Re-evaluation of tert-butyl method in crystal engineering of

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Re-evaluation of tert-butyl method in crystal engineering of salicylideneanilines by simultaneous observation of photochromism and thermochromism in single crystals Hirohiko Houjou, Taku Kato, Hongyi Huang, Yoshikazu Suzuki, Isao Yoshikawa, and Toshiki Mutai Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b01764 • Publication Date (Web): 03 Jan 2019 Downloaded from http://pubs.acs.org on January 6, 2019

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Crystal Growth & Design

Re-evaluation of tert-butyl method in crystal engineering of salicylideneanilines by simultaneous observation of photochromism and thermochromism in single crystals

Hirohiko Houjou,* Taku Kato, Hongyi Huang, Yoshikazu Suzuki, Isao Yoshikawa, Toshiki Mutai

Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 1538505, Japan.

*Corresponding author, E-mail: [email protected]

ORCID

Hirohiko HOUJOU: 0000-0003-3761-9221

Isao YOSHIKAWA: 0000-0002-9183-7636

ACS Paragon Plus Environment

1

Crystal Growth & Design 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Toshiki MUTAI: 0000-0002-5562-7121

Abstract Single-crystal microscopic absorption spectroscopy was used to clarify the effect of a bulky substituent on the photochromic properties of salicylideneanilines. We discovered that 3,5-di-tert-butylsalicylideneaniline (1), exhibiting debatable chromic properties, had at least three crystalline polymorphs, 1, 1, and  1-1 is thermochromic from −75 to 45 °C and also photochromic, particularly from −75 to 0 °C; 2-1 is photochromic but considerably unstable and transforms into the  form; -1 is thermochromic from −75 to 90 °C but not photochromic. The 4-methyl analog of 1 (2) is simultaneously thermochromic and photochromic, especially around 75 °C, and the 4-chloro analog (3) is thermochromic from −75 to 90 ℃ and is more strongly photochromic at lower temperatures. Comparison of the crystal structures facilitated the identification of some common features among the photochromic species. We demonstrated a quantitative temperature-dependent photochromic response profile for the first time. The observed temperature-dependence of photo-induced spectral changes presents an intrinsic


2

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challenge to the conventional dualistic classification of compounds as photo- or thermochromic.

Introduction The prototropic isomerization of salicylideneanilines (SAs) has been extensively studied to elucidate their photo- and thermochromism.1–4 Specifically, the color changes are attributed to the alteration of the ratio between the enol-imine (OH) and ketoenamine (NH) forms.5–10 While several theoretical approaches have contributed to our understanding of the photoreaction pathway of SAs,

11–15

the intra- and intermolecular

effects that enable an SA analog to exhibit photochromism are still unknown. According to earlier reports, SAs exhibited thermochromism when the phenyl ring was close to coplanar with the salicylidene ring, but otherwise exhibited photochromism.1 It was also widely accepted that a less densely packed structure tends to cause photochromism.2 However, these empirical rules have been broken by a number of exceptions;16–21 therefore, the prediction of the chromic properties of SAs is becoming less certain. The separation of intra- and intermolecular effects is significantly challenging; the


3


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introduction of substituents often causes a considerable change in the crystal packing, thereby inducing changes in the intermolecular interactions. One promising approach to separating the intra- and intermolecular effects is to comparatively examine the polymorphs of a compound, although it is not always possible to observe a sufficient number of polymorphs and pseudo-polymorphs.22–25 As a sophisticated resolution to this problem, co-crystallization21,26–29 and salification20,30,31 techniques have been proposed. With all these efforts, it is very important to study samples with sufficient crystallographic purity; however, single-crystal observation to elucidate chromic properties is not widely reported.32–34 Herein, we report our successful attempts at clarifying the chromic properties of wellidentified crystalline samples via microscopic absorption spectroscopy. We focused on the controversial empirical rules for predicting photochromism: Kawato et al.35–37 reported that all of 1–3 are photochromic, demonstrating that their “tert-butyl method” could effectively create an intermolecular cavity that allows the compounds to undergo

cis-trans isomerization, whereas Johmoto et al.38 reportedly classified 1 as purely


4

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thermochromic. To resolve this disagreement, we conducted quantitative observations of the crystals of the three polymorphs of 1.

X t

Bu O t

N H

Bu 1: X = H 2: X = Me 3: X = Cl

Chart 1. Structure of the molecules studied

Results and Discussion Compounds 1–3 were prepared by a conventional condensation reaction in a methanolic solution. Single crystals of these compounds were obtained under various conditions of recystallization, affording crystals with structures essentally identical to those reported previously (CSD refcodes IVOHIL01 (1),38 IVOHOR (2),39 and NIQTEN (3)40). However, during repeated attempts at recrystallization, we encountered two new polymorphs of 1. One, which is denoted as -1, was occasionally obtained by crystallization from the acetonitrile solution, while the other, -1, was quite rarely


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obtained from the methanol or hexane solutions. We refer to the known crystal (IVOHIL01 and its equivalents) as the -1 form. Figure 1 compares the packing structures of these polymorphs. The molecules in crystals of -1 and -1 had a C–N– C–C torsion angle of 35.6° and 45.0°, respectively, both of which satisfy the empirical criterion for photochromism.1–4 Meanwhile, the corresponding angle for -1 was 26.0°, narrowly failing that criterion. Although it is difficult to quantify the similarity in crystal packing, we can point out that 1-1 and 2-1 have some common features: two layers composed of aniline rings (denoted as the A-layer) and salicyl rings (the S-layer), respectively, are separated in the crystals. With this type of packing, a bulky substituent, such as a tert-butyl group, would effectively provide space around the cis-trans isomerization site. In contrast, the -1 crystal can be described as having a “meshinggear” arrangement of the aniline and salicyl rings, indicating that the tert-butyl groups might interfere with the free rotation at the isomerization site. While the crystal structures of 2 and 3 (Figure S1) were not isostructural to any of the polymorphs of 1, they were rather similar to that of 1-1. Some structure-related values of the crystals of -1, 2-1, -1, 2 and 3 were 1sted in Table 1. The packing factor41 of


6

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the 2 polymorph shows a looser packing than the others, which is thought to be necessary for photochromic activity. However, the values for the others were similar to each other, irrespective of their chromic property. These simply estimated packing factors were verified by isosurface calculations of electron density.42 The void surface and crystal fingerprints43 suggested no striking differences with respect to intermolecular contacts and voids in the packing structures (Figure S2). Moreover, we calculated C···C% values based on the fingerprints, which have recently been proposed by Carletta et al.21 as a new criterion for the photochromism of salicylideneanilines. The values predict that all of the compounds would be photochromic, but that does not agree with our present observations (vide infra). Consequently, the existing criteria seem to provide conflicting predictions, and hence none of them can decisively predict which crystals would be photochromic.


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(a) α1-1

S-layer

A-layer a b

c

(b) α2-1

A-layer

S-layer

a b

c


8

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(c) β-1

A-layer S-layer a b

c

Figure 1 Packing structures of the polymorphs of 1.

Table 1 Summary of the structural and chromic features of the crystals. packing

void

factor

ratio (%)

C…C%

73

notes

torsion

(%) 1-1

C–N–C–C angle (°)

12.1

1.0

35.6

photo/thermochromic, turns

to

-form

at

>60°C in ~10 min 2-1

67

20.5

0.3

26.0

photochromic, turns to -form at rt in ~5 min

-1

72

13.2

0.8

45.0

thermochromic

2

70

15.5

1.1, 2.4a)

37.1, 32.1a)

photo/thermochromic

3

70

16.0

2.7

29.1

photo/thermochromic


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a) Two values are for the two symmetrically independent molecules.

We performed some qualitative observations of the crystals of 1 with its 4-methyl(2) and 4-chloro- (3) analogs. At ambient temperatures, 1-1 crystals appeared yellow and exhibited orange luminescence upon UV irradiation, after which the crystals turned a slightly deeper color. Upon cooling with liquid nitrogen, the crystal color became paler while the UV-induced coloration became prominent, indicating that 1-1 exhibited simultaneous photo- and thermochromism. This was not surprising considering a recent view that these two properties are not always mutually exclusive.44 Moreover, we observed photochromism for 2-1 crystals. As for crystals of -1, the color changed from pale yellow to deep yellow as the temperature increased, whereas the UV irradiation had no influence on their color. These results suggest a possibility that Kawato et al.35– 37

and Johmoto et al.38 observed different polymorphs (or mixtures of polymorphs). In

addition, we observed simultaneous photo- and thermochromism for 2 and 3 although this has not been clearly mentioned in previous reports.4 At 90 °C, for example, 3 had an appearance of yellow plates, which turned slightly darker upon UV irradiation (Figure


10

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S3). The plates became almost colorless upon cooling down to −30 °C and turned orange upon UV irradiation at 30°C or lower. The above observations were also noted in Table 1. The spectral changes of 1-1, -1, 2, and 3 were recorded for single-crystal samples by microscopic UV-Vis absorption spectroscopy (Figure 2),34 and their absorbance in “dark-adapted”

(DA,

only

irradiated

with

the

halogen

lamp

for

absorption

measurements) and “photo-stationary” (PS, irradiated with a Hg lamp in addition to the halogen lamp) states was plotted against temperature (Figure 3). Comparing the spectra in Figure 2, we would suggest that the absorption band around 490 nm is attributed to trans-keto form, a photo-colored species, while the band around 460 nm is attributed to cis-keto form, a thermo-colored species. However, considerable overlap of the two bands makes it difficult to separate the contributions from these tautomers. Hereafter, we confine our focus to phenomenological aspects rather than speculative discussion into the mechanism of photo tautomerization. For 1-1, data could only be obtained up to 45 ℃ due to a solid-to-solid transformation; at 60 °C or higher, the emission color gradually changed from orange to


11


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greenish yellow similar to that of -1 crystals (Figures 4a–c). After 10 min, the emission spectrum became essentially identical to that of -1 crystals (Figure 4g), suggesting that the transformation was complete within this time scale. We did not observe any thermal anomalies related to this transition in DSC measurements, and only obtained the same melting point (111–112 °C) as that of -1. Crystals of -1 exhibited weak photochromism at 30 °C, similar to 1-1 (Figures 4 d and e), but they gradually lost this property and became less transparent during the microscopic observation over ca. 5 min. Unfortunately, the collapse of the crystallinity of 2-1 was so rapid that we could not conduct the spectral analysis for photo-/thermochromism that we performed for the crystals of 1-1. During this change, the 2-1 crystal started to emit greenish yellow luminescence upon UV irradiation (Figure 4f), and this change was accelerated by heating the sample to 60 °C. After the transformation was complete, the emission spectrum was essentially identical to that of -1 (Figure 4g). In contrast, -1 was purely thermochromic, as the UV irradiation had no influence on the spectral shape at any temperature from −75 to 90 °C (Figures 2b and 3b). From all these results, we can suggest that the photochromic 1-1 and 2-1 forms are only kinetically stable, and


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spontaneously convert to the thermodynamically stable -1 form at ambient temperature or higher.

0.7

(a)

0.5

Abs.

0.1

0.7

0.5 0.4

0.4 0.3

0.0 400

500

600

0.3

0.2

0.2

0.1

0.1

0.0 400

0.6 0.5

500

(c)

600

0.1

600

500

600

(d)

0.5 0.4

0.0 400

500

600

0.3

0.2

0.2

0.1

0.1

0.0 400

500

0.6

0.4 0.3

0.0 400 0.7

Abs.

0.7

(b)

0.6

Abs.

0.6

Abs.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


500

600

Wavelength (nm)

0.0 400

Wavelength (nm)

Figure 2 Microscopic absorption spectra of the single crystals in dark-adapted (solid lines) and photo-stationary (dotted lines) states: (a) 1-1, (b) -1, (c) 2, (d) 3. Blue lines are data obtained at −30 °C, and orange lines are data obtained at 90 °C (for b–d) or 45 °C (for a). The insets of (a) and (c) show the difference in the spectra between photostationary and dark-adapted states for each temperature.


13


0.4

(a)

(b)

0.3

Abs@484 nm

Abs@494 nm

0.2

0.1

0.2

0.1 0.0

0.0 -60 -30 0

0.4

30 60 90 120

-60 -30 0 1.0

(c)

30 60 90 120

(d)

0.8

Abs@484 nm

0.3

Abs@470 nm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.2

0.1

0.6 0.4 0.2

0.0 -60 -30 0

30 60 90 120

Temperature (oC)

0.0 -60 -30 0

30 60 90 120

Temperature (oC)

Figure 3 Temperature dependence of the absorbance at the selected wavelength before (filled symbols) and after (open symbols) UV irradiation: (a) 1-1, (b) -1, (c) 2, (d) 3.


14

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(a)

(b)

(c)

(d)

(e)

(f)

1.0 Normalized intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(a) (b) (c) (f)

(g)

0.8 0.6 0.4 0.2 0.0

500

600 700 Wavelength (nm)

800

Figure 4 Optical micrographs at 25 °C. Fluorescence images of an 1-1 crystal (a) as prepared and (b) after heating at 60 °C for 10 min, and (c) a -1 crystal; transmission images of an 2-1 crystal (d) before and (e) after UV irradiation; and (f) fluorescence image of 2-1 after being left at 25 °C for 1 h. (g) Solid-state microscopic fluorescence spectra, where the lines (a)–(c) and (f) correspond to the same samples as already described.


15


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Figure 2c shows the absorption spectra of 2 at −30 and 90 °C in the DA and PS states. In the DA state, the thermochromic response appears to become intense at 30– 105 °C (Figure 3c). Interestingly, the UV-induced coloration became prominent as the temperature increased to a maximum at 75 °C. This is an opposite tendency to the temperature dependence observed for 1-1 (Figure 3a). As for crystals of 3, the thermochromic response was much less notable than the photochromic response. The UV-induced coloration was strongest at −75 °C and rapidly diminished with increasing temperature. Comparing Figures 3a–d highlights the differences in the crystals’ chromic behavior: 1-1 is thermochromic from −75 to 45 °C and is photochromic, particularly below 0 °C; -1 is thermochromic from −75 to 90 °C but not photochromic; 2 is thermochromic and is simultaneously photochromic, especially around 75 °C; 3 is thermochromic from −75 to 90 ℃ and is more prominently photochromic as temperature decreases. The observed temperature dependence of the photochromism depicts a quantitative profile for a so-called “usable temperature range”, which Cohen and Schmidt referred to insightfully but only qualitatively.45 They explained this phenomenon based on two combined effects: (1) the rate of thermal fading increases with increasing


16

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temperature; and (2) the yield of the photo-product considerably decreases at low temperatures. In this regard, the lower limit of the usable temperature range of 3 is presumed to be much lower than −75 °C. Such variation in the temperature dependence of the absorption spectra reveals an intrinsic shortcoming in the conventional dualistic classification of compounds as photochromic or thermochromic; even though 1-1, 2, and 3 were all photochromic, the effects of the tert-butyl groups on their kinetics and photokinetics may have resulted in totally different mechanisms. To verify the “usable temperature range” hypothesis, we attempted a preliminary analysis of the kinetics of photo-coloration and photo/thermal decoloration. The decay of the photo-product(s) did not always follow a single exponential curve, in agreement with previous reports.17,18,21,46 Instead, we determined the half-lives for the DA state (TDA1/2, the time required for the absorbance to reach 50% of the maximum under UV irradiation) and the PS state (TPS1/2, the time required for the absorbance to reach 50% of the minimum under UV shielding), and plotted them against the temperature (Figure 5). At first sight, it can be seen that the trend in the half-lives of 1-1 and 3 is similar, although their values are different by a factor of about 10. For these compounds, the


17


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ratio of TPS1/2 to TDA1/2 becomes larger as temperature decreases, corresponding to the observation that their photochromic response became more prominent. In contrast, the trend in the half-lives of 2 is rather unusual: both TPS1/2 and TDA1/2 increase as temperature decreases from 100 °C to 45 °C, and they then decrease below 30 °C. There is a difference in the temperatures at which TPS1/2 and TDA1/2 are maximal, which is presumably responsible for the highest photochromic response at around 75 °C (Figure 3c). Interestingly, as opposed to the model conceived so far, the decline of the photochromic response at low temperature seems to be caused by accelerated decay of the photo-product(s), rather than its (their) decelerated generation. At this moment, we have no rational explanation for the accelerated decay: perhaps there are several factors determining the bleaching rate, including visible light absorption by the photoproduct. Because of the restriction of our measurement system, the “dark-adapted” state was not truly in the dark but was under irradiation from the measurement light (a halogen lamp), while “photo-stationary” indicates that there was simultaneous irradiation with the UV light and measurement light. Although the intensity of both lights was not quantified, it was kept constant during the measurements of all the samples. Therefore,


18

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the results presented here are semi-quantitatively (and at least qualitatively) correct. Even though the measured half-lives may not be quantitatively accurate, it is more important to observe the general trend of the kinetics of the photo-product(s) being dependent on both temperature and visible-light intensity.

500

Half-life (s)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(a) @494 nm

10

50

(b) @470 nm

400

8

40

300

6

30

200

4

20

100

2

10

0

0

0 -50

0

50

100

(c) @484 nm

-50

0

50

100

Temperature (oC)

-50

0

50

100

Figure 5 Plots of the half-lives of the dark-adapted state (close circles) and photostationary state (open circles): (a) 1-1, (b) 2, and (c) 3.

Conclusions


19


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We used single-crystal microscopic absorption spectroscopy to examine three polymorphs of 3,5-di-tert-butylsalicylideneaniline (1) and revealed that the debatable chromic behavior could be attributed to the observations of mixtures of its polymorphs. Moreover, we compared the polymorphs’ chromic behavior with the 4′-methyl- (2) and 4′-chloro- (3) derivatives of 1. We demonstrated that the tert-butyl groups on the salicyl ring do not always impart photochromic properties to salicylideneanilines. In addition, the temperature dependence of the photo-response was quantitatively demonstrated for the first time by means of the simultaneous observation of photo- and thermochromism in single crystals. Various parameters related to the photochromic properties were different among the compounds, which was presumed to be due to differences in their crystal packing as well as their molecular structures. Such variation in the temperature dependence of their absorption spectra presents an intrinsic challenge to the conventional dualistic classification of compounds as photochromic or thermochromic. Moreover, the origin of the usable temperature range could not be explained by a simple comparison of the half-lives of the photo-product(s), which encourages us to conduct detailed kinetic analyses. Careful comparison of the kinetic parameters and


20

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crystal structures will clarify the rational criteria for the prediction of the chromic behavior of SAs.

Experimental

Measurements The ultraviolet-visible (UV-Vis) absorption and emission spectra of the solid samples were acquired using a conventional optical microscope equipped with an optical fiber connected to a spectrometer (Ocean Optics USB4000) and a temperature-controlled stage (Linkam THMS600) (Figure S4). The incident light was guided from a highpressure Hg lamp via a 330–380 nm bandpass filter. A half mirror was used to pass the transmitted light of 

Re-evaluation of tert-butyl method in crystal engineering of

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