Polymorphism in Bulk and Thin Films: The Curious Case of

Article pubs.acs.org/JPCC

Polymorphism in Bulk and Thin Films: The Curious Case of Dithiophene-DPP(Boc)-Dithiophene Shabi Thankaraj Salammal,† Jean-Yves Balandier,† Jean-Baptiste Arlin,† Yoann Olivier,‡ Vincent Lemaur,‡ Linjun Wang,‡ David Beljonne,‡ Jérôme Cornil,‡ Alan Robert Kennedy,§ Yves Henri Geerts,† and Basab Chattopadhyay*,† †

Université Libre de Bruxelles (ULB), Faculté des Sciences, Laboratoire Chimie des Polymères, CP 206/1, Boulevard du Triomphe, 1050 Bruxelles, Belgique ‡ Université de Mons (UMons), Service de Chimie des Matériaux Nouveaux, Place du Parc 20, 7000 Mons, Belgium § Department of Pure and Applied Chemistry, WestCHEM, University of Strathclyde, 295 Cathedral Street, Glasgow UK G1 1XL, United Kingdom S Supporting Information *

ABSTRACT: Polymorphism is an interesting phenomenon critical to our understanding of structure−property relationships in solid-state functional materials. We report the synthesis and structural characterization of two polymorphic forms of a DPP-Boc derivative, DPP4T-α and DPP4T-β, as well as the study of their optical properties. Thin film studies have been carried out to identify the specific polymorphic form that exists in contact with the substrate and also to obtain a better understanding of the interface morphology with respect to crystal packing. The two polymorphs, DPP4T-α and DPP4T-β, have been structurally characterized using single crystal/powder X-ray diffraction data with a detailed analysis of Hirshfeld surfaces and fingerprint plots facilitating a comparison of the type and nature of intermolecular interactions in the supramolecular architectures. DPP4T-α crystallizing in a space group P21/c with Z′ = 0.5 interlinked via C−H···O/π and π···π interactions forms 2D herringbone sheets. In the polymorph DPP4T-β with space group P1 and Z′ = 1, the crystal packing is stabilized by CH···π and π···π interactions forming a columnar network. From considerations of density and from lattice energy calculations, it can be concluded that the αform is more stable in bulk. In thin films the β-form was found to be more stable. This work gives a unique example where the polymorphism could be identified and separated both in the bulk and in thin films. This study on the structural effects of polymorphism is useful for further development of DPP-based materials and also for targeted design of other functional materials.

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INTRODUCTION Polymorphism, a phenomenon critical to our understanding of crystal nucleation and growth through synthon evolution and to structure−property correlation, has been a subject of intense research in recent decades.1 This can be partially attributed to the serious implications that polymorphism has on industrial research and development.1e,f,2 Specifically, the identification and characterization of different polymorphic forms of a drug material is now an essential element in the pharmaceutical sector.2b,3 Given the importance of polymorphism in determining the property of materials, it is interesting to study how it affects the performance of functional materials in the field of organic electronics. In recent years, this has been addressed © 2013 American Chemical Society

with research being carried out to identify polymorphic phases of thin films, where the polymorphism exists near the substrate.4 Pentacene, perhaps the most widely studied organic semiconductor, is known to crystallize in several polymorphic forms.5 Polymorphisms in other materials such as rubrene, sexithiophene, and anthradithiophene have also been reported.6 The work of Jurchescu and coworkers,7 where they report the existence of two temperature-dependent polymorphs of fluorinated 5,11-bis triethylsilylethynyl anthradithiophene and Received: November 3, 2013 Revised: December 11, 2013 Published: December 12, 2013 657

dx.doi.org/10.1021/jp410824u | J. Phys. Chem. C 2014, 118, 657−669

The Journal of Physical Chemistry C

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how this affects the performance of field-effect transistors, is worth mentioning. The presence of several polymorphic forms facilitates the study of charge-transport behavior with respect to crystal packing. However, it may hinder the reproducibility, reliability, and stability of the devices fabricated using such materials. Where several crystalline forms exist, it is of utmost importance to identify the phases and to control the formation of the different forms. Complete structural characterization in such cases is essential to understand the structure−property relations. 3,6-Diaryl-2,5-dihydro-1,4-diketopyrrolo[3,4-c]pyrroles or DPPbased materials are known for their wide range of applications, ranging from high-performance pigments to use as functional materials in organic light-emitting diodes, field-effect transistors, and solar cells.8 Their extensive use can be attributed to several qualities including ease of synthesis and derivatization, chemical and thermal stability, broad optical absorption, and high charge carrier mobility. The electron-deficient DPP core is often used to synthesize low band gap materials suitable for optoelectronic applications by flanking the core with electron-rich aromatic rings such as thiophene, phenyl, or thienothiophene.9 Considering the wide range of solid-state applications of DPP-based materials, the presence/effect of polymorphism is thus a matter of utmost interest. The study of the structural effects of polymorphism on the basic properties is useful not only for further development of DPP-based materials but also for targeted design of other functional materials. As part of our ongoing recent research activity involving thiophene-based semiconducting materials,8b,10 we synthesized a DPP-Boc (Boc = t-butoxycarbonyl) with a dithiophene backbone. The nitrogen atoms of the lactame groups of the DPP core were substituted with Boc groups to improve the solubility of the DPP-based material. Boc moieties are preferred to alkyl chains to prevent the O-functionalization of the DPP core, as was previously reported for dialkylated DPP derivatives.8b,11 All examples previously mentioned have studied polymorphism in either the bulk or in thin films at the vicinity of the substrate. A search of the Cambridge Structural Database (version 5.34, CSD 2013 release)12 for DPP derivatives yielded only 54 results (excluding duplicate structures). If we restrict our search to cases where the NH− group has a Boc substitution, there are just five hits, out of which three are polymorphs of the same material.13 In all cases found, there is a phenyl ring substitution at the DPP core, but there are no reports of structural data for

DPP-based compounds with thiophene ring substitutions. This can be attributed to the fact that growing diffraction quality single crystals is not always possible for such compounds. In such cases, structural analysis can be accomplished via X-ray powder diffraction data,14 as was the case for phenylDPP(Boc)-phenyl,15 for which three different polymorphic forms13,15,16 are known. We report the synthesis of a DPP-Boc derivative, structural characterization of the polymorphic forms in the bulk and thin films, and the study of their optical properties. Serendipitously, we discovered that the as-synthesized compound could be separated into two polymorphic forms depending on the solvent used for recrystallization. Thin film studies have been carried out to identify the specific polymorphic form that exists near the substrate and to obtain a better understanding of the morphology with respect to crystal packing. To the best of our knowledge, this is the first work where the structural and spectroscopic signatures of both polymorphic forms have been identified in bulk and thin films.

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EXPERIMENTAL SECTION Synthesis. All chemicals and anhydrous solvents were purchased from Acros and Aldrich and were used without further purification unless otherwise stated. Thin layer chromatography (TLC): SiO2 Silica gel 60F254 on aluminum sheet (Merck). Column chromatography: Silica gel 60 (particle size 0.063 to 0.200 mm, Merck). 1H NMR (300 MHz) and 13C NMR (75 MHz) were recorded on Bruker Advance 300 spectrometer. Chemical shifts are given in ppm and coupling constants J are given in hertz. The residual signal of the solvent was taken as internal reference standard. MALDI-ToF measurements were made on a Waters QToF Premier apparatus. Di-tert-butyl3,6-di([2,2′-bithiophen]-5-yl)-1,4-dioxopyrrolo[3,4-c]pyrrole-2,5-(1H,4H)-dicarboxylate (DPP4T-5) was synthesized as depicted in Scheme 1. The intermediates 3,6-di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (2), di-tert-butyl 1,4-dioxo-3,6-di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-2,5(1H,4H)-dicarboxylate (3), and di-tert-butyl 3,6bis(5-bromothiophen-2-yl)-1,4-dioxopyrrolo[3,4-c]pyrrole-2,5(1H,4H)-dicarboxylate (4) were prepared according to procedures described in the literature.16 A mixture of 4 (2.31 g, 3.50 mmol), 2-tributylstannylthiophene (3.92 g, 8.94 mL, 10.50 mmol), and Pd(PPh3)4 (0.81 g, 0.70 mmol) was refluxed under an argon atmosphere for 3.5 h. After cooling to room

Scheme 1. Synthesis of 5 (DPP4T-5)a

a

Reagents and conditions: (i) diethyl succinate, tert-amyl alcoholate (1M), tert-amyl alcohol, reflux 2 h, Ar atmosphere, 72%; (ii) Boc2O, DMAP, THF, r.t., 24 h, 68%; (iii) NBS, CHCl3, r.t., 48 h, light exclusion, Ar atmosphere, 66%; (iv) 2-tributylstannylthiophene, Pd(PPh3)4, toluene, reflux, 3.5 h, Ar atmosphere, 55%. 658



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temperature (RT), the reaction was filtered on Celite and washed several times with chloroform (CHCl3). The filtrate was evaporated under reduced pressure. The solid residue was then purified by column chromatography on silica gel (CHCl3 then CHCl3 + 2% of AcOEt) to give pure 5 (1.27 g, 1.91 mmol) as a green solid in 55% yield. 1H NMR (300 MHz, CD2Cl2) δ 8.22 (d, J = 4.1 Hz, 2H, H3), 7.37 (m, 4H, H3′, H5′), 7.30 (d, J = 4.1 Hz, 2H, H4), 7.10 (dd, J = 5.1, 3.7 Hz, 2H, H4′), 1.62 (s, 18H, CH3). 13C NMR (75 MHz, CD2Cl2) δ 159.50, 149.4, 144.5, 137.2, 136.62, 135.57, 128.9, 128.7, 127.3, 126.2, 125.0, 111.0, 86.5, 28.1. Rf (CHCl3/AcOEt 98/2 v/v): 0.63. C32H28N2O6S4: MALDI-HRMS: (M+.): calc, 664.0830; found, 664.0856 (Figures S1−S3 in the Supporting Information). Thin Film Preparation. All films were prepared using Spin Coater, model P6700 Series from Specialty Coating Systems. In all cases, for spin coating, a volume of 170 μL was used with the same speed sequence: 60 s at 1000 rpm. Thin films were fabricated on glass substrates (Marienfeld cover glasses cat. no. 0101040, around 160 μm thick). The solvents used are choloroform (CHCl3) and tetrahydrofuran (THF). Films used in this study were prepared by spin-coating the solution with concentration of 10 mg/4 mL for CHCl3 and 10 mg/2 mL for THF. All substrates were sonicated for 15 min in dilute liquid detergent (RBS T 105), isopropyl alcohol, and acetone, then blown dry with nitrogen and were cleaned using a UV/ozone cleaner (BioForce Nanosciences, Ames, IA) for 20 min. Polarized Optical Microscopy. Single crystals of DPP4T-α and thin films of both of the polymorphs were studied with a Nikon Eclipse E200 microscope equipped with a digital Nikon DS-Fi1 camera and polarizing filters. The images have been treated with the NIS-Elements software (version 3.0). UV−vis Spectroscopy. Absorption spectra of both polymorphs in solutions and thin films were recorded from the range 200−900 nm under ambient conditions using an Agilent 8453 spectrometer. Atomic Force Microscopy. AFM was performed using Digital Instruments Nanoscope III microscope operated in tapping mode (TM). All TM-AFM images were recorded under ambient condition with a conventional silicon tip (silicon nitride). The images were recorded with a sampling resolution of 256 × 256 data points. X-ray Powder Diffraction. X-ray powder diffraction data for crystalline powder samples of DPP4T-5, DPP4T-α, and DPP4T-β were collected on a Bruker D8 Advance powder diffractometer using CuKα radiation (λ = 1.5418 Å). The diffraction patterns were recorded at 293(2) K with a step size of 0.02° (2θ) and counting time of 60 s/step in the angular range 4.0−50.0° (2θ) using the Bragg−Brentano geometry. The indexing of the powder patterns of DPP4T-α and DPP4T-β using the program DICVOL06 17 showed a monoclinic unit cell with a = 11.274(6) Å, b = 5.753(3) Å, c = 24.144(13) Å, and β = 101.08 (3)° for DPP4T-α and a triclinic unit cell with a = 11.84(1) Å, b = 10.18(1) Å, c = 6.678(1) Å, α = 77.65(9), β = 95.79(9), and γ = 92.59(9)° for DPP4T-β. The most probable space-group for DPP4T-α was identified as P21/c with two molecules in the unit cell (Z′ = 0.5), while for DPP4T-β the most obvious choice is P-1 (Z′ = 0.5). However, efforts to solve the structure using spacegroup P-1 were unsuccessful and so space-group P1 with one molecule in the asymmetric unit (Z′= 1) was used for structure solution. The crystal structures were solved by global optimization of the structural models in direct space using the simulated

annealing technique, as implemented in the program DASH.18 Rietveld refinement19 was carried out using the program GSAS20 with an EXPGUI interface.21 The background was described by the shifted Chebyshev function of the first kind with 36 points regularly distributed over the entire 2θ range. The lattice parameters, background coefficients, and profile parameters were refined initially, followed by refinement of the positional coordinates of non-hydrogen atoms with soft constraints on the bond lengths and bond angles and a planar restraint for the DPP-core and thiophene rings. A fixed isotropic displacement parameter of 0.04 Å2 for all non-hydrogen atoms was maintained. Hydrogen atoms were placed in the calculated positions with a common Biso value of 0.06 Å2. In the final stages of refinement, a preferred orientation correction using the generalized spherical harmonic model was applied. The final Rietveld refinement converged to Rp = 0.0370 and Rwp = 0.0491 for DPP4T-α and Rp = 0.0365 and Rwp = 0.0507 for DPP4T-β, with an excellent agreement between the observed and the calculated powder patterns (Figure S4 in Supporting Information and Figure 1, respectively). For DPP4T-α, it was possible to obtain diffraction quality single crystals, and the structure could be solved from single-crystal diffraction data. Accordingly, all structural discussions and comparisons are carried out using the single-crystal diffraction data of DPP4T-α. Specular X-ray diffraction patterns for the thin films of both polymorphs were recorded with a Bruker D8 diffractometer at 293(2)K with a step size of 0.02° (2θ) and counting time of 60 s/step in the angular range 4.0−50.0° (2θ). The X-ray rocking curve of films treated by solvent vapor annealing (SVA) was recorded by rocking the detector and the incoming beam for ±0.5° after fixing both the detector and the incoming beam at the Bragg angle (3.76°). The in-plane X-ray diffraction pattern of the SVA-film was recorded using a Rigaku Ultima IV diffractometer under grazing incidence geometry by fixing the incident angle of the beam at 0.22°. Single-Crystal X-ray Diffraction. For DPP4T-α, data collection was carried out with Oxford diffraction Xcalibur E diffractometer using MoKα radiation (λ = 0.71073 Å). The crystal structure was solved by direct methods using SHELXS22 and refined by full matrix least-squares methods based on F2 using SHELXL97.22 The displacement parameters of all non-H-atoms were treated anisotropically. H atoms were placed at calculated positions using suitable riding models with fixed isotropic thermal parameters [Uiso(H) = 1.2Ueqv(C) for CH groups and Uiso(H) = 1.5Ueqv(C) for CH3]. Crystal data for DPP4T-α are summarized in Table 1. Quantitative Phase Analysis. Rietveld refinement-based quantitative phase analyses (QPAs) of the powder diffraction data of DPP4T-5 was done using GSAS20 with the EXPGUI21 interface. The crystal structures of DPP4T-α and DPP4T-β were used as input structural models. Final global optimized parameters were: background coefficients, zero-shift error, cell parameters, and peak-shape parameters using a pseudo-Voigt function corrected for axial divergence.23 The final Rietveld refinement pattern is given in Figure 2. Hirshfeld Surface Analysis. Hirshfeld surfaces24 and the associated fingerprint plots25 were calculated using CrystalExplorer,26 which accepts a structure input file in the CIF format. Bond lengths to hydrogen atoms were set to typical neutron values (C−H = 1.083 Å). For each point on the Hirshfeld isosurface, two distances de, the distance from the point to the nearest nucleus external to the surface, and di, the distance to the nearest nucleus internal to the surface, are 659



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Figure 1. Final Rietveld refinement plot of DPP4T-β.

Table 1. Crystal Data and Structure Refinement Parameters for DPP4T-α and DPP4T-β DPP4T-α single crystal

powder C32H28N2O6S4 664.80

chemical formula Mr temperature (K) crystal system, space group Z a, b, c (Å) α, β, γ (deg) volume (Å3) D (mg/m3) wavelength (Å) crystal size (mm) diffractometer no. of measured, independent, and observed reflections Rint, θmax data/restraints/parameters/ Rp/R1 Rwp/wR(I) χ2/S

123(2) monoclinic, P21/c 2 11.1947(6), 5.6858(3), 23.9486(16) 90, 101.642(6), 90 1492.99 (15) 1.479 0.71073 0.35 × 0.03 × 0.02 Oxford Diffraction Xcalibur 6167, 2912, 1821 0.0673, 26.00 2912/0/202 0.0592 0.1001 1.024

defined. The normalized contact distance (dnorm) based on de and di is given by dnorm =

(d i − rivdW ) rivdW

DPP4T-β powder

+

293(2) monoclinic, P21/c 2 11.2908(7), 5.7526(3), 24.1276(16) 90, 101.071(4), 90 1537.9(2) 1.436 1.5418

293(2) triclinic, P1 1 11.864 (10), 10.197 (8), 6.677 (5) 77.785(5), 95.666(6), 92.402 (4) 785.4 (19) 1.406 1.5418

Bruker D8 Advance

Bruker D8 Advance

2251, 85, 123 0.0370 0.0491 4.752

2399/196/169 0.0365 0.0507 4.707

We have carried out single-point periodic boundary conditions (PBC) calculations using the B97D functional,28 which specifically includes dispersion corrections to the total energy. Because the stability of one polymorph with respect to the other is governed by the amplitude of the intermolecular interactions, we have extracted, by subtracting from the total energy of the cell the internal energy of the isolated molecules, the lattice energy mainly dominated by van der Waals interactions. In addition, lattice energy calculations have been performed at the molecular mechanics level using the Materials Studio 6.0 package.29 Specifically, we have adopted the Compass force field that was previously used for thiophene-based materials.30 Force-field methods are particularly suited because the largest contributions to the lattice energy of both polymorphs arise

(de − revdW ) revdW

where rivdW and revdW are the van der Waals radii of the atoms. The value of dnorm is negative or positive depending if the intermolecular contacts are shorter or longer than the van der Waals separations. The parameter dnorm displays a surface with a red−white−blue color scheme, where bright red spots highlight shorter contacts, white areas represent contacts around the van der Waals separation, and blue regions are devoid of close contacts. Theoretical Calculations. All quantum-chemical calculations have been performed with the Gaussian 09/A02 package.27 660



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Figure 2. Final Rietveld refinement plot of QPA of PXRD data of DPP4T-5. The crystal structures of DPP4T-α and DPP4T-β were used as input structural model.

that of the DPP4T-α or DPP4T-β (Figure S5 in the Supporting Information) reveals that the product DPP4T-5 could be a mixture of several phases, a fact that rationalizes our inability to index the powder pattern of DPP4T-5. The crystalline powder of DPP4T-α could be isolated from DPP4T-5 using liquid/ liquid diffusion technique using chloroform (CHCl3) and methanol (CH3OH). Needle-shaped single crystals of DPP4Tα were obtained using the same methodology (Figure S6 in the Supporting Information), whereas the other polymorphic form DPP4T-β could only be obtained in powder form if DPP4T-5 is recrystallized in acetonitrile solution. DPP4T-α crystallizes in monoclinic space group P21/c with half of the molecule in each asymmetric unit, that is, Z′ = 0.5, while DPP4T-β crystallizes in a triclinic unit cell (P1) with one molecule in the asymmetric unit. Molecular views of the two polymorphs are given in Figure 3. Apart from the difference in the crystal systems, DPP4T-α and DPP4T-β also differ in their molecular conformation. The thiophene-DPP-thiophene moiety is essentially planar in both of the polymorphs. DPP4T-α and DPP4T-β differ significantly with respect to the terminal thiophene rings and the Boc- group. In DPP4T-β, the terminal thiophene rings are planar with respect to the thiophene-DPPthiophene moiety, with the torsions S1−C7−C8−S2 and S3− C23−C24−S4 being −180.0(3) and −179.7(4)°, respectively, whereas in DPP4T-α the terminal thiophene ring is twisted with the torsion S1−C7−C8−S2 being 147.6(2)°. The Boc groups attached to the N atom of the heterocyclic ring are twisted in with respect to the heterocyclic system by 72.7(4)° (C1−N1−C12−O2) for DPP4T-α and by −62.9(6)° (C1− N1−C12−O2) and 41.9(6)° (C17−N2−C28−O5) for DPP4T-β. The torsion angles defining the conformations of the Boc groups in DPP4T-α and DPP4T-β are listed in Table S1 in Supporting Information and again indicate significant differences in this conformational aspect. For DPP-based molecules, similar conformational polymorphs have been observed in the case of 3,6-di-2-pyridylpyrrolo[3,4-c]pyrrole1,4(2H,5H)-dione,34 where the asymmetric unit contains multiple molecules each exhibiting a different conformation. Unlike the

from van der Waals interactions, which are usually welldescribed within this approach. The structures considered for these calculations correspond to the experimentally resolved unit cell of DPP4T-α and a supercell made of two unit cells along the crystallographic c direction for DPP4T-β to compare lattice energies evaluated for the smallest structures of both polymorphs with an equivalent number of molecules. The optical absorption spectrum of large DPP4T-α and DPP4T-β aggregates was simulated by solving a Holstein model including coupling to one quantum vibrational mode (in the so-called one-particle approximation, which has been shown to correctly reproduce the absorption spectra of multichromophoric systems in the intermediate regime coupling regime).31,9b The dimensionless electron−phonon coupling λ2 was set to 0.9, leading to a polaronic relaxation energy λ2hνvib of ∼0.16 eV for an effective, stretching-like, vibrational mode at hνvib = 0.18 eV. The excitonic couplings between molecules i and j in the aggregates have been computed on the basis of the electronic transition densities for the lowest optical transition in the isolated molecules (as calculated at the INDO/CCSD level32 with a 10 × 10 CI active space): Vij =

1 4πε0

∑ ∑ qi(k)qj(l)F(rkl) k

l

where qi(k) represents the transition density on atom k for the lowest electronic excitation on molecule i and F(rkl) is the electron−electron interaction potential, namely, here the Mataga−Nishimoto potential.33 The vibronic transitions obtained by solving the Holstein Hamiltonian (in the one-particle approximation and with two vibration quanta per molecule) were convoluted with Gaussian line-shape functions with standard deviation 0.08 eV and applying a gas-to-crystal shift D to allow for direct comparison with the experiment.

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RESULTS AND DISCUSSION Crystal Structure and Hirshfeld Surface Analysis. A comparison of the observed powder profile of DPP4T-5 with 661



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Figure 3. Molecular view of DPP4T-α and DPP4T-β. In DPP4T-α; the atoms marked with # are generated by symmetry.

Figure 4. (a) Perspective view of the 3D- crystal packing in DPP4T-α as viewed along the crystallographic b axis. Intermolecular interactions are shown with dotted lines. (b) Formation of 2D-herringbone sheet in DPP4T-α by a combination of C−H···π hydrogen bonds (shown in dark blue) and π···π interactions (shown in light blue). H atoms involved in H bonds are only shown for clarity.

Further reinforcement along the [010] direction is provided by several π···π interactions resulting in a columnar assembly (Figure 4). Interconnection between such columns is provided via C14−H(C14) ···π hydrogen bonds to form 2D herringbone sheets (Figure 4b). The S2···O1 interaction35 connects the 2D sheets to form a 3D supramolecular assembly (Figure 4a). In DPP4T-β, a columnar stack of the planar thiophene-DPP backbone is formed along the [001] direction via π···π interactions (Table 3). Each molecule is not exactly parallel to its neighbor in the column. The offset between the molecules is such that it facilitates interaction between the terminal thiophene rings with the rings adjacent to the DPP core. The π-stacked columns can be visualized as molecular steps, as shown in Figure 5b, with each molecule shown in a different color with respect to its neighbor. Adjacent columns are interconnected to each other by C16−H(C16)···π hydrogen bonds (shown in dark blue in Figure 5b) to form a 2D network in the crystallographic bc plane (Figure 5).

case presented herein, none of the molecules reported were planar, with the angle between the pyridyl rings and the DPP heterocycle being 1.2/10.9°. In both the α and β forms, the DPP4T molecules form columns (parallel to the b axis for DPP4T-α and the c axis for DPP4T-β), as shown in Figures 4 and 5. The major difference between the structures of α and β phases concerns the relative packing of these molecular columns. In DPP4T-α, the molecules in adjacent columns are packed relative to each other in a herringbone arrangement (Figure 4b), whereas in DPP4T-β, the molecules in adjacent columns are packed in a parallel arrangement (Figure 5b). Although there are some similarities in the packing of molecules along a given column in α and β phases, the details differ significantly. The crystal packing exhibits intermolecular C−H···O and C−H···π hydrogen bonds and S···O and π···π interactions (Tables 2 and 3). In DPP4T-α, the C9−H9···O3 and C15−H15C···O1 hydrogen bonds connect the molecules along the [010] direction forming fused R22(17) rings. 662



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Table 3. Intermolecular π···π Interactions in the Crystal Structures of DPP4T-α and DPP4T-βa Cg···Cgb

d[Cg···Cg] Å

symmetry

Cg1−Cg3 Cg1−Cg3 Cg1−Cg4 Cg1−Cg4

4.228(2) 3.732(2) 4.228(2) 3.732(2)

2 − x, −y, 1 − z x, y − 1, z 2 − x, −y, 1 − z x, y − 1, z

Cg1−Cg2 Cg1−Cg3 Cg3−Cg4

4.030(4) 4.685(4) 3.954(4)

x, y, z + 1 x, y, z − 1 x, y, z − 1

DPP4T-α

DPP4T-β

a Only relevant π···π interactions are mentioned. bCg1, Cg3, and Cg4 are the centroids of the rings S1−C4−C5−C6−C7, N1−C1−C2− C2#−C3#, and N1#−C1#−C2#−C2−C3, respectively, in DPP4T-α. Atoms marked with # are generated by symmetry. In DPP4T-β, Cg1, Cg2, Cg3, and Cg4 are the centroids of the rings formed by atoms S1− C4−C5−C6−C7, S2−C8−C9−C10−C11, S3−C20−C21−C22− C23, and S4−C24−C25−C26−C27, respectively.

spots marked as a, b, and c in Figure 6 correspond to the interactions between the C−H and the carbonyl O atoms. The spots marked as d are due to the S···O interactions. In DPP4T-β, the spots marked as a and b are C−H···π hydrogen bonds, while c and d are due to C−H···S interactions. Other visible spots in the Hirshfeld surfaces correspond to H···H contacts. In the 2D fingerprint plots (Figure 6, bottom panels), the C−H···O intermolecular interactions in DPP4T-α appear as two distinct spikes (di, de) regions of (1.4, 1.1 Å) and (1.1, 1.4 Å), labeled as I and I′. Prominent pairs of sharp spikes of almost equal lengths in the fingerprint plots (Figure 6) are characteristics of nearly equal C(donor)···O(acceptor) distances. The upper spike I (Figure 6) corresponds to the donor spike (H atoms interacting with O atoms), while the lower spike I′ is an acceptor spike (O atoms interacting with the H atoms). The C−H···S interactions appear as small wings marked as II and II′. In the 2D fingerprint plot of DPP4T-β, there are no spikes for O···H interactions due to the absence of any C−H···O hydrogen bonds. The presence of C−H···S interactions is manifested in the form of two spikes appearing in the (di, de) regions of (1.6, 1.0 Å) and (1.0, 1.6 Å) in the fingerprint plots of DPP4T-β, which are marked as I and I′ (Figure 6). The wings marked in Figure 6 (encircled in black) are due to the C···H contacts corresponding to C−H···π interactions. H···H contacts appear as spikes (marked as red circles in Figure 6). The relative contributions of H···H, O··· H/S···H, C···H, C···C, and other interactions to the Hirshfeld surface area are depicted in Figure S7 in the Supporting Information for both polymorphs. The quantitative analysis

Figure 5. (a) Perspective view of the 2D- columnar packing in DPP4T-β as viewed along the crystallographic c axis. Intermolecular interactions are shown with dotted lines. (b) View of the molecular steps (neighboring molecules are shown in red and green) formed by π···π interactions (shown in light blue). Adjacent steps are interconnected to each other by C−H···π (S4/C24−C27) hydrogen bonds (shown in dark blue) to form a 2D network. H atoms involved in H bonds are only shown for clarity.

To further investigate the significance of these long-range nonclassical interactions, we calculated the Hirshfeld surfaces of DPP4T-α and DPP4T-β and illustrated them in Figure 6 (top panels), showing surfaces that have been mapped over a dnorm range of −0.19 to 1.17 Å. Because Hirshfeld surfaces are directly related to a given molecular environment, their use can enable a rapid and easy visual comparison of polymorphs.36 The dominant interactions can be seen in the Hirshfeld surfaces as the bright red areas marked in Figure 6. In DPP4T-α, the red

Table 2. Intermolecular Hydrogen Bonds in the Crystal Structures of DPP4T-α and DPP4T-β D···A/D−H···A (Å)

d(D−H) (Å)

d(D−A) (Å)

d(H···A) (Å)

D−H···A (deg)

symmetry

C9−H9···O3 C15−H15C···O1 C14−H14C···Cg2a S2···O1

0.95 0.98 0.98

3.456(5) 3.431(5) 3.924(6) 3.122

2.51 2.49 2.96

175 162 168

2 − x, −y, 1 − z x, 1 + y, z x, 1/2 − y, −1/2 + z 1 − y, −y, 1 − z

C16−H16A···Cg4a C31−H31A···S2

0.98 0.98

3.780(4) 3.266(5)

2.89 2.61

151 125

x, y − 1, z x, y + 1, z

DPP4T-α

DPP4T-β

a

Cg2 is the centroid of the ring formed by atoms S2 and C8−C11 in DPP4T-α; Cg4 is the centroid of the ring formed by atoms S4 and C24−C27 in DPP4T-β. 663



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Figure 6. Hirshfeld surfaces (top panels) and fingerprint plots (bottom panels) for DPP4T-α and DPP4T-β.

Thin Film Analysis. UV−vis Absorption Spectra. UV−vis absorption spectra of the as-spin-coated, thermally annealed, and SVA films using two different solvents are shown in Figure 7. Thermal annealing of the films was done at a temperature of 90 and 120 °C for 60 and 5 min, respectively. During the annealing process, particular care has been taken to avoid removal of the Boc- moieties.38 In Figure 7a, the UV−vis absorption spectrum of DPP4T films spin-coated using CHCl3 is shown. Independent of the post-growth treatment, all of the films reveal HOMO−LUMO transition (0−0) and the vibronic side bands. These vibronic side bands are absent in DPP4Tsolution (Figure S8 in the Supporting Information), as was observed in the case of DPP grafted with alkyl side chains,9b,39 most likely due to conformational disorder in the DPP− thiophene and thiophene−thiophene torsion angles. Interestingly, we observe a red shift of ∼0.05 eV of the (0−0) transition in solvent vapor-annealed film as well as an increase in the intensity ratio between the (0−0) and (0−1) transitions compared with the thermally annealed and the as-spin coated films (spin-coated using CHCl3). For the film spin-coated using THF, no significant red shift was observed after the SVA. Moreover, the optical band gap extracted from the absorption edge of SVA film using THF is the same as the SVA film using CHCl3. The occurrence of the red shift after the SVA is not a consequence of the increase in crystallinity of the film. If this was the case, then a decrease in the intensity of the 0−0 transition would be expected31 (Figure 7a). To shed light on the origin of the spectral features observed in the optical absorption spectrum of the crystals and their

clearly shows that these molecular interactions in both polymorphs are accounting for 90 to 91% of the Hirshfeld surface area. The C···C contacts corresponding to π···π interactions vary significantly in the two polymorphs, from 6.8% in DPP4T-α to 9.1% in DPP4T-β. This can be attributed to the planar nature of the dithiophene DPP moiety in DPP4T-β with respect to DPP4T-α, where the terminal thiophene rings are twisted. QPA of the DPP4T-5 using Rietveld analysis revealed that the weight fractions of α and β forms are 54 and 46, respectively. From the quality of the Rietveld fit as well as the agreement factors, it can be unequivocally concluded that DPP4T-5 is essentially a mixture of DPP4T-α and DPP4T-β. The densities of forms α and β are 1.436 and 1.406 g/cm3, respectively, and according to the well-known “density rule”,37 the polymorph α with the higher density is most likely to be more stable. These observations are consistent with the lattice energy calculations obtained at the density functional theory (DFT) level on appropriately chosen cell sizes (see the Theoretical Calculations section), which show that the polymorph DPP4T-α is energetically more favorable than DPP4T-β by 39.98 kJ/mol per cell. The trends observed with the DFT lattice energies are further confirmed by molecular mechanics calculations using the COMPASS force field. In this case, DPP4T-α appears to be more stable by 24.3 kJ/mol. Altogether, our calculations support the fact that DPP4T-α is more stable and is consistent with our ability to obtain single crystals only for DPP4T-α. 664



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Figure 7. UV−vis absorption spectra of DPP4T films spin coated using solvents: (a) CHCl3 and (b) THF. The corresponding spectrum for 1 × 10−5 M solution of DPP4T is given in Figure S8 in the Supporting Information.

Figure 8. Absorption spectra for aggregates of DPP4T-α and DPP4T-β polymorphs (blue and black curves) as well as a single DPP4T molecule (red curve) obtained for a gas-to-crystal shift D equal to 1.81 eV.

relative changes going from the α to the β phase, we have performed aggregate absorption calculations (see Theoretical Calculations section) for both polymorphs. 2D aggregates made of 100 molecules in the densest crystalline planes (namely clusters of 10 ×10 molecules in the ab and bc planes for DPP4T-α and DPP4T-β, respectively) have been built (test calculations on larger clusters do not show significant differences in the electronic exciton bandwidths). As a consequence of their similar organization in these crystalline planes, the largest excitonic couplings in the two aggregates investigated have relatively similar magnitudes and signs (see Figures S9 and S10 in the Supporting Information). Most of these couplings are positive, consistent with H-like aggregates. This is in line with previous results on similar molecules containing four and six thiophene rings, while shorter oligomers (two thiophene rings) show a more complex J-H mixed behavior depending on the nature of the alkyl side chains.9b As a result, both the α and β phase clusters show a reduced ‘0−0’ intensity associated with dynamic symmetry breaking entailed by coupling to the molecular vibrations (the pure electronic transition to the bottom exciton state is symmetry-forbidden in H aggregates). The spectra predicted for the two polymorphs are essentially superimposable with no apparent spectral shift and in reasonable agreement with the experimental data obtained for the films casted from chloroform. The predicted 0−0 intensity is underestimated, which likely arises from static disorder effects not accounted for here.31 As expected for H-aggregates, turning

off all intermolecular excitonic couplings (i.e., simulating single molecules) yields an increased 0−0 intensity and a red shift of the optical absorption spectrum. Interestingly, the resulting spectrum now appears to be in closer correspondence with that measured for films exposed to solvent annealing; see Figure 8. We thus speculate that the SVA films are β-phase crystals that are either highly disordered or, more likely, include a large fraction of noninteracting molecules (possibly in stronger interaction with the substrate compared with the α polymorph). In favor of the latter hypothesis, we note that while the singlemolecule spectrum nicely reproduces the low-energy part of the SVA optical spectral (‘0−0’ and ‘0−1’ transitions up to 2.1 eV), it predicts a too-low intensity at higher energy (‘0−2’ at ∼2.2 eV), where the crystals (being either α or β) strongly absorb. Thus a superimposition of the single molecule and crystalline domains would fit the experimental data across the whole spectral range. Morphology Investigations. The morphology of the as-spin coated and SVA-films using various solvents was investigated using POM and AFM. The corresponding POM and AFM images of as-spin coated DPP4T films using CHCl3 and THF are shown in Figure 9. The POM and AFM images of as-spin coated DPP4T using CHCl3 depict the trapping of small crystallites into the amorphous matrix (Figure 9a,c). The film spin-coated using THF reveals the growth of needle like bent crystallites (Figure 9b). The AFM height image of the film spin coated using THF shows a clearer picture revealing the 665



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DPP4T-β polymorph. For the films spin-coated and solventvapor-annealed using THF, there were no major changes in morphology visible from POM. From the AFM image, it is evident that the semicircular crystallites seen in Figure 9d crack into small needles after SVA (Figure 9f). X-ray Diffraction Measurements on Thin Films. Figure 10a,b depicts the specular X-ray diffraction patterns (sXRD) of DPP4T films spin coated using CHCl3 and THF, respectively. The film as-spin-coated using CHCl3 reveals a considerably broad peak at 2θ = 7.40° that corresponds to the (002) reflection of the DPP4T-α phase. To increase the crystallinity, the films were annealed at 120 and 90 °C for 5 and 60 min, respectively. As seen from Figure 10a, annealing does not seem to have much effect on the film. The as-spin coated films were then subjected to SVA at RT for 1 day to increase the crystallinity of the films; the same solvent was chosen as the one used to spin coat. Interestingly, the crystallinity of the film increased significantly, accompanied by a shift of the (002) reflection to a higher angle (2θ = 7.52°), which corresponds to the (100) reflection of DPP4T-β (Figure 10a). The peak shift is the more prominent if we focus on the second-order reflections of both phases (Figure 10a), that is, (004) reflection of the DPP4T-α (2θ = 14.92°) and (200) reflection of the DPP4T-β (2θ = 15.08°). For the film spin-coated using THF, the first peak appears at 2θ = 7.52°, which corresponds to the (100) reflection of DPP4T-β. As seen from Figure 10b, exposing the films to THF vapor significantly increases the crystallinity, but there is no shift of the (100) reflection. From these findings, it can be concluded that SVA promotes the formation of only β-phase, which is the more stable phase in thin films. For further justification, X-ray rocking curve and grazing incidence in-plane X-ray diffraction pattern (GID) of SVAfilms of DPP4T were recorded. The X-ray rocking curve was recorded to understand the mosaicity of the crystallites on the substrate surface. The X-ray rocking curve corresponding to the (100) reflection [2θ = 7.52°] is shown in Figure 11a. The Lorentzian shape of the profile confirms the formation of crystallites that are predominantly oriented parallel to the substrate. The GID profile of SVA-film contains two reflections that correspond to the (010) and (020) reflections of DPP4T-β (Figure 11b). The absence of any other in-plane reflections that may correspond to the DPP4T-α confirms that SVA of DPP4T films exclusively promotes the formation and growth of the β-phase. The films solvent-vapor-annealed using various solvents such as methanol, THF, CHCl3, and acetonitrile showed the same results (Figure S11 in the Supporting Information).

Figure 9. POM images (a,b) and AFM (c,d) of as-spin coated DPP4T using CHCl3 (a,c) and THF (b,d) forming α- and β-form, respectively. AFM images of solvent-vapor-annealed DPP4T films using CHCl3 (e) and THF (f).

semicircular growth along a circular arc, which acts as a site of secondary nucleation (Figure 9d). This peculiar morphology of the films can be attributed to the stress imposed by spin coating. There were no significant changes in morphology when the films were annealed at 120 and 90 °C. When exposed to solvent vapor, the morphology of the films changes considerably, as seen in Figure 9e,f. The films that were solventvapor-annealed using CHCl3 crystallize to form small plateshaped crystallites (Figure 9e) confirming the improved crystallinity of the films after SVA with the formation of

Figure 10. Overlay of the specular X-ray diffraction patterns of thin films spin-coated using different solvents: CHCl3 (a) and THF (b). 666



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Figure 11. (a) X-ray rocking curve of (100) Bragg reflection corresponds to the β-phase. (b) GID pattern of DPP4T films spin coated and solventvapor-annealed with CHCl3 for 1 day.

Figure 12. Schematic representation of the orientation of the DPP4T molecules in thin films. SVA of DPP4T-α leads to the formation of DPP4T-β accompanied by increase in crystallinity.

the triclinic β-form is found to have the larger crystallization tendency and dominates the crystal formation in the spincoated films. We propose that the enhanced stability of the β-form in thin films can be attributed to substrate−molecule interactions. Although molecules in both forms are essentially edge-on, the molecules of the β-form are more titled with respect to the substrate than the α-form. The effects of film treatment by thermal and SVA were investigated. Additionally, UV−vis study of the films revealed the characteristic spectra for the two polymorphs, with a distinct red shift for the β-form and a vibronic progression with the 0−0 band being the most intense. Absorption spectrum calculations on large aggregates have revealed that α and β polymorphs have similar spectral behavior owing to the relatively similar organization of the molecules within their densest planes. The slight red shift and the distinct vibronic progression observed in the SVA films are reminiscent of the single-molecule optical spectra. This behavior is possibly due to the presence of amorphous regions in the films or, more likely, isolated molecules with preferential interactions with the substrate. This study has highlighted the importance of polymorphism in DPP-thiophene-based materials and the specific organization that could arise from the interaction with the substrate depending on the growing conditions. These are clearly factors to consider to induce the formation of a particular polymorph and to help to design deposition methodologies. In particular, this is really crucial in the field of organic electronics, where the charge-transport properties are highly dependent on crystal packing, especially for organic field-effect transistors where charge transport occurs

From the sXRD and the GID data, the orientation of the molecules of DPP4T-α and DPP4T-β in the films could be visualized, as shown in Figure 12. The (001) plane in the α-form is parallel to the substrate surface, while for the β-form it is the (100) plane. The essential difference between the two polymorphs is due to the tilt of the molecule with respect to the substrate. In the α-polymorph, the molecules are tilted by ∼81°, while for the β-polymorph, the tilt is ∼65°. The increased stability of the triclinic β-form in thin films can be rationalized if the higher degree of molecular tilt is considered, which effectively increases the interaction between the substrate and the molecules.

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CONCLUSIONS We have identified and isolated two polymorphic forms of a DPP-Boc derivative both in bulk and in thin films. The crystal structure of the polymorphs DPP4T-α and DPP4T-β obtained from single crystal/powder X-ray diffraction data revealed that dithiophene-DPP-dithiophene is essentially planar for the β-form, while for the α-form the terminal thiophene rings are twisted. The crystal packing in both polymorphs is stabilized via S···O, C−H···O/π, and π···π interactions forming herringbone sheets in α-form and a columnar network in β-form. From considerations of density and lattice energy calculations at both density functional theory and molecular mechanics levels, it can be concluded that the α-form is more stable in bulk, consistent with the fact that single crystals of only the α-form are obtained. Thin film studies have been carried out to identify the specific polymorphs favored by the substrate. Among the polymorphs, 667



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(3) (a) Snider, D. A.; Addicks, W.; Owens, W. Polymorphism in Generic Drug Product Development. Adv. Drug Delivery Rev. 2004, 56, 391−395. (b) Brittain, H. G. Polymorphism in Pharmaceutical Solids; Informa Healthcare: New York: 1999; Vol. 192. (c) Marques, M. P. M.; Valero, R.; Parker, S. F.; Tomkinson, J.; Batista De Carvalho, L. A. E. Polymorphism in Cisplatin Anticancer Drug. J. Phys. Chem. B 2013, 117, 6421−6429. (4) (a) Werzer, O.; Boucher, N.; De Silva, J. P.; Gbabode, G.; Geerts, Y. H.; Konovalov, O.; Moser, A.; Novak, J.; Resel, R.; Sferrazza, M. Interface Induced Crystal Structures of Dioctyl-Terthiophene Thin Films. Langmuir 2012, 28, 8530−8536. (b) Salzmann, I.; Nabok, D.; Oehzelt, M.; Duhm, S.; Moser, A.; Heimel, G.; Puschnig, P.; Ambrosch-Draxl, C.; Rabe, J. R. P.; Koch, N. Structure Solution of the 6,13-Pentacenequinone Surface-Induced Polymorph by Combining X-Ray Diffraction Reciprocal-Space Mapping and Theoretical Structure Modeling. Cryst. Growth Des. 2011, 11, 600−606. (5) Chou, W.-Y.; Chang, M.-H.; Cheng, H.-L.; Lee, Y.-C.; Chang, C.C.; Sheu, H.-S. New Pentacene Crystalline Phase Induced by Nanoimprinted Polyimide Gratings. J. Phys. Chem. C 2012, 116, 8619−8626. (6) (a) Moser, A.; Salzmann, I.; Oehzelt, M.; Neuhold, A.; Flesch, H.G.; Ivanco, J.; Pop, S.; Toader, T.; Zahn, D. R. T.; Smilgies, D.-M.; et al. A Disordered Layered Phase in Thin Films of Sexithiophene. Chem. Phys. Lett. 2013, 574, 51−55. (b) Siegrist, T.; Kloc, C.; Laudise, R. A.; Katz, H. E.; Haddon, R. C. Crystal Growth, Structure, and Electronic Band Structure of A-4t Polymorphs. Adv. Mater. 1998, 10, 379−382. (c) Jurchescu, O. D.; Meetsma, A.; Palstra, T. T. M. LowTemperature Structure of Rubrene Single Crystals Grown by Vapor Transport. Acta Crystallogr., Sect. B 2006, 62, 330−334. (7) Jurchescu, O.; Mourey, D.; Subramanian, S.; Parkin, S.; Vogel, B.; Anthony, J.; Jackson, T.; Gundlach, D. Effects of Polymorphism on Charge Transport in Organic Semiconductors. Phys. Rev. B 2009, 80, 085201. (8) (a) Chandran, D.; Lee, K.-S. Diketopyrrolopyrrole: A Versatile Building Block for Organic Photovoltaic Materials. Macromol. Res. 2013, 21, 272−283. (b) Stas, S.; Balandier, J.-Y.; Lemaur, V.; Fenwick, O.; Tregnago, G.; Quist, F.; Cacialli, F.; Cornil, J.; Geerts, Y. H. Straightforward Access to Diketopyrrolopyrrole (Dpp) Dimers. Dyes Pigm. 2013, 97, 198−208. (c) Chen, L.; Deng, D.; Nan, Y.; Shi, M.; Chan, P. K. L.; Chen, H. Diketo-Pyrrolo-Pyrrole-Based Medium Band Gap Copolymers for Efficient Plastic Solar Cells: Morphology, Transport, and Composition-Dependent Photovoltaic Behavior. J. Phys. Chem. C 2011, 115, 11282−11292. (9) (a) Qu, S.; Tian, H. Diketopyrrolopyrrole (Dpp)-Based Materials for Organic Photovoltaics. Chem. Commun. 2012, 48, 3039−3051. (b) Kirkus, M.; Wang, L.; Mothy, S.; Beljonne, D.; Cornil, J.; Janssen, R. A. J.; Meskers, S. C. J. Optical Properties of Oligothiophene Substituted Diketopyrrolopyrrole Derivatives in the Solid Phase: Joint J- and H- Type Aggregation. J. Phys. Chem. A 2012, 116, 7927−7936. (10) (a) Bronstein, H.; Chen, Z.; Ashraf, R. S.; Zhang, W.; Du, J.; Durrant, J. R.; Shakya Tuladhar, P.; Song, K.; Watkins, S. E.; Geerts, Y.; et al. Thieno[3,2-B]Thiophene−Diketopyrrolopyrrole-Containing Polymers for High-Performance Organic Field-Effect Transistors and Organic Photovoltaic Devices. J. Am. Chem. Soc. 2011, 133, 3272− 3275. (b) Balandier, J.-Y.; Quist, F.; Stas, S.; Tylleman, B.; Ragoen, C.; Mayence, A.; Bouzakraoui, S. D.; Cornil, J.; Geerts, Y. H. Dimers of Anthrathiophene and Anthradithiophene Derivatives: Synthesis and Characterization. Org. Lett. 2011, 13, 548−551. (c) Ashraf, R. S.; Chen, Z.; Leem, D. S.; Bronstein, H.; Zhang, W.; Schroeder, B.; Geerts, Y.; Smith, J.; Watkins, S.; Anthopoulos, T. D.; et al. Silaindacenodithiophene Semiconducting Polymers for Efficient Solar Cells and High-Mobility Ambipolar Transistors. Chem. Mater. 2010, 23, 768−770. (d) Zhang, X.; Richter, L. J.; Delongchamp, D. M.; Kline, R. J.; Hammond, M. R.; Mcculloch, I.; Heeney, M.; Ashraf, R. S.; Smith, J. N.; Anthopoulos, T. D.; et al. Molecular Packing of High-Mobility Diketo Pyrrolo-Pyrrole Polymer Semiconductors with Branched Alkyl Side Chains. J. Am. Chem. Soc. 2011, 133, 15073− 15084.

at the interface between the organic semiconductor and the dielectric. To end the article, it would be most apt to quote Joel Bernstein on his view on polymorphism “Obtaining new crystal forms, whether by systematic search or by serendipity, is an adventure into the crystallographic unknown, and preparing or recognizing a new crystal form is undeniably a chemical invention.”1a

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ASSOCIATED CONTENT

S Supporting Information *

Crystallographic files in CIF format (CCDC 958155, 958156); 1 H and 13C NMR; MALDI-ToF spectrum; final Rietveld refinement plot of DPP4T-α; comparison of the powder diffraction pattern of the as-synthesized compound DPP4T-5 with that of DPP4T-α and DPP4T-β; POM images of single crystals of DPP4T-α; relative contributions to the Hirshfeld surface areas for the various intermolecular contacts; UV−vis absorption spectra; representation of molecular organization and electronic coupling magnitudes; GID pattern of SVA films using different solvents and blended solvents. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; basab.chattopadhyay@ gmail.com. Notes

The authors declare no competing financial interests.

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ACKNOWLEDGMENTS Financial support from the Marie Curie IIF Scheme of the seventh EU Framework Program for the DISCO-project (project no. 298319) and from the ARC program of the Communauté Française de Belgique (grant no. 20061) is kindly acknowledged by B.C and Y.H.G. The work in Mons was supported by the European Commission/Région Wallonne (FEDER − Smartfilm RF project), the Programme d’Excellence de la Région Wallonne (OPTI2MAT project), MMM@HPC (FP7-RI-261594), and the Interuniversity Attraction Pole program of the Belgian Federal Science Policy Office (PAI 6/27). Y.O, D.B., and J.C. are FNRS Research Fellows.

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