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Photocatalytic Application of 4f-5f Inorganic-Organic Frameworks: Influence of Lanthanide Contraction on the Structure and Functional Properties of a Series of Uranyl-Lanthanide Complexes [ChemPlusChem](ChemPlusChem Via Acquire Media NewsEdge) A series of 4f-5f heteronuclear metallic complexes were synthesized by a hydrothermal route from mixtures of uranyl acetate, lanthanide nitrate, and 1,2,4,5-benzenetetracarboxylic acid (H4BTA) in an aqueous system. Single-crystal X-ray analysis revealed that the thirteen UO2 -Ln-BTA complexes ([(UO2)1.5 Ln-(C10 O8 H2)1.5 (H2 O)x].y H2 O; Ln=La (1), Ce (2), Pr (3), Nd (4), Sm (5), Eu (6), Gd (7), Tb (8), Dy (9), Ho (10), Er (11), Tm (12), Yb (13); x=6 for 1-4, x=5 for 5-13; y=0 for 4, y=1 for 3, y=2 for 2, y=3 for 5, 9, 10, 12, 13, y=4 for 1, 6, 7, 8, 11) are isomorphous with three-dimensional structures. For the purpose of developing potential applications of the UO2 -Ln-BTA complexes, we investigated their surface photovoltage and photo-catalytic properties. In addition, the thermogravimetric analysis and photoluminescent properties of these complexes were also studied in detail. Keywords : heterometallic complexes · lanthanides · metal- organic frameworks · photochemistry · surface photovoltage spectroscopy Introduction In recent years, the uranium element has attracted extensive attention because of its extraordinary fission ability and wide use as nuclear fuel or in nuclear weapons. However, the dis- posal of nuclear waste has not been well resolved. In nature, elemental uranium normally exists in uranium compounds with different oxidized states varying from + 4to+ 6 because of its active chemical properties.[1] In recent years, uranium compounds with different oxidized states have been investi- gated extensively.[2, 1b-d] In particular, uranyl-based complexes have been studied widely because of their extraordinary properties, such as the diversity of the structural topologies, dimen- sionalities, and connection modes.[3, 1a] The key to designing uranyl organic frameworks (UOFs) with desired networks and outstanding features mainly depends on the judicious and ra- tional choice of the bridging ligand because it can greatly in- fluence the molecular packing arrangement of the compound. One strategy that has been adopted to build up this kind of hybrid inorganic-organic building block is to link the uranyl cation with organic ligands,[4] such as rigid,[5] flexible,[6] and rigid-flexible polycarboxylic acid ligands ; rigid-flexible mixed polycarboxylic acid ligands;[7] and N-heterocyclic ring ligands ;[8] which lead to the formation of UOFs. For purposes of clarity, the UOFs reported previously were mainly classified into five types as follows : (1) rigid uranyl-polycarboxylate coordination polymers, such as [HNEt3][UO2(5-SIP)] (5-SIPH3 = 5-sulfoisoph- thalic acid)[5a] and [UO2(H2L)(H2O)2]·2H2O(L= biphenyl-3,3,4,4- tetracarboxylic acid) ;[5c] (2) flexible uranyl-polycarboxylate coor- dination polymers, such as [H2NMe2]2[(UO2)4O2(OH)2(C8)2- (CB6)]·8 H2Oand[H2NMe2]2[(UO2)4O2(OH)2(C9)2(CB6)]·5 H2O (CB6 = cucurbit[6]uril, H2C8 = HOOC(CH2)6COOH, H2C9 = HOOC- (CH2)7COOH);[6b] (3) U-rigid-flexible mixed polycarboxylate co- ordination polymers, such as [(UO2)3(H2O)2L] (H6L =hexakis[4- (carboxyphenyl)oxamethyl]-3-oxapentane),[7b] [Me4N]8[L(UO2)]4 (H4L= bis-methylterephthalamide);[7d] (4) U-N-heterocyclic ligand complexes, for example, [(UO2)2(2,2'-bipy)- (CH3CO2)(O)(OH)] (2,2'-bipy=2,2'-bipyridine);[8c] (5) U-M-L framework complexes, such as [Zn2(bipy)2(UO2)2(L)2(H2O)2] (H4L= 1-hydroxyethylidenediphosphonic acid)[8d] and [(ZnO)2- (UO2)3(NA)4(OAc)2](HNA= nicotinic acid; HOAc = acetic acid);[9] (6) U-Ln-L framework complexes, such as [(UO2)2Ln(L)- (C2O4)(OH)(H2O)]·3H2O (Ln= Gd or Eu, H4L = 1,4,7,10-tetraazacy-clododecane-N,N',N'',N'''-tetraacetic acid).[7e] It was found that the rich variety of coordination modes of the polycarboxylate groups of the above-mentioned organic ligands leads to differ- ent complexes with 1D chains, 2D sheets, or 3D network struc- tures. Previous studies found that polycarboxylic acid ligands are excellent linkers for the formation of coordination poly- mers. Among these polycarboxylate ligands, 1,2,4,5-benzenete- tracarboxylic acid (H4BTA) has attracted much attention[10] by virtue of its inherent chemical features : (1) the modality of its four potential binding sites belongs to the classical polycarbox- ylate O-donor ligand, which especially prefers bonding with metal ions ; (2) its variation and richness of coordination modes in binding to metal ions can bring about unforeseen structure patterns and packing frameworks ; (3) the protonated or depro- tonated carboxylate groups can easily behave as hydrogen- bond donors or acceptors, which are prone to yield supra- molecular structures through inter- or intramolecular hydro- gen-bonding interactions with solvent molecules or other or- ganic guest molecules, thereby resulting in higher-unit-cell di- mensionalities of open structures ; (4) its centrosymmetric for- mation may be helpful for improving its coordination capabili- ty with metal ions and for enhancing the stability of coordination polymers. Additionally, with their p-conjugated system composed of a benzene ring and four carboxylate groups, H4BTA ligands behave as efficient antenna linkers that favor fluorescence enhancement of the complexes, which is an interesting research area for the development of fluorescent materials. In contrast to the flourishing and prosperous transi- tion-/lanthanide-metal complexes, uranyl complexes with H4BTA ligands only using single rigid-type backbones BTA through covalent bonds as a linker to coordinate to uranyl ions[11] are not only relatively rare but also structurally simple. So, it is very necessary to develop new uranyl metal-organic frameworks (MOFs) involving the H4BTA ligand with extended structures and promising properties. Moreover, it is proposed that introducing a lanthanide atom into U-BTA framework complexes may be a more direct and effective route to creat- ing uranyl MOFs with double structural and functional proper- ties. So, in this study we will report the synthesis of U-BTA framework complexes with lanthanide atoms and their various properties, such as structure, surface photovoltage behavior, fluorescence, and photocatalytic properties. Uranyl-containing complexes possess promising properties for potential applications in catalysis,[12] magnetic materials,[13] gas separation/storage,[14] ion exchange,[15] proton conductivi- ty,[16] chiral materials,[17] biosensors,[18] and biosorption.[19] In particular, in the field of photochemistry, uranyl-containing complexes have been widely reported as a kind of photocata- lyst that is mainly caused by the uranyl bonds. Although the complex [(ZnO)2(UO2)3(NA)4(OAc)2] (HNA= nicotinic acid; HOAc= acetic acid) has been reported by the group of Jie- Sheng Chen,[8] there has been no comprehensive investigation of the surface photoelectronic properties of uranyl-containing complexes. Moreover, there have been no reports on the pho- toelectronic properties of 4 f-5f heteronuclear complexes. Herein, we report the surface photoelectronic properties of 4 f- 5 f heteronuclear complexes by using the surface photovoltage method (SPV), which is a well-established contactless tech- nique for the characterization of semiconductors. It relies on the analysis of illumination-induced changes in the surface voltage and can be performed by monitoring the surface pho- tovoltage spectroscopy (SPS). Research of the SPV method can be traced back to the late 1940s, and after several decades of study, it has became a highly sensitive tool for detecting the change in surface electronic behavior.[20] By following the preparation method of Ln+R+F(R=rigid ligand; F= flexible ligand), our group designed and synthe- sized several series of unique lanthanide organic frameworks with good luminescence properties, and parts of this research has already been reported.[21, 10a] Based on our previous investi- gations, we have designed and synthesized new uranyl-lantha- nide coordination polymers with the H4BTA ligand. In this study, we utilized the rigid H4BTA ligand and intro- duced lanthanide ions into the synthesis system to develop a new family of U-Ln MOFs with H4BTA as linker. As a result, a series of U-Ln MOFs (1 -13) with 3D structures were synthe- sized for the first time under hydrothermal conditions : [(UO2)1.5Ln(C10O8H2)1.5(H2O)x]·yH2O (Ln= La (1), Ce (2), Pr (3), Nd (4), Sm (5), Eu (6), Gd (7), Tb (8), Dy (9), Ho (10), Er (11), Tm (12),Yb(13);x=6 for1-4,x=5 for5-13;y=0 for4,y=1 for 3, y= 2 for 2, y = 3 for 5, 9, 10, 12, 13, y =4 for 1, 6, 7, 8, 11). It is worth mentioning that except for complexes 2 and 4, which have already been reported,[22] for the first time we have synthesized complexes containing uranium and a series of lan- thanide atoms (except for promethium and lutetium) by means of a hydrothermal method.[7e, 23] Moreover, we have in- vestigated the surface photoelectronic behavior to further de- velop the photochemical properties of the thirteen complexes. We also studied the photocatalytic properties of the thirteen complexes and discussed the effects of introducing the lantha- nide atoms into the U-Ln-BTA framework, structural parame- ters, and their functional properties. Results and Discussion Synthesis By using a hydrothermal reaction method, a novel family of U- BTA-Ln coordination polymers was successfully generated. In the reaction system, it is crucial to address the different physi- cal and chemical properties of 4 f and 5f metal atoms when co- ordinating with the polycarboxylate ligand of H4BTA. Herein, we endeavored to search for the optimal reaction conditions in which two metal cations are able to coordinate to the poly- carboxylate ligand at the same time. To prepare heteronuclear U-BTA-Ln coordination polymers, we initially planned to use a molar ratio value UO2(CH3COO)2/Ln(NO3)3/BTA of 1:1:1 (by using 0.25 mmol LnCl3·6 H2O and 10 mL H2O) with reaction conditions of 120 8C for three days in a Pyrex vial with high temperature (120 8C) and pressure glass, but only single Ln- BTA crystals were obtained. After this, various molar ratios were tested, but failed. Following this, we adopted the initial molar ratio value and reaction conditions, but adjusted the pH value of the medium to 2 by using 4 m HNO3, and in doing so we finally gained the products U-BTA-Ln. This indicated that the pH value might be one of the main factors influencing the formation of the Ln-BTA-U complexes. The compositions of 1-13 were confirmed by elemental analysis, IR spectroscopy, and powder X-ray diffraction (PXRD). All the peaks present in the measured patterns closely matched those of the simulated patterns generated from single-crystal diffraction data as shown in Figures S1-S13 in the Supporting Information. Structural description Single-crystal X-ray analyses revealed that the frameworks of 1-13 are isomorphous except for the number of lattice water molecules and the water molecules coordinated with the lan- thanide atoms. Therefore, complex 6 is taken as an example to present and discuss the structure in detail. The single-crystal X-ray study showed that complex 6 is a three-dimensional framework, crystallizing in the triclinic system with space group P1- . The asymmetric unit (Figure 1a) contains one eight-coordinated europium atom, one and a half uranyl cations, one and a half bta4 ^ ligands, five coordinated water molecules, and four lattice water molecules. Two oxygen atoms (O1, O2) bond axially to U1 to form a uranyl unit. Furthermore, three oxygen atoms (O8, O11, O12) from monodentate carboxylate groups of the three bta4^ ligands, and two oxygen atoms (O4, O5) from the chelating bidentate carboxylate group of one BTA2^ ligand coordinate with the uranium atom U1 equatorially in a distorted-pentagonal bipyr- amidal [U1O7] geometry (Figure 1b). One uranyl cation (O3-U2- O3#), two oxygen atoms (O7, O7#) from the monodentate car- boxylate groups of two BTA4^ ligands, and four oxygen atoms (O14, O15, O14#,O15#) from chelating bidentate carboxylate groups of two BTA4^ ligands in a distorted-hexangular [U2O8] bipyramidal geometry (Figure 1c). The U^Ouranyl bond is in the range of 1.762-1.778 ^ and the U^Ocarb bonds are in the range of 2.332-2.567 ^. The europium atom coordinated with two oxygen atoms (O9#,O10#) of two adjacent carboxylate groups come from one BTA4^ ligand, one oxygen atom (O6) of the car- boxylate group comes from another BTA4^, and the five oxygen atoms (O16 to O20) of the five coordinated water mol- ecules form a distorted-square antiprism EuO8 polyhedron con- figuration (Figure 1d). The bond lengths of Eu^Ocarb and Eu^ Owater are in the range of 2.385(2)-2.414(2) ^ and 2.355(2)- 2.462(3) ^, respectively. The BTA4^ ligand presents two coordinated modes in the structure of the complex, the m6-h1, h2, h2, h2 bridging mode (Figure 2a) and the m4-h1, h1, h1, h1 bridging mode (Figure 2b). When adopting the first coordination mode, the BTA4^ ligand chelating the bidentate carboxylate group and the para-posi- tioned monodentate carboxylate groups of the BTA4^ ligand link the [U1O7] polyhedrons to form an infinite one-dimension- al chain. Furthermore, two of these so-built chains intercon- nect with each other through the monodentate carboxylate groups that occupy the meta position of the chelating biden- tate carboxylate group to form a double self-assembled infinite chain (Figure 2c). When the BTA4^ ligand adopts the m4-h1, h1, h1, h1 bridging mode, two of the carboxylate groups coordi- nate with the U2 metal atom in the chelating bidentate mode and link with the [U2O8] poly- hedron to form a one-dimen- sional infinite chain (Figure 2d). Then, the two kinds of chain in- terconnect one by one with the remaining monodentate carbox- ylate groups of the BTA4^ ligand with the two coordination modes to form an infinite two- dimensional U-BTA sheet (Fig- ure 2e). Based on these U-BTA sheets, a cluster unit Eu(H2O)5, which contains the Eu3 + cation and five coordinated water mol- ecules (Figure 2 f), links the adja- cent U-BTA sheets to form a three-dimensional framework (Figure 2g) by connecting through the carboxylate group of the BTA ligands. This then generates a new topologic structure of a 3D (2,4,6)-connected 5-node network with a {44 ;62,88, 12}{44,62}2{8} Schl^fli symbol after analysis by the TOPOS4.0 software package (Figures S14 and S15).[24] Therefore, the overall structural construction of the complex can be summarized as that which forms a 1D U-BTA chain motif, then a 2D U-BTA sheet motif, and lastly a 3D U- BTA-Er(H2O)5 network structure. Lanthanide contraction By comparison and analysis of the lattice parameters, interlayer spacing (d), and Ln^Ocarb and Ln^OH2O (average value) bond lengths of complexes 1-13, most of the data reveal trends that decrease as a function of the increasing atomic sequence number from lanthanum to ytterbium. The correlation be- tween the lattice parameter, Ln^O bond length of com- plexes 1-13, and the atomic sequence number was statistically tested and the results are shown in Figure 3 and the detailed parameters are listed in Table S3. The bond lengths of Ln^Ocarb and Ln^OH2O (average value) linearly decreased as a function of the increasing atomic sequence number. Although relationship plots of the lattice parameters a and b, the interlayer spacing (d), and the volume of the crystal cell with atomic sequence number are not linear, the plots still present a decreasing trend with increasing atomic number. These phenomena can be regarded as a direct consequence of the so-called "lantha- nide contraction".[25] In addition, the nine-coordinated struc- tures of complexes 1-4 and eight-coordinated structures of complexes 5-13 should also be classed as evidence of "lantha- nide contraction". Thermal analysis Complexes 1-13 were studied by thermogravimetric analysis (TGA) and exhibited similar thermal behaviors upon compari- son of their thermogravimetric curves. Hence, complex 6 is taken as an example to illustrate the thermogravimetric behav- ior of these thirteen uranyl-BTA-lanthanide complexes. In the thermogravimetric curve of complex 6 (Figure 4), two weight-loss steps are observed. An immediate weight loss exists at 158 8C on heating, which corresponds to the loss of the four lattice water molecules and the five coordinated water molecules (obsd 85.52 %, calcd 85.18 %); this indicates that the interaction of these water molecules with the frame- work is weak. The second weight loss starts at 411 8C and shows a rather complicated profile that is complete at 935 8C. The final remaining weight was 54.29 %, which is in good agreement with the formation of stoichiometric mixtures (1/ 2)U3O8[1e,26]and (1/2)Eu2O3(calcd54.77%). It is worth noting that all of the TGA curves indicate an im- mediate weight loss of the lattice water molecules and the co- ordinated water molecules. These water molecules of com- plexes 5, 6, 7, and 10 (Ln = Sm, Eu, Gd, and Ho) were lost in one step, whereas those of the other nine complexes were lost through several steps. These water-loss processes are all com- plete within 340 8C. Another weight-loss behavior occurred at about 410 8C and was completed at about 8008C, which re- lates to the removal of the organic ligands. The weight-loss be- havior of the other twelve complexes and detailed weight-loss information are listed in Figures S16-S27 and Table S4. Infrared spectroscopy The IR spectra of complexes 1-13 are similar, so complex 6 is taken as an example. In the IR spectrum of complex 6, a broad band from 3651 to 3289 cm^1 is associated with free water molecules. The intense peaks at 1542 and 1396 cm^1 are due to the asymmetric and symmetric stretching vibration of the C=O bond, respectively. The asymmetric stretching vibration of the U=O bond of the uranyl cation is observed at 921 cm^1, whereas the symmetric stretching vibration of the U=O bond varies from 808 to 877 cm^1.[27] The detailed assignment and spectra of the other twelve complexes are listed in Table S5 and Figures S28 and S29. Photoluminescence properties The photoluminescent behaviors of complexes 1-13 were in- vestigated in the solid state at room temperature with an exci- tation wavelength of 439 nm. The photoluminescent spectra of complexes 1-13 are similar (Table S6 and Figures S30-S41). Again, we chose complex 6 as an example to illustrate the photoluminescent properties of the thirteen complexes. The photoluminescent spectrum of the benchmark compound UO2(CH3COO)2·2 H2O was also investigated for comparison. As shown in Figure 5, the fluorescence spectra of UO2- (CH3COO)2·2 H2O consists of six typical well-defined bands, which correspond to the electronic and vibronic transitions of S11!S00 and S11!S0n (n= 0-4), respectively.[28,27a] The fluores- cence spectra of 6 exhibit seven absorption bands at 459, 470, 496, 518, 540, 596, and 593 nm. The prominent emission band at 459 nm is attributed to the BTA ligand. The other six emis- sion bands are associated with the uranyl cation, and exhibited a slight redshift relative to the fluorescence spectra of the benchmark compound. The most intense peak of the bench- mark compound is located at 512 nm, whereas the emission bands at 459 and 470 nm of complex 6 are extraordinarily in- tense so that the peaks at 496, 518, 540, 596, and 593 nm could be hardly observed. Furthermore, as was known, EuIII should exhibit five emission bands at 577, 590, 618, 653, and 702 nm for 5D0!7Fn (n = 0, 1, 2, 3, and 4) transitions.[29,21b] How- ever, the emissions of EuIII were not observed in the spectra (the absorptions of SmIII,TbIII, and DyIII were not observed either). This phenomenon of fluorescence quenching could be attributed to the partial energy-level overlap of emission from the uranyl cation with the f-f* absorption band of LnIII,[27a] thereby yielding energy transfer and nonradiative decay. Photoelectronic properties UV/Vis spectroscopy The diffuse reflectance UV/Vis spectra of complexes 1-13 show five absorption peaks at very similar positions from 200 to 420 nm. For complex 6 (Figure 6), the peaks appearing at about 306 and 418 nm are attributed to the typical absorption peaks of the uranyl cation, which arise from the electronic transition of the U=O double bond.[8c, 30] The other two absorp- tion peaks appearing at about 270 and 336 nm are associated with the p-p* transfer of the BTA4^ ligand. For the lanthanide atoms, f-f* transition peaks are present in the visible light, in- frared, and far-infrared region. Detailed information and assign- ment of complexes 1-13 are listed in Table S7 and Figur- es S42-S53. Surface electronic behavior Surface photovoltage (SPV) measurements are the most common and sensitive way to study photoelectric properties. The character of the photovoltaic response of organic semi- conductors, inorganic semiconductors, organic-inorganic semi- conductors and the transition or diffusion of electrons on the surface of a solid sample can be detected by SPV.[20a, b] At present, SPV has been employed in the study of charge transfer in photostimulated surface interactions, dye sensitization process- es, photocatalysis electrons, charge transition between differ- ent phases, surface electronic behavior, and photoelectric con- versation. By detecting the photovoltage intensity change with the wavelength of the excited light source, the spectroscopy that reflects the relationship between the wavelength of excit- ed light and the photovoltage intensity is the so-called surface photovoltage spectroscopy (SPS). In other words, SPS is the detailed reflection of the SPV. SPS is an effective technique to investigate the surface charge behavior of a solid sample. It is significant to study the electronic transition of a surface and interface because it not only relates to the electronic transition under light-induced conditions, but also reflects the separation and diversion of photogenerated charge. By studying the SPS of the sample, we can not only know the electron transition behavior on a surface but also make a judgment about the type of semiconductor samples. In SPS, the detected signal is equivalent to the change in the surface-potential barrier on il- lumination (DVs). The value is calculated by the equation : DVs =Vs^Vs0, in which Vs and Vs0 are the surface-potential barri- ers before and after illumination, respectively. A positive re- sponse of SPV (DVs > 0) means that the sample is characterized as a p-type semiconductor, whereas a negative one means that the sample is an n-type semiconductor.[31, 32] By careful analysis of the UV/Vis spectra of the thirteen com- plexes, we think these complexes are extended semiconduc- tors. After treatment with the program Origin 7.0, the SPS spectrum shape of complexes 1-13 was divided into two types based on whether the spectra show one or two main peaks. The SPS of complexes 1, 3, 5, 7, 9, 12, and 13 (Ln = La, Pr, Sm, Gd, Tb, Dy, Tm and Yb) exhibit two sets of peaks (Figur- es S54-S61), whereas those of complexes 2, 4, 6, 8, 10,and11 (Ln = Ce, Nd, Eu, Ho, and Er) exhibit only one set of peaks (Fig- ures S62-S66). For complex 9 (Ln= Dy), the SPS spectra shows two main peaks that have five response signals at 329, 373, 462, 477, and 487 nm (Figure 7a). By comparison with the SPS spectrum of H4BTA, the signals at 329 and 373 nm can be at- tributed to the BTA4^ ligand, which is also consistent with the UV/Vis spectroscopy. The other three response signals can be attributed to the uranyl cation by comparison with the SPS spectrum of UO2(CH3COO)2·2 H2O, which is in good agreement with the absorption band at 420 nm from UV/Vis spectroscopy. For the SPS spectrum of complex 2 (Figure 7b), there is just one main peak (391 nm), which can be attributed to the BTA4^ ligand. The response signals (429, 444, 462, 475, and 486 nm) caused by the uranyl cation are not very clearly observed be- cause they just appear as a shoulder peak of the main peak. In summary, after comparison and analysis of the SPS spec- tra of the thirteen complexes, we observed that all of the re- sponse signals caused by the BTA4^ ligand are blueshifted more or less. This phenomenon can be attributed to the energy-level increase of the p-p* transition after coordination with central metal atoms. The response signals caused by the uranyl cation nearly agree with the signals in the SPS spectrum of UO2(CH3COO)2 ; the intensity of the signals at 459, 476, and 486 nm increase more or less and the other signals caused by the uranyl cation become weak or even disappear. Field-induced surface photovoltage electronic spectroscopy (FISPS) is an effective method for the detection of the type of semiconductor by applying an external electronic field to a sample. The intensity of the photovoltaic response signals is related to the efficiency of the separation of photoexcited elec- tron-hole pairs and a positive electric field is beneficial to in- crease the efficiency whereas a negative one has the opposite effect. For a p-type semiconductor, when a positive electric field is applied on the semiconductor surface, the SPV re- sponse increases since the external field is consistent with the built-in field. On the contrary, when a negative electric field is applied, the SPV response is weakened. In contrast to p-type semiconductors, the SPV response intensity of n-type semicon- ductors increases as a negative field is applied and reduces as a positive electric field is applied.[33] The SPS spectra of com- plexes 9, 2, and the others are shown in Figures 7c, and d and Figures S67 and S77 when the external electric field is ^0.2, 0, and + 0.2 V. For complexes 9 and 2, the SPV intensity increases when a positive field is applied, whereas it decreases when a negative field is applied. The FISPS spectra of the other twelve complexes also exhibit a similar phenomenon when the induced fields were applied. Hence, complexes 1-13 are all characterized as p-type semiconductors. Photocatalytic performance Photocatalysts have attracted great attention owing to their potential applications in purifying wastewater and air.[34] Among these photocatalysts, uranyl-containing complexes are extraordinarily important because of the thorough decomposi- tion of the organic ligand mainly caused by the special uranyl double bonds.[35, 3d] Furthermore, it is necessary to point out that the catalytic properties of uranyl-containing complexes can be tuned by the introduction of other metal ions[36] and the presence of lanthanide atoms in complexes 1-13 make comparison possible. During the degradation process, we chose rhodamine B (RhB) as a model dye pollutant to demon- strate the efficacies of complexes 1-13. A 18 W Hg lamp was used as the UV light source. The distance between the reaction vessel and the light source was 15 cm. During the decomposi- tion reaction, a UV-251 spectrophotometer was used to moni- tor the reaction at a specific wavelength. To rule out the possi- bility that the photocatalytic activity of complexes 1-13 arises from molecular or oligomeric species formed through dissolu- tion of the solid samples in the photocatalytic reaction sys- tems, control experiments were conducted. We filtered the re- action suspensions after 10 h of irradiation to remove the solid catalyst, and fresh RhB was added into the respective filtrates for catalysis testing. Without solid catalyst in the reaction system, the RhB was almost not degraded in 10 h of irradiation under a Hg lamp, which suggests that the solution contains no photocatalytically active species. The decomposition results of complexes 1-13 under UV irra- diation after 110 min are shown in Figure 8. The decomposition data imply that the photodegradation of the thirteen complexes is similar and they all exhibit good photolytic activi- ties. The slope of the curves (Figures S78-S90 and Table S8) re- flects the photodegradation rate of the complexes. During the earlier 20 min, the thirteen complexes have high photocatalytic activities for the degradation of RhB (6.05^ 10^2, 6.54 ^ 10^2, 8.00 ^ 10^2, 7.26 ^ 10^2, 7.26 ^10^2, 5.81 ^ 10^2, 12.59 ^ 10^2, 10.17 ^ 10^2, 11.62 ^ 10^2, 15.25 ^10^2, 14.77 ^10^2, 10.17 ^ 10^2, and 18.16 ^10^2 min^1 for complexes 1-13, respectively). Then the photodegradation rate decreases gradually and trends to- wards finally flattening. After 110 min, the photodegradation is almost finished. For the sake of comparison, Figure 9a and b are shown to illustrate the relationship between the photo- degradation rate and photodegradation degrees with atomic sequence numbers, respectively. It is clearly observed that the photodegradation rate and the photodegradation degree in- crease with the increase of the Ln atomic sequence number, al- though their changes are not linear. Lanthanide contraction could be the main reason causing this phenomenon. To our knowledge, the photocatalytic process could be affected by the irradiation intensity, the coordination environments of the central metals, the extent of the conjugation,[37] and the size of the metal-oxygen complexes.[38] The only difference among isomorphous complexes 1-13 is the distance between layers, which is mainly caused by the radii of the lanthanide atoms. With the decrease of the lanthanide atomic radius, the dis- tance between the layers also decreases, further leading to an increase of the specific surface area of the active site and then to an increase of the photocatalytic effects. A schematic illustration of the photocatalytic mechanism of the complex is shown in Figure 10. It was found that the pres- ence of LnIII ions in the complex might cause the steps[39] shown in Scheme 1. In the whole process, the charge separa- tion occurred in the complex when the UV light was absorbed, and the electrons in this system could be trapped by the LnIII ion to form the LnII ion. Owing to the adsorbed oxygen pres- ent in the system, LnII could be oxidized to LnIII and formed the active species O2C, which is responsible for the degradation of pollutants. Conclusion In summary, we have synthesized a series of 4 f-5 f heteronu- clear complexes [(UO2)1.5Ln(BTA)1.5(H2O)x]·y H2O by means of a hydrothermal method. Spectroscopic data, including IR, UV/ Vis, and fluorescence spectra, are fully consistent with these crystal structures. To exploit the potential photoproperties, we investigated the surface photovoltage spectra and the photo- catalytic properties of the thirteen complexes. For the first time, we have investigated the surface photovoltage spectra of 4 f-5 f heteronuclear complexes and our results demonstrate that the surface photoelectronic signals are caused by the uranyl cation and the H4BTA ligands, and they have no rela- tionship to the lanthanide elements of the complexes. More- over, the introduction of lanthanide atoms affected the photo- degradation rate of the thirteen complexes through the "lan- thanide contraction" phenomenon. In addition, the thirteen complexes exhibited a series of signals when irradiated by visi- ble light in the surface photovoltage spectra, ranging from about 300 to 650 nm, and high photocatalytic activities under the irradiation of ultraviolet light. Our study indicates that these complexes with high utilization ratio of ultraviolet and visible light could have extraordinary potential as photocatalyt- ic materials. Experimental Section General considerations IR spectra were recorded on a JASCO FT/IR-480 PLUS Fourier Trans- form spectrometer with pressed KBr pellets in the range 200- 4000 cm^1 at room temperature. UV/Vis spectra and absorption data were acquired for the two complexes and UO2(Ac)2·2 H2O from single crystals using a JASCO V-570 UV/Vis/NIR microspectro- photometer. The absorption data was collected in the range of 200-2500 nm at room temperature. TGA was performed on a Perki- nElmer Diamond TG/DTA under air from room temperature to 10008C (complexes1, 9-13) or 12008C (complexes 2-8) with a heating rate of 10 8 C min^ 1. PXRD patterns were obtained on a Bruker Advance-D8 equipped with CuK a radiation ( l = 1.5418 ^) in the range of 58 <2q < 608, with a step size of 0.028 (2q), and a count time of 2 s per step. The luminescence spectra were re- ported on a JASCO FP-6500 spectrofluorimeter (solid). The surface photovoltage spectra were measured with a home-built apparatus. The electrode was made of optical glass coated with indium and tin oxides (ITO). The crystals were powdered and put into a sample cell that consisted of an ITO/sample/ITO sandwich structure. Stan- dard p-type silicon flake was used to adjust the comparative phase and a xenon lamp was used as an illuminant to supply radiation in the range of 300-800 nm.[31] Synthesis method Caution! Because uranium is a radioactive and chemically toxic ele- ment, uranium-containing samples must be handled with suitable care and protection. In a typical preparation, an aqueous mixture (10 mL) containing UO2(CH3COO)2·2 H2O (106.0 mg, 0.25 mmol), H4BTA (63.5 mg, 0.25 mmol), Ln(NO3)3·6 H2O (0.25 mmol, Ln=La (1), Ce (2), Pr (3), Nd (4), Sm (5), Eu (6), Gd (7), Tb (8), Dy (9), Ho (10), Er (11), Tm (12), Yb (13)) and distilled water (5 mL) were sealed in a Teflon- lined autoclave (20 mL) and heated at 120 8C for three days. After filtration and washing thoroughly with distilled water to remove amorphous impurities, the sample was air-dried at ambient tem- perature. Light yellow crystals were obtained. C15H23LaO25U1.5 (1): Yield: 66%; IR data (KBr pellet): ñ=3421 (vs), 1617 (vs), 1568 (vs), 1397 (vs), 1135 (s), 916 (s), 873 (w), 838 (w), 826 (w), 812 cm^1 (w) ; elemental analysis calcd (%) for 1: C 16.39, H 2.09 ; found : C 16.00, H 2.25. C15H19CeO23U1.5 (2): Yield: 65%; IR data (KBr pellet): ñ=3421 (vs), 1615 (vs), 1568 (vs), 1397 (vs), 1136 (s), 916 (s), 873 (w), 838 (w), 826 (w), 812 cm^1 (w) ; elemental analysis calcd (%) for 2 : C 16.92, H 1.78 ; found : C 17.10, H 1.63. C15H17PrO22U1.5 (3): Yield: 64%; IR data (KBr pellet): ñ=3417 (vs), 1617 (vs), 1557 (vs), 1397 (vs), 1136 (s), 916 (s), 873 (w), 835 (w), 826 (w), 813 cm^1 (w) ; elemental analysis calcd (%) for 3 : C 17.20, H 1.62 ; found : C 17.35, H 1.70. C15H21NdO24U1.5 (4): Yield: 69%; IR data (KBr pellet): ñ=3417 (vs), 1617 (vs), 1565 (vs), 1400 (vs), 1136 (s), 916 (s), 873 (w), 838 (w), 825 (w), 812 cm^1 (w) ; elemental analysis calcd (%) for 4 : C 16.58, H 1.93 ; found : C 16.55, H 1.88. C15H17SmO22U1.5 (5): Yield: 58%; IR data (KBr pellet): ñ=3421 (vs), 1621(vw), 1539 (vs), 1394 (vs), 1138 (s), 917 (s), 876 (w), 838 (w), 810 cm^1 (w) ; elemental analysis calcd (%) for 5 : C 17.05, H 1.61; found : C 17.21, H 1.46. C15H21EuO24U1.5 (6): Yield: 61%; IR data (KBr pellet): ñ=3411 (vs), 1542 (vs), 1396 (vs), 1139 (s), 921 (s), 879 (w), 838 (w), 810 cm^1 (w); elemental analysis calcd (%) for 6: C 16.46, H 1.92 ; found: C 16.58, H 1.75. C15H21GdO24U1.5 (7): Yield: 62%; IR data (KBr pellet): ñ=3411 (vs), 1617 (vw), 1539 (vs), 1394 (vs), 1139 (s), 924 (s), 879 (w), 838 (w), 810 cm^1 (w) ; elemental analysis calcd (%) for 7: C 16.39, H 1.91; found : C 16.17, H 1.73. C15H21TbO24U1.5 (8): Yield: 70%; IR data (KBr pellet): ñ=3423 (vs), 1624(vw), 1542 (vs), 1397 (vs), 1139 (s), 920 (s), 879 (w), 838 (w), 810 cm^1 (w) ; elemental analysis calcd (%) for 8 : C 16.36, H 1.91; found : C 16.50, H 1.86. C15H19DyO23U1.5 (9): Yield: 68%; IR data (KBr pellet): ñ=3404 (vs), 1539 (vs), 1397 (vs), 1139 (s), 920 (s), 878 (w), 837 (w), 812 cm^1 (w); elemental analysis calcd (%) for 9: C 16.58, H 1.75 ; found: C 16.47, H 1.69. C15H19HoO23U1.5 (10): Yield: 60%; IR data (KBr pellet): ñ=3408 (vs), 1674 (vs), 1534 (vs), 1397 (vs), 1139 (w), 917 (s), 879 (w), 838 (w), 810 cm^1 (w) ; elemental analysis calcd (%) for 10 : C 16.54, H 1.74 ; found : C 16.33, H 1.69. C15H21ErO24U1.5 (11): Yield: 64%; IR data (KBr pellet): ñ=3430 (vs), 1615 (vs), 1544 (vs), 1382 (vs), 1136 (s), 920 (s), 879 (w), 835 (w), 813 cm^1 (w) ; elemental analysis calcd (%) for 11: C 16.24, H 1.89 ; found : C 16.35, H 1.97. C15H19TmO23U1.5 (12): Yield: 65%; IR data (KBr pellet): ñ=3436 (vs), 1615 (vs), 1568 (vs), 1378 (vs), 1136 (s), 920 (s), 879 (w), 835 (w), 810 cm^1 (w) ; elemental analysis calcd (%) for 12 : C 16.48, H 1.74 ; found : C 16.52, H 1.88. C15H19YbO23U1.5 (13): Yield: 58%; IR data (KBr pellet): ñ=3430 (vs), 1615 (vs), 1568 (vs), 1373 (vs), 1133 (s), 920 (s), 876 (w), 835 (w), 809 cm^1 (w) ; elemental analysis calcd (%) for 13 : C 16.42, H 1.73 ; found : C 16.59, H 1.90. X-ray crystallographic studies Suitable single crystals of the thirteen compounds were mounted on glass fibers for X-ray measurement. Reflection data were collect- ed at room temperature on a Bruker AXS SMART APEX II CCD dif- fractometer with graphite-monochromatized MoKa radiation (l = 0.71073 ^). All the measured independent reflections (I > 2s(I)) were used in the structural analyses, and semiempirical absorption corrections were applied using the SADABS program.[40] Crystal structures were solved by direct methods.[41] All non-hydrogen atoms were refined anisotropically. Hydrogen atoms on carbon and nitrogen were fixed at calculated positions and refined by using a riding model. The hydrogen atom of coordination water molecules were found in the difference Fourier map. Hydrogen atoms of the lattice water molecules in uranyl complexes were not located by difference Fourier maps. All calculations were per- formed using the SHELXS-97 program.[41] Crystal data and details of the data collection and the structure refinement of complexes 1- 13 are given in Table 1. The selected bond lengths and angles are listed in Tables S1 and S2, respectively. CCDC 951036 (1), 951037 (2), 951038 (3), 951039 (4), 951040 (5), 951041 (6), 951042 (7), 951043 (8), 951044 (9), 951045 (10), 951046 (11), 951047 (12), and 951048 (13) contain the supplementary crys- tallographic 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. Acknowledgements This study was supported by the grants of the National Natural Science Foundation of China (no. 21371086), Guangxi Key Labo- ratory of Information Materials, Guilin University of Electronic Technology, P. R. China (project no. 1210908-06K), and State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Col- lege of Chemistry, Jilin University, Changchun 130012, P. R. China (grant no. 2013-05) for financial assistance. [1] a) K. X. Wang, J. S. Chen, Acc. Chem. Res. 2011, 44, 531 - 540 ; b) L. Chate- lain, V. Mougel, J. P^caut, M. Mazzanti, Chem. Sci. 2012, 3, 1075 - 1079 ; c) V. Mougel, J. 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Received : April 22, 2014 Published online on June 24, 2014 Ya-Nan Hou,[a] Xue-Ting Xu,[a] Na Xing,[a] Feng-Ying Bai,*[b] Shi-Bo Duan,[a] Qiao Sun,[c] Si- Yue Wei,[a] Zhan Shi,[d] Huan-Zhi Zhang,[e] and Yong-Heng Xing*[a] [a] Y.-N. Hou, X.-T. Xu, N. Xing, S.-B. Duan, S.-Y. Wei, Prof. Y.-H. Xing College of Chemistry and Chemical Engineering Liaoning Normal University No. 850 Huanghe Road, Dalian 116029 (P. R. China) E-mail : [email protected] [b] Prof. F.-Y. Bai College of Life Science Liaoning Normal University No. 850 Huanghe Road, Dalian 116029 (P. R. China) [c] Dr. Q. Sun Centre for Theoretical and Computational Molecular Science Australian Institute for Bioengineering and Nanotechnology The University of Queensland, Brisbane (Australia) [d] Prof. Z. Shi State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry Jilin University, Changchun 130012 (P. R. China) [e] Dr. H.-Z. Zhang Guangxi Key Laboratory of Information Materials Guilin University of Electronic Technology Guilin 541004 (P. R. China) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cplu.201402121. (c) 2014 Blackwell Publishing Ltd. |