Pressure-Induced Conformational Change in Organic Semiconductors

Article pubs.acs.org/JPCC

Pressure-Induced Conformational Change in Organic Semiconductors: Triggering a Reversible Phase Transition in Rubrene Stefano Bergantin,† Massimo Moret,† Gernot Buth,‡ and Francesca P. A. Fabbiani*,§ †

Dipartimento di Scienza dei Materiali, Università di Milano-Bicocca, Via R. Cozzi 55, 20125 Milano, Italy Karlsruhe Institute of Technology, ANKA Synchrotron Radiation Facility, Hermann-von-Helmholtz-Platz 1, D-76344 Eggenstein-Leopoldshafen, Germany § GZG, Abteilung Kristallographie, Goldschmidtstr. 1, D-37077 Göttingen, Germany ‡

S Supporting Information *

ABSTRACT: A high-pressure polymorph of the organic semiconductor rubrene was obtained above 6.0 GPa by hydrostatic compression of the triclinic form. In the highpressure phase, rubrene adopts an unexpected and previously unobserved conformation, which is ca. 70 kJ/mol less stable than the planar one observed in the ambient-pressure phase and is characterized by a unique “double twisting” of the tetracene core and “scissoring” of the lateral phenyl groups, which favor the formation of C−H···π contacts. The evolution of the structure as a function of pressure is monitored and quantified by Hirshfeld surfaces analysis and calculations of lattice and intermolecular interaction energies. The isosymmetric single-crystal-to-single-crystal transition is fully reversible and is primarily driven by a reduction in molecular volume.

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INTRODUCTION Polycyclic aromatic hydrocarbons (PAHs) are presently attracting considerable interest because of their potential application as organic semiconductors in electronic and optoelectronic devices;1 among these compounds, rubrene (5,6,11,12-tetraphenyl-tetracene, Figure 1) is currently the most widely studied material. Single crystals of the orthorhombic form of rubrene have been reported to exhibit hole mobility of 20 cm2 V−1 s−1.2

organic-based semiconductor devices is hampered by the low solubility of this compound and by its poor stability toward oxygen and light.5 Several of the synthetic efforts directed toward addressing these problems produced rubrene derivatives displaying a twisting of the aromatic core of the molecule, with a consequent negative effect on the semiconductive properties.6−10 Unfortunately, without a full understanding of the interplay of intermolecular interactions in the solid state, the occurrence of this kind of distortions is unpredictable. The application of high pressure is a powerful method for modifying intermolecular interactions and exploring the polymorphic behavior of molecular compounds; refs 11−17 provide a good starting point for further reading. The structural response of several PAHs to high pressure has been investigated using both direct compression and in situ crystallization techniques.18,19 An enhancement of the charge transport properties of an orthorhombic rubrene single-crystal field-effect transistor up to ca. 0.6 GPa was previously documented.20,21 The reported pressure-dependent increase in mobility was attributed to a putative decrease of intermolecular distances; in addition, a decrease in the charge-carrier mobility, ascribed to possible molecular displacements in the crystal, was observed by Okada et al. at higher pressures, up to 0.9 GPa.20 Despite these interesting observations, to the best of our knowledge no high-

Figure 1. Chemical sketch of the rubrene molecule.

An additional monoclinic and a triclinic form of the compound are also known;3 the measured mobility of the latter was reported to be 1 order of magnitude lower than that of the orthorhombic one, whereas for the former no evidence of a semiconducting behavior was reported. The presence of a π−π stacking arrangement of the planar tetracene backbones in both conducting phases suggests that this feature is of primary importance for the semiconducting behavior displayed by the crystalline material.4 The application of rubrene for developing © 2014 American Chemical Society

Received: April 2, 2014 Revised: May 26, 2014 Published: June 3, 2014 13476

dx.doi.org/10.1021/jp503271h | J. Phys. Chem. C 2014, 118, 13476−13483

The Journal of Physical Chemistry C

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Table 1. Comparison between the Crystallographic Details of the Triclinic Rubrene Crystal throughout the Compression Study form I

form I

form I

form I

pressure (GPa) CCDC deposition number space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) volume (Å3) Vmol (Å3) Z/Z′ Dcalc (g cm−3) resolutionmax (Å) completeness to resolutionmax (%) parameters/restraints Rint R1 [F2 > 2σ(F2)], wR [F2] Δρmax/Δρmin (eÅ−3)

ambient pressure 991019 P−1 7.0883(7) 8.5994(8) 12.006(1) 93.486(5) 105.642(5) 95.977(5) 697.9(1) 697.9 1/0.5 1.267 0.90 99.2 167/156 0.0256 0.0524, 0.1341 0.17/−0.20 form I

0.15 991020 P−1 7.0478(3) 8.549(1) 11.948(1) 93.20(1) 105.501(5) 96.079(6) 687.2(1) 687.2 1/0.5 1.287 1.00 39.1 167/156 0.0530 0.0589, 0.1290 0.09/−0.10 form I

1.21 991021 P−1 6.8535(4) 8.264(1) 11.657(1) 91.61(1) 104.921(5) 96.311(7) 633.0(1) 633 1/0.5 1.397 0.85 35.9 167/156 0.0570 0.0431, 0.1281 0.13/−0.12 form I

2.42 991022 P−1 6.7392(4) 8.059(1) 11.464(1) 90.27(1) 104.627(6) 96.329(8) 598.4(1) 598.4 1/0.5 1.478 0.99 40.3 167/156 0.0505 0.0602, 0.1950 0.10/−0.13 form II

pressure (GPa) CCDC deposition number space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) volume (Å3) Vmol (Å3) Z/Z′ Dcalc (g cm−3) resolutionmax (Å) completeness to resolutionmax (%) parameters/restraints Rint R1 [F2 > 2σ(F2)], wR [F2] Δρmax/Δρmin (eÅ−3)

3.58 991023 P−1 6.6779(5) 7.926(2) 11.341(2) 89.37(2) 104.460(7) 96.321(9) 577.6(2) 577.6 1/0.5 1.531 1.00 41.4 167/156 0.0629 0.0659, 0.1845 0.11/−0.13

5.91 991025 P−1 6.6162(4) 7.676(2) 11.100(1) 87.40(2) 104.322(7) 96.289(9) 542.8(1) 542.8 1/0.5 1.629 0.90 39.1 167/156 0.0429 0.0561, 0.1681 0.14/−0.15

7.12 991026 P−1 14.239(4) 6.774(1) 11.281(1) 81.26(1) 100.38(1) 101.77(1) 1040.3(1) 520.1 2/1 1.700 0.90 35.1 332/357 0.0507 0.0599, 0.1610 0.20/−0.19

4.65 991024 P−1 6.6350(3) 7.799(2) 11.218(1) 88.44(2) 104.348(7) 96.316(8) 559.0(1) 559 1/0.5 1.582 0.85 35.9 167/156 0.0606 0.0639, 0.1743 0.13/−0.13

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EXPERIMENTAL AND COMPUTATIONAL METHODS Parallelogram-shaped crystals of triclinic rubrene were grown by slow evaporation of a saturated solution in 1,1,2,2tetrachloroethane. Preliminary screening of the samples by single-crystal X-ray diffraction revealed that all the crystals obtained were twinned with nearly equal volume fractions and a twin law corresponding to a 2-fold rotation about the [0 −1 1] reciprocal-lattice direction. A single crystal was loaded in a DAC modified from ref 22 using a 4:1 methanol/ethanol solution as hydrostatic pressure-transmitting medium. The DAC had an opening angle of 84° and was equipped with ca. 600 μm culet diamonds and a tungsten gasket. Single-crystal data were collected on beamline SCD at the ANKA synchrotron radiation facility of the Karlsruhe Institute of Technology (λ = 0.66100 Å) using a 3-circle Bruker APEX-I diffractometer. The pressure was monitored by the ruby fluorescence method with an accuracy of 0.05 GPa.23 Because of gasket failure, three different crystals were used to cover the entire pressure range. Data were collected with the DAC in two

pressure structural studies on rubrene have been reported in the literature. Indeed, in the absence of structural data, a comparison between the mobility increase ratio with pressure observed experimentally and that predicted on the basis of available structural parameters was performed by taking the compressibility or the Young’s modulus of similar compounds, such as anthracene or tetracene.20,21 In this context, we set out to investigate the high-pressure behavior of rubrene by means of hydrostatic compression in a diamond-anvil cell (DAC). This technique enables quantitative monitoring of structural changes as a function of pressure. The low solubility of rubrene in common organic solvents hampered our efforts with obtaining new phases by means of in situ high-pressure crystallization. On the basis of density considerations, we decided to focus our attention on the triclinic polymorph: by choosing the form with the highest density (1.294 g/cm3 at 175 K, cf. 1.284 and 1.279 g/cm3 for the orthorhombic and monoclinic polymorphs, respectively, at the same temperature), we hoped for a higher probability of triggering a phase transition to a completely new phase. 13477



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Figure 2. Optical microscopy images of a rubrene triclinic single crystal in the DAC, taken from ca. 0.1 to ca. 7.1 GPa at regular pressure intervals of ca. 1.2 GPa; the gradual darkening of the crystal as the pressure is increased is indicative of a change in the solid-state absorption spectrum of the material.

Figure 3. Directions of maximum (red), medium (green), and minimum (blue) compressibility and median compressibility values for triclinic rubrene-I throughout the compression study. Calculations performed with PASCal.28

7.5%), the compression of the b axis is substantially larger (ca. 11%). Using a Eulerian finite-strain model, the directions of the principal axes of the strain ellipsoid in form I, i.e., the directions of minimum, medium, and maximum linear compressibility, were calculated with the PASCal program28 and are depicted in Figure 3, together with the median principal compressibility values obtained over the whole pressure range up to 5.91 GPa. Compression of form I primarily affects the π−π stacking layer of the tetracene cores, as confirmed by the direction of maximum compressibility, leaving almost unaltered the piling scheme of these layers. Maximum compressibility occurs in a direction very close to that along which the outer rings of opposing tetracene cores face each other; the direction of minimum compressibility is close to that of the tetracene backbone short molecular axis. Using a third order Birch−Murnaghan-type equation of state29 we calculated the bulk modulus B0 and its derivative B′ for triclinic rubrene, 8.21(84) GPa and 9.37(89), respectively, in good agreement with the values reported in the literature for anthracene [8.4(6) GPa and 6.3(4)], tetracene [9.0(20) GPa and 7.9(12)], and pentacene [(9.6(10) GPa and 6.4(5)], although calculated using a different type of fitting.30 As a result of compression, the molecular volume is reduced by up to 23% at 5.91 GPa. To accommodate the increasing number of intermolecular contacts, the molecule itself is forced to increase the torsion angle between the tetracene core and each phenyl group, while reducing the opening angle among the two phenyl rings lying on the same side of the tetracene core moiety (Table S3 in the Supporting Information). This conformational change is associated with an energy penalty of ca. 40 kJ/mol, as calculated by a single-point MP2/6-31g** energy calculation. Changes in intermolecular interactions can be easily monitored

different orientations to improve data completeness. Further details on data reduction, structure solution, and structure refinement are given in Table 1 and in the Supporting Information. The pressure applied to the sample was gradually increased up to 7.2 at 1.2 GPa intervals. Figure 2 shows the evident darkening of the crystal, indicating that a red shift occurs in the absorption spectrum of the material as pressure is increased. A gradual quenching of the fluorescence of the crystal irradiated by the laser beam used for the pressure monitoring was observed concomitantly. The occurrence of both phenomena for rubrene under high-pressure conditions was previously documented in the literature24 and was not further investigated here. The final refined structures were used to calculate the molecular electron density at each pressure using the program Gaussian 9825 with the MP2/6-31G** basis set; these electron densities were used to calculate lattice and intermolecular energies by means of the PIXEL method26 as incorporated in the CLP program package.27 For the PIXEL calculation, a cluster of molecules with maximum distance from the central one of 20 Å, was employed.

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RESULTS AND DISCUSSION The effect of the applied pressure on the crystal structure of the ambient-pressure triclinic form, here denoted form I, is highly anisotropic, yielding an impressive volume reduction of ca. 22.8% in the 0−5.91 GPa pressure range. Unit-cell angle variations across this pressure interval are modest: the β and γ angles are only slightly affected by the compression, and α is gradually reduced by up to 6% of its original value. While the length of the a and c axes are subject to similar reductions (ca. 13478



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Figure 4. Relative atom-type contributions to the Hirshfeld surface for the intermolecular contacts within the structure of triclinic rubrene, at every pressure step.

Figure 5. Modification of the crystal structure of triclinic rubrene through the phase transition: perspective view of the (001) layer of molecules of form I at 5.91 GPa (left, gray) and of form II at 7.12 GPa (center, green); superimposed ball-and-stick model of a single molecule for the two structures (right).

by analyzing the dnorm Hirshfeld surface31 and derived fingerprint plots32 at each pressure step, in particular comparing the relative contributions to the Hirshfeld surface area for close intermolecular contacts, as depicted in Figure 4. For instance, the increase in C···C contacts with increasing pressure is ascribed to a shorter π−π stacking distance, together with smaller displacements of the opposing tetracene cores; at the same time, the rearrangement of the phenyl groups reduces the amount of H···H contacts, in favor of a larger percentage of C··· H contacts (see Figure S1 of the Supporting Information for a depiction of Hirshfeld surfaces and derived fingerprint plots). When the pressure is increased to 7.12 GPa, a phase transition to a different triclinic form, here denoted form II and depicted in Figure 5, clearly occurs: the isosymmetric phase transition is associated with a loss of rubrene’s inversion symmetry and doubling of the unit-cell volume. Each of the phenyl groups undergoes a considerable twist, increasing independently the torsion angle with the tetracene backbone, which is forced to bend and lose its planarity: this bending is perfectly compatible with the direction of maximum compressibility in form I, as depicted in Figure 3. In addition, in form II the torsion angle between the phenyl rings lying on the same side of the tetracene core moiety undergoes a dramatic decrease from the form I ambient pressure value of 22.7(2)° to 9.1(8)° and 1.4(8)°. To accommodate the new molecular conformation, the unit cell also undergoes some modifications. To maintain a good coherence with the description of the

crystal structure at ambient pressure found in the literature, we described the new unit cell as a cell with b and c axes similar to the ambient pressure cell and doubling of the a axis; according to our description, the values of both the α- and β-angles decrease, while the γ-angle widens. Hirshfeld surface analysis reveals a drastic drop of the percentage of C···C (3.2%) and H···H (8.2%) contacts, which is balanced by an increase of the C···H contacts of 11.4% during the phase transition, which favors the formation of a denser structure. The molecular volume of triclinic rubrene-II (see Table 1) clearly indicates that the structure of the new phase is much denser than would be expected from extrapolation of the trend for triclinic rubrene-I through the phase transition at 7.1 GPa. With the aim of providing a quantitative energetic description of the effects of compression on the intermolecular interactions, the lattice energy and its Coulombic, polarization, dispersive, and repulsive components were calculated by means of the PIXEL26 method for every pressure point. We focused on the eight molecular pairs having interaction energies greater than 2.0 kJ mol−1 at ambient pressure and labeled them 1−8 in increasing order of total energy. The least energetic interaction has almost completely a dispersive character and no repulsion: the molecules are related by translation along the [110] direction, and the distance between their centers of mass is 10.559 Å. Interactions 2, 3, 4, and 8 arise from contacts between phenylic hydrogen atoms, the latter also accounting 13479



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Figure 6. Energy ranking of the eight most energetic intermolecular interactions within the crystal. The molecular pairs are viewed perpendicularly to the tetracene unit of the central black reference molecule (left); plot of the energy of each interaction as a function of pressure (right, lines serve as a guide to the eye). The same color code is applied.

for the π−π stacking of the tetracene units. Interactions 6 and 7 involve C−H···π contacts between tetracenic hydrogens and lateral phenyls, as well as between phenylic hydrogens and the tetracene unit; interaction 6 also accounts for H···H contacts between hydrogen atoms on adjacent tetracene units. Interaction 5 arises both from H···H contacts among hydrogen atoms on the lateral phenyls and on the tetracene and from π···π interactions originating from the increasing proximity of the lateral phenyls of neighboring molecules. A pictorial description of the dimers and a plot of their corresponding energies as a function of applied pressure are depicted in Figure 6; dimer distances and single-energy components for each pressure are reported in Table S1 in the Supporting Information. Up to 5.9 GPa, interactions 1, 4, and 6 are only marginally affected by the compression. Interestingly, interaction 2 becomes more stabilizing as pressure is increased, as the sum of the Coulombic, polarization, and dispersion terms exceeds the repulsion. On the contrary, interactions 3, 5, 7, and 8 are gradually destabilized because the repulsive term dominates. Above 5.9 GPa, as the phase transition occurs, two effects can be clearly noticed regarding the energy ranking of the interactions: First, with the loss of the crystallographic inversion symmetry of the rubrene molecule, only the distance between the centroids of dimers 2, 3, and 7 survives unaltered on both sides of the central reference molecule. The corresponding interaction energies are similarly not affected by the conformational change of rubrene, whereas for all the other previously considered interactions two different dimers are found at opposite sides of the central molecule. Consequently, two distinct values for the energy of interactions 1, 4, 5, 6, and 8 are given in form II. Second, as a consequence of the profound alteration of intermolecular contacts within the crystal, the interactions energy ranking of form II is radically different from that of form I. For instance, interaction 6, relatively unaffected by the compression up to 5.9 GPa, is highly destabilized at 7.1 GPa: while the Coulombic, polarization, and dispersion terms all become more stabilizing, the repulsive term increases

dramatically, doubling in value for one of the two contacts in form II as both C···H and, more importantly, C···C contacts between adjacent units become significantly shorter. The trend for interaction 7 is actually reversed, with the contact energy first becoming more positive as pressure increases, then sharply getting more negative, thanks to a significant stabilization of the dispersive contribution to this interaction and only a very modest increase in repulsion. This interaction is predominantly responsible for the increased number of C−H···π contacts between tetracenic hydrogens and phenyl groups identified by the previous analysis of Hirshfeld surfaces and facilitated by the new molecular conformation. To rationalize the phase transition and identify possible driving forces, the method pioneered by Parsons et al.33−36 was followed. A correction for the internal energy variation resulting from the pressure-induced conformational changes in the isolated molecule was applied. The resulting Uadj values (corresponding to the total lattice energy minus the difference in internal energy of the molecule in its ambient-pressure conformation) were used to calculate the lattice enthalpy, H = Uadj + PV, where P is the applied pressure and V is the molar volume, as reported in Table S2 of the Supporting Information and graphically depicted in Figure 7. Although the conformational change of the rubrene molecules through the phase transition is substantial, it does not lead to a decrease in the internal energy of the molecule, as it occurs for example in the case of L-serine (40 kJ/mol for the pressure-induced phase transition), 33 but actually to an energy penalty: the conformation adopted by rubrene in form II is ca. 70 kJ/mol and ca. 30 kJ/mol less stable than that of form I at ambient pressure and at 5.91 GPa, respectively. This suggests that the phase transition is not driven by an optimization of molecular conformation, but more likely by a reduction of the PV term, which contributes to the lattice enthalpy; for similar phase transitions in other molecular crystals see, for example, L-serine monohydrate35 or salicyldoxime.36 By linear extrapolation of the relationship between H and P, the value of the lattice enthalpy for phase II seems to be, in fact, slightly below the 13480



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Figure 8. View of the buckling of the tetracene core in triclinic rubrene forms I and II and in the DFT-optimized structure with coordinates taken from ref 39.

Figure 7. Calculated enthalpy as a function of pressure for triclinic rubrene. The linear fit is calculated on the data points of form I only.

conformers similar to the one found in the DFT-optimized isolated molecule has also been observed experimentally (see Supporting Information for details of the CSD search): this conformation can be described as a “single helical twist” (Figure 8), which is usually reported in the literature as the torsion angle between the two opposite C−C bonds at each end of the tetracene backbone (42°). This torsion angle does not lend itself to description of the buckling in the high-pressure form II (1.38°), in which the lateral aromatic rings of the tetracene core rotate independently with respect to the almost planar inner two rings in a “double twist” and the buckling is described by dihedral angles formed between selected planes through the tetracene core (see Table S5 of the Supporting Information).

trend line of phase I, suggesting form II at 7.12 GPa is marginally more stable (see Figure 7 and Table S2 of the Supporting Information). This trend would be confirmed if further data above 7.12 GPa were available: unfortunately, in all our experiments higher pressures could not be reached without incurring into the risk of gasket failure. The negative change in the PV term is a characteristic feature of pressure-induced phase transitions, in which denser structures with a significantly more efficient packing are formed. The reverse phase transition to triclinic rubrene-I could be triggered by decreasing the pressure between 5.0 and 4.0 GPa, once again without any damage occurring to the crystal. Considering the ca. 2% increase in the amount of C···C contacts highlighted by the Hirshfeld surface analysis, together with the ca. 15% decrease in the π-stacking distance of the tetracene units, as applied pressure is increased in the 0−5.91 GPa range, an increase in the charge-transport properties of triclinic form-I should be expected. On the contrary, when compared to form I at 5.91 GPa, form II displays only a slightly further decrease (ca. 0.3%) in the average π-stacking distance, while the percentage of C···C contacts drastically drops by more than 3%. Overall, form II may not display enhanced charge-transport properties, although only in situ electrical measurement could confirm this. While the molecular conformation of rubrene form I changes gradually as a function of increasing pressure and becomes less stable (see Tables S1−S5 of the Supporting Information), it is radically altered in form II, where twisting of both the tetracene core and phenyl groups takes place. In this context, we note that the reported gas-phase conformation of rubrene (D2 symmetry), as computed and optimized by density functional theory (DFT),37−39 exhibits a significant twisting of the tetracene core (Figure 8); this conformer was reported to be ca. 20 kJ/mol more stable than the planar one observed in the crystalline state, i.e., as found in form I. An analysis of the Cambridge Structural Database (CSD, Version 5.35)40 indicates that the conformer adopted by form II is unusual, in particular in terms of the torsion angle that defines the “scissoring” of the lateral phenyl groups and in the buckling of the tetracene core (see Tables S4 and S5 of the Supporting Information). The energy penalty found in the form II conformer therefore appears to arise from both these conformational changes. Buckling of the tetracene core to

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CONCLUSIONS We have reported the first instance of a solid-state phase transition in one of the semiconducting polymorphs of the PAH rubrene. The transition, triggered by applying pressures in excess of 5.9 GPa, is fully reversible and proceeds in a singlecrystal-to-single-crystal fashion. The evolution of the structure as a function of pressure has been described and rationalized using qualitative and quantitative approaches. Both Hirshfeld surfaces analysis and intermolecular interaction energy calculations indicate a significant increase in stabilizing C− H···π contacts at the expense of π···π interactions, which from an energetic point of view become less stabilizing. This is different from what has been observed in the unbranched, flat PAH pyrene and phenanthrene,18 in which the herringbonetype structures of the ambient-pressure phases are characterized by C−H···π contacts, whereas the high-pressure structures are dominated by π···π interactions. The experimental observations and our detailed analysis not only reiterate the well-established view of crystal packing as a delicate balance of inter- and intramolecular forces but also illustrate the power of high-pressure techniques to probe the conformational space of a molecule in a crystal experimentally beyond the one accessible under ambient-pressure conditions. Our study provides structural data specific to rubrene for rationalizing at the structural level the charge carrier transport properties of the compound as a function of pressure without invoking and extrapolating elastic moduli of similar compounds and may prove useful for guiding new synthetic efforts of rubrene derivatives.41,42 A very recent study43 reported the results of crystal structure prediction of rubrene. Unfortunately, 13481



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(9) Schuck, G.; Haas, S.; Stassen, A. F.; Kirnerc, H.; Batlogg, B. 5,12Bis(4-tert-butylphenyl)-6,11-diphenylnaphthacene. Acta Crystallogr., Sect. E: Struct. Rep. Online 2007, 68, o2893. (10) Haas, S.; Stassen, A. F.; Schuck, G.; Pernstich, K. P.; Gundlach, D. J.; Batlogg, B. High Charge-Carrier Mobility and Low Trap Density in a Rubrene Derivative. Phys. Rev. B: Condens. Matter Mater. Phys. 2007, 76, 1−6. (11) Miletich, R.; Allan, D. R.; Kuhs, W. F. High-Pressure SingleCrystal Techniques. Rev. Min. Geochem. 2001, 41, 445−520. (12) Boldyreva, E. V. High-Pressure Diffraction Studies of Molecular Organic Solids. A Personal View. Acta Crystallogr., Sect. A: Cryst. Phys., Diffr., Theor. Gen. Crystallogr. 2008, 64, 218−231. (13) Fabbiani, F. P. A. In High-Pressure Crystallography: From Fundamental Phenomena to Technological Applications. Boldyreva, E. V., Dera, P., Eds.; Springer: Dordrecht, The Netherlands, 2010; 545. (14) Patyk, E.; Skumiel, J.; Podsiadło, M.; Katrusiak, A. HighPressure (+)-Sucrose Polymorph. Angew. Chem., Int. Ed. 2012, 51, 2146−2150. (15) Moggach, S. A.; Parsons, S.; Wood, P. A. High-Pressure Polymorphism in Amino Acids. Crystallogr. Rev. 2008, 14, 143−184. (16) Oswald, I. D. H.; Urquhart, A. Polymorphism and Polymerisation of Acrylic and Methacrylic Acid at High Pressure. CrystEngComm 2011, 13, 4503−4507. (17) Oswald, I. D. H.; Millar, D. I. A.; Davidson, A. J.; Francis, D. J.; Marshall, W. G.; Pulham, C. R.; Cumming, A.; Lennie, A. R.; Warren, J. E. High-Pressure Structural Studies of Energetic Compounds. High Pressure Res. 2010, 30, 280−291. (18) Fabbiani, F. P. A.; Allan, D. R.; Parsons, S.; Pulham, C. R. Exploration of the High-Pressure Behaviour of Polycyclic Aromatic Hydrocarbons: Naphthalene, Phenanthrene and Pyrene. Acta Crystallogr., Sect. B: Struct. Sci. 2006, 62, 826−842. (19) Fabbiani, F. P. A.; Allan, D. R.; David, W. I. F.; Moggach, S. A.; Parsons, S.; Pulham, C. R. High-Pressure RecrystallisationA Route to New Polymorphs and Solvates. CrystEngComm 2004, 6, 504−511. (20) Okada, Y.; Sakai, K.; Uemura, T.; Nakazawa, Y.; Takeya, J. Charge Transport and Hall Effect in Rubrene Single-Crystal Transistors under High Pressure. Phys. Rev. B: Condens. Matter Mater. Phys. 2011, 84, 245308−5. (21) Rang, Z.; Nathan, M. I.; Ruden, P. P.; Podzorov, V.; Gershenson, M. E.; Newman, C. R.; Frisbie, C. D. Hydrostatic Pressure Dependence of Charge Carrier Transport in Single-Crystal Rubrene Devices. Appl. Phys. Lett. 2005, 86, 123501−3. (22) Moggach, S. A.; Allan, D. R.; Parsons, S.; Warren, J. E. Incorporation of a New Design of Backing Seat and Anvil in a Merrill− Bassett Diamond Anvil Cell. J. Appl. Crystallogr. 2008, 41, 249−251. (23) Piermarini, G. J.; Block, S.; Barnett, J. D.; Forman, R. A. Calibration of the Pressure Dependence of the R1 Ruby Fluorescence Line to 195 kbar. J. Appl. Phys. 1975, 46, 2774−2780. (24) Nakashima, T. T.; Offen, H. W. Crystal Spectra of Tetracene and Rubrene under Pressure. J. Chem. Phys. 1968, 48, 4817−4822. (25) Frisch, M. J. et al., Gaussian 98, revision a.7; Gaussian Inc., Pittsburgh, PA, 1998 (26) Gavezzotti, A. Calculation of Intermolecular Interaction Energies by Direct Numerical Integration over Electron Densities. 2. An Improved Polarization Model and the Evaluation of Dispersion and Repulsion Energies. J. Phys. Chem. B 2003, 107, 2344−2353. (27) Gavezzotti, A. Efficient Computer Modeling of Organic Materials. The Atom−Atom, Coulomb−London−Pauli (AA-CLP) Model for Intermolecular Electrostatic-Polarization, Dispersion and Repulsion Energies. New J. Chem. 2011, 35, 1360−1368. (28) Cliffe, M. J.; Goodwin, A. L. PASCal: a Principal Axis Strain Calculator for Thermal Expansion and Compressibility Determination. J. Appl. Crystallogr. 2012, 45, 1321−1329. (29) Birch, F. Finite Elastic Strain of Cubic Crystals. Phys. Rev. 1947, 11, 809−824. (30) Oehzelt, M.; Aichholzer, A.; Resel, R.; Heimel, G.; Venuti, E.; Valle, R. D. Crystal Structure of Oligoacenes under High Pressure. Phys. Rev. B: Condens. Matter Mater. Phys. 2006, 74, 104103−7.

no information on lattice parameters of the predicted structures was given in the accompanying paper: it would be interesting to check whether triclinic rubrene form II was predicted as a stable, dense form in the crystal energy landscape, as it was for instance found in the case of halophenols44 and the pharmaceutical piracetam.45 Experimental structural data provides an essential benchmark for further developing potentials for crystal structure prediction or testing how accurately the pressure dependence of crystal structures can be reproduced computationally, as recently investigated for several PAHs.46

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details, structural parameters, CSD search, and details of the PIXEL computational study. This material is available free of charge via the Internet at http://pubs.acs.org. CCDC 991019−991026 contain the supplementary crystallographic data. These data can be obtained free of charge from the CCDC via www.ccdc.cam.ac.uk/data_request/cif

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AUTHOR INFORMATION

Corresponding Author

*E-mail: ff[email protected]. Tel.: (+)49 (0)551-39 33935. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are very grateful to Prof. Angelo Gavezzotti (University of Milan) for his precious help and advice with PIXEL calculations. M.M. and S.B. are grateful to Fondazione Cariplo (Grant 2009/2551) for financial support and gratefully acknowledge funding from the CALIPSO Project. F.P.A.F. thanks the DFG for funding, Emmy Noether Project FA 9649/ 1-1. The authors thank ANKA for the award of synchrotron beamtime.

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Pressure-Induced Conformational Change in Organic Semiconductors

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