In Situ Control of Photochromic Behavior through Dual Photo

Although both 3-cyanopropyl and 1-cyanopropyl group complex crystals showed the yellow (or orange) to orange (or light brown) photochromism upon UV li...
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In Situ Control of Photochromic Behavior through Dual PhotoIsomerization Using Cobaloxime Complexes with Salicylidene-3aminopyridine and 3‑Cyanopropyl Ligands Yuta Yamazaki, Akiko Sekine,* and Hidehiro Uekusa Department of Chemistry and Materials Science, Graduate School of Science and Engineering, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo 152-8551, Japan ABSTRACT: In situ control of crystalline-state photochromism was achieved using a photo-isomerization reaction of cobaloxime complexes. These dual photoreactive complexes that have the photochromic salicylideneaniline ligand and 3-cyanopropyl group as the axial alkyl group were synthesized and crystallographically characterized. When each crystal was exposed to visible light, the 3cyanopropyl group isomerized to a 1-cyanopropyl group while retaining the single crystal form. Although both 3-cyanopropyl and 1cyanopropyl group complex crystals showed the yellow (or orange) to orange (or light brown) photochromism upon UV light irradiation, the fading rate of the photochromic color was significantly changed after the 3−1 isomerization. The differences in fading rates can be rationalized by a change of the volume of the cavity around the carbon−nitrogen bonds of the three salicylideneaniline derivatives, which is caused by the 3−1 photo-isomerization in the crystal. Also, the reactivity of 3−1 photo-isomerization was explained by the cavity around the 3-cyanopropyl group.

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INTRODUCTION The reversible color change of a material induced by irradiation with light is known as photochromism.1 Organic photochromic compounds have attracted the attention of chemists due to their potential applications as materials for optical data storage, electronic display systems, optical switching devices, and ophthalmic glasses.2−4 In 1909, Senier and Shepheard discovered that N-salicylideneanilines (SAs) exhibited photochromism both in solution and in the solid state.5 Photochromic SA crystals are usually colorless or pale yellow colored and exist in the enol form (Scheme 1). When the crystals are irradiated with ultraviolet (UV) light, their color changes to deep red. The deep red color can be easily erased either by irradiation with visible light or by thermal fading in the dark. Both spectroscopic data and crystal structures of several structurally related compounds indicate that the hydrogen atom of the hydroxyl group is fully transferred to the imine nitrogen atom during exposure to UV light.6−12 The subsequent rearrangement of the molecular geometry in the excited state is assumed to form a red-colored product, the trans−keto form, as shown in Scheme 1. Although the exact structure of the red-colored species has been a matter of controversy for a long time, it was finally determined by X-ray crystal structure analysis after photoirradiation with the twophoton excitation technique. This experiment confirmed the reaction process shown in Scheme 1 in 1999.13 The lifetime of the metastable red-colored species represented another important problem to be solved. The © XXXX American Chemical Society

longest lifetime of ca. 1200 min for an SA derivative was observed at room temperature for a crystal of N-3,5-di-tertbutylsalicylidene-3-nitroaniline.13,14 The crystal structure of the red-colored species revealed that two molecules related by an inversion center in the crystal were connected through a pair of hydrogen bonds. These hydrogen bonds stabilize the redcolored species, thus contributing to its long lifetime.15 In order to utilize SAs as photochromic materials, it is essential to control the lifetime of their color changes. Although the coloration time is very fast upon exposure to UV light and is difficult to control, the lifetime of the metastable, red-colored species strongly depends on the nature and number of intermolecular interactions established within the crystal. That is, photochromism of SAs depends on the crystal environment surrounding the SA molecules. Therefore, it is possible that environmental changes around the SA molecules in the crystal could control the photochromism upon introduction of a different photoreactive group. Cobaloxime complexes with photoresponsive alkyl groups may represent excellent candidates to control the photochromic behavior of SAs because they show almost complete isomerization upon exposure to visible light without the destruction of their single crystal forms.16−19 If such a cobaloxime moiety can be added to the SA molecules and the corresponding crystals Received: April 20, 2016 Revised: November 30, 2016 Published: December 1, 2016 A

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Scheme 1. Mechanism of the Photochromic Process in N-Salicylideneaniline

Chart 1. Three Cobaloxime Complexes Investigated in This Work

display photochromism, it is expected that the lifetime of the red-colored species of SA should be influenced by the photoisomerization of the alkyl group coordinated to the cobaloxime moiety. In our previous papers,20,21 cobaloxime-type complexes containing a 3-cyanopropyl group in the axial position and azobenzene derivatives as base ligands were synthesized, and the photochromic characteristics of their azobenzene moieties before and after the photo-isomerization of the 3-cyanopropyl group were investigated. The rate of the color change coming from the azobenzene derivatives was significantly modified by the 3−1 cyanopropyl photo-isomerization. These experiments indicated that the reaction cavity around the azobenzene moiety was significantly expanded after the photo-isomerization of the 3-cyanopropyl group (denoted to as 3−1 photoisomerization) and that the expanded cavity well explained the increased rate of the color change. However, it was difficult to examine how the structural change caused by the 3−1 photoisomerization process affected the rate of the color change because the crystallinity was not kept during the trans−cis isomerization of the azobenzene moiety. As a result, the crystal structure after the color change could not be determined. In our previous paper,22 N-salicylidene-3-aminopyridine (SAP) derivatives were used as the photochromic moiety in the dual photoreactive 2-cyanoethyl complexes, and their photochromic behavior under the 2−1 photo-isomerization reaction activated by visible light was investigated. The fading rate of colored species changed proportionally to the irradiation time with visible light. Although the overall photochromic process was explained by considering the reaction cavities of the corresponding molecular models, the experimental structure of the keto−enol photoproduct was not obtained. This lack of an accurate molecular model somewhat obscure the calculation of the reaction cavity.

The aim of this work is to achieve not only the in situ control of photochromic behavior but also to clarify the mechanism of the reaction with the aid of accurate crystal structures. We thus aim to quantitatively establish the correlation between lifetime of the colored species and the corresponding crystal structures. Three cobaloxime complex crystals, that is, (3-cyanopropyl) bis(dimethylglyoximato)(N-3,5-di-tert-butylsalicylidene-3aminopyridine)cobalt(III) (I), (3-cyanopropyl) bis(dimethylglyoximato)(N-3,5-dibromosalicylidene-3aminopyridine)cobalt(III) (II), and (3-cyanopropyl) bis(dimethylglyoximato)(N-5-methoxysalicylidene-3aminopyridine)cobalt(III) (III) (Chart 1), were obtained, and their photochromic behavior before and after the 3−1 photoisomerization of the 3-cyanopropyl group is examined in detail.

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RESULTS AND DISCUSSION 3−1 Photo-Isomerization. Powder samples of photochromic complexes I, II, and III were irradiated at room temperature with visible light between 620 and 800 nm. The change of the CN bond stretching vibration in the 3cyanopropyl group was measured at constant time intervals with an infrared (IR) spectrometer. The intensity of the peak located around 2250 cm−1 from the 3-cyanopropyl group gradually decreased, while a new peak originating from the 1cyanopropyl group appeared around 2200 cm−1 and gradually increased in intensity. For the powder sample of I, the intensity of the peak at 2250 cm−1 became negligible, and only a peak at ca. 2200 cm−1 was observed after 24 h of light exposure, as shown in Figure 1. This indicated that the 3-cyanopropyl group was completely isomerized to the 1-cyanopropyl group. The powdered crystal after complete 3−1 photo-isomerization is denoted as I′. The changes of the IR spectra of the powdered samples of II and III upon visible-light irradiation were measured in the same B

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Figure 3. Crystal structure of I before visible-light irradiation viewed along the a axis. Hydrogen atoms are omitted for clarity.

Figure 1. Infrared spectra of I before and after 24 h irradiation with visible light.

way as that of I. The rates of the changes in the spectra were lower than that of I. After 72 h of visible-light exposure, both the original peak at 2250 cm−1 and the newly produced peak at 2200 cm−1 were observed, and their intensity ratio became constant. This indicated that further 3−1 photo-isomerization did not occur. The IR spectrum of II after 72 h of visible-light irradiation is shown in Figure 2. From the decrease in intensity

Figure 4. Molecular crystal structures of (a) I and (b) I′. Hydrogen atoms are omitted for clarity and thermal ellipsoids are drawn at the 50% probability level. The 3-cyanopropyl group in I is changed to a disordered 1-cyanopropyl group with R (yellow) and S (green) configurations with a ratio of 0.56(1) and 0.44(1), respectively, in I′.

The N-3,5-di-tert-butylsalicyliden-3-aminopyridine ligand is nonplanar, although the 3-aminopyridine and salicylidene moieties are both planar within ±0.04 Å. The dihedral angle between the two planar moieties is 30.6(1)°. The bond distances and bond angles are similar to those of related cobaloxime complexes with a 3-cyanopropyl group and those of the N-3,5-di-tert-butylsalicylideneaniline derivatives. The crystal structure of I′ is shown in Figure 4b. The N-3,5di-tert-butylsalicylidene-3-aminopyridine ligand of I′ adopts almost the same conformation as that of I. The dihedral angle between the two planar groups of the ligand is 32.6(1)°, which is close to the corresponding value of 30.6(1)° before the photoirradiation. Color Fading Rates of I and I′. The powder samples of I and I′ were irradiated with UV light, since the single crystal of I showed no significant color change when it was exposed to UV light. Considering from the single crystal structure analyses of salicylideneanilines,13,14 the occupancy factors of the redcolored species in the single crystal were ca. 10%. The yellow color rapidly changed to orange after UV irradiation began, as depicted in Figure 5a, which indicated that the cis−enol form of the SAP moiety transformed to the trans−keto form as summarized in Scheme 1. The orange color gradually faded and returned to the original yellow when the sample was kept in the dark at room temperature. This color change was measured by UV/vis spectroscopy, as shown in Figure 6. The absorption spectra before and just after UV irradiation clearly indicate that an orange species, which has an absorption band at ca. 500 nm, was produced in the powder sample. The intensity of the

Figure 2. Infrared spectra of II before and after 72 h irradiation with visible light.

of the 2250 cm−1 peak, it was assumed that about 30% of the 3cyanopropyl groups isomerized to the 1-cyanopropyl groups. For the powder sample of III, the change of its IR spectra also indicated that the 3−1 photo-isomerization stopped at about 30% conversion at room temperature. The powdered crystals of II and III after ca. 30% 3−1 photo-isomerization are denoted as II′ and III′, respectively. The crystal structure of I viewed down the a axis is depicted in Figure 3. There is only one molecule in the asymmetric unit of the P212121 cell. The molecules have a dog leg-like shape and are arranged along the b axis. The zigzag chains are closely packed along the c axis to form a sheet parallel to the bc plane. These sheets are stacked along the a axis. Neither intermolecular hydrogen bonds nor unusually short contacts between the molecules in the crystal are present. The molecular structure of I is illustrated in Figure 4a. The torsional angles Co 1−C9−C10−C11 and C9 −C10−C11−C12 are 174.17(19)° and −64.6(3)°, respectively, which indicate that the 3-cyanopropyl group coordinated to cobalt adopts a trans− gauche conformation. C

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from the 3-cyanopropyl group to the 1-cyanopropyl group affected the fading rate of the trans−keto form of the N-3,5-ditert-butylsalicylidene-3-aminopyridine ligand. Structures and Photochromism of II and II′. The crystal structure of II viewed down the a axis is shown in Figure 8.

Figure 5. Color changes of (a) I, (b) II, and (c) III before and after UV irradiation.

Figure 8. Crystal structure of II viewed along the a axis before visiblelight irradiation. Hydrogen atoms are omitted for clarity.

There is only one molecule in the asymmetric unit of the P212121 cell. The molecules are stacked along the b axis while adopting an antiparallel orientation with each other. This arrangement indicates that the 3-cyanopropyl group is in close contact with the SAP moiety of the neighboring molecule. However, neither intermolecular hydrogen bonds nor unusually short contacts between the molecules in the crystal are present. The molecular structure of II is depicted in Figure 9a. The SAP moiety is nonplanar, and the dihedral angle between the two six-membered rings is 48.39(8)°. The torsional angles Co1− C9−C10−C11 and C9−C10−C11−C12 are 166.0(3)° and −65.6(4)°, respectively, which indicate that the conformation of the 3-cyanopropyl group is trans−gauche, like that in I. The

Figure 6. UV/vis spectra of I before and after UV irradiation.

absorption band at 500 nm gradually decreased to that of the original one. A similar color change to that of I was observed for I′ upon exposure to UV light. The fading rates of I and I′ are summarized in Figure 7. As it appears from this figure, the fading rates of I and I′ are considerably different. For example, I and I′ show fading ratios of 8% after about 25 and 50 min, respectively. The fading rate of I is much faster than that of I′. It is thus clear that the structural change that occurs on going

Figure 9. Molecular structures of (a) II and (b) II′. The disordered 3cyanopropyl group of II′ and hydrogen atoms of II and II′ are omitted for clarity. The carbon atoms of the proposed 1-cyanopropyl group are shown in green. The thermal ellipsoid of each atom is drawn at the 50% probability level.

Figure 7. Fading rates of I (green) and I′ (red). D

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bond distances and angles are essentially the same as the corresponding ones of I. The crystal structure of II′ after partial 3−1 photoisomerization is almost the same as that of II. The molecular structure of II′ is shown in Figure 9b. The structure indicates that the occupancy factors of the original 3-cyanopropyl group and the newly produced 1-cyanopropyl group are 0.74(2) and 0.26(2), respectively. These values are consistent with the conversion of around 30% as determined from the IR spectral changes. The 1-cyanopropyl group possesses the S configuration, and no peak was observed for the R configuration.23 The structure of the SAP ligand is nearly the same as that before photoirradiation. The dihedral angle of II′ is 47.02(2)°, which is similar to the corresponding one of 48.39(8)° in II. A powder sample of II was irradiated with UV light, causing it to change from yellow to light brown, as shown in Figure 5b. The light brown color gradually faded and returned to the original yellow when the sample was kept in the dark at room temperature. The absorption spectra of II obtained before and just after UV irradiation clearly indicated that the light browncolored trans−keto form, which has an absorption maximum at ca. 500 nm, was produced in the sample. The intensity of the absorption band gradually decreased to that of the original one. The initial fading rate is shown in Figure 10.

Figure 11. Molecular structures of (a) III and (b) III′. The 3cyanopropyl group in III′ and hydrogen atoms in III and III′ are omitted for clarity. The produced 1-cyanopropyl groups are disordered and shown in green and yellow. The thermal ellipsoid of each atom is drawn at the 50% probability level.

C9−C10−C11 and C9−C10−C11−C12 are 169.90(15)° and −63.9(3)°, respectively. The bond distances and angles in III are essentially the same as the corresponding ones of I and II. The crystal structure of III′ is essentially the same as that of III. The molecular structure of III′ is provided in Figure 11b. The occupancy factors of the original 3-cyanopropyl, produced (S)-1-cyanopropyl, and produced (R)-1-cyanopropyl groups were 0.72(1), 0.17(1), and 0.11(1), respectively.24 The structure of the SAP ligand in III′ is nearly the same as that in III. The dihedral angle of III′ was 38.5(1)°, which is similar to the corresponding one of III of 38.71(5)°. The orangecolored sample of III changed to dark red upon UV irradiation (III′), as shown in Figure 5c. The intensity of the newly appeared absorption maxima of III′ at ca. 500 nm gradually decreased to that of the original one. The fading rate of III′ is much lower than that of III, as indicated in Figure 12. Size and Shape of the Reaction Cavity. The crystals of I, II, and III displayed 3−1 photo-isomerization with retention of the single crystal form. The structural changes of the cyanopropyl group viewed in the direction normal to the cobaloxime molecular plane, before and after irradiation with

Figure 10. Fading rates of II and II′.

The powder sample of II′ was irradiated with UV light under the same conditions as II. A color change similar to that of II was observed. The fading rate of II′ is also illustrated in Figure 10. The fading rate of II′ is much greater than that of II. It is clear from these results that the structural change of the cyanopropyl group influences significantly the fading rates of II and II′. Structural and Photochromic Changes of III and III′. The crystal of III belongs to the space group Pna21, and there is only one molecule in the asymmetric unit. The molecules are stacked antiparallel to each other. This means that the 3cyanopropyl group is in close contact with the SAP moiety of the neighboring molecule, as already observed in the crystal structure of II. Neither intermolecular hydrogen bonds nor unusually short contacts between the molecules were observed. The molecular structure of III is displayed in Figure 11a. The dihedral angle between the two six-membered rings of the SAP moiety is 38.71(5)°. The conformation of the 3-cyanopropyl group is trans−gauche, because the torsional angles of Co1−

Figure 12. Fading rates of III and III′. E

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III. As for the difference in the occupancy factors for R and S configurations between II and III, it can be explained by considering the shape of the cavity. For example, the shape of the cavity of II fits that of S-1-cyanopropyl group. The different steric repulsions exerted by the surrounding groups may produce two configurations of the 1-cyanopropyl group with different occupancy factors. Controlling Photochromic Behavior. The thermal fading rates of the trans−keto forms in the three crystals changed considerably when the 3-cyanopropyl group bonded to the cobalt atom was transformed to a 1-cyanopropyl group by visible light. Since the unit-cell volumes of I′, II′, and III′ increased by 32.5(2), 29.6(2), and 12.3(2) Å3 compared with those of I, II, and III, respectively, voids originating from 3 to 1 photo-isomerization should appear in the molecular crystal, which may assist the structural change from trans−keto to cis− enol forms. However, the fading rates of I′, II′, and III′ were smaller, greater, and smaller, respectively, than those of the corresponding materials with 3-cyanopropyl groups. This suggests that the local intermolecular interactions around the SAP moiety strongly influence the thermal fading rate of these complexes. As observed directly in the X-ray analyses of N-3,5di-tert-butylsalicylidene-3-nitroaniline13 and N-3,5-di-tert-butylsalicylidene-3-carboxyaniline,14 the cis−enol form changed to the trans−keto form upon exposure to UV light, changing the color of the crystals from yellow to red. The trans−keto form gradually returns to the cis−enol form upon irradiation with visible light or kept in the dark. The transformation from cis− enol to trans−keto and vice versa occurred with retention of the single crystal form, and the molecules before and after the transformation occupy approximately the same position. It was thus proposed that two rings of the SA moiety should move cooperatively, similar to the pedal motion of a bicycle.26 Therefore, it is plausible that the SAP moieties of I, II, and III should be transformed through a pedal motion. Other parts of the molecules such as the cobaloxime and cyanopropyl groups may have nearly the same positions before and after the transformation. Moreover, the intermolecular hydrogen bonds observed in the crystals of N-3,5-di-tert-butylsalicylidene-3nitroaniline and N-3,5-di-tert-butylsalicylidene-3-carboxyaniline are not formed in any crystals of I, II, and III. Only the structure around the central bond of C7−N1 may change markedly before and after the transformation. This change is shown in Figure 15, which was drawn by assuming that the transformation from cis−enol to trans−keto forms occurred in the crystal of I in the same manner as that observed in crystals of N-3,5-di-tert-butylsalicylidene-3-nitroaniline.13 In order to estimate the void space around the C7−N1 bond, we calculated the volume of the reaction cavity for the C7−N1 group before the transformation. The reaction cavities for the

visible light, are summarized in Figure 13. The 3-cyanopropyl group with trans−gauche conformation was completely trans-

Figure 13. Structures of the 3-cyanopropyl (3-cp) and 1-cyanopropyl (1-cp) groups of I, II, and III, viewed normal to the cobaloxime planes, before and after irradiation with visible light, respectively.

formed to the 1-cyanopropyl group with R and S configurations in the ratio of 56:44, respectively, in I. For the reactions keeping the single crystal form, it has been proposed that the reaction process and the reaction rate can be rationalized by considering the size and shape of the reaction cavity around the reactive group.25 To examine the reason why such a ratio was obtained in the photoreaction, the reaction cavity for the 3cyanopropyl group was drawn, as shown in Figure 14. The

Figure 14. Reaction cavities for the 3-cyanopropyl groups of I, II, and III.

volume of the cavity was calculated to be 33.2 Å3, which is large enough to accommodate the disordered photoproducts with R and S configurations. The structures of the produced 1cyanopropyl group with R and S configurations are drawn in the cavity in Figure 14. Since the shape of the cavity is nearly symmetric, both the 1-cyanopropyl groups with R and S configurations can be accommodated with an almost equal probability. The structural change of the 3-cyanopropyl group of II upon exposure to visible light is illustrated in Figure 13. Only 26% of 3-cyanopropyl groups changed to 1-cyanopropyl groups with S configuration. The cavity for the 3-cyanopropyl group of II (Figure 14) is much smaller than that of I. Although the cavity can accommodate the S-1-cyanopropyl group, it is very difficult for the cavity to host the R-isomer. This is the reason why only the 1-cyanopropyl group with S configuration was produced in the crystal of II. The reaction cavity volumes of II and III are almost the same. This could explain why approximately the same yields of photoproduced 1-cyanopropyl groups were obtained for II and

Figure 15. Predicted movement of atoms in the transformation from the cis−enol form to the trans−keto one in I and I′. F

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to visible light. Such a reaction is termed dual photoisomerization, which shows promise as an effective method to in situ control photochromism.

C7−N1 bond for the cis−enol form of I and I′ were calculated to be 5.9 and 4.9 Å3, respectively, as illustrated in Figure 16.

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CONCLUSION In this study we demonstrated how to control the photochromic behavior of the SAP derivatives using the crystallinestate photo-isomerization of cobaloxime complexes containing a 3-cyanopropyl group. The reaction rate of the SAP derivatives from trans−keto form, which was produced by UV irradiation, to the original cis−enol form in the dark can be markedly changed after the 3-cyanopropyl group coordinated to the cobalt center was isomerized to the 1-cyanoethyl group upon exposure to visible light, without destroying the single crystal form. Because the crystallinity was kept during the photoisomerization of the 3-cyanopropyl group, the mechanism of the modified reaction rate was clarified. Such a method is termed dual photoreaction. When the solid-state reactions will be applied to the organic substances, it is most important how to control their characteristics to match the conditions to utilize them. We already succeeded in changing the fading rate of the crystals of salicylideneanilines by introducing the intermolecular hydrogen bonds. However, it is worth noticing that the introduction of intermolecular hydrogen bonds into the molecular crystal is a difficult step in the overall design process aimed at controlling the fine-tuning of the photochromic characteristics. On the other hand, we expect the dual photoreaction approach to be useful since a variety of photoreactions can be combined together. Moreover, the substituents on the organic ligands can be easily modified, thus increasing the number of photochromic complexes that can be investigated. It is possible to say that the field of solid-state photoreactions will benefit by using the technique of dual photoreaction.

Figure 16. Cavities for the C7−N1 bonds in the cis−enol forms of I and I′.

Since the 1-cyanopropyl group in I′ has disordered 1cyanopropyl groups with R and S configurations, the reaction cavity should be calculated for the two configurations. The cavity volumes were essentially equal to each other for the two configurations because the disordered 1-cyanopropyl group is far away from the SAP moiety. The reaction cavity for the reactive C7−N1 bond of I′ was smaller than that of I, although the unit-cell volume of I′ is greater than that of I. This readily explains why the fading rate of I′ is lower than that of I. For the crystals of II and III, the 3−1 photo-isomerization only proceeded with 26% and 28% conversion rate, respectively. Therefore, when we calculated their reaction cavities, it was assumed that only 1-cyanopropyl groups were produced with 100% occupancy and the structure of the original 3-cyanopropyl group was removed in the same lattice of II′ and III′, respectively. The reaction cavities for the C7− N1 bonds of II and II′ were calculated in the same manner as those for I. The volumes of II and II′ were calculated to 4.4 and 4.9 Å3, respectively. The increase in volume of the reaction cavity after 3−1 photo-isomerization explains the increased fading rate of II′ compared with that of II. For the III and III′ crystals, the photoinduced 1-cyanopropyl group was disordered with the R and S configurations characterized by different occupancy factors. When the reaction cavity for the C7−N1 bond is calculated, it is necessary to consider the following three cases: (A) both sides of the C7− N1 bond are surrounded by two 1-cyanopropyl groups with S configuration, (B) both sides of the C7−N1 bond are surrounded by two 1-cyanopropyl groups with R configuration, and (C) one side contacts with S configuration and the other side with R configuration. The cavity volumes for A, B, and C were calculated to be 3.4, 6.3, and 3.2 Å3, respectively. Considering that the S and R configurations have the occupancy factors of 0.17(1) and 0.11(1), respectively, the cavity volume of III′ became 3.7 Å3 with weighted average. This value is much smaller than that of III of 4.5 Å3. This clearly explains the lower reaction rate of III′ in comparison to that of III. The rate of the photochromism of SAP derivatives induced by UV irradiation can be markedly changed by the isomerization of the 3-cyanopropyl group introduced on a different part of the molecule to a 1-cyanopropyl group upon exposure

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EXPERIMENTAL SECTION

Cobaloxime Complexes I, II, and III. The cobaloxime complexes I, II, and III were similarly synthesized by the previously reported method.22 Single crystals of I, II, and III were obtained by recrystallization from methanol. 3−1 Photo-Isomerization of Powder Samples I, II, and III. KBr discs containing a powdered sample of I, II, or III (1.2%) were irradiated with visible light using a Xe lamp (SAN-EI, Japan, SUPERBRIGHT-152S), the top of which was placed 7 mm from the disc. A glass filter (Toshiba R-62) was inserted between the disc and the lamp so that the disc was exposed to visible light in the range 620−800 nm at room temperature. The change of the CN bond stretching vibration was measured at constant intervals with the aid of an IR spectrometer (Bio-Rad FTS 3000). The intensity of the peak at ca. 2250 cm−1 caused by 3-cyanopropyl group gradually decreased, and a new peak from the 1-cyanopropyl group appeared around 2200 cm−1, which gradually increased in intensity. For the powdered sample of I, the photoirradiation was continued for 24 h because the intensity of the 2250 cm−1 peak became negligible and only that at 2200 cm−1 was observed. This result indicates that the 3-cyanopropyl group coordinated to cobalt isomerized completely to the 1-cyanopropyl group. The reaction rates of powder samples of II and III were lower than that of I. After 72 h of exposure, the isomerization became within the experimental error. However, the original peak remained along with the appearance of a new one after 72 h. It was assumed from the relative peak heights that about 30% of the 3-cyanopropyl groups changed to 1-cyanopropyl ones in II and III. Fading Rates of the Trans−Keto Forms. The powdered samples I, II, and III were irradiated at room temperature with UV light using an ultrahigh-pressure mercury lamp (SAN-EI UVF-352S) through a G

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Table 1. Crystal Data and Experimental Details Before and After Visible-Light Irradiation of Crystals of I, II, and III material formula temperature (K) crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) volume (Å3) Z dcalc (Mg m−3) crystal size (mm3) θ range (deg) reflections collected R(int) data/restraints/parameters GOF R1 (I > 2σ(I)) wR2 (I > 2σ(I)) R1 (all data) wR2 (all data) Δρmax/Δρmin (e·Å3)

I

I′

C32H46N7O5Co 123(2) 123(2) orthorhombic P212121 9.1510(2) 8.8232(4) 17.0419(3) 17.2575(7) 22.1178(6) 22.8662(9) 90 90 90 90 90 90 3449.30 (13) 3481.8(2) 4 1.286 1.274 0.23 × 0.09 × 0.07 0.18 × 0.09 × 0.07 3.0−27.4 3.2−27.5 33563 55666 0.076 0.095 7821/2/428 7967/35/435 1.09 1.03 0.036 0.043 0.080 0.11 0.038 0.047 0.082 0.11 0.57/−0.24 0.63/−0.31

II

II′

C24H28N7O5Br2Co 123(2) 123(2) orthorhombic P212121 8.7733(3) 8.8261(3) 12.6940(3) 12.6786(4) 25.3648(7) 25.5074(7) 90 90 90 90 90 90 2824.81(13) 2854.37(15) 4 1.677 1.660 0.29 × 0.20 × 0.06 0.30 × 0.20 × 0.06 3.2−27.4 3.2−27.4 27779 27976 0.091 0.105 6415/2/365 6477/13/380 1.03 1.05 0.035 0.069 0.078 0.18 0.037 0.075 0.079 0.19 1.12/−0.39 3.50/−1.13

glass filter (HOYA UV330) that allowed the wavelengths around 365 nm to pass through. The color of the samples changed upon UV irradiation because of the cis−enol to trans−keto transformation shown in Scheme 1. The trans−keto form thermally faded when the samples were kept in the dark at room temperature. To evaluate this color fading, I, II, or III (9.2 mg) was mixed with barium sulfate (43.8 mg) in a mortar to give a concentration of I, II, or III in the powdered matrix of about 17%. The UV/vis spectra of the powdered samples were recorded before and after UV irradiation for 30 s using a spectrometer (JASCO V-560). The absorption at 500 nm increased markedly because of the color change. To measure the rate of thermal color fading, time-dependent UV/vis spectra were measured in the dark for the trans−keto forms of I, II, and III at regular intervals of 5 min. The rate was estimated with the following formula: (A0 − At)/(A0 − Aini) × 100 (%) where Aini is the absorbance before UV irradiation, A0 is the absorbance after 30s UV irradiation, and At is the absorbance left after t minutes. Structural Changes of I, II, and III Before and After 3−1 Photo-Isomerization. A single crystal of I was exposed to visible light from the Xe lamp under the same conditions as the powder samples for 24 h. Complete 3−1 photo-isomerization occurred with retention of the single crystal form. The rates of 3−1 photoisomerization of the single crystals of II and III were lower than that of I, and further conversions were not obtained after 72 h of exposure keeping the single crystal form. Crystal Structure Analyses. Each single crystal was mounted on the diffractometer (Rigaku, R-AXIS RAPID or XtalLab), and the intensity data were collected using the MoKα radiation at 173 K. The crystal data and experimental details for the six crystals are presented in Table 1. The unit-cell volumes of I′, II′, and III′ increased by 32.5(2), 29.6(2), and 12.3(2) Å3 after photoirradiation, respectively. The crystal structures were solved with SHELXS9727 or SIR201128 and refined with SHELXL2013.29 All the non-hydrogen atoms and the disordered atoms in the unit cell were refined with anisotropic temperature factors. The hydrogen atoms were obtained geometrically and then refined with isotropic temperature factors by assuming rigid motion. For comparing the absolute configuration of the produced 1cyanopropyl group and the shape of the reaction cavity around it before and after the reaction, the overall crystal structures and absolute

III

III′

C25H32N7O6Co 123(2) 123(2) orthorhombic Pna21 8.9532(2) 9.0205(4) 23.8676(5) 23.8672(10) 12.8388(7) 12.8003(6) 90 90 90 90 90 90 2743.52(11) 2755.8(2) 4 1.418 1.411 0.27 × 0.16 × 0.14 0.34 × 0.26 × 0.11 3.0−27.4 3.0−27.5 26154 25468 0.052 0.044 5918/3/369 6197/24/205 1.09 1.06 0.031 0.060 0.071 0.15 0.031 0.067 0.072 0.16 0.52/−0.31 1.15/−0.33

configurations of I′, II′, and III′ are consistent with the corresponding ones of I, II, and III, respectively.

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ASSOCIATED CONTENT

Accession Codes

CCDC 1456428−1456433 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Akiko Sekine: 0000-0001-9220-944X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Ministry of Education, Culture, Sports, Science and Technology of Japan through a Grant-in-Aid for Scientific Research (C) (No. 24550014, for A.S.). The comments and suggestions of reviewers are gratefully acknowledged.

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REFERENCES

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DOI: 10.1021/acs.cgd.6b00602 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

Article

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I

DOI: 10.1021/acs.cgd.6b00602 Cryst. Growth Des. XXXX, XXX, XXX−XXX