Preparation and Structural Evaluation of the Conformational Polymorphs of r-[(4-Methoxyphenyl)methylene]-4-nitrobenzeneacetonitrile Ranko M. Vrcelj,† Evelyn E. A. Shepherd, Choon-Sup Yoon,‡ John N. Sherwood,* and Alan R. Kennedy
CRYSTAL GROWTH & DESIGN 2002 VOL. 2, NO. 6 609-617
Department of Pure and Applied Chemistry, University of Strathclyde, 295 Cathedral Street, Glasgow G1 1XL, Scotland, U.K. Received May 17, 2002
ABSTRACT: Crystals of R-[(4-methoxyphenyl)methylene]-4-nitrobenzeneacetonitrile (C16H12O3N2) are shown by X-ray crystallography to exist in three polymorphic trans forms (I-III) and one cis configuration (IV). Calorimetric studies show that at low temperature form I is thermodynamically the most stable phase, followed by form III and form II. On heating, form III undergoes a phase transformation to form II. There is no direct thermal transformation between forms II and I. Form I melts at 164.5 °C. Mechanical damage of form II initiates a phase transformation into form III, and structural studies confirm that forms II and III, both of which show attractive second-order nonlinear optical behavior, are closely related. Though clearly conformational polymorphs, they also exhibit the essence of orientational polymorphism. Although form I is the most stable of the three trans isomers, nevertheless forms II and III are indefinitely stable and may be useful for development for NLO purposes. Crystal growth studies show that growth from the melt and vapor phases could be the only routes for the preparation of specimens for optical examination. The results show well the difficulties involved in the preparation and isolation of the different polymorphic forms of such materials. 1. Introduction Over the past 20 years interest has grown in novel, highly polar, organic materials that show large secondorder nonlinear optical (NLO) susceptibilities. These materials have potential for applications in frequency conversion and related processes. Materials such as 3-methyl-4-nitroaniline (MNA)1,2 and (-)-2-((R-methylbenzyl)amino)-5-nitropyridine (MBANP)3 have been shown to have second- and third-order nonlinear optical properties that are favorable when compared with those of traditional inorganic NLO materials such as potassium titanyl phosphate (KTP)4 and lithium borate (LBO).5 In the search for new and improved organic materials many properties have to be defined, ranging from the ability to grow large, good-quality single crystals for possible use in NLO devices6 to damage thresholds for any given material.7 Much time and effort have been expended in crystal growth, whether from solution, the vapor phase, or the melt, to provide samples for the accurate evaluation of optical performance. However, unlike parallel developments in the related fine chemical and pharmaceutical industries, little or no importance is given to the existence of polymorphism, the undefined occurrence of which could severely limit the applicability of such materials.8 In the two mentioned fields, polymorph screening is commonplace; in the area of NLO materials such screening is minimal. Thus, 5-nitrouracil was believed to be a welldefined material with two forms, a monohydrate9 and a single anhydrous NLO-active orthorhombic phase.10,11 * To whom correspondence should be addressed. E-mail:
[email protected]. Tel: +44 141 548 2797. Fax: +44 141 548 4822. † Current address: Combinatorial Centre of Excellence, Department of Chemistry, University of Southampton, Highfield, Southampton SO17 1BJ, England, U.K. ‡ Current address: Department of Physics, KAIST, 373-1 KusongDong, Yusong-Gu, Taejon 305-701, Korea.
The most stable of its phases, however, is the recently discovered anhydrous monoclinic phase, which is inactive for NLO purposes.12 This polymorph has been shown to develop under much the same conditions as the first. Attempts to develop the former may thus yield the latter. The potential for polymorph formation rises with the complexity of the molecule. The relative flexibility of the more active, complex NLO molecules means that for any given molecule a number of relatively stable packing arrangements will be possible. Indeed, it is this flexibility which may be of use to crystal engineers for either designing or using organic NLO materials in the future. Although for any given set of polymorphs of a material, a number will not be of use (for example, due to either high thermodynamic instability or inappropriate space group), the understanding of how a set of polymorphs is structurally interrelated is important. Without this knowledge the growth of large perfect crystals can be problematic. R-[(4-Methoxyphenyl)methylene]-4-nitrobenzeneacetonitrile (C16H12O3N2), hereafter called CMONS, a nomenclature derived from the methoxy-nitrostilbene structure with an additional letter prefix (hence, there also exist AMONS, BMONS, etc.), is one of a family of yellow (in the terminology of Zyss13) related compounds which exhibit NLO behavior. The molecular structure is shown in Figure 1. The growth of CMONS from solution has proved to be difficult, since it is either insoluble or sparingly so in a wide range of solvents. Oliver et al.14 indicated by using optical absorption and UV luminescence spectroscopy that they had identified three polymorphs of CMONS following crystallization from toluene, acetic acid, and a dichloromethane/ industrial methylated spirit mix. Two of these polymorphs were active with respect to NLO behavior; the third was not. Gilmour,15 growing crystals by vapor
10.1021/cg025529h CCC: $22.00 © 2002 American Chemical Society Published on Web 09/07/2002
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Vrcelj et al. Table 1. Crystallographic Data for the Crystal Forms of CMONS Isomers
Figure 1. Connectivity diagram of the molecular structure of CMONS.
diffusion and utilizing differential scanning calorimetry, showed the existence of a possible fourth polymorph. Sherwood16 showed that good large crystals can be grown from 1,4-dioxane; however, these proved to be solvated and thermally unstable.17 Crystals of the NLOinactive form have also been grown from the vapor phase, and the structure has been elucidated.18 Even given these problems, CMONS has already proved to be of interest to those attempting to create optical devices using organic NLO materials, with its use in crystal-cored optical fibers or as thin films being possible applications.19 Because of this interest, we have examined the structural interrelationship between the forms and their relative stability on growth from the melt, the solution, and the vapor phase.
formula mol wt cryst syst a, Å b, Å c, Å β, deg V, Å3 T, K space group Z
form 1
form II
form III
cis
C16H12N2O3 280.28 monoclinic
C16H12N2O3 280.28 monoclinic
C16H12N2O3 280.28 monoclinic
C16H12N2O3 280.28 monoclinic
3.814(1) 12.371(3) 28.132(5) 91.50(2) 1327.1(6) 123 P21/n
4.6002(7) 31.947(4) 9.511(1) 92.81(1) 1396.1(3) 293 Cc
3.859(1) 30.971(5) 11.346(2) 91.17(2) 1355.7(4) 295 Cc
8.543(3) 12.697(4) 12.572(4) 101.52(3) 1336.2(7) 123 P21/c
4
4
4
4
was performed on a Siemens D500 diffractometer in BraggBrentano geometry, using Cu KR radiation and a graphite post-sample monochromator. X-ray Laue´ diffraction patterns were collected at station 7.6 of the Daresbury Laboratory, Daresbury, U.K. Calorimetric studies were carried out using a Mettler FP84HT TA microscopy variable-temperature hot stage provided with DSC facilities.
3. Results 2. Experimental Section CMONS, prepared by condensation of 4-methoxybenzaldehyde and 4-nitrobenzeneacetonitrile using piperidine as a basic catalyst,20 was purified by multiple recrystallizations from toluene and dried before use. All solvents were of analytical grade and were dried and distilled. 2.1. Growth from Solution. Small crystallites and powders of CMONS were grown by the slow evaporation of saturated solutions at constant temperature (25 °C) over periods of ca. 4 weeks. A number of solvents were used: toluene, acetic acid, methyl ethyl ketone (MEK), carbon tetrachloride, and acetone. 2.2. Growth from the Vapor Phase. Crystals were grown from the vapor phase by sublimation in an evacuated tube placed in a temperature gradient.21 The source temperature was set at 160 °C, and the operational part of the temperature gradient varied linearly from 160 to 60 °C, yielding an average gradient of 2.5 °C cm-1. Once established, the sublimation was allowed to proceed for several (3-5) days before the removal and examination of the growth. 2.3. Growth from the Melt. Melt crystallization was carried out by the Bridgman technique using the twocondenser system developed by McArdle et al.22 Heating of the two zones was achieved using the vapor of boiling diisobutyl ketone (167 °C) in the upper condenser and either boiling xylene (147 °C) or hexan-1-ol (157 °C) in the lower condenser. The crucible lowering rates lay in the range 0.1-1 mm h-1. 2.4. Single-Crystal XRD Measurements and Analysis. Data were collected on a Rigaku AFC7S four-circle diffractometer using a monochromatic wavelength of 0.711 Å. The crystal structures were solved using SIR and refined using TEXSAN. Atoms other than hydrogen atoms were located experimentally and refined anisotropically. The hydrogen atoms of forms II and III were refined isotropically, those of form IV had only Uiso refined, and in form I all hydrogen atoms were placed in calculated positions with riding modes. Crystallographic parameters are given in Table 1, and full details are available as Supporting Information. Further data for form I was collected on a Bruker AXS diffractometer with a SMART area detector using monochromatic radiation of wavelength 0.687 Å at station 9.8 of the Synchrotron Radiation Source (SRS) at the Daresbury Laboratory, Warrington, U.K. 2.5. Other Structural Analysis. Photography was performed using an Olympus OM2 camera mounted on an Olympus BO61 stereoscopic microscope and recorded on standard ASA 400 color film. X-ray powder diffraction (PXRD)
3.1. Crystal Growth. From Solution. All solvents yielded similar results, in that microcrystalline mixtures of the three trans polymorphs containing some larger (50-100 µm) crystals were produced. There was no distinction in composition which suggested that any of the solvents played a particular role in directing the polymorphic nature of the crystalline product other than what might be expected from the phase diagram and the kinetics of the crystallization process. In all cases, when left in contact with the solvent, the mixtures of polymorphs slowly converted to the most stable form I at a rate that reflected the solubility of the material in the solvent. Figure 2 presents typical powder X-ray diffraction (PXRD) traces of the solids retrieved from the evaporation of CCl4 solutions. Figure 2a shows the trace obtained for the separated yellow material, corresponding to form III, and Figure 2b that for the remaining powder, a mixture of forms I and II. These traces confirm the ease of distinction of the polymorphs. The colors of the separate forms were also distinctive and, once the structures were defined by XRD, could be used to identify the polymorphs even in mixtures. These were as follows: form I, deep yellow; form II, orange; form III, pale yellow. From the Vapor. Growth from the vapor phase of the purified CMONS yielded crystals of three of the forms: I, II, and IV. Form I was the predominant phase found throughout the sublimation tube at all supersaturations. A few crystals of form II were found at the hottest end of the tube. A scant amount of form IV was retrieved from the coolest end of the tube. No crystals of form III were observed to form in the growth tube. This polymorph could be formed, however, by the mechanical damage of crystals of form II. This mechanically induced transformation was first noted during attempts to record X-ray topographs23 of the small vapor-grown crystals of form II. For these experiments small perfect crystals were sandwiched between Mylar films for attachment to the goniometer of the X-ray
Conformational Polymorphs of C16H12O3N2
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Figure 2. Powder X-ray diffraction patterns of the transCMONS polymorphs crystallized from solution in carbon tetrachloride: (a) form III; (b) mixture of forms I and II.
Figure 4. Synchrotron X-ray Laue diffraction patterns of adjacent (a) form II and (b) form III sections of a partially mechanically transformed form II crystal of CMONS originally grown from the vapor phase.
Figure 3. Crystals of CMONS form II (orange) grown from the vapor phase which have converted in part to form III (yellow) under the influence of applied stress (Figure 7 shows a detailed diagram of crystal in part c).
camera. Suddenly, and following the mounting process, the thin crystals deformed by bending in plane to form a series of orange and yellow sections, each related to the other by a well-defined structural reorientation (Figure 3). It was later shown that the same change could be initiated immediately by cutting the form II crystals with a scalpel, by sticking them to adhesive tape, or by mechanical grinding. Being positioned on the topographic system allowed Laue diffraction patterns to be recorded for each of the sections, to define their single crystallinity and relation-
ship (Figure 4). Judging from the nature of the diffraction images, the product form III crystal is considerably less strained than the form II crystal from which it is developed. This is in accord with the fact that the structure is under some constraint to transform but requires nucleation to do so. From the Melt. Crystallization from the melt using the Bridgman technique21 (upper temperature 167 °C, lower temperature 147 °C, crucible lowering rate 1 mm h-1) immediately yielded a deep orange polycrystalline mass (form II). Following rapid lowering of the hot crystal to ambient temperature and during removal from the growth tube, the boule transformed into a fibrous mass of yellow crystals (form III). Reduction of the lowering rate to 0.2 mm h-1 under the same temperature gradient resulted in a strained singlecrystal boule of orange color. On this occasion the boule could be removed from the growth tube without transformation but converted to a mass of yellow needlelike crystals (form III) over the following 4 days. When the crucible lowering rate was reduced even further (
