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Crystal Engineering of Dibenzothiophenothieno[3,2-b]thiophene (DBTTT) Isomers for Organic Field-Effect Transistors Hung-Yang Chen, Guillaume Schweicher, Miquel Planells, Sean M. Ryno, Katharina Broch, Andrew J. P. White, Dimitrios Simatos, Mark Little, Cameron Jellett, Samuel J. Cryer, Adam Marks, Michael Hurhangee, Jean-Luc Brédas, Henning Sirringhaus, and Iain McCulloch Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b02757 • Publication Date (Web): 25 Oct 2018 Downloaded from http://pubs.acs.org on October 28, 2018
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Chemistry of Materials
Crystal Engineering of Dibenzothiophenothieno[3,2b]thiophene (DBTTT) Isomers for Organic Field-Effect Transistors Hung-Yang Chen,*,† Guillaume Schweicher,*,‡ Miquel Planells,† Sean M. Ryno,§ Katharina Broch,∥ Andrew J. P. White,† Dimitrios Simatos‡, Mark Little,† Cameron Jellett,† Samuel J. Cryer,†, Adam Marks,† Michael Hurhangee,† Jean-Luc Brédas,§ Henning Sirringhaus,‡ and Iain McCulloch†,# †
Department of Chemistry and Centre for Plastic Electronics, Imperial College London, London SW7 2AZ, UK Cavendish Laboratory, University of Cambridge, JJ Thomson Avenue, Cambridge CB3 0HE, UK § Solar and Photovoltaics Engineering Research Center, Division of Physical Science and Engineering, King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia ∥ Institut für Angewandte Physik, Universität Tübingen, Auf der Morgenstelle 10, 72076 Tübingen, Germany # King Abdullah University of Science and Technology (KAUST), KSC, Thuwal, 23955-6900, Saudi Arabia ‡
Department of Chemistry, University College London, WC1H 0AJ, London
ABSTRACT: Three thiophene ring-terminated benzothieno[3,2-b]benzothiophene(BTBT)-derivatives, C-C6-DBTTT, C-C12DBTTT, and L-C12-DBTTT, were designed and synthesized, differing in the isomerization of alkyl chain position as well as aromatic core construction. A study of crystal structure and electronic properties, combined with a theoretical investigation was performed to understand the structure-property relationships for the application of these molecules in organic field-effect transistors (OFETs). Different crystal packing structures were observed for these three isomers by single crystal X-ray diffraction as a result of a crystal engineering molecular design approach. The highest charge-carrier mobility was observed for the isomer with a co-linear core, L-C12-DBTTT. Preliminary results demonstrated a promising hole mobility of 2.44 cm 2 V−1 s−1, despite the polymorphism observed in ambient conditions.
INTRODUCTION Organic field-effect transistors (OFETs) have sparked intensive interest both in the academic and industrial fields due to their application in low-cost and flexible electronics. Charge carrier mobility μ (cm2 V-1 s-1), defined as the drift velocity of the charge carrier (cm s-1) per unit of applied electric field (V cm-1), is a key figure of merit for these OFET devices. The past three decades have witnessed a tremendous improvement in the charge carrier mobility both in conjugated polymer and small-molecule organic semiconductors. Mobility of >1-10 cm2 V-1 s-1 have been achieved both in p-type and n-type1 transport through intensive materials development and device optimization, exceeding amorphous silicon based FETs (0.5-1.0 cm2 V-1 s-1).2 Thienoacene-based polymers3 and small molecules4 have been prominent in the materials classes as p-channel semiconductors for OFETs due to their high chemical stability and potential for optimal electronic intermolecular coupling. For example, benzothieno[3,2-b]benzothiophene (BTBT) is one of the most studied molecular cores in small molecule OFETs. Owing to its poor film-forming property and high ionization potential (IP) (5.8 eV),4 structural modifications on the parent BTBT core have been necessary to successfully achieve high mobility in OFET devices. Two main approaches for the molecular design of BTBT-based small molecule OFETs have been applied, firstly grafting of aromatic rings or long hydrocarbon chains on the 2,7positions of the BTBT core, and secondly, π-conjugation exten-
sion. 2,7-Diphenyl[1]benzothieno[3,2-b]benzothiophene (DPhBTBT) was the first reported high mobility BTBT analogue with mobility up to 2.0 cm2 V-1 s-1 achieved in a vapor-processed OFET device.5 DPh-BTBT exhibits a reduced IP of 5.6 eV than the parent BTBT, which is beneficial for hole injection from electrodes, and can be attributed to the extension of its πconjugation. The substitution of long hydrocarbon chains does not adversely affect the intermolecular packing of the BTBT core structure; rather, these chains induce closer packing for the molecules due to the van der Waals intermolecular interaction between alkyl groups. This enhanced packing also results in significantly smaller IPs of 5.3 eV for alkylated BTBT molecules (Cn-BTBT).6 Illustrating the π-conjugation extension approach, several highly π-extended molecules based on the BTBT core, such as DNTT,7 DATT,8 BBTBDT,9 BNTBDT9 and BBTNDT,10 have also demonstrated high charge carrier mobility, with values in excess of 1.0 cm2 V-1 s-1. One of the crucial factors in the high charge carrier mobility of these BTBT derivatives is their herringbone molecular packing, realizing a two-dimensional (2D) electronic structure in the thin film morphology. This can be attributed to their effective intermolecular interactions through short S-S contacts by sulfur atoms in the central thienothiophene moiety of the BTBT core. Recently, extending the π-conjugation of BTBT core was achieved by adding thiophene rings as the terminal ring, dibenzothiopheno[6,5-b:6’,5’-f]-thieno[3,2-b]thiophene (DBTTT),11 to
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further enhance S-S and S-C interactions thus increasing the compactness of the packing. Compared with its electronically equivalent benzene analogue, DNTT, DBTTT molecules showed shorter intermolecular π-π distance and higher packing density, demonstrating a high mobility of 19.3 cm2 V-1 s-1 in a polycrystalline thin-film transistor. Previously, we have also reported a new polymer containing a repeat unit based on the BTBT core with two thiophene rings as the terminal rings, tetradodecylthieno[3’, 2’:6,7][1]benzothieno[3,2-b]-thieno[3,2g][1]benzothiophene (DTBTBT), which shows a mobility of 0.1 cm2 V-1 s-1 when copolymerized with thiophene.12 The crystal packing and OFET performance for the core structure of DTBTBT was not explored, therefore we designed and synthesized three isomers of DTBTBT by varying the alkyl side chain position (changing from the lateral position to the axial position) and the position of the terminal thiophene rings (from a curved core to a linear core). In this work, we demonstrate the crystal engineering of three thiophene ring-terminated BTBT derivatives for OFET applications by varying the isomerism of the alkyl side chain and the aromatic core. RESULTS AND DISCUSSION The chemical structures of the investigated three isomers are presented in Figure 1(a). For consistency, two different aromatic cores are both denoted as DBTTT with C- and L- representing curved and linear cores, respectively. The reported non-alkylated DBTTT (Dibenzothiopheno[6,5-b:6',5'-f]thieno[3,2-b]thiophene) is also included for comparison. Three alkylated DBTTT isomers
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were obtained according to the synthetic routes outlined in Scheme S1-S3 in the supporting information. The two curved DBTTT isomers (C-C6-DBTTT and C-C12-DBTTT) were synthesized from the same core structure, 2,5-di(thiophen-2yl)thieno[3,2-b]thiophene, with different substituents (compound 2 and 9 in Scheme S1 and S2, Supporting Information) to afford the final products, respectively. Compound 2 and 9 were separately synthesized from Negishi and Stille coupling reactions in good yields, respectively. C-C6-DBTTT was obtained by the palladium (Pd)-catalyzed double cross-coupling bis-annulation reaction13 via tetrabromo-tetraaryl compound 2 and vicdiborylated tetradec-7-ene compound 3. C-C12-DBTTT was synthesized from a platinum (Pt)-catalyzed intramolecular cyclization of two alkyne groups on the thieno[3,2-b]thiophene derivative 9. The synthesis of the linear DBTTT isomer, L-C12DBTTT, was carried out following a literature procedure.11 The key step in preparing L-C12-DBTTT employed an acidcatalyzed intramolecular cyclodehydration (i.e. hetero-Bradsher reaction14) from dicarboxaldehyde compound 13, which was obtained in three steps starting from the commercially available thieno[3,2-b]thiophene. The full synthetic procedures are detailed in the Supporting Information. These three isomers exhibit significantly different solubility in common organic solvents at room temperature, in descending order of solubility: C-C6DBTTT > C-C12-DBTTT > L-C12-DBTTT.
Figure 1. a) Molecular structure of different DBTTT derivatives. b) Side view of the packing structure highlighting the side chain behaviour. c) Lattice parameters of the crystalline structure [Å]. d) Packing of the aromatic cores underscoring the formation of a classical herringbone motif for DBTTT and L-C12-DBTTT, a slipped co-facial packing for C-C6-DBTTT and a slipped-stack sandwich packing for CC12-DBTTT (side chains are omitted for clarity).
The thermotropic behaviours of the three DBTTT isomers were investigated by differential scanning calorimetry (DSC) (Figure 2). All isomers exhibit good thermal stability with thermal decomposition temperature (Td) higher than 400 oC. Interestingly,
DSC scans of these isomers highlight several phase transitions in the range of 25-300 oC. Both C-C12-DBTTT and L-C12-DBTTT exhibit more than one phase transition, including the presence of liquid-crystal (LC) phases. As examined under polarized optical
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microscope (POM), a nematic LC phase was observed after the isotropic to LC crystallization at 176 oC during the cooling scan for C-C12-DBTTT. L-C12-DBTTT presents several crystalline phases differentiated by small enthalpy energies below the LC to crystal transition taking place at 267 oC.15 Between 267 and 280 o C, a LC phase assigned as a SmA phase16 persists before the isotropic melt at 282 oC. C-C6-DBTTT shows a sharp melting peak at 175 oC during the heating scan and exhibits unresolved crystallization events during the cooling scan. The strong difference of thermotropic behaviour of the investigated isomers is mainly attributed to the various anchored positions of alkyl chains. The LC phases of C-C12-DBTTT and L-C12-DBTTT arise from their rod-like molecular shapes with alkyl chains anchored axially to the rigid aromatic core. This tends to selfalign molecules and induce a long range directional order with their long axis roughly parallel. A similar phenomena was also observed recently in didodecyl[1]benzothieno[3,2b][1]benzothiophene isomers.17 C-C6-DBTTT 4.0
C-C12-DBTTT 3.0 2nd Heating/Cooling, 10 C/min.
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Chemistry of Materials
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Figure 2. DSC curves of the three alkylated DBTTT isomers. Inset shows polarized optical microscope (POM) images of the liquid crystal phases of C-C12-DBTTT and L-C12-DBTTT.
Single-crystals of the different DBTTT derivatives were grown by slow recrystallization from a saturated 1,2dichlorobenzene solution. Structure determination was realized by single-crystal X-ray diffraction and the complete crystal data are available in the Supporting Information (the crystal structure of DBTTT has previously been solved by Lee and co-workers11). L-C12-DBTTT, the axial alkylated version of DBTTT, adopts a classical layer-by-layer herringbone packing motif mostly stabilized by CH···π interactions (like DBTTT, see Figure 1). In comparison, co-facial interdigitated structures dominated by π-π interactions are observed for C-C6-DBTTT and C-C12-DBTTT that differs by the number of alkyl substitutions and the position of their lateral thiophene unit on the aromatic core. C-C6DBTTT and C-C12-DBTTT exhibit a slipped co-facial packing and a slipped-stack sandwich packing, respectively.18 The shortest stacking distance between molecular planes, 3.59 Å, is obtained for C-C12-DBTTT. Table S1 (Supporting Information) highlights the different distances of stacking and slippage from the structures. DBTTT and L-C12-DBTTT present consequently favorable close contacts between the aromatic cores in two dimensions, key component for good charge transport properties. Crystallographic information files (CIFs) for different DBTTT derivatives are available in the Supporting Information (these data can also be obtained free of charge from The Cambridge Crystallographic Data Centre; CCDC 1590306 (C-C6-DBTTT), CCDC 1590307 (C-C12-DBTTT) and CCDC 1590308 (L-C12DBTTT). To have a clear quantitative and visual insight into the intermolecular interactions, Hirshfeld surfaces of the DBTTT derivatives and their relative 2D fingerprints were calculated using Crystal Explorer (Figure S6, Supporting Information). Interestingly, the derivatives C-C6-DBTTT and C-C12-DBTTT do not present the dominant interactions found in DBTTT and L-C12-
DBTTT (S···S, C···S and C-H···π), as seen in the Hirshfeld surfaces as bright red areas (shorter contacts). A look at the 2D fingerprint plots (Figure S7) clearly reveals that the packing environment is completely different for the three observed molecular packings. The C-H···π hydrogen bonds in DBTTT and LC12-DBTTT appear as a pair of wings of almost equal length in the (di, de) regions of (1.8 Å, 1.2 Å) and (1.2 Å, 1.8 Å). The absence of such distinct features in C-C6-DBTTT and C-C12DBTTT points to the lack of such strong directional interactions. The presence of C-H···S interactions is manifested in the form of two spikes appearing in the (di, de) regions at (1.9 Å, 1.2 Å) and (1.2 Å, 1.9 Å) in the fingerprint plots. These C-H···S interactions act as additional reinforcements within the π-stacked columns of C-C6-DBTTT and C-C12-DBTTT, with the π-π interactions visible in the regions of di = de = 2.0 Å. A look at the relative contributions of the various close intermolecular contacts for the different alkylated DBTTT derivatives (Figure S7) highlights the driving role of the alkyl chains in the crystal packing of the alkylated DBTTT derivatives since the H···H contacts account for ~70% of the Hirshfeld surface area. The C···H contacts, corresponding to C-H···π interactions, vary significantly, from 5.8% in C-C6-DBTTT to 15.1% in L-C12-DBTTT due to the herringbone packing in the latter case. Moreover, the C···C contacts, corresponding to π···π interactions, are as low as 0.1% in L-C12-DBTTT. The ionization potential (IP) of the three investigated isomers, estimated using photoelectron spectroscopy in air (PESA), are 5.49 eV, 5.34 eV and 5.17 eV for C-C6-DBTTT, C-C12-DBTTT and L-C12-DBTTT, respectively. Despite the same conjugated core in the case of C-C6-DBTTT and C-C12-DBTTT, the different IP values arise from different molecular packing patterns and intermolecular interactions. Compared with the IP of nonalkylated DBTTT (5.32 eV),11 the smaller IP for L-C12-DBTTT may imply that long hydrocarbon chains induce a more closely packed molecular arrangement, which enhances intermolecular electronic coupling interactions between adjacent molecules. This difference is consistent with those observed in BTBT/CnBTBT and DNTT/Cn-DNTT molecules.4 To further reinforce this idea, we performed theoretical calculations to determine the molecular ionization energies and electronic polarization energies (P+).19 The gas-phase IEs suggest an ordering similar to the PESA measurements of 7.00 eV, 6.67 eV, and 6.63 eV for C-C6DBTTT, C-C12-DBTTT and L-C12-DBTTT, respectively. However, the IE differences between the molecules is largely different from that of the experimental values, and as the packing changes between each of the molecules the local molecular environment is expected to be different for each of the systems. The electronic polarization energies were calculated to be 0.97 eV, 0.85 eV, and 1.05 eV for C-C6-DBTTT, C-C12-DBTTT and L-C12-DBTTT, respectively, suggesting that the environment of charge carriers in L-C12-DBTTT is more stabilizing than either C-C6-DBTTT or C-C12-DBTTT and possibly due to denser packing of L-C12-DBTTT. Correcting the values of the molecular IEs with the calculated polarization energies gives final IE values of 6.03 eV, 5.82 eV, and 5.58 eV for C-C6-DBTTT, CC12-DBTTT and L-C12-DBTTT, respectively, in excellent agreement with the IE differences from PESA. Density function theory (DFT) calculations at the ωB97X-D/augcc-pvdz level was carried out to understand the electronic interactions between adjacent π-systems of three DBTTT isomers, to collect the theoretical estimates of two important energetic parameters for charge transport: (i) the charge transfer integral (J) and (ii) the reorganization energy (). Figure 3 shows the corre-
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Chemistry of Materials sponding indexation of the molecules within the bulk singlecrystal phase. Among three isomers, L-C12-DBTTT presents the most balanced transfer integrals along three directions (92 meV, 59 meV, and 17 meV) within a molecular layer compared with the other two isomers wherein couplings are only significant along one direction. This is due to its herringbone packing that favors a 2D charge transport compared to the 1D slip-stack packing present in C-C6-DBTTT. Moreover, the transfer integral along the edge-to-edge pair (the a-axis) is significantly larger in L-C12-DBTTT (92 meV) than in the well-known, highperformant C12-BTBT (79 meV) under the same calculations. This may be attribute to the extended π-conjugation for L-C12DBTTT that enhances the intermolecular orbital overlap.4 For CC6-DBTTT and C-C12-DBTTT, significant transfer integrals are calculated only along the π-π stacking axis, which means the spatial overlaps of HOMO wave functions between molecular columns are extremely small. In particular, C-C12-DBTTT shows a distinct larger transfer integral (149 meV) along the π-π stacking axis among three isomers due to its smaller displacement between adjacent molecules and shorter π-π stacking distance (Table S1). The reorganization energies were calculated to compare the core structures, with the linear isomer L-DBTTT (261 meV) presenting a smaller energy than the curved isomer C-DBTTT (336 meV). The smaller reorganization energy of the linear isomer combined with large electronic coupling could indicate an increased likelihood of band-like transport. C-C6-DBTTT
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Figure 3. Top view of the packing structures showing the indexation of the molecules used for calculations of the transfer integrals and their related values [meV] (side chains are omitted for clarity). Negligible couplings have been omitted for clarity. Calculations at the ωB97X-D/aug-cc-pvdz level.
First assessment for the performance of the alkylated DBTTT derivatives has been performed through the characterization of polycrystalline bottom-gate top-contact organic field-effect transistors (OFETs - 20 nm thick evaporated films, substrates kept at room temperature). Figure 4 shows representative characteristics of 100 μm long and 700 μm wide channel devices of the three alkylated DBTTT derivatives (forward and backward sweeps are always displayed to highlight the presence of any hysteresis). Thienoacenes are known to exhibit injection issues because of their deep IPs,20,21 therefore F4-TCNQ and MoO3 doped contacts were used for L-C12-DBTTT to assess our device architecture (Figure 3a-3d). The characteristics of the MoO3 doped contacts devices of L-C12-DBTTT exhibit relatively ideal behaviour through hysteresis-free, acceptable level of threshold voltage (~ -25V), linearity of the output characteristics at small drain voltages and the establishment of a stable mobility over the whole ON-state gate voltage range. On the contrary, F4-TCNQ doped contacts devices present less ideal characteristics (small hysteresis, large threshold voltage (~ -55V), presence of a “hump” non-linearity in the subthreshold regime, incoherence
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between the linear and saturation regime), attributed to a larger contact resistance: 1.1 kΩ cm and 4.6 kΩ cm for MoO 3 and F4TCNQ doped contacts respectively, evaluated by transfer line measurement (L = 50 and 100 µm - Figure S8). MoO3 doped contacts were thus selected to investigate the performance of the other two derivatives. Transfer characteristics measured in the saturation regime and relative gate-voltage dependence of the saturation mobility of C-C6-DBTTT, C-C12-DBTTT and LC12-DBTTT are presented in Figure 3e-3f for comparison (see Figure S9 and Figure S10 for the individual characteristics of CC6-DBTTT and C-C12-DBTTT). At first observation, it is clear that these two derivatives exhibit poor performance leading to highly non-ideal devices: hysteresis, extremely large threshold voltage (~ -75V), non-linearity of the characteristics, incoherence between the linear and saturation regime. This is most probably the result of a deeper IP combined with a molecular packing less optimal for charge transport. The hole transport in the linear and saturation regime from OFET devices are presented in Table 1. It is worth noting that despite extraction of parameters for the less ideal devices, these values should be viewed with caution. Indeed, only ideal devices can provide unambiguous information regarding the charge transport properties of a new material. The good agreement between the linear and saturation mobilities support however the reliability of the extracted values for our MoO3 doped contact devices of L-C12-DBTTT. All depositions were carried out at room temperature. Large grain size could be expected by performing the thermal evaporation at elevated temperature. X-Ray reflectivity (XRR) and grazing-incidence X-ray diffraction (GIXRD) performed at room temperature on thin films of the different DBTTT derivatives highlight however the presence of polymorphism for each molecule. The films consist of a mix of two polymorphs for C-C12DBTTT and L-C12-DBTTT while only one polymorph is present for C-C6-DBTTT (see Figure S11 and also extra information in the XRD section of the Supporting Information). This polymorphic behaviour is most likely the result of the stabilization within the thin film geometry of polymorphs present in the thermotropic behaviour of the different derivatives (all molecules present several solid-solid transitions as seen in the DSCs.22,23). Polymorphism most likely impacts charge transport and lead to trapping effects in our films. Atomic Force Microscopy (AFM) images confirm (see AFM section of the Supporting Information) that the morphology is not impacting the charge transport performance in our devices. Despite larger domains, CC12-DBTTT exhibits the poorest transport properties while CC6-DBTTT, presenting smaller domains, has slightly better performance.
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Chemistry of Materials combined theoretical and experimental study demonstrated an effective crystal engineering approach for the design of thiophene-ring terminated BTBT molecules in OFET applications. The isomer with the linear aromatic core, L-C12-DBTTT, shows the most optimal molecular packing for two-dimensional charge transport, with a herringbone arrangement. Evaluation of the charge transport properties of L-C12-DBTTT shows promising semiconducting behaviour despite polymorphism under the conditions used.
ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Materials, characterization and synthesis, crystallography and crystal analysis, fabrication of organic field-effect transistors (OFETs), computational methodologies, X-ray reflectivity (XRR) and grazing-incidence X-ray diffraction (GIXRD).
AUTHOR INFORMATION Corresponding Authors *E-mail:
[email protected] *E-mail:
[email protected] ACKNOWLEDGEMENTS Figure 4. a) Transfer characteristics measured in the linear and saturation regime (logarithmic scale), b) Output characteristics, c) Transfer characteristics in the linear and saturation regime (linear and square root scale respectively) and d) Gate-voltage dependence of the linear and saturation mobility for L-C12-DBTTT OFETs (L = 100 µm, W = 700 µm) presenting F4-TCNQ and MoO3 doped contacts. e) Transfer characteristics measured in the saturation regime and f) Gate-voltage dependence of the saturation mobility for C-C6-DBTTT, C-C12-DBTTT and L-C12-DBTTT OFETs (L = 100 µm, W = 700 µm) using MoO3 doped contacts.
Table 1. OFET performances in the linear and saturation regime for the different DBTTT derivatives in devices presenting a bottom-gate top-contact architecture with a 100 μm long and 700 μm wide channel. Material
C-C6DBTTTa C-C12DBTTTa L-C12DBTTTb L-C12DBTTTa aInjection
Regime
μhole (cm2 V−1 s−1)
Vth (V)
Ion/Ioff
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0.030.01
-79.74.0
~ 1*106
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0.020.01
-76.93.8
~ 7*105
Linear
0.0030.001
-74.12.3
~ 4*104
Saturation
0.0020.001
-71.52.3
~ 4*104
Linear
1.130.08
-54.82.9
~ 4*107
Saturation
1.760.12
-57.21.4
~ 7*106
Linear
2.390.05
-29.52.0
~ 2*108
Saturation
2.440.06
-21.90.8
~ 1*108
layer: MoO3. bInjection layer: F4-TCNQ.
In summary, we have designed and synthesized three isomers of thiophene-ring terminated BTBT derivatives with varying isomerism of the alkyl side chain and the aromatic core. Our
H.-Y.C. acknowledges postdoctoral fellowship support from Ministry of Science and Technology in Taiwan. G.S. acknowledges postdoctoral fellowship support from the Wiener-Anspach Foundation and The Leverhulme Trust (Early Career Fellowship supported by the Isaac Newton Trust). We acknowledge the European Synchrotron Radiation Facility for provision of synchrotron radiation facilities and we would like to thank Dr. Raja Znaiguia and Dr. Francesco Carla for assistance in using beamline ID03. K.B. acknowledges financial support from the Institutional Strategy of the University of Tuebingen (Deutsche Forschungsgemeinschaft ZUK63). IM, SC, CJ and ML acknowledge EC FP7 SC2 (610115), EC H2020 (643791) and EPSRC Projects EP/G037515/1 EP/M024873/1 and EP/M005143/1.
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Mobility
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Chemistry of Materials
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C6H13 S S C6H13
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Crystal Packing
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