CRYSTAL GROWTH & DESIGN
Surface Morphology Changes of a Salt Crystal of 4-(2,5-Diisopropylbenzoyl)benzoic Acid with (S)-Phenylethylamine via Single-Crystal-to-Single-Crystal Photocyclization
2008 VOL. 8, NO. 7 2058–2060
Hideko Koshima,* Yuya Ide, and Naoko Ojima Department of Materials Science and Biotechnology, Graduate School of Science and Engineering, Ehime UniVersity, Matsuyama 790-8577, Japan ReceiVed December 28, 2007; ReVised Manuscript ReceiVed March 13, 2008
ABSTRACT: The changes in the surface morphology of a salt crystal of 4-(2,5-diisopropylbenzoyl)benzoic acid with (S)phenylethylamine upon photoirradiation were determined under atomic force microscopy. The crystal underwent enantiospecific photocyclization of the benzophenone derivative to give (S)-cyclobutenol as the sole product via single-crystal-to-single-crystal transformation. Upon UV irradiation, numbers of hemispheric unevennesses appeared on the (001) and (010) single-crystal surfaces with heights of tens of nanometers. Prolonged irradiation caused the hemispheres to merge, restoring essentially flat surfaces at the completion of the reaction. In contrast, the roughness of the (100) surface resulting from cutting with a razor blade decreased on photoirradiation, and a smooth surface was obtained. The morphological changes likely resulted from the changes in the molecular structure of the benzophenone derivative; the movement of the salt bond chains near the crystal surface then became easier as the melting point decreased on photoirradiation. Numerous solid-state reactions in molecular crystals have been developed in recent decades that are correlated with the crystal structures.1–5 Crystalline state photoreactions should give rise to molecular motion, causing morphological changes at the crystal surfaces. Kaupp first reported that the photodimerization of trans-cinnamic acids and anthracenes in the crystalline phase induced morphological changes due to phase-rebuilding of the surface molecules.6 The appearance of surface relief gating on a single crystal of 4-(dimethylamino)azobenzene was seen on repeated irradiation with two coherent laser beams.7 Recent reports include the reversible change in both surface morphology and the shape of photochromic diarylethene single crystals on irradiation.8 However, the morphological changes of crystals have not been studied intensively. Recently, actuators based on molecular crystals that change morphology in response to light have attracted a great deal of interest. To develop crystal actuators, it is necessary to elucidate the correlation between the structural and morphological changes. Isopropylbenzophenone derivatives undergo photocyclization in the crystalline state on UV irradiation.9 Previously, we reported enantioselective cyclization utilizing the salt crystals of carboxylic acids with the chiral and achiral amines.10–13 Some of the crystals were found to react without any cracking or breaking because of small changes in the crystal structures, which involved single-crystal-to-single-crystal transformation. This study correlated the changes in surface morphology with those in crystal structure. The single-crystal-to-single-crystal reaction is indispensable because the reaction process can be traced using X-ray crystallographic analysis of a single crystal before and after irradiation. Scheme 1 shows the enantiospecific photocyclization of 4-(2,5diisopropylbenzoyl)benzoic acid 1 in the salt crystal 1 · (S)-2 with (S)-phenylethylamine 2 to give the cyclobutenol (S)-3 as the sole product with 98% enantiomeric excess (ee) and 100% chemical yield.11 The reaction proceeds smoothly in the crystal lattice via single-crystal-to-single-crystal transformation. Here, we investigated the changes in surface morphology on photoirradiation under atomic force microscopy (AFM). Single crystals of 1 · (S)-2 prepared by evaporating methanol solutions of both components at room temperature gave needle * To whom correspondence should be addressed. Fax: 81-89-927-8523. E-mail:
[email protected].
Scheme 1
crystals grown along the a-axis (Figure 1A). The (001) face was the most developed, followed by the (010) face. The (100) face was cut perpendicular to the (001) and (010) faces with a razor blade to afford a piece of single crystal (210 × 209 × 68 µm). The crystal was then irradiated at >290 nm with a high-pressure mercury lamp through Pyrex glass under argon at 15 °C. Figure 1B shows the AFM images of the changes in surface morphology during the reaction. Before irradiation, the (001) face was flat (a). Upon UV irradiation, numbers of hemispheric unevennesses appeared on the (001) surface and these gradually grew to heights of tens of nanometers after irradiation for 20 min (b). The relative unevenness was less than 0.01% of the crystal thickness (68 µm), indicating that the unevenness was limited to the crystal surface. Prolonged irradiation for 100 min resulted in merger of the hemispheres and the return to the initial smooth surface (c). Similar morphological changes appeared on the (010) face on photoirradiation (d-f). In contrast, the (100) face (g) was very rough with heights of several nanometers because of the cut with the razor blade. UV irradiation decreased the roughness gradually (h) and finally gave a very smooth surface (i).11 After UV irradiation for 100 min, the crystal was treated with CH2N2 and subjected to high-performance liquid chromatography (HPLC) using a chiral column to confirm completion of the reaction.
10.1021/cg701275k CCC: $40.75 2008 American Chemical Society Published on Web 06/03/2008
Communications
Crystal Growth & Design, Vol. 8, No. 7, 2008 2059
Figure 1. (A) Crystal of 1 · (S)-2 and (B) AFM images of the morphological changes on UV irradiation for 0, 20, and 100 min: (a-c) (001), (d-f) (010), and (g-i) (100) faces.
Figure 2. Molecular structures of the (a) reactant 1 · (S)-2 and (b) product (S)-3 · (S)-2, (c) a stereoview of 1 · (S)-2, and the molecular arrangements on the (d, e) (100) and (f, g) (010) faces in 1 · (S)-2 and (S)-3 · (S)-2, respectively.
2060 Crystal Growth & Design, Vol. 8, No. 7, 2008 The smooth progress of the reaction is consistent with the result obtained in the X-ray crystallographic analysis of a single crystal of 1 · (S)-2 (1280 × 210 × 90 µm) in which irradiation for 45 min gave the product crystal (S)-3 · (S)-2. To evaluate the molecular mobility during the morphological change, the melting point change was measured. The melting point of the microcrystals before photoirradiation was 163-164 °C, decreasing to a minimum of 92-106 °C on irradiation due to the coexistence of the reactant 1 · (S)-2 and the product (S)-3 · (S)-2 as a disordered structure in the crystal lattice. The melting point then reached an almost constant temperature of 102-107 °C because of the formation of (S)-3 · (S)-2 alone at the completion of singlecrystal-to-single-crystal photocyclization. The considerable decrease in the melting point should facilitate the movement of molecules in the photochemical process. The molecular structures of 1 · (S)-2 and (S)-3 · (S)-2 are shown in structures a and b in Figure 2, respectively. The distance between the methine hydrogen (H1) of the o-isopropyl group and the carbonyl oxygen (O1) of the benzophenone moiety of 1 was determined to be 3.15 Å, which is close enough for hydrogen abstraction to occur.14 After irradiation for 5 min, the H1 hydrogen atom was abstracted by the excited carbonyl oxygen O1 atom, but cyclization did not occur.11 The methine and ketyl radicals then approached each other and connected to give the cyclobutenol (S)-3. The ionic bond between the carboxylate anion of 1 and the ammonium cation of (S)-2 forms a 2-fold helical chain in the reactant crystal. The stereoview and molecular arrangements in the (100) and (010) planes are shown in panels c, d, and f in Figure 2, respectively. The helical salt bridges remain after the reaction and are arranged in a similar manner in the product crystal (panels e and g in Figure 2). This suggests that the intermolecular salt bridges are so strong that photocyclization takes place without breaking the salt bonds and destroying the crystal; i.e., singlecrystal-to-single-crystal transformation occurs. When the benzophenone 1 was converted to the cyclobutenol (S)-3, the molecular conformation changed with the conversion from the sp2 carbonyl carbon to the sp3 chiral carbon. The C3-C2-C1-C4 and C2-C1-C4-C5 torsion angles of 1 changed from 101.7 and 170.9° to -0.8 and -154.1° in (S)-3, respectively (structures a and b in Figure 2). This allowed the carboxybenzene plane to orient almost perpendicular to the (100) plane (Figure 2e). The sizes of the unit cells changed slightly after irradiation, increasing (+2.29%) along the a-axis, decreasing (-4.41%) along the b-axis, and decreasing (-0.77%) along the c-axis, resulting in an overall decrease (-2.97%) in cell volume. The crystalline-state reaction proceeds heterogeneously from the surface to the inside on UV irradiation. The crystal expands slightly along the a-axis and contracts along the b- and c-axes without destroying the crystal. Therefore, some stress is induced within the crystal lattice. The helical salt bond chains are strong, like molecular springs. In contrast, the intermolecular interaction among the neighboring salt bond chains is weak due to van der Waals forces alone, as shown in panels d and f in Figure 2, suggesting that the (001) plane can be cleaved in a thin layer.
Communications Each molecular chain near the (001) surface can move along the a-axis, and the decrease in the melting point on photoirradiation promotes the movement, which leads to the appearance of hemispheric unevennesses on the (001) surface shown in Figure 1b. When the reaction is completed, the inner stress disappears, and an essentially smooth surface is restored (Figure 1c). A similar explanation is possible for the morphological change of the (010) surface. In contrast, the roughness of the (100) face decreased on irradiation and the surface ultimately became flat (Figure 1g-i). The experimental results provide visual evidence that the molecules on the (100) surface moved inside or outside the crystal. The increase in molecular movement with the decrease in the melting point should lead to a decrease in the surface energy to a minimum, resulting in the formation of a very smooth surface. This is like photochemical annealing of a crystal. In conclusion, we found that the surface morphology of the salt crystal of the diisopropylbenzophenone derivative changed via single-crystal-to-single-crystal photocyclization.
Acknowledgment. This study was supported by a Grant-inAid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology of Japan, the Asahi Glass Foundation, and the Ehime University COE incubation program.
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CG701275K
