Solubility Measurement and Recrystallization Process Design for 1,1,2

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Solubility Measurement and Recrystallization Process Design for 1,1,2,2,9,9,10,10-Octafluoro[2.2]paracyclophane (AF4) Purification Published as part of a Crystal Growth and Design virtual special issue of selected papers presented at the 13th International Workshop on the Crystal Growth of Organic Materials (CGOM13, Seoul, South Korea) Hyeonjung Kim,†,# Kiho Park,†,# Ji Woong Chang,‡ Taeho Lee,† Sung Hyun Kim,† and Dae Ryook Yang*,† Downloaded via UNIV OF TEXAS AT DALLAS on February 14, 2019 at 09:27:37 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

†

Department of Chemical and Biological Engineering, Korea University, Seoul, Republic of Korea Department of Chemical Engineering, Kumoh National Institute of Technology, Gumi, Republic of Korea

‡

S Supporting Information *

ABSTRACT: In this study, a single-step recrystallization process for purifying 1,1,2,2,9,9,10,10-octafluoro[2.2]paracyclophane (AF4) was investigated. The existing AF4 purification method consists of three steps. Among the steps, methanol washing has a problem of dissolving AF4, so the production yield is around 60%. To improve the yield of AF4 without decreasing its purity, a recrystallization process is suggested, and its feasibility was analyzed by measuring AF4 solubility in each prescreened solvent. Both cooling and antisolvent crystallization were investigated to determine which method would be suitable for the recrystallization process by measuring the purity of the AF4 product. In the results, cooling crystallization with chloroform showed the best performance in terms of 92.31% of the maximum obtainable yield and 99.98% purity in the AF4 purification process. However, antisolvent crystallization showed a high maximum obtainable yield, but very low purity. Therefore, compared to the existing method (60% yield and 99% purity), the suggested cooling recrystallization with chloroform as the solvent is more appropriate for the AF4 purification process.

1. INTRODUCTION

Many reaction pathways have been suggested for synthesis of AF4. The most classical method for synthesizing AF4 is an unimolecular cyclization of diradical intermediate after generation of p-xylylene.17 However, a high dilution condition should be maintained to avoid bimolecular oligomerization. Since the high dilution condition can require a very large equipment size in a large-scale system, other ways of production without the high dilution condition should be searched. Recently, an innovative way for synthesis of AF4 was suggested, in which p-bis(chlorodifluoromethyl)benzene (BCFMB) dissolved in N,N-dimethylacetamide (DMA) is reacted together with a support of zinc dust as a catalyst under a N 2 environment.17 This method has many advantages such as a very simple reaction mechanism, higher yield of AF4 (∼60%) compared to the classical methods (30−40%), and no requirement of a high dilution condition. Thus, this method has been regarded as convenient, inexpensive, and easy to scaleup for manufacturing AF4.

Parylene polymers have been widely used as a coating material.1−3 The application has been expanded in the aerospace, military, medical, semiconductor, and display industries,4−8 due to many features and advantages of parylene such as low dielectric constant, high thermal stability, low moisture absorption, and long-term UV stability.3,9−14 There are many variations of parylene depending on their chemical structure: parylene C, parylene D, parylene N, and parylene HT (or parylene AF4).9 Among these polymers, parylene AF4 has especially received growing attention because the fluorinated structure in parylene AF4 improves its physicochemical properties significantly compared to other parylene polymers.3,9,15,16 Parylene AF4 is usually manufactured from the process of chemical vapor deposition (CVD) using 1,1,2,2,9,9,10,10-octafluoro[2.2]paracyclophane (known as AF4) as a precursor.3,14,17 Therefore, AF4 should be prepared to manufacture parylene AF4. However, the fluorinated structure in AF4 makes its manufacturing procedure more complicated. Thus, many problems can be encountered during the complex manufacturing process.3 Thus, it is very important to discover a simpler and more effective way for obtaining AF4. © XXXX American Chemical Society

Received: November 19, 2018 Revised: January 30, 2019

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Figure 1. Schematic diagrams of AF4 purification processes. (a) The existing purification method with methanol washing, benzene extraction, and hexane recrystallization, and (b) the proposed method in this study with the recrystallization process.

In the aforementioned AF4 manufacturing method from BCFMB, DMA, and zinc dust, byproducts are typically removed by the washing steps using water and cold methanol individually. AF4 cannot dissolve in water but has low solubility in methanol. Despite the low solubility of AF4 in methanol, excess methanol washing can reduce the yield of AF4. Therefore, the purification step in the aforementioned AF4 manufacturing method should be improved by replacing the cold methanol washing with another process. The new purification process should show high yield and high purity of AF4 compared to the conventional method. In this study, we improved the purification process via a recrystallization method to obtain both high yield and purity of AF4. The recrystallization process enabled the removal of the byproducts in the AF4 manufacturing process. Since crystallization processes have been regarded as one of the best separation processes with high purity and high yield,18−20 the crystallization process is very plausible to replace the existing methanol washing method. To establish the recrystallization process for purification of AF4, the solubilities of AF4 in many solvent candidates should be measured. This study investigated the applicability of cooling and antisolvent crystallization on the purification of AF4. Thus, the solubility data depending on temperature and antisolvent mixing ratio were collected by changing the prescreened solvent candidates. Then, the recrystallized AF4 product samples were analyzed to identify the yield and purity depending on the solvents and operating condition. Finally, the most appropriate operating condition and kind of solvent for recrystallization were identified, and the

purity and yield data from the recrystallization process were compared with the existing methanol washing method.

2. METHODS 2.1. Description of the Existing AF4 Manufacturing and Purification Method. The existing AF4 manufacturing process is started with DMA and BCFMB as reactants, and zinc dust as a catalyst. The BCFMB is dissolved in DMA with zinc dust. Under N2 conditions, the reactants are well mixed and heated until 100 °C. Then, the temperature of BCFMB solution is maintained and continuously stirred at 100 °C for 4 h to induce the AF4 reaction from BCFMB. After finishing the reaction, the solution is cooled down and filtered to remove zinc dust and byproducts at the solid phase. To recover a small amount of AF4 attached on zinc dust, the solid particles on the filter paper are washed by DMA. Then, a small amount of KMnO4 are added to the filtered solution, and continuously stirred for 1 day at the room temperature to remove impurities and byproducts. Then, the solution is filtered again. The filtered solution is evaporated at 100 °C under a vacuum condition to remove DMA. The remaining solid products are washed by deionized water and methanol. The washed products are extracted with benzene to remove byproducts and impurities further, and the benzene is removed by evaporation. Finally, the extracted residue is recrystallized from hexane to obtain AF4 with over 99% purity. There are some problems in the method of methanol washing. As will be reported in the following section, AF4 has solubility in methanol even though the extent of solubility is low. Therefore, the yield of AF4 can be reduced if an excess amount of methanol is used for increasing the purity of AF4. In addition, separation processes such as extraction with benzene and recrystallization from hexane are required to obtain the high purity of AF4. If these separation processes could be avoided, or the steps of the separation processes could be reduced, a cost B

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2.3. Experimental Procedure. In this study, to evaluate the feasibility of the suggested recrystallization process, two experiments were designed. The first one is for the AF4 solubility measurement in each prescreened solvent shown in Table 1. The second one is to measure yield and purity of AF4 in the product from the recrystallization process with each solvent. The AF4 solubility in each screened solvent was measured by changing temperature from −30 to 40 °C, and by changing the solvent/ antisolvent ratio from 1:1 to 1:10. The detailed experimental procedure is shown in Figure 2a. In the experiment of temperature-dependent solubility, there are five data points for each solvent (40 °C, 20 °C, 0 °C, −15 °C, −30 °C) except DMA (40 °C, 20 °C, 0 °C, −15 °C), because the melting point of DMA is −20 °C. In the experiment of antisolventdependent solubility, three data points (1:1, 1:5, 1:10) were measured. The temperature in the antisolvent-dependent solubility measurement was fixed at 25 °C. After an excess amount of pure AF4 (Yuan-Shin Materials Technology Corp., Taiwan, 99.45%) was dissolved in each solvent, the remaining solid was filtered to obtain a saturated AF4 solution. The saturated AF4 solution was placed onto a dry oven to evaporate all the solvent. Then, the AF4 solubility for each solvent was calculated from the amount of AF4 solid after the evaporation. Since our objective is to confirm high efficiency of the suggested recrystallization process as a purification process for AF4 manufacturing, the crude AF4 mixture before the purification process should be obtained, and the detailed experimental procedure is schematically shown in Figure 2b. A total of 150 mL of DMA and 15.25 g of zinc dust (HANCHANG Ind. Co. Ltd., Republic of Korea, 3−5 μm) were fed into the reactor. Under a N2 environment, these feeds were heated up to 100 °C and stirred at 700 rpm. After 40 min, 15 g of BCFMB (79.16% purity) was fed into the reactor, and the temperature and stirring conditions were maintained for 8 h for inducing the AF4 reaction. The BCFMB was manufactured by Korea Institute of Science and Technology (KIST), using the method in the reference (hexachloroparaxylene is reacted with anhydrous HF).24 After the completion of the AF4 producing reaction, the remaining reactants and products in the reactor were cooled down to room temperature. These materials were filtered by filter paper (retention range 7−9 μm). To recover the small amount of remaining AF4 on the solid particles, 25 mL of DMA was used additionally. To remove impurity 1 which is shown in Figure 1, 1.25 g of KMnO4 (DAEJUNG Chemical, Republic of Korea, 99.3%) and the filtered products were mixed and reacted at room temperature for 24 h. After finishing the reaction, the mixed AF4 and impurities were filtered using filter paper (retention range 7−9 μm) to remove KMnO4 and impurities. The filtered solution (AF4 + DMA + impurities) was evaporated to remove DMA at 100 °C under a vacuum. After the evaporation was finished, the obtained AF4 product had 79.08% purity. From the obtained AF4 product, the suggested recrystallization experiment was carried out as shown in Figure 2c. As changing solvents which are prescreened in Table 2, cooling and antisolvent crystallizations were performed to determine which method and which solvent would be the most effective to remove impurities in the obtained AF4 product. Since the maximum obtainable yield in the suggested recrystallization process can be calculated from the data of AF4 solubility, only the top four solvents with high yields among the prescreened solvents were selected and examined in the purity measurement. In the cooling crystallization method, the crude AF4 product was dissolved in each solvent at 40 °C. The undissolved solids were filtered to obtain a saturated AF4 solution. Then, the temperature of the filtered solution (40 °C saturated solution) was cooled down to −30 °C, and maintained for 3 h. In the antisolvent crystallization, the filtered solution was fed into the antisolvent. After enough time for crystallization (around 10 min), the recrystallized AF4 was filtered using filter paper, and the purity of the recrystallized AF4 was measured by GC mass (Agilent 7890A Series GC Custom, Agilent Technologies, U.S.A.).

reduction would be expected in a large-scale system for AF4 production. In this study, a single-step recrystallization process is suggested to avoid the above-mentioned problems. Figure 1 shows schematic diagrams for comparison of the existing purification method and the proposed method in this study. After AF4 manufacturing from BCFMB and DMA, two main impurities (tricyclo[8.2.2.2]hexadeca2,4,5,6,10,12,13-heptaene, 2,3,8,8,9,9-hexafluoro- and tricyclo[8.2.2.2]hexadeca-4,6,10,12,13,15-hexaene, 2,2,3,3,8,8,9,-heptafluoro- are denoted as impurity 1 and impurity 2, respectively) usually occurred simultaneously. Impurity 1 can be removed by KMnO4. To remove impurity 2, methanol washing, benzene extraction, and hexane recrystallization are utilized in the existing AF4 purification process. However, by selecting an appropriate solvent for recrystallization, the purification step of AF4 can be simplified without loss of purity and yield as shown in Figure 1. For this purpose, the AF4 solubility in each selected solvent was measured, and experimental validation of the suggested recrystallization process was carried out especially in terms of product purity and yield. 2.2. Solvent Screening for Suggested Recrystallization Process. The crystallization process can largely be classified into three groups depending on the generation method of supersaturation; cooling, antisolvent, and evaporation.21−23 Among the crystallization processes for our purposes, the evaporation method is not suitable as a purification process because it cannot remove impurities that are contained in solute. Therefore, in this study, cooling and antisolvent crystallization methods were selected and the applicability of the recrystallization process for purification of AF4 was investigated. To screen the appropriate solvent candidates, two selection criteria were applied. First, the solvents that are frequently used in the AF4 manufacturing process from the literature were selected, because these solvents have been already confirmed to be non-reactive toAF4. In addition, AF4 is soluble in these solvents. The second criterion is that the melting and boiling points of these solvents should be in an appropriate range for the recrystallization process. The screened solvents with these criteria are chloroform, methanol, DMA, n-hexane, ethanol, 1,2-dichloroethane (EDC), acetone, and toluene. In this study, a maximum 40 °C to a minimum −30 °C was considered as the temperature range for the recrystallization process to avoid unnecessary reactions. In the case of benzene, even though it is commonly utilized as a solvent in the AF4 manufacturing process, it cannot be used in the recrystallization process due to its high melting point (around 5 °C). The information on the selected eight solvents is shown in Table 1.

Table 1. Information of the Selected Solvents for the Suggested Recrystallization Processa name

assay (%)

melting point (°C)

boiling point (°C)

chloroform methanol DMA n-hexane ethanol toluene EDC acetone

99.5 99.5 99.5 96 99.9 99.5 99.5 99.5

−63 −98 −20 −95 −114 −93 −35 −94

60−61 64 164−166 69 78.5 110 81−85 56

a

All solvents are manufactured by DAEJUNG chemical in Republic of Korea. For development of the antisolvent crystallization method, an appropriate antisolvent should also be selected. In this study, water was selected as an antisolvent for the recrystallization process, because AF4 cannot be dissolved in water. The possible antisolvent should have characteristics to mix with solvent, and not to dissolve AF4. Therefore, three solvents (methanol, ethanol, DMA) were selected because these solvents can mix with the selected antisolvent (water). Then, these solvents and antisolvent were examined for the antisolvent recrystallization process.

3. RESULTS AND DISCUSSION 3.1. Solubility Measurement. Figure 3 shows the results of temperature-dependent solubility in each prescreened solvent. C

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Figure 2. Schematic diagrams for experimental procedure for solubility and purity measurement. (a) AF4 solubility measurement in each screened solvent, (b) AF4 manufacturing process from BCFMB, and (c) AF4 purity measurement in the product from the suggested recrystallization process.

Table 2. AF4 Solubility in Each Prescreened Solvent and Maximum Obtainable Yield from the Suggested Recrystallization Process (Cooling)

AF4 solubility at 40 °C (g/100 g solvent) AF4 solubility at −30 °C (g/100 g solvent) maximum obtainable yield by cooling from 40 °C to −30 °C (%)

chloroform

methanol

DMA

nhexane

2.34 0.18 92.31

1.64 0.18 89.02

13.82 5.19 (at −15 °C) 62.45

0.67 0.18 73.13

Solubility curves were obtained by fitting the second order polynomial in each experimental data. Since the cooling range of the suggested recrystallization was designed from 40 °C to −30 °C, the maximum obtainable yield by the suggested recrystallization process can be calculated from the solubility data, and the calculation method can be expressed as

1.59 0.98 38.36

7.02 2.87 59.12

EDC

acetone

1.69 0.49 71.01

16.35 5.17 68.38

From the calculated results of maximum obtainable yield, the top four solvents with high yield were screened, i.e., chloroform, methanol, n-hexane, and EDC. Since the rest of the solvents showed a maximum obtainable yield lower than 70%, these solvents are not efficient for the suggested recrystallization process. Chloroform would be the best solvent for the suggested recrystallization process (92.31% of yield) and followed by methanol (89.02% of yield). Since the existing AF4 purification shows around 60% of yield, the suggested cooling recrystallization with these solvents could be competitive compared to the existing process. In the results of antisolvent-dependent solubility, the solubility data and maximum obtainable yield in each solvent are displayed in Figure 4 and Table 3. In the case of ethanol as a solvent, the maximum obtainable yield showed the highest

maximum obtainable yield =

ethanol toluene

(AF4 solubility at 40°C − AF4 solubility at −30°C) AF4 solubility at 40°C (1)

Table 2 shows the AF4 solubility data at 40 °C and at −30 °C in each solvent, and the maximum obtainable yield from the temperature range of the suggested recrystallization process. D

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Figure 3. Temperature-dependent AF4 solubility in each prescreened solvent. Solid line is a second order polynomial fitting curve from the experimental data. (a) all the solvents and (b) enlarged graph for showing only chloroform, methanol, n-hexane, ethanol, and EDC.

drastically reduced through the suggested recrystallization process. Especially, in the case of chloroform, the impurity 2 was removed almost completely. Since chloroform already showed the highest maximum obtainable yield in AF4 solubility experiment, the results of purity examinations revealed that chloroform is the best suitable and efficient solvent among the prescreened solvents for the suggested recrystallization process. However, in the antisolvent crystallization method, impurity 2 cannot be removed effectively as shown in Figure 5b. It implies that antisolvent addition can reduce the solubility of AF4 in solvent, but also can reduce the solubility of impurities by almost the same degree. The purity of AF4 in the recrystallized product is even degraded compared to the crude AF4. Therefore, the suggested recrystallization with antisolvent (water) addition is not plausible unlike the cooling crystallization for purification of AF4. In the manufacturing process of crude AF4 product, a considerable amount of impurity 1 was remained. However, it has been reported that the impurity 1 can be almost completely removed in the step of KMnO4 reaction.17 Therefore, in this study, a case study was carried out by assuming that the impurity 1 would be removed completely. The results are shown in Figure 6. In the method of cooling crystallization, the purity of AF4 over than 99% can be obtained by utilizing solvent as chloroform and methanol. In the case of chloroform, over 99.9% AF4 purity can be achieved. The results showed that the recrystallization process using chloroform has a very high potential for the purification of AF4. However, in the method of antisolvent crystallization, AF4 purity cannot reach 90% in all cases. Therefore, for AF4 purification, the suggested recrystallization by utilizing water as antisolvent is not suitable. 3.3. Comparison with the Existing AF4 Purification. The feasibility of the suggested recrystallization process can be validated by comparing with the existing AF4 purification process. The comparison results are shown in Figure 7. As mentioned in section 2.1, the most significant problem of the existing AF4 purification method is low production yield due to the low AF4 solubility in methanol. Since the AF4 solubilities at −30 °C in chloroform and methanol are very low, the maximum available yield can be enhanced by applying the suggested recrystallization process. Furthermore, the purity results from the recrystallization process are comparable or higher than the existing AF4 purification method. These results revealed obviously that the suggested cooling recrystallization process

Figure 4. Antisolvent-dependent AF4 solubility in each prescreened solvent. The temperature condition is fixed at 25 °C.

Table 3. AF4 Solubility in Each Prescreened Solvent and Maximum Obtainable Yield from the Suggested Recrystallization Process (Antisolvent) DMA AF4 solubility at 1:0 ratio (g/100 g solvent) AF4 solubility at 1:10 ratio (g/100 g solvent) maximum obtainable yield by antisolvent feeding of 1:10 ratio (%)

10.00 1.51 84.90

ethanol methanol 1.52 0.21 86.18

1.22 0.32 73.77

value, while methanol showed the lowest. However, all the cases showed a high yield over 70%. Therefore, the method of antisolvent crystallization would be competitive for being utilized as the suggested recrystallization process. Even though the yields are improved in both cooling and antisolvent recrystallization, the purity must be examined, and it is followed in the next section. 3.2. Purity Examination of AF4 via Cooling and Antisolvent Crystallization. The purity examination was performed from the crude AF4 product which was manufactured from BCFMB. The purity of the crude AF4 was 79.08%. After the recrystallization process with the cooling method, the purity improvements by using each solvent are displayed in Figure 5a. The main purpose of the recrystallization process is to remove impurity 2. Therefore, the percentage of impurity 2 was E

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Figure 5. Purity examination results in crude AF4 and the product after the suggested recrystallization process (a) cooling crystallization, and (b) antisolvent crystallization.

Figure 6. Results of a case study with an assumption that impurity 1 can be removed completely by KMnO4. (a) Cooling crystallization and (b) antisolvent crystallization.

Figure 7. Purity and yield data of the suggested recrystallization process and comparison with the existing AF4 purification method. (a) Cooling crystallization and (b) antisolvent crystallization.

method, the required steps can be simplified from three steps to one step. Even though the suggested recrystallization process with the cooling method should require cooling energy to decrease the temperature of crystallizer lower than −30 °C, the energy cost is very insignificant compared to the price of high purity AF4. Therefore, the most important factor in the AF4 manufacturing process is high purity and high yield, so it implies that the suggested recrystallization process with the cooling method can be more efficient than the existing AF4 purification method.

with chloroform and methanol is more efficient to purify AF4 than the existing method. In the case of the antisolvent recrystallization process, the maximum yield can be enhanced. However, the purity results revealed that the recrystallized product has low purity compared to the existing purification method. Therefore, despite its high yield, antisolvent recrystallization is not recommended for the AF4 purification process. However, if any appropriate antisolvent could be researched, the applicability of the antisolvent recrystallization process could be discussed in the future. In addition, the suggested recrystallization process can reduce the number of steps for purifying AF4. Compared to the existing F

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4. CONCLUSIONS In this study, a new recrystallization process was suggested for improving the existing AF4 purification method. The existing method can obtain very high purity of AF4 but shows low yield (∼60%) due to methanol washing. In addition, the three-step process of the existing purification method suffers from high capital cost in a large-scale operation. The new recrystallization process not only can reduce the number of steps for AF4 purification, but also can improve the yield of AF4. To confirm the feasibility of the suggested recrystallization process, AF4 solubilities in each prescreened solvent were measured, and the purity and yield of AF4 from the recrystallization process were evaluated. Two crystallization methods (cooling and antisolvent) were investigated to identify which method is more appropriate for the suggested recrystallization process. In the cooling crystallization method, chloroform showed the best performance in terms of both purity (99.98%) and maximum obtainable yield (92.31%). Methanol can also show better results for purity (99.73%) and maximum obtainable yield (89.02%) than the existing purification method (60% of yield and 99% of purity). In the antisolvent crystallization method, the maximum obtainable yield showed a higher value than the existing method, but purity of the recrystallized product was quite low, around 85%. Thus, cooling crystallization is more suitable than the antisolvent crystallization method for AF4 purification. If chloroform would be utilized as solvent, and the operating temperature from 40 °C cooling down to −30 °C would be applied, the AF4 yield can be improved from 60% to 92.31% without loss of purity compared to the existing method.

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great appreciation to Dr. Honggon Kim in KIST for manufacturing the feed material (BCFMB) during this research.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.8b01723. Figure S1. Gas chromatography data of crude AF4. Figure S2. Gas chromatography data of the BCFMB for the feed material in AF4 manufacturing. Figure S3. Gas chromatography data of the recrystallized product after the cooling crystallization. Figure S4. Gas chromatography data of the recrystallized product after the antisolvent crystallization. Table S1. Purity data obtained from gas chromatography in the BCFMB for the feed material in AF4 manufacturing. Table S2. Purity data obtained from gas chromatography in feed and products. Table S3. Zinc dust specification (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Tel.: +82-2-3290-3298; fax: +82-2-929-9613; e-mail: dryang@ korea.ac.kr. ORCID

Kiho Park: 0000-0002-8303-0461 Ji Woong Chang: 0000-0002-5927-0780 Author Contributions #

H.K. and K.P. equally contributed to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financially supported by SHINSUNGFA Co., Ltd., and Korea University. In addition, we would like to express our G

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Crystal Growth & Design

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DOI: 10.1021/acs.cgd.8b01723 Cryst. Growth Des. XXXX, XXX, XXX−XXX