Polymorph Selection via Sublimation onto Siloxane Templates

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Polymorph Selection via Sublimation onto Siloxane Templates Marina A. Solomos, Christina Capacci-Daniel, Judith Faye Rubinson, and Jennifer A. Swift Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b01151 • Publication Date (Web): 08 Oct 2018 Downloaded from http://pubs.acs.org on October 12, 2018

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Crystal Growth & Design

Polymorph Selection via Sublimation onto Siloxane Templates Marina A. Solomos, Christina Capacci-Daniel, J. Faye Rubinson and Jennifer A. Swift Department of Chemistry, Georgetown University 37th and O Sts NW, Washington, DC 20057-1227 Abstract Diphenylurea (PU) crystallizes via slow evaporation from a range of solvents as phase pure α-PU or in a concomitant mixture with the less stable β-PU phase. Sublimation onto glass also yields α-PU in high phase purity. In contrast, sublimation onto siloxane-coated glass templates with various terminal groups (e.g. isocyanate, acetate, bromo) resulted in primarily β-PU. Scanning electron microscopy showed the individual crystals on siloxanes exhibit a number of unusual morphologies (e.g. plates, needles, hollow tubes, spirals) which are somewhat influenced by the terminal siloxane functionality. Under extended sublimation times (> 5 hours), mixtures of β and α become apparent on most siloxanes. Grazing incidence X-ray diffraction studies showed that the β:α ratio changes as a function of distance from the siloxane, with more α-PU appearing at higher angles. Diffraction line intensities also indicate strong preferred orientations for -PU crystallites within the polycrystalline layer with the fastest growth direction (b-axis) parallel to the template, but more varied orientations for α-PU. This suggests α-PU may nucleate elsewhere in the sublimation apparatus rather than on the -coated siloxane. This study demonstrates the utility of template-directed sublimation methods as a way to engineer metastable phases with high phase purity but also illustrates some of the challenges associated with achieving phase control on templates with limited surface area. Introduction The occurrence of polymorphism in organic compounds is a widespread phenomenon and has important implications in their material properties.1 In solution crystallization, the major variables which can lead to the polymorph observed—solvent choice, temperature and supersaturation—are usually examined through trial and error.

More rational strategies to

selectively prepare a given polymorph have been adopted including crystallization in the presence of soluble additives,2,3 polymers,4-6 or on the surfaces of other solid templates including selfassembled monolayers7-11 or seed crystals. The advantage of template-based approaches is that 1 ACS Paragon Plus Environment

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the phase is established at the earliest stages of crystallization by inducing heterogeneous nucleation events.

When successful, polymorph selectivity is typically ascribed to either

geometric matching (epitaxy) or complementary chemical interactions across the template/crystal interface. Solvent undoubtedly also plays a role in heterogeneous nucleation events, though its role in the process remains an open question. Sublimation has long been employed as a purification method for organic compounds under solvent-free conditions.12-15 Though approximately two-thirds of organic compounds are able to sublime,16 the use of sublimation as a crystal growth method for small molecule organics is less frequently used than solution growth methods. This may be in part because it often yields crystalline or polycrystalline thin films which are not amenable to characterization by single crystal X-ray diffraction, though high quality crystals have been obtained in some cases.17-19 Greater recognition of the advantages sublimation offers — such as eliminating the possibility of solvated phases16,20-22 and in some cases the realization of unexpected polymorphs23-26 could facilitate its wider use. Traditional sublimation apparatuses require a heating source for crude material and a colder glass surface where purified material will be deposited, with either carrier gases or a vacuum to facilitate the vapor diffusion. The vapor pressure27,28 and/or the temperature of the deposition surface16,29 are the variables most often controlled, since both can strongly affect the supersaturation, and ultimately, crystal phase and form. Vapor phase deposition onto various substrates has been extensively used as a means to prepare thin films of organic electronics,30-35 though the method has not been widely applied to other classes of organic compounds. A number of reports have achieved polymorph selectivity by sublimation directly onto crystal surfaces of epitaxially matched materials36-41 and in the presence of additives42,43 though we are aware of only one report44 where the relative distribution of polymorphs was affected by sublimation onto amorphous or less well-ordered substrates (e.g. bare glass,

trimethoxysilane and

chlorotriisobutylsilane). Herein we report on the phase selectivity observed when diphenylurea (PU), a plant growth promoter,45 is sublimed onto glass substrates coated with a chemically diverse library of siloxanes. Solvent-free sublimation of PU onto bare glass yields the thermodynamically stable α-PU phase, however, deposition onto siloxanes yields a less stable β-PU polymorph with high phase purity. The siloxane functionality was also found to have a significant effect on the crystal morphology, 2 ACS Paragon Plus Environment

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with deposition on some substrates reliably yielding conventional habits (e.g. needles, plates) and others consistently yielding non-equilibrium ones (e.g. hollow tubes, spirals). EXPERIMENTAL SECTION Materials. Phenyl isocyanate and aniline were purchased from Sigma Aldrich and used without further purification. Gold Seal No.1 glass microscope slide covers (12 x 24 mm, 0.13-0.17 mm thickness) were purchased from Electron Microscopy Sciences (Hatfield, PA). Silanes (3acetoxypropyl-trimethoxysilane, trimethoxysilane,

3-aminopropyl-trimethoxysilane,

3-chloropropyl-trimethoxysilane,

3-bromopropyl-

3-cyanopropyl-trimethoxysilane,

3-

isocyanato-propyltrimethoxysilane, and 2-phenethyl-trimethoxysilane) were purchased from Gelest with 95% purity or greater. Synthesis. Diphenylurea (PU) was synthesized from phenyl isocyanate and aniline according to previously reported methods.46,47 1H NMR (DMSO-d6, ppm) : 8.64 (s, 2H), 7.45 (m, 4H), 7.27 (m, 4H), 6.96 (m, 2H). The material was recrystallized from several different solvents (e.g. benzene, acetone, ethyl acetate, methanol, ethanol, acetonitrile). Phase-pure α-PU (mp = 239240C) was obtained from recrystallization in ethanol, ground, and stored in capped glass vials. Optical and hot stage microscopy images were collected on an Olympus BX-50 polarizing microscope fitted with a HCS302 optical hot-stage (INSTEC, Inc., Boulder, CO).

diphenylurea

Siloxane Preparation. Siloxane templates were prepared according to methods previously described.10,11 Microscope slide covers were cleaned in oxidizing piranha solution (70:30% H2SO4:H2O2) for 60 min, thoroughly rinsed with distilled H2O, and dried in a vacuum oven. The clean slides were functionalized by soaking in a 12 mM trimethoxysilane in anhydrous toluene solution for 2.25 hrs at 80°C. The siloxane templates were rinsed with toluene, sonicated in 3 ACS Paragon Plus Environment


methanol for 50 min (turned over part-way through) and stored overnight in methanol. The templates were removed from methanol and oven cured for 2 hrs at 135°C. The contact angle of each template was obtained by the half angle measuring method on a CAM-PLUS MICRO contact angle meter (Tantec, Inc., Schaumberg, IL). Siloxane templates are named for their carbon chain length and terminal functional group, e.g. amino-propyl (Si-3-NH2), bromo-propyl (Si-3-Br), chloro-propyl (Si-3-CN), acetoxy-propyl (Si-3-OAc), cyano-propyl (Si-3-CN), isocyanato-propyl (Si-3-NCO), and phenyl-ethyl (Si-2-Ph). Sublimation Apparatus. Sublimation experiments made use of a custom-built apparatus with a square bottomed cold finger (Figure 1). This geometry enabled the siloxane to be securely affixed to the cold finger and ensured that the temperature was uniform across the siloxane. In a typical experiment, approximately 25 mg of ground α-PU was placed in the apparatus base. The siloxane (or bare glass) substrate was affixed to the cold finger base with adhesive mounting pads (Bruker Nano Inc.) and positioned 2.5 cm above the source material. A KNF Neuberger Inc. (Trenton, NJ) vacuum pump (Model UN726 FTP, 115V, 60 Hz, 1.2A, 50 torr/ 28 in. Hg maximum) was connected to the apparatus. The base of the apparatus was placed in contact with a 40 mL silicone oil bath which was heated to 200°C and magnetically stirred at 140 rpm on a Corning PC420D hotplate. The cold finger temperature (near 0 – 50°C) and the distance between sample and cold finger were varied systematically, however, changes in these parameters did not affect the net outcome of any experiment. All results reported herein used a 20-22°C cold finger. After experiment completion, siloxane substrates were gently removed from the cold finger and stored in capped glass vials until further characterization. A separate thermal gradient apparatus was also assembled by affixing glass slides and/or siloxanes to an aluminum strip which was placed in a horizontal test tube with a side arm connected to a vacuum (Figure S1). A hot plate on one end and an ice bath on the other created a temperature gradient. Thermocouple measurements revealed six temperature zones (Zones #1 - #6) ranging from 200°C (Zone #1), 170°C, 125°C, 75°C, 60°C, and 25°C (Zone #6), each of which fell within a ±5°C range.

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Figure 1. Schematic of custom designed sublimation apparatus to allow for vapor phase deposition directly on template surfaces. The cold finger within the apparatus is rectangular (with a square end face) to allow for the template to be flush with the sides and bottom of the cold finger.

Characterization.

Optical microscopy was performed on an Olympus BX50 microscope

(OPELCO, Dulles, VA) fitted with a Lumenera Infinity 2.0 microscope camera. For hot stage microscopy, an HCS302 hot-stage (INSTEC, Inc., Boulder, CO) with a standard deviation of 1° was interfaced with the microscope. Differential Scanning Calorimetry (DSC) was performed using a TA Instruments Modulated DSC 2920. Samples were prepared in hermetically sealed aluminum pans (2-10 mg) and heated at 10°C/min. Raman microscopy was performed on 1 µm spots on single crystals using a Horiba LabRAM HR Evolution with a 532 nm laser and an 1800 I/mm grating filter. Scanning electron microscopy (SEM) images were obtained on a Zeiss SUPRA55-VP. Custom mounts were prepared to allow for edge-on analysis of the crystalline film, enabling visualization of crystal growth extending from the siloxane template.

X-ray Diffraction. All samples were analyzed using powder X-ray diffraction (PXRD) on a Rigaku Ultima IV diffractometer or Miniflex II X-ray diffractometer. Siloxane templated growth was characterized with crystalline samples and films attached to the substrate as grown. The 5 ACS Paragon Plus Environment


diffraction patterns were obtained over a 2θ range of 5-40º at 40 kV and 30 mA with a scan speed of 1.2-1.5°/min. Grazing incidence X-ray diffraction (GIXD) over incidence angles ranging from 0.25-2.00° was also performed on the Ultima IV. The PXRD data was compared against calculated powder diffraction patterns from single crystal cifs. The single crystal structure of β-PU was determined from a crystal grown from methanol. Single crystal diffraction data were collected at 100(2) K using the Bruker Apex DUO diffractometer (Mo Kα,  = 0.71073 Å). The structure was solved in SHELXS and refined using SHELXL with the OLEX 248 user interface.

Non-hydrogen atoms were solved using direct

methods and refined with anisotropic displacement parameters. Urea hydrogen atom positions were determined from the residual electron density, while other protons were placed in ideal positions and refined as riding atoms. CCDC deposition no. 1841857.

Results and Discussion A Dimorphic System. The Cambridge Structural Database Version 5.39 contains several entries for diphenylurea (refcode: DPUREA), all of which correspond to the same phase. Recrystallization of PU in assorted solvents (e.g. benzene, acetone, ethyl acetate, methanol, ethanol, acetonitrile) produced large, thick, colorless needles. PXRD of the crystalline material corresponded to the known phase in most cases. However, from methanol solutions with PU concentrations greater than 1.8 mM, additional diffraction lines were observed indicating the presence of a second phase. Physical inspection of the bulk sample showed two subtly different crystal morphologies (Figure 2). The thick needles with end faces that intersect at right angles corresponded to the previously reported phase, which we now refer to as α-PU. Elongated blades with angled end faces corresponded to a second polymorph, β-PU, whose structure was determined by single crystal X-ray diffraction (Table 1).


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Figure 2. (a) Concomitant growth of α- and β-PU from methanol. (b) α-PU, (c) β-PU. Scale bar = 100μm. The structures of α-PU and β-PU polymorphs are compared side-by-side in Figure 3. Both polymorphs exhibit the classic 1-dimensional hydrogen bonded urea chain motif (graph set [C(4) R 12 (6)]),

which parallels the fast-growing direction of the crystal. This corresponds to [100] in α-

PU and [010] in β-PU. Carbonyl and amine groups are exposed on the (100) end faces of α and on the (011) end faces of β (Figure S2). The most notable structural differences are in the molecular conformations and the relationship between molecules within each urea chain. Molecules in α-PU twist about a 2-fold urea chain axis, whereas molecules in β-PU are related by translation along the urea axis. In addition to clearly different PXRD patterns, the different molecular conformations also allow the α and β polymorphs to be distinguished by Raman spectral differences in the 300-500 cm-1 and 1500-1700 cm-1 regions (Figure S3). With a symmetrical molecular conformation, the Raman spectrum of β-PU has one prominent “ring breathing” peak49 at approximately 1250 cm-1. The unsymmetrical conformation in α-PU results in two ring breathing mode signals in the same region. Subtle differences are also evident between 100-850 cm-1, such as a small peak at 400 cm-1 present in α-PU. Both DSC and hot stage microscopy (HSM) confirmed that β-PU is the less stable of the two forms. Between 96-114°C the β form converts to α which later melts at 239-240C. HSM studies revealed that PU also begins to sublime at temperatures close to 220°C (Figure S4). PXRD of the material sublimed on glass coverslips consistently showed it to be phase pure α-PU.



Figure 1. Molecular packing in PU polymorphs viewed parallel and perpendicular to the urea chain axis. (top) Molecules in α-PU have C-N-C-C torsion angles of -42.9° and 38.2° and twist along the chain axis. (bottom) Molecules in β-PU all have C-N-C-C torsion angles of 54.9° and are related by translation along the chain axis.

Table 1. Crystallographic data for two polymorphs of diphenylurea.


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α-PU* β-PU Formula C13H12N2O C13H12N2O Morphology colorless needle colorless plate Crystal System orthorhombic monoclinic Space Group Pna21 C2/c a (Å) 9.091(8) 23.3264(13) b (Å) 10.535(9) 4.6067(3) c (Å) 11.768(10) 9.9153(5) β (º) 90 98.627(4) 3 vol. (Å ) 1127.0(6) 1053.42 Z 4 4 Temp. (K) 283-303 100 R 0.038 0.0421 Rw NA 0.1161 Torsion 38.90, 54.90, Angles (º) -43.99 54.90 *REFCODE: DPUREA Sublimation on Siloxane Templates. In our previous work with other molecular crystal systems, siloxane monolayers prepared on glass substrates were explored as templating agents for controlling polymorphism.10,11,50 While polymorph selectivity could be achieved under some conditions, the role of solvent in the heterogeneous nucleation process at the siloxane/crystal interface was not well understood. With two distinguishable phases and the ability to grow crystals from the vapor phase, PU seemed an ideal system to examine template directed nucleation in the absence of solvent effects. The present study employed seven different short 3-carbon chain siloxanes with various terminal functionalities. The greatest attention was given to siloxanes with terminal cyano (CN), isocyanate (NCO), acetate (OAc) and bromo (Br) groups, which have contact angles of 56 ± 5°, 61 ± 5°, 67 ± 5°, and 72 ± 4°, respectively. Plain (unmodified) glass slides were used as controls. The use of a flat-bottomed cold finger enabled the siloxane to be securely affixed and ensured that the temperature was uniform across the siloxane. The cold finger temperature (0 – 50°C) and the distance between sample and cold finger were varied systematically, however, changes in these parameters did not affect the major phase obtained. Typically, sublimation was evident after approximately 15 min with an obvious white film covering the surface after one hour. Deposition times were systematically increased in one hour increments, yielding a noticeable change in the morphology of the crystalline particles covering the surface. All experiments were performed a minimum of three times, with the text describing 9 ACS Paragon Plus Environment


only the consistent features observed. Figure 4 illustrates the typical morphologies observed on Si-3-Br surfaces after various sublimation times. At 1 hour, the siloxane surface is covered with plates. At 3 hours there is a significant density of hollow tubes (in addition to some plates), and at 5 hours the surface is covered with clusters of thin needles. Samples are viewed both from the top-down and side-on.

Figure 4. (left) SEM showing the progression of PU crystal morphologies with increasing sublimation time on Si-3-Br siloxanes. Scale bar = 10 µm. (right) Corresponding side-on view with scale bar indicated.

Sublimation onto siloxanes with other terminal functionalities also yielded a range of crystal morphologies, as shown in Figure 5. With long sublimation times, vapor phase growth on all templates resulted in clusters of thin needles over large areas of the siloxane surface (Figure 5A). By focusing on some of the darker (lower) regions between the needles or in areas with less needle coverage, the surfaces generally had a dense underlayer of plates, spirals or small tubular 10 ACS Paragon Plus Environment

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crystals (Figure 5B-D). The calculated Bravais, Friedel, Donnay and Harker (BFDH) crystal morphologies are much simpler, with prisms predicted for α-PU and plates for β-PU (Figure S5). Tubular organic crystals obtained from solution51,52 and via sublimation53,54 have been the subject of previous literature reports and are thought to result from rapid non-equilibrium growth conditions.

Figure 5. SEM images of different PU crystal morphologies vapor deposited on different substrates. (A) Clusters of thin α-PU needles appear on glass and most siloxane surfaces with long (>5 hr) sublimation times. Plates, spirals and hollow tubes of what proved to be β-PU were observed across multiple siloxanes. Representative ones on (B) Si-3-NCO, (C) Si-3-CN and (D) Si-3-Cl surfaces are shown. Scale bar = 10 µm.

Powder X-Ray Diffraction. A major advantage of deposition on siloxane-glass substrates is that the siloxane can be removed from the cold finger and subjected to further analysis. Oriented PXRD was performed on the polycrystalline sample as grown on the substrate. This yields not only phase information but also the preferred orientation(s) within the polycrystalline layer, since only X-ray diffraction lines corresponding to crystal planes parallel to the siloxane are observed.



Sublimation directly onto the cold finger or onto plain glass slides (controls) reproducibly yielded α-PU in phase pure form regardless of the deposition time (Figure 6). Orientations within the polycrystalline α layer varied, but usually corresponded to a mix of (011), (001) and (201) planes. In contrast, β-PU was observed on all the siloxanes, typically exhibiting a strong preferred orientation along (100). β-PU with high phase purity was obtained on Si-3-NCO siloxanes under short sublimation times (1-3 hrs). A very low intensity diffraction line at 2θ = 19.6° was also observed on these siloxanes and with the current instrument resolution, we are unable to confirm if this corresponds to β-(110) or α-(200) (Table S1). It may indicate the presence of very small amounts of α, given that α-(200) is one of the most intense reflections for this phase. With extended sublimation times (5+ hrs), the presence of α-PU becomes more apparent, as evidenced by other diffraction lines. Similar trends were seen when the sublimation time was varied for Si3-Br and Si-3-OAc (Figure S6). On Si-3-CN siloxanes, diffraction lines corresponding to both polymorphs were apparent regardless of deposition time (1–5 hrs), though the mixtures usually contained more β than α.


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Figure 6. Oriented PXRD of vapor deposited PU on different siloxane substrates compared to PXRD simulated from the single crystal cif. On plain glass substrates or when deposited directly on the cold finger, -PU is the only phase observed. PXRD of PU on other siloxanes are listed in order of increasing contact angle. As a general trend, the least hydrophilic surfaces tend to show -PU either predominantly or exclusively.

The time-dependent appearance of α coupled with the changing morphologies seen in SEM images (Figure 4) suggested that the plates and hollow tubes nearest the siloxane corresponded to β-PU, whereas the clusters of fine needles further from the siloxane corresponded to α-PU. To support this hypothesis, grazing incidence angle X-ray diffraction was performed on the polycrystalline films with incident angles ranging from θ = 0.25 – 2.00°. Representative data from Si-3-NCO siloxanes (Figure 7) indicated a larger β:α ratio at low incidence angles (scanning a larger, shallow sample surface area) than at higher angles (scanning a smaller, deeper portion of the sample). The changing ratio is consistent with β formation at the siloxane-vapor interface with the subsequent attachment of α needles to the polycrystalline β surface.



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Figure 7. GIXD of Si-3-NCO templated PU growth. Angles of incidence are shown on the right of each individual PXRD pattern. Reference powder diffraction patterns (calculated from single crystal structure data) are shown at the bottom of the overlay.

Siloxanes Required for β-PU. These experiments indicated that exposed siloxane is a necessary prerequisite to nucleating β-PU under vapor growth conditions. However, since β appeared on siloxanes with a range of functionalities, it seemed unlikely that the polymorph selection was entirely attributable to a specific type of intermolecular interaction. Given the advantages of selfepitaxy (seeding), it also seemed counter-intuitive that a β-PU coated surface would nucleate α rather than β. In previous work examining the solution crystallization of bis-(m-nitrophenyl)urea on siloxane substrates, we found that metastable polymorphs appeared in orientations that aligned the fastest crystallographic growth direction parallel to the siloxane substrate.10

In the vapor

deposition here, the β-PU crystals within the polycrystalline layer are again preferentially aligned 14 ACS Paragon Plus Environment

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with the urea chain (b-axis) parallel to the surface. Although alignment along any (h0k) plane would enable fast growth along the siloxane surface, the strong preference for (100) is evidenced by intense (200), (400) and (600) reflections in the oriented PXRD. The solution-grown and calculated β-PU crystal morphology indicates that growth normal to the (100) plane is the slowest direction. Alignment in this direction allows growth in the two fastest directions to occur parallel to the surface. With the slowest growth normal to the polycrystalline β-PU layer on the siloxane, nucleation elsewhere in the sublimation apparatus (e.g. on the exposed glass surface of the cold finger) may become more energetically favorable. Some fraction of needle-like clusters of α that nucleate elsewhere may spontaneously detach under vacuum conditions and subsequently adsorb onto the polycrystalline β-PU layer. The observation that crystals within the α-PU clusters adopt multiple orientations also supports this hypothesis.

Figure 8. β-PU crystals grown on siloxane templates exhibit preferred orientations along (100). X is the terminal functional group of the siloxane (X = Br, CN, NCO, OH, Ph, OAc, NH2)

CONCLUSIONS Vapor phase growth of small organic molecular crystals is an underutilized method to grow solvent-free crystals or nucleate new polymorphs. Sublimation growth of diphenylurea (PU) in the presence of siloxane templates was found to provide selective, oriented growth of the metastable β polymorph whereas sublimation onto glass yielded α. In reporting these results, we have attempted to illustrate some of the opportunities deposition onto functionalized templates affords as well as some of the challenges that can arise. The fact that the siloxane functionality does not appear to have a major effect on the phase obtained was not expected, but illustrates that 15 ACS Paragon Plus Environment


template-directed polymorph selectivity at interfaces remains a complicated process even in the absence of solvent. Polymorph selectivity may in part be due to differences in the mobility of gasphase molecules on glass and siloxane surfaces, though varying the cold finger temperature did not change the major phase obtained. It is less obvious how interfacial interactions lead to the different but reproducible crystal morphologies observed on siloxanes with different terminal functionalities. These unusual morphologies may also be of interest in relation to the development of theoretical models for predicting crystal habit and sublimation thermodynamics.55-58

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS The authors are grateful for financial support provided by the National Science Foundation under awards 1429079 and 0959546. MAS thanks the ARCS Foundation for a predoctoral fellowship. We additionally thank Jasper Nijdam and Leon Der for assistance with SEM, Earl Morris for making the glassware for the sublimation apparatus, and Jeffrey Bertke for his assistance with single crystal X-ray diffraction.

References (1) Bernstein, J.: Polymorphism in Molecular Crystals; Oxford University Press: New York, 2002. (2) Weissbuch, I.; Addadi, L.; Lahav, M.; Leiserowitz, L. Molecular Recognition at Crystal Interfaces. Science 1991, 253, 637-645. (3) Davey, R.; Blagden, N.; Potts, G.; Docherty, R. Polymorphism in Molecular Crystals: Stabilization of a Metastable Form by Conformational Mimicry. J. Am. Chem. Soc 1997, 119, 1767-1772. (4) Price, C.; Grzesiak, A.; Matzger, A. Crystalline Polymorph Selection and Discovery with Polymer Heteronuclei. J. Am. Chem. Soc 2005, 127, 5512-5517. 16 ACS Paragon Plus Environment

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Table of Contents Graphic & Synopsis For Polymorph Selection via Sublimation onto Siloxane Templates Marina A. Solomos, Christina Capacci-Daniel, J. Faye Rubinson and Jennifer A. Swift

Diphenylurea is dimorphic. Sublimation onto plain glass substrates yields the more stable α phase, while vapor deposition onto siloxane coated templates results in the less stable β as the major phase. The appearance of traditional and/or non-equilibrium crystal morphologies depends on the siloxane functionality.


Polymorph Selection via Sublimation onto Siloxane Templates

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