Tunable Light Emission and Multi-Responsive Luminescent

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Tunable Light Emission and Multi-Responsive Luminescent Sensitivities in Aqueous Solutions of Two Series of Lanthanide Metal-Organic Frameworks Based on Structurally Related Ligands Xiuna Mi, Dafei Sheng, Yu'e Yu, Yuhao Wang, Limin Zhao, Jing Lu, Yunwu Li, Dacheng Li, Jianmin Dou, Jingui Duan, and Suna Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 05 Feb 2019 Downloaded from http://pubs.acs.org on February 5, 2019

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ACS Applied Materials & Interfaces

Tunable Light Emission and Multi-Responsive Luminescent Sensitivities in Aqueous Solutions of Two Series of Lanthanide Metal-Organic Frameworks Based on Structurally Related Ligands Xiuna Mi †, Dafei Sheng †, Yu’e Yu †, Yuhao Wang †, Limin Zhao †, Jing Lu †, Yunwu Li †,*, Dacheng Li †, Jianmin Dou †, Jingui Duan ‡,*, Suna Wang †,* †

Shandong Provincial Key Laboratory of Chemical Energy Storage and Novel Cell Technology,

School of Chemistry and Chemical Engineering, Liaocheng University, Liaocheng, 252059, P.R. China. ‡

State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemical

Engineering, Nanjing Tech University, Nanjing, 210009, China.

KEYWORDS: Ln-MOFs, flexible phenoxyl-containing tricarboxylate ligands, luminescent sensors, Fe(III) and Cr(VI), tunable light emission, ABSTRACT: Two series of lanthanide metal-organic frameworks (Ln-MOFs) from two structurally related flexible carboxylate-based ligands were solvothermally synthesized. H3L2 with additional –CH2- group provides more flexibility, different coordination modes and conformations compared with H3L1. As a result, 2-Ln MOFs are modulated from 2D kgd of 1Ln to 3D rtl topological frameworks, and further achieve enhanced chemical stability. The Euand Tb-MOFs exhibit strong fluorescent emission at solid state due to the antenna effect of the

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ligands. Interestingly, the emissions can be tuned by simply doping Eu3+ and Tb3+ of different concentrations within the EuxTb1-x MOFs. Notably, 2-Ln MOFs realize nearly white-light emission by means of trichromatic approach (red of Eu(III), green of Tb(III) and blue of H 3L2 ligand). Furthermore, 2-Ln MOFs also present water stability and demonstrate high selective and sensitive sensing activities towards Fe(III) and Cr(VI) in aqueous solutions. The results further highlight the importance of the ligand flexibility on tuning MOF structures, improving structural stability, and ion-sensing properties. 1. INTRODUCTION Recently, significant efforts have been dedicated to lanthanide metal-organic frameworks (LnMOFs) because of the special optical properties, including characteristic spectrum, high quantum yields, large Stokes shift, long luminescence lifetimes and so on.1-5 The optical and biomedical applications thus incurred, such as light-emitting devices, bio-sensors, and cell imaging agents.612

For example, many Ln-MOFs have been developed into luminescent sensors towards cations,

anions, and organic molecules due to the high selectivity, fast response and high accuracy.13-18 Particularly, single-phase materials with multicolored photoluminescence exhibit great potential for display and security applications.19-22 A common method to synthesize multicolored photoluminescence MOFs requires the incorporation of different Ln ions into MOF frameworks to achieve doped heterometallic structures, which may exhibit dual-emission under single excitation wavelength. By adjusting the molar ratio of the starting metal salts to further modulate the proportion of two emissive Ln centers, the luminescence color may be precisely tuned. It is well known that in Ln3+ ions the 4f electrons are shielded by the outer 5s and 5p electrons, which are less affected by the coordination environment. According to Laporte-partity selection rule, most Ln3+ ions could not present effective luminescence resulting from the forbidden nature


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and low absorption coefficients of the f-f electron transitions.23 Through the introduce of organic chromophores into the lanthanide system, which is named antenna effect, has been proved to be a reliable strategy to overcome the light absorption.

24-27

Through coordinated interactions,

organic ligands are stabilized in solid crystals, and thus the ligands energy that absorbed via possible π-π* or n-π* process could be easily transferred to Ln3+ ions. In such way, a suitable energy gap should be present between the resonant level of the Ln3+ ions and the triplet state of the ligands. That is, the energy level of the lowest triple state of the ligands should be greater than or at least equal to the resonant energy level of Ln3+ ions.28 Fortunately, by changing the size, functional groups and rigidity as well as flexibility, the energy of ligands could be carefully adjusted and further maximize the transmission efficiency. As reported in the literatures, carboxylate-based ligands with π-conjugated aromatic rings as fluorescence cores are good candidates for blue-light sources. Combined with the unique coordination ability of the carboxyl groups and their outstanding sensitization to lanthanide ions, this type of ligands are commonly used for constructing lanthanide-based optical MOFs.29,30 Along with our previous research studying fluorescent MOFs,31,32 two structurally related flexible phenoxyl-containing tricarboxylate ligands, namely, H3L1, and H3L2, have been selected to build Ln-MOFs because of the following attractive features (Scheme 1): (a) three carboxylate groups of the ligands may offer strong affinity to capture lanthanide ions; (b) π-conjugated aromatic rings within them may lead to efficient antenna effect as well as improved light absorption and energy transition; (c) phenoxyl groups here can not only make the ligands flexible but also may serve as functional sites to sense analytes; (d) free –CH2- groups in H3L2 may mediate the structural diversity and fluorescent variations. Bearing these consideration, two series of Ln-MOFs were prepared by using the above two ligands under solvothermal conditions,



namely,

{[Ln(L1)(DMF)(H2O)]·DMF}n

(1-Ln

=

Eu

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1a,

Tb

1b,

Gd

1c),

and

[Ln(L2)(H2O)(DMF)]n (2-Ln = Eu 2a, Tb 2b, Gd 2c). The two ligands show different coordination modes and conformations, and both connect Ln2 units to form 2D kgd (1-Ln) and 3D rtl topological frameworks (2-Ln), respectively. All Eu- and Tb-MOFs show strong fluorescent emissions at solid state due to the antenna effect. Energy transfer pathways were studied by measuring the triplet state energy of the ligands through analyzing Gd-based complexes at low temperature. Moreover, doped EuxTb1-x MOFs can present better color transformation emission by adjusting molar ratio of reactants. 2-Ln series also present water stability and demonstrate high selective and sensitive sensing activities towards Fe(III) and Cr(VI) in aqueous solutions, indicating the potential applications of the MOFs as ion sensors.

Scheme 1. Structures of the ligands H3L1 and H3L2. 2. EXPERIMENTAL SECTION 2.1. Materials and characterization Materials, details of structures, sensing experiments and spectroscopic techniques are available from the Supporting Information.


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2.2. Syntheses of all MOFs Syntheses of {[Ln(L1)(DMF)(H2O)]·DMF}n (1-Ln = Eu 1a, Tb 1b, Gd 1c) H3L1 (0.1 mmol, 0.030g) and Ln(NO3)3·6H2O (0.1 mmol, 0.045g) were dissolved in 12 mL solution of DMF/H2O (6:6, v/v), which were added to a 25 mL Teflon-lined stainless steel vessel. Colorless needle-like crystals were isolated after the mixture was kept 80 °C for three days. These crystals were filtrated, rinsed by water and dried naturally. {[Eu(L1)(DMF)(H2O)]·DMF}n (1a). Yield: 63.4% (based on H3L1). C21H23N2O10Eu (Mr = 615.37). Calcd.: C 40.95, H 3.74, N 4.55 %. Found.: C 41.06, H 3.84, N 4.62 %. IR (KBr pellet, cm-1): 3417(br), 2931(w), 1656(s), 1609(s), 1588(s), 1549(s), 1451(s), 1390(s), 1252(m), 1213(w),

1095(m),

981(w),

856(w),

777(w),

734(m),

659(w),

446(w).

{[Tb(L1)(DMF)(H2O)]·DMF}n (1b). Yield: 27.2% (based on H3L1). C21H23N2O10Tb (Mr = 622.33). Calcd.: C 40.49, H 3.70, N 4.50 %. found. C 40.57, H 3.83, N 4.59 %. IR (KBr pellet, cm-1): 3417(br), 2931(w), 1656(s), 1610(s), 1589(s), 1549(s), 1452(s), 1392(s), 1253(m), 1213(w), 1095(m), 981(w), 941(w), 856(w), 786(w), 736(m), 680(w), 659(w), 447(w). {[Gd(L1)(DMF)(H2O)]·DMF}n (1c). Yield: 28.2% (based on H3L1). C21H23N2O10Gd (Mr = 620.66). Calcd.: C 40.60, H 3.71, N 4.51 %. Found.: C 40.45, H 3.82, N 4.66 %. IR (KBr pellet, cm-1): 3419(br), 2930(w), 1656(s), 1610(s), 1589(s), 1548(s), 1452(s), 1391(s), 1253(m), 1214(w), 1094(m), 982(w), 943(w), 856(w), 786(w), 735(m), 659(w), 482(w), 447(w). Syntheses of [Ln(L2)(H2O)(DMF)]n (2-Ln = Eu 2a, Tb 2b, Gd 2c) Similar procedure was performed with H3L2 (0.032g, 0.1mmol) instead of H3L1 in the reaction system. The obtained colorless needle-like crystals were isolated and dried naturally. {[Eu(L2)(H2O)(DMF)]n (2a). Yield: 53% (based on H3L2). C19H18NO9Eu (Mr = 556.30). Calcd.: C 41.02, H 3.26, N 2.52 %. Found.: C 42.21, H 3.70, N 2.54 %. IR (KBr pellet, cm-1):



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3430(br), 3081(w), 2935(w), 1675(s), 1605(s), 1579(s), 1546(s), 1441(s), 1385(s), 1279(m), 1254(m), 1105(m), 777(m), 752(m), 718(w), 675(w). {[Tb(L2)(H2O)(DMF)]n (2b). Yield: 48.2% (based on H3L2). C19H18NO9Tb (Mr = 563.27). Calcd.: C 40.51, H 3.22, N 2.49 %. Found.: C 40.49, H 3.18, N 2.50 %. IR (KBr pellet, cm-1): 3427(br), 3080(w), 2897(w), 1675(s), 1606(s), 1580(s), 1441(s), 1385(s), 1280(m), 1254 (m), 1104(m), 1055(w), 777(m), 673(w), 538(w). {[Gd(L2)(H2O)(DMF)]n (2c). Yield: 27.2% (based on H3L2). C19H18NO9Gd (Mr = 561.59). Calcd.: C 40.64, H 3.23, N 2.49 %. Found.: C 41.10, H 3.20 N 2.58 %. IR (KBr pellet, cm-1): 3426(br), 3080(w), 2897(w), 1676(s), 1580(s), 1547(s), 1385(s), 1254(m), 1154(w), 1055(w), 810(w), 777(m), 752(m), 719(m), 674(w), 580(w), 453(w). Syntheses of doped bimetallic EuxTb1-x MOFs Microcrystalline powders of the co-coped bimetallic complexes 1-EuxTb1-x and 2-EuxTb1-x were synthesized according to similar procedures as aforementioned only by changing the stoichiometric ratios of Ln(NO3)3·6H2O. The powder X-ray diffractions (PXRD) proved the isostructural structures of the doped complexes to1-Ln and 2-Ln complexes. The ICP spectroscopy was executed to determine the relative molar ratios within the doped MOFs. Table 1. Crystal and refinement data for two series of MOFs. MOFs

1a

1b

1c

2a

2b

Formula

C21H23N2O10Eu

C21H23N2O10Tb

C21H23N2O10Gd

C19H18NO9Eu

C19H18NO9Tb

FW

615.37

622.33

620.66

556.30

563.26

561.59

T (K)

298(2)

298(2)

298(2)

293(2)

293(2)

293(2)

Cryst. syst.

Triclinic

Triclinic

Triclinic

Monoclinic

Monoclinic

Monoclinic

Space group

P1

P1

P1

C2/c

C2/c

C2/c

a [Å]

9.3443(8)

9.5049(8)

9.2506(8)

24.4269(19)

24.491(4)

24.491(5)

b [Å]

10.2351(9)

10.4264(9)

10.1406(9)

13.6018(9)

13.587(2)

13.580(3)


2c C19H18NO9Gd

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c [Å]

14.0620(13)

14.3841(12)

13.9291(11)

14.9262(10)

14.903(3)

14.922(3)

α [˚]

89.791(2)

89.746(2)

89.775(2)

90

90

90

β [˚]

88.975(2)

88.922(2)

88.899(2)

124.984(3)

125.108(2)

124.998(2)

γ [˚]

68.1260(10)

68.2630(10)

68.1150(10)

90

90

90

V [Å3]

1247.86(19)

1323.89(19)

1212.25(18)

4063.2(5)

4056.9(12)

4065.7(14)

Z

2

2

8

8

8

Dcalcd [g·cm-3]

1.638

1.561

1.700

1.819

1.844

1.835

μ [mm-1]

2.568

2.722

2.792

3.139

3.538

3.314

θ range

2.52-25.02

2.31-25.02

2.55-25.02

2.73-25.02

2.74-25.02

2.73-25.02

Index ranges

-10

Tunable Light Emission and Multi-Responsive Luminescent

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