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Single Crystal X-ray, AFM, NEXAFS, and OFET Studies on Angular Polycyclic Aromatic Silyl-Capped 7,14Bis(ethynyl)dibenzo[b,def ]chrysenes Kerry B. Burke,†,‡ Ying Shu,§ Peter Kemppinen,§ Birendra Singh,§ Mark Bown,*,§ Irving I. Liaw,∥ Rachel M. Williamson,⊥ Lars Thomsen,⊥ Paul Dastoor,‡ Warwick Belcher,‡ Craig Forsyth,⊗ Kevin N. Winzenberg,§ and Gavin E. Collis*,§ †

CSIRO, Energy Technology, P.O. Box 330, Newcastle, NSW 2300, Australia Centre for Organic Electronics, University of Newcastle, Callaghan, NSW 2308, Australia § CSIRO, Materials Science and Engineering Division, Future Manufacturing Flagship, Private Bag 10, Clayton South, Victoria 3169, Australia ∥ School of Chemistry, University of Melbourne, Victoria 3010, Australia ⊥ Australian Synchrotron, Melbourne, Victoria 3168, Australia ⊗ School of Chemistry, Monash University, Melbourne, Victoria 3800, Australia ‡

S Supporting Information *

ABSTRACT: The impact of molecular packing and alignment of 7,14-bis((triethylsilyl)ethynyl)dibenzo[b,def ]chrysene (TES-DBC) and 7,14-bis((triisopropylsilyl)ethynyl)dibenzo[b,def ]chrysene (TIPS-DBC) on OFET performance was investigated. The bulk solid state packing of these angular polycyclic aromatic hydrocarbons (PAHs) was analyzed via single crystal X-ray analysis, and their molecular stacking arrangements on HMDS modified SiO2 substrate were studied using near edge X-ray absorption fine structure spectroscopy (NEXAFS) at the carbon K-edge. Our studies found that TESand TIPS-DBC have significantly different solid state packing arrangements and tilt angles, yet have OFET mobilities that are comparable at 1.6 × 10−3 and 1.0 × 10−3 cm2/Vs, respectively. The deposition method also has a dramatic impact on film morphology.

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INTRODUCTION Organic electronics is an expanding area, which is realizing new opportunities and applications in optoelectronic devices, such as organic field effect transistors (OFETs), organic light emitting diodes (OLEDs), and organic photovoltaics (OPVs).1−3 Devices are comprised of either single or multiple semiconducting components that are typically fabricated in layered (i.e., OFET, OLED, and OPV-bilayer) or blended (OPV-bulk heterojunction) configurations.4 The ability to effectively and efficiently migrate charges through the device structure is fundamental to achieving high performing and stable devices. Polycyclic aromatic hydrocarbon (PAH) based small molecules and conjugated polymers are the current semiconductor materials of choice. Compared with polymers, small molecules offer added advantages in that they are easily accessible, easy to purify by varied techniques, tunable in terms of their physical and electronic properties through facile modification of functional groups, and can be amenable to both evaporative and solution processing methods. © 2011 American Chemical Society

In OFETs, the performance of the device is dependent on the intrinsic charge mobility of the organic semiconductor material, the formation of this material into ordered films with a high degree of π-overlap to facilitate charge migration, and minimal defects at the organic/organic and organic/inorganic interfaces.4 While different OFET architectures5 are available to minimize interface problems and optimize material properties, the formation of an ordered π-overlapping network is highly dependent upon the molecular structure of the organic semiconducting material and the processing conditions used to deposit the material onto the substrate surface. The challenge for researchers is to understand how to design materials that pack well in thin films, with the required alignment, and are processable by solution and evaporation methods. Received: August 6, 2011 Revised: December 11, 2011 Published: December 14, 2011 725

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nonlinear or angular PAH dibenzo[b,def ]chrysene (DBC) framework that exhibits improved chemical stability compared to TIPS-PEN and has shown promising results as a p-type material in a bulk heterojunction solar cell.16 Surprisingly, there is limited information on angular PAH OFET materials, in terms of how they assemble on a substrate surface and how the structure may impact device performance.17 To increase our understanding and further our aims of designing highperforming OFET materials, we undertook studies on the angular PAHs 7,14-bis((triethylsilyl)ethynyl)dibenzo[b,def ]chrysene (TES-DBC) and 7,14-bis((triisopropylsilyl)ethynyl)dibenzo[b,def ]chrysene (TIPS-DBC), encompassing single crystal packing, processing methods, AFM, near edge X-ray absorption fine structure (NEXAFS) thin film analysis, and OFET mobilities.

To date, some of the best performing PAH small molecule OFETs have been produced using linear acenes and heteroacenes.6,7 The single crystal structure, thin film morphology, and OFET properties of nonfunctionalized and functionalized pentacene and heterocyclic pentacene derivatives have been studied extensively.8,9 Although not a proven model, analysis of the single crystal X-ray data does provide a good indication of whether an organic material may have the necessary packing to facilitate charge transport in thin films. Single X-ray crystal data for pentacene (PEN) show that the material adopts a herringbone packing motif in the solid state,10,11 whereas thin film studies indicate that PEN adopts a slightly different tilt-free herringbone motif that gives rise to moderate mobilities in transistor devices.12 In the case of 6,13bis-((triisopropylsilyl)ethynyl)pentacene (TIPS-PEN) (Figure

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EXPERIMENTAL SECTION

TES-DBC and TIPS-DBC were synthesized according to previous literature methods.16 Crystals suitable for X-ray structure analysis of TIPS-DBC were obtained from chloroform/light petroleum ether, while TES-DBC crystals were grown from toluene. Data for X-ray structure determination was collected on a Bruker X8 Apex CCD diffractometer using graphite monochromator Mo Kα radiation at a temperature of 123 K. The structures of both compounds were solved by direct methods and refined using SHELXL-97.18 A doped (N ≈ 3 × 1017 cm−3) silicon wafer was used as the substrate and as the gate electrode. Discrete bottom contact OFETs were fabricated on thermally grown silicon dioxide (230 nm). Interdigitated source and drain were photolithographically patterned from a 50 nm sputtered gold layer. The substrate was first cleaned with acetone, 2-propanol, and then UV ozone treated. Hexamethyldisilazane (HMDS) primer was then immediately applied, by spin coating at 4000 rpm, followed by heating at 115 °C for 5 min. The sample was then immersed in 15 mM pentafluorobenzenethiol (PFBT) in ethanol for 40−60 min and sonicated in neat ethanol before being blow-dried with N2. The active organic layer was deposited by solution deposition or thermal evaporation. Solution processing was completed by spin coating the semiconductor layer at 1500 rpm in air from solution (15 mg/mL) in chloroform. The evaporated films of DBC were obtained by vacuum sublimation at a base pressure of 1 × 10−6 mbar with substrate temperature controlled at 25 °C. Recorded film thickness was 50 nm. Samples for NEXAFS analysis were prepared following the same process but on doped SiO2 substrates without gold contacts. OFET measurements were performed under an inert atmosphere (N2) in a glovebox. Carbon K-edge NEXAFS spectroscopy was performed in an ultra high vacuum (UHV) endstation attached to the Soft X-ray Spectroscopy Beamline at the Australian Synchrotron.19 The UHV chamber, which had a base pressure lower than 2 × 10−10 mbar, was equipped with a SPECS Phoibos 150 hemispherical electron energy analyzer. This analyzer allowed for the detection of carbon K-edge photoelectrons in auger electron yield (AEY) mode by setting its kinetic energy to 230 eV. The AEY signal is surface sensitive to within approximately 1 nm,20 and as such, the molecular conformation obtained from the NEXAFS experiments will reflect the tilt angle of the molecules of the top surface layers. NEXAFS spectra were recorded at angles of 20°, 30°, 40°, 50°, 55°, 60°, 70°, 80°, and 90°, measured between the direction vector of the incident light and the surface plane of the sample. All spectra acquired were normalized following the procedures discussed by Watts et al.21 To allow for the effects of synchrotron radiation beam damage on the thin films, a sequence of scans was measured on the same spot for a significant length of time until significant changes were observed in the Carbon K-edge spectra. The scan time was subsequently restricted to an appropriate interval in which beam damage during a single scan was negligible. Furthermore, between each angle change the beam was moved to a fresh sample area before data collection.

Figure 1. Chemical structures of TES-PEN, TIPS-PEN, TES-DBC, and TIPS-DBC.

1), the bulk single crystals and thin film molecular structures are nearly identical and show a 2D brickwork packing that favors high OFET mobilities.13 For both PEN and TIPS-PEN, the molecular alignment on the device substrate has a direct correlation with OFET device performance; semiconductor materials that display an edge-on orientation to the substrate, parallel with respect to source and drain electrodes have higher OFET mobilities than those that adopt a face-on orientation.14 These linearly fused PAH systems have some of the highest mobilities (1−5 cm2/Vs) reported for small molecules. The incorporation of solubilizing groups, as in TIPS-PEN, has improved the stability and solution processability of these materials. However, the linear structure of acenes, consisting of repeat units of butadiene subunits, makes these materials only moderately stable and susceptible to degradative chemical reactions.15 We have recently developed a solution processable 726

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Figure 2. Optical microscopy images of crystals grown from (a) TIPS-DBC by slow diffusion from chloroform/petroleum ether, (b) TES-DBC by slow diffusion from chloroform/petroleum ether, and (c) TES-DBC by slow evaporation from toluene. AFM topographic maps were performed directly on the active layer of the OFETs using an Asylum Research MFP-3D-SA instrument in tapping mode. Optical microscopy images were taken with a Nikon InfinityX camera at 0.7× magnification.

packing of these two materials in single crystal X-ray crystallography studies has changed going from the TES to the TIPS end capping group (Figure 3).

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RESULTS AND DISCUSSION Crystals of TIPS-DBC suitable for X-ray structure analysis were easily obtained from chloroform/light petroleum via slow diffusion (Figure 2a). However, attempts to grow suitable crystals of TES-DBC also from chloroform/light petroleum surprisingly gave two visibly different crystals (Figure 2b) that could not be unambiguously resolved by X-ray diffraction. The appearance of two distinct crystal forms of TES-DBC would suggest the presence of polymorphs. Suitable crystals of TESDBC were grown by slow evaporation from toluene (Figure 2c). TES-DBC and TIPS-DBC vary by having different end capping groups; triethylsilyl (TES) and triisopropylsilyl (TIPS), respectively. The key crystallographic parameters for TES-DBC and TIPS-DBC are given in Table 1. Changing the capping Table 1. Crystal Data for TES-DBC and TIPS-DBC TES-DBC

TIPS-DBC

prism, red C40H42Si2 Mr 578.92 monoclinic, P21/c a = 9.4638 (18) Å b = 13.389 (2) Å c = 13.149 (3) Å α = 90.00° β = 97.521 (7)° γ = 90.00° V = 1651.8 (5) Å3 Z=2 Dx = 1.164 Mg m−3 R1 = 0.0417 wR = 0.1241

plate, orange C46H54Si2 Mr 663.07 triclinic, P1̅ a = 8.0322 (4) Å b = 8.4306 (5) Å c = 16.3624 (10) Å α = 88.660 (3)° β = 81.815 (3)° γ = 61.977 (2)° V = 966.93 (10) Å3 Z=1 Dx = 1.139 Mg m−3 R1 = 0.0749 wR = 0.1355

Figure 3. (a) TES-DBC side view along long axis; (b) TIPS-DBC side view along long axis; (c) TES-DBC side view along short axis; (d) TIPS-DBC side view along short axis. Top view of (e) TES-DBC and (f) TIPS-DBC to illustrate π-overlap between adjacent molecules in the same stack. Trialkylsilyl groups have been omitted for clarity.

TIPS-DBC adopts a 1D slipped stacked packing arrangement (Figure 3b) and TES-DBC adopts a face-to-edge herringbone packing motif with minimal π-overlap (Figure 3a). The TIPSderivative exhibits close π−π contacts between adjacent stacks with the closest aromatic carbon−carbon contact of 3.396 Å and an interstack distance of ∼3.2 Å, well within that of C−C van der Waals radii (∼3.4 Å). The closest aromatic C−C contact distance denotes the closest distance between carbon atoms in the conjugated DBC cores of closest neighboring molecules. The interstack distance refers to the closest absolute distance between the aromatic planes of neighboring molecules. The TES-derivative has a deceptively close interstack distance of ∼3.2 Å. However, the closest aromatic contacts between neighboring TES-DBC molecules are solely on the ends of the molecules; this distance is larger, with a close contact distance

group from triethylsilyl to triisopropylsilyl has little impact on the alkyne carbon−carbon bond length: 1.209(2) Å for TESDBC and 1.201(3) Å for TIPS-DBC. The C−Si bond length is similarly unaffected; 1.848(1) Å for TES-DBC versus 1.844(3) Å for TIPS-DBC. The silyl groups of both TES-DBC and TIPS-DBC are noncoplanar with the DBC core; the increase in steric bulk of TIPS-DBC corresponding to an increase in displacement of the Si atom from the plane of the DBC core (0.449 Å c.f. 0.250 Å for TES-DBC). The TES and TIPS groups have roughly spherical diameters of 6.6 Å and 7.5 Å, respectively. Consistent with similar studies on the trialkylsilylacetylene pentacene series, we observe here that the solid state 727

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of 3.504 Å (Figure 3c). There is considerably more π−π overlap in TIPS-DBC than TES-DBC according to solid state data gleaned from single crystal X-ray crystallographic studies. In addition to crystal packing, the subtle changes in the end capping groups also have an impact on the solubility and thus the solution processability of these materials. We were interested to see if changes to the end capping group would also have an impact on how these materials align themselves on the substrate surface and influence device performance. All the OFET devices were fabricated with a bottom contact, bottom gate configuration on doped silicon dioxide wafer with W = 10 mm and L = 10 μm (W/L ratio of 1000). Charge carrier mobilities were extracted from the regime where drain current, ID, followed square root dependence on applied gate voltage, VG. From the respective slopes, δ√ID/δVG, and device parameters, L and W, capacitance per unit area, Ci of 10 nF/ cm2, the saturated field effect mobility μ was calculated using eq 1 below

μe,h = (2L /WC i)(δ I D /δVG)2

Table 2. Comparison of Tilt Angles and OFET Mobilities of TES-DBC and TIPS-DBC Deposited by Spin Coating and Evaporation deposition method tilt angle OFET hole mobility (cm2/Vs)

TES-DBC

TIPS-DBC

TIPS-DBC

solution 70° 1.6 × 10−3

solution 61° 1 × 10−3

evaporation 73° 8 × 10−6

root-mean-squared (rms) roughness of ∼7 nm and large domains on the order of 400 nm (Figure 5a). In contrast, TIPSDBC solution deposited films are smoother (rms ≈ 2−3 nm) with smaller domains of around 200 nm (Figure 5b). Interestingly, the film surface morphology of TIPS-DBC is significantly different depending on whether it is deposited by evaporation or solution processed. The evaporated films show a different surface morphology with slightly rougher films (rms ≈ 3−4 nm) with fibrillar features of about 50 nm in width and several microns in length (Figure 5c). NEXAFS is a well established technique for quantifying molecular alignment at film surfaces and makes use of incident polarized X-rays to excite electrons from the core shell states to molecular orbitals that are usually π* or σ* in nature. The dipole of these transitions give rise to an angular dependence of the transitions’ absorption cross-section, the alignment of which can be quantified by collecting NEXAFS spectra with the incident X-ray beam at varying angles to the substrate surface.20 Figure 6 shows the structure of TES-DBC along with some important features and orbitals that generate key attributes in a NEXAFS scan. These important features and orbitals will be highly similar for TIPS-DBC. As shown in Figure 6, the pz orbitals of the many CC π* bonds of the PAH template of both TES- and TIPS-DBC are all aligned perpendicularly to the plane of the ring structure. As such any preferential alignment of the TES- and TIPS-DBC PAH template near the surface of a thin film will be revealed by an angular dependent NEXAFS spectrum in the π* region (∼284−287 eV). Attached to the conjugated core ring structure are two alkyne bonds, which also have π* character. The central ring structure and the triple bonds both have C−C σ* bonds aligned in the same plane as the ring structure, while the C−C σ* bonds in the silyl-groups will have no strong alignment. The C−Si σ* bonds and the C−H σ* bonds are all highly unaligned because of the rotational conformers present. Shown in Figure 7 is the normalized set of NEXAFS spectra of TES-DBC deposited by solution methods. The π* region of the NEXAFS spectra (∼284−287 eV)23 is distinct from the σ* region and step-edge. The π* region is highly featured showing strong angular dependence. By contrast, the higher energy region (>287 eV) shows little angular dependence and broader features. The slight reverse angular dependence at ∼305 eV is consistent with the C−C σ* bonds in the core being orthogonal to the pz orbitals of the CC π* bonds. Because of the weak alignment of the C−H and C−C σ* regions, the C−C π* region was analyzed in more detail to determine the overall molecular alignment. A truncated section of NEXAFS spectra showing the π* features is shown in the inset on the upper right corner of Figure 7 with Gaussian fits. As can be observed from Figure 7, the shape of the C−C π* region changes with the angle of incidence of the X-ray beam. The spectra for TIPS-DBC (solution deposited) were very similar to those of TES-DBC, as expected by their chemical similarity. Six Gaussian peaks were fitted to the π* region of

(1)

The output and transfer curves for solution processed TESDBC are represented in Figure 4. Other output and transfer

Figure 4. Output (I/V) (top) and transfer curves (bottom) for OFETs with spin-cast TES-DBC on HMDS/PFBT treated devices.

curves for OFET devices of TIPS-DBC (solution) and TIPSDBC (evaporation) are presented in the Supporting Information. From these curves and eq 2, the hole mobility for each of these devices, TES-DBC (solution), TIPS-DBC (solution), and TIPS-DBC (evaporation), were determined and are presented in Table 2 below. The films used in the NEXAFS study were initially characterized by tapping mode atomic force microscopy (AFM). Images of film profiles show well ordered, uniform, polycrystalline films. TES-DBC solution deposited films have a 728

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Figure 5. AFM profiles of thin films of TES- and TIPS-DBC deposited on HMDS treated SiO2. (a) TES-DBC deposited via solution spin casting from chloroform, (b) TIPS-DBC deposited by solution spin casting from chloroform, and (c) TIPS-DBC deposited by evaporation. Each image is 5 × 5 μm.

is the average tilt angle of the central ring structure, and θ is the angle of the NEXAFS scan.

I∝1+

1 (3 cos2 θ − 1)(3 cos2 α − 1) 2

(2)

The two largest peaks at 284.4 and 285.6 eV as well as the small peak at 283.6 eV showed a very similar alignment. The peaks at 284.8 and 285.1 showed very little alignment, and the peak at 285.5 eV showed a small amount of alignment in the direction orthogonal to the two largest peaks. Because of the fact that the two largest peaks represent the majority of the peak area and show the same angular dependence, we have assigned these bonds to the central aromatic ring structure, which has the largest number of C−C π* bonds in the molecule. To our knowledge, no NEXAFS studies have been performed on dibenzo[b,def ]chrysene previously. However, the assignment of the two largest peaks to the conjugated ring structure is supported by NEXAFS studies of fused benzene ring PAH templates such as naphthalene, anthracene, tetracene, pentacene,24 chrysene, perylene, and coronene,25 which all exhibit a characteristic double peak in the C−C π*region. The peak at 285.5 eV showing a small amount of alignment orthogonal to the larger peaks is likely associated with the alkyne bonds. Normally, alkyne bonds in symmetric molecules such as acetylene have rotational symmetry about the bond axis.20 However, in a case such as phenylacetylene where the rotational symmetry is broken, the conjugation extends onto the triple bond.23 The local chemical structure of phenylacetylene is replicated in the dibenzochrysenes studied in this work with pz orbitals from the alkyne bond participating in the molecular conjugation. The px orbitals from the alkyne bond form their own orbital with an angular dependence orthogonal to the conjugated orbital’s angular dependence and in the same plane as the ring structure. Furthermore, the large peaks at 284.4 and 285.6 eV are not likely to be due to C−H or C−C σ* bonds as all literature reports found that mentioned peak assignment assigned C−C or C−H σ* to peaks higher than 286 eV.20,22,23,26,27 Hence, we can reliably determine the overall alignment of TES-DBC to be 71° from the angular dependence of the two peaks at 284.4 and 285.6 eV. For TIPS-DBC, NEXAFS was collected on both a spin coated and an evaporated film to establish whether the deposition method influences the alignment of the molecule on the film surface. The determined molecular alignment of TIPS-DBC deposited by evaporation was 73°, whereas the alignment of solution processed TIPS-DBC was 61° (Table 2).

Figure 6. Alkane, alkene, and alkyne bonds and orbitals found in TESDBC.

Figure 7. NEXAFS spectra for the TES-DBC sample solution deposited. The color order blue-green steps through 20°, 30°, 40°, 50°, 55°, 60°, 70°, 80°, and 90°, respectively (measured as the angle between the incident linear polarized light and the surface). The CC π*, C−H σ*, and C−C σ* regions have been indicated on the figure along with a NEXAFS step edge and a highlighted peak in the π* region, which are used to determine the orientation of the conjugated ring structure with respect to the surface. The inset shows the π* region up to 286 eV with fitted Gaussian peaks.

both TES-DBC and TIPS-DBC at energies of 283.6, 284.4, 284.8, 285.1, 285.5, and 285.6 eV (see Supporting Information). This was the minimum number of peaks that was able to adequately fit the data. The peak width and peak position were constrained to be the same across all nine angles in the fitting process but were not constrained between the two molecules. To each peak, we have fitted the relationship (eq 2),20 where α 729

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Linear acenes with fewer π−π stacking interactions typically have lower FET mobilities when compared with molecular analogues with greater π−π overlap.7 For example, single crystal X-ray studies for TES-PEN and TIPS-PEN have shown that the former exhibits 1D packing with moderate π−π overlap, whereas the latter shows 2D brickwork packing with high π−π overlap. Device measurements performed on these compounds are consistent with the crystal packing; TES-PEN shows only moderate mobilities (0.001 cm2/Vs), whereas TIPS-PEN achieves considerably higher values (0.4−1.8 cm2/Vs). It was, therefore, surprising that the TES- and TIPS-DBC analogues, when deposited onto a surface via spin coating, produced such similar FET mobilities (Table 2) when tested in devices despite their significantly different packing motifs in the single crystal state. The similar mobilities of solution processed TES-DBC and TIPS-DBC may suggest that the different packing motifs observed in the bulk single crystal studies may not hold true in the thin film state with alternative crystallographic polymorphs, and consequently, different packing motifs predominate. Polymorphs have been identified with TIPS-difluoroanthradithiophene and connected with deposition processing conditions.28 More enlightening is the difference in tilt angles for the spin cast films; TES-DBC and TIPS-DBC had tilt angles of 70° and 61°, respectively. The NEXAFS data would suggest that the silyl-capping group has a substantial impact on the alignment of the DBC structure near the surface of the thin film. The investigation into the impact of the deposition method of TIPS-DBC on tilt angle and transistor mobility measurements found some significant differences (Table 2). When TIPS-DBC was deposited onto a surface via evaporation rather than by spin casting, we observe a dramatic increase in the average molecular tilt angle from 61° to 73°. This difference in tilt angle was also reflected in a significant decrease in mobility by over 3 orders of magnitude. However, further work is needed to understand whether this decrease in mobility is directly related to the alignment of the molecule at the substrate surface, poor film morphology and connectivity, or a combination of these factors. Modeling of the NEXAFS data may also provide a better understanding of the band structures observed in DBCs that seem different from linear acenes and may assist in the design of materials with better crystal packing and film morphology.

hypothesis is further supported by a difference in the average molecular tilt angles observed for solution cast TES- and TIPSDBC as generated by the NEXAFS studies. The data shows the DBC core of TES- and TIPS-derivatives are aligned at 70° and 61° to the substrate surface, respectively. The change in film morphology is further influenced by the deposition method and is reflected in the AFM images, NEXAFS, and OFET data. TIPS-DBC deposited by solution methods produces mobilities that are far superior to evaporation methods by an order of three magnitudes when measured in OFET devices and as observed by AFM images. This significant variation in device performance is identified in the NEXAFS data, which shows a significant shift in tilt angles between the two different deposition methods.

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ASSOCIATED CONTENT

S Supporting Information *

Additional crystallographic information on TES-DBC and TIPS-DBC X-ray data (CIF files) and detailed NEXAFS and OFET data for TES-DBC and TIPS-DBC are included. 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] (M.B.); [email protected] (G.E.C.).

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ACKNOWLEDGMENTS We gratefully acknowledge support from the Future Manufacturing Flagship and Flexible Electronics Theme at the Materials Science and Engineering Division, CSIRO, Clayton, Melbourne. K.B. thanks CSIRO for financial support and mentoring and the University of Newcastle for their Ph.D. program. Y.S. thanks CSIRO for an OCE Postdoctoral Fellowship, and G.E.C. acknowledges the CSIRO OCE Julius Award. This NEXAFS research was undertaken on Soft X-ray beamline at the Australian Synchrotron, Victoria, Australia.

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REFERENCES

(1) Shirota, Y.; Kageyama, H. Chem. Rev. 2007, 107, 953−1010. (2) Pron, A.; Gawrys, P.; Zagorska, M.; Djurado, D.; Demadrille, R. Chem. Soc. Rev. 2010, 39, 2577−2632. (3) Arias, A. C.; MacKenzie, J. D.; McCulloch, I.; Rivnay, J.; Salleo, A. Chem. Rev. 2010, 110, 3−24. (4) Ma, H.; Yip, H.-L.; Huang, F.; Jen, A. K.-Y. Adv. Funct. Mater 2010, 20, 1371−1388. (5) Cosseddu, P.; Bonfiglio, A. Appl. Phys. Lett. 2010, 97, 203305(1− 3). (6) Anthony, J. E. Chem. Rev. 2006, 106, 5028−5048. (7) Anthony, J. E. Angew. Chem., Int. Ed. 2008, 47, 452−483. (8) Pedio, M.; Doyle, B.; Mahne, N.; Giglia, A.; Borgatti, F.; Nannarone, S.; Henze, S. K. M.; Temirov, R.; Tautz, F. S.; Casalis, L.; Hudej, R.; Danisman, M. F.; Nickel, B. Appl. Surf. Sci. 2007, 254, 103− 107. (9) Yang, H.; Shin, T. J.; Ling, M.-M.; Cho, K.; Ryu, C. Y.; Bao, Z. J. Am. Chem. Soc. 2005, 127, 11542−11543. (10) Mattheus, C. C.; Dros, A. B.; Baas, J.; Meetsma, A.; de Boer, J. L.; Palstra, T. T. M. Acta Crystallogr. 2001, C57, 939−941. (11) 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. (12) Mannsfeld, S. C. B.; Virkar, A.; Reese, C.; Toney, M. F.; Bao, Z. Adv. Mater. 2009, 21, 2294−2298.

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CONCLUSIONS NEXAFS has been shown to be a useful diagnostic tool to interpret the alignment of these angular PAHs at the surfaces of thin films for correlation with single crystal X-ray, AFM images, and OFET device data. Current single crystal X-ray data suggest that by selecting the appropriate capping group, it is possible to control packing and achieve good π-overlap. However, unlike the linear acenes, the angular PAH system may be more susceptible to perturbation by the capping group. TIPS-DBC shows a 1D packing confirmation, whereas TESDBC has a significantly disordered herringbone structure and shows potential to form other crystal packing structures. The OFET data for TES- and TIPS-DBC deposited by solution methods show very similar mobilities (10−3 cm2/Vs), which is surprising considering the significant difference in crystal packing determined from the single crystal X-ray results. This implies that the molecular packing in the thin films may vary substantially from the crystal packing observed in the single crystal X-ray structures for TES-DBC and TIPS-DBC. This 730

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dx.doi.org/10.1021/cg201020w | Cryst. Growth Des. 2012, 12, 725−731