Photochromism Control of Salicylideneaniline Derivatives by Acid

Aug 29, 2012 - The series of co-crystals N-salicylidene-3-carboxyaniline (1) with 2-aminopyridine (a), guanylthiourea (b), cytosine (c), 4,4′-bipyri...
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Photochromism Control of Salicylideneaniline Derivatives by Acid− Base Co-Crystallization Kohei Johmoto, Akiko Sekine, and Hidehiro Uekusa* Department of Chemistry and Materials Science, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo, 153-8902, Japan S Supporting Information *

ABSTRACT: Acid−base co-crystallization has been used to control the photochromic reactivities of salicylideneaniline derivatives in co-crystals. The series of co-crystals Nsalicylidene-3-carboxyaniline (1) with 2-aminopyridine (a), guanylthiourea (b), cytosine (c), 4,4′-bipyridyl (d), piperazine anhydrous (e), 1,3-di-o-tolylguanidine (f), and dibenzylamine (g) and N-salicylidene-4-carboxyaniline (2) with 4,4′-bipyridine (d) and N,N-dibenzylamine (g) have been synthesized. The weak photochromic compound 1 becomes nonphotochromic or strongly photochromic in the co-crystals and the nonphotochromic compound 2 becomes photochromic in the co-crystal 2g. The photochromic properties of compounds 1 and 2 change because of the conformational changes induced in the salicylideneaniline moieties in the crystal structure. The lifetimes of the colored species formed in the photochromic reaction are also affected by the changes in the environment around the molecule in the crystal. As shown in this study, acid−base type co-crystallization may be a promising method to control the photochromic reactivities of salicylideneaniline derivatives.

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INTRODUCTION Photochromism causes reversible color changes in substances upon photoirradiation.1,2 Chemical transformations upon photoirradiation, such as proton transfer, covalent bond formation or cleavage, and isomerization, induce changes in the physicochemical properties of the material, that is, the absorption, fluorescence, refraction index, and electric permittivity2−4 are altered. Because of their reversible properties, photochromic compounds have attracted considerable attention in applications such as photochromic lenses,5 rewritable papers,6 photoswitching materials,7 optical data storage,7,8 and biological sensors.9 Salicylideneaniline (SA) derivatives, which change from yellow to red upon irradiation with UV light and back to the original yellow upon exposure to heat (thermal fading) or visible light, are one of the oldest investigated organic photochromic compounds. SA derivatives are prepared according to a procedure first reported in 1909, involving the condensation reaction between salicylaldehydes and anilines.10,11 In the 1960s, Cohen and Schmidt investigated the relationship between structural and physical properties in detail, and suggested that crystals containing the molecule in a nonplanar conformation were photochromic and those that contained a planar conformation molecule were nonphotochromic and would exhibit thermochromic properties.12−15 There has been a lively debate about the structure of the colored species2 in SA derivatives generated by photochromic reactions. In 1995, Harada et al. directly observed the X-ray structure of the colored species, generated using two-photon excitation, and revealed that the colored species is the transketo form.16 The mechanism of the photochromic reaction is as © 2012 American Chemical Society

follows: The proton transfer from the phenolic OH group to the N atom of the imine produces the cis-keto form. Subsequently, isomerization occurs from the cis-keto form to the trans-keto form through a molecular twist called “pedal motion”17,18 (Scheme 1). Scheme 1. Photochromism Reaction Mechanism of NSalicylideneanilinea

a

Hydrogen transfer (i) and cis−trans isomerization (ii).

In our previous study, the photochromism and crystal structures of three polymorphic crystals (α, β, and γ forms) of an SA derivative, N-3,5-di-tert-butylsalicylidene-3-carboxyaniline, were compared.19 Using polymorph crystals with different crystal and molecular structures allows for the direct comparison of their photochromic properties without interference from differences in chemical structures.20,21 Among the three polymorphs, the α and β crystals exhibit photochromism upon UV irradiation. The molecules in these polymorphs are nonplanar because of their crystalline environment. Thermal fading is also an interesting property in photochromism and Received: April 3, 2012 Revised: August 17, 2012 Published: August 29, 2012 4779

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varies between SA crystals. In that study, the β form faded more slowly than the α form. This lower fading rate is due to the stability of the colored species which is determined from the intermolecular hydrogen bonds between the trans-keto form (colored species) and neighboring molecules. This finding is also supported by ab initio calculations.22 Photochromic properties, including thermal fading, are highly dependent on the crystal structure, i.e., the molecular conformation and crystalline environment around the molecule. Using the same SA derivative compound is important for detailed crystal structure comparisons and for elucidating the crystalline environment factors on the molecular conformation and photochromic properties. Therefore, in this study, we aim to control the photochromic properties of the same SA derivative in a series of crystals by changing their crystalline environments and the conformation of the molecules within them. Further elucidation of the photochromic properties, including thermal fading, is possible by comparing their crystal structures. There are three methods in the literature to modify the SA derivatives’ molecular conformation with an aim to change photochromic reactivity. The first method is chemical modification via the introduction of a bulky tert-butyl group which allows open-spaced crystal packing,23 or by adding an alkyl group to the ortho position of the aniline ring which twists the molecular conformation by inducing intramolecular steric repulsions.24 However such chemical modifications are not always applicable to the target SA molecule, and may also lead to extreme molecular structure conformations that are not suitable for structural comparisons. The second method is mixing the SA compound with polymers,25,26 zeolites,27 and bile acids.28 Unfortunately, crystal structure determinations are difficult in such systems and do not allow us to compare photochromic properties and crystal structures. The third method involves confining the SA molecule in a metal organic framework (MOF).29 This method cannot allow us to make various conformational changes of SA molecules because available MOFs are limited. In this study, to control the photochromic property of SA derivatives, we introduce an effective crystal engineering method to change the molecular conformation and the crystalline environment, using co-crystal formation through acid−base interactions. Co-crystal formation is used successfully in the pharmaceutical industry to improve many physicochemical properties such as solubility and hygroscopicity.30−32 Here, co-crystallization modifies the crystalline environment around the SA molecule, resulting in planar to nonplanar SA derivative conformations, which have different photochromic reactivities. Hydrogen-bonded co-crystals formed by acid−base interactions are promising because the intermolecular interactions are strong, due to the ionic bonds which overcome the drive toward single component crystallization. In this study, acid− base co-crystallizations were conducted on two SA derivatives containing carboxylic acid groups (compounds 1, 2). They formed many co-crystals with various bases and adopted various molecular conformations. The relationships between their photochromic properties and their molecular and crystal structures are discussed.

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Chart 1. Chemical Structures of N-Salicylideneaniline Derivatives and Secondary Amines

the condensation of salicylaldehyde with 3-aminobenzoic acid and 4aminobenzoic acid according to the literature.2 Crystals were obtained by slow evaporation from methanol. Co-Crystallization of 1a−g, 2d, and 2g. SA derivative 1 was mixed with the appropriate base, 2-aminopyridine (a), guanylthiourea (b), cytosine (c), 4,4′-bipyridyl (d), piperazine anhydrous (e), 1,3-dio-tolylguanidine (f), or dibenzylamine (g), and SA derivative 2 was mixed with 4,4′-bipyridyl (d) or dibenzylamine (g) (Chart 1), in molar ratios of 2:1 (1d and 2d) or 1:1 (the other co-crystals). The resulting solids were precipitated by evaporation in a vacuum. Co-crystals were obtained by slow evaporation from methanol (1b, 1c, 1e, 2d), methyl acetate (1a, 1f), acetone (1d), or THF (1g, 2g). Photoirradiation. Photoirradiation was carried out in air with a 350 W, ultrahigh-pressure Hg lamp (SAN-EI Electric UVF-352F), through a quartz fiber and a band-pass filter (HOYA UV-360) that allows the mercury emission line of 365 nm to pass through.. UV/Vis Spectra. The ultraviolet−visible spectra of all SA derivatives and co-crystals were measured using a JASCO V-560 spectrometer equipped for diffuse reflectance spectroscopy (ISV-469). The samples were prepared by mixing crystals (7 mg) with MgSO4 powder (350 mg). Spectra were measured before and after UV irradiation. To measure the rate of thermal fading, time-dependent UV/vis spectra were measured for the photochromic samples, 1, 1e, 1f, and 1g. These colored samples faded in the dark at ambient temperature. The fading rate was calculated using the following equation:

⎛ A − A∞ ⎞ kt = − ln⎜ t ⎟ ⎝ A 0 − A∞ ⎠ where k and t represent a rate constant and time (in seconds), respectively, A0 and At represent the value of the absorbance of the spectra at 0 and t, respectively [seconds] around 520 nm, where the maximal decrease occurred in the dark. X-ray Measurements and Refinements. Single crystal X-ray diffraction data were collected at ambient temperature in ω-scan mode using either a R-AXIS RAPID Imaging plate camera (Rigaku) with Mo Kα radiation obtained from a rotating anode source with a graphite monochromator, a R-AXIS RAPID II Imaging plate camera (Rigaku) with Cu Kα radiation from a rotating anode source with a confocal multilayer X-ray mirror, or a XtaLab mini (Rigaku) with Mo Kα radiation sealed in a tube source with a graphite monochromator. The integrated and scaled data were empirically corrected for absorption effects using ABSCOR.33 The initial structures were solved by using direct methods with SHELXS 9734 and refined on Fo2 with SHELXL 97.34 All the non-hydrogen atoms were refined anisotropically; the hydrogen atoms between the carboxylic group and N atoms of the base and the hydrogen atoms of intramolecular hydrogen bond and

EXPERIMENTAL SECTION

Synthesis of SA Derivatives. N-Salicylidene-3-carboxyaniline (1) and N-salicylidene-4-carboxyaniline (2) (Chart 1) were prepared by 4780

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Table 1. Crystal Data and Experimental Details of SA Derivative 1 and Co-Crystals 1a−g 1 chemical formula formula weight temp/K wavelength/Å cryst syst space group a/Å b/Å c/Å β/deg vol/Å3 Z calcd density/Mg m−3 abs coeff/mm−1 F(000) cryst size/mm3 θ range for data collection index ranges

reflns collected indep reflns Rint completeness to θ abs correction max and min transm refinement meth data/restraints/params goodness-of-fit on F2 final R indices [I > 2σ(I)] R indices (all data) largest diff peak, hole, e·Å−3 chemical formula formula weight temp/K wavelength/Å cryst syst space group a/Å b/Å c/Å β/deg vol/Å3 Z calcd density/Mg m−3 abs coeff/mm−1 F(000) cryst size/mm3 θ range for data collection index ranges

reflns collected indep reflns Rint completeness to θ abs correction

1b

1c

C19H12N3O3 C14H11NO3 241.24 335.36 293(2) 293(2) 0.71075 1.54186 monoclinic monoclinic P21/c P21/c 24.618(3) 18.7827(3) 3.8451(6) 6.1948(1) 12.3857(16) 15.6531(3) 91.330(3) 111.743(10) 1172.1(3) 1691.74(5) 4 4 1.367 1.317 0.097 0.745 504 704 0.36 × 0.22 × 0.21 0.36 × 0.34 × 0.04 3.29° to 27.48° 5.07° to 68.22° −31 ≤ h ≤ 31 −21 ≤ h ≤ 22 −4 ≤ k ≤ 4 −7 ≤ k ≤ 7 −16 ≤ l ≤ 15 −18 ≤ l ≤ 18 15904 18664 2641 3102 0.0492 0.0304 99.5% 99.9% semiempirical from equivalents 0.9802, 0.7851 0.9687, 0.8319 full-matrix least-squares on F2 2641/0/172 3102/0/241 1.030 1.141 R1 = 0.0500 R1 = 0.0397 wR2 = 0.1327 wR2 =0.1114 R1 = 0.0711 R1 = 0.0522 wR2 = 0.1441 wR2 = 0.1250 0.257, −0.173 0.136, −0.119 1d 1e

1a

C16H17N5O3S 359.41 293(2) 1.54186 monoclinic P21/c 23.3532(5) 5.58131(12) 13.0587(3) 93.4884(12) 1698.93(6) 4 1.405 1.930 752 0.19 × 0.11 × 0.02 3.79° to 68.21° −28 ≤ h ≤ 28 −6 ≤ k ≤ 6 −15 ≤ l ≤ 15 18545 3103 0.0465 100.0%

C18H16N4O4 352.35 293(2) 0.71073 orthorhombic P21 11.8523(11) 5.7183(5) 12.3445(12) 101.500(7) 819.85(13) 2 1.427 0.104 368 0.51 × 0.16 × 0.09 3.37° to 27.48° −15 ≤ h ≤ 15 −7 ≤ k ≤ 7 −16 ≤ l ≤ 16 8777 2071 0.0288 99.8%

0.9716, 0.7626

0.9911, 0.8530

3103/0/253 1.146 R1 = 0.0609 wR2 = 0.1835 R1 = 0.0924 wR2 = 0.2083 0.336, −0.358 1f

2071/1/253 1.024 R1 = 0.0374 wR2 = 0.0874 R1 = 0.0451 wR2 = 0.0921 0.148, −0.206 1g

C19H15N2O3 C16H16N2O3 319.33 284.31 293(2) 293(2) 0.71073 0.71075 monoclinic orthorhombic P21/c Pbca 4.4760(5) 7.8579(7) 13.4666(14) 10.9170(9) 26.426(3) 32.253(3) 93.712(7) 1589.5(3) 2766.8(4) 4 8 1.334 1.365 0.092 0.096 668 1200 0.41 × 0.30 × 0.15 0.28 × 0.26 × 0.11 1.54° to 27.54° 3.26° to 27.47° −5 ≤ h ≤ 5 −10 ≤ h ≤ 10 −17 ≤ k ≤ 17 −14 ≤ k ≤ 14 −34 ≤ l ≤ 34 −41 ≤ l ≤ 41 15685 24323 3656 3161 0.0423 0.0608 99.9% 99.7% semiempirical from equivalents

C29H28N4O3 480.55 293(2) 1.54186 orthorhombic Pbca 19.7104(4) 10.8346(2) 24.3804(4)

C28H26N2O3 438.51 293(2) 0.71075 orthorhombic Pna21 9.0694(6) 17.9222(17) 14.5011(13)

5206.54(17) 8 1.226 0.650 2032 0.12 × 0.08 × 0.03 3.63° to 68.21° −23 ≤ h ≤ 23 −12 ≤ k ≤ 12 −29 ≤ l ≤ 29 56428 4744 0.0652 99.8%

2357.1(3) 4 1.236 0.081 928 0.30 × 0.10 × 0.10 3.20° to 25.35° −9 ≤ h ≤ 10 −21 ≤ k ≤ 21 −17 ≤ l ≤ 17 17115 2246 0.0345 99.7%

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Table 1. continued 1d max and min transm refinement meth data/restraints/params goodness-of-fit on F2 final R indices [I > 2σ(I)] R indices (all data) largest diff peak, hole, e·Å−3

1e

1f

0.9864, 0.9634 0.9896, 0.8135 full-matrix least-squares on F2 3656/0/243 3161/0/202 1.140 1.056 R1 = 0.0545 R1 = 0.0516 wR2 = 0.1459 wR2 = 0.1090 R1 = 0.0766 R1 = 0.0790 wR2 = 0.1775 wR2 = 0.1199 0.202, −0.206 0.163, −0.215

azomethine were located in the differential Fourier map and were refined isotropically; and all other hydrogen atoms were obtained geometrically. The dihedral angles (ϕ) between the two benzene rings on the SA molecule were calculated using SHELXL 97.34 Ambient temperature conditions in the X-ray diffraction measurements were employed to maintain the same conditions as in the photoirradiation and photochromic experiments.

1g

0.9789, 0.7858

0.9920, 0.8818

4744/0/345 1.045 R1 = 0.0584 wR2 = 0.1470 R1 = 0.1263 wR2 = 0.1872 0.300, −0.179

2246/1/310 1.126 R1 = 0.0399 wR2 = 0.0957 R1 = 0.0433 wR2 = 0.0978 0.107, −0.151

Table 2. Crystal Data and Experimental Details of SA Derivative 2 and Co-Crystals 2d, 2g chemical formula formula weight temp/K wavelength/Å cryst syst space group a/Å b/Å c/Å β/deg vol/Å3 Z calcd density/Mg m−3 abs coeff/mm−1 F(000) cryst size/mm3

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RESULTS AND DISCUSSION In this study, SA derivatives 1 and 2 with carboxylic groups were selected. They were chosen because of their stability under the basic conditions of the co-crystallizing solution, and for their easy formation of acid−base type co-crystals through hydrogen bonds. If an SA derivative with a basic substituent was selected, it may have been decomposed by the acidic co-crystal former molecule in solution. The selected molecules are free from bulky substituent groups (such as tert-butyl groups) which have strong steric effects on the molecular conformation,23 making a comparison of the effects of the crystalline environment on the SA molecular conformation and the photochromic property difficult.23 Thus, the simplest SA derivatives containing a carboxyl substituent group were the most suitable for this study. Basic molecules (a−g) were selected as co-crystal formers, as they give large ΔpKa values [the difference of pKa (conjugate acid of base) and pKa (acid)]. In the pharmaceutical industry, there is a widely accepted guideline that a proton transfer ionic co-crystal can be formed if the ΔpKa is greater than 2 or 3.35 Crystal Structure. The crystallographic data and experimental details of all samples (1, 1a, 1b, 1c, 1d, 1e, 1f, 1g, 2, 2d, 2g) are shown in Tables 1 and 2. In the crystal structure of SA derivative 1, there was one independent molecule in an asymmetric unit. The carboxylic group of the molecule is situated near an inversion center and forms hydrogen-bonded dimer structures via O2···H3O′−O3′ (Figure 1). The dihedral angle between the two benzene rings is ϕ = 26.9(1)°. In the co-crystal structures 1a, 1b, 1c, 1d, 1e, 1f, and 1g hydrogen bonds between the carboxyl group and the nitrogen atom of the basic co-crystal formers are observed, and no carboxylic dimers are formed between the SA derivatives. In all co-crystals except 1d, the proton of the carboxylic group was transferred to the nitrogen atom of the basic co-crystal former to form O−···HN+ charge-assisted hydrogen bonds. However, this proton transfer would not affect the photochromic reactivity because it strongly depends on the molecular conformation. In 1d, no such proton transfer was observed and the usual OH···N type hydrogen bonds were formed, resulting from a small ΔpKa in these co-crystals (see Supporting Information). The co-crystallization induces conformational changes in the SA moieties of 1a to 1g and especially affects the dihedral angle between the two benzene

θ range for data collection index ranges

reflns collected indep reflns Rint completeness to θ abs correction max and min transm refinement meth data/restraints/ params goodness-of-fit on F2 final R indices [I > 2σ(I)] R indices (all data) extinction coeff largest diff peak, hole, e·Å−3

2

2d

2g

C17H11NO3 241.24 293(2) 1.54186 monoclinic P21/c 5.8687(3) 4.7866(2) 41.133(2) 92.208(3) 1589.5(3) 4 1.388 0.814 504 0.28 × 0.06 × 0.05 4.30° to 68.25°

C19H15N2O3 319.33 293(2) 1.54186 monoclinic C2/c 50.8968(12) 4.91775(12) 12.5961(3) 90.5364(13) 3152.64(13) 8 1.346 0.755 1336 0.32 × 0.11 × 0.08 3.47° to 68.25°

C28H26N2O3 438.51 293(2) 0.71075 monoclinic P21/c 15.5992(14) 8.7313(7) 18.0848(15) 110.397(4) 2308.7(3) 4 1.262 0.082 928 0.25 × 0.25 × 0.03 3.16° to 25.35°

−7 ≤ h ≤ 7 −60 ≤ h ≤ 60 −5 ≤ k ≤ 5 −5 ≤ k ≤ 5 −48 ≤ l ≤ 49 −15 ≤ l ≤ 15 12134 16507 2103 2873 0.0568 0.0649 99.8% 99.9% semiempirical from equivalents 0.9620, 0.7982 0.9455, 0.7941 full-matrix least-squares on F2 2103/1/173 2873/1/226

−18 ≤ h ≤ 18 −10 ≤ k ≤ 10 −21 ≤ l ≤ 21 17307 4223 0.0351 99.7%

1.009 R1 = 0.0483

1.145 R1 = 0.0588

1.151 R1 = 0.0549

wR2 = 0.1193 R1 = 0.0679 wR2 = 0.1339 0.0025(6) 0.200, −0.143

wR2 = 0.2032 R1 = 0.0811 wR2 = 0.2161

wR2 = 0.1159 R1 = 0.0748 wR2 = 0.1252

0.190, −0.178

0.276, −0.212

0.9975, 0.6913 4223/1/311

rings in the SA molecule. The SA moiety conformation varied from planar to nonplanar, and the dihedral angles were ϕ = 5.5(1)°, 8.9(3)°, 10.9(2)°, 14.0 (1)°, 32.9(1)°, 37.9(1)°, and 49.8(1)° for 1a, 1b, 1c, 1d, 1e, 1f, and 1g, respectively, and the dihedral angle of the SA molecule 1 was ϕ = 26.9(1)°. It can be seen from Figure 2 that the SA moieties are twisted in the cocrystals. 4782

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angle between the two benzene rings is ϕ = 5.1(1)°. In the cocrystal of 2d, the molecular conformation is planar and the dihedral angle in 2 is ϕ = 9.7(2)°. In 2g, however, the molecular conformation is nonplanar and the dihedral angle in 2 is ϕ = 40.2(1)°. Thus, the co-crystals of SA derivative 2 with two co-crystal formers showed conformational variation in SA moieties. Photochromic Properties. The UV/vis spectra before and after UV irradiation (Figures 4−6) were measured and compared to investigate the photochromic properties of SA derivatives (1, 2) and the co-crystals (1d, 1g, 2d, 2g, used as representative samples). The spectra of the other co-crystals are in the Supporting Information. In Figure 4c, the before and after irradiation spectra for SA derivative 1 are different in the range 450−550 nm, and a weak photochromic color change from yellow to red is observed. In the spectra for SA derivative 2 (Figure 4d) no difference is seen in the before and after irradiation spectra. This confirms that the nonplanar conformation of SA derivative 1 [ϕ = 26.9(1)°] is weakly photochromic and the planar conformation in SA derivative 2 [ϕ = 5.1(1)°] is not, which agrees with the reported literature.36 The co-crystals 1g and 2g showed photochromism, unlike co-crystals 1d and 2d, which did not (Figures 5, 6). The weak photochromic behavior in SA derivative 1 changes and exhibits photochromic behavior in 1g [nonplanar, ϕ = 49.5(1)°] and is nonphotochromic in 1d [planar, ϕ = 14.0(1)°]; the nonphotochromic SA derivative 2 exhibits photochromic behavior in 2g [nonplanar, ϕ = 40.2(1)°] and is nonphotochromic in 2d [planar, ϕ = 9.7(2)°]. The photochromic behavior in co-crystals 1a−1g depend on the molecular conformation adopted by the SA moiety.36 The planar conformational co-crystals, 1a−1d, have nonphotochromic reactivity, however the more twisted conformational cocrystals, 1e−1g, are photochromic. The threshold value of the dihedral angle which determines whether photochromic or nonphotochromic behavior exists is between co-crystal 1d [ϕ = 14.0 (1)°] and the SA derivative 1 [ϕ = 26.9(1)°], which is good agreement with the previously reported value36 (20° < ϕ < 30°). It is also known that the UV absorbance intensity changes in photochromic reactions, in which there is no correlation with the dihedral angle of SA molecules in cocrystals. Moreover, these results clearly show that it is possible to control the photochromic properties of the same SA molecule by conformational changes caused by co-crystallization through acid−base interactions. Thermal Fading Rate. The rates of thermal fading of the photochromic species, i.e., the time-dependent change of the UV/vis spectra corresponding to a red to yellow color change, were measured for SA derivative 1 and the co-crystals 1e, 1f, and 1g. These systems contain the same SA molecules in different crystalline environments. Photochromic 2g was excluded because it is a regioisomer of them. The rates were approximated by a first-order reaction. The rate constants, k, were calculated from a linear least-squares fitting of the firstorder reaction expression, as shown in Figure 7. The very different lifetimes (τ) were calculated from rate constants, τ = 1/k, 4.85 × 104 s for SA derivative 1, 2.30 × 103 s for 1e, 5.49 × 103 s for 1f, and 4.67 × 102 s 1g, illustrating that SA derivative 1 fades more slowly, by 2 orders of magnitude, than 1g (Figure 7). As the same SA molecule is in 1, 1e, 1f, and 1g, these differences must be caused by the crystalline environment of the colored species in the trans-keto form. As we reported in

Figure 1. Conformation of the hydrogen-bonded carboxylic acid dimer of 1 in the crystal structure.

Figure 2. Molecular conformational changes induced by the acid−base co-crystallization (SA moieties are shown in yellow).

In the crystal structure of SA derivative 2, there is one independent molecule in an asymmetric unit and the two carboxylic groups situated around an inversion center form hydrogen-bonded dimer structures via O2−H2O···O3′ (Figure 3). The molecular conformation is planar, and the dihedral

Figure 3. Conformation of the hydrogen-bonded carboxylic acid dimer of 2 in the crystal structure. 4783

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Figure 4. Molecular structures (top, viewed edge-on to accentuate the dihedral angle) and UV/vis spectra [bottom, before (black line) and after (red line) photoirradiation] of SA derivatives 1 (left) and 2 (right).

Figure 5. The molecular structures (top, viewed edge-on to accentuate the dihedral angle) and UV/vis spectra [bottom, before (black line) and after (red line) photoirradiation] of co-crystals 1d (left) and 1g (right).

the previous paper,19 the trans-keto form is generated in the crystal by photoirradiation. A newly generated N−H group projects to the opposite side of the OH group in the enol form (Scheme 1). If an intermolecular hydrogen bond is formed between the N−H group and its neighboring molecule, this interaction would stabilize the trans-keto form, slowing down thermal fading and thus increasing the lifetime. In these cocrystals, the trans-keto form was not observed in crystal structure analysis, so we assume that the trans-keto form has essentially the same structure as that of N-di-3,5-tertbutylsalicylidene-3-carboxyaniline and N-di-3,5-tert-butylsalicylidene-3-nitroaniline.16,19 In the construction of the trans-keto form, the central −NH−CH moiety of the cis-keto form

which was calculated from the enol form is rotated 180° around the molecular axis following the pedal motion (Supporting Information). The other atoms shift depending on the position of these atoms. These calculated structures were used to locate the intermolecular hydrogen bonds without geometry optimization by theoretical calculations. Part of the crystal structure containing one trans-keto molecule surrounded by several other trans-keto molecules was calculated for 1, 1e, 1f, and 1g (Figure 8).19 In these structures, all possible intermolecular hydrogen bonds between the N−H group of the trans-keto molecule and the oxygen atom of the neighboring trans-keto molecule were located. However for 1g no intermolecular 4784

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Figure 6. The molecular structures (top, viewed edge-on to accentuate the dihedral angle) and UV/vis spectra [bottom, before (black line) and after (red line) photoirradiation] of co-crystals 2d (left) and 2g (right).

Figure 7. Kinetic plots of absorbance at wavelengths where the maximum decrease occurred for 1 (red), 1e (yellow), 1f (blue), and 1g (green). Solid lines show the linear least-squares regression fits. Figure 8. Calculated structures of the trans-keto form in (a) 1, (b) 1e, (c) 1f, and (d) 1g. Green dashed lines in (a), (b), and (c) represent the intermolecular hydrogen bonds involving the hydroxy group of the neighboring molecule.

interactions were observed. Therefore, we infer that the hydrogen bonds present in the calculated structures of 1, 1e, and 1f give rise to the longer lifetimes of the colored form. Furthermore, the lifetime differences among the three SA crystals, 1, 1e, and 1f, correlate to the strength of the intermolecular hydrogen bonds; the longer lifetime SA crystals have a more linear intermolecular hydrogen bond formation. The intermolecular hydrogen bond distances are not significantly different (2.9, 2.7, and 2.9 Å for 1e, 1f, and 1, respectively), however, as the hydrogen bond angles are 120, 142, and 157° for 1e, 1f, and 1, respectively, we infer that the hydrogen bond angles correspond to the increase in the colored form lifetimes, 2.30 × 103, 5.49 × 103, and 4.85 × 104 s, respectively. Thus, we conclude that the stronger intermolecular hydrogen-bond stabilized colored form results in the longer lifetimes of the colored form. On the other hand, the dihedral angles do not correlate to the lifetime, because both 1g and N-3,5-di-tert-butylsalicylidene-3-nitroaniline (3)16 have large dihedral angles [ϕ = 49.5(1) and 49.8°, respectively], however, 1g has a much shorter lifetime (τ = 4.67 × 102 s)

compared to 7.2 × 104 s for 3. The dihedral angles of 1, 1e, 1f, and 1g do not correlate to their lifetimes.

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CONCLUSION In this study, we have succeeded in controlling the photochromic reactivities of SA derivatives by changing the dihedral angle between the benzene rings using acid−base cocrystallization. Using this method, a nonphotochromic SA derivative 2 was made photochromic, and vice versa. Furthermore the thermal fading rates of 1, 1e, 1f, and 1g are shown to depend on the surrounding environment with the intermolecular hydrogen bonds of the trans-keto form, the colored species, playing a key role. Therefore, it is possible to control the thermal fading rate of photochromic SA molecules by co-crystallization using an appropriate base former compound. 4785

dx.doi.org/10.1021/cg300454q | Cryst. Growth Des. 2012, 12, 4779−4786

Crystal Growth & Design

Article

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The conformational variety was possible because of the inherent flexibility of the SA molecule around the N−C bond between the N imine and the aniline moiety, which allows crystal-packing forces to alter the conformation. There are a variety of base molecules which can form co-crystals with SA derivatives, enabling the SA molecules to adopt different conformations in various crystalline environments. Therefore, the acid−base co-crystallization method is very useful to control the photochromic property, and may lead to the development of new functional chromic materials.

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ASSOCIATED CONTENT

* Supporting Information S

The X-ray crystallographic information files (CIF) of the cocrystals, as well as the molecular structures, UV/vis spectra, pKa values, and how to generate the calculated trans-keto form from enol form. 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] Tel/fax: +81-3-5734-3529. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Grant-in-Aid for Scientific Research and JSPS Fellow from the Japan Society for Promotion of Science.

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REFERENCES

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