Photochromism of Fulgide Crystals: From Lattice-Controlled Product

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Photochromism of fulgide crystals: from latticecontrolled product accumulation to phase separation Jun Harada, Masaya Taira, and Keiichiro Ogawa Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b00182 • Publication Date (Web): 20 Mar 2017 Downloaded from http://pubs.acs.org on March 23, 2017

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Crystal Growth & Design

Photochromism of fulgide crystals: from lattice-controlled product accumulation to phase separation Jun Harada,*,† Masaya Taira,‡ and Keiichiro Ogawa‡ †

Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo 060-0810, Japan. Department of Basic Science, Graduate School of Arts and Sciences, The University of Tokyo, Tokyo 153-8902, Japan. ‡

Abstract. Fulgides are photochromic in solution and in the solid state. This study revealed that the photochromism of fulgide crystals proceeds in two stages. Based on a previously developed two-photon excitation method, a single-crystal X-ray diffraction study on (2E,3Z)-2-[1-(2,5dimethyl-3-furyl)-2-methylpropylidene]-3-sec-butylidenesuccinic anhydride (3) allowed monitoring the structural changes in the solid state, and confirmed that the photocyclization proceeded in a conrotatory fashion. The lattice-controlled photoreaction was, however, limited to the early photoconversion stages, where only a small amount of photo-generated colored species was present in the lattice of the reactant crystals. Upon further accumulation of the photoproduct, the reaction proceeded to a second stage, where the crystals of the photoproduct separated from the reactant-like crystals to form a different solid phase. The in situ observation of the solid-state photochromism was thus also limited to the early stages of the photochromic conversions, where the product yield is low and the irradiated crystals retain the original crystal lattice.

Corresponding Author: Jun Harada Department of Chemistry, Faculty of Science Hokkaido University, Sapporo 060-0810, Japan Phone/Fax: +81 11 706 3563 E-mail: [email protected]

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Photochromism of fulgide crystals: from latticecontrolled product accumulation to phase separation Jun Harada,*,† Masaya Taira,‡ and Keiichiro Ogawa‡ †

Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo 060-0810, Japan.

‡

Department of Basic Science, Graduate School of Arts and Sciences, The University of Tokyo,

Tokyo 153-8902, Japan. *E-mail: [email protected]

ABSTRACT: Fulgides are photochromic in solution and in the solid state. This study revealed that the photochromism of fulgide crystals proceeds in two stages. Based on a previously developed two-photon excitation method, a single-crystal X-ray diffraction study on (2E,3Z)-2[1-(2,5-dimethyl-3-furyl)-2-methylpropylidene]-3-sec-butylidenesuccinic anhydride (3) allowed monitoring the structural changes in the solid state, and confirmed that the photocyclization proceeded in a conrotatory fashion. The lattice-controlled photoreaction was, however, limited to the early photoconversion stages, where only a small amount of photo-generated colored species was present in the lattice of the reactant crystals. Upon further accumulation of the photoproduct, the reaction proceeded to a second stage, where the crystals of the photoproduct separated from the reactant-like crystals to form a different solid phase. The in situ observation of the solid-state


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photochromism was thus also limited to the early stages of the photochromic conversions, where the product yield is low and the irradiated crystals retain the original crystal lattice.

INTRODUCTION Photochromism describes the reversible color change of substances upon irradiation with light. This phenomenon has attracted considerable interest in the areas such as physical and organic chemistry as well as photochemistry.1–7 Fulgides, i.e., derivatives of dimethylene-succinic anhydride, represent one of the most investigated classes of photochromic compounds.8,9 One of the reasons why fulgides have been examined so extensively is the fact that furylfulgides yield thermally stable colored species.10,11 Exposure of furylfulgides in solution to irradiation with UV light converts the stable E-form (typically pale yellow), into the strongly colored C-form (typically red) via a 6 π-electrocyclization (Scheme 1). The C-form is stable in the dark and retroconverts into the E-form upon irradiation with visible light. Furylfulgides with bulky substituents (R1 in Scheme 1) have been reported to increase the quantum yield of the cyclization by suppressing undesired E-Z isomerizations,12 which resulted in the quantitative transformation into the C-form upon irradiating such furylfulgide solutions with UV light.10,13 Scheme 1. The photochromic reactions of furylfulgides examined in this study. R1

O

R1 O

O

UV Vis

O O

O

O

R2 O C-form

R2 E-form 1: R1 = Me, R2 = Me 2: R1 = i-Pr, R2 = Me 3: R1 = i-Pr, R2 = Et


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Furylfulgides exhibit photochromism in the crystalline bulk,14–18 and in one case, photomechanical effects have been reported.19 Even though colored furylfulgide-derived photoproducts have not yet been detected by X-ray diffraction analysis of crystals irradiated with steady light on account of the usually surface-confined reactions, pulsed-laser light induces twophoton excitation and homogenous photoreactions in crystalline samples.20 In-situ X-ray diffraction studies on crystals of (E)-2-[1-(2,5-dimethyl-3-furyl)-ethylidene]-3isopropylidenesuccinic anhydride (1) and (E)-2-[1-(2,5-dimethyl-3-furyl)-2-methylpropylidene]3-isopropylidenesuccinic anhydride (2) have previously shown that the crystal lattice of the starting materials may be maintained during the photochromic reactions in certain cases.20,21 Unfortunately, such observations are limited to reactions with low conversion rates. For example, the solid-state photochromic transformation of 1, which shows a high conversion rate, results in the formation of non-crystalline amorphous materials.17 The details of the solid-state photochromism of furylfulgides remain accordingly unknown when the products are obtained in moderate to high yields. This lack of understanding of the photoconversion pathway in solid photochromic systems hampers the design and development of materials suited for future applications, in e.g. molecular switches and optical data storage devices. In this work, we have broadened our investigation on the solid-state photochromic reactions of furylfulgides 1 and 2. We also provide an insight into the behavior of the novel furylfulgide (2E,3Z)-2-[1-(2,5-dimethyl-3-furyl)-2-methylpropylidene]-3-sec-butylidenesuccinic anhydride (3). In order to confirm the conrotatory photo-cyclization mode, single-crystal X-ray diffraction studies were carried out on a crystal of 3 after approximately 10% of its C-form had been produced by two-photon excitation. Powder X-ray diffraction studies on crystalline samples of 13 revealed that their original crystal lattices were not maintained during the photoreactions.


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Instead, these reactions proceed to completion under the formation of a new solid phase, which was in the case of 1 identified as the crystal of the C-form. In addition, an overall perspective of the solid-state photochromism of furylfulgides is discussed below. EXPERIMENTAL SECTION Materials. Compounds 1–3 were prepared according to previously published methods,10,22–24 and microcrystalline powders were obtained from the recrystallization from hexane solutions. (2E,3Z)-2-[1-(2,5-dimethyl-3-furyl)-2-methylpropylidene]-3-sec-butylidenesuccinic anhydride (3). Mp: 95 °C (from hexane). δ H NMR (500 MHz; CDCl3; Me4Si): 0.88 (3H, br d), 1.04 (3H, t), 1.31 (3H, br d), 1.36 (3H, s), 1.90 (3H, s), 2.27 (3H, s), 2.57 (1H, m), 2.82 (1H, m), 4.28 (1H, sept), 5.93(1H, s). Elemental analysis: Found: C, 71.41; H, 7.44. Calc. for C18H22O4: C, 71.50; H, 7.33%. UV-Vis spectroscopy. Diffuse UV-Vis reflectance spectra were measured on a Jasco V-550 spectrometer equipped with an integrating-sphere accessory (ISV-469). Ground NaCl powder (MERCK Suprapur) was used as the diluent and reference. Solution UV-Vis absorption spectra in toluene were measured on a Jasco V-560 spectrometer. Spectrophotometric grade toluene (Spectrosol, Dojin) was used as received. Photo-irradiation. A Nd:YAG pumped optical parametric oscillator (Quanta-Ray Pro 250 and MOPO SL; pulse duration: 5 ns; repetition rate: 10 Hz) was used for the laser irradiation of the crystals. Prior to interacting with the samples, the laser beam was depolarized to minimize the effects of optical anisotropy. The temperature of the irradiated crystals was controlled using a Cryostream (Oxford Cryosystems) open-flow gas cryostat. For the photocoloration of the crystals by irradiation with steady light, a SAN-EI UVF-352S high-pressure Hg lamp was used,


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whose 365 nm emission line was isolated by using a band-pass filter (ASAHI Spectra MX0365) and a heat absorption filter (HOYA HA50). A SAN-EI UVF-202S Hg-Xe lamp with a HOYA Y52 filter and a HA50 filter was used for the bleaching of the colored crystal. A glass plate with a square shallow cavity (depth: ~0.05 mm) was used as sample holder to allow the incident light to penetrate the entire microcrystalline samples during the powder X-ray diffraction measurements. The sample holder was placed on a thermoplate, which was kept at 10 °C during the photo-illumination. Once the powder was packed on the plate, it remained untouched during the series of measurements. The yields of the photochromic reactions of 1–3 were determined by solution UV-Vis absorption spectra of aliquots of the irradiated powder samples according to previously described procedures.20 Single-crystal X-ray diffraction analysis. All diffraction measurements were carried out using a Bruker SMART 1000 CCD area detector system with graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å). The frame data were integrated with a Bruker SAINT V6.45A (2θmax = 60.0°). Structures were solved by a direct method using SHELXS-97,25 and refined by fullmatrix least-squares on F2 using SHELXL-2014.26 All C and O atoms in non-disordered crystals were refined anisotropically, while all H atoms were refined using riding models. The laserirradiated crystal of 3 showed disorder and the details of the refinement are given in the Supporting Information (SHELXL-2014 res file). The crystal and refinement data are summarized in Table 1. Powder X-ray diffraction measurements. Powder X-ray diffraction patterns were measured on a Rigaku MultiFlex with Cu-Kα radiation (λ = 1.54184 Å). Microcrystalline powders of 1 – 3 used for photochromic reactions exhibited identical crystal structures to those obtained from


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single-crystal X-ray diffraction studies at room temperature (Figure S1).27,28 The diffraction patterns of 1 – 3 and their changes during the photoreactions are shown for the whole range of the measurement (5° < 2θ < 40°) (Figures S2-S4). Table 1. Crystal data and structure refinements for 3 Before irradiation

Before irradiation

Irradiated with laser light (742 nm)

formula

C18H22O4

C18H22O4

C18H22O4

formula weight

302.35

302.35

302.35

temperature (K)

300

90

90

crystal system

monoclinic

space group

P21/c

a (Å)

8.2537(9)

8.1843(3)

8.224(3)

b (Å)

18.852(2)

18.6679(8)

18.685(7)

c (Å)

11.3355(12)

11.0971(5)

11.072(4)

β (deg)

108.310(2)

108.2730(10)

109.046(7)

V (Å3)

1674.5(3)

1609.96(12)

1608.3(10)

Z

4

reflections collected

26045

25606

24549

unique reflections

4898

4725

4761

Rint

0.0270

0.0223

0.0379

data/restraints/parameters

4898/0/205

4725/0/205

4761/174/237

goodness-of-fit on F2

1.056

1.054

1.069

R1 [F2 > 2σ(F2)]

0.0485

0.0358

0.0557

wR2 (all data)

0.1523

0.1028

0.1437


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∆ρmin (e Å-3)

0.19

0.36

0.39

∆ρmax (e Å-3)

–0.20

–0.20

–0.27

CCDC number

1471623

1471624

1471625

Table 1. (Continued) Irradiated with i) laser light (742 nm) and ii) visible light formula

C18H22O4

formula weight

302.35

temperature (K)

90

crystal system

monoclinic

space group

P21/c

a (Å)

8.171(3)

b (Å)

18.623(6)

c (Å)

11.074(4)

β (deg)

108.333(6)

V (Å3)

1599.5(9)

Z

4

reflections collected

25270

unique reflections

4755

Rint

0.0364

data/restraints/parameters

4755/0/205

goodness-of-fit on F2

1.017


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R1 [F2 > 2σ(F2)]

0.0398

wR2 (all data)

0.1097

∆ρmin (e Å-3)

0.40

∆ρmax (e Å-3)

–0.17

CCDC number

1471626

RESULTS AND DISCUSSION Photochromic reactions of 3 and crystal structure changes. Compound 3 exhibited photochromism in the solid state, i.e., yellow powder samples changed to red upon irradiation with UV light (λex = 365 nm). The changes in the diffuse reflectance spectra (Figure 1) indicate that this photochromic reaction proceeds in high yield, similarly to that in toluene (Figure S5), i.e., the absorption band at ~345 nm, which is associated with the E-form, disappeared after irradiation. It should be noted here that the regions of the powder sample which can contribute to the diffuse reflectance spectra are limited to those available to the incident light for the spectral measurements on the surface, and accordingly, the observed spectra do not represent the progress of the reaction in the whole powder sample. The spectral changes indicate that the photocoloration reaction in the crystals of 3 proceeds in high yield only where light can penetrate the crystals. Accordingly, the reaction yield for the whole powder sample was not necessarily high.


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Figure 1. Kubelka-Munk spectra for 3 in NaCl (0.05 wt%) before and after irradiation (λex = 365 nm). Single-crystal X-ray diffraction studies were carried out on 3 in order to monitor changes in its molecular and crystal structures associated with the photocoloration. The photoreaction was induced by two-photon excitation and yielded homogeneously distributed photoproducts in the bulk of the crystals.20,29 A single crystal of 3 (0.4 × 0.4 × 0.3 mm3) was irradiated with the output of a nano-second-pulsed laser (wavelength: ~742 nm; 30 mJ/cm2 per pulse) for 180 min. The reaction was carried out at low temperature (T = 200 K) in order to minimize the crystal damage that was frequently observed upon irradiation at room temperature. Although light at 742 nm is neither absorbed by the E- nor the C-form of 3 through one-photon excitation, it resulted in photocoloration and afforded a red-black crystal, i.e., the two-photon absorption of 742 nm light provides a molecule with the same energy as that of one-photon absorption of 371 nm light. The diffraction data for the red-black crystal was collected at T = 90 K in order to reduce the thermal motion of the atoms and the blur of their electron densities in the resulting analyzed structures. Prior to laser irradiation of the crystal, the molecular structure of 3 corresponded to the E-form (Figure 2a). However, after irradiation with pulsed laser light, several new peaks appeared in the


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difference Fourier electron density maps, which were assigned to the atoms of the photo-induced C-form. The refinement, using a disorder model for the overlap of the E- and C-forms, showed a site occupancy factor of 9.1(2) % for the C-form (Figure 2b). Additionally, the structural relationship between the E- and C-forms at the crystal sites proved that the photocyclization proceeded via a conrotatory mode of the 1,3,5-hextatriene moiety of fulgide 3, which is consistent with the Woodward-Hoffmann rules.30

Figure 2. Molecular structures of 3 obtained at T = 90 K (a) before and (b) after irradiation with laser light (atomic displacement parameters set at 50% probability; hydrogen atoms omitted for


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clarity). In (b), a superposition of the E- and the photo-induced C-form is shown, whereby the latter consists of the white atoms and red bonds. The aforementioned red-black crystal resumed its original yellow color upon irradiation with steady visible light (λ > 500 nm, 42 mW/cm2) for 180 min at room temperature. Subsequently, X-ray diffraction data were collected for the yellow crystal at T = 90 K. The crystal structure showed only the E-form (Figure S6), while residual peaks for the C-form were not detected in the difference Fourier maps, which suggests a full retroconversion of the latter into the former. These results provide further evidence that photochromic reactions of furylfulgides can proceed reversibly and maintain their crystal lattice, at least to some extent, and that the structural changes can be monitored by single-crystal X-ray diffraction studies if the reaction is induced by two-photon excitation. In spite of the accessibility of lattice-controlled photochromic reactions of 3 for low product yields, further progress of the photochromic transformation always resulted in the destruction of the crystal lattice. The diffuse reflectance spectra (Figure 1) clearly show that the generation of the C-form in high yields should be expected if the incident light penetrates the crystals. Even though irradiation with pulsed laser light at 742 nm guarantees two-photon absorption and high penetration of the light, prolonged irradiation resulted - at least in our hands - in the unavoidable deterioration of the reacting crystals. For the corresponding reactions involving 1 and 2, we were also unable to obtain good-quality single crystals with a higher amount of the photo-induced Cform, similar to the case of 3. In these cases, the yields of the C-forms were determined by the refinement of the site occupancy factors of the C-form in the reacted crystal structures, which achieved a maximum of 5.5(3)% (1) and 27.8(2)% (2).20 The problems to obtain single crystals for high photoconversions can be rationalized in terms of a separation of the crystal phases of the


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products and a subsequent destruction of the single crystals, induced by the accumulation of the products in the crystal lattice of the reactants. Phase separation during the photochromic reactions. The progress of the solid-state photochromic reactions of furylfulgides 1–3 was further examined by monitoring the changes in their powder diffraction patterns during the reactions. Powder X-ray diffraction studies on 1 were performed repeatedly while the sample was irradiated with steady UV light (λex = 365 nm) (Figure 3). As the solid-state reaction proceeded, several new peaks emerged in the powder Xray patterns, and their intensity increased with irradiation time. The diffraction angles of the new peaks could not be reproduced by simulating the diffraction pattern of 1. These additional peaks thus clearly show that the photoreactions produce a new solid phase, which significantly differs from that of the reactant crystal. The new phase was assigned to the C-form, given that the diffraction angles of the emerging peaks were identical to those simulated for the reported crystal structure (296 K ) of the C-form of 1 (Figure 3, bottom).17 The yield of the C-form after 40 h of irradiation (34 %) was determined from the UV-vis absorption spectra of a solution obtained from dissolving a small portion of the irradiated powder sample in toluene (for details see Supporting Information).20


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Figure 3. Superimposed experimental (top) and simulated (bottom; light blue; based on the crystal structure of the C-form of 1) X-ray diffraction patterns for 1. Miller indices of some simulated reflections are added to the corresponding peaks of the C-form crystal. The powder sample was irradiated with 365 nm light, and the peaks that emerged after irradiation are marked by red arrows. The photoreactions of 2 and 3 in the solid state also led to the formation of new solid phases. The additional peaks that emerged after irradiation with UV light are shown in Figure 4. Based on an analogy with the reaction of 1, the newly developed phases should be assigned to crystalline phases of the C-forms, which are likely to exhibit molecular arrangements and crystal lattices different from those of the reactant crystals of the E-forms.31 The yields of the C-forms


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after 40 h irradiation were 50% (2) and 47% (3). The slight increase in background signals for the powder samples of 3, which were observed in the diffraction patterns after irradiation (Figure 4b), might be attributed to the formation of amorphous materials.

Figure 4. Superimposed powder X-ray diffractograms for (a) 2 and (b) 3, showing the changes of the diffraction patterns caused by irradiation with 365 nm light. Peaks that emerged after irradiation are marked with red arrows. The photoreaction from the E- to the C-forms involves only minor changes of the molecular structures of the starting materials (Figure 2), which allows i) topochemical photo-cyclizations,32 and ii) lattice control at the crystal sites, at least in the early stages of the photoreaction. Studies


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on 1-3, based on the combination of single-crystal and powder X-ray diffraction techniques, have thus allowed us to describe the progress of photochromism in crystalline samples of furylfulgides as a two-step process, which consists of an initial formation of mixed crystals with reactant-like crystal lattices, followed by a phase separation of the products (Figure 5). In the early stages of the photoreaction, where the yield of the C-form is still low, small amounts of the photoproduct combined with the remaining reactant molecules form a solid solution (Figure 5b). The solubility of the product in the crystal of the reactant should vary depending on a variety of factors that include: i) the similarity between the structures of the E- and C-forms for each compound, and ii) the flexibility of the molecular packing of the reactants. Initially, the mixed crystals should exhibit crystal lattices that are similar to those of the starting materials and, accordingly, provide powder-diffraction peaks at almost identical angles. With increasing irradiation time, and probably due to the limited solubility of the C-form in the crystal lattice of the E-form, the reaction reaches a point, where the mixed crystal lattice cannot accommodate the molecules of the products anymore, i.e., the gradual increase of the photoproducts leads to a saturation of the mixed crystals, and eventually to a precipitation of the C-form. Subsequently, this precipitate should split from the reactant lattice to form a new solid phase (Figure 5c).33,34 The product molecules usually crystallize with packing arrangements and lattice parameters that are significantly different from those of the reactant molecules. The product crystals therefore provide diffraction peaks at angles different from those of the reactant crystals, which results in the emergence of new diffraction peaks.


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Figure 5. Progress of the photochromic reactions of furylfulgide crystals focusing on the crystal lattice and its disruption: (a) Crystals of the E-form; (b) mixed crystals with a reactant-like lattice in the early stages of the photoreaction, containing the reactant (E-form) and small amounts of the product (C-form); (c) phase separation of the crystal phases of the C-form, which exhibit a different crystal lattice, from the mixed crystal. The progress of the reactions, in which new solid phases are generated under concomitant deterioration of the original reactant crystals, does not correlate with the yields of the products in the bulk of the crystalline samples, as irradiation of crystals that strongly absorb UV light results in a reaction confined to the surface of the crystals. As a direct consequence, the concentration of C-forms near the surface of the crystal is higher than that in the inside, and this heterogeneous distribution of the C-form facilitates phase separation, even at stages where the overall conversion of the product is still low. This is often the case when conventional one-photon excitation is used for solid-state photoreactions, and explains why in-situ X-ray diffraction studies on single crystals are not always successful.20,35 In contrast, two-photon excitation takes advantage of the very weak absorption and high penetration of light, which enables a homogeneous distribution of the photoproducts in the crystal. Thus, photoproducts may accumulate sufficiently in the crystals without disrupting the reactant lattice, to be detected by single-crystal X-ray diffraction analysis. As shown for furylfulgides 1–3, the observation of the photoreactions by diffraction techniques was only limited by the amount of the C-form that could be accommodated by the crystal lattice of the E-form, and prolonged irradiation times beyond that limit resulted in the destruction of the crystals. CONCLUSIONS


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In the present study, we have investigated solid-state photochromic reactions of crystalline furylfulgides. The changes of the crystal structures associated with the photocoloration of 3 were successfully observed by X-ray diffraction analyses, which proved that the photocyclization proceeded in a conrotatory fashion. Although the furylfulgides examined in this study maintained their crystal lattices in the early stages of the photochromic reactions, an increase in the amount of photoproducts induced phase separations of the product crystals, and ultimately led to the inevitable destruction of the reactant crystal lattices. Therefore, phase separations during photochromic transformations should be taken into account when designing photochromic crystals as functional materials in e.g. optical memory elements, as they can exert a significant influence on the properties of the materials, such as the reactivity and the quantum yields of the photocoloration/discoloration, and the fatigue resistance of the crystals. The full picture of solidstate reactions can only be obtained by careful examination throughout the course of the reaction. We have observed that the detection of the products by X-ray diffraction studies on single crystals does not guarantee that the reactions achieve completion in a single-crystal-to-singlecrystal fashion. However, even if a solid-state photochromic reaction results in the destruction of good quality crystals, it still can be monitored by single-crystal X-ray diffraction studies on single crystals by avoiding high conversions. ASSOCIATED CONTENT Supporting Information. The SHELXL-2014 res file of a laser-irradiated crystal of 3; experimental and simulated X-ray diffraction patterns for 1–3; UV-vis spectra for 3 in toluene; ORTEP drawing of the crystal structure of 3 after irradiation with visible light. This material is available free of charge via the internet at http://pubs.acs.org.


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Accession Codes CCDC 1471623–1471626 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 e-mailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was partially supported by JSPS KAKENHI Grant Number 16H04126 and a Grant-inAid for Scientific Research on Innovative Areas “π-System Figuration: Control of Electron and Structural Dynamism for Innovative Functions” (Grant Number 15H00980) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. REFERENCES (1) Photochromism. Techniques of Chemistry; Brown, G. H., Ed.; Wiley-Interscience: New York, 1971; Vol. 3. (2) Photochromism. Molecules and Systems; Dürr, H.; Bouas-Laurent, H., Eds.; Elsevier: Amsterdam, 2003.


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For Table of Contents Use Only

Photochromism of fulgide crystals: from lattice-controlled product accumulation to phase separation Jun Harada,*,† Masaya Taira,‡ and Keiichiro Ogawa‡ †

Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo 060-0810, Japan. ‡ Department of Basic Science, Graduate School of Arts and Sciences, The University of Tokyo, Tokyo 153-8902, Japan.

SYNOPSIS. To investigate the mechanism of solid-state photochromism, single-crystal and powder X-ray diffraction studies were conducted on crystals of furylfulgides. In the early stages, photocoloration reactions produced mixed crystals that contain product and reactant molecules, and exhibit reactant-like crystal lattices. Prolonged irradiation induced a phase separation of the photoproduct phases, which eventually resulted in a physical deterioration of the crystals.


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Photochromism of Fulgide Crystals: From Lattice-Controlled Product

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