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Highly Efficient White-light Emission and UV-visible/NIR Luminescence Sensing of Lanthanide Metal-Organic Frameworks Xinyu Wang, Peng-Fei Yan, Yuxin Li, Guanghui An, Xu Yao, and Guang-Ming Li Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b00112 • Publication Date (Web): 01 Mar 2017 Downloaded from http://pubs.acs.org on March 2, 2017

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Crystal Growth & Design

Highly Efficient White-Light Emission and UV-Visible/NIR Luminescence

Sensing

of

Lanthanide

Metal-Organic

Frameworks Xinyu Wang, Pengfei Yan, Yuxin Li, Guanghui An, Xu Yao and Guangming Li* Key Laboratory of Functional Inorganic Material Chemistry (MOE), P. R. China; School of Chemistry and Materials Science, Heilongjiang University; Harbin 150080, P. R. China;E-mail: [email protected]; Fax: +86-451-86673647. ABSTRACT

A series of new isostructural lanthanide metal-organic frameworks (Ln-MOFs), [Ln2(L)2(DMAC)2]·nH2O [Ln= La (1), Pr (2), Nd (3), Sm (4), Eu (5), Gd (6), Tb (7) and Tm (8);

H4L

=

5-(bis(4-carboxybenzyl)amino)-isophthalic

acid,

DMAC

=

N,N′-dimethylacetamide], have been isolated. Single-crystal X-ray diffraction analysis reveals that all complexes exhibit a rare (4,8)-connected alb-4,8-P topology with binuclear [Eu2(COO)8] n secondary building units as 8-connected nodes and H4L ligands as 4-connected nodes. Two mixed-lanthanide analogues of single-lanthanide MOFs were design and prepared [Ln = La0.93Eu0.02Tb0.05 (9) and Tm0.47Eu0.18Tb0.35 (10)] by way of careful regulation of the relative concentration of lanthanide ions which are able to emit pure white light. Luminescent sensing of complexes 3 and 5 has been investigated. Strikingly, complex 5 exhibits an excellent luminescent sensing to TNP with a high Ksv value of 3.58 × 104 M−1 and low detection limit of 4.66×10−4 mM. Complex 3 reveals high selectivity and sensitivity toward benzaldehyde (Ksv of 4.9 × 104 M−1; detection limit of 3.4×10−4 mM). It represents the first example of NIR luminescent MOF for sensing of benzaldehyde.

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INTRODUCTION In a wide variety of MOFs-based functional materials, lanthanide metal-organic frameworks (Ln-MOFs) are fascinating category because of the unusual coordination modes, high coordination number and connectivity of lanthanide ions, which could facilitate the formation of multi-dimensional networks with high thermal and chemical stability.1-3 Meanwhile, lanthanide ions as the metal component of Ln-MOFs are excellent candidates displaying luminescent properties due to their large Stokes shift, sharp emission, and high color purity.4, 5 In particular, single-component white-light-emitting Ln-MOFs have attracted great interest owing to their advantages over multiple component systems such as greater reproducibility, low-cost preparation, ease of modification, and simpler fabrication processes.6-8 The white-light emission of MOFs is commonly generated from three pathways, including the luminescence of monochromatic, in situ doping of dichromatic and tirchromatic.9-14 The treble emission pathway generally gives rise to finer color-rendering properties and higher luminescence quantum yields, thus gaining more popularity.15, 16 However, exactly adjusting the ratio of multi-chromatic components to realize pure white-light emission with high quantum yield is still great challenge.17 On the other hand, the unique photoluminescent properties and high recognition capability for guest molecules make Ln-MOFs a candidate for luminescent sensing, including the sensing of metal ions, small organic molecules even the sensing of nitro explosives.18-24 For example, Li et

al.

reported

a

highly

luminescent

MOF

[Zn2(bpdc)2(bpee)]

(bpdc

=

4,4′-biphenyldicarboxylate; bpee = 1,2-bipyridylethene) for the detection of 2,4-DNT and DMNB with high sensitivity and reversible in 2009, represents the first MOFs-based sensor for detecting nitro aromatic compounds (NACs).25 Notably, almost all the luminescence sensor occurred in UV-visible region,3, 26, 27 seldom resulting from a response of near infrared (NIR) luminescence, although the NIR emitting from Ln3+ ions have high permeability through biotissues and thus significantly improves deep-penetrating imaging and reduces photo damage.28, 29 For the first time, Chen et al. report the first NIR luminescent MOF, Yb(BPT)(H2O)(DMF)1.5(H2O)1.25 (BPT = biphenyl-3,4′,5-tricarboxylate), for the highly


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selective and sensitive sensing of small molecules in 2011, indicates the promise of this type of NIR luminescent materials for the sensing of substrates in biological systems.28 We have been studying the structure and luminescence of lanthanide complexes for a long standing.13, 30-32 To design novel Ln-MOF, realize pure white-light emission and develop new luminescent

sensor

of

lanthanide

MOF,

an

aromatic

tetracarboxylate

ligand

5-(bis(4-carboxybenzyl)amino)-isophthalic acid was employed in the solvothermal reactions with La(NO3)3·6H2O. As a result, a series of isostructural 3D single-lanthanide MOFs [Ln2(L)2(DMAC)2]·nH2O [Ln = La (1), Pr (2), Nd (3), Sm (4), Eu (5), Gd (6), Tb (7) and Tm (8); L = 5-(bis(4-carboxybenzyl)amino)-isophthalic acid; DMAC = N,N′-dimethylacetamide] have been isolated. Two mixed-lanthanide analogues [Ln = La0.93Eu0.02Tb0.05 (9) and Tm0.47Eu0.18Tb0.35 (10)] were designed and prepared which emit pure white-light emission with impressive lifetime and quantum yield. In comparison with previously reported result, luminescent sensing of complex 5 have been investigated which exhibit essential sensitivity for TNP with a high Ksv value of 3.58 × 104 M−1 and low detection limit of 4.66 ×10−4 mM, respectively. On the basis of high permeability through bio-tissues and thus significantly improves deep-penetrating imaging and reduces photo-damage of the NIR emission of Ln3+ ions, complex 3 was employed as a NIR luminescent sensor detecting of benzaldehyde.

EXPERIMENTAL SECTION Materials and Instrumentations Ln(NO3)3·6H2O was obtained by the reactions of Ln2O3 and nitric acid. The H4L ligand and other chemicals were purchased from commercial sources and used without purification. 5-(bis(4-carboxybenzyl)amino)-isophthalic acid (H4L), was purchased from Jinan Henghua Sci. & Tec. Co. Ltd., 99%. All organic solvent and nitro compounds were purchased from Aladdin Industrial Inc. FT-IR spectra were collected on a Perkin-Elmer Spectrum 100 spectrophotometer by using KBr disks in the range of 4000−500 cm–1. UV spectra were recorded on a Perkin–Elmer Lambda 35 spectrometer (in DMAC). Thermogravimetric analyses (TGA) were conducted on a Perkin-Elmer STA 6000 in the temperature range of 30 ºC to 800 ºC with a heating rate of 10 ºC·min–1. Powder X-ray diffraction (PXRD) data were performed on a Rigaku D/Max-3B X-ray diffractometer with



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CuKα as the radiation source (λ = 0.15406 nm) in the angular range 2θ = 5−50º at room temperature. The photoluminescence (PL) spectra were measured with an Edinburgh FLS 920 fluorescence spectrophotometer. The steady-state near-infrared (NIR) emission spectrometer equipped with a Hamamatsu R5509-72 supercooled photomultiplier tube at 193 K and a TM 300 emission monochromator with NIR grating blazed at 1000 nm. The corrected spectra were obtained via a calibration curve supplied with the instrument. Luminescence lifetimes were recorded on a single photon counting spectrometer from Edinburgh Instrument (FLS 920) with microsecond pulse lamp as the excitation. The absolute quantum yield was calculated using the following expression:

Φ=

∫L

emission

∫

(1)

∫

E reference − E sample

Where Lemission is the emission spectrum of the sample, collected using the sphere, Esample is the spectrum of the incidence light used to excite the sample, collected using the sphere, Ereference is the spectrum of the light used for excitation with only the reference in the sphere. The NIR luminescence quantum yield can be obtained by the ratio the observed luminescence decay time τ and the radiative “natural” decay time τR of that specific Ln3+ ions. The Commission International de I’ Eclairage (CIE) colour coordinates were calculated on the basis of the international CIE standards. The general CRI (colour rendering index) were designated by the symbol Ra, which is the average value of R1 to R8.33, 34 The numbers in parentheses indicate the Munsell colour system.35 The CCT (correlated colour temperature) values were obtained based on the corresponding CIE colour coordinates.

Synthesis of Complexes1−10 Complexes 1−6 were synthesized by solvothermal reactions. A mixture of La(NO3)3·6H2O (0.25 mmol, 82mg), H4L ligand (0.25 mmol, 112mg), DMAC (3.5 ml), and water (1.5 ml) were added to a Teflon-lined autoclave at 100 °C for 2 days and then cooling to room temperature gradually. The resulting brown crystal was filtrated, and washed by DMAC for several times. The yield was 57%, 55%, 61%, 49%, 54% and 68% for 1−6 (based on H4L),


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respectively.

[La2(L)2(DMA)2]n·nH2O (1). Elemental analysis (%): Calcd for C28H24LaN2O9 (671.4), C 50.09, H 3.60, N 4.17; found: C 50.01, H 3.55, N 4.13. IR (KBr, cm−1): 3471 (w), 3065 (w), 1647 (s), 1596 (s), 1393 (s), 1167 (w), 768 (m), 709 (w), 593 (w). UV/Vis [DMAC, λ]: 217 nm. [Pr2(L)2(DMA)2]n·nH2O (2). Elemental analysis (%): Calcd for C28H24PrN2O9 (673.4), C 49.94, H 3.59, N 4.16; found: C 49.91, H 3.51, N 4.08. IR (KBr, cm−1): 3486 (w), 2933 (w), 1640 (s), 1589 (s), 1392 (s), 1021 (w), 768 (m), 716 (w), 593 (w). UV/Vis [DMAC, λ]: 218 nm. [Nd2(L)2(DMA)2]n·nH2O (3). Elemental analysis (%): Calcd for C28H24NdN2O9 (676.73), C 49.69, H 3.57, N 4.14; found: C 49.81, H 3.48, N 4.05. IR (KBr, cm−1): 3479 (w), 2933 (w), 1639 (s), 1596 (s), 1378 (s), 1007 (w), 768 (m), 702 (w), 585 (w). UV/Vis [DMAC, λ]: 217 nm. [Sm2(L)2(DMA)2]n·nH2O (4). Elemental analysis (%): Calcd for C56H48Sm2N4O23 (1445.7), C 49.25, H 3.54, N 4.10; found: C 49.01, H 3.44, N 4.01. IR (KBr, cm−1): 3470 (w), 2960 (w), 1627 (s), 1580 (s), 1369 (s), 1047 (w), 760 (m), 711 (w), 590 (w). UV/Vis [DMAC, λ]: 217 nm. [Eu2(L)2(DMA)2]n·nH2O (5). Elemental analysis (%): Calcd for C56H48Eu2N4O19 (1384.92), C 49.13, H 3.53, N 4.09; found: C 48.90, H 3.41, N 3.98. IR (KBr, cm−1): 3469 (w), 2947 (w), 1617 (s), 1579 (s), 1388 (s), 1089 (w), 757 (m), 706 (w), 579 (w). UV/Vis [DMAC, λ]: 217 nm.

[Gd2(L)2(DMA)2]n·nH2O (6). Elemental analysis (%): Calcd for C56H48Gd2N4O23 (1459.48), C 48.76, H 3.51, N 4.06; found: C 48.27, H 3.39, N 3.97. IR (KBr, cm−1): 3480 (w), 2921 (w), 1657 (s), 1569 (s), 1374 (s), 1034 (w), 764 (m), 703 (w), 585 (w). UV/Vis [DMAC, λ]: 217 nm.



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Complexes 7 and 8 were synthesized by a room temperature reaction, a mixture of corresponding lanthanide nitrate (0.3 mmol), H4L (0.3 mmol), DMAC (4 ml) and H2O (1.5 ml) were added to a round-bottom flask with a magneton, and stiring at room temperature for 24 h. The resulting white powder was filtrated, and washed by DMAC for several times. The yield was 72% and 65% for 7 and 8 (based on H4L), respectively. For the isostructure complexes 9 and 10, the synthetic methods are the same as above mentioned except using a mixture lanthanide nitrate as the starting materials in stoichiometric ratios. The yield was 61% and 67% for 9 and 10 (based on H4L), respectively. The identical structures of complexes 1−10 are demonstrated by the corresponding PXRD patterns (Figure S4). Luminescence sensing experiments. To obtain a stable suspension, the crystal samples of complex were finely ground and immersed in DMAC by ultrasonication treatment for 30 min. Then it was made into a 0.2 mg/mL suspension. Finally, the stable suspensions were used for luminescence sensing measurement at room temperature. The excitation and emission slit widths were stay the same always. X-ray Crystallographic Analysis. Crystal data for complexes 1−6 were collected on an Oxford Xcalibur Gemini Ultra diffractometer using graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å) at room temperature. Structures of complexes 1−6 were solved by using Patterson methods (SHELXS-97), expanded using Fourier methods and refined using SHELXL-97 (full-matrix least-squares on F2) and WinGX v1.70.01 programs packages.36 All non-hydrogen atoms were refined anisotropically. Empirical absorption corrections based on equivalent reflections were applied. For complexes 1−3 and 5, the contribution of HIGHLY disordered anions/solvent molecules were treated as diffuse using the Squeeze procedure implemented in the PLATON program.37 The resulting new files were used to further refine the structures. The Squeeze results are consistent with TG and elemental analysis, which indicate that there is a water molecule in a structure unit as free guest. These water molecules are added in the molecular formula. The crystal data and structure refinements of complexes 1−6 are summarized in Table 1. Crystallographic data for complexes 1−6 have been deposited in the Cambridge Crystallographic Data Center with CCDC No. 1509300−1509305.


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Table 1. Crystal data and structure refinement for complexes 1−6. 1

2

3

4

5

6

Empirical

C28H24N2O9

formula

La

C28H24N2O9

C28H24N2O9

C56H48N4O23

C56H48N4O19

C56H48N4O23

Pr

Nd

Sm2

Eu2

Formula weight

Gd2

671.40

673.40

676.73

1445.70

1384.92

1459.48

Crystal system

Monoclinic

Monoclinic

Monoclinic

Monoclinic

Monoclinic

Monoclinic

space group

P 2/c

P 2/c

P 2/c

P2/c

P2/c

P2/c

a /Å

15.861(5)

15.837(5)

15.838(5)

15.858(5)

15.854(5)

15.894(5)

b /Å

10.599(5)

10.568(5)

10.548(5)

10.522(5)

10.515(5)

10.491(5)

c /Å

21.235(5)

21.099(5)

21.074(5)

20.970(5)

20.947(5)

20.909(5)

α (deg)

90

90

90

90

90

90

β(deg)

106.914(5)

107.098(5)

107.247(5)

107.520(5)

107.587(5)

107.841(5)

γ (deg)

90

90

90

90

90

90 3319(2)

3

V(Å )

3415(2)

3375(2)

3362(2)

3337(2)

3329(2)

Z

4

4

4

2

2

2

Dc(g⋅cm-3)

1.306

1.325

1.337

1.439

1.382

1.460

T (K)

293(2)

293(2)

293(2)

293(2)

293(2)

293(2)

λ (Mo Kα)(Å)

0.71073

0.71073

0.71073

0.71073

0.71073

0.71073

Reflections

6025

5969

5941

5890

5882

5859

collected

µ(mm-1)

1.296

1.490

1.591

1.816

1.933

2.055

F(000)

1340.0

1348.0

1352.0

1440.0

1380.0

1448.0

FinalR1a,wR2b[I

0.0707,

0.0509,

0.0453,

0.0506,0.148

0.0375,

0.0475,

> 2σ(I)]

0.1851

0.1547

0.1422

6

0.1096

0.1366

FinalR1a,

0.0812

0.0595,

0.0509,

0.0628,

0.0436,

0.0544,

wR2b(all data)

0.1908

0.1612

0.1458

0.1586

0.1138

0.1422

1.143

1.074

1.097

1.095

1.099

1.106

2

GOF on F a

||

|

||

b

R1 = Σ F0|− Fc /Σ|F0|. wR2 =

Σ[w(F0 −Fc2)2]/ 2

2 2 1/2

Σ[w(F0 ) ] .

RESULTS AND DISUSSION

Crystal Structure. X-ray crystallographic analysis reveals complexes 1–6 are isostructural with P 2/c space group. In a typical complex 5, each asymmetric unit consists of one crystallographically independent Eu3+ ion, one deprotonated L4− ligand, and one coordinated DMAC molecule (Figure S5). Each Eu3+ ion is eight-coordinated by seven oxygen atoms from carboxyls of six H4L ligends and one oxygen atom from one DMAC molecule. Meanwhile, the neighboring Eu3+ ions were bridged through oxygen atoms from the carboxylate groups of branched organic ligand and further form the [Eu2(COO)8]n secondary building unit (SBUs) (Figure 1a). The neighboring dinuclear [Eu2(COO)8]

n

units were

connected by L4− in (k1-k1-µ2)-(k1-k1-µ2)-µ4 mode forming 1D chain (Figure 1b). The adjacent 1D chains were linked by ligand L4− in (k1-k1-µ2)-(k1-k1-µ1)-µ3 mode forming 2D network



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(Figure 1c). Furthermore, the adjacent networks further accumulation into 3D frameworks in the vertical direction (Figure 1d). In the past report, the H4L ligands have been used as a 4-connected node, and combined with the four-connected [In(COO)4] group building a zeolitic metal-organic frameworks (ZMOFs) with a SOD topology.38 In this work, the dinuclear [Eu2(COO)8]n SBUs connected by eight carboxylate groups, can be considered as an 8-connected node. As a result, the framework can be viewed a (4,8)-connected 3D framework with a topological point symbol of {44·62}2{48·617·83} (Figure 1e). Such a topology is usually associated with high connectivity and long intermodal bridges.39-41 PLATON calculation shows that there is about 26% accessible volume of the unit cell volume when the guest molecules were removed.

Figure 1. (a) Coordination environment of central Eu3+ ions in complex 5 with hydrogen atoms omitted for clarity. (b) 1D chain. (c) 2D layer. (d) 3Dframework. (e) Schematic representation with 4,8-connected alb-4,8-p topology.


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Luminescence. Photoluminescent spectra for all single-lanthanide complexes 1–8 were conducted in solid state at room temperature (Figure 2 and S6–S10). Complexes 1 and 8 exhibit lanthanide ion tuned ligand-centered emission at 426 and 421 nm under excitation at 398 and 394 nm, respectively. Complex 4 exhibits co-luminescence of Sm3+ ion and the ligand excited at 417 nm, implying that the H4L ligand can only partly sensitize Sm3+ ion. Complex 5 exhibits an intense characteristic red emission of Eu3+ with peaks located at 590, 614, 648 and 698 nm upon excited at 398 nm, respectively, which are attributed to the 5

D0→7FJ (J = 1–4) transitions of Eu3+ ion (Figure 2b). The lifetime and quantum yield of

complex 5 are τ1 = 0.26 ms (4.67%), τ2 = 1.10 ms (95.33%) and Φ = 21.78%. In contrast, complex 2 reveals the NIR luminescence around 1020 and 1227 nm when excited at 420 nm, which are attributed to f-f transitions of 1D2→3F4 and 1G4→3H4 for Pr3+ ion, respectively (Figure S7).42, 43 Complex 3 exhibits the characteristic NIR luminescence around 890 nm, 1054 nm and 1326 nm under excitation at 386 nm that can be attributed to the 4F3/2→4F9/2, 4

F3/2→4F11/2 and 4F3/2→4F13/2 transitions of Nd3+ ion, respectively (Figure S8). The lifetime

and quantum yield are τ1 = 2.11 µs (41.83%), τ2 = 7.51 µs (58.17%) and Φ = 1.9%. Notably, the lifetimes and yields of complexes 3 and 5 are comparable to those of the reported neodymium and europium complexes (Table 2).

Figure 2. Excitation and emission spectra of complexes 1 (a), 5 (b), 7 (c) and 8 (d) at room temperature. Insert: the luminescence photograph of solid state complexes 1 (a), 5 (b), 7 (c) and 8 (d) excited under a UV lamp at 365 nm.



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In view of that complexes 1–8 are isostructural and complexes 1 and 8 exhibit intense blue emission (Figure 2a and 2d), it will therefore be possible to construct white light-emitting materials by doping of both Eu3+ ions and Tb3+ ions into the host frameworks of complexes 1 or 8 with an appropriate molar ratio. In fact, the designed mixed lanthanide complexes 9 and 10 have been realized the white-light emission when excited at 320 nm and 322 nm (Figure 3). The optimal concentrations of the components are 93 mol% for La3+ ions, 2 mol% for Eu3+ ions and 5 mol% ions for Tb3+ ions in complex 9, and 47 mol% for Tm3+ ions, 18 mol% for Eu3+ ions and 35 mol% for Tb3+ ions in complex 10. The CIE coordinates (0.329, 0.333) and (0.331, 0.336) for complexes 9 and 10, respectively, fall within the white-light region of the 1931 CIE chromaticity diagram. The quantum yield Φoverall is 15.3% for complex 9 and 13.9% for complex 10. The lifetime are τTb = 785 µs (monitored at 542 nm) and τEu = 436 µs (monitored at 614 nm) for complex 9, τTb = 705 µs (monitored at 542 nm) and τEu = 405 µs (monitored at 614 nm) for complex 10, respectively (Figure S12 and S13). The CRI index (80.2 and 78.7) and CCT value (5652 K and 5558 K) could also readily realized for complex 9 and 10. Notably, the quantum yield of complex 9 is slightly higher than that of complex 10, attributed to the poor sensitivity of the ligand to Tm3+ ion. In comparison the quantum yields with previously reported three-component analogs (Table 2), the quantum yields of complexes 9 and 10 are higher than those of La0.6Eu0.1Tb0.3CPOMBA (H3CPOMBA

=

4,4

′

-(((5-carboxy-1,3-phenylene)bis(oxy))bis(methylene)

),44

[Sm41.26Gd44.72Eu14.02(4-SBA)(IP)OH] and [Sm27.93Tb53.13Eu18.94(4-SBA)(IP)OH] (4-SBA = 4-sulfobenzoateand IP = 1H-imidazo[4,5-f][1,10]-phenanthroline),45 but lower than those of La0.6Eu0.1Tb0.3-BTPCA(H3BTPCA (1,1

′

,1-(benzene-1,3,5-triyl)tripiperidine-4-carboxylicacid)10

[H2NMe2]3[Gd1-x-yEuxTby (L)3] (L= Pyridine2,6-dicarboxylic acid).46


= and

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Figure 3. Emission spectra of complexes 9 (a) and 10 (b) at room temperature and CIE-1931 chromaticity diagram. Insert is optical image of a powder sample of complexes 9 (a) and 10 (b) excited with a Xe lamp at 320 nm (a) and 322 nm (b). Table 2. Luminescence lifetime and quantum yield of complexes 1–10 and their analogs. Materials λex/λem (nm) τ (µs) Φ (%) Ligand 396/424 Complex 1 398/428 τ1 = 1.3; τ2 = 9.6 3.0 Complex 2 420/1227 τ = 1.7 1.4 Complex 3 382/1054 τ1 = 2.1; τ2 = 7.5 3.5 Complex 4 417/642 τ1 = 1.1; τ2 = 7.0 2.7 Complex 5 398/614 τ1 = 258.3; τ2 = 1101.6 21.8 Complex 6 385/490 τ1 =1.0; τ2 = 8.4 2.4 Complex 7 379/542 τ = 1029.7 20.6 Complex 8 380/421 τ1 = 1.0; τ2 = 6.5 2.3 320/542,614 τTb = 785.1/τEu = 436.3 15.3 Complex 9 Complex 10 322/542,614 τTb = 705.4/τEu = 405.1 13.9 Nd(TFI)3(bpy)47 343/1060 τ = 9.9 4.0 Nd(TFI)3(phen)47 358/1060 τ = 11.3 4.6 Nd(5,6-DTFI)3(phen)48 336/1058 τ = 7.9 2.9 [Eu2L3(CH3OH)2(H2O)413 320/614 τ = 269.8 15.0 [NaEu-(pztc)(H2O)3]·H2O30 334/614 τ = 395.2 17.2 [Eu(4-SBA)-(IP)OH]·1.5H2O45 347/612 τ = 464.0 20.7 Luminescent sensing. The efficient energy transfer form ligands to Eu3+ ions encourage us to investigate the luminescent sensing capacity of complex 5. To obtain a stable suspension, the crystal samples of complex 5 were finely ground and immersed in DMAC by ultrasonication treatment for 30 min. Then the luminescent intensity of the suspension were investigated by addition of various nitro aromatic compounds, including NB, m-DNB, 2,4-DNT, 2,6-DNT and TNP in DMAC solution (10 mM). As a result, the maximum luminescent intensity of



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complex 5 was reduced by 22.27%, 25.88%, 28.03%, 28.78% and 75.08%, respectively, when the content of nitro aromatic compounds is 120 µL (10 mM, in DMAC solution) (Figure 4a). It suggests that the quenching efficiencies of complex 5 follow the sequence of TNP > 2,6-DNT > 2,4-DNT > NB > m-DNB (Figure 4b).

Figure 4. (a) Quenching efficiency of complex 5 for various nitro aromatic compounds. (b) The relationships between emission intensities and nitro aromatic compounds (c) Emission spectra of complex 5 dispersed in DMAC upon incremental addition of TNP solution in DMAC. (d) Reproducibility of complex 5 dispersed in DMAC with TNP solution of 120 µL.

The quenching efficiency can be quantitatively explained by the Stern-Volmer (SV) equation: I/I0= Ksv[Q] +1, where I and I0 are the luminescent intensity of the suspension of complex 5 before and after for sensing nitro aromatic compounds, [Q] is the molar concentration of analytes (M−1), Ksv is the Stern-Volmer constants. In this work, the SV plots for all analytes are nearly linear in the range of low concentrations, the Ksv can then be estimated as 1.39 × 103 M−1 for NB, 1.9 × 103 M−1 for m-DNB, 2.33 × 103 M−1 for 2,4-DNT, 2.4 × 103 M−1 for 2,6-DNT and 3.58 × 104 M−1 for TNP, respectively (Figure S15−S19). Particularly, the Ksv value of complex 5 is comparable to most of the MOF-based probes toward TNP reported recently (Table S3). On the basis of Ksv values and the standard deviations for five repeated


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luminescent measurements of blank solutions, the detection limits (3σ/Ksv) of complex 5 toward five nitro aromatic compounds are also calculated (Table S1).49 For complex 5, TNP is the most efficient quencher with a detection limit of 4.66 × 10−4 mM. The high quenching constant and low detection limits indicates that complex 5 exhibits extremely high detection sensitivity towards TNP (Table 3). In addition, complex 5 can be recycled by centrifuging the suspension after use and washing several times with DMAC, the luminescent intensity and quenching ability of complex 5 exhibited negligible changes after being used for four repeated cycles (Figure 4d). The identical structure of the regenerative sample is demonstrated by the corresponding PXRD patterns (Figure S20). Similarly, The H4L ligand was also employed as luminescent sensor toward the detection of TNP in DMAC. The emission spectra of H4L dispersed in DMAC were investigated upon incremental addition of TNP contents. The Ksv value of H4L ligand is 2.9 × 104 M−1 for TNP, which is smaller than 3.58 × 104 M−1 of complex 5 (Figure 5). This indicated that the framework of complex 5 is more efficient than the H4L ligand for the detecting of TNP, which may be attributed to the presence of pores in the framework to facilitate possible host-guest interactions.50

Figure 5. Emission spectra of H4L dispersed in DMAC upon incremental addition of TNP solution, the inset is Stern-Volmer plot for the fluorescence quenching of H4L upon addition TNP and corresponding photographs under UV-light irradiation at 365 nm.



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Table 3. Summary of quenching constants (Ksv) and Detection limit of complex 5 and ligand for different NACs. Materials Complex 5

Ligand

NACs NB m-DNB 2,4-DNT 2,6-DNT TNP TNP

Ksv (M−1) 1.4×103 1.9×103 2.3×103 2.4×103 3.6×104 2.9×104

Detection limit (mM) 1.2×10−2 8.8×10−3 7.2×10−3 7.0×10−3 4.7×10−4 5.8×10−4

In order to better understand the luminescent quenching effect of complex 5 toward nitro aromatic

compounds,

the

quenching

mechanism

was

analyzed.

Generally,

the

conduction-band (CB) of a MOF lies at a higher energy than the lowest unoccupied molecular orbitals (LUMOs) energy of nitro aromatic compounds, and leads to a luminescent quenching.51, 52 With the LUMOs energy gets lower, the electron accepting efficiency of the nitro aromatic compounds and luminescent quenching become higher. It was found that the LUMOs energies are in good agreement with the maximum quenching efficiency observed for TNP, however, the order of observed quenching efficiency is not fully in accordance with the LUMOs energies of other nitro aromatic compounds (Figure S21 and Table S2). The results indicate that there may be other luminescent quenching mechanism in this work, such as fluorescence resonance energy transfer (FRET).49

Figure 6. (a) NIR emission spectra of complex 3 dispersed in DMAC with different analytes (10 mM) of 200 µL upon excitation at 370 nm. (b) Emission spectra of complex 3 dispersed in DMAC upon incremental addition of benzaldehyde. The inset is Stern-Volmer plot for the fluorescence quenching of complex 3 upon addition of benzaldehyde.

In comparison with UV-visible luminescent sensing, NIR luminescent sensor is scarce. In


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view of the excellent sensitization effect of H4L ligand on Nd3+ ion, complex 3 was employed for the sensing of small organic molecules. As show in Figure 6a and S22, the NIR light-emitting intensity largely depends on the type of solvents, particularly in the case of benzaldehyde. It exhibits the most significant quenching effects. The sensing properties of complex 3 toward benzaldehyde were further investigated by monitoring a series of emissions of complex 3 in DMAC with gradually increased benzaldehyde content. As predicted, the luminescent intensity of the suspended solution of complex 3 in DMAC gradually decreases with increasing content of benzaldehyde. At a content of 200 µL (10 mM), the luminescence intensity is nearly completely quenched with a high quenching efficiency of about 94% (Figure 6b). Similarly the Ksv constant is estimated to be 4.9 × 104 M−1 (Figure 6b) and the detection limit of complex 3 toward benzaldehyde is estimated to be 3.4 × 10−4 mM. To the best of our knowledge, it is the first example that the luminescence MOFs was applying on benzaldehyde detection with high Ksv and low detection limit. Previously, only {[Sm2Zn(abtc)2(H2O)4]·2H2O} (abtc = 3,3′,5,5′-azo-benzenetetracarboxylic acid) was documented for detecting benzaldehyde with a Ksv value of 1.36 × 104 M−1.53

To further explore the mechanism of the luminescence quenching to benzaldehyde, some necessary measures are performed. Firstly, the PXRD spectrum of the complex 3 before and after benzaldehyde treatment verifies that the framework of complex 3 remained intact. Therefore the luminescence quenching was not caused by alteration of crystallization structure (Figure S23). Secondly, according to the static quenching mechanism, the static quenching does not change the lifetime. In contrast, the dynamic quenching will cause comparable decrease in lifetime.17,26 The emission lifetime of complex 3 in the absence and presence of benzaldehyde were measured, and remained almost unchanged in the average lifetime (Figure S24), implying that there are no interactions between the Nd3+ ions and benzaldehyde. Thus the quenching in the emission intensity is supposed to be the interactions between benzaldehyde and the other sections of frameworks, such as ligands. Finally, the FT-IR spectra of complex 3 before and after treatment with benzaldehyde (Figure S25) reveal that the peak belonging to skeleton vibration of ligands’ benzene rings shifted from 796 cm−1



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to 782 cm−1, which suggest that strong hydrogen bonds interactions exist between framework and benzaldehyde. Therefore, we speculate that the hydrogen bonds between benzaldehyde and frameworks affect the energy transfer efficiency from the ligands to Nd3+ ions, which lead to a luminescent quenching.14

CONCLUSION

Isolation of a series of isostructural lanthanide metal-organic frameworks 1−8 demonstrates that 5-(bis(4-carboxybenzyl)amino)-isophthalic acid is able to reacted with Ln(NO3)3·6H2O forming Ln-MOF with unique (4,8)-connected alb-4,8-P topology in solvothermal conditions. With Eu3+ ions and Tb3+ ions were doped into the isostructural complexes 1 and 8, the highly-pure white-light emission were realized. This lays the foundation for the subsequent fabrication of nanostructure materials and light-emitting devices. In addition, the characteristic luminescence of Nd3+ and Eu3+ ions in complexes 3 and 5 suggests that the energy transfer between the ligand and lanthanide ions are effective. The typical luminescence of Eu3+ ions in complex 5 could be quenched efficiently by trace amounts of TNP, which could be employed as luminescence sensor for TNP with high selectivity, sensitivity and reproducibility. The characteristic NIR luminescence of complex 3 shows solvent-dependent changes with a significant quenching effect towards benzaldehyde. It represents the first NIR luminescent sensor for detecting benzaldehyde. This approach may provide potential materials for WLED and luminescent sensing application. ASSOCIATED CONTENT

Supporting Information

UV, IR, TG, and PXRD data. The Supporting Information is available free of charge on the ACS Publications website.

AUTHOR INFORMATION

Corresponding Author


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*E-mail: [email protected]. Notes The authors declare no competing financial interest. Acknowledgments This work is financially supported by the National Natural Science Foundation of China (No. 51472076 & 51473046). REFERENCE

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For Table of Contents Use Only Highly Efficient White-Light Emission and UV-Visible/NIR Luminescence Sensing of Lanthanide Metal-Organic Frameworks Xinyu Wang, Pengfei Yan, Yuxin Li, Guanghui An, Xu Yao and Guangming Li*

A series of new isostructural lanthanide metal-organic frameworks with unique (4,8)-connected alb-4,8-P topology perform highly pure white light emission and detect TNP and benzaldehyde in UV-visible and NIR region with high selectivity and sensitivity.


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NIR

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