giunchi,[a] Matteo Masino,[b] Natalia Bedoya-Martinez,[c] Raffaele G. Della Valle,[a] Aldo Brillante,[a] Alberto Girlando,[b] and Elisabetta Venuti*[a] [a]

Department of Industrial Chemistry “Toso Montanari” and INSTM−UdR Bologna, University of Bologna, Viale del Risorgimento, 4 – 40136 Bologna (Italy). [b] Dipartimento di Scienze Chimiche, della Vita e della Sostenibilità Ambientale (SCVSA) and INSTM-UdR Parma, Università di Parma, Parco Area delle Scienze, IT-43124 Parma (Italy). [c] Institute of Solid State Physics, NAWI Graz, Graz University of Technology, Petersgasse 16, 8010 Graz (Austria).

The use of a highly accurate DFT calculations coupled to low-frequency Raman spectroscopy has been applied to the polymorphs recognition of Coronene. The astonishing fidelity of the Raman calculated spectra to the experiments allows to unambiguously identify the low temperature phase transition from γ to β polymorph reported recently as new obtained applying a magnetic field.


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Scheme 1. Coronene (hexabenzobenzene) molecular structure. 24x24mm (600 x 600 DPI)


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Crystal Growth & Design

Figure 1. Lattice phonon Raman spectra of a coronene crystal recorded during the thermal cycle of a sample across the phase transition occurring above 158 K in this experiment. In red the spectra of the γ polymorph, stable at high temperature; in blue the spectra of the low temperature polymorph. On the right hand side the images of a coronene crystal subjected to cooling (top) and of one of its fragments selected for the measurements after the transition (bottom). 418x312mm (96 x 96 DPI)



Figure 2. Calculated lattice phonon Raman spectra of γ (right) and β (left) polymorphs are compared to the experiments at high and low temperature, respectively. The corresponding Lorentzian band FWHMs are 4.5 and 2.5 cm-1. The scattering at 19.6 cm-1, indicated with an asterisk in the experimental pattern of β, is due to the glass slide in the cryostat. 695x624mm (96 x 96 DPI)


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An alternative strategy to polymorph recognition at work: The em

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