Photocatalytic Properties and Luminescent Sensing for Cr3+ Cations

Article pubs.acs.org/crystal

Photocatalytic Properties and Luminescent Sensing for Cr3+ Cations of Polyoxovanadates-Based Inorganic−Organic Hybrid Compounds with Multiple Lewis Basic Sites Hong-Mei Zhang,† Jin Yang,*,† Wei-Qiu Kan,‡ Ying-Ying Liu,† and Jian-Fang Ma*,† †

Key Lab of Polyoxometalate Science, Department of Chemistry, Northeast Normal University, Changchun 130024, People’s Republic of China ‡ School of Chemistry and Chemical Engineering, Huaiyin Normal University, Huaian 223300, People’s Republic of China S Supporting Information *

ABSTRACT: A new family of polyoxovanadates-based inorganic−organic hybrid compounds with multiple Lewis basic sites, namely, [Zn5(Htrb)2(H2O)2(V5O15)2]·11H2O (1), [Zn2(Htrb)(HV5O15)]·6H2O (2), [Co3(Htrb)(H2O)4(V3O9)2]·4H2O (3), and [Ag3(Htrb)(H4V5O16)]·H2O (4), have been hydrothermal synthesized by using a multidentate N-containing hexakis(1,2,4triazol-ylmethy1)benzene (Htrb). In 1, [V4O12]4− and [V6O18]6− rings are linked by Zn(II) ions into a two-dimensional (2D) inorganic layer, which are pillared by Htrb ligands to afford a unique three-dimensional (3D) framework. In 2, rod-shaped [H2V10O30]8− clusters are bridged by Zn(II) ions to generate one-dimensional inorganic hybrid motifs, which are joined by Htrb ligands to yield a 3D framework. In 3, Co(II) ions bridge [V6O18]6− rings to result in a 2D inorganic sheet, and the adjacent sheets are extended by the Htrb ligands into a 3D motif. In 4, adjacent 12-membered vanadium rings, composed of [VO4] tetrahedra and [VO5] trigonal bipyramids, form a unique 2D inorganic layer via sharing corner O atoms. Strikingly, neighboring layers are further extended by Htrb ligands and Ag(I) ions into a 3D framework. Moreover, photocatalytic degradation of compounds 1, 3, and 4 toward methylene blue (MB) and methyl orange (MO) was studied under UV light irradiation. A possible photocatalytic mechanism was also speculated by introducing t-butyl alcohol as a widely used ·OH scavenger. In addition, luminescent selective and sensitive sensing of Cr3+ compared with other metal ions such as Zn2+, Al3+, Co2+, K+, Na+, and Pb2+ were investigated by using 1 and 4. Finally, their electrochemical behaviors were also studied.

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INTRODUCTION Polyoxovanadates (POVs), as a significant ramification of polyoxometalates (POMs), have attracted the increasing interest of chemists owing to variable oxidation states of vanadium (+3, +4, and +5) and diverse coordination spheres of vanadium oxide polyhedra including VO4 tetrahedron, VO5 square pyramid or trigonal bipyramid, and VO6 octahedron.1−7 Generally, vanadium oxide polyhedra can be self-condensed to form a variety of vanadium oxide units, including clusters, chains, layers, and three-dimensional (3D) frameworks.8−16 As a result, POVs are usually utilized as excellent building blocks for constructing POVs-based hybrid compounds.17−21 During the construction of POVs-based inorganic−organic hybrid materials, organic components introduced as ligands for the metal sites play a significant role in the structural modification of inorganic oxides or overall © XXXX American Chemical Society

frameworks. Thus, decorating POVs by incorporating rational organic ligands is an effective synthetic approach to prepare POVs-based hybrid materials with fascinating structure motifs.22−26 Apart from the structure features, the POMs clusters usually show similar semiconductor photochemical behaviors because of analogous electronic characteristics between band gap transition of semiconductors and HOMO−LUMO transition of POMs.27−33 As a result, POMs are good candidates as cheap and green photocatalysts for removing organic pollutants from water. Currently, organic dyes and heavy metal cations have been given tremendous attention for their adverse effects on human Received: August 24, 2015 Revised: November 17, 2015

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DOI: 10.1021/acs.cgd.5b01226 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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health and the environment.34−39 In this regard, organic dyes are not very biodegradable in water because of their complex structures.40−43 Photocatalysis has been widely applied as an effective and economical degradation technique for the removal of organic dyes.44−48 Very recently, POMs have been explored as one kind of potential photocatalyst in the degradations of organic dyes.49−52 For example, Maggard and co-worker reported a series of silver−vanadate hybrid solids that exhibit photocatalytic activity for the decomposition of methylene blue (MB) under both UV and visible light.53 Nevertheless, systematic investigations on the typical POVs-based inorganic−organic hybrid materials as photocatalysts have received relatively less attention.5,54,55 On the basis of the above considerations, four novel inorganic−organic hybrid compounds with multiple Lewis basic sites, namely, [Zn5(Htrb)2(H2O)2(V5O15)2]·11H2O (1), [Zn 2 (Htrb)(HV 5 O 15 )]·6H 2 O (2), [Co 3 (Htrb)(H 2 O) 4 (V3O9)2]·4H2O (3), and [Ag3(Htrb)(H4V5O16)]·H2O (4), have been successfully synthesized by using a multidentate N-containing ligand (Htrb = hexakis(1,2,4-triazol-ylmethy1)benzene). Remarkably, compounds 1, 3, and 4 show highly photocatalytic degradation of MB and methyl orange (MO) under UV light irradiation, where the possible photocatalytic mechanism was also speculated by introducing TAB as a widely used ·OH scavenger. Moreover, their fluorescence probe for heavy metal ions56−59 and electrochemical behaviors were also studied.

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RESULTS AND DISCUSSION Structural Description of [Zn5(Htrb)2(H2O)2(V5O15)2]· 11H2O (1). Single crystal X-ray diffraction analysis reveals that 1 crystallizes in monoclinic group P21/c. In the asymmetric unit, there exist two and a half Zn(II) ions, one Htrb ligand, a half tetranuclear [V4O12]4− ring, a half hexanuclear [V6O18]6− ring, one coordination water molecule, and five and a half lattice water molecules. In both tetranuclear [V4O12]4− and hexanuclear [V6O18]6− rings, each vanadium center shows a VO4 tetrahedral sphere, coordinated by two terminal O atoms and two bridging O atoms. As shown in Figure 1a, three unique Zn(II) ions (Zn1, Zn2, and Zn3) exhibit tetrahedral (N2O2), trigonal-bipyramidal (N3O2) and octahedral (N2O4) coordination geometries, respectively. It is worthwhile to note that V1, V2, and their symmetry-related species form a four-membered [V4O12]4− cycle by corner-sharing O atoms, while V3, V4, V5 and their symmetry-related ones generate a six-membered [V6O18]6− cycle via sharing the corner O atoms. Notably, although some inorganic−organic hybrid compounds containing the POV isomers have been reported, examples containing two types of coexisting POV clusters in the same compound are rarely observed.60,61 Further, adjacent [V4O12]4− and [V6O18]6− rings are linked by Zn(II) ions through sharing their terminal O atoms to yield a two-dimensional (2D) inorganic hybrid layer (Figure 1b). Neighboring inorganic hybrid layers are further pillared by the Htrb ligands in a 1,2,3-up/4,5,6-down manner to afford a unique 3D motif (Figure 1c). Structural Description of [Zn2(Htrb)(HV5O15)]·6H2O (2). The asymmetric unit of 2 is composed of two crystallographically independent Zn(II) ions, one Htrb ligand, a half [H2V10O30]8− cluster, and six free water molecules. Each Zn(II) ion is in a trigonal bipyramid coordination geometry, surrounded by three N donors from three Htrb ligands, and two O atoms from two [H2V10O30]8− clusters. Each vanadium atom exhibits a distorted tetrahedral sphere (Figure 2a).

Figure 1. (a) View of the coordination environments of Zn(II) ions in 1. (b) View of the 2D inorganic layer of 1 formed by Zn(II) ions, [V4O12]4− and [V6O18]6− rings. (c) View of the 3D motif of 1.

Noticeably, V1, V2, and V3 form a V3O10 unit via sharing the corner O atoms, while V4, V5, and their symmetry-related species generate a four-membered V4O12 cycle by sharing corner O atoms. As illustrated in Figure 2b, two diagonal V3O10 units are joined together by a V4O12 cycle through corner-sharing O atoms to yield a rod-shaped [H2V10O30]8− cluster. Nevertheless, in the known [{CoIII(phen)2}2V8O23] (phen = 1,10-phenanthroline), B



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Figure 2. (a) View of the coordination environments of Zn(II) ions in 2. (b) View of the 1D inorganic motif of 2. (c) View of the 3D framework structure of 2.

alternating V4O12 rings and V4O13 units are joined together by corner-sharing O atoms to form a [V8O23]n chain.62 In 2, neighboring [H2V10O30]8− rod-shaped clusters are further linked by Zn(II) ions to result in a one-dimensional (1D) inorganic hybrid motif. Moreover, adjacent 1D inorganic hybrid motifs are bridged by the Htrb ligands in 1,2,3-up/4,5,6-down modes to form a rare 3D framework (Figure 2c). It is worthwhile to note that the assembly process of 1 and 2 is highly pH-dependent. 1 and 2 were achieved at the final pH values of 6.0 and 4.2, respectively. From their structures, it can be seen that the effect of the pH value on their structures is in fact mainly based on the formation of different vanadium polyoxoanion units.63,64 Structural Description of [Co3(Htrb)(H2O)4(V3O9)2]· 4H2O (3). The asymmetric unit of 3 consists of one and a half Co(II) ions, a half Htrb, a half [V6O18]6− ring, two coordination water molecules, and two free water molecules. As illustrated in Figure 3a, two crystallographically independent octahedral Co(II) ions show different coordination environments. Co1 is surrounded by two nitrogen atoms of two Htrb ligands, and four oxygen atoms of four different VO4 tetrahedra,

Figure 3. (a) View of the coordination spheres of Co(II) ions in 3. (b) View of the 2D inorganic layer of 3 constructed by Co(II) ions and the [V6O18]6− rings. (c) View of the 3D framework structure of 3.

while Co2 is coordinated by two N atoms from two Htrb ligands, and four O atoms from two water molecules and two VO4 tetrahedra. Notably, V1, V2, V3, and their symmetryrelated species form a six-membered [V6O18]6− ring via sharing the corner O atoms. Adjacent [V6O18]6− rings are linked by Co(II) ions to generate a 2D inorganic layer (Figure 3b). The Htrb ligands in a 1,3,5-up/2,4,6-down manner further bridge adjacent inorganic layers to yield a 3D framework (Figure 3c). Structural Description of [Ag3(Htrb)(H4V5O16)]·H2O (4). Single crystal X-ray diffraction analysis shows that 4 crystallizes in trigonal group R3c̅ . As illustrated in Figure 4a, Ag1 is in a distorted tetrahedral sphere, coordinated by two N atoms of two Htrb ligands, and two O atoms of two VO4 tetrahedra. Six [VO4] tetrahedra and six [VO5] trigonal bipyramids are C



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Figure 4. (a) View of the coordination environment of Ag(I) ion in 4. (b) View of the 2D POV inorganic layer of 4. (c) View of the 2D inorganic layer constructed by 12-membered vanadium rings and Ag(I) ions. (d) View of the 3D framework of 4.

Table 1. Crystallographic Data and Structure Refinements for Compounds 1−4 formula Mr crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z Dcalc (g cm−3) F(0 0 0) Rint GOF on F2 R1a [I > 2σ(I)] wR2b (all data) a

1

2

3

4

C48H74N36O43V10Zn5 2679.68 monoclinic P21/c 13.0737(4) 20.2192(9) 19.6196(1) 90 115.529(3) 90 4679.9(4) 2 1.902 2676 0.0312 1.062 0.0463 0.1367

C24H37N18O21V5Zn2 1299.16 triclinic P1̅ 8.9711(5) 14.1439(8) 19.8782(1) 82.151(5) 82.397(5) 74.901(5) 2400.0(2) 2 1.798 1300 0.0320 1.197 0.0775 0.2172

C24H40Co3N18O26V6 1479.17 triclinic P1̅ 7.9490(5) 10.6780(6) 14.4030(7) 91.118(4) 103.636(5) 91.170(5) 1187.44(12) 1 2.069 737 0.0287 1.064 0.0361 0.0916

C24H30Ag3N18O17V5 1420.97 trigonal R3̅c 10.5197(12) 10.5197(12) 66.084(5) 90 90 120 6333.4(11) 6 2.235 4152 0.0990 1.058 0.0593 0.1639

R1 = Σ||Fo| − |Fc||/Σ|Fo|. bwR2 = {Σ[w(Fo2 − Fc2)2]/Σw(Fo2)2]}1/2.

alternately joined together by sharing the corner O atoms, yielding a 12-membered vanadium ring. Each 12-membered

vanadium ring connects its six neighbors to result in a 2D inorganic POV layer (Figure 4b). In the 12-membered vanadium D



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Figure 5. Absorption spectra of the MO solution during the decomposition reaction under UV irradiation in the presence of compounds 1 (a), 3 (b), and 4 (c). (d) Photocatalytic decomposition rate of the MO solution under UV irradiation with the use of compounds 1, 3, 4 and the control experiment without any catalyst under the same condition.

Figure 6. Absorption spectra of the MB solution during the decomposition reaction in the presence of 1 (a), 3 (b), 4 (c). (d) Photocatalytic decomposition rate of the MB solution under UV irradiation with the use of compounds 1, 3, 4 and the control experiment without any catalyst under the same condition. E



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ring, three Ag(I) ions bridge three oxygen atoms of three [VO4] tetrahedra (Figure 4c). Further, the Htrb ligands in a 1,3,5-up/ 2,4,6-down fashion connect neighboring inorganic layers by sharing the Ag(I) ions to give a 3D framework (Figure 4d). Coordination Modes of The Htrb Ligand in 1−4. The six triazole groups of the Htrb can adopt various coordination fashions owing to the flexible −CH2− spacers. From the structure descriptions above, it can be observed that the Htrb mainly shows two kinds of coordination modes in 1−4 (Scheme S1). In 1 and 2, each Htrb links six metal ions in the same 1,2,3-up/4,5,6-down mode (mode I in Scheme S1). In 1, the 2D inorganic hybrid layers built with Zn(II) ions, [V4O12]4− and [V6O18]6− rings are linked by Htrb ligands in mode I into a rare 3D framework, while in 2 the Htrb ligands in mode I link adjacent 1D inorganic hybrid motifs composed of Zn(II) ions and rod-shaped [H2V10O30]8− clusters into a 3D framework. However, in 3 and 4, six triazole groups of the Htrb dispose alternately above and below the benzene group to form the 1,3,5-up/2,4,6-down coordination fashion (mode II). In 3, neighboring sheets built with Co(II) ions and [V6O18]6− rings are further connected by the Htrb ligands in mode II to afford a striking 3D motif. In 4, adjacent inorganic layers are linked by Htrb ligands (mode II) and Ag(I) ions into a 3D framework. Notably, the Htrb ligand connects six metal ions in 1−4, yielding relatively high dimensional structures. On the basis of the above result, we also can deduce that six identical triazole groups are difficult to locate on the same side of the central benzene due to the effect of steric hindrance. Photocatalytic Activity. Photocatalytic degeneration of organic pollutants is one of the effective methods to remove organic pollutants.65−67 Recently, much effort has been devoted to studying POMs as photocatalysts and applying them for degradation of organic dyes.68−72 In this regard, heterogeneous POM-based inorganic−organic hybrid compounds as photocatalysts are ideal candidates for the degradation of organic dyes in water.73−75 Here, we selected MO and MB as the model pollutants of dye contaminants in water to evaluate the photocatalytic effectiveness of insoluble compounds 1, 3, and 4. The MO and MB are usually employed as a representative of organic dyes which are difficult to decompose in wastes.76−78 The photodegradation experiments under UV light irradiation were performed after the adsorption equilibrium was reached. For comparison, the photocatalytic process of MO or MB solution without any photocatalysts was also investigated under the same condition (Figure S1). As shown in Figure 5, the absorbance decreased, and the changes in the concentration (Ct/C0) of MO solutions are plotted versus irradiation time using 1, 3, and 4 as photocatalysts. The photocatalytic rates increase from 10% (without any catalyst) to 39% for 1, 27% for 3, and 58% for 4 after 2.5 h of irradiation. It is evident that the photodegradation performance of 4 toward MO under UV light is much better than those of 1 and 3. Moreover, the photocatalytic performances of 1, 3, and 4 for the degradation of MB are also conducted, as illustrated in Figure 6. After 2.5 h of irradiation, the photocatalytic effectiveness increased from 24% (without any catalyst) to 84% for 1, 73% for 3, and 60% for 4. The result demonstrates that 1, 3, and 4 are excellent candidates for photocatalytic degradation of organic pollutant MB. In addition, from the above photocatalytic results of MB and MO, it also can be seen that 1 and 3 exhibit an advantage for photocatalytic decomposition of MB under the identical condition (Figure 7).

Figure 7. Comparison of the photodegradation rate for 1, 3, and 4 in the MO and MB solutions, respectively.

Figure 8. PXRD patterns of 1 (a), 3 (b), and 4 (c) simulated from single-crystal X-ray data (black line), as-synthesized (red line) and after the photocatalytic reaction using MB (blue line) and MO (green line).

It is noteworthy that the photocatalytic effectiveness of 1, 3, and 4 toward MO after 2.5 h of irradiation is much higher than those of the known compounds [Ni(3-atrz)2V2O6]·2H2O (5), [Ni3(4-atrz)6V6O18]·4H2O (6), [Co3(4-atrz)6V6O18]·4H2O (7), and [Zn3(4-atrz)6V6O18]·4H2O (8) (atrz = amino-1,2,4triazole) under the similar condition.79 The percent conversions of MO for 5−8 are only 18.2%, 11.7%, 12.4%, and 19.6%, respectively, after 3 h of irradiation. Moreover, the photocatalytic activity of 1 toward MB is also higher than those of 5−8, where the decomposition rates of MB for 5−8 are 27.8%, 29.6%, 75.8%, and 76.6%, respectively. Moreover, the specific surface areas are measured for the compounds. The relatively low Brunauer−Emmett− Teller surface areas (SBET) for 1, 3, and 4 are 13.85, 8.51, and 4.92 m2/g, respectively (Figure S2). Among these three compounds, the SBET value of 1 is slightly higher than the ones F



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Figure 9. Cyclic voltammograms of 1-CPE (a), 3-CPE (c), and 4-CPE (e) in 1 M H2SO4 at different scan rates and plots of the anodic and the cathodic peak currents of II−II′ against scan rates for 1 (b), 3 (d), and 4 (f).

toward MO than those of 1 and 3. The results indicate that the variable POV motifs, framework structures, and the specific surface areas are the potential factors that influence the photocatalytic activities of the compounds.38,71,79,80 To elucidate the possible photocatalytic reaction mechanism, further photodegradation experiment was conducted in the presence of TAB, a widely used ·OH scavenger.47,80,81 In this study, effect of the TAB on the photocatalytic degradation of MB with 1 as a typical photocatalyst was evaluated. From the photodegradation curve (Figure S3), we can observe that the presence of TAB greatly suppressed the degradation activity of catalyst 1 toward MB. The ·OH quenching experiment indicates that the photodegradation process of MB with 1 as catalyst predominantly involves attack by ·OH radicals. As a result, a possible model of photocatalytic reaction mechanism was deduced.82 As illustrated in Scheme S2, under UV light irradiation, the active [V4O12]4− and [V6O18]6− unites of the

of 3 and 4. For the degradation of MB solution, the photocatalytic performances of 1, 3, and 4 are almost consistent with their specific surface areas. Nevertheless, the degradation of MO solution does not correspond well to this regulation. On the other hand, Maggard and co-worker reported the photocatalytic degradation of organic dyes by silver-vanadate hybrid compounds [Ag(bpy)]4V4O12·2H2O (9), [Ag(dpa)]4V4O12· 4H2O (10), and Ag4(pzc)2V2O6 (11) (bpy = 4,4′-bipyridine, dpa = 1,2-bis(4-pyridyl)-ethane, pzc = pyrazinecarboxylate). The study indicated that the vanadate chains and the much larger clusters of Ag-oxide/organic chains in 11 favored the transport of excited holes/electrons to the surface to initiate the effectively photocatalytic degradation of organic dyes.53 This viewpoint was also supported by the photocatalytic degradations of compounds 1, 3, and 4 toward MO. In contrast to 1 and 3, compound 4, composed of the much larger 12-membered vanadium rings, exhibits a much higher photocatalytic efficiency G



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Cyclic Voltammetry. The electrochemical behaviors of 1, 3, and 4 were investigated in 1 M H2SO4 aqueous solution. Because of the poor solubility of the POMs-based compounds in aqueous media and common organic solvents, modified carbon paste electrodes (CPEs) of 1, 3, and 4 were fabricated as the working electrode to monitor their electrochemical behaviors (Figure 9). As shown in Figure 9a, in the potential from +1.7 to −1.3 V, two pairs of redox peaks (II−II′ and III−III′) are found for 1, with peak potentials E1/2 = (Epa + Epc)/2 of approximately 0.438 and −0.589 V (vs Ag/AgCl), which corresponds to the vanadium-centered reductions, (VV → VIV) and (VIV → VIII), respectively.24 There also exists an irreversible anodic peak (I) for 1, which could be attributed to the oxidation of the Zn(II) ion.83 The cyclic voltammetry (CV) diagram of 3 is also similar to that of 1 except for some slight potential shift. Two reversible redox peaks II−II′ and III−III′ in the potential from +1.7 to −1.3 V are attributable to the redox process of VV centers with the mean peak potentials at 0.590 (II−II′) and −0.487 V (III−III′), respectively. Moreover, an irreversible anodic peak (I) found in 3 corresponds to the oxidation of the Co(II) ion (Figure 9c). For 4, two pairs of redox peaks (II−II′ and III− III′) are located at 0.280 and −0.551 V, respectively, versus the Ag/AgCl electrode. The two redox couples are attributed to redox process of the V centers (VV → VIV) and (VIV → VIII). An irreversible anodic peak (I) is observed for 4, which is ascribed to the oxidation of Ag(I) (Figure 9e). Additionally, the CV diagrams of 1, 3, and 4 at different scan rates show that the peak currents of the redox process were proportional to the scan rates, indicating that the redox process of the 1-CPE, 3-CPE, and 4-CPE are mainly surface-controlled (Figure 9b,d,f).24,71

Figure 10. Emission spectra of compound 1 at different temperatures.

POV-based 1 transformed into the excited state transition species [*V4O12]4− and [*V6O18]6− (*1). Then the [*V4O12]4− and [*V6O18]6− species immediately captured electrons from water molecules to yield the ·OH active species along with the formation of [V4O12]5− and [V6O18]7−. On the other hand, the MB dye is also excited by UV light to generate *MB molecule. Finally, the highly oxidizing ·OH radicals could cleave MB effectively to complete the degradation process. After photocatalysis, PXRD patterns of 1, 3, and 4 are measured, and they are nearly identical to those of the original complexes, indicating that their basic frameworks remain unchanged (Figure 8). Thus, 1, 3, and 4 are the potential stable photocatalysts with improved photocatalytic activities in the degradation of organic dyes.

Figure 11. Luminescent intensities of 1 (a and b) and 4 (c and d) with the addition of 10 mM of the various metal ions in aqueous solutions. H



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Figure 12. Comparison of the luminescence intensities of 1 (a) and 4 (b) in aqueous solutions with the introduction of other metal ions (Al3+, Cd2+, Co2+, K+, Na+, Pb2+) in the absence and presence of 10 mM Cr3+.

Figure 13. Room-temperature luminescent intensities of 1 (a) and 4 (b) with adding 5 equiv of Cr3+ and 1 equiv of other metal ions in aqueous solution (5 equiv is equal of 10 mM).

Luminescent Sensing for Cr3+ Cations. The luminescent properties of 1 and 4 were studied in the solid state at room temperature (Figure S4). The main emission peak of the free Htrb ligand is at 501 nm upon excitation at 340 nm, which is probably from the π* → π or π* → n transition.84 For 1 and 4, their emission peaks appear at 579 (λex = 337 nm) and 418 nm (λex = 357 nm), respectively. With respect to the free Htrb ligand, the emission band of 1 is red-shifted by 78 nm, which may be attributed to a mixed contribution of intraligand and ligand-to-ligand charge transition (LLCT).85,86 Nevertheless, the emission of 4 is blue-shifted by 83 nm relative to the Htrb ligand. The observed blue-shift is probably ascribed to ligandto-metal charge transfer transition (LMCT) involving the Htrb ligands and Ag(I) ions.84,85 Moreover, variable temperature luminescent property of compound 1 as a representative example was explored in the temperature range from 80 to 300 K. As illustrated in Figure 10, upon cooling, the emission intensities drastically increase. Notably, the maximum emission peak has a weak blue-shift with increasing temperatures from 80 to 300 K. Usually, this phenomena that led to emission intensity enhancement with the reduction of temperature can be probably ascribed to the thermally active of phonon-assisted tunneling from the excited states between the low-energy site and the highenergy site.87,88

Attributing to the characteristic structure of the Htrb ligand, the free N atoms from six functional triazole groups provide potential open Lewis base sites for specific coordination sites of the additional metal ions that can tune the luminescent properties.89,90 So, the quenching effects of the metal ions, such as Al3+, Cd2+, Co2+, K+, Na+, Pb2+ and Cr3+, on the fluorescent intensities of 1 and 4 were investigated. As depicted in Figure 11, upon the Cr3+ ions being added in, the emission intensities of 1 and 4 decreased remarkably. Nevertheless, other metal ions have no evident effects on the emission intensities of 1 and 4. Most interestingly, the luminescent intensities of 1 and 4 decreased pronouncedly with the increasing of the concentration of Cr3+ ions. Further, to check the excellent quenching sensitivity of 1 and 4 toward Cr3+ over other metal ions, the experiments were carried out by introduction of other metal ions (Al3+, Cd2+, Co2+, K+, Na+, Pb2+) into the system. As illustrated in Figures 12 and 13, the quenching selectivity of 1 and 4 to Cr3+ is not interfered by the addition of other metal ions. Thus, 1 and 4 are capable of selectively sensing the exoteric Cr3+ through the luminescent emission quenching. The reduction of luminescent intensity may account for less effective intramolecular energy transfer between Htrb ligand and metal ions (Zn2+ and Ag+).91−93 It is well-known that several factors can influence the fluorescence attenuation efficiency of the compounds, such as I



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energy constraints, the binding strength to the analytes, its porosity, etc.94,95 As far as our compounds 1 and 4 are concerned, six nitrogen atoms from six triazole groups of each Htrb ligand are free to the Zn(II) and Ag(I) ions and thus extensively decorate the framework as free-standing donors for guest metals (Figure 14). The further interaction between guest

CCDC 1412476−1412479 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J.Y.). *E-mail: [email protected]. Fax: +86-431-85098620 (J.-F.M). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant Nos. 21277022, 21371030, 21301026, and 21401063) and Jiangsu Province NSF (Grant No. BK20140452).

Figure 14. View of the Lewis based site of 1.

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metal ions and the readily accessible Lewis basic nitrogen atoms minimize the energy transfer efficiency from π → π* orbital within the Htrb ligand, thus reducing the energy transfer from the organic ligands to metal ions.92

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CONCLUSIONS In summary, a new family of inorganic−organic hybrid compounds built with various POV units and a flexible hexadentate Htrb ligand have been hydrothermally synthesized and structurally characterized. These hybrid compounds display intriguing 3D frameworks with variable POV clusters, chains, and layers. The result indicates that multiple coordination sites and the flexible nature of the Htrb ligand exhibit significant effects on the construction of the inorganic−organic hybrid complexes. Strikingly, compounds 1, 3, and 4 show highly photocatalytic activity for the degradation of organic pollutants MB and MO under UV light irradiation. The ·OH quenching experiment using the TAB as a ·OH scavenger implies that the photodegradation of catalysts 1, 3, and 4 toward MB is predominately through attack of ·OH radicalsn. Remarkably, 1 and 4 show the efficient fluorescent probe for Cr3+ ions and high selectivity with strong emission quenching effects over other metal ions.

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REFERENCES

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.5b01226. Experimental section, selected bond lengths and angles, coordination modes of the Htrb ligands in 1−4, the proposed photodegradation mechanism of MB with 1 as catalyst under UV light irradiation, absorption spectra of the MO and MB solutions during the decomposition reaction under UV irradiation without any catalyst, absorption spectra of the MB solution during the decomposition reaction in the presence of 1/TAB and photocatalytic decomposition of MB solution under UV with the use of compound 1 and 1 in the presence of TAB, emission spectra of compounds 1 and 4, TG curve of compounds 2 and 4 (PDF) J



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