Anion−π Interaction-Directed Assembly of Polyoxometalate-Based

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Anion−π Interaction-Directed Assembly of Polyoxometalate-Based Host−Guest Compounds and Its Contribution to Photochromism Jian-Zhen Liao,†,‡ Xiao-Yuan Wu,† Jian-Ping Yong,† Hai-Long Zhang,†,‡ Wen-Bing Yang,† Rongmin Yu,† and Can-Zhong Lu*,† †

Key Laboratory of Design and Assembly of Functional Nanostructures, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, P. R. China ‡ Graduate University of Chinese Academy of Sciences, Beijing 100049, P. R. China S Supporting Information *

ABSTRACT: Two rare copper-based host−guest crystalline compounds were synthesized with different photoresponsive reversible visible light photochromism as a result of blending of distinctively different functional components, naphthalenediimides (NDIs) tectons and polyoxometalates. A slight adjustment of the substituent of the diimide nitrogens, from rigid to semirigid ligands, will result in different modes of anion−π interactions, which eventually lead to different structures and different sensitivities to visible light. The unique anion−π interactions impart significant impacts on the structures of the compounds and, to some extent, the ability to gain or lose electrons of the NDIs of the compounds, which is evident from subtle changes in photochromic sensitivity and reduction potentials; if compounds bear more and stronger anion−π interactions, this might dramatically enhance the compounds’ stability and diminish the π-electron-deficient property of the core naphthalene ring of the NDIs.

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INTRODUCTION Photochromic materials, whose instant property changes occur easily only by photoirradiation without tedious processing, have been attracting much attention, and their application in various optoelectronic devices, such as optical memory, photo-optical switching, and display, has been proposed. 1−6 Several conditions, including low fatigue (can be cycled many times while performance is maintained), rapid response, high sensitivity, and thermal stability, are very important for practical photochromic materials,1,7,8 and these conditions are the most often considered and studied problems for basic research into photochromic materials. In particular, the photoresponsive time (the rapid reaction) is crucially important under the given conditions.6 The cognition and comprehension of the influence factors on photoresponsive processes contribute to the fundamental understanding of the effect factors that control electron or energy transfer with photochromic compounds. Thus, detailed research into the structures of the photochromic compounds as well as the affecting factors of the photoresponsive time is particularly important for achieving the goal of designing practical photochromic materials. The system, by forming neoteric polyoxometalate (POM) anion−π interactions between the electron reservoirs (POMs)9 and π-electron-deficient naphthalenediimide (NDI) dye derivatives, not only can construct polyoxometalate-based host− guest supramolecular compounds by different functional modules but also can effectively realize quickly responsive reversible photochromism,10,11 but it is still unclear whether the © XXXX American Chemical Society

anion−π interactions affect the photoresponsive time of photochromic compounds. We envisioned whether it can adjust the substituent of the diimide nitrogens, from rigid to semirigid ligands, which might be able to form different anion−π interactions, and then control the different anion−π interaction sites, to observe the photochromic speed. Thus, through elaborate modification of the embedded photochromic molecules to change the noncovalent interaction sites of the self-assembling modules and the selected proper center metal Cu(II) cation, which is easy to coordinate with ligands bearing nitrogen, two rare supramolecular crystal compounds based on π-electron-deficient naphthalenediimide tectons and electronrich POMs molecules were synthesized with different photoresponsive reversible photochromism, which obviously derived from the photoinduced organic radicals of naphthalenediimide. These two new naphthalenediimide-based coordination compounds were formulated as [Cu2(DPNDI)2(H2O)7(OH)· ( P W 1 2 O 4 0 ) ] n ( c o m p o u n d 1 ) a n d [ Cu ( DP M NI ) (C 4 H 9 NO) 2 (CH 3 OH)·(HPW 12 O 4 0 )] n (compound 2) [DPNDI, N,N′-di(4-pyridyl)-1,4,5,8-naphthalenediimide; DPMNI, N,N′-bis(4-pyridylmethyl)-1,4,5,8-naphthalenediimide]. It should be pointed out that the electron-rich POM molecules were embedded into the coordination compounds through ingenious anion−π interactions and C−H···O contacts, Received: June 19, 2015 Revised: August 24, 2015

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structure. The distances of the C−H···anion interactions are similar to the reported values.10−12 Moreover, each POM was closed surrounded by four π-electron-deficient DPNDI tectons (Figure 1) through anion−π interactions and C−H···anion interactions to form a three-dimensional supramolecular framework structure (Figure 2a). Interestingly, the one-

which not only effectively immobilize and disperse the POMs but also promote the charge transfer or exchange among naphthalenediimide centers, coordinated other molecules, and POMs’ surface to yield the reversible photochromic materials with different response times. Furthermore, the anion−π interactions impart significant impacts on structures of the compounds and, to some extent, the ability of gaining or losing electrons of the NDIs of the compounds, which is evident from subtle changes in photochromism and reduction potentials; if compounds bear more and stronger anion−π interactions, this might dramatically enhance the compounds’ stability and diminish the π-electron-deficient property of the core naphthalene ring of the NDIs.

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RESULTS AND DISCUSSION Structure of Compound 1. Single-crystal X-ray analysis reveals that compound 1 crystallized in the I41/amd space group. The center Cu(II) cation that is located in the site with crystallographically imposed 4-fold symmetry is octahedrally coordinated by two nitrogen atoms from two bridging πelectron-deficient N,N′-di(4-pyridyl)-1,4,5,8-naphthalenediimide (DPNDI) tectons and four end-coordinated water molecules (Figure S1 of the Supporting Information). The other nitrogen atoms from the bridging DPNDI tectons, mentioned above, coordinated with neighboring copper ions to generate a one-dimensional chain structure. That is, one metal cation links to two tectons and every ligand bridges two metal centers. The Keggin polyanions ([PW12O40]3−) typically reside directly over the π-electron-deficient naphthalenic ring centroid [centroid distance of ∼2.8 Å (Figure 1)], which not only act as

Figure 2. (a) View of the three-dimensional framework structure of compound 1 along the c axis: Cu, turquoise; C, black; H, gray; O, red; N, blue; P, purple; W, teal; POMs, red and purple polyhedron. (b) Simplified graphic of the three-dimensional framework structure of compound 1.

dimensional chain was arranged alternately (Figure S2 of the Supporting Information). That is, horizontal rows were arranged after the vertical rows to generate square grids and form a one-dimensional channel along the c axis (Figure 2b), which were filled with electron-rich POMs. The fraction of volumes accessible for the inclusion of guest solvents molecules is 57.5% (calculated by PLATON). Several disordered solvent molecules filled in the void of compound 1. Direct evidence of the inclusion of the guest solvent molecules was obtained by thermogravimetric analyses, which showed expected losses of mass in the range of 30−100 °C. Structure of Compound 2. Single-crystal X-ray analysis reveals that compound 2 crystallized in the P21/n space group. The center Cu(II) cation is square-pyramidally coordinated by two nitrogen atoms from two bridging π-electron-deficient cisDPMNI [N,N′-bis(4-pyridylmethyl)-1,4,5,8-naphthalenediimide (DPMNI)] tectons, two N,N′-dimethylacetamide (DMA) molecules, and one end-coordinated methanol molecule (Figure S3 of the Supporting Information). The other nitrogen atoms from the bridging cis-DPMNI tectons, mentioned above, coordinated with neighboring copper ions to generate a one-dimensional wave-type chain structure. Similarly, the Keggin polyanions typically reside directly over the π-electron-deficient naphthalenic ring centroid of the cisDPMNI tectons, but each POM was only surrounded by two cis-DPMNI tectons (Figure 3). The dihedral angle between the DPMNI core and N-pyridyl groups is ∼89.18°. Combined with the middle methyl, it generates an appropriate orientation to allow pyridyl-Hs to form additional C−H···anion interactions (Figure 3) with encapsulated POM anions [dC···O = 3.484− 3.583 Å, and ∠C−H···O = 152.2−164.9° (see Table S2 of the Supporting Information)],10−12 which wrapped the POMs in the cis-DPMNI tectons. The intermolecular C−H···anion interactions between the coordinated dimethylacetamide molecules and POMs were also properly generated [dC···O = 3.525−3.687 Å, and ∠C−H···O = 152.2−164.9° (see Table S2 of the Supporting Information)].10−12 Furthermore, the intermolecular C−H···O hydrogen bonds between the coordinated dimethylacetamide molecules and adjacent cisDPMNI tectons were formed [dC···O = 3.280−3.394 Å, and ∠C−H···O = 125.3−130.6° (see Table S2 of the Supporting

Figure 1. Anion−π interactions (blue dashed line) and C−H···anion interactions (yellow dashed line) between DPNDI and POMs in 1: Cu, turquoise; C, black; H, gray; O, red; N, blue; P, purple; W, teal; POMs, red and purple polyhedron.

counterions but also serve as electron reservoirs. The dihedral angle between the DPNDI core and N-pyridyl groups is ∼71.60°, and this orientation allows pyridyl-Hs to form additional C−H···anion interactions (Figure 1) with encapsulated POM anions [dC···O = 3.359 Å, and ∠C−H···O = 126.4° (see Table S2 of the Supporting Information)], which help to immobilize the POMs and enhance the stability of the B

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compounds (Figure 4a). It must be pointed out that two oxygen atoms from POMs face the π-electron-deficient naphthalenic ring centroid of the back of the cis-DPMNI tectons [distances of ∼2.8 and ∼3.0 Å (Figure 3)]. On the other hand, the electron-rich POM was surrounded by the πelectron-deficient naphthalenic ring centroid of the cis-DPMNI tectons [centroid distances of ∼3.6 Å (Figure 3)]. Photochromism. Interestingly, compounds 1 (Figure 5) and 2 (Figure 6) are sensitive to visible light and undergo a

Figure 3. Anion−π interactions (blue dashed line) and C−H···O contacts (yellow dashed line) between DPMNI and POMs in 2: Cu, turquoise; C, black; H, gray; O, red; N, blue; P, purple; W, teal; POMs, red and purple polyhedron.

Information)], which are similar to the reported values.11−13 Neighboring chains connected with each other by anion−π interactions and C−H···O hydrogen bonds (Figure 4b). Each

Figure 4. (a) View of the three-dimensional supramolecular structure of compound 2. (b) Adjacent chains connected with each other by anion−π interactions (blue dashed line) and C−H···O contacts (yellow dashed line). (c) Each chain connected adjacent POMs by anion−π interactions and C−H···O contacts: Cu, turquoise; C, black; H, gray; O, red; N, blue; P, purple; W, teal; POMs, red and purple polyhedron.

Figure 5. Photo images of 1 with slow recovery after color change. (a) ESR spectra of 1 (black line, before irradiation; red line, after irradiation for 5 min). The inset shows enlarged ESR spectra of 1. (b) UV−vis spectra of 1 (black line, before irradiation; red line, after irradiation for 5 min). The inset shows the estimated energy band gap by the UV−vis diffuse reflectance spectroscopy based on the Kubelka− Munk function.

wave-type chain also connected adjacent POMs by anion−π interactions and C−H···O contacts (Figure 4c). Notably, more C−H···anion interactions exist in compound 2, which can lead to stabilization of the three-dimensional supramolecular structure of 2 (Figure 4) with a 52.5% void space as calculated by PLATON. Similar to compound 1, POMs act as counterions and electron reservoirs. C−H···O contacts as well as anion−π interactions facilitate the self-assembly process and control the structures of the polyoxometalate-based solid host−guest

photochromic transformation upon irradiation by visible light. A 300 W light source (xenon arc lamp, PLS-SXE 300/300UV, Beijing Perfectlight, Co. Ltd.) equipped with one filter to keep the wavelength in the range of 400−780 nm was employed as the irradiation source. Both 1 and 2 are stable in air; accordingly, the color of the samples can gradually return to the almost original color in a dark room for several days at ambient temperature, which means the photochromic processes C

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generate radicals upon light irradiation.18−21 In addition, there are no explicit W(V) signal peaks in ESR spectra of compounds 1 and 2,22,23 which suggests that the W atoms of the POMs are not changing valence state after irradiation by visible light. Thus, the photochromic process may mainly arise from photoinduced radical generation of organic tectons. Before visible light irradiation, the UV−vis spectrum of 1 shows board absorption bands at ∼400 nm, corresponding to the n−π* and π−π* transitions of the aromatic organic tectons (Figure 5b). In addition, a weak shoulder around 480−500 nm can likewise be observed, which can be attributed to an intermolecular electron-transfer transition. The appearance of this band can be mainly attributed to the intermolecular metalto-ligand charge-transfer transition.14 Besides these, it also shows a broad absorption band in the region 550−800 nm, which may originate from characteristic absorption of the charge-transfer transition of blue Cu(II) derivatives.24,25 On the basis of the previous works of NDI derivatives,26,27 these spectral features are in accord with the formation of the radical NDI species, which is indeed substantiated by ESR studies. That is, two mainly absorption bands of a photoinduced electron-transfer transition from NDI derivatives are located at ∼490 and ∼700 nm.28,29 Figure 5b shows that the major effect of the irradiation by visible light (400−780 nm) is not the appearance of a new absorption band, but the enhanced absorptions in the regions of 480−500 and 550−800 nm, which may be due to the absorption of the photoinduced electrontransfer transition overlapping the characteristic absorption of the charge-transfer transition of blue Cu(II) derivatives. The UV−vis spectrum of 2 is shown in Figure 6b; it almost retains a spectral region similar to that of the spectrum of compound 1. Because of the different coordination spheres present in compounds (octahedral in 1 and square-pyramidal in 2), 2 possesses two broad bands at 450 and 490 nm that might due to the transition from metal-based d orbitals and ligand-based π molecular orbitals to the ligand-based π* orbitals,14 that is, metal-to-ligand and ligand-to-ligand charge-transfer transition, which can be mainly attributed to the intermolecular electrontransfer transition. Both kinds of compounds are, more or less, colored blue (before or after irradiation), as witnessed by a broad absorption band in the 600−800 nm region, likely formed by an envelope of several partly overlapping transitions, whose correct locations and attributions are too difficult to assign. Nevertheless, it may include the absorption of the photoinduced electron-transfer transition15,18 and characteristic absorption of the charge-transfer transition of blue Cu(II) derivatives.14,16,17,24,25 Furthermore, the estimated band gap values of compounds 1 and 2 based on the Kubelka−Munk function are 2.73 eV (Figure 5b) and 2.96 eV (Figure 6b), respectively, which further indicate that both 1 and 2 can be bandgap photoexcited by visible light irradiation. As expected, they are indeed sensitive to visible light and undergo a photochromic transformation upon irradiation by visible light; the major photochromic process arises from the photoinduced radical generation of organic ligands. The X-ray photoelectron spectroscopy (XPS) spectra of compounds 1 and 2 are almost the same before and after visible light irradiation (Figures S5 and S6), perhaps for the following reasons. It may be a result of electron delocalization in the whole organic tectons after the generation of organic radicals,27,30 or the complicated bonding environments that lead to the XPS spectra of compounds 1 and 2 could not be

Figure 6. Photo images of 2 with slow recovery after color change. (a) ESR spectra of 2 (black line, before irradiation; red line, after irradiation for 30 min). The inset shows enlarged ESR spectra of 2. (b) UV−vis spectra of 2 (black line, before irradiation; red line, after irradiation for 30 min). The inset shows the estimated energy band gap determined by UV−vis diffuse reflectance spectroscopy based on the Kubelka−Munk function.

are reversible. This reversible photochromic transformation can be repeated for several cycles by alternated visible light irradiation and dark treatment while keeping the structure unchanged (Figure S4). This phenomenon indicates that the photoresponsive behaviors may be a result of the electrontransfer chemical process in the structure rather than from a structural transformation. Moreover, the compounds are also sensitive to ultraviolet light (xenon arc lamp, 300 W, PLS-SXE 300/300UV, 320−400 nm). The electron spin resonance (ESR) spectra of the virgin samples (compounds 1 and 2) are both broad, which is the typical characteristic of a normal Cu(II) compound.14−17 No organic radical signal peaks were detected before irradiation (Figures 5a and 6a); however, after visible light irradiation, a noticeable peak radical signal with a g value of 2.0039 [in 1 (Figure 5a)] or 2.0045 [in 2 (Figure 6a)] was observed, whereas this signal disappeared after dark treatment at ambient temperature, which suggests that it will generate organic radicals after irradiation by visible light. It is generally known that NDI derivatives are redox active and can D

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are slightly more cathodic than the potentials for the DPNDI (E1/2 values of −0.91 V and the second reduction at −1.33 V, respectively),19 but the difference in their first reduction potentials is very small and less obvious. Thus, the π-acidities of these two organic tectons are considered to be extremely similar. It is observed from Figure 7 that 1 shows one quasi-reversible process in the cathodic region at an E1/2 of −0.853 V and 2

separated. Thus, the variations of the XPS spectrum of the organic tectons are undetectable. The slight adjustment of the substituent of the diimide nitrogens, from rigid to semirigid ligand, will result in different modes of anion−π interactions, which eventually lead to different structures and different sensitivities to visible light. Compounds 1 and 2 both contain POM anion−π interactions and can generate stable NDI derivative radical anions after visible light irradiation. However, in 1, the POMs typically reside directly over the electron-deficient naphthalenic ring centroid, and one POM anion was closely surrounded by four π-acidity DPNDI tectons; the POMs in 2 were surrounded by only two π-acidity DPMNI tectons, which are loosely stacked. The distances of anion−π interactions between oxygen atoms from POMs and electron-deficient imide rings of DPNDI ligands of the neighboring chain, which could be the chargetransfer or charge-exchange pathway between the organic tectons and POMs in 1, are 2.702 and 2.813 Å (Figure 1), respectively, which are similar to those of 2 [2.793 and 3.024 Å, respectively (Figure 3)] but much shorter than that between POMs and the electron-deficient naphthalenic ring centroid [3.569 Å (Figure 3)] in 2. In other words, the anion−π interactions in 1 are closely bonding, and it has more anion−π interactions sites; however, in 2, there are fewer anion−π interactions as well as anion−π interactions sites, which have much longer distance and weaker interactions. This might explain why 1 shows a much faster response to visible light (5 min) and easier decoloration (for 3 days) than in 2 (∼30 min for coloration and ∼13 days for decoloration). The different photochromic sensitivities exhibited by the two compounds indicate that the photoinduced generating stable radical of NDIs might be strongly influenced by the surrounding anion−π interactions, or probably the conjugation effects of the DPMNIs in compound 2 are weaker than those of 1, which result in the inactivation of the organic radicals. It must be pointed out that the anion−π interaction, which is the interactions between electron-rich POMs and π-acidity NDIs, will probably affect the electron transfer among the components to form the stable organic radical during irradiation. The more anion−π interactions there are in the structure, the more closely they are packing. Then the photoresponsive time of the compound is reduced, thus effectively enhancing the sensitivity to light rather than determining the photochromic properties. After all, the coordination compounds based on NDI derivatives may display photochromic properties because of the naphthalenediimide chromophore.18 As we all know, the π-acidity of NDI receptors can be easily manipulated by installing different electron-rich and -deficient substituents on two imide N centers30,31 as well as in the core naphthalene ring.32−35 The electron-withdrawing groups at the NDI core dramatically enhance its π-acidity, but N-substituents have modest impacts on NDI’s π-acidity.26,30,31 In other words, the electron-rich N-substituents slightly diminish the π-acidity of NDIs, and they might display more negative first reduction potential. Because the difference in the electronic effect of two organic tectons is extremely small, the π-acidity of DPMNI does not appear to be much different from that of DPNDI. The cyclic voltammogram of DPMNI exhibits two quasi-reversible processes in the cathodic region at E1/2 values of −0.98 and −1.51 V, which correspond to the reduction of DPMNI to its radical anion and dianion states, respectively (as shown for DPMNI in Figure S8). The potentials for molecular DPMNI

Figure 7. Solid-state cyclic voltammograms (10 cycles, overlapping) of 1 (a) and 2 (b) at 100 mV s−1 in a 0.1 M nBu4NPF6/MeCN electrolyte.

shows one quasi-reversible process in the cathodic region at an E1/2 of −0.802 V, which are consistent with the [DPNDI]0/•− (that of free ligand; E1/2 = −0.91 V) and [DPMNI]0/•− (that of free ligand; E1/2 = −0.98 V) redox couples, respectively. The slight shifts of the DPNDI reductions in compound 1 (or the DPMNI reductions in compound 2) relative to the NDI radical anions are stabilized in the crystalline solid material by delocalization of the unpaired electron over several molecules such as coordinated solvent molecules, POMs, and NDI molecules through anion−π interactions and C−H···O contacts. Notably, both 1 and 2 showed excellent stability toward electrochemical cycling with no decrease in the peak current observed over 10 cycles at a rate of 100 mV s−1 (Figure 7). The anion−π interactions have significant impacts on structures of the compounds and, to some extent, the ability to gain or lose electrons of the NDIs, which is evident from subtle changes in reduction potentials; more and stronger anion−π interactions might dramatically enhance the comE

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pounds’ stability and diminish the π-electron-deficient property of the core naphthalene ring of the DPNDIs. In 1, electron-rich POMs are tightly besieged by π-electron-deficient naphthalene rings of the DPNDIs through anion−π interactions and C−H··· anion interactions, which effectively diminish the π-electrondeficient property of the core naphthalene ring of the DPNDIs; thus, it displays a first reduction potential [E1/2 = −0.853 V (Figure 7a)] more negative than that of 2 [E1/2 = −0.802 V (Figure 7b)]. Compared with 1, there are fewer anion−π interactions and anion−π interaction sites in 2. Moreover, the distance between one of the DPMNI tectons and the POM is ∼3.569 Å, which is much longer than that of the anion−π interactions in 1. Probably because of these weaker anion−π interactions as well as the fluffy packing structure, the core naphthalene ring of the DPMNIs is easy to reduce. Because the opposite was observed, it can be supposed that DPMNI radical anions are stabilized in 2 by delocalization of the unpaired electron over several molecules such as methanol molecules, N,N′-dimethylacetamide (DMA) molecules, POMs, and DPMNI molecules through anion−π interactions and C−H··· O contacts.32,36

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the 973 Key Program of the MOST (2010CB933501 and 2012CB821705), the National Natural Science Foundation of China (21373221, 21221001, 91122027, 51172232, and 21403236), the Chinese Academy of Sciences (KJCX2-YW-319 and KJCX2-EW-H01), and the Natural Science Fo undatio n of Fujian Province (2011HZ0001-1, 2012J06006, and 2006L2005).

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CONCLUSIONS In conclusion, two extremely rare POM-based host−guest crystalline compounds were synthesized, which have reversible photochromic properties with different response times. The substituent groups at N centers of NDIs could influence the intermolecular anion−π interactions and thus affect the sensitivity of photochromic properties and the π-electrondeficient property of the core naphthalene ring of the NDIs of the compounds. The trapped POM anions connected with πacidity NDI tectons effectively via directional anion−π and C− H···anion interactions, which indicate POM anion−π interactions, not only will be applicable to the stabilization and immobilization of the POMs anions but also will promote the charge transfer or exchange among components to lead to reversible photochromic materials with different response times. These results illustrate that one would be able to produce further different photoresponsive photochromic supramolecular compounds with elaborately hybridize photochromic molecules by regulating the noncovalent interaction sites to realize specifically designed supramolecular self-assemblies. We believe that insight into the construction concept of such supramolecular self-assemblies should help in the realization of practical smart photochromic materials.

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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.cgd.5b00857. Experimental materials and physical measurements, PXRD, TGA, XPS core-level spectra of 1 and 2, additional figures, and and C−H···O contact geometry for compounds 1 and 2(PDF) Crystallographic data for 1 (CIF) Crystallographic data for 2 (CIF)

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Crystal Growth & Design

Article

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DOI: 10.1021/acs.cgd.5b00857 Cryst. Growth Des. XXXX, XXX, XXX−XXX