Nucleation and Growth of Metastable Polymorphs on Siloxane

DOI: 10.1021/cg9012697

Nucleation and Growth of Metastable Polymorphs on Siloxane Monolayer Templates

2010, Vol. 10 952–962

Christina Capacci-Daniel,†,§ Karen J. Gaskell,‡ and Jennifer A. Swift*,† †

Department of Chemistry, Georgetown University, 37th and O Streets NW, Washington, DC 20057, and ‡Department of Chemistry & Biochemistry, University of Maryland, College Park, Maryland 20742 Received October 13, 2009

ABSTRACT: The development of methods aimed at polymorph screening and effecting phase control is of broad interest to the field of molecular materials. The present work is the most extensive systematic combinatorial study of molecular crystal growth in the presence of modified siloxane templates. 1,3-Bis(m-nitrophenyl)urea (MNPU) is known to crystallize from solution in three polymorphic forms (R, β, δ) as well as a hydrate (γ), typically concomitantly. Previous growth studies of MNPU on goldthiol self-assembled monolayers (SAMs) (J. Am. Chem. Soc. 2005, 127, 18321-18327) demonstrated phase selectivity and preferred orientations which could be rationalized on the basis of thermodynamic stabilization at the SAM/crystal interface. Crystallization studies of MNPU on 11 different siloxanes in 3 unique solvents showed that in general siloxane SAMs tended to favor the nucleation and growth of metastable phases (β, δ, γ) and completely suppress the growth of the most stable R phase. A new ε-MNPU phase not previously observed in either solution or template-directed studies was also identified. Differences in the preferred orientations and phase selectivity relative to Au-S SAMs growth studies suggest that kinetic factors play a more pronounced role during interfacial stabilization on siloxane SAMs. This method serves as an effective complement to existing strategies used in the discovery and reliable generation of polymorphs, and may have useful applications in the polymorph screening of other molecular crystal systems.

Introduction The identification and characterization of all possible polymorphs of a given compound remains a chief concern for solid-state chemists, particularly when phase control is required in the production of patentable materials (e.g., pharmaceuticals, energetics, foodstuffs, etc.). No one method, including high throughput screening,1,2 can reliably generate all polymorphic phases of a compound. Therefore, the use of multiple screening methods which explore different phase spaces offers an increased probability that the greatest number of polymorphs will be discovered. There is a practical limit to the extent to which one can afford to screen for polymorphs, such that new approaches which generate and identify phases early in the process through relatively simple and cost-effective means are often the most desirable. In the search for polymorphs, it is common to screen crystallization variables such as solvent, temperature, and saturation conditions, usually by empirical and trial-anderror means. A number of rational methodologies aimed at the selective crystallization of organic polymorphs have emerged in recent years. Many focus on directed nucleation on ordered two-dimensional (2D) surfaces, for example, single crystals,3,4 Langmuir-Blodgett films,5,6 and Au-S self-assembled monolayers (SAMs).7-11 Siloxane SAMs have been used as substrates in the vapor deposition of thin metallic films for many years12,13 but have only more recently been employed as substrates for the heterogeneous nucleation and growth from solution of inorganic crystals,14-16 metalorganic frameworks,17 and molecular crystals.18,19 The degree of 2D geometric ordering on siloxane SAMs is generally thought to be less than that of other 2D surfaces; however, § Present address: Novartis Institutes for BioMedical Research, 250 Massachusetts Ave, Cambridge, MA 02139. *Author to whom correspondence should be addressed.

pubs.acs.org/crystal

Published on Web 11/18/2009

polymer heteronucleation studies have clearly demonstrated that lattice matching is not a formal prerequisite for templatedirected growth.20-23 These methods collectively offer the distinct advantage of identifying polymorphs at an early nucleation stage in the crystallization process. Specific advantages of siloxane SAMs include their low cost, capacity for in situ functionalization, and more robust covalent attachments relative to other types of 2D surfaces. The covalent nature of the monomer assembly enables siloxane monolayers to survive with minimal damage in detergent solutions, water, and a broad range of organic solvents.13,24 The fact that siloxanes can be assembled on glass slides in conventional organic solvents at relatively low temperatures also makes them accessible to a wide range of experimentalists and amenable to scale-up. Collectively, these properties make them highly desirable support materials for crystallization applications. The first step toward understanding and developing their true potential and general use as templating agents for a broader set of crystalline materials is the systematic examination of molecular crystal growth on a range of siloxanes under a variety of different solvent conditions. First described over a century ago, 1,3-bis(m-nitrophenyl) urea (MNPU), has been the subject of several polymorphism studies.25-28 It was selected as a model compound in the present study precisely because its polymorphism has been well characterized. Etter et al.29,30 investigated MNPU for its interesting H-bond patterns and its ability to form donoracceptor cocrystals. Structures of the yellow needle R phase (CSD: SILTOW)29 and white needle β phase (SILTOW01)30 resulted from that work. Bernstein et al.28 more recently reported the structures of white needle δ (SILTOW11) and a yellow plate γ hydrate (SAYQEP) phases, as well as characterized the phase relationships between these various MNPU forms. The most thermodynamically stable phase is R, which melts and/or decomposes between ∼231-250 °C. r 2009 American Chemical Society

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Table 1. Observed (lit.)a Contact Angle Measurements for Siloxane SAM Used in This Study Si-Y-X

Bare SiO2

2-CH3

2-CF3

3-Cl

Obs. Lit.

range (32)32

68 ( 4 (54)33 (80)34

71 ( 4 (72.5)35

79 ( 2 (80)36

Si-Y-X

3-NH2

3-Br

3-I

2-Ph

Obs.

36 ( 3 (23)13 (40)37 (42)38,15

74 ( 2

72 ( 3

69 ( 4

(82)39

(80)40

(87.9)41 b (69)42 b

Si-Y-X

3-CN

3-NCO

3-urea

2-Py

Obs.

58 ( 3

61 ( 3

77 ( 1

Lit.

(82)39

(80)40

61 ( 3 (87.9)41 b (69)42 b

Lit.

Figure 1. Trimethoxy silanes used for siloxane SAM formation. (Top row, L to R) Si-2-CH3, Si-2-CF3, Si-3-Cl, Si-3-Br, Si-3-I, Si-3-NH2; (bottom row, L to R) Si-3-CN, Si-3-NCO, Si-3-urea, Si-2-Py, Si-2-Ph.

Both β and δ forms undergo a phase transition to R at ∼160-221 °C. The γ phase also transforms to R upon dehydration ∼70-101 °C. Notably, conventional solution crystallization typically results in the formation of two or more phases concomitantly.

In previous work,9 we demonstrated that template-directed nucleation and growth of MNPU on Au-S monolayers was a highly selective means to generate phase pure material. Regardless of solvent and temperature conditions, only the R phase grew from unsubstituted biphenyl thiol SAMs, β from 40 -iodobiphenyl thiol SAMs and γ from 40 -bromobiphenyl thiol SAMs. The observed selectivity was rationalized on the basis of thermodynamic stabilization at the interface through both chemical complementarity and geometric lattice matching. Many attempts at solution growth of δ in our laboratory at that time were unsuccessful, though the ability to grow a particular polymorph in one time and location and not another is not unheard of in the field of polymorphism.31 The current study takes a systematic approach toward the development and utilization of siloxane SAMs as crystallization templates for molecular crystals using MNPU as a model polymorphic system. A series of 11 short chain siloxane SAMs (Figure 1) with a wide range of terminal functional groups halogens, hydrogen-acceptor/donors, phenyl urea analogues was assembled, thoroughly characterized and used in MNPU crystallization experiments. Silanes used in this study are named Si-Y-X where Y is the number of chain spacer carbons (ethyl = 2 or propyl = 3) and X is the terminal functionality. The results are compared and contrasted with previous work both in solution and on Au-thiol SAMs. The detailed analysis of the orienting effects and phases obtained on siloxane surfaces supports a templating mechanism that is based more in kinetic stabilization, rather than the thermodynamic stabilization asserted for Au-S monolayer templates.

a Values often fall over a broad range, presumably due to the variance in their assembly procedures. Some values correspond to SAMs fabricated on silicon rather than silica. b Corresponds to SAM of (2-ethylphenyl)trichlorosilane monolayer.

Experimental Section Materials. n-Propyltrimethoxysilane (Si-2-CH3), 3,3,3-trifluoro propyl(trimethoxy)silane (Si-2-CF3), 3-chloropropyltrimethoxysilane (Si-3-Cl), 3-bromopropyltrimethoxysilane (Si-3-Br), 3-iodopropyltrimethoxysilane (Si-3-I), 3-aminopropyltrimethyoxysilane (Si-3-NH2), 3-cyanopropyltrimethoxysilane (Si-3-CN), 3-isocyanotopropyltrimethoxysilane (Si-3-NCO), 3-ureidopropyltrimethoxysilane (Si-3-urea), 2-(trimethoxysilylethyl)-pyridine (Si-2-Py), and trimethyoxy(2-phenylethyl)silane (Si-2-Ph) were purchased from Gelest, Inc. (Morrisville, PA) and Sigma-Aldrich (St. Louis, MO) and used without further purification. Gold Seal No.1 glass microscope slide covers (12 mm 24 mm) were purchased from Electron Microscopy Sciences (Hatfield, PA). Solvents of the highest purity were obtained from Sigma-Aldrich, Fisher Scientific, EM Science and Warner-Graham. 1,3-Bis(m-nitrophenyl) Urea (MNPU) Synthesis. MNPU was synthesized according to previously reported methods.9,29 The material was recrystallized from ethanol, resulting in concomitant formation of yellow needles (R) and white needles (β) which were used together in all crystal growth experiments. Silane SAM Formation. Microscope slide covers were trimmed to 1.2 cm2, submerged in oxidizing piranha solution (70% H2SO4:30% H2O2) for 80 min, rinsed with distilled H2O, and dried under N2. Substrates were soaked in 4 mL of a freshly prepared ∼12 mM solution of a given trimethoxysilane in anhydrous toluene for 135 min at 80 °C. The use of anhydrous solvents appeared to greatly enhance surface coverage. The substrates were thoroughly rinsed with fresh toluene then sonicated twice in methanol for 20 min. After being soaked overnight in methanol and dried under N2, they were cured for 2 h at 135 °C before use. Bare silica control substrates were cleaned in piranha solution for 80 min, rinsed with distilled water, and dried under N2 before use. Siloxane SAM Characterization. SAMs were characterized by contact angle, atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS). The contact angle (Table 1) was determined by the half angle measuring method on a CAM-PLUS MICRO contact angle meter (Tantec, Inc., Schaumberg, IL). As both sides of the substrates were functionalized by this procedure, contact angles reported are the average of 18 measurements for each siloxane SAMs (nine on each side of the substrate). The accuracy of the measurements is (0.8°. AFM was performed in tapping mode on a Nanoscope IIIa (Digitial Instruments, Santa Barbara, CA) using silicon cantilevers tips (Nanosensors, Inc.) with spring constants of 312-376 kHz. All images were collected in air at room temperature at a scan rate of 3.05 Hz. XPS on Si-3-Br and Si-3-NH2 SAMs was performed on a Kratos Axis 165 operating in the hybrid mode using Al KR radiation (1486.6 eV). Survey spectra were collected with a pass energy of 160 eV, while high resolution spectra were collected at a pass energy of 20 eV. The pressure of the system was 5 10-8 Torr or better

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during data collection. Data were collected at electron takeoff angles (TOA) of 90°, 40°, and 20° or 10° (relative to the surface plane) with the iris closed in order to minimize the acceptance angle and thus improve angular depth resolution. The smallest angles possible were 10° and 20° for Si-3-Br and Si-3-NH2, respectively, to avoid an unintentional signal from the sample holder. A charge neutralizer was used to minimize sample charging and was set so as to give the most intense peak with the narrowest full width at halfmaximum. The amount of charge neutralization required varied with TOA. Data analysis was carried out using CASAXPS, and relative atomic sensitivity factors were taken from the Kratos Vision library. All peaks were fit with a 70% Gaussian, 30% Lorentzian product function after application of a Shirley background. Crystal Growth on SAMs. 6.5 mM MNPU solutions were prepared in ethyl acetate, ethanol, or 2-propanol from an initial mixture of R and β crystals. Small vials (15 mm diameter, 45 mm in height) were filled with 4 mL of solution. Prepared SAMs substrates were placed in the vials vertically, leaning against the vial walls. Vials were covered with Parafilm and 2-3 small holes were cut into the film. Samples took approximately 3-7 days to evaporate at room temperature on the benchtop. Single crystals were harvested from the SAMs before complete solvent evaporation occurred. Crystal Characterization. MNPU crystals grown on SAMs were characterized by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), hot stage microscopy (HSM), and X-ray diffraction (XRD). Melting points and phase transitions were determined by DSC using a TA Instruments Modulated DSC 2920 at a heating rate of 5 °C/min and 2-4 mg of single crystals in hermetically sealed aluminum pans. TGA was performed on a Q600 TGA (TA Instruments, New Castle, DE) at 5 °C/min. HSM was accomplished on an Olympus BX50 polarizing microscope equipped with an HCS302 optical hot-stage (INSTEC, Inc., Boulder, CO). The hot-stage was calibrated against several standards with known melting points and has a standard deviation of (1° up to 250°. X-ray Diffraction. Powder X-ray diffraction was carried out at room temperature on a Rigaku RAPID XRD. Ground samples were prepared in 0.5 mm glass capillary tubes (Charles Supper Company, Natick, MA). Using a 0.5 mm collimator, data were collected for 2 h and integrated over a 2θ range of 4-45° with a step size of 0.010°. Data were processed using Rigaku RAPID/XRD Version 2.3.3 and Rigaku AreaMax software. Face indexing was determined by X-ray diffraction goniometry. Using a Bruker single crystal SMART CCD at 173 K or a Bruker APEX II diffractometer at 100 K, 30-50 diffraction peaks were collected and a unit cell determined. SMART or APEX 2 software was used to assign Miller indices to video images of single crystals while still oriented on the goniometer head. All morphologies reported are reproducible and based on samples grown from multiple surfaces. On occasions when surface grown crystals were too small or thin to determine the contacting growth face by single crystal XRD goniometry, oriented powder X-ray diffraction was employed. Diffraction data from crystals still attached to the siloxane SAMs were collected on a Rigaku MiniFlex II desktop X-ray diffractometer (Tokyo, Japan) at George Washington University.

Results and Discussion SAM Assembly and Characterization. Reproducible template assembly was viewed as a necessary prerequisite if siloxanes were to be used reliably in crystallization applications. A survey of the current literature showed there is a high degree of variability in the assembly procedures of siloxane SAMs. Therefore, a standard protocol was developed so that siloxane SAMs with a variety of terminal chemistries could be assembled in a way that would ensure adequate batch to batch consistency. Typically, siloxane SAMs are prepared by introducing a hydroxylated surface into low concentration (∼1030 mM)37,43 silane solution in toluene32 or hexanes44 at

Capacci-Daniel et al.

elevated temperature45-47 for ∼1.5-2 hours,48,49 followed by some post-adsorption curing37,50 or an extended drying period.34 Monomer chain length can affect the homogeneity of the surface, though short chain silanes are generally less influenced by van der Waals chain effects. Propyl silanes have previously been shown to form monolayers via homogeneous but disordered intermediates.51,52 Variables in the assembly conditions such as solvent,32,53 temperature,45,54 water concentration48,55 and silane concentration37,43 have been examined in other studies and each of these parameters was also considered here. Silane SAMs are typically characterized by contact angle,32 conoscopy,56 ellipsometry,53,57 infrared spectroscopy,48,58,59 grazing incidence x-ray diffraction,55,60 x-ray photoelectron spectroscopy (XPS),32,38,52 and atomic force microscopy (AFM).44,51,61,62 In the present work, contact angle measurements were used routinely to determine the surface properties of the siloxane SAMs and to check for reproducibility in the sample preparation (Table 1). A series of control experiments showed that even the contact angle for bare silica glass can vary widely depending on its treatment. For example, bare silica substrates washed in piranha solution and dried under N2 can have very low contact angles on the order of 10-20°. After two hours of oven curing, the contact angle rose to 30 ( 4°, due to the oxidation of surface hydroxyl groups. Cleaned silica samples heated in toluene for 2.25 hrs at 80 °C, washed in methanol and oven cured had a contact angle of 41 ( 2°. This increase is attributed to reactions with solvent, which add some carbons on the surface making the surfaces less hydrophilic. A detailed analysis of siloxane assembly as a function of time was performed using AFM and XPS for two siloxane SAMs, Si-3-Br and Si-3-NH2. Si-3-Br SAMs were allowed to assemble in silane solution for 0, 30, 60, 90, 120, 180, and 240 min, cleaned, and cured as described in the Experimental Section. AFM and XPS were used to track the homogeneity and completeness of the assembly process. As other studies34 have found, AFM showed the cleaned silica surfaces to have the lowest rms roughness (2.6 A˚ ( 0.4). Assembly for 30 min resulted in an increase in roughness (=4.2 A˚ ( 0.6), but after 120 min, the values decreased (=2.7 A˚ ( 0.3) (see Supporting Information). Closer examination of the surfaces revealed the presence of small circular pits with depths corresponding to that of a monolayer. Others have reported that monomeric species can fill these pits with additional assembly time;48 however, in our experience longer assembly times resulted in more irregular surface topographies and the presence of large microstructures ∼22-50 A˚ in height scattered across the surface. We interpret these features as evidence of either excess vertical surface polymerization or the deposition of solution polymerized clusters onto the substrate. Assembly times of just over 2 h produced the most consistent and homogeneous SAMs. Similar AFM trends were observed on an analogous set of Si-3-NH2 SAMs synthesized under anhydrous conditions. XPS was next used to assess the order and density on the surface. Previous studies63 showed that the absolute bromine signal is relatively low for Si-3-Br, but that monitoring the relative ratio of C:Si, C:Br, and Br:Si over time can be instructive.64 Figure 2 is a plot of the X/Si (X = Br or N) intensity as a function of assembly time. Monomer attachment seems to occur most rapidly within the first hour, then to slow significantly. The Br/Si or N/Si signal is not expected to strictly plateau with expanded assembly times due to the

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Figure 2. XPS data showing the relative concentration of Br (blue diamonds) and N (red squares) - normalized to the Si signal - during the SAM assembly of Si-3-Br and Si-3-NH2 over time. A general trend is suggested by the black dotted line.

possibility of vertical polymerization leading to the formation of multilayers or nanoparticle deposition. Together with the AFM data, this again confirmed an optimal assembly time of just over 2 h, which was the protocol subsequently used for all SAMs used in this study. XPS at different takeoff angles provided some insight into the extent of long-range 2-D ordering on Si-3-Br and Si-3-NH2 SAMs (Figure 3). At low angles, XPS intensity is more sensitive to the topmost surface atoms. In a tightly packed SAM with chains nearly perpendicular to the substrate, it reflects the terminal chemical functionality. XPS intensity at higher angles is increasingly more indicative of the bulk composition of the surface monolayer and substrate.65 The trend for both SAMs is such that after 1 min of silane assembly the bromine or amine signal is approximately equivalent at all three takeoff angles. After 2 h of assembly, the intensity of terminal Br or N at the smallest angle (10° or 20°) is significantly greater than what was observed at the mid-angle (40°) or high-angle (90°). Similar data have been observed in other siloxane SAM studies.43,66,67 The angle resolved data for the 180 min samples suggested greater disorder and/or multilayer formation. For example, the normalized N signal ratio (low°/high°) in Si-3-NH2 for the 60, 120, and 180 min samples was 2.3, 3.72, and 2.13, respectively. Likewise, the low angle to high angle Br ratio in Si-3-Br at 60, 120, and 180 min was 2.29, 2.58, and 1.38, respectively. This supports the AFM observation that disordered siloxane aggregates deposit on the surface with extended 180 min time periods. While admittedly it is not possible to establish the exact 2D lattice geometry of siloxane surfaces as is the case in other types of 2D templates, the chemistry of the surfaces is established. We found that so long as the siloxane templates were assembled following a rigid protocol, the crystallization results obtained on these templates were highly consistent and very reproducible. MNPU Growth on Siloxane SAMs. The crystal structures and solution growth faces of MNPU crystals are discussed in detail elsewhere9 such that only a brief summary is needed here. MNPU polymorphs and other bis-substituted diphenylureas68-70 share a propensity for the urea groups to

Figure 3. XPS ratios obtained on (a) Si-3-Br and (b) Si-3-NH2 collected at three takeoff angles and after three periods of assembly: blue = 1 min; red = 30 min; green = 120 min. Low angle (10° or 20°) measurements are surface sensitive, while a high angle (90°) intensity reflects a greater contribution from the bulk.

align in 1D hydrogen bonded chain motifs (Figure 4). The simplest hydrogen bonding pattern is found in β-MNPU where the C(4) chain consists of bifurcated N-H 3 3 3 O hydrogen bonds linking neighboring ureas. Both the R and δ phases share this C(4) chain, although only one of the N-H 3 3 3 O (urea) hydrogen bonds to a neighboring amide. The other amide hydrogen is bonded to a neighboring NO2 group, N-H 3 3 3 O (NO2) thereby creating longer C(7) and C(9) chains, respectively, in directions different than that of the urea chain. The urea chain is parallel to the c-axis in R, the b-axis in β and γ, and the a-axis in the δ phase. In the γ hydrate, the 1D hydrogen bonded chain is expanded and includes bifurcated (water)O 3 3 3 HN hydrogen bonds to urea as well as (water)H 3 3 3 O2N hydrogen bonds to nitro groups. Antiparallel alignment of neighboring urea chains results in the centrosymmetric phases, R and γ. Parallel alignment of the chains leads to polar phases, β and γ. The rotational barrier28 about the amide bond is low; this conformational flexibility may at least in part contribute to this compound’s ability to crystallize in so many various forms.

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MNPU was crystallized from ethanol, 2-propanol, and ethyl acetate in the presence of the 11 short-chain siloxane monolayers in Figure 1. Growth solutions were prepared by initially dissolving a mixture of R and β-MNPU. SAMs were positioned vertically against the growth vial wall to minimize the possibility that any particles nucleated in solution would deposit on the SAMs and lead to false positive results. Each SAM-solvent condition was tested in at least triplicate with similar results. Phase determination and growth face characterization for each set of conditions that generated crystals was made by single crystal X-ray diffraction goniometry and/or by oriented powder diffraction while attached to the siloxane SAM. Comparison of the oriented diffraction patterns with the 2θ values and calculated spacings of known

crystal structures was used for phase identification. In cases where further differentiation was required, for example, 2θ= 12.96° corresponds to reflections in both R or δ, samples were examined by HSM. A summary of the phases obtained from each SAM/solvent condition and their observed orientations appears in Table 2. Before discussing the details of the growth observed, it is worth noting conditions that did not promote growth, as these too can be informative when analyzed in context. There were no indications of crystallization on bare silica controls, Si-2-CF3 or Si-2-Py SAMs from any of the three solvents. The absence of crystals on the trifluoro surface was not unexpected since highly fluorinated surfaces are both hydrophobic and lipophobic. Others have used fluorinated glass vessels to effect solution crystallization of less stable phases.71 Interestingly, when MNPU was grown in a fluorinated glass vial (synthesized similarly to 2D substrates), a mixture of β and γ was observed both from ethanol and ethyl acetate and a mixture of δ and γ from 2-propanol. Prior to these siloxane studies,9 we were unable to obtain δ from conventional solution crystallization methods and were unable to characterize their morphology. Needles of δ grown in fluorinated vials were elongated along the a-axis (i.e., the C(4) axis) and are bounded by (011) or (012), (011), (100) and symmetry equivalent faces. For SAMs that did support crystallization, all crystals were found oriented along specific crystallographic planes regardless of the phase obtained. Occasionally multiple orientations of the same phase were observed. Even in cases where two or more phases grew simultaneously on the same template, all crystals showed a limited number of preferred orientations. The Si-3-NH2 SAM was uniquely interesting, because in addition to supporting the oriented growth of β, δ and γ crystals simultaneously, a new ε-MNPU phase not previously seen from conventional growth methods was identified. The absence of the most stable R polymorph on any of the siloxane SAMs examined was unexpected, but regarded as significant. Analysis of the relevant crystal/SAM interfaces and comparison with previous work on Au-thiol

Figure 4. Hydrogen bonding networks in previously reported MNPU phases. One-dimensional C(4) chains (green) link urea groups in either a bifurcated (β) or singular manner (R, δ) manner, while C(9) chains (orange) in δ and C(7) chains (red) in R link urea and nitro groups. The one-dimensional urea chain in γ is expanded to include water molecules (blue).

Table 2. MNPU Crystallization on Siloxane SAMsa silane

ethanol

solution Si-2-CF3 vials clean SiO2 Si-2-CH3

R, γ β, γ β-(100) δ-(110) & (012) γ-(100) β-(201) & (100) δ-(011) & (012) β-(201) & (100) δ-(110) & (012) -

R, β, γ, δb δ, γ β-(100) δ-(110) & (012) β-(100) δ-(110) δ-(110)

γ-(100) δ-(110) & (012) β-(100) & (201) ε -

β-(100) δ-(110) & (012) & (011)

β-(100) δ-(110) & (012)

β-(100) δ-(110) & (012)

Si-2-CF3 Si-3-Cl Si-3-Br Si-3-I Si-3-CN Si-3-NH2

Si-3-NCO Si-3-urea Si-2-Py Si-2-Ph

2-propanol

δ-(110)

-

ethyl acetate δb, γ β, γ β-(100) δ-(110) & (011) β-(100) δ-(110) & (012) β-(100) δ-(110) & (011) β-(100) δ-(011) β(100) δ-(110) & (012) & (011) ε β-(100) & (201) δ-(110) & (011) δ-(011) β-(100) δ-(110)

a The phase(s) and plane(s) at the crystal/SAM interfaces are noted. Dashes indicate no growth was observed. b The δ phase was obtained from solution growth only after siloxane SAM studies were initiated in the laboratory.

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Figure 5. β-MNPU and γ-MNPU viewed down their respective b-axes. (left) Siloxane templates supported the oriented growth of β-MNPU on (100), (201) and (20-1) faces but not (001). Created from fractional coordinates of CSD entry: SILTOW01.30 (right) γ-MNPU viewed down the b axis. γ-MNPU growth was observed on only the (100) plane. Schematic constructed from fractional coordinates of SAYQEP28 with water molecules enlarged for clarity.

SAMs provides insight into some thermodynamic and kinetic aspects of template-directed crystal growth. Growth conditions for each phase are addressed in the separate sections that follow. β-MNPU Growth. Of all the MNPU phases, β-MNPU growth was observed most frequently. It appeared on 7 siloxane SAMs (terminal groups: CH3, Br, I, NH2, CN, NCO and Ph) and from all three growth solvents. In many cases, β growth occurred concomitantly with δ. From every SAM-solvent condition in which β was observed, the majority of crystals were in interfacial contact with the template through the (100) face. On select SAMs, mixed orientations were observed. On Si-3-NH2 and Si-3-NCO, the β (201) orientation was additionally found, while on Si-3-Br and Si3-I SAMs β (201) orientations were observed. Interestingly, these are presumably not low energy faces, as solution grown β-MNPU crystals typically adopt a needle-like morphology in which (100) is a very small end face and (201) and (201) faces are generally not observed.9 The most obvious common structural feature shared by the β (100), (201), and (201) planes is that in each, the 1-D hydrogen bonded urea chain (along the ( b-axis) is oriented parallel to the surface (Figure 5, left). Interestingly, orientation along the other low index face, β (001), which would have also aligned urea chains parallel to the surface, was not observed under any of the SAM/solvent conditions tested. These results are quite different than previous MNPU studies on Au-biphenylthiol SAMs9 in which template directed growth of the β-phase resulted in urea chain orientation perpendicular to the template. Also, while β growth was selectively observed on iodo-terminated biphenyl thiol SAMs it was frequently observed over many siloxanes SAMs often in combination with other phases. This suggests that the phase discrimination in Au-S and siloxane SAMs is quite different. A number of aspects were examined to try to understand the preference for the β orientations observed. Hydrogen bonding is typically assumed to be among the strongest type of intermolecular interactions, but interfacial hydrogen

bonding interactions either to the urea or the NO2 groups on the most frequently observed β (100) is not a compelling argument given the orientation of molecules on the surface. They may, however, be a factor in the less frequently observed (201) and (20-1) orientations. On β (201), observed on two of the three SAMs bearing hydrogen bonding groups (Si-3-NH2 and Si-3-NCO), the urea functionalities reside closer to the outer van der Waals surface than in any of the other possible surfaces shown in Figure 5. With only minor steric adjustment, H-bonds between the ureas and the SAM could potentially form across this interface. We do not know the extent to which this might destabilize the urea chain within the crystal, but presumably any small energetic penalty within the crystal would be offset by the interfacial stability gained. Nitro group interactions at the template interface are also an unlikely factor in the most common β (100) orientation. Nitro groups on this face are oriented toward the crystal interior, and a significant change in torsion angle would be required for them to interact with any SAM surface. Nitro groups are present on the (201), and (20-1) faces, and the C-NO2 bond orientation with respect to the two surfaces are fairly comparable at 41.8° and 44.8°, respectively. This should in principle allow NO2 groups on either of these surfaces to interact with the SAMs. Previous template directed growth work9,11,72 on halo-aromatic Au-S SAMs showed NO2 3 3 3 X (X = halogen) to be a desirable interaction in engineering interfaces. A search of the Cambridge Structural Database (v5.30, Nov. 2008 with Update 1) yielded 40 C-NO2 3 3 3 X-C interactions which have an average interaction angle of 94.8° ( 9.2°. The preference for the β (20-1) orientation on halogenated SAMs might be partially explained based on the basis of its high NO2 density. The density of NO2 groups is higher on (20-1) than (201) - 1 per 53.69 A˚2 vs 1 per 67.03 A˚2. However, the β (001) plane, which is not observed, has a marginally higher NO2 density (1 per 49.35 A˚) and comparable C-NO2 orientation at 46.7°, so this rationale is also incomplete.

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Figure 6. (Top) Oriented powder diffraction pattern of MNPU grown on Si-3-I from ethanol. The δ (110) peak at 2θ = 19.44° is significantly more intense than the other observed diffraction lines: 7.65° δ (011), 8.77° β (200), 12.99° δ (012), 17.43° β (400). (lower left) Schematic of the δ (110) face. (lower right) Schematic of the δ (011) and (012) faces. Carbonyl oxygens are enlarged to illustrate C(4) chain direction. The C(4) urea chain is denoted with green dots and the C(9) in orange.

By examining other molecular crystal systems73 in addition to MNPU, we found that frequently in cases where preferred orientations have no obvious stabilizing interfacial bonding, preferred orientations tended to be along planes with the smallest cell parameters. Among all the hypothetical low index β-MNPU faces with k = 0, (100) has the smallest lattice parameters. Though we do not yet fully understand how this might play a definitive role, we suspect that either larger cell parameters make it more difficult to achieve lattice registry with the underlying substrate, or if the relative size of the cell dimensions and growth kinetics are related, then orientations which enable fast growth parallel to the surface might be preferred. γ-MNPU Growth. As the only other known polar phase, γ-MNPU is structurally most similar to β-MNPU. Both have similar molecular conformations and 1-D hydrogen bonded chain motifs parallel to the b axis. Unlike the β phase which grew on many different siloxane SAMs, γ growth was observed on only Cl- and NH2-terminated templates (Figure 5, right). The same γ-(100) orientation was observed on both surfaces, which aligns the 1-D hydrogen bonded chains parallel to the interface. That γ appeared only from ethanolic growth solutions is not particularly significant, since solution growth of γ in the absence of a template also occurs most readily from ethanol. γ-MNPU crystals grown on both SAMs were typically elongated along the urea chain direction ((b). In contrast, growth in the absence of a template yielded morphologies elongated along (c with a combination of (100), (001) and higher order (hkl) faces.9 The right half of Figure 5 is a schematic of γ-MNPU viewed down the H-bonding axis. The observed γ-(100) plane is indicated as well as some other low index (h0l) planes, e.g., (001), (101), (201), and (401), which would also orient the urea chain axis parallel to the SAM, but were not observed.

It is curious that the same (100) orientation was observed on these two templates, as they have seemingly little in common in terms of the potential chemical interactions that can be made at the interface. On chloro-terminated SAMs, γ-MNPU was the only phase observed, whereas aminoterminated SAMs also supported the simultaneous growth of several other MNPU phases. Based on contact angle measurements, NH2 terminated SAMs are fairly hydrophilic, and Cl-terminated ones are hydrophobic. The γ (100) face, the water molecules within the hydrogen bonded chain are positioned as far from the crystal/SAM interface as possible. All of the other hypothetical SAM/crystal interfaces would require that water molecules occupy positions directly at the interface, albeit with different densities and orientations. Interestingly, of all the (h0k) planes considered, the γ (100) surface has the smallest 2-D cell parameters (3.74 7.38 A˚, 90°) and is the most densely packed. All of the others share the same 7.38 A˚ axis, but the second cell dimension ranges from ∼24.6-27.45°. Although this does not fully explain the orientation observed, it is the most obvious similarity when comparing the β and γ growth results. δ-MNPU Growth. δ-MNPU was grown from seven siloxane SAMs (CH3, Br, I, CN, NH2, NCO and Ph). It was exclusively obtained from Si-3-I and Si-3-CN SAMs in ethanol and Si-3-urea in ethyl acetate. On other SAMs, it was found as the minor component in concomitant mixtures with β. Preferred orientations were (110), (011) and (012) faces, often with two or more appearing simultaneously. Qualitatively crystals oriented on the (110) face were more abundant and/or larger as determined by their relative oriented PXRD intensity (Figure 6). The (011), (110), and (012) reflections have relative intensities of 13.4:5:1 in the calculated PXRD spectrum. In the oriented PXRD spectrum, the δ (110) reflection intensity is ∼5 times that of

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Figure 7. Thin colorless needles of ε grown on Si-3-NH2 SAM in ethanol. Scale bar = 200 μm.

Figure 8. ε-MNPU crystals under polarized hot-stage microscopy before and after their phase transformation ∼170-174° to R-MNPU. (a) 165.6 °C and (b) 181.0 °C. Scale bar = 100 μm.

δ (012) and markedly larger than (011), which is the most intense reflection in unoriented samples. The surface chemistry on the (110) face is particularly complicated as this plane transects the C(4) urea chain and molecules occupy a variety of orientations (Figure 6, lower left). Relative to the (110) plane, the urea chain (a-axis) emerges at 75.7° and C-N(O2) bonds are oriented at 20.5°, 34.6°, and 41.1°. Interestingly, the C(9) chain (nitro 3 3 3 amine) chain lies nearly parallel to the (110) plane. It is not clear whether the C(4) or C(9) chains are more influential in the assembly of δ. Similar to the β and γ cases previously described, a 1D H-bonded chain is oriented parallel to the template interface. The preferred orientation along δ-(110) is difficult to explain on the basis of chemical interactions alone. Previously, we hypothesized that the prevalence of β (100) on a wide variety of SAMs may be a consequence of small lattice dimensions. However, the same justification cannot be extended to δ (110), which has relatively larger lattice parameters compared to some other low index δ planes.74 We considered the possibility that δ (110) could result from cross-nucleation on β (100) since the two crystal orientations are almost always simultaneously observed. Epicalc75 was used to evaluate potential lattice matching between δ (110) and β (100). Using a β (100) substrate with fixed cell parameters and a δ (110) overlayer whose parameters were allowed to vary by (5% in each unit cell dimension, several coincident matches were found. This suggests that δ (110) nucleation from β (100) is geometrically possible; however, we have not experimentally seen direct evidence confirming this possibility. The δ (011) and (012) orientations more closely resemble the growth of β and γ-MNPU from siloxane SAMs, since urea chains on (011)/(012) align parallel to the SAM surface and C-NO2 bonds are oriented 36.6° and 32.6° relative to the plane, respectively. In general, crystals oriented on the (011) and/or (012) faces were minor components of surface growth when (110) is observed. The chemistry along the (011) and (012) faces is somewhat less complicated than (110). On both surfaces, the molecules reside in two different

orientations - nearly perpendicular and nearly parallel relative to the surface (Figure 6). Molecules lying nearly parallel to the surface can potentially form urea hydrogen bonds to the SAMs, as in β (201). Likewise, the perpendicular molecules may form NO2 3 3 3 halogen interactions on I and Br terminated SAMs, as in β (201). The appearance of crystals oriented on (011) and (012) both together and separately suggests that some thermodynamic optimization occurs at the surface at least in some cases. Reasons for this preference are unclear, but appear to be effected by the solvent choice. The (011) and (012) orientations are not as frequently observed from 2-propanol compared to ethyl acetate and ethanol growth solutions. New ε-MNPU Phase. A new feathery, colorless needle phase, ε-MNPU, was observed on Si-3-NH2 SAMs from growth in ethanol and ethyl acetate (Figure 7). The material appeared crystalline by both optical microscopy and PXRD. By hot-stage microscopy (Figure 8) and DSC, ε-MNPU was found to undergo a phase transition between 170.8 and 174.5 °C. The crystals appeared to retain their transparency throughout the heating process and TGA showed no corresponding weight loss, suggesting that ε-MNPU is a true polymorph and not a solvate. The transformed material exhibited a melting point and Raman signal consistent with R-MNPU. Crystals of ε-MNPU were too small for single crystal X-ray diffraction and several attempts to seed MNPU solutions in the hopes of generating larger single crystals were also not successful. Refinement of pure, nonoriented PXRD data of ε (see Supporting Information) using the pattern indexing option in JADE (Materials Data, Inc., Livermore, CA) suggested two different unit cells: a = 9.246 A˚, b = 9.484 A˚, c = 6.948 A˚, β = 95.15° (V = 606.80 A˚3) or a = 13.836 A˚, b = 15.716 A˚, c = 7.302 A˚, β = 126.10° (V = 1282.92 A˚3). The molecular volume of a single MNPU molecule in the three anhydrous phases, R (1318.53 A˚3, Z=4), β (640.41 A˚3, Z=2), and δ (1271 A˚3, Z=4), ranges from 317.8-329.6 A˚3. Assuming a Z=2 for the first indexed PXRD cell and Z = 4 for the latter, the latter seems significantly more probable. It corresponds to a molecular volume of 320.73 A˚3, which falls squarely in the same range

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as the other known phases. The first cell gives a molecular volume ∼5-8% smaller than the others, which seems less likely for a metastable phase. Although we do not known the full structure of ε, oriented powder diffraction suggests that crystals of this phase also adopt preferred orientations on Si-3-NH2 SAMs (see Supporting Information). Recall that Si-3-NH2 also supports the growth of other phases concomitantly in both ethanol and ethyl acetate. Some of the observed oriented diffraction peaks from Si-3-NH2 grown from these solvents clearly correspond to β, δ, and γ phases. However, diffraction at 2θ = 9.30°, 18.51° (the most intense), and 28.16° does not match any of the known phases and can be attributed to ε. Diffraction at 2θ = 9.30° is not an intense reflection in the bulk PXRD pattern of ε, suggesting this reflection is a result of strong orientational enhancement. Nucleation on Siloxanes: Kinetic Factors. The variety of siloxane SAMs used in this study showed that templatedirected growth favored the nucleation of β, δ, γ, and ε metastable phases; however, the notable absence of the most thermodynamically stable R phase is curious, since it grows readily from solution. Oriented powder X-ray diffraction data were thoroughly and repeatedly examined but showed no trace of R. Even minor amounts of R should be detectable by this in situ technique, since even systematically absent low index reflections have an observable angle multiple within the 2θ range surveyed (4-40°). Direct analysis of siloxane SAMs by HSM also showed all crystalline material on the template undergoes a phase transformation prior to the melting/decomposition of R (∼230 °C). Using the logic applied to the growth of the metastable phases, some R growth faces should be theoretically advantageous. Orientation along any of the R (hk0) planes would position the C(4) chain parallel to the surface. If one argues that the C(4) urea chain motif is relatively less important than the C(9) motif of nitro-amine H-bonding, R (101) orientations might be preferred, since it aligns the C(9) motif parallel to the surface. Most of these planes are also at least in principle capable of forming advantageous chemical interactions with a SAM across the interface. R-MNPU crystals were readily grown on biphenyl thiol SAMs,9 and consistently exhibited (010) orientations. The absence of R on siloxane SAMs suggests some other more subtle structure directing factors may be operative. A number of recent studies76-78 have advanced the idea that nucleation in solution occurs via a two-step process whereby disordered clusters initially form and later reorganize into structured aggregates. In a two-step model, both the initial cluster formation and the cluster restructuring have their own associated thermodynamic energy barriers, the latter suspected to be larger. Under template-directed growth conditions, any phase and/or orientational selectivity observed is a direct consequence of reducing the energetic barrier to either step (or perhaps both). The present work demonstrates that crystal growth on siloxane templates does not require strict chemical matching and in fact is more consistent with physical stabilization of kinetically favored aggregates. If either the lifetime of the initially formed aggregates or the reorganization kinetics to structured entities is enhanced through interfacial interactions, then oriented crystal growth would follow. Etter79 asserted that the kinetically fastest forming aggregates (or in a two-step model, faster forming structured aggregates) will become the first observed crystal form,


regardless of its thermodynamic stability relative to other polymorphs. Over time, less stable phases present in solution can transform, perhaps through Ostwald’s ripening, and more stable but slower nucleating and/or slower growing phases appear.80 If kinetically favored aggregates are stabilized on a siloxane SAM and allowed to subsequently grow, the kinetically favored polymorph should be observed over many SAM and solution conditions. One might expect fast nucleating phases to orient in ways that enable fast growth to occur parallel to the template surface. It is reasonable to assume that the phase with the fastest nucleation kinetics might also be that with the simplest H-bonding motif(s). β-MNPU contains the simplest C(4) motif of all the known phases and is the most frequently observed over a range of siloxanes and solvents. Its preferred orientation along (100) cannot be rationalized based on interfacial chemical interactions, although it does possess the smallest 2D lattice dimensions and aligns the short urea axis parallel to the interface. β (201) and β (20-1) orientations, which preserve the parallel urea chain orientation with respect to the SAM surface, are observed under select conditions where complementary chemical interactions at the interface are most plausible. δ-MNPU also frequently appears across many siloxanes. With two orthogonal H-bonding motifs C(4) and C(9), its H-bonding network is arguably more complex than β. In previous solution growth studies, β and δ were originally observed to be determinately concomitant.28 On siloxane SAMs this is also the case, yet in mixtures of the two phases, δ is typically the minor component. The preferred δ (110) orientation positions the C(9) chain parallel to the surface and may also be influenced by factors related to the possible cross-nucleation on the major face of β. The minor orientations observed, δ (011) and (012), both align the C(4) chain parallel to the interface. Similarly, the appearance of γ is rare, but its (100) orientation places the simplest 1D H-bonding motif parallel to the surface. The siloxane SAM growth results are particularly interesting considering the growth observed from gold-biphenyl thiol SAMs.9 In that study, varying solvent and temperature conditions did not affect the resulting crystal phase; rather, it was the terminal chemical functionality of the SAM which seemed to dictate selectivity. Nucleation was more selective on these templates with only one unique phase per SAM observed. In comparison, siloxanes nucleated multiple phases simultaneously often in orientations different from those observed on gold-biphenyl thiol SAMs. While the goldbiphenyl thiol SAMs results suggested a template effect dependent on interfacial thermodynamic, the results on the siloxane SAMs seem to reflect a greater sensitivity to the kinetics of nucleation and growth of individual phases. Conclusion This crystallization study demonstrates that directed nucleation on siloxane templates is an effective and reproducible means to screen polymorphs, which in some cases can lead to the appearance of phases not typically observed with conventional growth methods (e.g., ε-MNPU). However, the absence of R-MNPU on siloxane templates (and absence of δ in the previous Au-S SAM study) provides a cautious reminder that polymorph screening using any one method can lead to an incomplete set of phases. It was not obvious at the outset of this study that siloxane and Au-S templates would yield such

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disparate results, although we have seen analogous differences in investigations of other molecular crystal systems. Collectively these studies suggest there may be fundamental differences in how template-directed nucleation and growth occurs on these two surface types. Siloxane and gold-thiol monolayer templates admittedly have many differences in terms of lattice parameters, degree of surface ordering, and chemical properties. We do not at this time know which of these factors or combination of factors is most influential, though future studies may help elucidate this. Acknowledgment. This work was made possible through the financial support of DTRA (P2-06B-0022) and the Camille & Henry Dreyfus Foundation. We also express our appreciation to Chris Cahill of George Washington University (and his group) for allowing us significant time on his Rigaku Miniflex II PXRD instrument. Supporting Information Available: AFM characterization of Si-3Br as a function of assembly time (Figure A); oriented PXRD of material grown on Si-3-NH2 in solvent and PXRD of ε-MNPU (Figure B); calculated 2θ values for known MNPU phases. This material is available free of charge via the Internet at http://pubs. acs.org.

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Nucleation and Growth of Metastable Polymorphs on Siloxane

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