Metal-dependent chromic properties of three isostructural 1D coordination polymers based on 1-(2-carboxyethyl)-4,4′-bipyridinium ligand
Introduction
As multifunctional materials with different structures, CPs (coordination polymers) can be used to cater for the increasing demands in the fields of energy storage, gas storage and catalysis, and so on [1][1], [1](a), [1](b), [1](c), [1](d). In recent years, as one of the most common and useful species, CPs with chromic properties have attracted tremendous attention [2][2], [2](a), [2](b), [2](c), [2](d). CPs have gradually become an interesting platform for designing chromic materials, which can be endowed with various responsive properties by functionalizing metal-based joints, organic and inorganic units, pore spaces or by utilizing the synergy between the components [3][3], [3](a), [3](b), [3](c), [3](d). In recent years, the development of photochromic inorganic-organic CPs has attracted increasing attention. It is conceivable that the introduction of active components into the CPs may not only incorporate functionality from both organic and inorganic units, but also provide the opportunity to produce new properties by the synergic interactions between these components [4][4], [4](a), [4](b), [4](c), [4](d).
Electron-deficient functional 4,4′-bipyridinium derivatives (viologens) [5][5], [5](a), [5](b), [5](c) are known to be a class of excellent candidates of organic ligands for construction of chromic CPs due to the electron transfer (ET) leading to the formation of organic radicals and the controllability of the substituent group on the pyridine N. Up to now, many viologen-based CPs have been reported as chromic compounds with excellent responsive properties by using viologen derivatives as ligands [6][6], [6](a), [6](b), [6](c). Hence, an effective strategy to establish the chromic CPs is the incorporation of viologen electron acceptors (D) and other electron donors (A) into structure through self-assembly [7][7], [7](a), [7](b), [7](c). As an electron donor component, 1,2,4,5-benzenetetracarboxylic acid (H4BTEC) has been widely used in the synthesis of CPs, because it has various valence states due to deprotonation of carboxyl groups [8][8], [8](a), [8](b). It is widely reported that many factors, such as the packing type and orientation between D-A units, the distance of the pathway for ET, suitable donors and acceptors, can influence the chromic properties [9][9], [9](a), [9](b), [9](c). In addition, the metal cations can also play a significant role in tuning the chromic properties. However, to the best of our knowledge, limited systematic investigation of the relationship between the metal cations and the chromic properties of viologen-based CPs has been reported hitherto [[10]]. It will be interesting if metal cations can be tuned to adjust the different responsive properties.
An understanding of the effect of metal cations on the chromic properties may therefore aid in the rational design of novel chromic CPs. For this purpose, comparison of a series of related viologen-based chromic CPs with the same structure containing different metal cations is required. In this study, we select a viologen-like ligand 1-(2-carboxyethyl)-4,4′-bipyridinium (CBbpy) with terminal N and O atoms simultaneously serving as coordination sites and H4BTEC as electron donors to build the same structure with different metal cations. Fortunately, we successfully synthesize three new isostructural viologen-based CPs, {[TM0.5(CBbpy)(H2O)][TM(H2O)6]0.5(H2BTEC)·H2O}n (TM = Zn (1), Ni (2) and Co (3)), which exhibit different chromic properties due to the different metal cations. The crystal structures, photochromism and hydrochromism of the three compounds, as well as the modulated effect of metal cations, will be represented and discussed in this paper.
Section snippets
Materials and methods
All chemicals used in the synthesis were purchased from commercial sources without further purification. 1-(2-Carboxyethyl)-4,4′-bipyridinium bromide (HCBbpyBr) was synthesized according to the literature [11]; X-ray diffraction data (XRD) was collected on an Oxford Gemini diffractometer at 293 K using graphite monochrome Mo-Kα (λ = 0.71073); X-ray powder diffraction (PXRD) pattern was collected using a Rigaku Ultima IV-185 diffractometer; using Vario EL III CHNOS elemental analyzer for
Structural description
Compounds 1–3 are isostructural, 1 is discussed in detail. Single crystal X-ray diffraction data shows that compound 1 crystallizes in the triclinic space group P-1 and features a one-dimensional (1D) framework structure. The asymmetric unit consists of one Zn2+ cation, one CBbpy ligand, one charge-balancing anion (H2BTEC)2-, four coordinated and one lattice waters. Both pyridine rings of the CBbpy ligand are not coplanar, which makes a dihedral angle of 27.51°. There are moderate π···π
Conclusions
In summary, we have synthesized three isostructural 1D coordination polymers based on a viologen-carboxylate ligand 1-(2-carboxyethyl)-4,4′-bipyridinium bromide (HCBbpyBr), 1,2,4,5-benzenetetracarboxylic acid (H4BTEC) and different metal cations. Due to the different electron-withdrawing capabilities of metal cations, three compounds exhibit different photosensitive properties. The progressive decrease in the photosensitive properties observed for the three CPs with a decrease in the
Declaration of competing interest
The authors declare that they have no conflicts of interest to this work.We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.
Acknowledgements
This work is supported by the Characteristic Discipline of Chemistry in 1331 Projects of Shanxi. There are no conflicts to declare.
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