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Redox‐Responsive Photochromism and Fluorescence Modulation of Two 3D Metal–Organic Hybrids Derived from a Triamine‐Based Polycarboxylate Ligand

2011; Wiley; Volume: 17; Issue: 12 Linguagem: Inglês

10.1002/chem.201003274

1521-3765

Qi‐Long Zhu, Tian‐Lu Sheng, Ruibiao Fu, Shengmin Hu, Ling Chen, Chaojun Shen, Xiao Ma, Xin‐Tao Wu,

Metal-Organic Frameworks: Synthesis and Applications

A radical change: Two 3D ZnII complexes with pillared structures have been synthesized from a triamine-based polycarboxylate ligand under hydrothermal conditions. The complexes exhibit reversible radical-based redox photochromism that varies from yellow to bluish-violet and dark blue, respectively, under UV/blue irradiation, and fluorescence with adjustable intensity. The ESR and X-ray photoelectron spectroscopy studies verify photochromic mechanisms based on electron transfer. Photochromism is defined as the reversible transformation of a chemical species, induced by the absorption of electromagnetic radiation, to a form with a different vibronic absorption spectra.1 Photochromic materials have attracted considerable interest owing to prospective real or potential applications in many fields, including protection (in spectacles, photobarriers, anti-fake, and camouflage), decoration, information storage, displays, optical switches, photomechanics, and so forth.2 Although numerous photochromic families have been reported to date, those based on an electron-transfer (redox) chemical process, especially for metal–organic complexes (MOCs), are rare.3 One of the biggest challenges in photochromic MOCs without photochromic organic ligands is to design photoinduced bistable systems based on different electron-transfer mechanisms,4 such as metal-centered electron transition (MC), ligand-to-metal charge transfer (LMCT),5 metal-to-metal charge transfer (MMCT), intraligand charge transfer (ILCT), and ligand-to-ligand charge transfer (LLCT).3b Structurally well-defined hybrids will give us better insight into the structure–photochromism relationships; nevertheless, few hybrids based on transition metals have been explored with simultaneous characterization of their structure and photochromism.3, 6 Chopoorian and Loeffler have discovered the electron-attractive ability of a porous glass matrix indicated by the blue radical-ion species after the irradiation with UV light, in which an aqueous solution of p-phenylenediaminetetraacetic acid (p-PTDA) is absorbed.7 Thus, in the presence of electron-accepting components, organic ligands containing N(CH2CO2H)2 groups in MOCs may undergo a similar electron-transfer process. However, no coordination polymer constructed from ligands containing N(CH2CO2H)2 groups shows photochromism so far.8 Guo and coworkers have reported the only metal-assisted LLCT photochromic MOC, [Cd2(ic)(mc)(4,4′-bipy)3]⋅4H2O (ic=itaconate, mc=mesaconate, bipy=bipyridine), which undergoes an interesting photochromic transformation from yellow to blue upon UV irradiation.3b Herein, we report two photochromic MOCs with adjustable fluorescent intensities, [Zn3(TTHA)(4,4′-bipy)1.5(H2O)2]⋅6H2O (1 a) and [Zn3(TTHA)(4,4′-bipy)1.5(H2O)2]⋅3H2O (2 a), derived from a novel triamine-based polycarboxylate ligand containing the N(CH2CO2H)2 group, 1,3,5-triazine-2,4,6-triamine hexaacetic acid (H6TTHA), as electron donor, although the ligand H6TTHA itself does not exhibit photochromism, and 4,4′-bipy as electron capturer. The photochromic mechanisms based on electron-transfer chemical processes have been verified with direct and powerful ESR and X-ray photoelectron spectroscopy (XPS) measurements. Yellow needlelike crystals of 1 a and prismatic crystals of 2 a were obtained by the hydrothermal reactions of Zn(NO3)2, H6TTHA, and 4,4′-bipy in a molar ratio of 3:1:2 at 140 °C and 120 °C for 72 h, respectively. The single-crystal X-ray diffraction data reveal that complexes 1 a and 2 a are 3D networks. As shown in Figure 1 and S3 in the Supporting Information, the 3D frameworks can be described as 2D layers constructed of ZnII ions and TTHA6− pillared by the coordinated 4,4′-bipy groups. 2D layer (left) and 3D framework along the c axis (right) in complex 1 a. The biggest difference between the structures of complexes 1 a and 2 a is the coordination modes of the ligands (Scheme 1). Due to the flexibility, six of the arms show significant deviation from the central triazine ring. In complex 1 a, the adjacent two couples of arms are arranged above and below the triazine plane, respectively; the other two arms bend in opposite directions to the triazine plane. Consequently, enantiomeric pairs of ligands are formed in spite of no asymmetric carbon atom in complex 1 a (Figure S4 in the Supporting Information). However, the configurations of the ligands in complex 2 a are of mirror symmetry. Two couples of arms bend in the same direction to the triazine plane, whereas the other couple of arms bend in the opposite direction. Coordination modes of the ligands in complex 1 a (top) and complex 2 a (bottom). Complex 1 a undergoes an interesting photochromic transformation from yellow to bluish violet (complex 1 b) upon irradiation with UV/blue light (Figure 2). Such a process is rather sensitive to light and can even be fulfilled upon irradiation by sunlight for a few minutes. An activation analysis showed that coloration was induced by radiation in the 260–410 nm region with an activation maximum between 310 and 380 nm. The blue sample (1 b) is stable in air in a dark room for 2 days at ambient temperature followed by a slow reversion back to yellow over two weeks in a refrigerator (ca −10 °C). Complex 1 b can be decolored by heating at 80 °C for several minutes in air or in an argon atmosphere. Photochromism and ESR spectral differences of samples 1 a and 1 b. Complexes 1 a and 1 b show different ESR and UV/Vis spectra but similar IR spectra. Complex 1 a exhibits no ESR signal, but 1 b shows a single-peak radical signal with a g value of 2.0014 (Figure 2). These phenomena indicate that the photoresponsive behavior should result from the in situ production of free radicals induced by irradiation. Figure 3 shows that1 a stimulated by UV light produces three new absorption bands at 380, 545, and 585 nm. After 30 min of irradiation, these bands reached maximums and no further color change was visible to the naked eye. Meanwhile, the intensities of the fluorescence decrease with the duration of UV irradiation and reach a minimum after 30 min. Crystals 1 a display light-blue luminescence with maximum and shoulder bands at 460 and 424 nm, and a very weak band at 600 nm, upon excitation at 375 nm. The bands in the region 400–500 nm with nanosecond lifetimes should be assigned to ILCT because the similar emission peaks (at 425 and 412 nm) were also observed in the emission spectrum of free ligand H6TTHA in the solid state and in dilute solution (Figure S5 in the Supporting Information). The second emission band (600 nm) probably results from the interaction between the TTHA6− and bipy ligands and can be assigned to exciplex emission.9 UV/Vis (top) and fluorescent (bottom) spectral changes upon irradiation of sample 1 a to form 1 b. Complex 2 a exhibits similar photochromic properties to 1 a with longer times of decoloration and a slightly different photoinduced color (Figure 4). Upon irradiation with UV/blue light for several minutes, complex 2 a turns from yellow to dark blue. The dark blue sample 2 b is stable at ambient environment for about two weeks. The reverse reaction occurs upon irradiation in the UV region from 250 to 400 nm with an activation maximum between 280 and 320 nm. The ESR measurements show that 2 b also generates the radical signal with a g value of 1.9988. The changes in the absorption spectra shown in Figure 5 should correspond to the photochromic reaction. Photochromism and ESR spectral differences of samples 2 a and 2 b. UV/Vis (top) and fluorescent (bottom) spectral changes upon irradiation of sample 2 a to form 2 b. Irradiation of 1 a under vacuum has no effect on the photochromism, indicating that the photochromism is due to a solid-state transformation and such a coloration–decoloration is not the result of a surface oxidation reaction.3b, 10 Furthermore, the XPS measurements also support the photoinduced electron transfer. The N 1s core-level spectrum of crystal 1 a could be fitted by two well-resolved components at 397.7 and 399.2 eV, respectively (Figure 6 a). The signal at 397.7 eV corresponds to the tertiary amine nitrogen atom, whereas the peak at 399.2 eV represents the characteristic emission from the NC moieties in the triazine and pyridine rings.11 After irradiation (Figure 6 b), the two signals at 397.7 and 399.2 eV became weak and a new signal appeared at 398.4 eV, which was assigned to the nitrogen atom of the tertiary amino and pyridyl radicals; this may suggest that electrons transfer from the tertiary amine groups to the 4,4′-bipy moiety.12 Thus, the bluish violet of 1 b could be related to the radicals generated by electrons dissociating from tertiary amine nitrogen atoms of the TTHA6− and captured by pyridyl groups. A similar process can also be found in p-PTDA, which possesses the same main active groups when it is absorbed into porous glass and irradiated with UV light.7 The XPS O 1s spectra of samples 1 a and1 b are illustrated in Figure 6 c. Because of the complicated bonding environment, the O 1s spectra of 1 a and 1 b could not be separated successfully by peak-resolution techniques. However, the overall signal of the binding energy of 1 b lies at 531.6 eV, 0.6 eV higher than that of 1 a, which should be attributed to the electron dissociation of oxygen atoms and indicates that carboxylate oxygen atoms of TTHA6− also participate in the formation of radicals. Similar changes of the N 1s and O 1s XPS core-level spectra of samples 2 a and 2 b could also be observed under identical conditions. N 1s and O 1s XPS core-level spectral differences of samples 1 a and 1 b (a–c) and samples 2 a and 2 b (d–f). The recent studies of other related photochromic MOCs have confirmed that the 4,4′-bipy can act as electron acceptor.4 Thus, the electron-transfer photochromism of the two samples may result from the synergetic action of the TTHA6− and 4,4′-bipy ligands and can be interpreted as the metal-assisted photoinduced LLCT. LLCT photochromic MOCs with non-photochromic components, which requires the coexistence of electron-donating and electron-accepting groups in the same or different ligands, are rather rare.4 For example, the carboxylate groups in [Cd2(ic)(mc)(4,4′-bipy)3]⋅4H2O act as electron donors, whereas the vinyl bonds of the ic ligand and 4,4′-bipy act as electron acceptors. Our experimental results could fit the specific interpretations for the overall reaction: The tertiary amine nitrogen atoms and carboxylate oxygen atoms of the triamine ligands dissociate electrons under UV/blue light. The electrons are captured by the 4,4′-bipy ligands and, on interruption of the activating light, optical bleaching can occur by electron-radical recombination (Figure 7). The electron-transfer (redox) chemical process of metal-assisted photoinduced LLCT in samples 1 a and 1 b. In summary, we have prepared two new 3D ZnII complexes with pillared structures from the same precursors at different temperatures under hydrothermal conditions. The two complexes exhibit reversible radical-based redox photochromism varying from yellow to bluish violet and dark blue, respectively, and fluorescence with adjustable intensity. The redox photochromic mechanisms of these complexes have been explored. Further research on other photochromic compounds is in progress. Preparation of H6TTHA: A solution of iminodiacetic acid (12.64 g, 95 mM) and sodium hydroxide (12 g, 300 mM) in water (40 mL) was added dropwise into cyanuric chloride (5.53 g, 30 mM) in water (40 mL) at 0–5 °C under stirring. After 1 h, the mixture was warmed to room temperature and reacted during stirring for 3 h. And then the mixture was heated to reflux at 110 °C for another 12 h. After cooling, the pH value was adjusted to about 2 by concentrated HCl. The white solid was collected by filtration, washed with alcohol and water, and dried under vacuum at 60 °C to give 76 % yield. 1H NMR (400 MHz, D2O): δ=4.18 ppm (s, 2 H); elemental analysis calcd (%) for C15H18N6O12: C 37.98, H 3.82, N 17.72; found: C 38.04, H 3.86, N 17.64. Preparation of 1 a: Zn(NO3)2⋅6H2O (178.5 mg, 0.6 mmol) and 4,4′-bipy (46.8 mg, 0.3 mmol) were added to the solution of H6TTHA (94.9 mg, 0.2 mmol in 10 mL of water), which was firstly adjusted to an acidity of pH≈5.5 by adding sodium hydroxide. The suspension was then transferred into a Teflon-lined autoclave (20 mL) and heated to 140 °C for 120 min. The autoclave was kept at 120 °C for 3 days and then slowly cooled to 30 °C at about 5 °C h−1. Yellow, needle-like single crystals of 1 a (ca 85 % yield on the basis of H6TTHA) were obtained. The crystals were filtered, washed with distilled water, and used for XRD determination. Elemental analysis calcd (%) for Zn3C30H40N9O20: C 34.55, H 3.87, N 12.09; found: C 34.84, H 3.64, N 12.13. Preparation of 2 a: Complex 2 a can be obtained by the same synthetic procedures as 1 a, but at 120 °C instead of 140 °C. Large, yellow, prismatic single crystals were collected in 72 % yield on the basis of H6TTHA. Elemental analysis calcd (%) for Zn3C30H34N9O17: C 35.44, H 3.47, N 12.55, found: C 35.70, H 3.27, N 12.65. CCDC-777289 (1 a) and CCDC-777290 (2 a) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. This work was supported by grants from the 973 Program (2007CB815301), the National Science Foundation of China (21073192, 20733003, 20871114, and 20801055), the Science Foundation of CAS (KJCX2-YW-H20) and the Science Foundation of Fujian Province (2009HZ0006-1 and 2006L2005). Detailed facts of importance to specialist readers are published as "Supporting Information". Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.