A series of Ln4III clusters: Dy4 single molecule magnet and Tb4 multi-responsive luminescent sensor for Fe3+, CrO42−/Cr2O72− and 4-nitroaniline

Five tetranuclear lanthanide clusters of compositions [Ln4L4(NO3)2(Piv)2]·2CH3OH (Ln = Gd (1), Tb (2), Dy (3), Ho (4), Er (5); H2L = 2-(((2-hydroxy-3-methoxybenzyl)imino)methyl)-6-methoxyphenol; Piv = pivalic acid) were synthesized under solvothermal conditions. The structures of 1–5 were characterized by single-crystal X-ray crystallography. Complexes 1–5 possess a zig-zag topology with [Ln4O6] cores being formed by the fusion of oxygen atom-bridged two [Ln2O2] moieties. Direct-current magnetic susceptibility studied in the 2–300 K range revealed weak antiferromagnetic interactions in 1, 2, 4, 5 and ferromagnetic interactions in 3. Complex 3 exhibits single molecule magnet (SMM) behavior. The luminescence studies indicated that complex 2 can serve as highly sensitive and selective luminescent materials for Fe3+, CrO42−, Cr2O72− and 4-nitroaniline (4-NA), demonstrating that complex 2 should be a potential candidate for multi-responsive luminescent sensor.


Introduction
Polynuclear lanthanide clusters have been attracting considerable attention due to their fascinating structures 1 as well as potential applications in single molecule magnets (SMMs), 2 luminescent devices 3 and magnetocaloric materials. 4 SMMs are of considerable promise as molecular spintronic devices for high-density data storage. 5 The signicant anisotropy of lanthanides arising from large unquenched orbital angular momentum 2a,2u has made lanthanides to be attractive candidates for SMMs. Recently, the research efforts towards lanthanide SMMs have been devoted to polynuclear complexes with variable nuclearities. 2 Moreover, owing to the characteristic sharp emission peaks, a large Stokes shi and a wide emission range, the utilization of lanthanide complexes as luminescent materials, particularly luminescent sensor, 6 also attracts intensive interest.

Physical measurement
Elemental analyses for C, H, and N were carried out with a Perkin-Elmer 2400 analyser. Fourier transform (FT) IR spectra were recorded with a Bruker VERTEX 70 FTIR spectrophotometer in the range of 600-4000 cm À1 . Magnetic susceptibility measurements were performed in the temperature range of 2-300 K, using a Quantum Design MPMS XL-7 SQUID magnetometer. Powder X-ray diffraction (PXRD) was obtained from a Rigaku D/Max-2500 diffractometer at 40 kV and 100 mA with a Cu-target tube and a graphite monochromator. Fluorescence spectra for the samples were recorded on a FLS-920 uorescence spectrophotometer. The UV-Vis absorption spectra were determined with an Agilent Cary-60 spectra photometer at room temperature. X-ray photoelectron spectroscopy (XPS) experiments were carried out by a ESCALAB 250Xi spectrometer using an Al Ka source at room temperature.
X-ray single-crystal data collection and structure determination The diffraction data for 1-5 were collected on a Bruker D8-VENTURE (120 K) CCD X-ray diffractometer equipped with a graphite monochromated Mo Ka radiation (l ¼ 0.71073 A). The u-2q scan technique was applied. The crystal structures of all complexes were solved with the Olex2 solve solution program 18 using Intrinsic Phasing and rened by full-matrix least-squares minimization using the ShelXL renement package. 19 All H atoms were placed on appropriate positions in theory and their positions were rened using the riding model. The details of the crystal parameters, data collection and renements for the complexes are summarized in Table 1. Selected bond lengths and angles with their estimated standard deviations are listed in Table S1. †

Synthesis of complexes
The tetranuclear complexes 1-5 were synthesized under solvothermal conditions. When a mixture of Ln(NO 3 ) 3 $6H 2 O (Ln ¼ Gd, Tb, Dy, Ho, Er), H 2 L (the synthesis of H 2 L was shown in Scheme 1), and pivalic acid, in a 1 : 1 : 1 molar ratio in MeOH/ Et 3 N was sealed in a Pyrex-tube and heated under solvothermal conditions, yellow block crystals were generated aer being heated at 353 K for one day. The experimental powder X-ray diffraction patterns matched well with those simulated from the crystal structures, demonstrating the purity of complexes 1-5.
Description of crystal structure of 1-5 Single crystal X-ray analyses and powder X-ray diffraction analyses indicated that complexes 1-5 are isostructural. They crystalize in the monoclinic P2 1 /c space group (Table 1, Fig. 1). Thus, complex 3 was chosen as a representative for 1-5 to describe the structure in detail. The molecular structure of 3 contains four Dy(III) ions in a zig-zag topology, four L 2À ligands, two deprotonated pivalic acids, two NO 3 À anions and two CH 3 OH solvate molecules ( Fig. 1). Two L 2À ligands coordinate in a m 2 : h 1 , h 2 , h 1 , h 1 fashion whereas the other two L 2À ligands coordinate in a m 3 : h 1 , h 2 , h 1 , h 2 , h 1 mode (Scheme 2). The structure of 3 is composed by two asymmetric [Dy2] units which are connected by phenolate bridges from the Schiff base ligands with Dy/Dy distances of 3.697(2) A (Dy1/Dy2) and 3.839(2) A (Dy2/Dy2A). Dy1 centre is coordinated by two oxygen atoms (O1 and O3) from one ligand, one nitrogen atom (N1) from the same ligand, two oxygen atoms (O5 and O6) from another ligand set, two oxygen atoms (O11 and O12) from one NO 3 À ion, and one oxygen atom (O10) from a deprotonated pivalic acid, forming a NO7 coordination environment. Dy2 centre was coordinated by two oxygen atoms (O7 and O9) from one ligand, one nitrogen atom (N2) from the same ligand, four oxygen atoms (O7A, O8A, O1 and O2) from another two ligand sets and one oxygen atom (O9) from deprotonated pivalic acid. It also forms a NO7 coordination environment. The Dy1-O bond  (15) 13.457 (03) 13.4321 (16) 13.4131(7) b/Å 11.9674(06) 11.9193 (13) 11.905 (02) 11.8480 (14) 11.8637 (7)    The exact coordination geometries of the octacoordinated Ln(III) ions were analysed by SHAPE 2.1 soware 21 and resulting data were shown in Table S2. † The resulting data from the closer analysis reveal that Ln1(III) ion is in a square antiprism (D 4d ) conguration while Ln2(III) is in a biaugmented trigonal prism (C 2V ) conguration with a minimum continuous shape measures (CShM) value.

Magnetic studies
The magnetic properties of 1-5 were investigated by directcurrent (DC) magnetic susceptibility studies under a magnetic eld of 1000 Oe in the temperature range 2-300 K. The experimental c M T values at 300 K and magnetic behaviours of 1-5 are shown in Table 2. The c M T versus T plots for 1-5 are shown in Fig. 2. For 1, the room temperature c M T value of 33.41 cm 3 K mol À1 is slightly higher than value of 31.52 cm 3 K mol À1 expected for four magnetically isolated Gd III ions ( 8 S 7/2 , g ¼ 2). At 300 K, the c M T values of 46.91 and 58.50 cm 3 K mol À1 for complexes 2 and 3 are close to the expected values of 47.20 cm 3 K mol À1 for uncoupled Tb III ions ( 7 F 6 , g ¼ 2) and 56.68 cm 3 K mol À1 for uncoupled Dy III ions ( 8 H 15/2 , g ¼ 4/3). The room temperature c M T values for 4 and 5 are 57.47 and 44.21 cm 3 K mol À1 , respectively, which are slightly lower than values of 58.28 cm 3 K mol À1 expected for four magnetically uncoupled Ho III ions ( 5 I 8 , g ¼ 5/4) and 45.92 cm 3 K mol À1 expected for four magnetically uncoupled Er III ions ( 4 I 15/2 , g ¼ 6/5). Upon cooling, the c M T product of 1 stays almost constant in the temperature range of 300-30 K and then decreases rapidly, reaching minimum value of 13.80 cm 3 K mol À1 at 2 K. This magnetic behaviour suggests weak antiferromagnetic exchange interactions. The application of a Curie-Weiss law gives values of q ¼ À1.98 K, C ¼ 32.86 for 1 (Fig. S6 †). With the decrease of temperature, the c M T values for 2, 4 and 5 rstly decrease slowly and then decline sharply to minimum values at 2 K. The decrease of c M T values for 2, 4 and 5 is typical for Ln III ions and is due to several factors, namely, the thermal depopulation of the excited m J sublevels of the 2s+1 G J ground state of the Ln III ion originated by a crystal eld symmetry, in combination with the weak Ln III -Ln III antiferromagnetic interactions in 2, 4 and 5. 22 Upon cooling, the c M T value of 3 decreases imperceptibly and reaches minimum value of 55.9 cm 3 K mol À1 at 23 K, which is attributed by the thermal depopulation of Stark effect for a single Dy III ion. Continuous being cooled down to 2 K, the c M T product sharply increases to a maximum value of 69.6 cm 3 K mol À1 . This magnetic phenomenon may be caused by the presence of ferromagnetic interactions between the spin carriers. 2g, 23 To investigate the dynamic magnetic properties of 3, alternating-current (ac) magnetic susceptibilities were determined under zero-dc eld with a 2 Oe oscillating ac eld (Fig. 3). The out-of-phase susceptibility (c 00 ) displays frequencydependent phenomenon at low temperatures, which suggests the presence of slow relaxation of the magnetization, typical of SMM behaviour. 24 However, the peak maxima are not found. Ac susceptibility measurements of 3 under 0-10 000 Oe dc eld at 1000 Hz were conducted (Fig. S7 †). Unfortunately, no optimum eld was found. The energy barrier (Ea/K B ) and pre-exponential factor (s 0 ) values were calculated from the frequency-dependent ac susceptibility data by using the Debye model based on the equation ln(c 00 /c 0 ) ¼ ln(us 0 ) + Ea/K B T. 24a The best tting results give Ea/K B z 1.61 K and s 0 z 7.42 Â 10 À6 s (Fig. 4). Apparently, the s 0 value is comparable to the expected values 10 À6 to 10 À11 for typical SMMs. 25

Luminescent properties
The solid-state luminescent properties of the H 2 L ligand and 2 were measured at room temperature (Fig. S8 †). The H 2 L ligand displays a broad emission around 544 nm (l ex ¼ 467 nm), which is due to the p / p* transition between the ligand orbitals. Complex 1 exhibits a ligand-centred broad band emission at 557 nm and a shoulder at 483 nm (l ex ¼ 396 nm), which is attributed to the ligand centred p-p* and charge-transfer transition between ligands and metal centers 26 (Fig. S8b †). As shown in Fig. S8, † the solid-state emission spectra of 3-5 (Dy 4 , Ho 4 , Er 4 ) display emission bands centred at 482, 482, 480 nm respectively under excitation at 324 nm. The blue shis of 3-5 are tentatively assigned to the electrostatic interaction between the ligand and metal ions. 26b Complex 2 displays four characteristic emission peaks at 490, 544, 584, and 618 nm (l ex ¼ 360 nm), corresponding to 5 D 4 / 7 F 6 , 5 D 4 / 7 F 5 , 5 D 4 / 7 F 4 and 5 D 4 / 7 F 3 , 3b respectively (Fig. S9 †). The strongest emission of 2 at 544 nm is responsible for the green emission, which can be observed with the naked eyes. The p-conjugated multidentate organic ligand can act as antennae to sensitize the weak luminescent of Tb(III). Therefore, the strong and sharp luminescence emission of 2 can be visually observed, which allows it to be considered as a uorescent sensor. The results of the luminescence sensing experiments revealed that complex 2 can be served as a luminescent senior for selectively sensing 4-NA, Fe 3+ , CrO 4 2À and Cr 2 O 7

Detection of metal ions
Based on the fact that compound 2 possesses excellent stability in the aqueous solutions with broad pH range, its potential orescent sensing properties towards metal ions were investigated. Firstly, a series of aqueous solutions of 2 were prepared by adding 2 mg of complex 2 into 2 mL of deionized water. Then, the resulting suspensions were sonicated for 8, 10 and 30 minutes, respectively. The sizes of suspensions of particles of 2 were analysed by scanning electron microscopy. The sizedistribution histograms were obtained from measuring the  diameters of 100 randomly selected particles. According to the histograms of the particle size distributions in Fig. S12 (2) aer being sonicated for 30 minutes displays the smallest particle size. The smaller size of the 2 leading to larger surface areas and more accessible active sites on their surface would be benecial to realize highly sensitive and fast-response luminescent sensing. 27 Therefore, the resulting suspensions were sonicated for 30 minutes. Next, aqueous solutions of nitrate salts (200 mL, 10 À2 M) of Na + , K + , Mg 2+ , Ca 2+ , Fe 2+ , Co 2+ , Ni 2+ , Cu 2+ , Zn 2+ , Cd 2+ , Pb 2+ , Al 3+ , Cr 3+ and Fe 3+ were added in the above suspensions for uorescence testing. As shown in Fig. 5, the metal salts slightly altered the luminescent intensity of 2, whereas Fe 3+ ion exhibited a signicant inuence on the luminescence of 2. Concentration-based studies were performed by adding different amount of aqueous solutions of Fe 3+ ion into the suspensions of complex 2 (2 mg in 2 mL deionized water). As shown in Fig. 6, the luminescent intensities of 2 progressively decreased as the concentrations of Fe 3+ ion increased. The luminescent intensities and the concentrations of Fe 3+ ions show good linear relationships at low concentrations. The limit of detection (LOD) value is calculated to be 1 Â 10 À5 M by 3d/s, where d is the standard deviation of uorescent test for 10 blank measurements and s is the slope of the calibration curve (Fig. S13b †). 12i,j In addition, the quenching effect can be explained by the Stern-Volmer equation: I 0 /I ¼ K SV [Q] + 1, where I 0 and I are the luminescent intensities before and aer addition of the target analyte, respectively. K SV is the Stern-Volmer quenching constant (M À1 ) and [Q] is the concentration of the analyte. 12p As can be seen from Fig. S13a, † the Stern-Volmer (SV) plot for Fe 3+ ion displays nearly linear relationships at low concentration with K SV value of 1.86 Â 10 4 M À1 , which is comparable to those of reported uorescent sensors based on Ln-complexes (Table S3 †). The S-V plot deviates from linearity at high concentration (Fig. 6b), which may be attributed to both the occurrence of static and dynamic quenching. Nonlinear S-V curve of 2 can be tted well by an exponential quenching equation, I 0 /I ¼ 2.118 exp (4.327[Fe 3+ ]) + 0.199. The possible interference experiments were carried out for complex 2. The selectivity of 2 towards Fe 3+ ion over other metal cations was veried by adding Fe 3+ ion (10 À2 M, 0.2 mL) to the suspensions of 2 (2.4 mL) in which other competitive metal ions (10 À2 M, 0.2 mL) were introduced. Then the changes of the emission intensities were recorded. No obvious changes were observed, which suggested that uorescence detection of 2 could not be interfered by the presence of other cations. Competitive experiments indicated that 2 has great potential as a highly selective sensor for Fe 3+ ion (Fig. 7). To study the recycling performance of the uorescent sensor, Fe 3+ ion (10 À2 M, 0.2 mL) was introduced to the suspension of complex 2 (2 mL), then washed by water (aer Fe 3+ ion being detected) with ve cycles of sensing experiments. As shown in Fig. 8, the uorescent intensity of the samples did not drop signicantly aer ve cycles. The resulting K SV , LOD value, high selectivity, stability and recyclable property reveal that complex 2 can be considered as a uorescent sensor for Fe 3+ ion in aqueous system.
As reported, the possible uorescence quenching mechanism for 2 towards Fe 3+ ion mainly rises from three aspects: (a) the collapse of the structure; 28 (b) the weak interactions between Fe 3+ ion and the methoxy group at the terminal of 2; 29 (c) the energy transfer between 2 and Fe 3+ ion. 30 In order to understand which part was the main contributor, the PXRD, X-ray photoelectron spectroscopy (XPS) and UV-Vis absorption spectra of  analytes were determined. By comparing the PXRD pattern ( Fig. S15 †) of 2 before and aer the detection of Fe 3+ ion, we found that no obvious changes occurred in this process, which indicated that the structure of 2 did not shatter. Depending on the result of the PXRD patterns, the uorescence quenching mechanism of structural collapse can be ruled out. As shown in XPS spectrum of 2 (Fig. S14 †), aer the detection of Fe 3+ ion, the O1s peak did not change, demonstrating the nonexistence of the weak interactions between Fe 3+ ion and complex 2. However, as shown in Fig. S14, † the absorption band of Fe 3+ ion from 250 nm to 400 nm remarkably overlaps the excitation band of 2 from 300 to 400 nm. Consequently, the solution of Fe 3+ may absorb the energy of the excitation wavelength, which leads to the luminescence quenching.

Detection of anions
The aqueous solutions (200 mL, 10 À2 M) of common sodium salts Na y X (X ¼ F À , Cl À , Br À , I À , NO 3 À , OAc À , SCN À , SO 4 2À , CO 3 2À , Cr 2 O 7 2À , CrO 4 2À , and PO 4 3À were added in the suspensions of 2 (2 mg complex 2 in 2 mL water, and then being sonicated for 30 minutes) to investigate the luminescence quenching effects. An alluring nding is that CrO 4 2À and Cr 2 O 7 2À ions showed remarkable turnoff quenching effect on orescent intensities of 2, whereas other tested anions displayed minor effects on the luminescent intensity (Fig. 9). In order to further investigate the sensitivity of 2 toward CrO 4 2À and Cr 2 O 7 2À , we implemented the titration experiments by adding aqueous solutions of CrO 4 2À or Cr 2 O 7 2À (2 Â 10 À3 M) to the suspension of complex 2 (2 mg in 2 mL water). The emission titration data were collected. As expected, the uorescent intensity of 2 gradually decreased with the increase of the concentrations of CrO 4 2À or Cr 2 O 7 2À (Fig. S16 †). Moreover, the S-V curve of 2 (Fig. 10)  . The S-V plot shows good linear relationships at low concentrations ( Fig. S17a and S17b †), suggesting that both static and dynamic quenching may happen simultaneously. The quenching constants, K SV , are calculated to be 2.998 Â 10 3 M À1 (CrO 4 2À ) and 7.44 Â 10 3 M À1 (Cr 2 O 7 2À ) for 2 ( Fig. S17a     or Cr 2 O 7 2À were performed. Aer ve cycles of quenching experiments and regenerations, the quenching efficiency experiences only a minor decrease (Fig. 12). These results showed that complex 2 could be a potential Detection of nitrobenzene derivatives Complex 2 was dispersed in ethanol and then stirred for three days. It was ltered and dried. The PXRD of 2 (Fig. S20 †) aer being stirred for three days matched with that of the simulated data, which indicated that complex 2 was stable in ethanol solution. The stable nature of 2 in ethanol solution affords a prerequisite for the detection of nitroaromatics (NACs). Nitrobenzene (NB), 4-nitrophenol (4-NP), 4-nitroaniline (4-NA), 4-nitrotoluene (4-NT), 4-nitrochlorobenzene (4-Cl-NB), 2,4dinitrotoluene (2,4-DNT) and 2,4-dinitrophenol (2,4-DNP) were selected to explore the potential of 2 as a chemosensor in ethanol solution. Firstly, complex 2 (2 mg) was homogeneously dispersed in a series of ethanol solution (2 mL) and sonicated for 30 minutes to form a uniform suspension. Then, different   NACs (10 mL, 10 À1 M) was added to the above-mentioned suspensions, respectively. Next, the uorescent intensities of the resulting mixture were measured. As shown in Fig. 14, the order of quenching extend is as follows: 4-NA > 2,4-DNP > 4-NP > 4-NT > NB > 4-Cl-NB > 2,4-DNT. Concentration-dependent titration experiments of 4-NA were performed to test the inuence of concentrations on uorescent intensity. As the concentration of 4-NA increases, the uorescent intensity gradually decreased (Fig. S18 †). The liner tting of the S-V plot for 4-NA at low concentrations (Fig. S19a †) gives K SV value of 1.14 Â 10 4 M À1 (4-NA). Nonlinear concentration curve (Fig. 15) can be tted well by an exponential quenching equation: I 0 /I ¼ 3.776 exp(5.380[Q]) À 3.131 (Q ¼ 4-NA). The detection limit value (Fig. S19b †) towards 4-NA was calculated by 3d/s, generating 1.17 ppm (8.5 Â 10 À6 M) for 2. Recyclability experiments on the detection of 4-NA were performed. Aer ve cycles of quenching experiments and regenerations, the quenching efficiency experiences only a minor decrease (Fig. 16). To our knowledge, few uorescent complexes acted as sensors for 4-NA in ethanol system have been reported in the literature    ( Table S4 †). 12q-s,13b Compared to luminescent sensors based on transition metal complexes, lanthanide complex 2 has the advantage of characteristic emissions and bright luminescent colour of Tb(III) ion. No lanthanide clusters as sensors for 4-NA have been reported. As reported, the quenching mechanism of NACs sensors is ascribed to two factors: photo-induced electron transfer (PET) and resonance energy transfer (RET) or their cooperative effect. According to the molecular orbital theory, nitroaromatic compounds are good electron acceptors, due to the substitution of the electron-withdrawing nitro groups on the aromatic ring, which can stabilize the lowest unoccupied molecular orbital (LUMO) of the system via conjugation effect, ultimately leading to the complete or part quenching of luminescence-based sensor. 13b, 31 The electron transfer originated from the phenyl rings of the ligands of 2 to excellent electron donor nitrobenzene leads to luminescence quenching upon excitation. On the other hand, the UV-Vis results reveal that there are obvious overlaps between the absorption spectrum of NACs especially 4-NA and the emission spectrum of 2 (Fig. 17). This would permit energy transfer from 2 to NACs, thus further improving the luminescence quenching efficiency towards NACs. 30b

Conclusions
In summary, ve tetranuclear lanthanide clusters have been synthesized by using Ln(NO 3 ) 3 $6H 2 O (Ln ¼ Gd, Tb, Dy, Ho, Er), pivalic acid and Schiff base ligand 2-(((2-hydroxy-3methoxybenzyl)imino)methyl)-6-methoxyphenol (H 2 L) as starting materials under solvothermal method. Complexes 1-5 possess [Ln 4 O 6 ] cores formed by the fusion of two phenoxide oxygen bridged two [Ln 2 O 2 ] moieties. The magnetic properties of 1-5 were investigated. Antiferromagnetic interactions were determined for 1, 2, 4 and 5. Complex 3 displayed typical single molecule magnet behaviour. Considering its low detection limits and high quenching constant K sv , complex 2 may potentially be utilized as luminescence sensors for quantitative detection of 4-NA, Fe 3+ and CrO 4 2À /Cr 2 O 7 2À ions. Although the design of chemosensors is more based on academic or research interest, the exceptional stability of Tb 4 clusters makes it applicable to extreme industrial application.

Conflicts of interest
There are no conicts to declare.