Influence of Coordination on Photochromism - ACS Publications

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Two New Coordination Compounds with a Photoactive PyridiniumBased Inner Salt: Influence of Coordination on Photochromism Pei-Xin Li, Ming-Sheng Wang, and Guo-Cong Guo Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.6b00195 • Publication Date (Web): 04 May 2016 Downloaded from http://pubs.acs.org on May 5, 2016

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Crystal Growth & Design

Two New Coordination Compounds with a Photoactive Pyridinium-Based Inner Salt: Influence of Coordination on Photochromism Pei-Xin Li, Ming-Sheng Wang,* Guo-Cong Guo* 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. E-mail: [email protected] (G.C. Guo); [email protected] (M.S. Wang) KEYWORDS: Coordination compounds; Electron transfer photochromism; Impact factor; Luminescence switch

ABSTRACT: The past years have evidenced the rapid development of photochromic coordination compounds; however, impact of coordination on photochromic behavior of organic dyes has never been explored in the pyridinium derivative photochromic system. In this work, two new coordination compounds with a photoactive pyridinium-based inner salt, [Zn(H2O)6](PTA)·(CEbpy)2·2H2O (1, PTA = terephthalate, CEbpy = 1-carboxyethyl-4,4’bipyridine) and [Zn(H2O)2(CEbpy)2]nBr2n·[Zn(H2O)4(PTA)]n (2), were selected as model compounds for this purpose. Compound 1 features an isolated structure, where uncoordinated photoactive CEbpy ligands connect to hexahydrated zinc ions through hydrogen bonds.

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Compound 2 features a 1-D chain structure with CEbpy ligands coordinating to zinc ions. Compound 1 shows faster coloration speed upon irradiation than 2, demonstrating that coordination of the electron donor in CEbpy is not in favor of photochromic behavior. Both compounds show significant photoluminescence quenching after coloration, and the intensity contrast before and after coloration for 1 is larger than that for 2. This finding will help to design and synthesize new photochromic compounds with high performance.

■ INTRODUCTION Photochromic materials is of great importance because of their appealing applications in various fields.1,2 In the past decades, much effort has been paid in the families of organic photochromic compounds to perform their best merits, because they are easily modified by careful design of functional groups.3,4 Among the various types of organic photochromic dyes, those based on photoinduced electron-transfer (redox) chemicals, such as the typical pyridinium derivative system, have attracted more and more interesting not only for their applications in such fields as catalysis and optic, electric, or magnetic switch,5 but also their compatibility to solid-state media owing to the tiny configurational transition between two stable states.6 As for the pyridinium derivative system, the insight into structure–property relationships has been focused on the electronic structures of electron donors/acceptors and the packing structure (distance, angle, etc.) between electron donors and electron acceptors.7 Incorporation of such photochromic dyes into metal complexes as ligands has also been found to offer rich properties.8 The Zhang group et al. found that the types of metal centers can influence the photosensitivity of complexes.9 However, impact of coordination of the dyes themselves on photochromic behavior has never been explored.


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During the study of structure–property relationships for electron-transfer (ET) photochromic species,10

we

obtained

two

new

crystalline

coordination

compounds,

[Zn(H2O)6](PTA)·(CEbpy)2·2H2O (1, CEbpy = 1-carboxyethyl-4,4’-bipyridine, PTA = terephthalate) and [Zn(H2O)2(CEbpy)2]nBr2n·[Zn(H2O)4(PTA)]n (2). Both compounds have the same space group P1 and similar packing structures but different coordination modes of the photoactive CEbpy ligand, and thus were selected to study the impact of coordination on photochromic behavior in this work. It was found that compound 1 with isolated CEbpy showed faster coloration speed than compound 2 with coordinating CEbpy. Photomodulated photoluminescence offers a promising tool for data storage,11a bioimaging11b and barcode11c because of high sensitivity, selectivity and spatial information of photoluminescence. Both 1 and 2 show significant photoluminescence quenching after coloration and the intensity contrast before and after coloration for 1 is larger than that for 2. Herein, we report their syntheses, crystal structures, and photochromic and photoluminescence properties. ■ RESULTS AND DISCUSSION


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Figure 1. For 1: a) ball-and-stick graph representation of a selected unit; b) 2-D hydrogenbonding layer viewed along the b-axis; c) packing structure viewed along the a-axis. Hydrogen bonds in the 2-D hydrogen-bonding layers and the hydrogen-bonding rings were connected with green dash lines, while others dark green dash lines. Symmetry code: A, –x, 2–y, –z. Structure Description. As shown in Figure 1, PTA and CEbpy in 1 are all isolated without coordination to zinc(II) ions. The asymmetric unit contains one zinc ion, one CEbpy, half one PTA, and three coordinated (O1W, O2W, O3W) and one lattice (O4W) water molecules. Each zinc ion locates at a centre of inversion and is six-coordinated by six water molecules. The hexahydrated zinc ions bond to PTA through O–H···O hydrogen bonds (O1W···O3, 2.694(7) Å;


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O2W···O4, 2.653(6) Å), to produce a 1-D hydrogen-bonding chain. These chains are further connected by O3W through O–H···O hydrogen bonds (O···O, 2.899(7) Å) to yield a 2-D hydrogen-bonding layer (Figure 1b). Two CEbpy units and two lattice water molecules are held together by O–H···O (O1···O4W, 2.831(7) Å) and O–H···N (O4W···N1, 2.849(9) Å) bonds to generate a hydrogen-bonding ring (Figure 1a). π···π stacking interaction exists between two pyridine groups from two CEbpy ligands in the ring with the nearest atom-to-atom distance of 3.236(2) Å. Finally, the 2-D hydrogen-bonding layers are pillared by the hydrogen-bonding rings through O–H···O bonds (O1W···O2, 2.670(2) Å; O2W···O1, 2.738(2) Å; O3W···O2, 2.823(2) Å) to form the whole 3-D crystal structure (Figure 1c).


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Figure 2. For 2: a) ball-and-stick graph representation of a selected unit; b) 2-D hydrogenbonding layer viewed along the b-axis; c) packing structure viewed along the a-axis. Hydrogen bonds in 2-D hydrogen-bonding layers were connected with green dash lines, while others dark green dash lines. Symmetry code: B, –x, –y+1, –z. When zinc(II) bromide is used instead of Zn(NO3)2·6H2O in the preparation, compound 2 with a different 3-D supramolecular structure is isolated. The structure of 2 features two types of 1-D


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chains, i.e. –[Zn(H2O)4(PTA)]n– and –[Zn(H2O)2(CEbpy)2]n– (Figure 2), in which PTA and CEbpy all coordinate to zinc ions. There are two zinc(II) ions, one CEbpy, half one PTA and three coordinated (O1W, O2W, O3W) water molecules in the asymmetric unit of 2. The Zn1 ion locates at a centre of inversion and is six-coordinated by two water molecules, two oxygen atom and two nitrogen atoms from four CEbpy ligands, respectively. Comparing with those in 1, two CEbpy units in 2 are linked by Zn1 ions through the coordination bonds (Zn–O, 2.078(4)– 2.197(5) Å) instead of the water molecules through hydrogen bonds, yielding a 1-D – [Zn(H2O)2(CEbpy)2]n– chain along the c axis. The nearest atom-to-atom distance between two pyridine groups from two adjacent CEbpy ligands is of 3.543(1) Å, implying the presence of

π···π stacking interaction. The Zn2 ion also locates at a centre of inversion but is sixcoordinated by four water molecules and two oxygen atoms from two PTA ligands. They are bridged by PTA with the coordination bond (Zn–O, 2.031(4)–2.171(5) Å) to form a 1-D – [Zn(H2O)4(PTA)]n– chain along the c axis. These chains are further connected by Br ions through O1W–H···Br hydrogen bonds (O···Br, 3.262(6) and 3.362(6) Å) to yield a 2-D hydrogen-bonding layer (Figure 2b). Finally, such 2-D hydrogen-bonding layers are bridged by the 1-D –[Zn(H2O)2(CEbpy)2]n– circulating chains through O–H···O bonds (O···O, 2.720(7) Å) and O3W–H···Br hydrogen bonds (O···Br, 3.291(5) Å) to form the whole 3-D crystal structure (Figure 2c).


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Figure 3. Photochromism of 1 (top) and 2 (bottom) in air.

Figure 4. Time-depended UV-vis absorption spectra of 1 (top) and 2 (bottom) upon photoirradiation at ambient environment.


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Figure 5. ESR spectra of 1 and 2 before and after coloration at ambient environment. Compound 1 easily undergoes a photoinduced color change at ambient environment. As shown in Figure 3, rod-like colourless crystals of 1 turned blue after irradiation by an 8 W energy saving lamp (480 lm/w). Concomitantly, two new broad structured bands peaked at ∼403 and 609 nm raised gradually with the duration of irradiation in the UV-visible (UV-vis) absorption spectrum (Figure 4). An electronic spin resonance (ESR) study of the photoproduct (hereafter named as 1P) showed that a symmetric radical signal with a g value of 2.0036 emerged (Figure 5), closing to that of a free electron found at 2.0023. An obvious decolouration happened when 1P was kept in the dark at ambient environment for at least half a year, and was supported by the obvious weakening of the absorption intensity (Figure S3, Supporting Information). Such slow reversible transformation shows compound 1 has a photoinduced long lived charge-separated state, which was regarded to be important for conversion from solar energy to chemical energy.12 The widely used heat-treatment method for accelerating the decoloration of photochromic samples was not suitable for 1P because of the easy loss of lattice water molecules. Compound 2 also turned blue after photoirradiation by the same energy saving lamp at ambient environment (Figure 3). Two new structured absorption bands peaked at ∼404 and 603 nm


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appeared in the UV-vis absorption spectrum (Figure 4) of the photoproduct (hereafter named as 2P). The ESR signal at g = 2.0028 for 2P demonstrates the generation of radicals (Figure 5). The new absorption bands were weakened obviously after annealing at 100 °C for 2 h in air (Figure S3). The photoinduced coloration–decoloration processes are reversible. In one CEbpy ligand, the nearest distance between the oxygen and nitrogen atoms of the pyridinium ring is approximately 2.73 Å for 1 and 2.66 Å for 2. In comparison, the nearest distance between the oxygen atom from PTA and the nitrogen atom of the pyridinium ring in CEbpy is about 4.127(2) Å for 1 and 4.436(2) Å for 2, and that between the bromine atom and the nitrogen atom of the pyridinium ring is about 4.068(1) Å for 2. A distance above 4 Å are not favorable for electron transfer.6 The peak positions for the radical products of 1 and 2 in the UVvis absorption spectra (Figure 4) are similar to those for CEbpy·3H2O, which locate at around 401 and 607 nm (Figure S1), and the known viologen compounds.13 Therefore, the photoinduced electron transfer should mainly occur in CEbpy.


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Figure 6. Solid state first order rate plot of coloration upon photoirradiation of 1 (top) and 2(bottom) at ambient environment. A linear fit of the time-dependent UV-vis absorption data monitored at 607 nm for CEbpy·3H2O (Figure S1), 609 nm for 1 and 603 nm for 2 (Figure 6) indicates that their photoinduced electron-transfer processes follow first-order reaction kinetics. The kinetic rate constants are determined qualitatively by the literature calculation method.14 The following equation is used for data treatment: ln / = Kt


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where A0, At, are the observed absorption data at the beginning, versus time, and at the end of the reaction, respectively. The simulated rate constants (kobs) of CEbpy·3H2O, 1 and 2 are ∼1.58 × 10−3, 1.67 × 10−3 and 1.22 × 10−3 s−1, respectively. Clearly, the coloration speed of 1 is closer to that of CEbpy·3H2O but clearly faster than that of 2. As mentioned above, the nearest distance between the oxygen and nitrogen atoms of the pyridinium ring in the same CEbpy ligand for 1 (2.73 Å) is slightly longer than that for 2 (2.66 Å). In addition, the 4,4’-pyridine group of the CEbpy ligand, i.e. the electron acceptor, is coordinated by the electron-withdrawing Zn atom in 2. It seems that the coloration speed of 1 should be smaller than that of 2, but the fact is opposite. An obvious difference between the crystal structures of 1 and 2 is that the carboxylate groups, as electron donors, in the CEbpy ligands are isolated in the former but coordinated by the Zn ions in the latter. The coordination may significantly weaken the electron donating ability of carboxylate group. This should be the major reason why the coloration of 1 is faster than that of 2. We have performed an ESR testing for the spin concentration of 1P and 2P with same moles after irradiation for 40 min by an 8 W energy saving lamp (480 lm/w) at ambient environment. The spin concentration is ∼6.8 × 1017 spins/mol for 1P and 9.3 × 1016 spins/mol for 2P. Theoretically, one mole 1 and 2 could generate 4 mole radicals (∼2.41 × 1024 spins/mol), given all the CEbpy ligands were involved in the photoreaction. Clearly, the spin concentration of 1P and 2P is significantly smaller than the theoretical values. This is probably ascribed to the common phenomenon, that is, photochromic reactions mainly happen in the surfaces of crystals.15


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Figure 7. In situ time-dependent PL spectra of 1 (λex = 465 nm) and 2 (λex = 280 nm) in the solid state at room temperature. The utilization of photochromism to photosmodulate photoluminescence (PL) has aroused intense interests in the past years. We found that compounds 1 and 2 exhibit clear PL quenching after coloration. The solid-state PL spectra recorded at ambient environment show broad emission bands centered at 390, 440 nm for 1 (λex = 340 nm) and 385, 420 nm for 2 (λex = 280 nm), respectively (Figure 7). H2PTA and CEbpy·3H2O display PL emission with peak values at 380 nm (λex = 320 nm) and 425 nm (λex = 280 nm), respectively (Figure S5). By comparing the locations and profiles of emission peaks, the PL of 1 and 2 can be tentatively attributed to the combination of ligand-self emission from PTA and CEbpy. In situ PL studies showed that the PL


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intensity went down dramatically with an increase of irradiation time and reached ∼42.98% for 1 and 62.1% for 2 of the starting value after illumination for 40 min by the 8 W energy saving lamp, respectively (Figure 7). The PL quenching was accompanied by the deepening of colors of the samples. Such phenomenon is common for pyridinium derivative-based compounds (such as viologens) that can undergo ET photochromism.16 The different PL intensity decrease is also corresponding to the coloration processes in 1 and 2, which further supports that ET is significantly affected by the coordination of the electron-donating group of the photoactive ligand. ■ CONCLUSIONS In summary, this work has revealed the photochromic and photomolulated luminescence properties of two new photochromic compounds with the same space group and similar packing structures but different coordination modes of the photoactive ligand. Structure and spectroscopy analyses showed that coordination of electron-donating groups does not facilitate to photochromism of photochromic dyes. This finding will help to design and synthesize new photochromic compounds with high performance. ■ EXPERIMENTAL SECTION Materials. ZnBr2, Zn(NO3)2·6H2O, terephthalic acid (H2PTA), 4,4′-bipyridine, and 2BrCH2COOH in AR grade were purchased commercially and used without further purification. Water was deionized and distilled before use. A 0.1 mol/L aqueous solution of Na2(PTA) used for the syntheses of 1 and 2 was obtained by reaction of H2PTA (0.16 g, 1 mmol) and NaOH (0.08 g, 2 mmol) with the ratio of 1: 2 in 10 mL water. The ligand CEbpy was obtained as a hydrated compound CEbpy·3H2O according to the same procedure reported in the literature.17


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Physical Measurements. UV−vis absorption spectra were recorded at room temperature in the reflectance diffuse mode on a PerkinElmer Lambda 900 UV/vis/NIR spectrophotometer equipped with an integrating sphere and BaSO4 plates as a reference (100% reflection). Powder X-ray diffraction (PXRD) patterns were collected on a Rigaku MiniFlex II diffractometer powered at 30 kV and 15 mA using Cu Kα (λ = 1.54056 Å). The simulated patterns were produced

using

the

Mercury

Version

1.4

software

(http://www.ccdc.cam.ac.uk/products/mercury/) and single crystal reflection diffraction data. ESR spectra were recorded on a Bruker ER-420 spectrometer with a 100 kHz magnetic field in X band at room temperature. X-ray Crystallographic Study. X-ray diffraction measurement of 1 and 2 were performed on a Rigaku SATURN70 CCD diffractometer using graphite monochromated Mo Kα radiation (λ = 0.71073 Å). Intensity data sets were collected using an ω scan technique, and corrected for Lp effects. The primitive structure of 1 and 2 were solved by the direct method using the Siemens SHELXTL Version 5 package of crystallographic software.18 Difference Fourier maps based on these atomic positions yielded other nonhydrogen atoms. The final structures were refined using a full-matrix least-squares refinement on F2. All non-hydrogen atoms were refined anisotropically. Hydrogen atoms on carbon atoms were generated geometrically, while others were included from difference Fourier maps. The entries of CCDC-1446793 and 1446794 contain the supplementary crystallographic data for

1

and

2.

These

data

can

be

obtained

free

of

charge

at

http://www.ccdc.cam.ac.uk/conts/retrieving.html or from the Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, U.K. Fax: (Internet) +44-1223/336-033. Email: [email protected].


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Synthesis of [Zn(H2O)6](PTA)·(CEbpy)2·2H2O (1). Compound 1 was obtained by a solution reaction of Zn(NO3)2·6H2O (44.6 mg, 0.15 mmol), the 0.1 mol/L Na2(PTA) solution (1.5 mL, 0.15 mmol), and CEbpy·3H2O (86 mg, 0.3 mmol) in 6 mL of water in the ratio of 1:1:2. The solution was filtered after stirring for 10 min and allowed to stand in the dark for several days to produce colourless rod-like crystals of 1 (ca. 45% yield based on Zn). The phase purity of its crystalline sample was checked by PXRD (Figure S2) and an elemental analysis. Calcd (%) for compound 1: C, 47.92; H, 4.99; N, 6.9. Found (%): C, 47.92; H, 4.93; N, 6.8. Synthesis of [Zn(H2O)2(CEbpy)2]nBr2n·[Zn(H2O)4(PTA)]n (2). Typically, the reaction of ZnBr2 (45 mg, 0.2 mmol), the 0.1 mol/L Na2PTA solution (1 mL, 0.1 mmol), and CEbpy·3H2O (86 mg, 0.3 mmol) in 6 mL of water yielded a pale yellow solution, which was filtered and allowed to stand in the dark for several days to produce light yellow prism crystals of 2 (ca. 60% yield based on Zn). The phase purity of its crystalline sample was checked by PXRD (Figure S2) and an elemental analysis. Calcd (%) for compound 2: C, 38.79; H, 3.67; N, 5.65. Found (%): C, 39.15; H, 3.82; N, 5.58. ■ ASSOCIATED CONTENT Supporting Information. Figures giving the color of CEbpy·3H2O, PXRD patterns of 1 and 2, UV-vis absorption spectrum of 1 and 2 before, after irradiation and the decoloration under ambient conditions, UVvis absorption spectrum of CEbpy·3H2O before and after irradiation under ambient conditions, PL spectra of H2PTA and CEbpy·3H2O, thermogravimetric analysis curves of 1 and 2. This material is available free of charge via the Internet at http://pubs.acs.org. ■ AUTHOR INFORMATION Corresponding Authors


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* (M.S. Wang) E-mail: [email protected]. *(G.C. Guo) E-mail: [email protected]. Notes The authors declare no competing financial interest. ■ ACKNOWLEDGMENTS We gratefully acknowledge the financial support by 973 program (2013CB933200), the NSF of China (21373225, 21521061, 21471149), the NSF of Fujian Province (2014J07003, 2014J01065), and Youth Innovation Promotion Association, CAS. ■ REFERENCES (1) Yildiz, I.; Deniz, E.; Raymo, F. M. Chem. Soc. Rev. 2009, 38, 1859. (2) Wang, M.-S.; Xu, G.; Zhang, Z.-J.; Guo, G.-C. Chem. Commun. 2010, 46, 361. (3) Irie, M.; Fukaminato, T.; Sasaki, T.; Tamai, N.; Kawai, T. Nature 2002, 420, 759. (4) Dong, H.-L.; Zhu, H.-F.; Meng, Q.; Gong, X.; Hu, W.-P. Chem. Soc. Rev. 2012, 41, 1754. (5) (a) Zahavy, E.; Willner, I. J. Am. Chem. Soc. 1996, 118, 12499. (b) Bechinger, C.; Ferrere, S.; Zaban, A.; Sprague, J.; Gregg, B. A. Nature 1996, 383, 608. (c) Jhang, P. C.; Chuang, N. T.;Wang, S. L. Angew. Chem., Int. Ed. 2010, 49, 4200. (d) Cai, L.-Z.; Chen, Q.-S.Zhang, C.-J.; Li, P.-X.; Wang, M.-S.; Guo, G.-C. J. Am. Chem. Soc. 2015, 137, 10882. (6) (a) Xu, G.; Guo, G.-C.; Wang, M.-S.; Zhang, Z.-J.; Chen, W.-T.; Huang, J.-S. Angew. Chem. Int. Ed. 2007, 46, 3249. (b) Wang, Y.-Z.; Li, H.-L.; Wu, C.; Yang, Y.; Shi, L. Wu, L.-X. . Angew. Chem. Int. Ed. 2013, 52, 4577. (7) (a) Sun, J.-K.; Jin, X.-H.; Cai, L.-X.; Zhang, J. J. Mater. Chem. 2011, 21, 17667. (b) Álvaro, M.; Ferrer, B.; Fornés, V.; García, H. Chem. Commun. 2001, 2546. (b) Park, Y. S.; Um, S. Y.;


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Table of Contents Graphic and Synopsis

Two new photochromic coordination compounds with a photoactive pyridinium-based inner salt are presented. Research on the impact of coordination on photochromic behavior of organic dyes showed that electron-transfer speed for the compound with isolated photoactive ligands (top) is faster than that for the compound with coordinating photoactive ligands (bottom). Photomodulated fluorescent quenching speed agrees with this result.


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Influence of Coordination on Photochromism - ACS Publications

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