18878
J. Phys. Chem. C 2007, 111, 18878-18881
Enantiotropic Polymorphism in Di-indenoperylene Michael A. Heinrich,† Jens Pflaum,‡ Ashutosh K. Tripathi,‡ Wolfgang Frey,§ Michael L. Steigerwald,| and Theo Siegrist*,† Bell Laboratories, Alcatel-Lucent, 600 Murray Hill, New Jersey 07974, 3.Physikalisches Institut, UniVersita¨t Stuttgart, D-70550 Stuttgart, Germany, Institut fu¨r Organische Chemie, UniVersita¨t Stuttgart, D-70550 Stuttgart, Germany, and Department of Chemistry, Columbia UniVersity, New York 10027 ReceiVed: June 22, 2007; In Final Form: September 6, 2007
The enantiotropic polymorphic phase transformation of di-indenoperylene (DIP) at 403 K has been structurally characterized, using single-crystal X-ray diffraction. Both the low temperature R- and the high-temperature β-phase have a herringbone-type structure, with the R-phase being triclinic with doubled unit cell volume compared to the monoclinic β-phase. In the latter, the molecules have a more upright orientation in the herringbone plane. The epitactic transformation from β- to R-phase involves strong shearing displacements as well as bending and torsional deformations of the DIP molecules.
Organic electronic materials have been the focus of considerable interest in the past decade. In particular, oligoacenes, such as anthracene, tetracene, and pentacene, show high charge carrier mobility in single crystal form1-4 and thin films.5 Keys to their performance in the case of single crystals are high purity as well as high structural perfection. High purity may be achieved via multiple zone refining steps,1,6 and, to some extent, via multiple sublimation steps.7 High structural perfection may be achieved by different crystallization techniques, such as Bridgman growth where possible1,8, or vapor phase transport, either in a carrier gas or in vacuum.7 However, polymorphism, in bulk and thin film form may hamper crystal quality, as, for example, is the case in pentacene.9 A desired material may therefore exhibit low vapor pressure for good room-temperature stability and ambipolar charge transport characteristics, which is for instance found for di-indenoperylene (DIP).10 If any structural phase transformations may occur, their respective transition temperatures should be high enough to ensure stability over a temperature region from about 250 to 350K, which is the range of operation for most of the molecular film based devices. DIP, a reddish perylene derivative (optical gap about 2.3 eV) with two indeno endgroups (see Figure 1) meets many of these requirements. DIP decomposes before melting, but the shape of the conjugated π system is expected to provide sufficient stability against oxidation11 for sublimation purification and crystal growth. Gradient sublimed material shows electron as well as hole transport in bulk.10 Furthermore, the low vapor pressure at room temperature of DIP ensures stability of thin films of DIP which have proven to be suited for thin film device applications.12-14 However, the reported thin films showed a different unit cell than bulk crystals, indicating polymorphism. Time-of-flight (TOF) studies indicate the occurrence of a phase transition at around 400 K which was further verified by temperature-dependent XRD measurements along the transport direction (crystallographic c′ direction) exhibiting a reversible * To whom correspondence should be addressed. E-mail: tsi@bell-labs. com. Fax: 908 582 4868. † Bell Laboratories. ‡ Physikalisches Institut, Universita ¨ t Stuttgart. § Institut fu ¨ r Organische Chemie, Universita¨t Stuttgart. | Columbia University.
Figure 1. Molecular structure of di-indenoperylene (DIP) and picture of a crystal.
structural phase transition.10 Here, we will discuss the enantiotropic polymorphic structural phase transformation in DIP observed at 403 K. In DIP, the triclinic low temperature R-phase (EI (403 K), β) with lattice parameters of a ) 7.1709(8) Å. b ) 8.5496(9) Å, c ) 16.7981(18) Å, and β ) 92.416(11)°, cell volume of 1028.9(3) Å3, and space group P21/a, at 423 K, containing one type of molecule. In Figure 2, we show the molecular packing in the two polymorphic structures and, in addition, in Figure 3 a graphic comparison of the two unit cell molecules at 110, 293, and at 423 K with the size of the carbon atoms reflecting the atomic thermal motion at 50% probability level. Remarkably, the β-phase is identical to the thin film phase observed, e.g., on weakly interacting substrates like SiO2 at room
10.1021/jp0748967 CCC: $37.00 © 2007 American Chemical Society Published on Web 11/30/2007
Enantiotropic Polymorphism in DIP
Figure 2. Herringbone layers of the R- phase (at left) and the β-phase (at right). The twist and bend in the DIP molecules in the R-phase is clearly visible. The larger tilt away from the c* axis in the R-phase is reflected in the unit cell indicated. In the lower part, the herringbone layers are depicted from the side, showing the alignment of the rows of molecules.
temperature.12,13 For easier understanding, we shall discuss the higher symmetry β-phase first and subsequently compare both phases. The β-phase crystal structure of DIP is shown in the right part of Figure 2. The individual DIP molecules pack in a standard 2-dimensional herringbone-type layered structure, with pseudohexagonal packing within the layers. The herringbone angle is 53.25°, similar to the value observed for pentacene.15-18 The molecular planes are almost perpendicular to the crystal (a,b) plane, with tilt angles of 7.5°. The 2-dimensional herringbone layers are plane and are very similar to the oligoacenes tetracene and pentacene, a notable exception being a slight (1.4°) misalignment of the molecular axes. Due to the higher space group symmetry with inversion center in the middle of the DIP molecule, the molecule is now flat in the β-phase with a maximum deviation of 0.23 Å from a least-squares plane. However, the larger thermal parameters (as compared to the R-phase) may hide some small residual distortion. The phase transition of DIP from the high-temperature β- to the low-temperature R-phase involves a doubling of the unit cell volume and differentiates two symmetrically identical molecules into a bent and a twisted DIP conformational isomer (conformers; Figure 2 and 3). The a- and b-lattice parameters of the R-phase are related to the [1h,1h,0] and [1,1h,0] diagonal directions of the β-phase. The herringbone angle is increased to 55.65°, and the molecular planes are more strongly tilted against the (a,b) plane, with angles of 20.4° and 32.0°. Due to the lower, triclinic, space-group symmetry in the R-phase, the molecules themselves are no longer located on a center of symmetry, and two differently distorted molecules, one bent, the other twisted, are observed. The bent DIP molecule tilts one terminal phenyl ring by about 14° out of the plane defined by the rest of the molecule. In contrast, the twist angle of the terminal indenogroup of the second DIP isomer is about 4.6° (see Figure 3). Within a herringbone-packed layer, the two DIP molecules form centrosymmetric quadruples including two twisted and two
J. Phys. Chem. C, Vol. 111, No. 51, 2007 18879 bent isomers each. In this quadruple, the molecules are slightly misaligned by about 0.7 Å. As a result, the bending of the bent isomer around its twisted counterpart, rather than being uniform, is restricted to the more exposed end of the molecule. The molecular packing is easily described in terms of these quadruples by comparing it to a staggered packing of lego blocks. This packing, shown in Figure 2, produces layers with steps. Upon stacking of these corrugated layers, the steps interlock. In terms of the studied crystal structure, the DIP quadruples are misaligned by approximately one aromatic ring (2.8 Å), allowing a slight interdigitation of adjacent layers. Structures composed of two or more symmetrically independent molecules are not uncommon, one example being the lowtemperature structure of pentacene.9 Typically though, the conformational differences of the molecules within such structures appear to be very small: In the case of pentacene, the program PLATON19 was used to calculate a maximum distance of 0.011 Å between homologous atoms. The substantial difference found in DIP however is very rare. The closest example to date is that of terpyrydine, a tridentate ligand with interesting photochemical properties, which also crystallizes in a herringbone-packed, monoclinic structure with two conformationally distinct molecules.20 For terpyridine, PLATON yields a difference of 0.16 Å, which is somewhat smaller than the 0.407 Å found in DIP. Another example is 1,4-diketo-3,6-diphenylpyrrolo-[3,4-c]-pyrrole,21 a pigment precursor, where the R-form contains a unit cell with two conformers present simultaneously. It is to be noted that both these cases involve deformation of single bonds rather than deformation of aromatic rings as is the case in DIP. The complete phase transformation in DIP from the room temperature, triclinic, R-phase to the high-temperature, monoclinic, β-phase occurs at approximately 403 K. Although the increase in the molecular volume is only about 2.6%, the transformation involves strong shearing (around 40%) of the crystal structure, frequently leading to cracking of the crystals along the (a,b) plane. The single crystalline nature of the β-phase crystal obtained from the transformation of a single crystal of R-phase material suggests an epitactic growth mechanism.22 Also, the entire transformation can be broken up into three hierarchic subtransformations, which would proceed simultaneously through the transformation. The first subtransformation takes place on the level of the unit cell and smoothes/flattens the corrugation of the herringbone planes through movement of the molecules along their long molecular axes as shown in Figure 2. In addition, the distortions of the molecules are lifted. The second transformation involves the same direction of movement but shears a whole herringbone plane. The third transformation on the level of many herringbone planes is a shearing realignment of these planes with respect to each other. These sub-transformations establish the structural analogy between the two phases, which is notably a prerequisite for epitactic growth.22 Differential scanning calorimetry (DSC) measurements were performed on both crystalline DIP flakes as well as powders and consistently yield a latent heat ∆H of 1.0 ( 0.2 kJ mol-1. As can be seen in Figure 4, the DSC spectra showed many individual peaks, corresponding to different parts of the sample undergoing the phase transition, with the lowest transition temperature being 403 K. We adjusted an entropy contribution of 2.4 ( 0.5 J/(mol K) by referring the latent heat to this transition temperature. According to Herbstein,22 the necessity of nucleation frequently leads to large hysteresis of solid-solid transformations,
18880 J. Phys. Chem. C, Vol. 111, No. 51, 2007
Heinrich et al.
Figure 3. Comparison of the two molecular isomorphs at 110, 298, and at 423 K; the thermal ellipsoids are drawn at the 50% probability level.
Figure 4. Differential scanning calorimetry spectra measured on DIP powder (mass 2.71 mg) and flakes (mass 2.17 mg) respectively.
especially at high crystalline perfection. We assume these to be the cause of the different transition temperatures measured in small-grained DSC samples and in those used for diffraction experiments and also accounts for the “spikes” visible in the DSC. These are assumed to represent the (nucleation-dependent) transformation temperature of a single grains. The triclinic organic semiconductor DIP shows an enantiotropic phase transition at 403 K to a higher symmetry monoclinic structure by an epitactic mechanism. The thermodynamic data indicates a phase transformation of first order. Due to the necessity of nucleation, small grain samples show a large hysteresis, whereas the transition temperature of single crystals, although well defined, is dependent on sample history. In terms of molecular arrangement, the transition can be described by three simultaneous molecular movements: (1) bending and twisting distortions of the DIP molecules present in the R-phase, disappear in the β-phase, where the DIP molecules are flat, (2)
the whole molecules of the 2-dimensional herringbone layers swivel to a more upright orientation, leading to smoother surfaces of the herringbone layers and thus loss of the interdigitation between the layers, and (3) the layers rearrange by shearing against each other. This solid-state phase transformation produces large stress/strain, inducing cracks in the crystal. Multiple passes through the phase transformation result in an oriented polycrystalline sample. The phase transformation imposes limitations on the use of DIP single crystals in organic semiconductor devices, since crack formation at temperatures above 375 K will severely affect the electronic transport properties. Luckily, these problems can be avoided by using thin films, in which the β-phase can be stabilized at room temperature.12 Such films have shown excellent crystallinity and electronic properties,12,13 holding great promise for DIP as a semiconductor material. Furthermore, ambipolar devices are feasible, with good charge carrier mobility for both holes and electrons.10 In conclusion, although the metastability of the β-phase of DIP in thin films requires some caution, our findings confirm this system as an excellent candidate for organic semiconductor devices. Acknowledgment. The authors acknowledge the help of S. Hirschmann for the calorimetric measurements as well as financial support by the Deutsche Forschungsgemeinschaft (Project PF385/2). Work at Columbia University and Bell Laboratories was supported in part by the Department of Energy, Office of Science, under Contract DE-FG02-04ER46118. Supporting Information Available: Single crystals of diindenoperylene (DIP) were grown by physical vapor-phase transport under flowing H2 and forming gas, yielding crystals with similar electronic and structural properties. The crystals
TABLE 1: Unit Cell Parameters at Temperatures between 110 and 423 K T [K]
a [Å]
b [Å]
c [Å]
R [°]
β [°]
γ [°]
V [Å3]
110 200 298 363 383 403 423
11.4616(6) 11.5164(6) 11.5848(6) 11.6284(8) 11.6432(8) 11.6592(8) 7.1709(8)
12.8653(7) 12.9066(7) 12.9624(7) 12.9916(9) 13.0004(9) 13.0102(9) 8.5496(9)
14.7649(7) 14.8197(7) 14.8847(7) 14.9311(9) 14.948(1) 14.966(1) 16.798(2)
97.696(4) 97.872(4) 98.136(4) 98.294(5) 98.360(6) 98.440(6) 90.0
98.263(4) 98.190(4) 98.089(4) 98.050(5) 98.042(6) 98.023(6) 92.416(11)
114.376(5) 114.443(5) 114.531(5) 114.543(7) 114.545(7) 114.548(7) 90.0
1916.9(2) 1931.2(2) 1963.0(2) 1979.7(3) 1985.2(3) 1991.3(3) 1028.9(2)
Enantiotropic Polymorphism in DIP are formed at temperatures above the phase transition, but since the phase transition phenomenon is reversible, crystals of sufficient size for the X-ray analysis survive the cooling to room temperature. Single crystals of DIP grown from solution in 1,2,4trichlorobenzene at a temperature of ∼380 K displayed the identical structure. The crystal structure was fully characterized at temperatures of 110, 200, 298, 363, 383, 403, and 423 K, using a conventional CCD equipped diffractometer (Oxford Diffraction Xcalibur2, Mo KR, with Cryojet temperature control system). The high-temperature monoclinic phase was characterized structurally at 423 K. Unfortunately, the large strain accompanying the phase transformation produced cracking of the samples, often leading to the loss of the crystal being measured. In addition, irreversible degradation of the samples at 430 K severely hampered the data collection, with the crystal no longer present after about 5 h. CCDC 642476 to 642482 contain the supplementary crystallographic data for this paper. To further estimate the thermodynamic quantities characterizing the phase transition, differential scanning calorimetry (DSC) measurements were carried out on several samples using a Perkin-Elmer DSC-2 setup. The temperature range from 343 to 453 K (thus containing the expected transition region) was scanned at a ramping speed of 2.5 K/min and a time resolution of 0.05 s. Latent heat was measured by integrating over all peaks and scaled to an indium standard. Entropies were calculated with respect to the lowest measured phase transition peak of 403 K. Thermal effects caused by the sample stage were accounted for by a background correction using a polynomial fit. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Karl, N. Mol. Cryst. Liq. Cryst. 1989, 171, 157.
J. Phys. Chem. C, Vol. 111, No. 51, 2007 18881 (2) Lang, D. V.; Chi, X.; Siegrist, T.; Sergent, A. M.; Ramirez, A. P. Phys. ReV. Lett. 2004, 93, 086802. (3) Warta, W.; Stehle, R.; Karl, N. Appl. Phys. A 1985, 36, 163-170. (4) Goldmann, C.; Haas, S.; Krellner, C.; Pernstich, K. P.; Gundlach, D. J.; Batlogg, B. J. Appl. Phys. 2004, 96, 2080-2086. (5) Dimitrakopoulos, C. D.; Brown, A. R.; Pomp, A. J. Appl. Phys. 1996, 80, 2501-2511. (6) Pflaum, J.; Niemax, J.; Tripathi, A. K. Chem. Phys. 2006, 325, 152-149. (7) Laudise, R. A.; Kloc, C.; Simkins, P.; Siegrist, T. J. Cryst. Growth 1998, 187, 449-454. (8) Tripathi, A. K.; Heinrich, M.; Siegrist, T.; Pflaum, J. AdV. Mater. 2007, 19, 2097-2101. (9) Siegrist, T.; Besnard, C.; Haas, S.; Schiltz, M.; Pattison, P.; Chernyshov, D.; Batlogg, B.; Kloc, C. AdV. Mater. 2006, 19, 2079-2082. (10) Tripathi, A. K.; Pflaum, J. Appl. Phys. Lett. 2006, 89, 082103. (11) Clar, E. The Aromatic Sextet; J. Wiley: London, 1972. (12) Du¨rr, A. C.; Schreiber, F.; Mu¨nch, M.; Karl, N.; Krause, B.; Kruppa, V.; Dosch, H. Appl. Phys. Lett. 2002, 81, 2276-2278. (13) Mu¨nch, M. Fakulta¨t Mathematik und Physik; University of Stuttgart: Stuttgart, 2001. (14) Karl, N. Synth. Met. 2003, 133, 649-657. (15) Holmes, D.; Kumaraswamy, S.; Matzger, A. J.; Vollhardt, K. P. C. Chem. Europ. J. 1999, 5, 3399-3412. (16) Mattheus, C. C.; Dros, A. B.; Baas, J.; Oostergetel, G. T.; Meetsma, A.; de Boer, J. L.; Palstra, T. T. M. Synth. Met. 2003, 138, 475-481. (17) Mattheus, C. C.; Dros, A. B.; Baas, J.; Meetsma, A.; de Boer, J. L.; Palstra, T. T. M. Acta Crystallogr. 2001, C57, 939-941. (18) Siegrist, T.; Kloc, C.; Scho¨n, H. J.; Batlogg, B.; Haddon, R. C.; Berg, S.; Thomas, G. A. Angew. Chem. 2001, 40, 1732-1736. (19) Spek, A. L. J. Appl. Cryst. 2003, 36, 7-13. (20) Bowes, K. F.; Clark, I. P.; Cole, J. M.; Gourlay, M.; Griffin, A. M. E.; Mahon, M. F.; Ooi, L.; Parker, A. W.; Raithby, P. R.; Sparkes, H. A.; Towrie, M. Cryst. Eng. Commun. 2005, 7, 269-275. (21) Mizuguchi, J. J. Imaging Sci. Technol. 2005, 49, 35-40. (22) Herbstein, F. A. Acta Crystallogr. 2006, B62, 341-383.
