The effects of molecular configuration on the photochromism and decolorization of two bipyridinium-based coordination polymers

    The effects of molecular configuration on the photochromism and decolourization of two bipyridinium-based coordination polymers Xiao-Dong Yang, Cheng Chen, Ya-Jun Zhang, Li-Xuan Cai, Jie Zhang PII: DOI: Reference:

S1387-7003(15)30036-8 doi: 10.1016/j.inoche.2015.07.022 INOCHE 6060

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Inorganic Chemistry Communications

Received date: Revised date: Accepted date:

19 June 2015 18 July 2015 25 July 2015

Please cite this article as: Xiao-Dong Yang, Cheng Chen, Ya-Jun Zhang, Li-Xuan Cai, Jie Zhang, The effects of molecular configuration on the photochromism and decolourization of two bipyridinium-based coordination polymers, Inorganic Chemistry Communications (2015), doi: 10.1016/j.inoche.2015.07.022

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The effects of molecular configuration on the photochromism and decolourization of two bipyridinium-based coordination polymers.

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Xiao-Dong Yangab, Cheng Chenb, Ya-Jun Zhangb, Li-Xuan Caib and Jie Zhangab* a

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College of Chemistry, Fuzhou University, Fuzhou, 350116, People’s Republic of China State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, People’s Republic of China

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* Corresponding author. Tel.: (+86) 591-83792871; fax: (+86) 591-83710051. E-mail addresses: [email protected]

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Abstract: Two layer-like coordination polymers, {[Cd(Bpyen)0.5(m-BDC)(H2O)Br]·4H2O}(1) and {[Cd(Bpyen)0.5(o-BDC)Br]·H2O}(2), have been solvothermally synthesized and structurally characterized. The two compounds own common rapid photochromic response but different decolourization behaviors. The photoproduct of compound 2 is more stable in air. The relationship between structure and photoresponsive behavior has been studied in detail.

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Keywords: Cadmium(II) complex; Bipyridinium; (6,3) topology; Photochromism; Decolourization; Single crystal

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In recent years, the photochromic coordination polymers have became a very hot research topic because of their well-defined crystalline networks and potential applications in a wide range of fields, such as photoswitchable biomaterials [1], photochromic decorations [2], displays [3], non-linear optics materials [4], and so on [5]. Compared with the simplex photochromic organic molecules, the coordination polymers can not only preserve the distinctive properties of the individual components in one complex, but also create neoteric properties by the synergetic interactions among the individual components [6]. An ever-increasing interest on this domain for scientists is to develop high-performance photoactive coordination polymers, but so far the insight into structure–property relationships and the rational design for constructing tunable photosensitive coordination polymers are still two difficult challenges [7]. Since the photochromic phenomenon was firstly discovered by Michaelis in 1932 [8], the viologen/4,4′-bipyridinium derivatives have been widely researched in adsorption/separation [9], supramolecular chemistry [10], and photochemistry [11]. Up to now, many efforts have been made on designing and synthesizing the bipyridinium-based coordination compounds with photochromism based on its photochemical activity [12]. One of the burning questions of the bipyridinium-based coordination polymers is how to construct a compound with rapid photoresponse and excellent reversibility though rational design and synthesis. In order to improve the present situation, a particular research on the relationship between the structure and the photochromic property is extremely necessary. Based on this, we attempt to introduce some auxiliary ligands with carboxylic groups into the bipyridinium-based coordination polymers to explore how the molecular packing influences the photochromic property because they may serve as electron donor to participate in photoinduced electron transfer reaction and can provide diverse molecular configurations and coordination modes [13]. Herein, we choose the m-H2BDC and o-H2BDC (m-H2BDC = 1,3-benzenedicarboxylic acid, o-H2BDC = 1,2-benzenedicarboxylic acid) as the auxiliary ligands to construct two two-dimensional (2D) layer-like coordination polymers, {[Cd(Bpyen)0.5(m-BDC)(H2O)Br]·4H2O}(1) and {[Cd(Bpyen)0.5(o-BDC)Br]·H2O}(2), via the molecular self-assembly with the bipyridinium-based ligand BpyenBr2 (BpyenBr2 = 1,2-bis(4,4'-bipyridinium)ethane dibromine). The two title compounds owning the same metal cation and bipyridinium-based ligand but different auxiliary ligands are perfect model compounds for exploring the key factors impacting on the photochromism properties. In this paper, we report the syntheses, crystal structures, and analysis of the relationship between structure and photochromism, decolourization of the two compounds in detail. [Insert Fig. 1 about here] The compound BpyenBr2 was synthesized following our previously reported literature [14]. Compound 1 and 2 were synthesized by the reaction of Cd(NO3)2·4H2O, BpyenBr2 and auxiliary ligand (m-H2BDC for 1, o-H2BDC for 2) under hydrothermal condition, as detailed in Supporting Information. Single crystal X-ray diffraction analysis revealed that the two compounds own a similar 2D laminar network with (6, 3) grid topological structure but obviously different spatial configurations. Compound 1 crystallizes in the monoclinic space group P21/c with one Cd2+ center, one half Bpyen2+ ligand, one m-BDC2– ligand, one Br–, one coordinated and four dissociative water molecules in the asymmetric unit. As shown in Fig. 1a, the Cd(II) ion is six-coordinated featuring a distorted octahedron bonded by one N atom from the Bpyen2+ ligand, three O atoms from two

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individual m-BDC2– ligands, one O atom from the coordinated water molecule and one Br– anion. The two carboxylate groups of the m-BDC2– ligand show two different coordination modes that one carboxylate is a monodentate while another one is a diatomic chelating group. Each Cd2+ ion coordinates with two μ2-m-BDC2– bridging ligands to form a 1D zigzag {[Cd(m-BDC)]}n chain, which is further connected by the Bpyen2+ ligands to form a 2D layer in the ab plane. Taking the Cd2+ as nodes and the ligands (Bpyen2+ and m-BDC2–) as connecting rods, the 2D grid layer can be considered to be a (6, 3) topological structure with hexagonal holes (Fig. 2a). There is no interpenetration in compound 1, the adjacent 2D layers pack in –ABCD– fashion along the c axis (Fig. S1a), which are interconnected through hydrogen bonds and π-π stacking to give a 3D framework, the geometrical parameters of hydrogen-bonds are enumerated in Tab. S1.

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[Insert Fig. 2 about here]

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Compound 2 is also a 2D layer-like coordination polymer crystallizing in the monoclinic space group P21/c. The asymmetric unit contains one Cd2+, one half Bpyen2+ ligand, one o-BDC2– ligand, one Br– and one lattice water molecule. Unlike compound 1, the Cd2+ in compound 2 is five-coordinated by one N atom from the Bpyen2+ ligand, three O atoms from two different o-BDC2– ligands and one Br– anion, as depicted in Fig. 1b. The five-coordinated geometry can be regarded as a square pyramid with the trigonality index τ = 0.0012 [15]. The bridging o-BDC2– and the Bpyen2+ ligands coordinate to the Cd2+ ion in a manner similar to compound 1 to form a 2D layer of a (6,3) net (Fig. 2b). Unlike the nearly coplanar layers in compound 1, the 2D laminar networks in compound 2 display a ladder-shaped configuration due to the high distortion between the carboxylate groups and the benzene ring of the o-BDC2– ligand (Fig. S1c, S1d). These 2D layers are further packed in –ABC– fashion along the c axis (Fig. S1b) via the hydrogen bonding and π-π stacking interactions to yield the whole crystal structure. The geometrical parameters of hydrogen-bonds are enumerated in Tab. S2. The powder X-ray diffraction patterns of the as-synthesized samples are in good agreement with the simulated ones from the single-crystal structure analysis, confirming the formation of pure phases (Fig. S4). The thermal stabilities of compound 1 and 2 have been evaluated by thermalgrametric analysis (TGA) under the air atmosphere in the temperature range 30-800°C. The TGA curve of compound 1 shows a weight loss of 14.46% from 30 to 115 °C, corresponding to the release of four free water molecules and one coordinated water molecule in the lattice (calcd: 14.61%), and then the framework collapses upon further heating (Fig. S3a). For compound 2, a weight loss of 3.71% is observed in the temperature from 30 to 178 °C corresponding to the release of one dissociative water molecule in the lattice (calcd: 3.33%) and then it begins to loss the components of the main framework (Fig. S3b).

[Insert Fig. 3 about here] [Insert Fig. 4 about here] Upon illumination by Xenon lamp, two compounds exhibit rapid photochromic behaviors. Compound 1 turns from light-yellow to green within 10 seconds while compound 2 changes from brown to dark-green within 5 minutes (Fig. 3). UV-vis diffuse reflectance spectrum of the

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compounds. In the two compounds, the carboxylate oxygen atoms of the BDC2 anions are situated at a location approximately perpendicularly to the N atom of the pyridinium core in the Bpyen2+ ligand with the O2···N2···C14, O3···N2···C14 angle of 103.43° and 78.65°, the O2···N2, O3···N2 distance of 3.17 Å and 3.83 Å in compound 1, (Fig. 5a); and the O1···N2···C6 angle of 112.78°, the O1···N2 distance of 3.29 Å (Fig. 5b) in compound 2. Such orientations and distances are thought to be favorable for the photoinduced electron transfer reaction [16-17].

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[Insert Fig. 5 about here]

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Although the two compounds can show quick color change upon light irradiation, the decolourization processes are obviously different. The photoproduct of compound 1 in crystalline state can be completely decolored within one day in the dark while the photochromic product of compound 2 needs about a week to be decolored. We can also observe the obviously different decolourization in the UV-Vis reflectance spectra of these two compounds. The characteristic absorption bands of Bpyen·+ radical of compound 2 is much stronger than those of compound 1. For compound 1 and 2, the decolourization originates from the oxidation process from Bpyen·+ to Bpyen2+ as observed commonly in viologen/4,4′-bipyridinium derivatives [14,16-17]. Based on this, we can rationally propose that the Bpyen·+ radicals in compound 1 are not as stable as that in compound 2 and can be easily oxidized to the initial Bpyen2+ state during the measurements of UV-Vis diffuse reflectance spectra due to increased exposure to atmospheric oxygen, thus leading to the weak signals. The different stabilities of the Bpyen·+ radicals may be attributed to the difference in molecular configurations and stacking of the Bpyen2+ ligands and auxiliary ligands in the two compounds. The dihedral angles between the two adjacent pyridine rings are 13.28°, 12.70° in compound 1 and 2, respectively. The better coplanarity of the adjacent pyridine rings in compound 2 can be a reason for the stability of the free radicals as found in the reported viologen/4,4′-bipyridinium derivatives [18]. Additionally, it is known that the π···π interactions among viologen/4,4′-bipyridinium derivatives can be an important factor to stabilize the generated radicals [19]. In compound 2, the interplanar angle and centroid distance between the adjacent pyridinium and pyridine rings is 12.70° and 3.87 Å, such weak π-π stacking interaction between the Bpyen2+ ligands is also favorable for the delocalization of electron and thus improve the stability of the free radicals (Fig. S2b). Whereas in compound 1, the π-π interaction mainly occurs between the pyridine ring of the Bpyen2+ ligand and the benzene ring of the m-BDC2 ligand with the interplanar angle and centroid distance of 2.31° and 3.66 Å (Fig. S2a), to disadvantage the delocalization of the pyridinium radical electron. As a result, the photoproduct of compound 2 is more stable while compound 1 is better reversible in the photochromic process. The current results reveal that the packing pattern of the photoactive centers can be adjusted by controlling the position of the substitution groups and thus enable the compounds to show different photochromic behaviors. In summary, we have synthesized and characterized two photochromic coordination polymers based on the bipyridinium derivative. Compounds 1 and 2 exhibit similar color development upon

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light irradiation, but different decolourization behaviors. Compound 1 is better reversible while the photoproduct of compound 2 is more stable in air. The influence of the molecular configuration and packing mode on the photochromism and decolourization process of the two compounds has been explored. These results not only give us a further insight into the influence of the structure on the photoresponsive behavior but also provide us a promising approach to construct a coordination polymer with different photochromic stability.

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Acknowledgements

This work was supported by the grants from the NNSF of China (Grant Nos. 21271173/20973171)

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Figure captions

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Fig. 1 The coordination environment of a mononuclear Cd 2+ ion in compound 1 (a) and compound 2 (b).

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All hydrogen atoms and lattice water molecules are omitted for clarity.

Fig. 2 The (6, 3) 2D laminar networks of compound 1 (a) and 2 (b).

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Fig. 3 UV-Vis diffuse-reflectance spectra and photographs showing the photoinduced color development

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of compound 1 (a) and 2 (b).

Fig. 4 The ESR spectra of compound 1 (a) and 2 (b) before (black line) and after irradiation (red line). The small ESR signal can be observed for the virgin samples due to ambient light-induced generation of radicals.

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Fig. 5 (a) The orientation diagram of m-BDC2 and Bpyen2+ in compound 1. (b) The orientation diagram

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of o-BDC2 and Bpyen2+ in compound 2.

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Graphical Abstract - Pictogram (for review)

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Graphical Abstract - Synopsis (for review)

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Two two-dimensional bipyridinium-based coordination polymers have been synthesized, and the structure–property relationship involving photochromism and decolourization has been studied detailedly.

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Highlights (for review) Two layer-like coordination polymers with bipyridinium-based ligand have been solvothermally synthesized and structurally characterized.

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The two compounds exhibit rapid photochromic response upon light irritation. The influences of the molecule packing pattern on the decolourization of the two compounds have been investigated in detail.

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