Thermosensitive Phase Behavior of Benzo-21-crown-7 and Its

Institute of Microbiology, Technische Universität Dresden, Zellescher Weg 20b, 01217 Dresden, Germany. Langmuir , 2017, 33 (48), pp 13861–13866. DO...
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Thermo-sensitive Phase Behavior of Benzo-21-Crown-7 and Its Derivatives Shengyi Dong, Li Wang, Jianfeng Wu, Lin Jin, Yan Ge, Zhenhui Qi, and Changzhu Wu Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b03431 • Publication Date (Web): 02 Nov 2017 Downloaded from http://pubs.acs.org on November 6, 2017

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Thermo-sensitive Phase Behavior of Benzo-21-Crown-7 and Its Derivatives Shengyi Dong,a Li Wang,a Jianfeng Wu,b Lin Jin,b Yan Ge,b Zhenhui Qib* and Changzhu Wuc* aCollege

of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, Hunan, P. R. China

bSino-German

Joint Research Lab for Space Biomaterials and Translational Technology, School of Life Sciences, Northwestern Polytechnical University, Xi’an, Shaanxi 710072, P. R. China cInstitute

of Microbiology, Technische Universität Dresden, Zellescher Weg 20b, 01217 Dresden, Germany

Supporting Information Placeholder ABSTRACT: For designing water-soluble responsive materials, utilizing crown ethers as main building blocks has been rarely explored in contrast to their linear poly(ethylene glycol) counterparts. In current study, we report the robust thermoresponsive properties of benzo-21-crown-7 (B21C7) family with lower critical solution temperature (LCST) and upper critical solution temperature (UCST) behavior. Different substituent groups on the benzene ring exhibit significant effects on water solubility and thermo-responsiveness. B21C7 and its cyano derivative display LCST phenomena, while B21C7-based carboxylic acid derivative presents UCST followed by LCST phase behavior. Supramolecular interactions with KCl provide an additional tuning approach for this crown ether system. These results demonstrate that B21C7s can serve as an easily accessible toolbox to develop new thermo-sensitive systems and prepare thermally responsive materials. Water is the most important material, not only as the reaction solvent and buffer solution for life activities, but also as an integral source for the ecosystem.1-3 In chemistry, using water is of great significance to mimic nature and realize sustainability.4-6 Supramolecular chemistry, by adopting noncovalent interactions, offers a powerful platform to realize complicated chemical/biological activities in water solution.7-15 For instance, supramolecular self-assembly and the corresponding assemblies in water endow us with an alternative choice to fabricate advanced functional materials that can be widely applied for biomedicine treatment, information technologies, and environmental sceience.16-22 Various structure-forming synthetic or natural compounds are prepared to construct water-soluble supramolecular assemblies.2331 Crown ethers,32-44 the first generation of macrocyclic hosts, are widely used as building blocks to prepare supramolecular assemblies toward with complementary guest species. Over the past two decades, the major focus was the fabrication of crown ether-based threaded or interlocked assemblies in organic solvents.45-47 In most cases, crown ethers with large cavities sizes, including dibenzo-24-crown-8 (DB24C8), bis(m-phenylene)-32crown-10 (BMP32C10) and cryptands, were utilized, due to cavity dimension matching with organic guest molecules. Unfortunately, compared with their linear glycol chain counterparts, these large crown ethers usually have poor water solubility, which limits their availability in aqueous solutions, thus prohibiting them from further applications.29,32-44 Decreasing cavity sizes can improve their water solubility, such as 15-crown-5 and 18-crown-6, but that dramatically restricted further applications as building blocks along with organic molecules (ammonium salts or pyridine salts).25,32,36,39,44 Considering the advantages and disadvantages of

crown ethers with different ring sizes, it is highly interesting to achieve a balance between a large cavity size and good water solubility. Among many different crown ethers, benzo-21-crown-7 (B21C7) derivatives are selected as the candidates for the development of water-soluble supramolecular structure with decent cavity size. This is because48-55: a) A crown ether ring with 21 atoms endows B21C7 with a sufficiently large cavity size to complex various guest molecules, including secondary ammonium salts and alkali metal cations; b) Compared with DB24C8 and dibenzo-21-crown-7 (DB21C7), B21C7 only has one benzene ring and a long glycol chain, thus the main portion of B21C7’s molecular area is occupied by hydrophilic ethylene glycol chain (seven equally distributed hydrophilic oxygen atoms thus provide abounding hydrogen bonding receptors to attract water molecules nearby). Moreover, since linear glycol chains have been widely used to display LCST (lower critical concentration temperature) behavior in water, we also envisioned that B21C7, as the closedloop analogue, might have the ability to realize unique thermoresponsiveness in water.56-58 Herein, we report a new class of water-soluble B21C7-based crown ethers and their unique thermoresponsive properties in water.

Scheme 1. Chemical structures of B21C7s. Table 1. Solubility of B21C7s and their thermal responsiveness in water (the test temperature is 25 ºC).

Macrocycle Solubility(g/L, Responsiveness B21C7 >1500 >4.21 -CN 166.2 4.36×10−1 -COOH 5.8 1.44×10−2 -NH2 197.8 5.13×10−1 DB21C7 insoluble

mol/L)

Thermo-

LCST LCST UCST then LCST noa nob

a

Though it is water soluble, neither LCST nor UCST phase transition behavior is observed. b This host shows poor solubility in water at both room temperature and high temperature.

As the basis of water-soluble supramolecular assemblies, the solubility of B21C7 was firstly investigated in water. After adding B21C7 to water, a transparent and colorless solution is immediately formed (Figure 1, Supporting Video S1). Detailed information

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about the solubilizing process was obtained by proton NMR (Figure S1). In deuterium oxide (D2O), B21C7 displays peaks of all proton signals with a good resolution. The solubility of B21C7 in water at room temperature reaches over 1500 g/L (4.21 M-1, Table 1). Although its water-solubility is lower than that of linear poly(ethylene glycol),59 it is definitely superior to many classic water-soluble supramolecular macrocycles such as cucurbitu[n]rils, α-cyclodextrin, β-cyclodextrin and γ-cyclodextrin in water.23,60-62

Figure 1. B21C7-induced LCST transition behavior in water solution: a) Temperature-dependence of light transmittance of B21C7 water solution; b) concentration-dependence of cloud point of B21C7 water solution; c) a cartoon representation of the process of LCST phase behavior.

In order to expand the B21C7-based water-soluble crown ether family, B21C7 was respectively functionalized by cyano group (B21C7-CN), amino group (B21C7-NH2) and carboxylic group (B21C7-COOH) on benzene rings (Scheme 1). These B21C7 derivatives are all water-soluble as confirmed by proton NMR spectra and macroscopic tests (Figure S1-S5). For example, B21C7-CN and B21C7-NH2 show moderate solubility in water (166.2 and 197.8 g/L, respectively). Compared with B21C7, the introduction of theses substituents on benzene rings reduces the water solubility (Table 1). For B21C7-COOH, the saturated solubility is relatively low, 5.8 g/L in water at room temperature (Table 1). Interestingly, when more B21C7-COOH is added to its saturated water solution, we get gel-like supramolecular assemblies instead of solid-type precipitate. Since a similar phenomenon is not observed for other B21C7s, this gelation process is mainly ascribed to the presence of carboxylic groups that have stronger intermolecular interactions with other carboxylic groups than with water molecules. Eventually, the lack of hydration forces B21C7COOH molecules assembling together to form a supramolecular gel (Figure S5 and S6).23, 60 In order to gain insight to the structural reason why B21C7 derivatives with polar substituents are less soluble in water than B21C7, we performed a series of concentration-dependent nuclear overhauser effect spectroscopy (NOESY) and diffusion-ordered spectroscopy (DOSY) NMR experiments (Figure S11-S18, Table S1) and theoretical calculations (Figure S20). NOESY experiments demonstrate that no obvious intermolecular interactions exist in the solutions, and combining with the DOSY results (DOSY shows

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very close diffusion constants at both high and low concentrations) and concentration dependent NMR experiments (neither new peaks nor obvious chemical shift changes are found at either high or low concentrations), it is reasonable that all the B21C7s dissolve as monomers in solution, and no aggregation processes exist. The polarity obtained from molecular electrostatic potential (MEP) maps is B21C7 > B21C7-NH2 > B21C7-COOH > B21C7-CN, which indicates that the substitutes of -CN, -COOH and -CH2NH2 change the electron distributions, and lead to the differences of solubility. The solubility of experimental result is B21C7 > B21C7NH2 > B21C7-CN > B21C7-COOH. Both, experiment and theory thus agree that B21C7 is higher in solubility than its derivatives (see Theoretical calculations in supporting information). Encouraged by the water-solubility properties, the B21C7 family is further explored for potential applications in aqueous medium. Considering that glycol chains have special thermo-sensitive properties in aqueous solution (LCST behavior), we speculated that these crown ethers, as the analogues of closed-loop glycol chains, might be able to display unique thermo-sensitive behavior in water.57, 58 Indeed, B21C7 (120 mg/mL) shows an obvious phase transition behavior when heated up to 60 ºC. Typically, as temperature increases, the homogenous and transparent water solution turns to turbid, suggesting it is macroscopical LCST phase behavior (Figure 1 and S3). After cooling down for a while, the turbid mixture returns to a transparent solution again. By applying UV/Vis measurements, such LCST phase behavior is confirmed with Tcloud at 55.1 ºC (120 g/L, Figure 1a). Further experiments show the phase transition to be a concentration-dependent LCST phase behavior (Figure 1a, b): with the decrease of concentrations of B21C7, Tcloud gradually increases and the transition windows become broader. For example, at a concentration of 40 g/L, Tcloud is 64.2 ºC (9.1 ºC higher than Tcloud of B21C7 at 120 g/L (55.1 ºC).8,59,60 Impressively, increasing B21C7 concentration to 80 - 120 g/L results in very shape transitions (less than 1 ºC) during the heating process, and meanwhile give rise to narrow hysteresis in the followed cooling process (less than 1 ºC). These LCST transition processes are all highly reversible, without any sign of fatigue after many repeating cycles of heating and cooling, indicating the structure integrity of B21C7 in water (Figure 2g). In order to get more detailed insight into the detailed phase transition behavior, we performed temperature-dependent proton NMR of B21C7. As shown in Figure S8 and S9, all proton signals move downfield upon heating: For example, signals of protons in the benzene ring shift from 6.94 ppm at 30 ºC, to 7.15 ppm at 50 ºC and 7.44 ppm at 80 ºC (the concentration of B21C7 is 67 g/L), respectively. At high temperature, we also observed the existence of new peaks, which might be assigned to some aggregated species.8,59,60 Interestingly, whenever B21C7 solution is diluted or concentrated, the proton signal intensity does not decay along with increased temperature. Its normalized intensity remains constant in process, indicating that the aggregated B21C7 molecules at high temperature are still detected by the NMR detector (Figure S10). Next, we turned to investigate the influence of substitute effect on thermo-sensitive properties. In case of the cyano-substituted derivative, we observe a typical LCST phase transition behavior (B21C7-CN, Figure 2a-f and Figure S4). Compared with B21C7, B21C7-CN shows lower LCST temperatures and shaper transition windows at the same molar concentrations (Figure 2e and f). For example, when the molar concentration is at 140 mM (about 50 g/L

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for B21C7, 54 mg/mL for B21C7-CN), Tcloud are 59.9 and 51.2 ºC for B21C7 and B21C7-CN, respectively.

LCST phase transition behavior in water solution. a-d) LCST phase separation of B21C7-CN (128 g/L): Sample b is obtained by heating and resting sample a, then fluorescein sodium is added to sample b in order to get a clear view of the phase separation, and sample d is irradiated by UV at 365 nm; e) comparison of cloud points between B21C7 and B21C7-CN; f) temperature-dependence of light transmittance of B21C7-CN; g) the reversible test of B21C7-based LCST phase behavior (the concentration of B21C7 is 120 g/L). Figure 2.

Interestingly, further substituting B21C7 with an amino methyl group shows totally different thermo-responsive behavior (Figure S5-S7). When heating concentrated B21C7-NH2 solutions even to the boiling point, we still get a homogenous and transparent solution without any LCST phase behavior, which is further characterized by UV/Vis measurements. As shown in Figure S7, the transmittance of a B21C7-NH2 solution keeps almost unchangeable upon heating, indicating no thermal self-assembly occurrence in the water solution. Such unusual thermoresponsiveness may be attributed to the following two aspects: a) Amines on crown ethers may provide additional hydrogen bonding interactions between B21C7-NH2 and adjacent water molecules, which leads to an enhanced hydration effect; b) the introduction of amine group probably changes the molecular geometries, which makes B21C7-NH2 more difficult to accumulate together to form macroscopic assemblies.8, 58 As mentioned above, B21C7-COOH formed a turbid gel-like supramolecular assembly in water at room temperature (Figure 3, S5). Therefore, it is highly interesting for us to investigate if it displays a different phase transition behavior in water. To our surprise, macroscopic tests and UV/Vis measurements show that B21C7-COOH has not only typical LCST but also UCST phase transitions. When we heat a gel-like sample (150 mg B21C7COOH in 1 mL water) to an appropriate temperature (around 58 ºC), the former turbid mixture turns to be a transparent and homogenous solution within a very short time. The light transmittance measurements provided us the detailed information about this UCST phase behavior. The transmittance of B21C7COOH is low at room temperature, due to the opaque gel. After heating, the gel disappears quickly and eventually the sample has a high transmittance, showing a unique UCST phase behavior, with

a transition temperature at about 58.6 ºC. Interestingly, further heating this solution to a higher temperature can trigger LCST phase transition behavior. For example, at temperature higher than 62.4 ºC, the former high value of light transmittance drops dramatically to a low value (closed to 10%, Figure 3), indicating a typical homogeneous-heterogeneous transition. These macroscopic changes from a turbid supramolecular gel to transparent solution phase, then to a white and turbid mixture are also observed by naked eyes (Figure 3). These observations clearly demonstrate that supramolecular interactions of B21C7-COOH molecules are sensitive to temperature, particularly at high temperature that can easily disassemble. The released “free” B21C7-COOH is subsequently solvated by water molecules, thus forming a homogenous solution (UCST process). With the temperature further increased, the hydration process is strongly retarded, and B21C7-COOH thereby starts aggregating and shows LCST phase transition behavior in water. It is noteworthy that such LCST phase transition is also concentration-dependent. For instance, at B21C7COOH concentration of 50 g/L (50 mg B21C7-COOH in 1 mL water), we only observe the UCST phase transition behavior but not LCST phase behavior (Figure S5).8,58

B21C7-COOH displays unique UCST behavior followed by LCST behavior, Temperature-dependence of light transmittance of B21C7-COOH. Small photos at the top-right: a-c) The reversible transitions from a gel state to a homogenous solution, then to a turbid mixture. The concentration of B21C7-COOH is 150 mg in 1 mL water.

Figure 3.

Based on the above observations, it is clear that the substituent groups on B21C7 exert great influence on its thermal responsiveness, and even cause completely different phase transition behavior. For examples, cyano groups on B21C7 lead to lower Tcloud and sharper phase transition, while carboxylic acid groups allows both UCST and LCST in water. Considering that B21C7 shows host-guest interactions with many guest molecules,48-55 such as metal cations and ammonium salts, we further investigated the cation-responsiveness of B21C7based LCST phase transitions by potassium chloride (Figure S19, Table S2 and S3). After the addition of potassium chloride to B21C7 water solution, higher Tcloud is observed. For example, when the concentrations of potassium chloride are 10 and 30 g/L, the corresponding Tcloud of B21C7 (120 g/L) are 60.9 ºC and 78.1 ºC, respective, which are much higher than Tcloud of B21C7 without any

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salt (55.1 ºC). Higher concentration of potassium chloride (> 60 g/L) even leads to the complete quench of LCST phenomena of B21C7.15 B21C7-CN and B21C7-COOH show similar phenomena after the addition of KCl (the more salt, the higher Tcloud). After the addition of KCl to B21C7-NH2, no obvious changes of thermo-responsiveness occur. Additionally, we also performed the pH induced thermo-responsiveness of B21C7s. The relevant result was listed in Table S3, suggesting that B21C7COOH and B21C7-NH2 lost the thermo-sensitive phase behavior at both lower and higher pH (pH = 3 and 11). In conclusion, we developed a new class of water-soluble crown ethers, B21C7 and its derivatives that show good water solubility in comparison with other macrocyclic hosts. Their thermo-sensitive properties of B21C7s have been thoroughly investigated. B21C7 and B21C7-CN display LCST phase transition behavior; while B21C7-COOH shows unique UCST followed with LCST phase behavior and B21C7-NH2 does not have any obvious thermal responsiveness. Supramolecular interactions are introduced to adjust the thermally dynamic behavior. All these studies suggest that the B21C7 family is a class of promising supramolecular species that can be applied as building blocks to construct thermo-responsive materials for controlled release systems, temperature-sensitive gels and sensors.

ASSOCIATED CONTENT Supporting Information Experimental details, NMR spectra, UV/Vis measurements, and other materials. These materials are available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author [email protected] [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the Fundamental Research Funds for the Central Universities from Hunan University. The authors thank Prof. Christoph. A. Schalley for the fruitful discussion. Prof. Zhenhui Qi acknowledges financial support from One Thousand Talents Program, and Northwestern Polytechnical University of China (1800-16GH030121). Dr. Changzhu Wu acknowledges the financial support from DFG.

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