Bipyridinium-Bearing Multi-stimuli Responsive Chromic Material

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Bipyridinium-bearing Multi-stimuli Responsive Chromic Material with High Stability Xiao-Dong Yang, Rui Zhu, Jian-Ping Yin, Li Sun, Rui-Yun Guo, and Jie Zhang Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00390 • Publication Date (Web): 28 Mar 2018 Downloaded from http://pubs.acs.org on April 2, 2018

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Crystal Growth & Design

Bipyridinium-bearing Multi-stimuli Responsive Chromic Material with High Stability Xiao-Dong Yang, Rui Zhu, Jian-Ping Yin, Li Sun, Rui-Yun Guo and Jie Zhang* MOE Key Laboratory of Cluster Science, Beijing Key Laboratory of Photoelectronic/Electrophotonic Conversion Materials, School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing 102488, P. R. China ABSTRACT: Interests in chromic materials are rapidly increasing for their potential applications as smart responsive materials. However, the majority of the existing ones are triggered by merely one or two outer stimuli and the few multi-stimuli responsive examples still suffer from their confining functioning state such as solution or polymer. To address these challenges, a new generation of multi-stimuli responsive chromic materials is being intensively pursued. In this article, we report a neoteric supramolecular solid material by assembling Co(II) ion with bipyridinium-functionalized ligand in solvothermal condition. This material can perform reversible chromic behaviors in response to 365 nm light, heat, vacuum, ammonia, ethanediamine and some organic solvents. Meanwhile, it exhibits excellent structure stability and reversibility to cope with these stimuli.

1. Introduction Nowadays, smart responsive materials based on chromic phenomena which can recognize specific external stimuli and then give readable corresponding signals with regenerated and reused characters have received tremendous scientific attention owing to their various potential applications in a wide range of high-tech fields, such as smart windows, rewritable copy papers, sensors, secret writing, brand protection and anti-counterfeiting, etc.1-5 Compared with other signal transduction such as luminescence quenching or deformation, chromism is more direct, convenient and efficient. Traditional chromic materials referring to a special kind of chemicals which can undergo a reversible change in electronic configuration from the thermodynamically stable form to a less one in response to outer stimuli such as light, heat, electric field, pressure, as well as solvent, etc.6-9 The two forms own disparate electronic absorption spectra, thus can exhibit different colors. Up to now, various kinds of responsive chromic materials containing inorganic, organic or hybrid species have been reported in literatures.10-12 Nevertheless, to date, the majority of the existing ones can be driven by merely one or two outer stimuli and the few reported multi-stimuli responsive examples still suffer from their confining functioning state such as solution or polymer.13-16 These urgently addressed problems generate a demand for a new generation of multi-stimuli responsive chromic materials. Viologen or 4,4´-bipyridinium derivatives are noted for their particular electron-deficient nature and can undergo oneelectron reduction to produce intensely colored free radicals.1718 Differing from other common organic chromophore units such as dithienylethene, spiropyrans and azobenzene which mainly perform photoinduced chromic behaviors based on the configurational changes involving ring-opening/closing or cistrans isomerization, viologens or 4,4´-bipyridinium derivatives exhibit chromism through stimuli-induced electron-transfer reaction between donor and acceptor, in which the donor-acceptor contact and orientation that greatly effect electron transfer efficiency, can be modulated by substituent modification or molecular self-assembly.19-20

Although, various kinds of chromic materials based on viologens or 4,4´-bipyridinium derivatives have been synthesized so far by our and other groups.21-25 However, most of these chromic materials function in isolation and it is still a challenge to integrate more than three different response behaviors into one compound. For another, as a classical substitution-active metal ion, Co(II) ion is well known for its coordination geometry triggered chromic phenomena. Rearrangement of the coordination sphere can considerably affect its d-orbital configuration and then exhibit obvious color change.26-28 Therefore, if fusing viologen or 4,4´-bipyridinium derivatives and Co(II) ion into the same complex, neoteric functionalized materials with diverse chromic properties could be envisioned. To give full play to the advantages of the two species, choosing an appropriate operating desk is of utmost importance. As a very broad and well-developed research area, supramolecular networks have aroused vast amounts of interest in the past decade.28-29 Electrostatic intermolecular interactions (e.g. van der Waals, dipole–dipole, metal–ligand, and hydrogen bonds) exist in these structures as the main connecting force.31-33 These interactions possess larger range of strengths, directionality and reversibility allowing effective modification over the structure and properties of materials and enabling the synthesis of large and complex structures with diverse functions.34-35 If functionalized bipyridinium unit and Co(II) ion are embedded into the same supramolecular networks, multi-stimuli responsive chromic materials could be expected. Alone this line, we have developed a novel bipyridinium-bearing supramolecular solid material called [Co(DBCbpy)2(H2O)4]·6H2O (Co-SN, H2DBCbpyBr = 1-(3,5dicarboxylbenzyl)-4,4´-bipyridinium bromine). Attractively, this material can perform reversible multi-stimuli responsive chromic behaviors when encountering 365 nm light, heat, vacuum, ammonia, ethanediamine and some organic solvents. Especially, the rich aromatic rings and carboxylate oxygen atoms in DBCbpy- ligand facilitate the formation of π–π stacking and hydrogen-bond, endowing the system with high structure stability to cope with these stimuli.

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2. Experimental section 2.1 Materials and instruments All chemicals and reagents were obtained from commercial sources and used without further purification. IR (KBr pellet) spectrum (400-4000 cm−1 region) was measured from KBr pellet on a Nicolet iS10 FT-IR spectrometer. Powder X-ray diffraction (PXRD) pattern was recorded with a Bruker D8 Advance X-ray diffractometer with Cu Kα radiation (λ = 1.54056 Å). Thermogravimetric analysis (TGA) was collected on a Mettler Toledo TGA/DSC 1/1100 analyzer in a flowing air atmosphere at a heating rate of 10 °C min−1 from 25 to 800 °C. UV-vis diffuse reflectance spectral measurement was carried out at room temperature by using a PE Lambda 900 spectrometer. Elemental analysis (C, H, N) was performed using a PE2400 II elemental analyzer. Electron spin resonance (ESR) signal was collected with a JES-FA200 spectrometer. 1 H NMR spectrum was performed on a Bruker AV-400 NMR spectrometer. Vacuum-treated experiment was taken on an Intelligent Gravimetric Sorption Analyser IGA100B from the Hiden Corporation. 2.2 Synthesis Ligand: H2DBCbpyBr was synthesized according to our previously reported literature.36 1H NMR (400 MHz, DMSOD6): δ 6.04 (s, 2H), δ 8.13 (d, J = 4 Hz, 2H), δ 8.47 (s, 3H), δ 8.68 (d, J = 4 Hz, 2H), δ 8.93 (d, J = 4 Hz, 2H), δ 9.46 (d, J = 4 Hz, 2H) (Figure S1). Co-SN: CoSO4·7H2O (0.056 g, 0.20 mmol) and H2DBCbpyBr (0.040 g, 0.10 mmol) were dissolved in a mixed solvent of DMF (2 mL) and distilled H2O (2 ml), then sealed in a 25 mL glass bottle after stirring for 20 min. The glass bottle was heated at 90 °C for 2 days and then slowly cooled to 30 °C within 2 days. Orange block crystals were obtained in about 60% yield based on Co. Elemental analysis (%): Calculated for CoN4O18C38H46 (905.72): C, 50.39; H, 5.12; N, 6.19; Found: C, 50.52; H, 4.87; N, 6.48. 2.3 X-Ray crystallographic analysis The X-ray diffraction data of Co-SN was collected on a Gemini A UItra diffractometer with graphite monochromated Mo Kα radiation (λ= 0.71073 Å) at room temperature. The absorption correction was performed by using the multi-scan program and the structure was solved by direct methods and refined on F2 by full-matrix least-squares methods using the SHELXL-2016 program package. All non-hydrogen atoms were refined with anisotropic displacement parameters. The hydrogen atoms except those attached to coordinated water molecules were calculated in ideal positions and refined by riding on their respective carbon atoms, while the hydrogen atoms of coordinated water molecules were first determined by difference Fourier map and then fixed at the calculated positions. For Co-SN, dissociative water molecules were subtracted from the reflection data by the SQUEEZE routine in PLATON due to their high disorder. Its final formula was derived from crystallographic data combined with elemental and thermogravimetric analysis data. Crystal data for Co-SN: CoN4O18C38H46; Mr = 905.72; monoclinic C2/c; a = 24.5144(3) Å, b = 16.5285(17) Å, c = 13.3333(16) Å, β = 110.645(13)°, V = 5055.5(11) Å3; T = 296 K; Z = 4; Dcalcd = 1.190 g·cm–3; µ = 0.407 mm–1; F(000) = 1892; 0.0627); GOF = 1.023; R1 = 0.0720 and wR2 = 0.1845 [I > 2σ(I)]. CCDC 18130712

Figure 1. (a) The coordination environment of Co(II) ion in Co-SN. Symmetry codes: #1 0.5-x, 0.5-y, -z. (b) The –ABCD– stacking mode of adjacent DBCbpy– ligands. (c) The π–π interactions between the benzene rings of adjacent DBCbpy− ligands. (d) The 3D stacking structure with 1D parallelogramshaped channels viewed along the c axis. All hydrogen atoms and lattice water molecules are omitted for clarity.


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Figure 2. (a) Photographs Co-SN before and after irradiation with 365 nm light. (b) UV-vis diffuse reflectance spectral changes of CoSN after irradiation. The insert shows ESR spectra of Co-SN before (black line) and after irradiation (red line). (c) Total and partial DOS of Co-SN. The Fermi levels are located on 0 eV (dashed lines). The insert shows the distance and orientation between the carboxylate oxygen atom (O2) and the pyridinium nitrogen atom (N2) in Co-SN.

2.4 Computation description The density functional calculations on the periodic structure came from the crystallographic data were performed in the DMOL3 package.37 A fine accuracy for the numerical integration of the Hamiltonian and a fine (10−5 eV/atom) tolerance for SCF convergence were applied. The Perdew−Burke−Eruzerhof (PBE) functional with the generalized gradient approximation was used for exchange correlation.38 The Tkatchenko−Scheffler (TS) scheme was applied for dispersion corrections. All-electron calculations were performed with the double numerical basis sets plus polarization functional (DNP) accompanied by a fine orbital cutoff quality. 3. Results and discussion 3.1 Crystal structure X-Ray single-crystal diffraction analysis reveals that the asymmetric unit of Co-SN contains one half crystallographically independent Co(II) ion, two coordinated water molecules, one DBCbpy− ligand and some highly disordered dissociative water molecules (Figure S2). The Co(II) ion is located on a symmetry center and the asymmetric unit further forms a centrosymmetric linear basic building unit by symmetry operation (symmetry code: 0.5-x, 0.5-y, -z). Around the central Co(II) ion, four O atoms (O1W, O1W#1, O2W, O2W#1) of water molecules and two N atoms (N1, N1#1) of DBCbpy− ligands complete an slightly distorted octahedral coordination sphere with Co−O bond lengths in the range of 2.067−2.097 Å and Cd−N bond length 2.167 Å (Figure 1a). These basic building units are further packed in – ABCD– fashion via π–π stacking and hydrogen-bond (Figure 1b & S3), extending the structural motif to a 3D porous supramolecular framework. The π–π stacking forms between the benzene rings of adjacent DBCbpy− ligands with centroid···centroid distances of 3.68 Å and 3.74 Å, and interplanar angles of 0° and 1.14°, respectively (Figure 1c), while the hydrogen-bond interactions occur among the carboxylate oxygen, water oxygen and aromatic carbon atoms [O1W (H1WA)···O4, d = 2.62 Å; O2W (H2WA)···O3, d = 2.60 Å; C10 (H10)···O3 d = 3.08 Å] (Table S1), which plays a

key role in enhance the structure stability of the framework.3940 When viewed along the c axis, 1D parallelogram-shaped channels with a pore size of 14.78 Å × 14.78 Å (the distances are measured between Co(II) ions located on the diagonal position) are presented (Figure 1d). The solvent-accessible volume of Co-SN is 1829.6 Å3 (5055.5 Å3), with solventaccessible voids of 36.2%, as calculated by PLATON. 3.2 Photochromism As exhibited in Figure 2a, Co-SN shows an efficient photochromic transformation from orange to purple in response to the irradiation of 365 nm light. In original UV-vis diffuse reflectance spectrum, besides the intense bands at 205 nm and 265 nm corresponding to the π→π* transition of DBCbpy- ligand, an additional band at 520 nm that is attributed the 4T1g→4T1g(P) transition of high-spin Co(II) ion in octahedral coordination also appears.41-42 The timedependent solid state UV-vis diffuse reflectance spectra are correlated with its photochromic behaviour. As shown in Figure 2b, two new absorption peaks centered at 405 and 610 nm appear, of which the intensity rises with the duration of irradiation. For another, Co-SN is ESR silent before irradiation due to the very short spin-lattice relaxation time related to the strong spin−orbit coupling typical of the orbitally degenerate Co(II)-4T1g state.43 However, after irradiation, a strong ESR signal at g = 2.0034 can be observed (the inset of Figure 2b). Such results are akin to these bipyridinium or viologen-based materials, suggesting that this chromic process should originate from the generation of free radicals via electron transfer.44-48 The colored samples can be bleached to the initial state in air within 3 min owing to the quenching effect of oxygen.44-48 These chromic and bleaching processes can be repeated for at least six times without appreciable variations in PXRD and IR spectra (Figure S4 & S5), indicating its good tolerance to photo-stimulus.



Figure 3. (a) Photograph showing the color change of Co-SN before and after dehydration (b) UV-vis diffuse reflectance spectral changes of Co-SN in a hydrochromic process. The insert shows ESR spectra of Co-SN before (black line) and after (red) dehydration. (c) The linear variation of the fading time for Co-SN treated in different RH conditions ranging from 50% to 90% (30 °C).

To better understand the electron-transfer process, density functional theory calculations have been carried out based on its single crystal structure. In Co-SN, both Co(II) ion and carboxyl O atoms of DBCbpy− ligands can theoretically donate electrons to pyridinium N atoms. However, the oxidation of Co(II) to Co(III) is very hard and only happens in some harsh conditions. And the nearest Co···N distance is 5.37 Å (Figure S6), which is much long for electron transfer. Besides, the characteristic UV-vis absorption band of 1A1g → 1 T2g (320 nm - 420 nm) and 1A1g → 1T1g (about 660 nm) transitions of octahedrally coordinated low-spin Co(III) ion cannot be observed during all the irradiation process.49-50 Therefore, the calculations were performed on a periodical structure obtained by replacing Co(II) with Zn(II) due to its complex effects of the unpaired electrons and no contribution for electron transfer. As depicted in Figure 2c and Figure S7, the HOMO of Co-SN mainly resides in the carboxylate group, whereas the LUMO is dominated by the bipyridinium ring. From the structural data, it can be found that nearest distance between carboxylate oxygen atom (donor: O2) and pyridine nitrogen atom (acceptor: N2) is 3.49 Å, O2···N2···C6 angle is 72.84° (the insert of Figure 2c). Such distance and geometry are conducive for the photo-induced electron transfer (PET) reaction through σ–π interaction to generate free radicals.51-53 More interestingly, such photochromic behaviors of Co-SN are distinguishing under the irradiation of light with different wavelengths (Figure S8). Upon continuous irradiation of 450 nm light, only a tiny color evolution from orange to orangered and small variations in UV-vis spectra can be observed. However, if the activation wavelength is shifted to 550 nm and 635 nm, no photochromic phenomenon can be observed even under prolonged irradiation. As is well known, coloration degree is closely related to the amount of photoinduced radicals. Due to the lower excited energy, the light in visible region can only trigger little or even no electron transfer. These characteristics endow Co-SN with potential for application in ultraviolet light sensors. 3.3 Heat and vacuum induced chromism Upon heat-treatment under 80 °C just for 3 min, Co-SN turns violet quickly (Figure 3a). However, no detectable ESR signal characterized for bipyridinium radicals after being heated manifests that this chromic process is not relevant to

the generation of free radical (the inset of Figure 3b). The insitu IR spectrum of the heat-treated Co-SN reveals the presence of dehydration due to the increased transmission intensity of stretching vibrations of H2O at 3432 cm-1 and 3230 cm-1 (Figure S9). In addition, its UV-vis diffuse reflectance spectrum shows a new band at 630 nm, which approximates the 4A2´ (F) → 4E´(F) transition of Co(II) ion in a trigonalbipyramidal geometry (Figure 3b).54-55 Thus, this chromic phenomenon can be reasonably assigned to the change of the coordination environment of partial Co(II) ions due to dehydration. Regrettably, the X-ray single-crystal data cannot be obtained due to the amorphous state after dehydration, as revealed by absent XRPD lines (Figure S10). However, after an exposure in air (22.1 °C, 48.1% RH) for just about 9 min, not only the violet color and UV-vis absorption band at 630 nm but also the XRPD pattern can recover to its initial state. The switching time of Co-SN is faster than those found in some common Co(II) chromic compounds (e.g. CoCl2·6H2O: 1 day, CoSO4·7H2O: 1.5 days, silica gel desiccant: more than 30 days) (Figure S11). More interestingly, the switching time of Co-SN has a linear response (R2 = 0.9806) to RH from 50% − 90% (30 °C) and the shortest time was obtained with RH = 90% (Figure 3c), suggesting that it could be used for humidity detection. These dehydration and rehydration induced color switching can be cycled for at least six times without a noticeable loss of UV-vis absorption intensity (Figure S12). Apart from heat, the vacuum-treatment also facilitates dehydration and triggers the color switching from orange to purple, and a fast bleaching process in air can be observed (Figure S13 & S14). After several cycles, no obvious variation in PXRD patterns appears (Figure S15), suggesting an excellent reversibility of such chromic process. 3.4 Lewis base induced chromism


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Figure 4. (a) Photograph showing the color change of Co-SN in presence of ammonia, ethanediamine, triethylamine, 1,2propanediamine and 1,3-butanediamine vapors. (b) Solid state UV-vis diffuse reflectance spectra of Co-SN after fumigated with ammonia, ethanediamine, triethylamine, 1,2propanediamine and 1,3-butanediamine vapours.

As a common kind of lewis basic chemical species, ammonia or organic amine is widely used for industrial applications, such as dying, fertilizer, refrigeration, etc.56-61 However, due to their toxicity, corrosivity and difficult handling, developing new convenient and sensitive materials is vital. For Co-SN, the most outstanding structure feature is the 1-D regular parallelogram-shaped channels along the c axis in which the pore wall is decorated with cationic bipyridinium group. The lewis acidic and redox nature of cationic bipyridinium group in Co-SN provide a possibility to sense ammonia or organic amine via acid−base interaction. Upon fumigated with ammonia, ethanediamine, triethylamine, 1,2-propanediamine and 1,3-butanediamine vapors respectively, it is interesting to find that Co-SN shows selective response to ammonia and ethanediamine with a visual color change from orange to purple within 30 seconds, while others induce no color change even under prolonged exposure (> 30 minutes) (Figure 4a). Meanwhile, CoSN@ethanediamine and Co-SN@ammonia exhibit strong ESR signals at g = 2.0036 and 2.0024 respectively (Figure S16 & S17). Such ESR signals are typical for these photoinduced bipyridinium radicals, suggesting that this ammonia or ethanediamine induced chromism is more likely to arise from the generation of bipyridinium radicals which can be further confirmed by their characteristic UV-vis absorption bands (625 nm in Co-SN@ethanediamine and 680 nm in Co-

Figure 5. (a) Photograph showing the color change of Co-SN after immersed in different organic solvents (b) PXRD patterns of Co-SN and the samples after immersed in the relevant organic solvents.

SN@ammonia) (Figure 4b).44-48 Ammonia or ethanediamine is not only a strong lewis base but also a reducing agent, which can reduce the cationic bipyridinium unit to radical state.59-61 As for triethylamine, 1,2-propanediamine and 1,3butanediamine, no redox reaction happens during the fumigating process owing to their weak lewis base property and stereo-hindrance effect. The colored samples can be maintained in air for about two days, and then bleached to the initial state due to the quenching effect of oxygen.44-48 In addition to ammonia or ethanediamine, some polar organic solvent can also be detected for acid−base interaction. When the as-synthesized samples of Co-SN were immersed in some common organic solvents, selective solvatochromic phenomena happened. As narrated in Figure 5a, Co-SN evolves to modena in DMSO and DMF, purple in CH3OH and CH3CH2OH, orange-red in CH3COCH3 and CH3CN. For organic solvents, donor number (DN) is the best parameter to assess their basic properties. As for the organic solvents mentioned above, their donor number are in the order of DMSO(29.8)> DMF(26.6)>CH3CH2OH(19.2)>CH3OH(19.0)>CH3CN(17.0 )>CH3COCH3(14.1).62-63 Chemical species with strong lewis basicity can make the electron-transfer transition to bipyridinium unit happen more easily. As is well known, in many radical-induced photochromic systems, the coloration degree is closely related to the amount of radicals. Obviously, due to the stronger lewis basicity, more bipyridinium radicals can be produced after the fresh samples immersed in DMSO or DMF which lead to their darker color. In previous



sheds light on the design and synthesis of multi-stimuli responsive smart material in the future.

ASSOCIATED CONTENT Supporting Information is available free of charge on the ACS Publications website. Additional data (PDF) Crystallographic data for Co-SN (CIF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

Notes The authors declare no competing financial interest. Figure 6. (a) The quadrate piece of Co-SN@aluminum foil thin film. (b) The PXRD patterns of Co-SN@aluminum foil thin film. (c) The chromic behaviors of Co-SN@aluminum foil thin film in response to outer stimuli.

ACKNOWLEDGMENT This work was supported by the grants from the NNSF of China (Grant No. 21573016/21271173).

REFERENCES literatures, solvatochromic supramolecular networks are quite few because of the challenge of combining both high stability and chromic behaviors together. Even though some supramolecular networks with high solvent durability have been reported such as HOF-8 and SOF-7,64-65 no solvatochromic phenomena were observed in these compounds. By comparison, the current work not only enriches the variety of solvatochromism but also provides a novel routine to synthesize solvatochromic supramolecular networks. When placed in air, these colored samples can undergo self-recovery to its original state. Unexpectedly, after being soaked in these organic solvents for more than 24 hours or fumigated with ammonia and organic amine, their measured PXRD patterns show retained crystallinity and unchanged structure, implying its excellent stability (Figure 5b & Figure S18). 3.5 Preparation of Co-SN@aluminum foil thin film In order to improve its practicability, we attempted to blend Co-SN with binders to fabricate thin film. Epoxy adhesive as a kind of polymer binders has been widely used in industrial production and daily life for its high adhesiveness even placed in some harsh environments. Alone this line, CoSN@aluminum foil thin film was prepared by mixing epoxy adhesive and powdered crystalline Co-SN in a w/w ratio of 1 : 1 and then casted onto aluminum foil. After drying in air, the aluminum foil was cut into quadrate pieces (Figure 6a). CoSN@aluminum foil thin film maintains the integrity of its framework and can also exhibit chromic behaviors in response to the outer stimuli mentioned above (Figure 6b & 6c). 4. Conclusions In summary, a novel bipyridinium-bearing supramolecular network has been solvothermally synthesized and structurally characterized. The title compound possesses multiple-in-one chromic behaviors which can recognize many specific external stimuli such as 365 nm light, heat, vacuum, ammonia, ethanediamine and some organic solvents. Due to the π–π stacking and hydrogen-bond interactions within the framework, it exhibits excellent structure stability and reversibility to cope with these stimuli. Such work not only provides a strategy for enhancing the stability of supramolecular framework but also

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Table of contents (TOC)

A bipyridinium-bearing Co(II) supramolecular network with high stability has been synthesized. This compound can perform reversible chromic behaviors in response to 365 nm light, heat, vacuum, ammonia, ethanediamine and some organic solvents.


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Bipyridinium-Bearing Multi-stimuli Responsive Chromic Material

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