Designing Polymorphic Bi3+-Containing Ionic Liquids for Stimuli

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Designing Polymorphic Bi3+-Containing Ionic Liquids for StimuliResponsive Luminescent Materials Nannan Shen,† Jun Li,‡ Ge Li,§ Bing Hu,*,† Jianrong Li,† Tianqi Liu,§ Liaokuo Gong,†,∥ Fuquan Huang,† Zeping Wang,† and Xiaoying Huang*,†

Downloaded by UNIV AUTONOMA DE COAHUILA at 19:04:00:911 on May 29, 2019 from https://pubs.acs.org/doi/10.1021/acs.inorgchem.9b00813.

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State Key Laboratory of Structure Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, China ‡ Liaocheng University, Liaocheng 252000, China § Department of Chemistry, School of Chemical Science and Engineering, KTH Royal Institute of Technology, Stockholm 10044, Sweden ∥ University of Chinese Academy of Sciences, Beijing 100049, China S Supporting Information *

ABSTRACT: Solid-state luminescent materials that possess reversible fluorescence changes toward external multistimuli are of immense interest because of their potential applications in data storage and sensors. While the recent developments in this field are mainly focused on the π-conjugated organic molecules. Herein two polymorphic luminescent ionic liquid (IL)-based stimuli-responsive materials were designed by the supramolecular assemblies of an organic-decorated chlorobismuthate anion and a rotationally flexible imidazolium cation, namely, α (1)/β (2)-[Bmmim][BiCl4(2,2′-bpy)] (Bmmim = 1-butyl2,3-dimethylimidazolium; 2,2′-bpy = 2,2′-bipyridine). Because of the different conformations of the n-butyl chains on the imidazolium cations, tuning of the supramolecular packing structures as well as luminescent colors for 1 and 2 was realized. Single-crystal X-ray diffraction and Hirshfeld surface analyses disclose that the polymorphism-dependent emission may be attributed to the different weak interactions, especially to the π−π interactions between adjacent [BiCl4(2,2′-bpy)]− anions in two compounds. Additionally, compound 2 could be transformed into 1 spontaneously at ambient conditions, which could be triggered by the moisture in the air. Both of the title compounds could detect NH3 vapor selectively through the luminescence “turn-off” method rapidly and reversibly because of the destruction of intermolecular interactions, indicating their stimuli-responsive property toward NH3.

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INTRODUCTION Tuning and switching of the solid-state luminescence properties are attractive targets for both fundamental studies and practical applications.1 Great progress has been made in regulating the luminescent properties by triggering the changes of the noncovalent interactions such as hydrogen-bonding, anion−π, π−π, and C−H−π interactions.2 Because the noncovalent interactions that determine the organization of fluorophores are weak and flexible, their making and breaking become more realistic even under ambient conditions, thus having the potential to achieve reversible structural transformations and hence external stimuli-responsive and switchable molecular fluorescent materials.3 At present, changes of the noncovalent bonds in molecules can be achieved through many approaches, especially through building conformational polymorphism,2b triggering phase transition,4 disturbing the long-range molecular ordering,5 modulating molecular arrangement,5a and so on. Tang et al. have established the concepts of aggregation/crystallization-induced emission enhancement © XXXX American Chemical Society

(AIEE/CIEE); that is, emission enhancement occurred in the aggregated or crystalline states relative to the monomeric or amorphous states.6 Polymorphs are the compounds that are characterized as more than one crystalline form with the same chemical composition. In the area of CIEE, the construction of crystal polymorphs is an effective method to obtain compounds with different luminescence properties.7 The ionic liquid (IL) crystals containing imidazolium cations with rotationally flexible alkyl chains have been confirmed to be excellent candidates for constructing crystal polymorphism.8 In addition, the mercury-like main-group metal ions with external electrons of the s2 configuration (such as Pb2+, Sb3+, Bi3+, and Te4+) possess unique luminescent properties in both coordination polymers and inorganic materials.9 Particularly, the bismuth-based compounds with low toxicity, low cost, and good chemical stability have gained Received: March 21, 2019

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DOI: 10.1021/acs.inorgchem.9b00813 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry ever-growing interest in potential luminescent applications.10 Thus, it should clearly be possible to create polymorphismdependent luminescent materials by assembling IL imidazolium cations with rotationally flexible alkyl chains and bismuth(III) complex anions. Very recently, we observed two luminescent polymorphs based on rotational isomeric 1butyl-3-methylimidazolium ([Bmim] + ) cations and [BiCl4(2,2′-bpy)]− (2,2′-bpy = 2,2′-bipyridyl) anions.11 However, because of the structural disorder of the butyl chains, the conformational differences between the cations within two polymorphs are not distinct, finally leading to similar emission properties of the two polymorphs. In the present work, we continue to work on the construction of external-stimuli responsive luminescent materials based on polymorphic Bi3+-containing ILs. By assembling an organically modified halobismuthate anion and another cation of flexible 1-butyl-2,3-dimethylimidazolium ([Bmmim]+), two target polymorphs were obtained, namely, α (1)/β (2)-[Bmmim][BiCl4(2,2′-bpy)] (Figure 1). Com-

of the noncovalent interactions among the internal crystalline structures, the luminescent properties of the polymorphs can be changed by the external stimuli, that is, compound 2 could convert into 1 at ambient conditions; moreover, both compounds could detect the NH3 vapor rapidly and reversibly through the luminescence “turn-off” method because of the destruction and re-formation of weak interactions. This work may provide unique access to novel multifunctional polymorphism-dependent and external stimuli-responsive luminescent materials by using the ILs.

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RESULTS AND DISCUSSION Both compounds 1 and 2 were isolated as colorless blocklike crystals by the slow evaporation of a homogeneous pink solution produced by a solvothermal method from a mixture of BiCl3, [Bmmim]Cl, 2,2′-bpy, and acetonitrile. Single-crystal structural analyses reveal that the two polymorphs crystallize in different crystal space groups; that is, 1 belongs to the triclinic space group P1̅, while 2 crystallizes in the monoclinic space group of P21/c. Both of their asymmetric units consist of one [Bmmim]+ cation and one isolated [BiCl4(2,2′-bpy)]− anion (Figure S1). The Bi atom is octahedrally coordinated by four Cl− anions and two N atoms from a bidentate 2,2′-bpy. The Bi−Cl bond lengths are in the range of 2.6562(12)− 2.7363(11) Å in 1 and 2.6495(12)−2.6938(12) Å in 2. The distances of Bi−N are 2.487(4) and 2.498(4) Å in 1 and 2.490(4) and 2.505(3) Å in 2 (Table S2). The n-butyl chains on the [Bmmim]+ cations in 1 exhibit a TG conformation, in which T refers to the trans conformation of the C6−C7 bond and G indicates the gauche conformation of the C7−C8 bond, while in compound 2, the n-butyl chains display GT conformation, as listed in Table S3. Likely the unique template effects of the conformationally different [Bmmim]+ cations direct the formation of two different crystalline phases. Because the electronic properties of the fluorophores are extremely sensitive to the environment of weak interactions, detailed structural analyses have been carried out to investigate the intermolecular contacts around [BiCl4(2,2′-bpy)]− in the crystalline phases of 1 and 2. The packing modes of the two polymorphs are depicted in Figure 2a,b, clearly showing the short contacts (e.g., hydrogen bonds, anion−π, and π−π) that direct the three-dimensional (3D) supramolecular organiza-

Figure 1. Diagrams of the two title polymorphs with different luminescent colors. T and G refer to the trans and gauche conformations of the C−C bond on the butyl chains, respectively.

pared with the reported α/β-[Bmim][BiCl4(2,2′-bpy)], the luminescent colors of the title polymorphs are distinctly different. Compounds 1 and 2 possess greenish-blue and greenish-yellow emissions, respectively, with quantum yields of 44.98 ± 1% and 36.36 ± 1%, which are higher than those of the reported α/β-[Bmim][BiCl4(2,2′-bpy)] and most of the reported hybrid halobismuthate compounds.12 Single-crystal structural measurements and Hirshfeld surface analyses reveal that their different packing motifs and weak intermolecular interactions caused by the template effects of rotationally isomeric [Bmmim]+ cations contribute greatly to their different emission properties. Because of the easy making and breaking

Figure 2. Side views of 1 (a) and 2 (b) along the a axis illustrating packing diagrams of the [Bmmim]+ cations (pink) and [BiCl4(2,2′-bpy)]− anions (cyan). 1D supramolecular chains in 1 (c) and 2 (d) assembled by a [BiCl4(2,2′-bpy)]− anion via hydrogen-bonding (yellow dashed lines) and π−π (pink and cyan dashed lines) interactions. B

DOI: 10.1021/acs.inorgchem.9b00813 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry tions of 1 and 2. Similarly, every two centrosymmetric [BiCl4(2,2′-bpy)]− anions in 1 and 2 are stacked parallel along the a axis, leading to the formation of one-dimensional (1D) supramolecular chains via the connection of hydrogen bonds, as depicted in Figure 2c,d. Viewed along the a axis, it shows different arrangements of the anionic chains in 1 and 2. In compound 1, the anion chains are further linked through C11−H11···Cl2 hydrogen bonding, generating two-dimensional (2D) supramolecular layers along the (01−1) plane (Figure S2), while there is no short contact extending the anion chains into 2D supramolecular layers in 2. In compound 1, apart from hydrogen bonds, the 2,2′-bpy ligands of adjacent anions show π−π interactions with a distance of 3.873(3) Å (pink dashed lines in Figures 2c and 3a and Table S4), while in

Figure 4. 2D finger plots for the [Bmmim]+ cation (left) and [BiCl4(2,2′-bpy)]− anion (right) in compounds 1 (a) and 2 (b).

greenish and yellow-greenish broad emission peaking at 490 nm (for 1) and 520 nm (for 2), respectively, corresponding to the wide excitation ranges from 250 to 450 nm, centered at 370 nm (Figure 5a,b), which are in agreement with the optical Figure 3. Diagrams of the π−π contacts between adjacent [BiCl4(2,2′-bpy)]− in compounds 1 (a) and 2 (b) viewed from different directions.

compound 2, the projection of the pyridine rings features large slippage and the distance from center to center is more than 4.5 Å (cyan dashed lines in Figures 2d and 3b and Table S5), indicating that the π−π interaction in 2 is negligible. The [Bmmim]+ cations are situated in the interstices of the anionic supramolecular chains and interact with them through hydrogen bonds. Every cation in 1 and 2 interacts with the surrounding anions through seven different hydrogen bonds, finally linking the anionic chains into 3D supramolecular networks. Detailed hydrogen bonds in compounds 1 and 2 are shown in Figure S3. The C···Cl distances are in the ranges of 3.336(5)−3.836(5) and 3.512(5)−3.889(5) Å (Table S6), respectively. In addition, the Hirshfeld surface analyses have been performed on the two polymorphs, and comparative 2D fingerprint plots of the cation and anion in 1 and 2 with different shapes are depicted in Figure 4a,b, visually exhibiting the different packing motifs and intermolecular interactions in 1 and 2. The green sharp spikelike structures mainly represent the weak interactions with relatively short distances. It is remarkable that the average distances of weak interactions in 1 are slightly shorter than those in 2. Another distinct difference in the two compounds is the contribution of C···C contacts (defined as π−π interactions). As circled by red/yellow dashed lines in Figure 4a,b, the π−π interactions in 2 could be ignored, in contrast with 1. All of the above indicates that compound 1 features a more densely packed structure than 2. Examination of the solid-state photoluminescence (PL) properties reveal that the two title compounds exhibit blue-

Figure 5. (a) Solid-state emission images under 365 UV light. (b) Excitation (dashed lines) and emission (solid lines) spectra of compounds 1 (cyan) and 2 (light green): λex = 370 nm for 1 and 2, λem = 490 nm for 1, and λem = 520 nm for 2. (c) CIE (1931) chromaticity diagrams. (d) Time-resolved PL spectra of compounds 1 and 2.

absorption spectra in Figure S4. The CIE (1931) chromaticity coordinates of the two polymorphs are totally different, located at (0.235, 0.377) and (0.303, 0.466) at room temperature (Figure 5c), respectively. The quantum yields are characterized as 44.98 ± 1% for 1 and 36.36 ± 1% for 2, which are higher than the values of the reported [Bmim][BiCl4(2,2′-bpy)] polymorphs (26.07% and 36.59%) and most of the reported hybrid halobismuthate compounds but lower than that of [TBA][BiBr3(bp4mo)] (TBA = tetrabutylammonium; bp4mo C

DOI: 10.1021/acs.inorgchem.9b00813 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry = N-oxide-4,4′-bipyridine)12a (Table S7). The time-resolved single-photon-counting measurements indicate that the emission of polymorphs originates from triplet excited states exhibiting phosphorescence with the lifetimes of 190.01 μs for 1 and 38.78 μs for 2 (Figure 5d). According to the previous studies, the organic ligands play vital roles in the emissions of hybrid halobismuthate compounds because of their relatively low energy of empty antibonding orbitals. The highly efficient luminescence of the title compounds is mainly attributed to the excited states of metal-to-ligand charge transfer.12 Additionally, the weak intermolecular interactions contribute greatly to the emissions. It is anticipated that the dissimilar luminescent properties exhibited by the title polymorphs could be dictated by the differences in weak interactions. Particularly, the different π−π contacts among the adjacent luminescent centers ([BiCl4(2,2′bpy)]− anions) in 1 and 2 greatly affect their luminescent colors. In compound 1, the center-to-center distance of the pyridine ring is 3.873(3) Å with a slippage of 1.905 Å (Figure 3a and Table S4), while in 2, the slippage is up to 3.024 Å and the π−π contacts (more than 4.5 Å; Figure 3b and Table S5) can be ignored. Additionally, the single-crystal structure and Hirshfeld surface analysis reveal that the average distances of weak interactions of 1 are slightly shorter than those of 2, indicating the stronger intermolecular interactions in 1. Given all this, compound 1 possesses more densely packed structures in comparison with 2 (ρ = 1.919 for 1 and ρ = 1.808 g·cm−3 for 2). The stronger interactions in 1 play great roles in stabilization of the structures at excited states and reduce the energy reduction caused by intramolecular vibrations, resulting in its higher quantum yields and longer lifetimes. Interestingly, compound 2 could transform into compound 1 spontaneously at ambient conditions, while 1 could be stable at ambient conditions. Figure 6a shows the changes of the

crystal appearance and luminescent colors of compound 2 with increased time. The freshly prepared crystals of 2 were colorless and transparent, exhibiting greenish-yellow emission. After 20 min, some parts of the crystals began to become opaque, and the emission of these parts changed from greenish-yellow to greenish-blue color. Over time, the opaque area on the crystals extended gradually, and the greenish-blue emission increased accompanied by a reduction of the greenish-yellow luminescence. About 1 day later, the whole crystal became white and opaque, and all parts of the crystals displayed greenish-blue emission. Additionally, the powder Xray diffraction (PXRD) patterns have confirmed the structural transformation from 2 to 1. As depicted in Figure 6b, after the crystals were exposed to the ambient conditions, new peaks that belong to compound 1 appeared, as labeled by triangles and four-point stars. Meanwhile, the diffraction intensities of some other peaks (noted by five-point stars and blue and green dashed lines) were obviously weakened. In addition, two peaks at 10.36° and 10.85° (as circled by a purple dashed line) slightly shifted to the low-angle region. As time went by, such variations of the patterns became more obvious. Finally, compound 2 was totally transformed into compound 1. In order to get further insight into the transformation process, the total energy of compounds 1 and 2 based on one unit cell was calculated using Materials Studio (MS). The results show that the ground-state energy of 2 (−24875.50 eV) is lower than that of 1 (−12437.13 eV). The chemical reaction based on the unit cell of the two polymorphs has been shown in Figure 7a. The calculated results show that the trans-

Figure 7. (a) Comparison of the relative stabilities of the two title compounds based on each unit cell as a function of energy. (b) Images of freshly prepared crystals of 2 sealed in a vacuum glass tube under daylight (top) and 365 UV light (bottom).

formation from 2 to 1 (ΔE = 1.24 eV) is an endothermic process with a small enthalpy change, indicating the feasibility of this transformation process. Additionally, it is anticipated that kinetic factors may also contribute largely to the transformation at ambient conditions. It is well-known that some hybrid metal halides (e.g., hybrid lead halide perovskites) are very sensitive to the air.13 Therefore, we sealed the freshly prepared crystals of 2 in a vacuum glass tube, as depicted in Figure 7b. Interestingly, the crystals could remain transparent for a long time (more than 6 months), and the luminescence color of greenish-yellow remains unchanged, indicating no structural transformation into compound 1. Thus, we suspect that the moisture in the air may be the main factor that triggers the structural conversion from 2 to 1. It further confirms that

Figure 6. (a) Images under daylight (top) and 365 nm UV light (bottom) showing the process of transformation into compound 1 at ambient conditions as time goes by. (b) PXRD patterns showing the structure transformation into compound 1 at ambient conditions. D

DOI: 10.1021/acs.inorgchem.9b00813 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 8. (a) Images under daylight and UV light showing the luminescent quenching and recovery of compounds 1 and 2. (b) Kinetic studies of compound 1 under saturated NH3 vapor. (c) PL intensities [490 nm for 1 (pink bar) and 520 nm for 2 (blue bars)] after exposure to the vapors of various solvents. (d) PXRD pattern changes of compound 1 before and after exposure to NH3 vapor.

compounds assembled by rotational flexible IL cations and halobismuthate anions are easily stimulated by external stimuli and exhibit corresponding responses. More significantly, both title compounds exhibited a rapid and reversible response toward NH3 vapor through a luminescence quenching approach. Ammonia (NH3) is considered to be one of the most common analytes in the areas ranging from food freshness determination to clinical diagnosis.14 Additionally, NH3 vapor is also highly toxic as part of industrial effluvia in the production of fertilizers, chemicals, and refrigeration systems.15 High-concentration NH3 can cause irritation to human skin, eyes, nose, throat, and the respiratory tract because of its corrosive properties. Exposure to a massive concentration of NH3 (>5000 ppm) may be deadly within minutes. Therefore, the sensing and detection of NH3 is of great significance for both human health and environmental protection. Recently, the luminescence-based methods have gained considerable interest because of their ease of operation, high sensitivity, and real-time monitoring with a rapid response time. As shown in Figure 8a, when samples 1 and 2 were exposed to NH3 vapor, their emission was completely quenched, while after removal of NH3 vapor, the greenishblue emission of 1 and greenish-yellow emission of 2 could be recovered slowly at ambient conditions or by heating. Kinetic studies of the more stable compound 1 under saturated NH3 vapor were performed. As depicted in Figure 8b, the luminescence intensity was reduced by 40% in just 16 s and up to 96% in 60 s. The sensing selectivity was measured by exposing the samples of the two title compounds to saturated vapors of commonly used volatile solvents, such as nitrobenzene, toluene, methanol, ethanol, chloroform, dichloromethane, acetonitrile, ethyl acetate, acetone, and diethyl ether. As shown in Figure 8c, after exposure to methanol, the luminescence intensity of compounds 1 and 2 increased by

40%, while for other solvents, the luminescent intensities were decreased slightly, indicating that both title compounds possess good selectivity to NH3 vapor. In order to investigate the sensing mechanism, the PXRD patterns of powder samples of compounds 1 and 2 have been recorded. Figures 8d and S6 and S7 show the structural changes of 1 and 2 before and after exposure to NH3 vapor. Upon exposure in the NH3 vapor for a period of time, the crystallinity of the sensing samples was severely weakened, along with the formation of NH4Cl (Figure S6) and amorphous powders. Because the two title compounds possess the nature of crystallization-induced emission, their PL was then completely quenched, while after removal of NH3 vapor, the samples could be recrystallized to their original states (Figures 8d and S7). To date, a wide range of metal−organic frameworks (MOFs)16 or organic fluorophores17 have been reported in the amine vapor detection through fluorescence “turn-on” (or enhancement) and “turn-off” methods due to the charge transfer between MOFs and analytes. This work shows two novel amine vapor sensors, the sensing mechanism of which was based on the amine-induced destruction of crystallization, further indicating that the luminescent properties of these two polymorphs could be easily changed by triggering the changes of the noncovalent interactions.

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CONCLUSIONS In conclusion, by the deliberate selection of cations, two Bi3+containing ILs, α- (1)/β-[Bmmim][BiCl4(2,2′-bpy)] (2), with polymorphism-dependent emission and potential applications as amine sensors were reported in this work. The template effects of the rational isomeric IL cations play significant roles in the formation of two different supramolecular networks as well as the different emission properties. This work may open up a new avenue for exploration of the noncovalent E

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interaction-based luminophors with multiple functions via the introduction of rotational flexible IL cations.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b00813. Materials and methods, Hirshfeld surface analysis, computational method, single-crystal structure determination, more structural details and figures, PXRD patterns, and TGA (PDF) Accession Codes

CCDC 1897659 and 1897660 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Fax: (+86)591-63173145 (B.H.). *E-mail: [email protected]. Fax: (+86)591-63173145 (X.H.). ORCID

Bing Hu: 0000-0001-7153-7372 Jianrong Li: 0000-0002-6354-4852 Xiaoying Huang: 0000-0002-3514-216X Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grants 21671187, 21601181, and 21521061). We acknowledge the Supercomputing Center of Computer Network Information Center, Chinese Academy of Sciences, for providing the computer resources. G.L. and T.L. are thankful for support from the China Scholarship Council. We also thank Diamond Light Source for access to beamline I19 (MT20805), especially the technical support from Dr. Lucy Saunders.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.9b00813 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.9b00813 Inorg. Chem. XXXX, XXX, XXX−XXX