Multifunctional naphthalene diimide-based coordination polymers: Photochromism and solventchromism
Graphical abstract
Three novel NDI-based coordination polymers were prepared by solvothermal reaction, which exhibit reversible photochromic properties and solvatochromic behaviour in the presence of different solvents.
Introduction
As a class of breakthrough materials, coordination polymers (CPs) have received increasing attention owing to their fascinating structures, designable networks, and various potential applications in catalysis, photoluminescence, chemical sensing, gas storage, and conductivity [1][1], [1](a), [1](b), [1](c), [1](d), [1](e), [1](f), [1](g), [1](h), [1](i). As a new aspect, photochromic CPs have become a hot research topic owing to their well-defined structure and potential application in various fields, such as optical memory, displays, sensors, switches, and erasable copy papers [2][1], [1](a), [1](b), [1](c), [1](d), [1](e), [1](f), [1](g), [1](h), [1](i), [2], [2](a), [2](b), [2](c), [2](d), [2](e), [2](f), [2](g). In general, the photochromic properties of the CPs are derived from either the structural transformation or electron transfer of photoactive organic ligands, in which the photoactive organic ligands can be chemically modified and anchored for their desirable specific properties to the resulting structures [3][3], [3](a), [3](b), [3](c), [3](e), [3](f). However, insight into structure-property relationships and guidance for the rational construction of CPs with adjustable photochromic properties still remains a challenge [4][4], [4](a), [4](b), [4](c), [4](d).
Naphthalenediimide derivatives (NDIs), an attractive class of π-electron-deficient, planar, highly redox-active dye with a substituent on the diimide nitrogen, are often considered as excellent candidates for the construction of photochromic CPs, as they can undergo single one-electron reduction forming colored organic radicals [5][5], [5](a), [5](b), [5](c), [5](d), [5](e), [5](f), [5](g), [5](h). NDI-based ligands with pyridyl substituent have multiple potential coordination sites, including terminal O and N atoms. This diverse coordination mode between the NDIs ligand and metal cations not only provides more opportunities for constructing various network structures, but also adjusts the photochromic properties of the resulting CPs [6][6], [6](a), [6](b), [6](c), [6](d). Up to now, much research has been made on designing and synthesizing the NDI-based compounds, exhibiting interesting luminescence and catalysis [7][7], [7](a), [7](b), [7](c), [7](d), [7](e), [7](f). There have been several examples involving NDI-based CPs reported, but their photochromic properties have rarely been investigated [[8], [8](a), [8](b), [8](c)]. Especially, almost no solvatochromic behaviour of NDI-based CPs has been reported.
Inspired from the crystal engineering and photochromic mechanism of electron-transfer CPs, we envisaged that it should be suitable to select electron-deficient NDIs as the organic ligands and electron-rich aromatic carboxylic acids as the secondary building linkers to construct photochromic CPs, and then further study the effect of secondary building unit on the photochromic properties of CPs. Hence, an NDI derivative- N,N′-bis(4-pyridylmethyl)-1,4,5,8-naphthalene diimide (DPMNI) and three different aromatic carboxylic acids were selected to construct three coordination polymers [Zn2 (DPMNI) (TPDC)2] (1), [Zn (DPMNI) (BPC)] (2), Zn (DPMNI) (HBTC) (3) (H2TPDC = thiophene-2,5-dicarboxylic acid, H2BPC = 4,4′-biphenyldicarboxylic acid, H3BTC = 1,3,5-benzenetricarboxylic acid, Schemes S1–S3). Compounds 1–3 all exhibit photochromism after irradiation with a Xe lamp and the resulting photoproducts were stable for several days. Notably, the different aromatic carboxylic acid linkers formed divergent supramolecular interactions (π–π and lone pair–π interactions), and thus significantly affected the structures and photochromic properties. The crystal structures, photochromism, and solvatochromism of these compounds, will be represented and discussed in this study.
Section snippets
Materials and measurements
All chemicals were purchased from commercial sources and used without further purification. Thermogravimetric analysis (TGA) was carried out using a Mettler Toledo TGA2 system analyzer under nitrogen atmosphere from 30 °C to 800 °C at a ramp rate of 10 °C/min. Elemental analyses (C, H and N) were carried out using a PerkinElmer 240C elemental analyzer. Powder X-ray diffraction (PXRD) patterns were recorded using a Rigaku Ultima IV diffractometer equipped with Cu Kα (λ = 1.5418 Å) radiation.
Results and discussions
Single-crystal X-ray diffraction analysis shows that compound 1 crystallizes in the monoclinic space group C2/c, and the asymmetric unit consists of a crystallographically independent Zn cation, one TPDC2− ligand, and one half of DPMNI ligand. As shown in Fig. 1a, the geometry at the Zn center is a distorted tetrahedron formed by three O atoms from three TPDC2− ligands with Zn–O distances of 1.952 (3)–1.997 (3) Å and one N atom from the DPMNI ligand with Zn–N distance of 2.012 (3) Å. The
Conclusion
In summary, three photochromic coordination polymers assembled from a NDI related ligand and different aromatic carboxylic acids were prepared and characterized. Interestingly, these three coordination polymers exhibit reversible photochromic properties upon irradiation in the solid state. Due to different π–π interactions between the NDI core and aromatic carboxylic acid, compounds 1–3 exhibit different photosensitive properties. Furthermore, compound 1 exhibits reversible solvatochromic
CRediT authorship contribution statement
Jian-Jun Liu: Writing - review & editing. Shu-Biao Xia: Writing - review & editing. Dan Liu: Writing - review & editing. Juying Hou: Writing - review & editing. Hongbo Suo: Writing - review & editing. Fei-Xiang Cheng: Writing - review & editing.
Declaration of competing interest
None.
Acknowledgements
This work has been supported by National Natural Science Foundation of China (21961030, and 51764048), Yunnan Province Thousand Youth Talents Plan, Program of Innovative Research Team (in science and technology) in University of Yunnan Province, and Application Basis Research Project of Yunnan Province Science and Technology Department (award No. 2017FD144).
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