A Dual Photochemically Active Molecule Showing Distributable

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A Dual Photochemically Active Molecule Showing Distributable Reactivity Based on Anion-Assisted Coordination Assembly Ya-Jun Zhang,† Cheng Chen,† Li-Xuan Cai,† Xiao-Dong Yang,‡ and Jie Zhang*,†,‡ †

State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, P. R. China ‡ 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 S Supporting Information *

ABSTRACT: Distributable reactivity of a dual photochemically active molecule functionalized by a carboxyl coordination unit has been achieved based on anion-assisted coordination assembly. The anions induce the photoactive ligand to align at different orientations in three-dimensional metal−organic frameworks, thus allowing the supramolecular systems to show photocycloaddition with simultaneous luminescence transformation or undergo photoinduced electron transfer accompanied by coloration−decoloration under alternating light and thermal stimuli.

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INTRODUCTION Stimuli-responsive photofunctional molecules always attract intense attention because of their widespread applications in sensing, controlled release, information processing, optical devices, etc.1 Integration of multiple photoactive units into one system has become an important strategy to achieving synergistic modulation between functional groups and a multistate response for developing novel intelligent materials. Although several molecules combining two or more identical photoactive units have been developed,2 it remains challenging to integrate different photoactive units into a single molecule and explore their stimuli-responsive ability under different environmental conditions. Since G. M. J. Schmidt systematically studied the photoinduced [2 + 2] cycloaddition in the solid state,3 the photochemical reactions of organic crystals, especially trans1,2-bis(4-pyridyl)ethylene (4,4′-bpe) and analogues as one kind of photoactive molecule, have been an appealing area of study4 because of their remarkable performance in synthetic organic photochemistry, photodriven mechanical movement,5 structure reinforcement,6 conductivity improvement,7 fluorescence switching,8 etc. Efforts have been made to manipulate the molecular alignment effective for cycloaddition through the strategies involving cation−π interaction,9 hydrogen-bonding,10 and coordination- and halogen-bond-driven molecular assemblies.10b For practice, the quaternization of one terminal pyridine group may not only provide cation−π interaction to facilitate proper alignment for regio- and stereoselective [2 + 2] photocycloaddition9 but also endow 4,4′-bpe potential photochromism resembling the bipyridinium family,11 owing to their similar large conjugated structure. However, such a dual © 2017 American Chemical Society

photochemically active molecule has rarely been explored until now. Especially, for a better understanding and utilization of the two photoactive segments, mere synthesis of the compound combined with two kinds of photoactivities is not enough.12 Herein we report an exploration into manipulating such dual photochemical properties through an anion-assisted coordination assembly based on a new quaternized 4,4′-bpe derivative containing carboxyl coordination units (H2DBCbpe· Br; Figure 1a). The DBCbpe ligand shows similar coordination linkages but differently oriented alignments on anions and thus allows two kinds of photoactivities of the DBCbpe ligand to be distributed to two compounds, [Zn(DBCbpe)(H2O)]·NO3· 2H2O (1-NO3−) and [Zn(DBCbpe)Br0.6(H2O)0.4]·1.9H2O· 0.4Br (2-Br−). 1-NO3− undergoes photoinduced cycloaddition with simultaneous luminescence transformation, while 2-Br− shows photocoloration under a xenon lamp or solo 365 nm light. Interestingly, a photoinduced single-crystal-to-singlecrystal structural transformation has been detected during light irradiation of 2-Br− and renders the new phase 2′-Br−, which shows reversible coloration−decoloration cycles upon alternation of the light and thermal stimuli.

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EXPERIMENTAL SECTION

Materials and Physical Measurements. All chemicals were purchased from commercial sources, were of GR/AR grade, and were used without further treatment. 1H NMR spectra were taken on a Bruker AV-400 NMR spectrometer (400 MHz). The chemical shifts were measured relative to tetramethylsilane (0.00 ppm) for deuterated Received: June 14, 2017 Published: August 15, 2017 10529

DOI: 10.1021/acs.inorgchem.7b01497 Inorg. Chem. 2017, 56, 10529−10534

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C21H19.6BrN2O6.3Zn: C, 46.20; H, 3.60; N, 5.13. Found: C, 46.52; H, 3.81; N, 5.41. Synthesis of 2′-Br−. Crystal 2-Br− was transformed into 2′-Br− after irradiation with xenon for 30 min, accompanied by a color change from yellow to purple. X-ray Crystallography. Data collection was carried out on an Agilent Diffraction SuperNova dual diffractometer with Cu Kα radiation (λ = 1.54178 Å) at 173 K for 1-NO3− and 100 K for 2Br− and 2′-Br−. Absorption corrections were performed using a multiscan method. The structures were solved by direct methods and refined by full-matrix least squares on F2 using the SHELXTL-2014 program package. All non-hydrogen atoms were refined with anisotropic displacement parameters. Hydrogen atoms of the ligands were located by geometrical calculations, and their positions and thermal parameters were fixed during structural refinement. The hydrogen atoms on the water molecules were not included in the refinement. Crystal data for 1-NO 3−: C21H15N3O10Zn; Mr = 534.73; orthorhombic Pnna; a = 17.8019(5) Å, b = 31.6604(9) Å, c = 7.8938(3) Å, and V = 4449.1(2) Å3; T = 173(2) K; Z = 8; Dc = 1.597 g cm−3; μ(Cu Kα) = 2.112 mm−1; F(000) = 2176; 10981 reflections collected, of which 4422 were unique (Rint = 0.0249); GOF = 1.063; R1 = 0.0717 and wR2 = 0.2203 [I > 2σ(I)]; CCDC 1553016. Crystal data for 2-Br−: C21H15BrN2O6.3Zn; Mr = 541.43; monoclinic C2/c; a = 16.0657(4) Å, b = 10.2017(3) Å, c = 28.0479(7) Å, β = 95.184(2)°, and V = 4578.2(2) Å3; T = 100 K; Z = 8; Dc = 1.571 g cm−3; μ(Cu Kα) = 3.888 mm−1; F(000) = 2163; 14814 reflections collected, of which 4599 were unique (Rint = 0.0220); GOF = 1.027; R1 = 0.1057 and wR2 = 0.2960 [I > 2σ(I)]; CCDC 1553017. Crystal data for 2′-Br−: C21H15BrN2O6.3Zn; Mr = 541.43; monoclinic C2/c; a = 16.0364(16) Å, b = 9.9830(12) Å, c = 28.123(2) Å, β = 96.278(9)°, and V = 4475.2(8) Å3; T = 100 K; Z = 8; Dc = 1.607 g cm−3; μ(Cu Kα) = 3.977 mm−1; F(000) = 2163; 8968 reflections collected, of which 4555 were unique (Rint = 0.0332); GOF = 1.067; R1 = 0.1122 and wR2 = 0.3214 [I > 2σ(I)]; CCDC 1553018.

Figure 1. Synthesis of the dual photochemically active ligand and anion-assisted coordination assembly strategy of crystals 1-NO3− (left) and 2-Br− (right).

dimethyl sulfoxide (DMSO-d6) as indicated. Powder X-ray diffraction measurements were performed on a Rigaku MiniFlex 600 diffractometer using Cu Kα (λ = 1.5406 Å) radiation with a scan speed of 2° min−1 in the angular range of 2θ = 5−55°. Elemental analysis of carbon, hydrogen, and nitrogen atoms was determined by a Vario EL III CHNOS elemental analyzer. UV−vis diffuse-reflectance spectra were measured on a Perkin-Elmer Lambda 950 spectrometer with BaSO4 as the reference. The IR spectra were taken on a Bomem MB102 IR spectrometer with samples prepared as KBr pellets in the range of 4000−400 cm−1 . Thermogravimetric analysis (TGA) was performed on a Mettler-Toledo TGA/SDTA851e thermal analyzer in a flowing air atmosphere at a heating rate of 10 °C min−1 from 30 to 800 °C. Electron spin resonance (ESR) signals were obtained on a Bruker A300 EPR spectrometer. Photoluminescence spectra were recorded on FLS920 and FLS980 fluorescence spectrometers at room temperature. Synthesis of H2DBCbpe·Br. DBCAbpe·Br was synthesized as previously reported.12 A mixture of DBCAbpe·Br (4.69 g, 10 mmol), HBr aqueous solution (1 mL, 33% w/w), and distilled water (1 mL) was stirred for 10 min, then sealed in a 20 mL vial, kept at 100 °C for 24 h, and then air-cooled to room temperature. The precipitate was collected by filtration, washed with distilled water, and then dried in a vacuum, giving the product as a yellow powder, and the purity was verified by 1H NMR (Figure S1 in the Supporting Information). Yield: 4.95 g, 95% based on DBCAbpe·Br. Synthesis of 1-NO3−. A mixture of H2DBCbpe·Br (27 mg, 0.06 mmol), Zn(NO3)2·6H2O (30 mg, 0.1 mmol), KNO3 (20 mg, 0.2 mmol), N,N-dimethylformamide (3 mL), and water (2 mL) was stirred for 30 min, then sealed in a 20 mL vial, kept at 90 °C for 2 days, and finally allowed to cool slowly to room temperature within 2 days. The yellow rhombus crystals of 1-NO3− were obtained in 25% yield. Elem anal. Calcd for C21H21N3O10Zn: C, 46.60; H, 3.88; N, 7.77. Found: C, 47.09; H, 4.13; N, 7.33. Synthesis of 2-Br−. Crystal 2-Br− was synthesized in the same conditions as those of 1-NO3−, except that there is no additional KNO3 in the synthesis process of 2-Br−, as shown in Figure 1. The yield is 40% based on H2DBCbpe·Br. Elem anal. Calcd for

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RESULTS AND DISCUSSION Crystal Structure Description. Compound 1-NO3− is a three-dimensional (3D) 2-fold interpenetrated structure crystallizing in the orthorhombic space group Pnna. The asymmetric unit contains one zinc(II), one DBCbpe ligand, one coordinated water molecule, two lattice water molecules, and one nitrate ion to balance the charge (Figure S2 in the Supporting Information). Energy-dispersive X-ray spectroscopy excludes the existence of Br− anions in compound 1-NO3− (Figure S3 in the Supporting Information). The zinc(II) ion is four-coordinated by three oxygen atoms from two carboxylate groups of two DBCbpe ligands and one water molecule and one pyridyl nitrogen atom from the third DBCbpe ligand, resulting a distorted tetrahedral geometry. Because the Vshaped ligand DBCbpe is semirigidly jointed, the dicarboxylbenzyl segment can rotate around the center of the methylene carbon. As shown in Figure 1b, the two V-shaped DBCbpe ligands connected to the same zinc(II) ion by carboxylate oxygen atoms are crisscross-arranged in the structure of 1NO3−. When methylene carbon and zinc(II) are selected as the connecting nodes to simplify the structure, an irregular hexagonal channel can be observed, as viewed along the caxis direction. Two independent frameworks are interpenetrated through the hexagonal windows with a crossed arrangement to give a 2-fold entanglement (Figure 1c). Interestingly, in 2-Br−, although each zinc(II) ion is also coordinated by two carboxylate oxygen atoms from two different DBCbpe ligands and one pyridyl nitrogen atom from the third ligand, an additional position is occupied by 0.6 10530

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Inorganic Chemistry statistical Br− and 0.4 statistical water. Replacement of the bromide anion causes a parallel arrangement of two V-shaped DBCbpe ligands connected to the same zinc(II) ion by carboxylate oxygen atoms (Figure 1d), and thus allows a more regular hexagonal channel to be formed in the 3D framework. The simplified structure obtained in the same manner as crystal 1-NO3− shows that two independent frameworks are 2-foldinterpenetrated through the parallel-arranged hexagonal windows (Figure 1e). Distribution of the Two Photochemical Activities Photocycloaddition and Photochromism to Crystals 1NO3− and 2-Br−. An anion-induced structural change definitely endows two compounds with distinct properties. Compound 1-NO3− shows a remarkable luminescence transformation from faint purple to yellow upon continuous irradiation with UV light (365 nm, 2 mW cm−2) at room temperature in air (Figure 2), whereas compound 2-Br−

Figure 3. (a) Structural and color transformation of the crystal 2-Br−. (b) Dihedral angle (α) change of the pyridine and pyridinium rings in the sample 2-Br− from 7.97° to 9.90° and ∠N2−C9(methylene)−C7 angle change from 112.15° to 113.05°. Hydrogen atoms are omitted for clarity.

structure undergo a photocycloaddition reaction, forming cyclobutane rings (conversion of ca. 95% based on the methylene proton within 12 h; Figure S4 in the Supporting Information). Obviously, the proton signals of the cyclobutane ring at 5.54 ppm and the corresponding phenyl, pyridyl, and pyridinium proton signals (9.15, 9.13, 8.79, and 8.77 ppm) of the cycloaddition product already existed in the 1H NMR spectrum of the as-synthesized compound 1-NO3−, revealing that 1-NO3− is highly active and about 5% has reacted under ambient light. Furthermore, the in situ IR spectra also confirm the structural transformation during the irradiation of 365 nm light, based on the disappearance of the characteristic C−H vibration of RCHCHR′ at 981 cm−1 (Figure S5 in the Supporting Information).13 These results indicate that the observed luminscence evolution should be closely related to the distortion caused by photoinduced cycloaddition. As seen in Figure 2, the emission peak in the UV region shifts from 402 to 395 nm and the emission peak centered at 601 nm is enhanced 6.7 times, which may derive from destruction of the regional conjugation in the structure.5c,8 Single-crystal structure analysis shows that the DBCbpe molecules in 1-NO3− are antiparallelaligned to each other with a centroid distance of the adjacent CC double bond of 3.85 Å. Also, the adjacent CC double bond is in a crisscrossed arrangement with an angle of 62.89° between them (Figure 4). This kind of geometry and packing of the double bonds cannot undergo photocycloaddition reaction directly according to the rules of [2 + 2] photocycloaddition, but the conformation of double bonds might alternate with each other between crisscross and parallel styles through a pedal motion.14 In contrast to compound 1-NO3−, compound 2-Br− cannot be triggered to display visible yellow fluorescence. The study of structure 2-Br− reveals that the centroid distance of 4.64 Å between the adjacent CC double bonds in crystal 2-Br− is beyond the critical length of 4.20 Å (Figure S6 in the Supporting Information). The 1H NMR spectrum of the photoirradiated sample 2′-Br− in DMSO-d6 further shows that no additional compound is yielded in the process of irradiation (Figure S7 in the Supporting

Figure 2. Emission spectra of the sample 1-NO3− before (black) and after (red) irradiation by 365 nm UV light. Inset: Compound 1-NO3− shows luminescence transformation from faint purple to yellow after irradiation.

exhibits a color evolution from yellow to purple after irradiation by the same 365 nm light. The intense xenon lamp light source can facilitate the corresponding photoresponse process but cannot inverse the photoreaction behaviors of the two compounds. These results are different from our previous DBCAbpe·Br molecule, which shows photocoloration under visible light and turn-on luminescence under UV light.12 The current observation reveals that the photoresponse behavior of the olefin-containing pyridinium derivatives can be manipulated to meet different requirements through anion-induced molecular assembly. Remarkably, the colored sample 2′-Br− will fade and reverse to yellow under 90 °C in air within 30 min (Figure 3a). Such results demonstrate that compounds 1-NO3− and 2-Br− undergo different photoreactions when exposed to light irradiation. The 1H NMR and IR spectra of the photoirradiated samples 1′-NO3− and 2′-Br− measured in DMSO-d6 clearly demonstrate the occurrence of different photoreactions in these two compounds. After exposure of 1-NO3− to 365 nm light, the olefinic proton signals at 8.21 and 8.23 ppm begin to weaken gradually and the proton signal of the cyclobutane ring at 5.54 ppm is enhanced, while the proton signal of methylene shifts from 6.03 to 5.89 ppm, indicating that the double bonds in the 10531

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xenon lamp (Figure S9 in the Supporting Information). These results reveal that the color change of crystal 2-Br− arises from photoinduced electron transfer (PET) to produce DBCbpe• radicals like the well-known bipyridinium derivatives. In compound 2-Br−, the closest distance between the O1 atom of the carboxylate group and the N2 atom of the pyridinium ring is 3.16 Å (Figure S10 in the Supporting Information), which is the closest donor−acceptor distance ever reported in the bipyridinium-based PET system,15 and the carboxylate oxygen atoms are approximately perpendicular to the nitrogen atoms with an O1···N2···C9 angle of 84.30°, indicating that the free radical can be easily obtained through the PET process.16 The purple crystal 2′-Br− can maintain the colored state in the dark in air for at least 12 h, while fading to the yellow state at 80 °C in the dark in air within 30 min. That is to say, the heating process has greatly accelerated quenching of the free radicals. The thermoinduced color-fading process monitored by the UV−vis diffuse-reflectance spectra (Figure 5b) clearly shows that the four newly emerged peaks weaken gradually in nearly the similar ratio as the coloring process upon increasing heating time, demonstrating that crystal 2′-Br− possesses excellent photochromic reversibility. However, it is noted that the absorption band at 328 nm does not reverse to the original state, so we speculate that the free radicals induced by xenon light are quenched by oxygen in air, but the structure has generated some irreversible changes under xenon light. By a comparison with the crystal data of photoirradiated sample 2′-Br−, the interplanar angle between the pyridine and pyridinium rings in sample 2-Br− changes from 7.97° to 9.90° and the ∠N−C(methylene)−C angle changes from 112.15° to 113.05° (Figure 3b), which suggests that the irreversible absorption change at 328 nm is mainly induced by variation of the torsion angle between the pyridine and pyridinium rings. In order to further examine the fatigue resistance of crystal 2′-Br− on the reversible photochromism, cycle experiments have been carried out by alternating irradiation by a xenon lamp and heating at 80 °C in the dark in air (Figures S11 and S12 in the Supporting Information). Especially, after five cycles, the ESR signal intensity can even reach the first cycle level, indicating a potential practical use in real life (Figure S9b in the Supporting Information). By contrast, crystal 1-NO3− shows no color change under either 365 nm light or a xenon lamp (inset in Figure S13 in the Supporting Information). Through a study of the crystal structure of 1-NO3−, it is not hard to find that the closest distance between the O2 atom of the carboxylate group and the N2 atom of the pyridinium ring is 3.50 Å (Figure S10b in the Supporting Information) and the O1···N2···C9 angle is 110.10°. Although the parameters are proper for the occurrence of the PET process in crystal 1-NO3−, the diffuse-reflectance spectra show that there is almost no new adsorption in the long-wavelength region, indicating that no free radical is generated during the irradiation process (Figure S13 in the Supporting Information). It is speculated that there might be two processes occurring at the same time in crystal 1-NO3− under light irradiation: one is PET, and the other is photocycloaddition. Photocycloaddition occurs more easily than PET. Cycloaddition heavily destroys the conjugated structure significant for the stability of free radicals, causing radicals to be quenched by atmospheric oxygen so quickly that no radical signal shows up during light irradiation.

Figure 4. (a) Crisscrossed arrangement of the adjacent CC double bonds with the angle θ1 between them. (b) Centroid distance (D) of the double bonds in the structure of 1-NO3−.

Information). Fourier transform infrared spectra of crystal 2Br− before and after irradiation by 365 nm light show that there is no vanishing peak, once again confirming that it is inert for photocycloaddition (Figure S8 in the Supporting Information). In order to explore the photochromism mechanism of the 4,4′-bpe derived pyridinium compound, the diffuse-reflectance spectra of crystal 2-Br− under irradiation of a xenon lamp were recorded. UV−vis diffuse-reflectance spectra show that four new absorption bands gradually emerge at the visible-light region with band maxima centered at 537, 604, 728, and 814 nm along with an increase in the irradiation time (Figure 5a). Meanwhile, it was found that ESR spectrum displays a strong signal at g = 2.0032 for compound 2-Br− after irradiation of a

Figure 5. (a) UV−vis−near-IR diffuse-reflectance spectra of crystal 2Br− after irradiation by xenon light. (b) UV−vis−near-IR diffusereflectance spectra of colored crystal 2′-Br− after heating at 80 °C. 10532

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with dual dithienylethene-acetylides. J. Am. Chem. Soc. 2012, 134, 16059−16067. (3) (a) Cohen, M. D.; Schmidt, G. M. J. Topochemistry. Part I.A survey. J. Chem. Soc. 1964, 1996−2000. (b) Cohen, M. D.; Schmidt, G. M. J.; Sonntag, F. I. Topochemistry. Part II.The Photochemistry of trans-Cinnamic Acids. J. Chem. Soc. 1964, 2000−2013. (c) Schmidt, G. M. J. Topochemistry. Part III.The Crystal Chemistry of some transCinnamic Acids. J. Chem. Soc. 1964, 2014−2021. (4) (a) Ramamurthy, V.; Venkatesan, K. Photochemical reactions of organic crystals. Chem. Rev. 1987, 87, 433−481. (b) Liu, D.; Wang, H. F.; Abrahams, B. F.; Lang, J. P. Single-crystal-to-single-crystal transformation of a two-dimensional coordinational polymer through highly selective [2 + 2] photodimerization of a conjugated dialkene. Chem. Commun. 2014, 50, 3173−3175. (c) Hu, F. L.; Wang, H. F.; Guo, D.; Zhang, H.; Lang, J. P.; Beves, J. E. Controlled formation of chiral networks and their reversible chiroptical switching behaviour by UV/microwave irradiation. Chem. Commun. 2016, 52, 7990−7993. (d) Hu, F. L.; Wang, S. L.; Lang, J. P.; Abrahams, B. F. In-situ X-ray diffraction snapshotting: Determination of the kinetics of a photodimerization within a single crystal. Sci. Rep. 2015, 4, 6815. (e) Liu, D.; Ren, Z. G.; Li, H. X.; Lang, J. P.; Li, N. Y.; Abrahams, B. F. SingleCrystal-to-Single-Crystal Transformations of Two Three-Dimensional Coordination Polymers through Regioselective [2 + 2] Photodimerization Reactions. Angew. Chem., Int. Ed. 2010, 49, 4767−4770. (5) (a) Nath, N. K.; Panda, M. K.; Sahoo, S. C.; Naumov, P. Thermally induced and photoinduced mechanical effects in molecular single crystalsa revival. CrystEngComm 2014, 16, 1850−1858. (b) Medishetty, R.; Husain, A.; Bai, Z.; Runcevski, T.; Dinnebier, R. E.; Naumov, P.; Vittal, J. J. Single crystals popping under UV light: a photosalient effect triggered by a [2 + 2] cycloaddition reaction. Angew. Chem., Int. Ed. 2014, 53, 5907−5911. (c) Sun, J. K.; Li, W.; Chen, C.; Ren, C. X.; Pan, D. M.; Zhang, J. Photoinduced bending of a large single crystal of a 1,2-bis(4-pyridyl)ethylene-based pyridinium salt powered by a [2 + 2] cycloaddition. Angew. Chem., Int. Ed. 2013, 52, 6653−6657. (6) (a) Medishetty, R.; Koh, L. L.; Kole, G. K.; Vittal, J. J. Solid-state structural transformations from 2D interdigitated layers to 3D interpenetrated structures. Angew. Chem., Int. Ed. 2011, 50, 10949− 10952. (b) Sun, J. K.; Tan, B.; Cai, L. X.; Chen, R. P.; Zhang, J.; Zhang, J. Polycatenation-driven self-assembly of nanoporous frameworks based on a 1D ribbon of rings: regular structural evolution, interpenetration transformation, and photochemical modification. Chem. - Eur. J. 2014, 20, 2488−2495. (7) Hutchins, K. M.; Rupasinghe, T. P.; Ditzler, L. R.; Swenson, D. C.; Sander, J. R. G.; Baltrusaitis, J.; Tivanski, A. V.; MacGillivray, L. R. Nanocrystals of a metal−organic complex exhibit remarkably high conductivity that increases in a single-crystal-to-single-crystal transformation. J. Am. Chem. Soc. 2014, 136, 6778−6781. (8) Chung, J. W.; You, Y.; Huh, H. S.; An, B. K.; Yoon, S. J.; Kim, S. H.; Lee, S. W.; Park, S. Y. Shear- and UV-induced fluorescence switching in stilbenic π-dimer crystals powered by reversible [2 + 2] cycloaddition. J. Am. Chem. Soc. 2009, 131, 8163−8172. (9) (a) Yamada, S.; Uematsu, N.; Yamashita, K. Role of Cation-π Interactions in the Photodimerization of trans-4-styrylpyridines. J. Am. Chem. Soc. 2007, 129, 12100−12101. (b) Yamada, S.; Tokugawa, Y. Cation-π controlled solid-state photodimerization of 4-azachalcones. J. Am. Chem. Soc. 2009, 131, 2098−2099. (10) (a) Atkinson, M. B. J.; Bucar, D. K.; Sokolov, A. N.; Friscic, T.; Robinson, C. N.; Bilal, M. Y.; Sinada, N. G.; Chevannes, A.; MacGillivray, L. R. General application of mechanochemistry to templated solid-state reactivity: rapid and solvent-free access to crystalline supermolecules. Chem. Commun. 2008, 5713−5715. (b) Nagarathinam, M.; Peedikakkal, A. M. P.; Vittal, J. J. Stacking of double bonds for photochemical [2 + 2] cycloaddition reactions in the solid state. Chem. Commun. 2008, 5277−5288. (c) Peedikakkal, A. M. P.; Koh, L. L.; Vittal, J. J. Photodimerization of a 1D hydrogen-bonded zwitter-ionic lead(II) complex and its isomerization in solution. Chem. Commun. 2008, 441−443. (d) Sokolov, A. N.; Bucar, D. K.; Baltrusaitis, J.; Gu, S. X.; MacGillivray, L. R. Supramolecular catalysis

CONCLUSIONS In summary, we have successfully incorporated photoactive DBCbpe molecules into 3D frameworks to give 1-NO3− and 2Br‑ by molecular self-assembly, in which the two compounds show similar coordination linkages but different ligand alignment orientations upon varying anions. Compound 1NO3− displays yellow luminescence after light irradiation owing to the occurrence of photocycloaddition, while compound 2Br− shows remarkable photocoloration from yellow to purple accompanied by single-crystal-to-single-crystal structural transformation into a new species with good photochromic reversibility and high fatigue resistance. The current work not only provides an example to show the importance of anions in the construction of MOFs but also gives insight into the separation of cycloaddition and photochromism of MOFs based on 4,4′-bpe pyridinium ligands.

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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.inorgchem.7b01497. Additional structural and spectral analyses (PDF) Accession Codes

CCDC 1553016−1553018 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 Author

*E-mail: [email protected]. ORCID

Jie Zhang: 0000-0002-6195-8525 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the NNSF of China (Grants 21573016, 21403241, and 21271173) and the NSF of Fujian Province (Grant 2014J01066). The authors gratefully acknowledge Prof. Xiao-Ying Huang for his helpful suggestions on structure refinement.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.7b01497 Inorg. Chem. 2017, 56, 10529−10534

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DOI: 10.1021/acs.inorgchem.7b01497 Inorg. Chem. 2017, 56, 10529−10534