Communication pubs.acs.org/cm
Co-Crystals of a Salicylideneaniline: Photochromism Involving Planar Dihedral Angles Kristin M. Hutchins, Saikat Dutta, Bradley P. Loren, and Leonard R. MacGillivray* Department of Chemistry, University of Iowa, Iowa City, Iowa 52242-1294, United States S Supporting Information *
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cocrystallization offers potential to alter properties of SAs for practical applications. Whereas packing of single-component SAs is predicated on interactions between molecules, cocrystal formation has considerable scope to alter packing environments of SAs involving the cocrystal former (CCF), or additional components, in the lattice. Here, we show the application of cocrystallization to induce photochromism in a SA derivative that is thermochromic and photoinactive. Specifically, we use the SA 2-((4hydroxyphenylimino)methyl)phenol (SHA) as a hydrogenbond donor through use of a hydroxyl group located in the para position. We employ the robust O−H···N synthon via pyridinebased CCFs to form cocrystals of SHA with 4,4′-dipyridyl (BIPY), trans-1,2-di(4-pyridyl)ethylene (BPE), and 4,4′-(1,2ethynediyl)bispyridine (BPA) to afford (BIPY)·2(SHA), (BPE)·2(SHA), and (BPA)·2(SHA), respectively (Scheme 2). We show that SHA is thermochromic, changing from
hotochromic compounds undergo a reversible color change when exposed to light of different wavelengths. The materials have attracted interest for applications in optical data storage,1 rewritable papers,2 and photochromic lenses.3 Specifically, salicylideneaniline (SA) derivatives are well documented to undergo both photochromic4 and thermochromic5 reactions in the solid state. At room temperature, SA derivatives exist in the enol form in solids.6 The accepted mechanism for photochromism in SA derivatives is proton transfer to the nitrogen (cis-keto form), followed by isomerization to the trans-keto form7 (Scheme 1). Scheme 1. Mechanism of Photochromism and Dihedral Angle in SA Derivatives
Scheme 2. OH···N Synthon: CCFs in SHA Co-Crystals
The dihedral angle between the two benzene rings in a SA is mainly a predictor of photochromic properties (Scheme 1). Generally, if the dihedral angle is greater than 30°, the compound is photochromic; if the angle is less than 20°, the compound is nonphotochromic.8 If the dihedral angle falls between 20° and 30°, the crystal can be either photochromic or nonphotochromic.8 In addition to dihedral angle, the interplanar spacing of the SA unit in the crystalline state contributes to photochromism. Close interplanar distances on the order of 3.3 Å to 3.5 Å (i.e., “closed structures”) are thought to prevent photoinduced isomerizations in solids and, thus, result in nonphotochromism.9 While much work has been directed to study photochromic and thermochromic SA derivatives as pure components, very little has been studied in two-component solids.7 In this context, Fujita et al. have recently described the inclusion of a SA in a porous coordination network wherein a SA becomes photochromic owing to twisting of chromophores (∼25°) upon inclusion.4 Cocrystallization, which involves combining two or more neutral molecules to form a crystal lattice,10 has been used to modify properties related to pharmaceutical materials,11 organic semiconductors,12 and solid-state reactivity.13 Given that the chromic properties of SAs are identified to be dependent on geometric considerations and packing, crystal engineering via © 2014 American Chemical Society
orange to pale yellow upon exposure to low temperatures (ca. 77 K), and nonphotochromic. The SA possesses dihedral angles of 10.5° and 15.7°,14 which is consistent with the lack of photochromism. Remarkably, we demonstrate all resulting cocrystals of SA to be photochromic, despite the fact that the resulting dihedral angle of the SA in each cocrystal is significantly less than 20°.8 We attribute the origin of the photochromism to the presence of nearest-neighbor edge-toface interactions between the SA and CCF, which serve to prohibit the generation of “closed structures”.9 The photochromism, in effect, arises from a structural mismatch15 between the CCF and SHA that supports an open structure. Received: March 7, 2014 Revised: April 23, 2014 Published: April 29, 2014 3042
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Chemistry of Materials
Communication
Table 1. Hydrogen-Bond Lengths, Dihedral Angles, and Spacings between SHAs in Co-Crystals co-crystal
(BIPY)·2(SHA)
(BPE)·2(SHA)
(BPA)·2(SHA)
Intramolecular O···N separation (Å) Intermolecular O···N separation (Å) dihedral angles (Φ) interplanar spacing (Å)
O1−N1: 2.638(4), O3−N2: 2.640(4) O2−N3: 2.735(4), O4−N4: 2.760(4) O1 ring: 1.30°, O3 ring: 2.21° 5.89
O2−N3: 2.640(4), O4−N4: 2.631(4) O1−N2: 2.737(5), O3−N1: 2.737(5) O2 ring: 0.08°, O4 ring: 1.56° 6.03
O1−N1: 2.603(2) O2−N2: 2.603(2) O1 ring: 1.79° 6.0
SHA was synthesized as reported.16 SHA has been described as two polymorphs: P21/c16 (Form A) and C2/c14 (Form B). In Form A the dihedral angles are 10.5° and 15.7°, and in Form B the angle is 14.9°. Owing to ease of preparation, Form A of SHA was used in our experiments. It has been demonstrated that SAs are thermochromic in the solid state regardless of photochromic properties.17 When a bulk powder of Form A of SHA was exposed to liquid nitrogen (77 K) for 15 min, a color change from orange to pale yellow was observed. Upon standing at room temperature for 1 h, the powder returned to the original color. From these observations, we concluded that SHA is thermochromic. When a bulk powder of Form A of SHA was exposed to UV light (365 nm, 12 h), no color change was evident. From this observation, we concluded that the compound is nonphotochromic. We next attempted to synthesize cocrystals of SHA by dissolving the CCF and SHA (1:2 ratio) in nitromethane. Yellow crystals, in all cases, formed upon slow cooling of the solutions, and cocrystal formation was confirmed by powder Xray diffraction (PXRD). 1H NMR spectroscopy confirmed CCF and SHA to be present in a 1:2 ratio. The cocrystals exhibited either prism or blade morphologies. Since photochromic properties are highly dependent on the dihedral angle,8 a single-crystal X-ray analysis was performed for each cocrystal. Single-crystal X-ray analyses18 of the three cocrystals revealed linear three-component assemblies with each sustained by two O−H···N hydrogen bonds (Table 1). An intramolecular hydrogen bond is present between the ortho hydroxyl group and imine N atom in all SHAs. Two cocrystals, (BIPY)· 2(SHA) and (BPE)·2(SHA), crystallized in noncentrosymmetric space groups (Pbc21 and Pca21, respectively), while (BPA)·2(SHA) crystallized in a centrosymmetric space group (P21/c). Importantly, all SHAs in all three cocrystals exhibit nearly planar dihedral angles that range from 0.1° to 2.2°. The cocrystals would, thus, be expected to be nonphotochromic. In each case, the three-component assemblies stack offset and parallel, with the shortest SHA separations being on the order of 6 Å (Figure 1). Alternating layers of 1D assemblies are rotated by 180° relative to each other and linked through C− H···π interactions. In (BIPY)·2(SHA) and (BPE)·2(SHA), a herringbone mode of packing is present in the ab-plane. In (BPA)·2(SHA), SHA molecules of a same layer display headto-head packing. Whereas the cocrystals were expected to be nonphotochromic owing to small dihedral angles, each cocrystal was photochromic. In a typical experiment, irradiation using a UV lamp (365 nm) was performed on samples of both single crystals and bulk powders. Visual inspection of each cocrystal clearly demonstrated a change in color from yellow to orange (Table 2, Figure 2). The cocrystal (BPA)·2(SHA) was weakly photochromic by visual inspection. A photoisomerization in each solid was confirmed by FTIR spectroscopy4 wherein the band at 1618 cm−1 attributable to the CN stretch of SHA decreased and a band at 1652 cm−1 attributable to the CO stretch of the keto tautomer of SHA appeared (see Supporting
Figure 1. Stacking of 1D assemblies of cocrystals: (a) (BIPY)·2(SHA) highlighting staircase motif, (b) (BIPY)·2(SHA), (c) (BPE)·2(SHA), and (d) (BPA)·2(SHA).
Table 2. Colors of Co-Crystals, Times of UV Irradiations, and Color Lifetimes co-crystal
color (R.T.)
UV time (h)
color (after UV)
time color persisted (h)
(BIPY)·2(SHA) (BPE)·2(SHA) (BPA)·2(SHA)
yellow yellow yellow
4 4 12
orange orange light orange
4 4 2
Figure 2. Images of cocrystals before and after UV exposure: (a) (BIPY)·2(SHA), (b) (BPE)·2(SHA), and (c) (BPA)·2(SHA).
Information Figures S8−S10). UV−vis absorption spectra demonstrated significant differences in the 325−600 nm region7,8 (see Supporting Information Figures S11−S13). All three cocrystals are also thermochromic. When bulk powders of the solids were exposed to liquid nitrogen (77 K) for 15 min, color changes from yellow to pale yellow were observed. Upon standing at room temperature for 1 h, the powders returned to original colors, confirming that thermochromism is maintained in the two-component solids. That all three cocrystals are photochromic despite SHA exhibiting a nearly planar dihedral angle in each solid can be attributed to the presence of the CCF. More specifically, whereas past studies on SAs involve a single component that is, a priori, dominated exclusively by contacts between SAs, the components of a cocrystal will also be dominated by contacts involving both the SA and the CCF. In the present cases, each SHA is effectively insulated from interacting with adjacent SHA molecules by the CCFs in the solids (Figure 3). Specifically, 3043
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Communication
(5) Ogawa, K.; Kasahara, Y.; Ohtani, Y.; Harada, J. J. Am. Chem. Soc. 1998, 120, 7107. (6) Harada, J.; Uekusa, H.; Ohashi, Y. J. Am. Chem. Soc. 1999, 121, 5809. (7) Johmoto, K.; Sekine, A.; Uekusa, H. Cryst. Growth Des. 2012, 12, 4779. (8) Johmoto, K.; Ishida, T.; Sekine, A.; Uekusa, H.; Ohashi, Y. Acta Crystallogr., Sect. B 2012, 68, 297. (9) Fukuda, H.; Amimoto, K.; Koyama, H.; Kawato, T. Org. Biomol. Chem. 2003, 1, 1578. (10) Eddleston, M. D.; Sivachelvam, S.; Jones, W. CrystEngComm 2013, 15, 175. (11) Sander, J. R. G.; Bucar, D. K.; Henry, R. F.; Zhang, G. G. Z.; MacGillivray, L. R. Angew. Chem., Int. Ed. 2010, 49, 7284. (12) Kapadia, P. P.; Ditzler, L. R.; Baltrusaitis, J.; Swenson, D. C.; Tivanski, A. V.; Pigge, F. C. J. Am. Chem. Soc. 2011, 133, 8490. (13) MacGillivray, L. R.; Reid, J. L.; Ripmeester, J. A. J. Am. Chem. Soc. 2000, 122, 7817. (14) Wang, Y. F.; Yu, Z. X.; Sun, Y. X.; Wang, Y. S.; Lu, L. D. Spectrochim. Acta, Part A 2011, 79, 1475. (15) Morimoto, M.; Irie, M. J. Am. Chem. Soc. 2010, 132, 14172. (16) Ersanli, C. C.; Albayrak, C.; Odabasogly, M.; Erdonmez, A. Acta Crystallogr., Sect. E 2004, 60, O389. (17) Fujiwara, T.; Harada, J.; Ogawa, K. J. Phys. Chem. B 2004, 108, 4035. (18) Crystal data for (BIPY)·2(SHA): (C10H8N2)·2(C13H11NO2), M = 582.64, orthorhombic, Pbc21, a = 5.8872(7), b = 13.1634(14), c = 38.795(4), V = 3006.44(6) Å3, Z = 4, T = 290(2) K, ρcalc = 1.29 g/cm3, and R1 = 0.0462 for 3553 reflections with I > 2σ(I). Crystal data for (BPE)·2(SHA): (C12H10N2)·2(C13H11NO2), M = 608.68, orthorhombic, Pca21, a = 13.6410(14), b = 6.0316(7), c = 37.772(4), V = 3107.77(6) Å3, Z = 4, T = 293(2) K, ρcalc = 1.30 g/cm3, and R1 = 0.0473 for 3501 reflections with I > 2σ(I). Crystal data for (BPA)· 2(SHA): (C12H8N2)·2(C13H11NO2), M = 606.66, monoclinic, P21/c, a = 19.0963(19) Å, b = 5.9947(6) Å, c = 13.5893(14), β = 99.325(5)°, V = 1535.10(13) Å3, Z = 2, T = 290(2) K, ρcalc = 1.31 g/cm3, and R1 = 0.0515 for 1661 reflections with I > 2σ(I). CCDC 975740-975742 contains the supplementary crystallographic data for this paper. This data can be obtained free of charge from the Cambridge Crystallographic Data Center via www.ccdc.cam.ac.uk/data_request/cif. (19) Arod, F.; Pattisont, P.; Schenk, K. J.; Chapuisl, G. Cryst. Growth Des. 2007, 7, 1679. (20) Ohashi, Y. Crystallogr. Rev. 2013, 19, 2. (21) We note very recent work with two cocrystals of a SA that generated planar structures that are nonphotochromic (ref 7).
Figure 3. SHA rings, CCF, and packing of SHA: (a) (BIPY)·2(SHA), (b) (BPE)·2(SHA), and (c) (BPA)·2(SHA) showing open structures. CCFs are pink, SHA rings are green or blue, and one SHA in spacefill to view close packing.
edge-to-face forces dominate between SHA (spacefill) and each CCF (see Supporting Information, Table S1). Each CCF, thus, serves to effectively eliminate a close mode of stacking normally present in planar, photoinactive SA derivatives.19 In doing so, a photoinactive SA has been switched to being photochromic. The cocrystallization process has, moreover, been used as a means to support “open structures” wherein edge-to-face packings can allow sufficient room20 for isomerization.21 In conclusion, photochromism involving a planar SA has been achieved using a cocrystal approach. We attribute the photochromism to the introduction of a CCF that supports an “open structure”, effectively insulating the SHA from close interactions with adjacent SHA molecules. We are investigating the incorporation of additional substituents on the rings of the SA as well as determining how additional CCFs affect photochromic and thermochromic properties of SA-based materials.
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ASSOCIATED CONTENT
S Supporting Information *
Experimental details, single-crystal and powder X-ray diffraction data, 1H NMR, FTIR spectra, and UV−vis spectra. This material is available free of charge via the Internet at http:// pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS L.R.M. acknowledges the National Science Foundation (DMR1104650) for generous support. REFERENCES
(1) Fukaminato, T.; Sasaki, T.; Kawai, T.; Tamai, N.; Irie, M. J. Am. Chem. Soc. 2004, 126, 14843. (2) Sousa, J. A.; Kashnow, R. A. Rev. Sci. Instrum. 1969, 40, 966. (3) Armistead, W. H.; Stookey, S. D. Science 1964, 144, 150. (4) Haneda, T.; Kawano, M.; Kojima, T.; Fujita, M. Angew. Chem., Int. Ed. 2007, 46, 6643. 3044
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