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Polymorphism in a trans-Cinnamic Acid Derivative Exhibiting Two Distinct β‑type Phases: Structural Properties, [2 + 2] Photodimerization Reactions, and Polymorphic Phase Transition Behavior Manal A. Khoj, Colan E. Hughes, Kenneth D. M. Harris, and Benson M. Kariuki* School of Chemistry, Cardiff University, Main Building, Park Place, Cardiff, CF10 3AT, U.K. S Supporting Information *

ABSTRACT: We report the discovery of a rare case of polymorphism among trans-cinnamic acid derivatives in which both polymorphs are classified as β-type structures based on their solidstate structural properties and their solid-state photoreactivity. Specifically, for 3-fluoro-transcinnamic acid, crystallization from many solvent systems leads to formation of one polymorph (denoted β1), although, in a few cases, concomitant crystallization of another polymorph (denoted β2) is also observed. On heating the β1 polymorph, a solid-state phase transition occurs at ca. 119 °C to produce the β2 polymorph. This polymorphic phase transition is irreversible, and the β2 polymorph remains stable on subsequent cooling to ambient temperature. Both the β1 and the β2 polymorphs undergo a topochemical [2 + 2] photodimerization reaction upon UV irradiation to produce 3,3′difluoro-β-truxinic acid as the photoproduct in almost 100% yield. However, these reactions are associated with complete loss of crystallinity, preventing the structural properties of the directly produced solid photoproduct from being determined by singlecrystal or powder X-ray diffraction techniques. ture and photoreactivity in these materials, with the α-, β-, and γ-type crystals each having a characteristic mode of molecular packing. A general observation from these studies was that, for all photoreactive (α-type and β-type) crystals, the distance between the centers of the CC bonds of potentially reactive monomer molecules is less than ca. 4.2 Å, whereas the corresponding distance in all photostable (γ-type) crystals is greater than ca. 4.7 Å. On the basis of these well-defined structure−reactivity correlations, the solid-state photodimerization reactions of trans-cinnamic acid and its derivatives represent a classic illustration of the topochemical princi ple,5−13 according to which the product obtained in a solidstate reaction can be rationalized directly from knowledge of the spatial arrangement of molecules in the reactant crystal structure.4 Polymorphic phase transitions provide a means of exploring the delicate balance between interactions in crystalline solids, as they yield insights on the changes in thermodynamic stability of different crystal structures of the same molecule in response to relatively small perturbations in environmental conditions, such as temperature or pressure. The prospect that polymorphic phase transitions may occur in trans-cinnamic acids arose from the seminal work of Schmidt and co-workers,2 although this issue received little attention until relatively recently.16,17 Transformations between two members of the same polymorphic classification (i.e., α, β, or γ) are of particular relevance

1. INTRODUCTION Solid-state transformations encompass a range of different types of processes, including solid-state chemical reactions, polymorphic phase transitions and solvation/desolvation processes. In most cases, such transformations proceed with loss of singlecrystal integrity, such that a single crystal of the starting solid phase transforms into a polycrystalline sample of the product. In extreme cases of this type, however, the transformation is associated with complete loss of crystallinity and the product phase is amorphous. At the other extreme, some solid-state transformations can occur with retention of single-crystal integrity, particularly when there are only small structural differences between the initial and final structures and the structural reorganization involved in the process is minimal. Historically, an important class of solid-state chemical reactions is the [2 + 2] photodimerization reactions of transcinnamic acid and its derivatives. Rationalization of these reactions led to an understanding of the direct relationship between crystal structure and photoreactivity1−15 and contributed significantly to the development of the topochemical principle for solid-state reactivity. It was found1−3 that crystals of these materials can be classified into three types (denoted α, β, and γ) according to their behavior in [2 + 2] photodimerization reactions. Thus, UV irradiation of α-type crystals produces a centrosymmetric (α-truxillic acid) dimer, UV irradiation of β-type crystals produces a mirror-symmetric (β-truxinic acid) dimer, whereas no reaction occurs when γ-type crystals are exposed to UV radiation. Single-crystal X-ray diffraction (XRD) studies4 demonstrated well-defined correlations between crystal struc© XXXX American Chemical Society

Received: June 19, 2013 Revised: July 11, 2013

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dx.doi.org/10.1021/cg4009202 | Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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2. EXPERIMENTAL SECTION

to the present study; examples are the transformation of the triclinic α polymorph of 2-ethoxy-trans-cinnamic acid to a second triclinic form on heating above 333 K,18 transformation of the triclinic β polymorph of 4-trifluoromethyl-trans-cinnamic acid to a second triclinic form on cooling below 132 K,19 and transformation of the monoclinic β polymorph of 4-bromotrans-cinnamic acid to a second monoclinic form on cooling below ca. 250−260 K.20 In all cases, the transformations are reported to be reversible. However, to our knowledge, the coexistence (as stable and metastable polymorphs) of two members of the same polymorphic classification (α, β, or γ) for a simple trans-cinnamic acid has not been reported previously. In the present work, we report the solid-state structural properties of 3-fluoro-trans-cinnamic acid (3-FCA; Scheme 1),

Crystallization. The sample of 3-FCA (>98%) used in this work was purchased from Alfa Aesar. For crystallization experiments, 3-FCA was dissolved in a solvent or mixture of solvents and left at ambient temperature (usually for several days) to allow slow evaporation of solvent until crystals appeared. The solvents used were ethanol (Aldrich, >99.8%), methanol (Fisher, >99.5%), acetone (Fisher, >99%), deuterated acetone (Aldrich, 99.5% atom % D), acetonitrile (Fisher, >99.99%), dimethylformamide (Alfa Aesar >99%), dichloromethane (Fisher, >99%), and glacial acetic acid (Fisher, >99%). In addition, the following mixed solvent systems (∼1:1 ratio) were used: ethanol/methanol, methanol/acetone, acetone/acetonitrile, methanol/ acetonitrile, and ethanol/acetonitrile. The crystals obtained in each case were colorless plates. Powder XRD studies of the crystallization products were carried out at ambient temperature on a Bruker D8 instrument operating in transmission mode (CuKα1 radiation, Ge monochromated). Powder XRD analysis of the crystallization products indicated that, with the exception of crystallization from acetonitrile or deuterated acetone, all crystallization experiments gave the same monophasic product, which we designate as β1. Crystallization from acetonitrile and crystallization from deuterated acetone, on the other hand, produced β1 together with smaller amounts of a second phase that we designate as β2 (see the Results and Discussion section for more details). Thermal Analysis. Differential scanning calorimetry (DSC) was carried out on a TA Instruments DSC Q100. About 2−4 mg of sample was placed in a covered aluminum pan. Heating/cooling rates within the range of 10−20 °C min−1 were used, with an equilibration time of 1 min between heating and cooling cycles. The thermal behavior of larger amounts of sample was studied by placing them in vials on hot plates set at appropriate temperatures. UV Irradiation. The solid material was ground to a fine powder and spread in a thin layer on a glass dish and exposed to UV radiation

Scheme 1. Molecular Structure of 3-Fluoro-trans-cinnamic Acid (3-FCA)

including the discovery of two polymorphs that are both classified as β-type structures. We explore both a polymorphic phase transition between these phases and the photoreactivity of each polymorph. In the tradition of the cinnamic acids, 3FCA proves to be a model material for such studies.

Table 1. Crystallographic Data for the β1 and β2 Polymorphs of 3-FCA and for the Acetic Acid Solvate of 3,3′-Difluoro-βtruxinic Acid

formula formula weight T (K) λ (Å) crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) volume (Å3) Z density calculated (Mg/m3) absorption coefficient (mm−1) F(000) crystal size (mm3) Θmax collected (deg) reflns collected independent reflns R(int) goodness-of-fit on F2 final R1 indices [I > 2σ(I)] final wR2 R1 indices (all data) wR2 (all data)

β1 polymorph

β2 polymorph

3,3′-difluoro-β-truxinic acid/acetic acid solvate

C9H7FO2 166.15 150(2) 1.5418 triclinic P1̅ 3.8430(3) 6.2681(4) 15.9607(12) 93.547(5) 96.858(6) 94.011(5) 379.82(5) 2 1.453 1.011 172 0.48 × 0.19 × 0.06 73.88 2238 1448 0.0139 1.058 0.0556 0.1549 0.0645 0.1646

C9H7FO2 166.15 150(2) 1.5418 monoclinic P21/c 6.5290(4) 3.7750(2) 31.1015(18) 90 91.250(4) 90 766.38(8) 4 1.440 1.002 344 0.20 × 0.12 × 0.03 73.42 2253 1462 0.0160 1.096 0.0513 0.1442 0.0597 0.1518

C20H18F2O6 392.34 150(2) 1.5418 triclinic P1̅ 7.6159(5) 8.6685(4) 15.1547(9) 77.057(5) 77.346(5) 74.730(5) 926.91(9) 2 1.406 0.999 408 0.19 × 0.13 × 0.02 73.91 6905 3618 0.0470 1.018 0.0520 0.1244 0.0859 0.1474

B



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from a high-pressure mercury vapor lamp (λ = 300−700 nm). The irradiation was continued for up to 106 h with periodic agitation of the powder (typically every 45 min to 6 h), and samples were collected periodically (usually every 6 or 12 h). Spectroscopic Analysis. Samples were analyzed by IR spectroscopy, solution-state 1H and 13C NMR, and electrospray mass spectrometry (ES-MS). IR spectra were recorded on a Shimadzu IRAffinity-1 Fourier transform infrared spectrometer. Solution-state NMR spectra were recorded on a Bruker AM 400 MHz instrument, with the sample dissolved in deuterated methanol [CD3OD] for 1H NMR and deuterated acetone [(CD3)2CO] for 13C NMR. ES-MS was carried out using a Waters LCT Premier XE mass spectrometer. Crystal Structure Determination. Single-crystal XRD data were collected on an Agilent SupaNova Dual Atlas diffractometer with a mirror monochromator [Cu (λ = 1.5418 Å) and Mo (λ = 0.7107 Å)], equipped with an Oxford Cryosystems cooling apparatus. Crystal structures were solved and refined using SHELX.21 In each structure reported here, the 3-fluorophenyl groups exhibit orientational disorder, and geometric restraints were applied during refinement. Nonhydrogen atoms were refined with anisotropic displacement parameters. Hydrogen atoms were inserted in idealized positions, and a riding model was used with Uiso set at 1.2 or 1.5 times the value of Ueq for the atom to which they are bonded. Crystallographic and structure determination data are summarized in Table 1.

3. RESULTS AND DISCUSSION Structural Properties of the β1 and β2 Polymorphs of 3-FCA. Crystallization of 3-FCA revealed two distinct crystalline phases that we denote as β1 and β2 for consistency with the α, β, and γ classification used for polymorphs of transcinnamic acids. Crystallization from all solvents listed in the Experimental Section (followed by analysis of the crystallization products by powder XRD) was found to favor the formation of the β1 polymorph, yielding monophasic samples of β1 in all cases with the exception of crystallization from acetonitrile or deuterated acetone; from these solvents, the β2 polymorph was also present as a minor second phase. The two polymorphs were indistinguishable based on crystal morphology, and initial structural characterization of β2 was by a trial-and-error analysis of crystals from the mixed-phase sample. The crystal structure of the β1 polymorph of 3-FCA is triclinic (space group P1)̅ . The asymmetric unit comprises one molecule, with the 3-fluorophenyl group disordered between two orientations related by 180° rotation about the C(phenyl)−C(vinyl) bond. The refined occupancies of the major and minor components are 0.927(4) and 0.073(4) (Figure 1a). Crystallographic data are shown in Table 1. The crystal structure comprises pairs of planar molecules linked by hydrogen bonds, forming the dimeric motif commonly observed for carboxylic acids [O(2)−H(2A)···O(1): O···O, 2.623(2)Å; O−H···O, 171.8°]. The dimers are stacked parallel to the a axis (Figures 1b and 2a), and the long axis of the molecule forms an angle of 107° with the stacking direction. The arrangement of the 3-FCA molecules is such that the structure contains fluorine-rich layers parallel to the ab plane. The planes of all molecules are parallel, and the distance between the centers of the CC bonds of neighboring molecules along the a axis is 3.843 Å. The crystal structure of the β2 polymorph of 3-FCA (Figures 1c and 2b) is monoclinic (space group P21/c) with one molecule in the asymmetric unit. The 3-fluorophenyl group is disordered between two orientations in a similar manner to β1, with the two orientations related by 180° rotation about the C(phenyl)−C(vinyl) bond. The refined occupancies for the major and minor components are 0.831(3) and 0.169(3). As

Figure 1. (a) The asymmetric unit in the β1 polymorph of 3-FCA showing the disorder and (b) the crystal structure viewed along (11̅0) showing the parallel orientation of the planes of the molecules. (c) Crystal structure of the β2 polymorph of 3-FCA viewed along (110), showing the two orientations of the molecular planes. Only the orientation of the fluorophenyl rings corresponding to the major disorder component is shown in the packing diagrams. For clarity, hydrogen atoms are omitted from (b) and (c).

Figure 2. (a) Crystal structure of the β1 polymorph of 3-FCA viewed along the b axis with red arrows indicating the rotation about the C−F bonds that would lead to the structure of the β2 polymorph. (b) Crystal structure of the β2 polymorph of 3-FCA viewed along the a axis. Only the major component is shown.

for the β1 polymorph, the crystal structure of the β2 polymorph contains hydrogen-bonded dimers [O(2)−H(2A)···O(1): C



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O···O, 2.619(2) Å; O−H···O, 170.9°]. These dimers are stacked along the b axis to form layers parallel to the ab plane and separated by regions of high fluorine density. The distance between the centers of the CC bonds of neighboring molecules along the b axis is 3.775 Å. The main difference between the crystal structures of the β1 and β2 polymorphs concerns the structural relationship between neighboring layers. The layers are related by c-glide symmetry in the β2 polymorph, and hence the long axes of the molecules in adjacent layers make different angles (101° and 79°) with the stacking direction. Consequently, the planes of the molecules in adjacent layers are not parallel, and the angle between the planes is 59.5°. In contrast, in the structure of the β1 polymorph, the planes of all molecules are parallel. The β2 polymorph of 3-FCA is isostructural with the β polymorph of trans-cinnamic acid.22 The crystal structures of β polymorphs of simple transcinnamic acid derivatives are found to fall into two general structure types: a monoclinic group and a triclinic group. In the monoclinic group, there are two orientations of the molecular planes, and examples in this group are trans-cinnamic acid, 4formyl-trans-cinnamic acid,23 2-fluoro-trans-cinnamic acid, 4fluoro-trans-cinnamic acid,24 3-nitro-trans-cinnamic acid,25 3bromo-trans-cinnamic acid,26 and 2-nitro-trans-cinnamic acid.27 In the triclinic group, the planes of all molecules are parallel to each other; this group includes 3-chloro-trans-cinnamic acid29 and 2-bromo-trans-cinnamic acid.24 Polymorphic Phase Transition. On heating the β1 polymorph from ambient temperature to 175 °C, DSC reveals two endothermic processes with onset temperatures of 119− 121 and 167 °C (Figure 3a). The second endotherm is consistent with the independently determined melting temperature. After melting the sample, fast cooling (immediate cooling to ambient temperature) is found by powder XRD to produce crystals of β1 only, whereas slow cooling (e.g., 10 °C/min) gives monophasic β2.28 The thermal event observed at ca. 119−121 °C on the heating cycle of the DSC experiment is attributed to a solidstate phase transition from the β1 polymorph to the β2 polymorph. Provided the sample is not heated above the melting point (Figure 3b), subsequent cooling indicates that this phase transition is irreversible; after cooling the sample to ambient temperature, powder XRD confirms that the sample is the β2 polymorph. Visual examination of this sample shows that it contains crystals suitable for single-crystal XRD. However, direct observation of the phase transition by single-crystal XRD is difficult because of significant sublimation of the crystal as the temperature is increased toward the phase transition temperature. However, for a crystal of the β1 polymorph “protected” in a drop of perfluoropolyether oil (to reduce the rate of sublimation), single-crystal XRD studies confirm that the phase transition at ca. 119−121 °C occurs with retention of singlecrystal integrity. In further DSC studies to explore the behavior of the β1 polymorph and the β2 polymorph at low temperature, no thermal events are observed for either polymorph on cooling from ambient temperature to 150 K. The structural rearrangement required to convert the β1 polymorph to the β2 polymorph can be visualized in terms of two hypothetical steps (although we emphasize that we are not implying in any way that this discussion represents a proposed mechanism for the polymorphic phase transition). The first step involves rotation of the molecules in one layer of hydrogen-bonded dimers by ca. 60° about the C−F bond

Figure 3. DSC data (with exotherms shown as positive peaks) starting from a sample of the β1 polymorph: (a) heated to 175 °C, followed by cooling (the inset shows the endotherm due to the polymorphic phase transition from β1 to β2), and (b) heated to 150 °C, followed by cooling (note that the reverse phase transition from β2 to β1 does not occur on cooling).

(red arrows in Figure 2a) without disrupting the hydrogen bonding. The second step involves readjustment of the unit cell, including doubling of the c axis, as required to obtain the monoclinic β2 structure from the triclinic β1 structure. Solid-State [2 + 2] Photodimerization Reactions. The crystal structures of both the β1 and the β2 polymorphs of 3FCA have the ideal geometry for solid-state [2 + 2] photodimerization reactions (Scheme 2). Potentially reactive Scheme 2. Photodimerization Reaction of 3-FCA To Give 3,3′-Difluoro-β-truxinic Acid

neighboring molecules along the stacks are related by unit cell translation (a axis for β1 and b axis for β2). The CC bonds of neighboring molecules along the stack are parallel to each other, and the distance between the centers of the CC bonds is 3.843 Å for the β1 polymorph and 3.775 Å for the β2 polymorph. As both polymorphs of 3-FCA fulfill the structural criteria characteristic of β-type structures, it would be expected on topochemical grounds that the photoproduct obtained in D



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each case would be the mirror-symmetric dimer (3,3′-difluoroβ-truxinic acid). We have studied the photochemical reactivity of both the β1 and the β2 polymorphs of 3-FCA under UV irradiation. An initial indication that a photoreaction occurs in each case is the observation of a color change from white to yellow, which becomes increasingly noticeable as the irradiation time increases. For each polymorph, analysis of the photoproduct by ES-MS confirms that a dimer is indeed obtained, characterized by a significant M+ peak at m/z = 331.06 (together with residual monomer at m/z = 156.02 for incomplete reactions). Furthermore, solution-state 13C NMR data show that the 13C resonances at ca. 122 and 138 ppm, due to the CC carbons of the monomer, are lost and are replaced by peaks at ca. 43 and 46 ppm, characteristic of a cyclobutane ring in the photoproduct. The results from IR spectroscopy also demonstrate the progressive consumption of the monomer and formation of the dimer as a function of irradiation time. The CC stretching band (1633 cm−1, Figure 4, band 4) decays, consistent with loss

Figure 5. Powder XRD patterns for (a) the β1 polymorph of 3-FCA, (b) the β2 polymorph of 3-FCA, (c) the product following UV irradiation of the β1 polymorph, and (d) the product following UV irradiation of the β2 polymorph. From (c) and (d), it is clear that crystallinity is lost after irradiation of both the β1 and the β2 polymorphs.

Figure 4. IR spectra showing the region from 1350 to 1800 cm−1 for (a) the β1 polymorph of 3-FCA and for samples collected following UV irradiation for (b) 6, (c) 12, (d) 18, (e) 24, (f) 30, (g) 36, and (h) 42 h.

of the CC bond on reaction, the CO stretching band (ca. 1696 cm−1, Figure 4, band 3) concurrently shifts to higher frequency (ca. 1710 cm−1, Figure 4, band 2), and another band grows at 1750 cm−1 (Figure 4, band 1). A sample of the photoproduct crystallized from acetic acid displays only band 2 (i.e., no bands 1 or 3), indicating that the shift from ca. 1696 to ca. 1710 cm−1 is due to loss of conjugation during the dimerization reaction. Band 1 (1750 cm−1) may be associated with changes in the crystal environment and disruption of hydrogen bonding that accompany the loss of crystallinity revealed by powder XRD (Figure 5). To confirm that the photoproduct is indeed 3,3′-difluoro-βtruxinic acid, the crystal structure of a recrystallized sample of the photoproduct was determined from single-crystal XRD data. The sample for this study was prepared by crystallization of the photoproduct from acetic acid. In the crystal structure, the asymmetric unit comprises one molecule of 3,3′-difluoro-βtruxinic acid and one molecule of acetic acid (Figure 6a). Crystallographic data are shown in Table 1. The two 3fluorophenyl groups of the 3,3′-difluoro-β-truxinic acid molecule are disordered between two orientations related by 180° rotation about the C(phenyl)−C(cyclobutane) bond, with

Figure 6. (a) The asymmetric unit in the crystal structure of the acetic acid solvate of 3,3′-difluoro-β-truxinic acid. (b) Part of the crystal structure showing a layer of chains. Only the orientation of the fluorophenyl rings corresponding to the major disorder component is shown, and dashed lines represent hydrogen bonds.

occupancies of 0.564(5) and 0.435(5) for F1/F1A and occupancies of 0.584(4) and 0.416(4) for F2/F2A. The C−C−C bond angles within the cyclobutane ring deviate E



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cyclobutane ring at ca. 4.3−4.7 ppm (labeled ii in Figure 7) appear and grow as a function of irradiation time. Investigation of microscopic samples is informative,24,35 but the effectiveness of solid-state photochemistry in synthesis, for example, depends on its applicability to bulk samples. In the present work, solution-state 1H NMR provides a reliable method for monitoring the reaction in bulk samples, and we focus on the results of two experiments starting from the β1 polymorph of 3-FCA. In the first experiment, ca. 100 mg of material was spread evenly in a Petri dish with a diameter of 5.5 cm, and in the second experiment, about 300 mg of sample in a dish with a diameter of 7.5 cm was used (the density of material in each case was ca. 6.5 mg/cm2). For the first experiment using the β1 polymorph, 10 samples were collected over a period of 12 h. The first sample was collected 60 min after initiating UV irradiation, followed by sampling at intervals of 45, 45, 45, 45, 90, 90, 90, 90, and 120 min. The solid was mixed thoroughly before collecting each sample or at 45 min intervals between sample collections. The collected samples were dissolved in deuterated methanol, and 1 H NMR spectra were recorded. The percentage of unreacted monomer was determined from integrated peak intensities and is shown in blue in Figure 8a [reactant % = 100 × reactant/ (reactant + product)]. The results show that over 93% of the monomer was consumed after 12 h. A plot of ln(reactant %) versus time is linear (blue line in Figure 8b) with R2 = 0.9986, consistent with a first-order reaction.

only slightly from 90°, ranging from 87.86(15)° to 90.59(16)°. The carboxylic acid groups of neighboring 3,3′-difluoro-βtruxinic acid molecules are linked to each other through hydrogen bonding [O(2)−H(2A)···O(1): O···O, 2.680(2) Å; O−H···O, 172.9°; O(4)−H(4)···O(3): O···O, 2.655(2) Å; O− H···O, 178.3°] to form zigzag chains. Adjacent chains interact in a ziplike fashion through the fluorophenyl groups to form layers parallel to the (112) plane (Figure 6b). The loss of crystallinity that accompanies the photodimerization reactions for both the β1 and the β2 polymorphs (see Figure 5) is possible evidence that the 3,3′-difluoro-βtruxinic acid molecules are not readily accommodated within a (nonsolvate) crystal structure and, thus, acetic acid molecules readily occupy spaces located above and below the layers of dimer molecules in the sample of the photoproduct crystallized from acetic acid. The acetic acid molecules exist as hydrogenbonded pairs [O(6)−H(6)···O(5): O···O, 2.641(4) Å; O− H···O, 165.4°], and there is no direct hydrogen bonding between acetic acid molecules and dimer molecules. The incorporation of other molecules (in this case, acetic acid) into the crystal structure is a common feature of the structures of other derivatives of β-truxinic acid.20,29,30 The loss of crystallinity during the photodimerization reactions for both the β1 and the β2 polymorphs of 3-FCA prevents the use of single-crystal XRD or powder XRD for monitoring the progress of the solid-state reaction and for determining the crystal structure of the directly produced photoproduct in each case. Under such circumstances, solidstate NMR spectroscopy has often been used to monitor the evolution of solid-state reactions of trans-cinnamic acids31−34 due to the sensitivity of this technique to probe changes in the local environment. In the present work, solution-state 1H NMR has proved to be an effective technique for monitoring the progress of the photoreaction. The 1H NMR spectra recorded (ex situ) for samples before and during irradiation of the β1 polymorph show that the 1H resonances due to vinyl hydrogens from ca. 6.8 and ca. 8.0 ppm (labeled i in Figure 7) disappear during the reaction, whereas 1H resonances due to the hydrogens of the

Figure 8. (a) The percentage of reactant phase remaining as a function of UV irradiation time for small (blue) and large (black) samples of the β1 polymorph of 3-FCA. (b) The corresponding plot of ln(reactant %) as a function of time.

For the second experiment using the β1 polymorph, the same conditions for irradiation were replicated and the same sample collection procedure was followed. After initiating irradiation, eight samples were collected at intervals of 6 h, with the final sample collected after a total of 106 h. In this case, over 96% of the monomer had been consumed after 36 h (black line in

Figure 7. Solution-state 1H NMR spectra recorded (ex situ) for (a) the β1 polymorph of 3-FCA and for samples collected after UV irradiation for (b) 60, (c) 105, (d) 195, (e) 510, and (f) 720 min. F



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119 °C, with further heating leading to melting at 167 °C. The polymorph obtained on cooling the melt depends on the rate of cooling, with β1 resulting from fast cooling and β2 resulting from slow cooling. The β1 and β2 polymorphs satisfy the structural requirements for topochemical solid-state [2 + 2] photodimerization, and such reactions occur in each case to produce 3,3′-difluoro-β-truxinic acid as the photoproduct in close to 100% yield. However, the photodimerization reaction in each case is associated with complete loss of crystallinity, leading ultimately to an amorphous photoproduct, which limits the opportunity to apply single-crystal XRD or powder XRD techniques to monitor the progress of the solid-state reaction and to determine the structural properties of the directly produced dimer phase in each case. In this regard, an interesting question is whether experimental conditions may be found that would allow the solid-state photodimerization reactions of the β1 and β2 polymorphs to proceed without loss of crystallinity, at least to obtain samples of the photoproduct phases that give good-quality powder XRD patterns. If such conditions are indeed found in the future, an interesting subsequent question is whether the photodimerization reactions of the β1 and β2 polymorphs yield the same polymorph or different polymorphs of 3,3′-difluoro-β-truxinic acid as the directly obtained photoproduct. In this regard, the application of modern techniques for analysis of powder XRD data36−40 may prove to be essential for carrying out complete crystal structure determination of the photoproduct phases.

Figure 8a), and a plot of ln(reactant %) against time (black line in Figure 8b) is also linear (R2 = 0.9986) for the first 30 h. For longer exposure times, the data deviate from linearity, possibly due to increasing sampling errors as the amount of reactant diminishes. Interestingly, the reaction takes about 3 times longer for this sample, although the sample density was the same in each experiment. Nevertheless, the final conversion is again close to 100%, and the slower rate may be attributed to the fact that the intervals between sample collections were longer and the mixing of the material during irradiation was inferior. The implication is that fast reaction rates may be achievable for relatively large quantities of sample with continuous agitation. The progress of the reaction was also investigated by solution-state 1H NMR for a sample of β2 obtained by thermal phase transformation of β1. The results indicate that over 90% of the reactant was consumed after 12.5 h (Figure 9a) and the

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

S Supporting Information *

CIF files containing crystallographic information for the β1 and β2 polymorphs of 3-fluoro-trans-cinnamic acid, and 3,3′difluoro-β-truxinic acid/acetic acid solvate. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author

*E-mail: KariukiB@cardiff.ac.uk. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We wish to thank the Umm Al-Qura University, Saudi Arabia (M.A.K.) and Cardiff University for financial and material support.

Figure 9. (a) The percentage of reactant phase remaining as a function of UV irradiation time for the β2 polymorph of 3-FCA. (b) The corresponding plot of ln(reactant %) as a function of time.

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plot of ln(reactant %) against time (Figure 9b) is also linear (R2 = 0.994) for the first 10 h (again, the deviation from linearity for longer exposure times may be attributed to sampling errors). Although the reaction appears to proceed more slowly for the β2 polymorph, we refrain from drawing any conclusions concerning the intrinsic relative reaction rates for the β1 and β2 polymorphs on the basis of the present data, due to the dependence of the observed reaction rates on the efficiency of sample mixing.

REFERENCES

(1) Mustafa, A. Chem. Rev. 1952, 51, 1. (2) Cohen, M. D.; Schmidt, G. M. J.; Songtag, F. I. J. Chem. Soc. 1964, 2000. (3) Schmidt, G. M. J. In Reactivity of the Photoexcited Organic Molecule, Proceedings of the 13th Conference on Chemistry at the University of Brussels, Brussels, Belgium, Oct 1965; Interscience: New York, 1967; p 227. (4) Schmidt, G. M. J. J. Chem. Soc. 1964, 2014. (5) Kohlschutter, V.; Tuscher, J. L. Z. Anorg. Allg. Chem. 1920, 111, 193. (6) Cohen, M. D.; Schmidt, G. M. J. J. Chem. Soc. 1964, 1996. (7) Hirschfeld, F. L.; Schmidt, G. M. J. J. Polym. Sci., Part A 1964, 2, 2181. (8) Cohen, M. D. Pure Appl. Chem. 1964, 9, 567. (9) Schmidt, G. M. J. Pure Appl. Chem. 1971, 27, 647. (10) Thomas, J. M. Philos. Trans. R. Soc. London 1974, A277, 251. (11) Cohen, M. D. Angew. Chem., Int. Ed. Engl. 1975, 14, 386.

4. CONCLUDING REMARKS Two β-type polymorphs of 3-FCA (β1 and β2) have been discovered and structurally characterized, representing a rare case among derivatives of trans-cinnamic acid for which two distinct polymorphs coexist within the same structural classification (in this case, β-type). The β1 polymorph transforms irreversibly to the β2 polymorph on heating above G



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Structural Properties - American Chemical Society

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