Phase-Selective Crystallization of Perylene on Monolayer Templates

Article pubs.acs.org/crystal

Phase-Selective Crystallization of Perylene on Monolayer Templates Jessica H. Urbelis and Jennifer A. Swift* Department of Chemistry, Georgetown University, Washington, D.C. 20057-1227, United States S Supporting Information *

ABSTRACT: The planar aromatic hydrocarbon perylene serves as the core unit in many photoconductive and electronic materials. Perylene crystallizes in two polymorphic forms, α and β, which grow concomitantly from many solvents. Crystallization in the presence of a small library of gold− thiol self-assembled monolayers (SAMs) and functionalized siloxanes identified conditions under which phase pure material can be obtained. Au-biphenylthiol SAM templates nucleated phase pure α-perylene with preferred alignment along the α-(100) plane. Siloxanes with terminal amino functionalities proved to be effective templates for the generation of phase pure β-perylene aligned along β-(100). Phase mixtures grew on most other templates examined. Comparison of the morphologies and nucleation densities of crystals grown under various conditions suggests that the phase selectivity observed on these templates is also sensitive to solvent choice and solute concentration.

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INTRODUCTION

In electronic and optoelectronic applications where flexibility and low-cost are important, organic semiconductor materials (OSCs)1 offer many advantages over inorganic materials. The structure of OSCs can be more readily modified through synthesis, and crystalline forms are commonly grown either via lower temperature vapor deposition or solution-based processing methods. The anisotropic nature of the intermolecular interactions in organic molecular solids means that charge mobility is direction-dependent.2 Controlling the size, shape, and orientation of OSC single crystals during device fabrication is therefore an important consideration for improving their electrical and optical properties.3,4 Perylene is a good prototype molecule for solution-based crystallization studies given its relatively small size which conveys greater solubility in conventional organic solvents than many other polyaromatic hydrocarbons. Functionalized perylenes,5−7 pentacenes,8 and tetracenes9 are among the highest performing OSCs. The fluorescent properties of perylene also impart it and its derivatives with a wide variety of uses ranging from organic light emitting diodes (OLEDs)10−14 to field effect transistors (OFETs)15−20 to biological staining and sensor applications.21−24 Unsubstituted perylene, like many organic molecular crystals, is known to be polymorphic.25 The structure of α-perylene was first reported in 195326 and has been redetermined multiple times at room temperature27−29 and at low temperature (PERLEN05, P21/c: a = 10.239, b = 10.786, c = 11.132, β = 100.92°).25 The β-perylene structure has also been reported multiple times at room temperature30,31 and low temperature (PERLEN06, P21/c: a = 9.763, b = 5.843, c = 10.608, β = 96.77°).25,32 The thermodynamically stable α-phase crystallizes in what Desiraju and Gavezzotti33 previously referred to as a © 2014 American Chemical Society

“sandwich herringbone” structure where a pair of molecules is arranged about a center of symmetry (Z′ = 1) (Figure 1). The β-phase adopts a “flattened out herringbone” (a.k.a. γstructure33) in which a molecule sits on a center of symmetry

Figure 1. Packing diagrams of α (left) and β (right) perylene constructed from fractional coordinates in PERLEN05 and PERLEN06, respectively. Received: July 9, 2014 Revised: August 12, 2014 Published: September 4, 2014 5244

dx.doi.org/10.1021/cg501030g | Cryst. Growth Des. 2014, 14, 5244−5251

Crystal Growth & Design

Article

(Z′ = 0.5). Sublimation typically yields crystals of α, but β has been reported from solution conditions requiring high temperature and saturation levels and is sometimes found as a minor product in α growth.34 Important in any device fabrication via solution is the need to control both the phase and orientation of crystals on surfaces. Previous reports of perylene and its derivatives have involved growth on glass, metal, and metal oxide surfaces35−48 and Authiol49−54 and Ru-thiol55 self-assembled monolayers (SAMs), but only as vacuum-deposited thin films. Crystal growth from solution allows for fewer crystal defects, new morphologies, as well as opportunities for stabilizing phases through heterogeneous nucleation driven by interactions at the crystal/ monolayer interface. In the current work, we examine perylene growth via template-directed nucleation on a library of Au−S and siloxane monolayers and compare the phase purity and crystal morphologies obtained against material grown under more conventional solution methods.

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and oxidized in piranha solution (70% H2SO4, 30% H2O2) for 30 min/ side, thoroughly rinsed with distilled water, and dried under a stream of N2 for 20 min. The glass substrates were then submerged in a Xtrimethoxysilane in toluene solution (12 mmol, 4 mL) and heated in a dry bath (80 °C) for 135 min. The templates were removed from the silane solution, cooled, and briefly stirred in pure toluene at RT. They were sonicated in methanol for 20 min/side and soaked in the same solution for an additional 12 h at RT. Finally, templates were dried for 10 min under N2 and cured in the oven (110 °C) for 2 h. Silanes (10− 14) used in this study are shown in Figure 2. Contact Angle. Contact angle measurements on each Au−S and siloxane template were obtained using the half angle measuring method on a CAM-PLUS MICRO contact angle meter (Tantec, Inc., Schaumburg, IL).65 Contact angle measurements for all monolayers fell within the following ranges: bare gold 90−94°; (1) 96−98°; (2) 29−33°; (3) 39−41°; (4) 48−51°; (5) 78−82°; (6) 74−78°; (7) 72− 76°; (8) 60−64°; (9) 78−81°; piranha cleaned glass 0−5°; (10) 89− 92°; (11) 37−40°; (12) 59−62°; (13) 44−48°; (14) 49−52°. Additional characterization of these surfaces has been described elsewhere.57,58 Crystal Growth. Saturated perylene solutions (0.1−1.0 mg/mL) were prepared in benzene, toluene, THF, acetone, hexanes, acetonitrile, pyridine, dichloromethane, and N,N-dimethylformamide (DMF). For solution growth experiments, the solution was placed in small 1 dram glass vials and covered with parafilm with small holes to allow for slow evaporation at RT. In template-directed experiments, monolayers were added to the solution-filled vials so that they were fully submerged in solution and slightly angled against the vial walls. Templates were removed from solution before full solvent evaporation. Hot Stage Microscopy. Melting points and phase transitions of perylene crystals were examined using a HCS302 optical hot stage (INSTEC, Inc., Boulder, CO) on an Olympus BX50 polarizing microscope. Samples were heated over the temperature range of 25− 300 °C at a ramping rate of 10 °C/min. The hot stage was calibrated against triphenylphosphine (mp 79−80 °C; Aldrich, 99%), 3aminophenol (mp 120−122 °C; Aldrich, 98%), triphenylphosphine oxide (mp 152−153 °C; Aldrich, 98%), and L-cysteine (decomposes 220 °C; Aldrich 97%). Atomic Force Microscopy (AFM). A Vecco multimode IIIA atomic force microscope was used to measure step heights and thickness of plates grown on monolayers (8) and (13) using contact mode in air. Samples were dried under N2 prior to analysis to remove residual solvent droplets. X-ray Diffraction. Single crystal unit cells of both solution and template-grown crystals were obtained on a Bruker SMART single crystal diffractometer with an APEX II CCD detector and Mo Kα source (λ = 0.7107 Å). Miller indices were assigned to the faces of several representative single crystals using X-ray goniometry based on videos taken over a 360° rotation in phi in conjunction with predetermined unit cells. WinXMorph59 software was used to create the morphology diagrams. Powder X-ray diffraction (PXRD) data were obtained using a Rigaku R-AXIS RAPID curved imaging plate multipurpose diffractometer with Cu Kα source (λ = 1.5418 Å) and analyzed with Jade v9.0 software. Oriented PXRD of crystals grown on siloxane surfaces was collected on a Rigaku Mini Flex II desktop X-ray diffractometer with measurement monitor v.1.0.0.0 and analyzed with PDXL software. Powder data were compared against simulated patterns of room temperature structures of α (PERLEN04) 2 9 and β(PERLEN06).25 Calculations. Gas phase Bravais, Friedel, Donnay and Harker (BFDH) crystal morphologies were calculated in Mercury 3.1 for α (PERLEN05) and β (PERLEN06). The 2D cell dimensions of the biphenyl monolayers60 were taken as 4.8 × 10.0, α = 90°. Nucleation densities on siloxane templates were calculated by counting the number of crystals within a (1 mm)2 area on one side of the template (crystals grow on both sides but are easily counted by focusing the microscope). Densities are reported as crystals/mm2 based on averages

EXPERIMENTAL SECTION

Materials. Toluene, dichloromethane, benzene, tetrahydrofuran (THF), acetone, and methanol were obtained from Sigma-Aldrich. Ethyl acetate and 1-propanol were obtained from Fischer. All recrystallization solvents and starting materials were reagent grade or higher. Alkanethiols 1-dodecanethiol (1), 11-mercapto-1-undecanol (2), and 11-mercaptodecanoic acid (3), and arene thiols 4mercaptobenzoic acid (4), 3-nitrobenzenethiol (5), and 4-nitrobenzenethiol (6) were purchased from Sigma-Aldrich and used without further purification. Biphenyl thiols 4-mercaptobiphenyl (7), 4′-nitro-4-mercaptobiphenyl (8), and 4′-iodo-4-mercaptobiphenyl (9) were synthesized according to the literature.56,57 N-Propyltrimethoxysilane (10), 2-phenethyltrimethoxysilane (11), 3-bromotrimethoxysilane (12), 3-aminopropyltrimethoxysilane (13), and 3-cyanotrimethoxysilane (14) were purchased from Gelest (>95% purity) and used without purification. High-purity perylene samples were obtained from Dr. Joseph Melinger, U.S. Naval Research Laboratory. Gold−Thiol Monolayer Preparation. Freshly cleaved mica squares (18 mm2) were sputtered with 30 Å of chromium at a rate of 0.8 Å/s followed by 1500 Å of Au at a rate of 1.5 Å/s. Gold substrates were cleaned in piranha solution (70% H2SO4, 30% H2O2) for 1 min, rinsed with distilled water and 200 proof ethanol and dried under N2. Substrates were then placed in amber vials of ethanolic thiol solution (2 mmol, 10 mL) and stored at RT for 24 h. After thiol assembly, all templates were rinsed with 200 proof ethanol and fully dried under N2. Thiols (1−9) used in the study are shown in Figure 2. Siloxane Monolayer Preparation. Gold Seal No. 1 rectangular glass coverslips (12 × 24 mm, Electron Microscopy Sciences) were used as substrates, and cut into 1 cm2 squares. Substrates were cleaned

Figure 2. Thiols (1−9) and silanes (10−14) used in the preparation of Au−S and siloxane monolayers, respectively. 5245



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Figure 3. Perylene single crystal morphologies obtained from various solvents depicted using Winmorph.59 Calculated BFDH morphologies of (a) α- and (b) β-perylene are compared against α-perylene from (c) benzene, (d) acetone, (e, f) toluene; and β-perylene from (g) toluene and (h) THF. of three sections of each template and three templates in the same solution.

The morphologies of the solution grown crystals ranged from plates to prisms, and elongated prisms and needles in the case of THF. Plates of both α and β grown from toluene had large (100) faces, as well as (011) and (1̅11) faces, similar to the prominent faces predicted by BFDH. However, the large faces observed on crystals grown from other solvents often differed from the predictions. Miller indices were assigned to several representative single crystals using X-ray goniometry (Figure 3c−h). Crystals grown from hexanes exhibited unusual needle-like morphologies, while THF gave a mixture of plates and needles. The β-needles grown from THF had prominent (102̅) faces which was a small face in the calculated morphology. The large (101) and (101̅) faces observed on αcrystals from benzene and acetone are unique and not among the predicted faces. The α prism grown from low concentrations in toluene (Figure 3f) was the only crystal to not exhibit the low energy (100) face. Heterogeneous Nucleation and Growth on Surfaces. Nucleation on surfaces provides a means to control both the crystal phase and the growth orientation through heterogeneous interactions at the crystal−substrate interface. When considering the orientation of aromatic perylene molecules crystallizing on a surface, the π−π stacking of the electron clouds could orient perpendicular, parallel, or at some intermediate angle. The orientation of the π stacking is important because it dictates the direction with the largest charge mobility as well as the aspect ratio of the crystals.61 Similarly, the phase and/or phase purity are expected to have a profound effect on the electrical properties. The electron mobility of the α phase (μelectron = 67.2 cm2/V·s) is about three times higher than for β (μelectron = 20.4 cm2/V·s).62 A better understanding of the substrate/crystal interface may enable the growth orientation to be rationally controlled for desired applications.

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RESULTS AND DISCUSSION Crystal Growth from Solution. The calculated BFDH morphology of α-perylene is a prism bounded by large (100) faces, medium-sized (011) and (110) faces, and small (1̅11), (102̅), and (002) faces (Figure 3a,b). This is consistent with vacuum deposited thin film growth studies which show the lowest surface energy to be the densely packed (100) surface.54 On this surface, the aromatic plane of perylene molecules are oriented edge-on (see Figure 1). The calculated β-perylene morphology is a hexagonal prism with a large (100) face, medium-sized (011) and (002) faces, and small (110), (111)̅ , (1̅11), and (102̅) faces. Most previous growth studies focus on α-perylene, which crystallizes as rectangular plates by sublimation and/or solution growth. Though generally harder to grow selectively, β-perylene crystals obtained from fast cooling of toluene were rhomboidal in shape.34 Perylene single crystals were grown by RT slow evaporation from a variety of solvents in order to identify optimal solution conditions and to serve as point of comparison for heterogeneous nucleation studies on 2D templates. Solvent choice had an obvious effect on the observed crystal size. In general, growth from acetone, benzene, and toluene yielded the largest single crystals of varying plate-like habits ranging from ∼900−6000 μm2, while dichloromethane produced smaller (100−300 μm2) plates and needles (Figure S1, Supporting Information). PXRD analysis showed that recrystallization from acetone, benzene, and toluene most consistently yielded welldefined crystals of α with small amounts of β impurities. Recrystallization in methanol, acetonitrile, THF, and hexanes yielded fewer single crystals, aggregates of crystals, and occasionally gels. 5246



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All single crystals on 7−9 were α-perylene, and regardless of the macroscopic morphology observed, the contacting plane was always (100). Previous studies of perylene thin films vacuum deposited on bare Au surfaces54,65 showed that metal-π interfacial interactions result in the parallel alignment of perylene’s aromatic plane with respect to the surface. In thicker perylene films, molecule−molecule interactions become dominant over molecule−surface interactions, and the α-perylene molecules typically stand upright with respect to the surface.46 In the assembly of biphenylthiol monolayers, strong Au−S interactions and van der Waals interactions between the aryl substituents direct the aromatic planes to align orthogonal to the surface.66,67 Aromatic monolayers are also of interest for use in molecular electronics because of their higher conductivity over alkanethiols.68 The degree of order on these surfaces is enhanced by increasing the number of aromatic rings in the backbone;69 more ordered monolayers also exhibit higher electron transfer efficiency for electrode applications. Epitaxial relationships between the biphenyl SAM and the (100) plane of α-perylene also likely contribute to this preferred orientation. Although phase-pure α-perylene appears to nucleate in the center of SAMs (7−9), near the edges of these templates, aggregates of crystals were sometimes observed. Hot stage microscopy of the aggregated material harvested from these areas showed that some individual crystals underwent a phase transition around ∼145 °C, consistent with the irreversible β to α transformation which is known to occur in this range33 (Figure S3, Supporting Information). PXRD confirmed the presence of small amounts of β-phase impurities in these areas. We could not, however, unambiguously determine whether the β-crystals present in these aggregates were physically in contact with the underlying monolayer or not. Given that all of the well-formed crystals on the surface were α, it may be more likely that β crystals present in the aggregates nucleated in solution rather than on the surface. Alternatively, it may suggest that the quality and/or 2D ordering of the monolayer near the periphery of the 18 mm2 SAM is reduced. Siloxane Templates. Compared to Au−S monolayers, the 2D ordering of organosilanes on glass is not as well-defined; however, the strength of the Si−O−Si bonds, low cost, and spectroscopic transparency of the amorphous substrate impart these templates with many desirable properties.58,70−75 Our earlier perylene crystallization experiments in solution often showed crystal growth on the sides of the glass vial, making heterogeneous nucleation on glass surfaces not atypical. Growth on plain glass templates varied from small crystals to larger aggregates, while crystallization on piranha-cleaned glass templates (very hydrophilic) yielded small plates and prisms from toluene, THF, and hexanes. Consistent growth was observed on all siloxane monolayers in toluene, but again with varying morphologies. Growth on

The gold−thiol and siloxane monolayers were chosen based on their potential for a variety of chemical and electrostatic interactions (differences in hydrophilicity are shown by contact angle). Acetone, benzene, and toluene were used in templatedirected growth experiments based on the size of the crystals grown from conventional solution experiments. Au-Thiol Templates. Crystal growth was attempted on alkyl and arene thiols shown in Figure 2 as well as bare gold surfaces as a control. Growth on the Au control surface was rare. On the alkyl and benzyl-thiol monolayers (1−6), only ∼1/3 of the SAMs yielded very small and sporadic prisms across the surfaces. The most reproducible crystal growth was observed on biphenylthiol SAMs (7−9). The improved growth observed on biphenylthiol monolayers is consistent with our previous crystallization studies of other small molecule aromatic compounds.57,63 This is likely due to the increased conjugation of π-electrons, which likely aids in the long-range stabilization and 2D order of the monolayer, which in turn affects the perylene−substrate interactions at the interface.64 Perylene crystals nucleated on monolayers (7−9) adopted simpler growth morphologies compared to what was typically observed from solution. Needles, plates, and prisms were observed on the unsubstituted biphenyl SAM (7), large plates and small prisms were observed for the nitro-terminated SAM (8), and small needles and prisms were observed for iodoterminated SAM (9) when the growth solvent was benzene, as shown in Figures 4 and S2. The smallest observable platelike

Figure 4. Gold-biphenylthiol SAMs of varying tail groups to illustrate preferred morphologies observed from benzene growth in the presence of templates. (a) (7) needles and prisms; (b) (8) large plates and small prisms; (c) (9) small needles and small prisms. Scale bar = 100 μm. Miller Index assignments of several representative crystals appear in Figure S2, Supporting Information.

crystals that nucleated on the biphenyl SAMs had a fairly consistent thickness, ranging from 1.6 to 2.5 μm, according to atomic force microscopy measurements. The top surfaces of these crystals were also relatively smooth, with a calculated roughness of only 15 Å, which is consistent with the topologies of sublimed crystals.46 To determine the absolute orientation and phase of the crystals grown on SAMs (7−9), several single crystals (2−3 per surface) were removed from the monolayer, and both the phase and the contacting face were determined by X-ray goniometry.

Figure 5. Solvent effect on the size and density of perylene crystals grown on monolayer (13). (a) Rectangular plates from acetone (77 ± 3 crystals/ mm2), (b) plates with some multilayer growth from benzene (60 ± 3 crystals/mm2), (c) tiny plates from dichloromethane (142 ± 11 crystals/mm2), and (d) hexagonal plates from toluene (11 ± 3 crystals/mm2). Scale = 100 μm. Miller Index assignments of several representative crystals appear in Figure S2, Supporting Information. 5247



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(10) yielded plates from benzene, acetone, and toluene. Irregularly shaped crystals were observed on (11) from all solvents. In one growth experiment on monolayer (11) in benzene, very thin extended rectangular plates ∼600 μm in diameter were observed (Figure S4, Supporting Information). We presume this is related to the presence of a highly ordered monolayer of aryl groups on the substrate, though the exact conditions that led to these extra-large plates proved difficult to reproduce. Siloxanes (12), (13), and (14) yielded the most regular and well-defined needles, prisms, and plates. AFM measurements of several typical crystals grown on these substrates had thicknesses ranging from 1.29 to 1.38 μm, ∼ 25% less than the smallest ones grown on Au−S SAMs under comparable solution conditions. The effect of solvent on the nucleation density, crystal orientation, and phase was probed in greater detail using the amino-terminated siloxane (13), which generally yielded the most consistent growth across multiple solvent conditions. Figure 5 shows typical micrographs of perylene crystals grown on (13) from (a) acetone, (b) benzene, (c) dichloromethane, and (d) toluene. In general, an increase in nucleation density coincided with smaller crystal sizes (as nuclei compete for solute) and was inversely proportional to a solvent’s boiling point. For example, toluene (bp = 111 °C), benzene (bp = 80.1 °C), acetone (bp = 56 °C), and dichloromethane (bp = 39.5 °C) had nucleation densities of 11 ± 3, 60 ± 3, 77 ± 3, and 142 ± 11 crystals/mm2, respectively. One advantage of using functionalized siloxanes as templates is that it allows for the crystal phase and orientation to be determined simultaneously using oriented PXRD since the glass does not diffract. With the crystalline sample still attached to the siloxane substrate, only X-ray diffraction lines corresponding to crystal planes parallel to the surface are observed. In the vast majority of solvent/siloxane combinations examined, concomitant growth of α and β was observed, though the relative intensities of α and β diffraction lines varied among different crystallization conditions. Representative oriented PXRD data are shown in Figure 6. Both polymorphs exhibited strong preferred orientations along (100) (Figure 7). The two phases are easily distinguishable, since the α-(100) and β-(100) reflections do not overlap, appearing at 2θ = 8.7 and 9.1, respectively. Higher order (200) reflections also do not overlap with other diffraction lines. It is not clear whether the preferred growth along α-(100) and β-(100) is due to an intrinsically favorable alignment of molecules edge-on to the siloxane, or if it comes from perylene−perylene interactions and the drive to close-pack into a dense layer with the direction of fastest growth parallel to the surface. A previous drop growth study76 of a saturated xylenes solution onto Si-octadecyltrichlorosilane showed selective orientation along α-(100), which is consistent with the major peak observed here. Growth under a few template/solvent combinations such as (10) in acetone showed a second low intensity diffraction line corresponding to α-(011). This high density plane is also a low energy face that was regularly observed on α-crystals. Perylene molecules exist in two orientations with respect to the (011) surface; in half, the aromatic plane is nearly parallel and in the other half the long edge is nearly perpendicular to the surface. No other diffraction lines were observed between 2θ = 21 to 40°. Unlike the Au−S biphenyl templates which generally yielded phase pure α-perylene, attempts to grow α selectively on siloxanes were generally unsuccessful. Although some individual

Figure 6. Oriented PXRD patterns for perylene grown under representative siloxane/solvent combinations. Simulated PXRD of (a) α-perylene (blue) and (b) β-perylene (red) are shown for comparison. Diffractograms collected on samples grown on (c) (10)/ acetone, (d) (11)/benzene, (e) (12)/toluene, (f) (13)/acetone, (g) (13)/CH2Cl2, and (h) piranha cleaned glass/toluene all show concomitant growth of α- and β-perylene. α-Perylene showed preferred orientation along (100) 2θ = 8.8 (with higher order (200) 2θ = 17.6) in a small number of cases orientation along (011) 2θ = 11.5. β-Perylene was also oriented along (100) 2θ = 9.1 (with higher order (200) 2θ = 18.2). In the special case of (i) (13)/toluene, selective growth of β (100) was observed.

templates nucleated only α, it was not reproducible across triplicate samples under any of the conditions examined. On the other hand, conditions were identified to selectively grow the metastable β-phase. Selective β-growth was regularly observed on both siloxane (13) and piranha cleaned glass in toluene solutions, with crystals adopting a preferred (100) orientation. Crystals of β grown on (13) displayed a hexagonal morphology (Figures 5d and S2e), which is markedly different from the rectangular plates of α observed from other solvents and also different from the rhomboidal forms previously reported for β.34 The template-bound crystals were reanalyzed after 16 months, and there was no indication of a phase change to the thermodynamic α-form. The phase purity of β grown on (13) in toluene was somewhat sensitive to concentration, but in general, the β-phase was dominant when the solution concentration was between 0.3 and 0.6 mg/mL. Similar concentration effects on polymorph selectivity were recently reported for carbamazepine.77 Piranha cleaned glass substrates in toluene also yielded phase pure β-perylene, but only when the concentration of the growth solution fell within a certain range and/or evaporation rate. The evaporation rate in these experiments was not well controlled; however, trends were observed over a concentration range of 0.1−1.0 mg/mL. In dilute solutions 0.5 mg/mL, mixtures of α and β were observed, with β as the major phase but with some α impurities. Modeling the siloxane/perylene interface presents many challenges. The glass slides as purchased have a contact angle of ∼69°. After the slides were cleaned in piranha solution followed by drying, the contact angle dropped considerably to 0−10°. 5248



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Figure 7. Perylene crystals nucleated on siloxane templates adopt preferred orientations along α-(100), α-(011), and β-(100). The β-(100) phase was exclusively observed from toluene solution when R = CH2CH2CH2NH2 or OH within select concentration ranges.

relative strength of perylene−perylene interactions is greater than the substrate interactions, and the preferred orientations observed result from kinetic stabilization of the nuclei which align in ways that allow them to grow faster parallel to the surface. A similar rationale was invoked for a different templatedirected study on a diphenylurea system.58 Considering that electron transport occurs most efficiently through the π−π stacking of perylene molecules, the (100) orientation may be particularly beneficial for devices requiring conductance parallel to the surface. A key to a more thorough understanding of the nucleation mechanism on the molecular level is a detailed picture of the crystal/substrate. The process is still heavily influenced by the solvent choice. Experimental and computational methods that provide a molecular-level understanding of the crystal/substrate interface will be instrumental in achieving the long-term goal of being able to make a priori predictions for polymorph selectivity on a given substrate.

When freshly cleaned glass slides are submerged in toluene solution for 15 min, the contact angle rises to 51 ± 2.4°. This clearly indicates that the surface changes over time, presumably through adsorption and/or chemical reaction with the solvent. Piranha cleaned surfaces subjected to refluxing toluene for 1 h had an even higher contact angle of 55 ± 4.1°. The contact angle of functionalized siloxanes subjected to these same solvent conditions does not result in appreciable changes. Siloxane substrates in different solvents may well have different properties which cannot be probed with contact angle measurement. A better understanding of how the siloxane surface responds to solvent is critical to understanding template-directed nucleation in solution on a molecular level.

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CONCLUSION In summary, this work demonstrates that 2D gold−thiol monolayers and siloxanes can act as nucleation templates to influence the crystal morphology, size, and phase purity of perylene compared to conventional solution growth conditions. By screening a library of templates and solvent combinations, conditions leading to the growth of phase pure α-perylene (the thermodynamic form) and phase pure β-perylene (metastable) were identified. This is in sharp contrast to conventional growth methods which routinely yield polymorph mixtures. α-Perylene crystals nucleated on Au-biphenylthiol monolayers grow such that their (100) faces are in contact with the underlying substrate. This is typically the largest face observed on conventionally grown crystals. This preferred alignment may be a consequence of maximizing perylene−perylene interactions and/or epitaxial matching with the substrate. In several cases, crystals grown on these Au−S monolayers adopted less complex morphologies which were generally in agreement with their predicted gas phase forms. Au−S monolayers with alkanethiol or smaller arenthiol units were inadequate templates for perylene nucleation and growth, confirming that the dimensionality and chemistry at the interface are crucial to template performance. Supersaturated toluene solutions in the presence of amino terminated siloxanes and freshly cleaned glass templates yielded hexagonally shaped β-perylene crystals with large (100) faces. In contrast, most other siloxane/substrate combinations examined nucleated mixtures of both α and β crystals. In general, higher nucleation densities tended to correlate with lower phase specificity, although even in the case of mixtures the crystals adopted highly preferred orientations; α-crystals aligned preferentially along (100) and sometimes (011), whereas β-crystals aligned along (100). Unlike the Au−S monolayer case, epitaxial interactions at the siloxane/crystal interface are presumably much less a factor. It may be that the

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ASSOCIATED CONTENT

* Supporting Information S

Optical and hot stage micrographs of perylene crystals obtained under conventional growth from various solvents, indexed single crystals grown on Au−S and siloxane templates, oriented powder X-ray diffraction data for perylene grown on glass substrates. 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]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for financial support provided by the National Science Foundation under DMR-0521170 and from DTRA under (P2-06B-0022). We additionally thank Dr. Joseph Melinger at Naval Research Labs for supplying perylene samples as well as Cameron Mohammadi and Adam Hoy for their assistance with contact angle measurements.

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REFERENCES

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