Triple-Wavelength-Region Luminescence Sensing Based on a Color

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Triple-wavelength-region luminescence sensing based on a color-tunable emitting lanthanide metal organic framework Xinyu Wang, Xu Yao, Qiang Huang, Yuxin Li, Guanghui An, and Guang-Ming Li Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b00494 • Publication Date (Web): 15 May 2018 Downloaded from http://pubs.acs.org on May 15, 2018

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Analytical Chemistry

Triple-Wavelength-Region Luminescence Sensing Based on a Color-Tunable Emitting Lanthanide Metal Organic Framework Xin-yu Wang, Xu Yao, Qiang Huang*, Yu-xin Li, Guang-hui An and Guang-ming 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], [email protected]; Fax: +86-451-86673647. ABSTRACT A series of isomorphic lanthanide metal-organic frameworks (Ln-MOFs), namely [La(dcbba)(DMF)2]n·H2O·0.5DMF [Ln = La (1), Eu (2) and Tb (3); H3dcbba = 4-(3, 5-dicarboxylatobenzyloxy)benzoic acid; DMF = N,N′-Dimethylformamide] have been designed and synthesized by the solvothermal reactions of H3dcbba and La(NO3)3·6H2O. Single-crystal X-ray diffraction analysis reveals that the complexes 1−3 exhibit a (3,6)-connected open framework structure with binuclear [Ln2(COO)6] n secondary building units as 6-connected nodes and H3dcbba ligands as 3-connected nodes. The isostructural mixed La/Eu/Tb-dcbba (4) was obtained via the in-suit doping of different Ln3+ ions into the host framework, which is able to emit pure orange, white and blue light when excited at 300, 305 and 350 nm, respectively. Subsequently, a novel and multifunctional sensing process was designed based on the excitation wavelength sensitive color tunable luminescent sample 4, which can detect HS– ions, THF (tetrahydrofuran) and Ag+ ions via different luminescence color change mechanism. The remarkable color change, excellent selectivity and high sensitivity further indicate the promise of this type of multifunctional luminescent materials for the sensing of anion, cation and organic small molecule.

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1. INTRODUCTION Lanthanide metal-organic frameworks (Ln-MOFs), the perfect unity of the porosity of MOFs and the high efficient luminescence of lanthanide ions, have received increasing attention in recent years.1-4 By the rational designs of synthesis and measurement routes, Ln-MOFs as the platform can hold up diverse luminescence process for different demands, including the Ln3+ ions tuned ligand-centered emission,5-7 co-luminescence of ligand and Ln3+ ions,8-11 as well as the characteristic emission of Ln3+ ions.12-14 Therefore, a color-tunable luminescence or white-light emission can be obtained predictably by reasonable modification of the energy transfer process which occurred in the interior of the frameworks.15, 16 A mass of color-tunable or white-light materials were designed based on the in-suit doping different Ln3+ ions into the host MOFs or by the post-synthetic modification (PSM) of the frameworks, and have performed an extremely important function in lighting and display territory.17-20 However, the applications of the color-tunable or white-light materials in other aspects are also urgent to be developed. In the past few years, Ln-MOFs have been frequently employed for the sensing of anions and cations, organic small molecules or temperature due to their unique photoluminescent properties and high recognition capability for guest molecules.21-30 However, the majority of sensing signals response are based on the monotonous enhancement or quenching of luminescent intensity, which is generally influenced by environmental interference, including the system noise, excitation power fluctuations and light source, etc.31,

32

The

luminescence-color change (LCC) sensing approach can effectively avoid these drawbacks in comparison with the traditional turn-on/off sensing. Recently, some LCC sensors based on lanthanide coordination polymers (LnCPs) have been reported.33-35 For example, a white-light-emitting lanthanide coordination polymer that could be effectively applied to the detection of Ag+ and Mn2+ ions by way of the LCC process with unique higher selectivity and sensitivity have been reported in our previous work. It represents the first white-light-emitting material at the luminescent sensing territory.36 Similarly, some mixed-component Ln-MOFs have also been employed as the LCC platform for the come true of ratiometric sensing, e.g. Zaworotko et al. have reported a mixed-crystal variants EuxTb1−x-ZMOF (ZMOF = zeolite metal-organic framework) for the LCC sensing of LPA (lysophosphatidic acid, a biomarker for ovarian cancer and other gynecologic cancers).37 Although some LCC path can be realized by exactly adjusting the multiple energy transfer process among the different components, the misty sensing mechanism and simplex sensing property is still great barrier for the development of this type of sensor.


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To the best of our knowledge, multifunctional luminescence sensors based on the LCC process

of

color

tunable

material

have

never

been

explored.

Herein,

a

triple-wavelength-region sensing procedure was designed and utilized as a trifunctional LCC path for the detection of anion, cation and organic small molecule. In this system, La3+, Eu3+ and Tb3+ ions are three independent luminescence centers, which can perform synergism effect on the final luminescence color. Along with the change of the energy transfer paths which from ligand to metal (LMET) or from Ln3+ ions to other Ln3+ ions (MMET), the corresponding luminescent color from the sample was changed too. When complex 4 was excited at 300, 305 and 350 nm, the specific energy transfer mechanism provide diverse LCC sensing process for HS– ions, THF and Ag+ ions, respectively. Notably, the complex 4 is the first example that the color-tunable emitting material was used at luminescent sensing application as well as the first multifunctional material that can sense anion, cation and organic small molecule.

2. EXPERIMENTAL SECTION Materials and Instrumentations. Ln(NO3)3·6H2O was obtained by the reactions of Ln2O3 and nitric acid. The DBCCA ligand and other chemicals were purchased from commercial sources and used without purification. 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 CuKα as the radiation source (λ = 0.15406 nm) in the angular range 2θ = 5−50º at room temperature. Elemental analyses of C, H, O and N for desolvent samples 1-4 were collected on a Perkin-Elmer 2400 analyzer. Inductively coupled plasma atomic emission spectroscopy (ICP-AES) was performed on a HK-2000 spectrometer. The La/Eu/Tb ratios of complex 4 were determined by ICP analysis. The photoluminescence (PL) spectra were measured with an Edinburgh FLS 920 fluorescence spectrophotometer. 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



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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. Synthesis of Complexes 1−4. Complexes 1−3 were synthesized by solvothermal reactions of Ln(NO3)3·6H2O (Ln = La, Eu and Tb; 0.1 mmol), H3L ligand (0.1 mmol, 32mg), DMF (3.5 ml), and water (1.5 ml) were prepared in a Teflon-lined autoclave at 100℃ for 48 hours and then cooling to room temperature gradually. The resulting colorless crystals were filtrated, and washed by DMF and water for several times. The yield was 70%, 65% and 61% for 1−3 (based on H3dcbba), respectively. {[La(DCBBA)(DMF)2]n·H2O·0.5DMF}n

(1).

Elemental

analysis

(%):

Calcd

for

C22H21LaN2O9 (desolvent samples; Mr = 596.32), C 44.31, H 3.55, N 4.70, O 24.15; found: C 44.09, H 3.50, N 4.58, O 24.11. IR (KBr, cm−1): 3456 (m), 3074 (w), 2932 (m), 1661 (s), 1389 (s), 1241 (m), 1110 (w), 1012 (w), 849 (w), 783 (m), 690 (m). UV/Vis [ACN, λ]: 216 nm. {[Eu(DCBBA)(DMF)2]n·H2O·0.5DMF}n

(2).

Elemental

analysis

(%):

Calcd

for

C22H23EuN2O9 (desolvent samples; Mr = 611.39), C 43.22, H 3.79, N 4.58, O 23.55; found: C 43.14, H 3.61, N 4.60, O 23.46. IR (KBr, cm−1): 3463 (m), 3061 (w), 2923 (m), 1678 (s), 1391 (s), 1236 (m), 1131 (w), 1023 (w), 856 (w), 778 (m), 681 (m). UV/Vis [ACN, λ]: 216 nm. {[Tb(DCBBA)(DMF)2]n·H2O·0.5DMF}n

(3).

Elemental

analysis

(%):

Calcd

for

C22H23TbN2O9 (desolvent samples; Mr = 618.35), C 42.73, H 3.75, N 4.53, O 23.29; found: C 42.69, H 3.67, N 4.62 O 23.14. IR (KBr, cm−1): 3471 (m), 3065 (w), 2947 (m), 1696 (s), 1393 (s), 1267 (m), 1168 (w), 1009 (w), 873 (w), 785 (m), 693 (m). UV/Vis [ACN, λ]: 216 nm. Complex 4 was synthesized by a room temperature reaction, a mixture of corresponding lanthanide nitrate (0.4 mmol; La: Eu: Tb = 0.88: 0.02: 0.10), H3L (0.4 mmol), DMF (3.5 ml) and H2O (1.5 ml) were added to a round-bottom flask with a magneton, and allowed to stir at room temperature for 24 h. The resulting white powder was filtrated, and washed by DMF for several times. The yield was 70% for 4 (based on H3L). For Complex 4, the molar ratio of the different Ln3+ ions are mainly identical to that of mixture lanthanide nitrate, which were completely demonstrated by ICP, distinctly revealing that in situ doping of the lanthanide ions was successful (Table S1). In addition, the identical structures of complexes 1−4 are


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demonstrated by the corresponding PXRD patterns too (Figure S4). Luminescence sensing experiments. To obtain a stable suspension, the crystal samples of complex were finely ground and desolved in DMF forming a suspension 0.2 (mg/mL) by ultrasonication treatment for 30 min. Then the stable suspension was applied for luminescence sensing analyses at room temperature. For the sensing experiments of anions and cations, different anions and cations aqueous solutions (1 × 10-4 M) were added to the suspension (2 mL) with the same quantity to examine the potential of complex 4 selectively recognize anions and cations. For the sensing of small organic molecules, these SOMs were slowly introduced into the DMF suspension (2 mL). The slit widths of excitation and emission were kept the same all the time. X-ray Crystallographic Analysis. Crystal data for complexes 1−3 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 1−3 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.38 All non-hydrogen atoms were refined anisotropically. Empirical absorption corrections based on equivalent reflections were applied. For complexes 1−3, the contribution of HIGHLY disordered anions/solvent molecules were treated as diffuse using the SQUEEZE procedure implemented in the PLATON program.39 The resulting new files were applied to further refine the structures. The Squeeze results are consistent with TG and elemental analysis, which indicate that there are half of the DMF and one water molecules in a structure unit as free guest. These solvent molecules are added in the molecular formula. The crystal data and structure refinements of complexes 1−3 are summarized in Table 1. Crystallographic data for complexes 1−3 have been deposited in the Cambridge Crystallographic Data Center with CCDC No. 1818054-1818056. Table 1. Crystal data and structure refinement for complexes 1−3. 1

2

3

Empirical formula

C23.5H26.5LaN2.5O10.5

C23.5H28.5EuN2.5O10.5

C23.5H28.5TbN2.5O10.5

Formula weight

650.88

665.95

672.91

Crystal system

Monoclinic

Monoclinic

Monoclinic

Space group

C 2/c

C 2/c

C 2/c

a /Å

28.492 (5)

28.220 (5)

28.614 (5)

b /Å

14.539 (5)

14.540 (5)

14.383 (5)

c /Å

13.492 (5)

13.394 (5)

13.644 (5)



α (deg)

90

Β (deg)

101.870 (5)

100.648 (5)

97.284 (5)

γ (deg)

90

90

90

Volume (Å3)

5470 (3)

5401 (2)

5570 (2)

Z

8

8

8

Dc (g⋅cm )

1.448

1.504

1.475

T (K)

293 (2)

293 (2)

293 (2)

λ (Mo Kα)(Å)

0.71073

0.71073

0.71073

-3

90

90

Reflections collected

4796

4749

4901

µ(mm-1)

1.608

2.370

2.585

2368.0

2432.0

2448.0 0.0275, 0.0693

F(000) Final R1 ,wR2 [I > 2σ(I)]

0.0303, 0.0732

0.0449, 0.1084

Final R1a, wR2b(all data)

0.0349, 0.0763

0.0491, 0.1119

0.0330, 0.0743

GOF on F2

1.107

1.074

1.004

a

a

Page 6 of 17

b

R1 = Σ||F0|−|Fc||/Σ|F0|. bwR2 = Σ[w(F02−Fc2)2]/ Σ[w(F02)2]1/2.

3. RESULTS AND DISCUSSION Crystal Structure Analysis of Complexes 1–3. X-ray crystallographic analysis reveals complexes 1–3 are isostructural with C 2/c space group. In a typical structure of complex 1, each asymmetric unit consists of one crystallographically independent La3+ ion, one deprotonated dcbba ligand, and two coordinated DMF molecules (Figure S5a). Each La3+ ion is nine-coordinated by seven oxygen atoms from carboxyls of five H3dcbba ligends and two oxygen atoms from two DMF molecules (Figure S5b). Meanwhile, the neighboring La3+ ions were bridged through oxygen atoms from the carboxylate groups of branched organic ligand and further form the [La2(COO)6]n secondary building unit (SBUs) (Figure 1a). The neighboring dinuclear [La2(COO)6]n units were connected by L3− in (k1-k1-µ2)-(k1-k1-µ1)-µ3 mode forming 1D chain (Figure 1c). The adjacent 1D chains were linked by the isophthalic sections of ligand L3− in (k1-k2-µ3) mode forming 2D network (Figure 1d). The adjacent 2D networks further accumulation into 3D frameworks in the vertical direction (Figure 1e). Notably, the framework exhibits an open 1D window with the size of about 9.8 × 9.5 Å2 (opposite distance of O7 to O7 and C14 to C14). PLATON calculation shows that there is about 18.7% accessible volume of the unit cell volume when the guest molecules were removed. From a topological point of view, the dcbba ligand bridge three carboxylate groups, can be considered as 3-connected node, and the dinuclear [La2(COO)6]n SBUs connected by six carboxylate groups, can be considered as an 6-connected node. As a result, the framework can be viewed a (3,6)-connected 3D topology, and the structure of complex 1 can be simplified as a new topology with the topological point symbol of {42·6}2{44·62·87·102} calculated with TOPOS.40


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Figure 1. (a) Coordination environment of central La3+ ions in complex 1 with hydrogen atoms omitted for clarity. (b) Coordination mode of H3dcbba ligand. (c) 1D chain. (d) 2D network. (e) 3D framework. Luminescence Properties of Complexes 1–3. The solid-state 3D luminescent spectra of free ligand and complexes 1–3 were conducted at room temperature. As shown in Figure 2, the dcbba ligand exhibited strong blue-green emission with the broad emission band from 400 to 500 nm when excited at 300–380 nm. Similarly, complex 1 exhibits Ln3+ ions tuned ligand-centered emission around 400–550 nm upon excited at 300–380 nm. However, the emission intensity exhibited an obvious enhancement with the increasing excitation wavelength. In contrary, complex 2 completely exhibits a characteristic emission of Eu3+ ions at 593, 616, 650 and 699 nm, respectively, which corresponding to the 5D0→7FJ (J = 1–4) transitions of Eu3+ ion upon excited at 300–380 nm. The lifetime and quantum yield for complex 2 are τ = 1.25 ms (Ex = 368 nm) and Φ = 28.36% (Figure S6b). Similarly, complex 3 exhibits four peaks at 490, 544, 582 and 621 nm corresponding to the 5D4→7FJ (J = 6–3) transitions of Tb3+ ion (Fig. 3a). The quantum yield and lifetime are 1.28 ms and 25.11%, respectively (Figure S6c). The long luminescence lifetimes and high quantum yields of complexes 2 and 3 suggest that the energy transfer between the H3dcbba ligand and lanthanide ions is effective (Table S2).



Figure 2. 3D solid state emission spectra of dcbba ligand (a) and complex 1 (b), complex 2 (c) and complex 3 (d). Color-Tunable Luminescence and White-Light. To design novel multiple-waveband response luminescent material and explore the multifunctional luminescent sensing application, primarily, an excitation wavelength sensitive color tunable luminescent sample should be afforded. For the coordination system of metal-organic ligand, the most direct and efficient path, is the in-suit doping of the trichromatic.41-43 Considering that complexes 1–3 are isostructural and exhibit pure blue, red and green emission, respectively, the color-tunable and white-light emission sample would be possible via the multiple and synergetic energy transfer between ligand and metal (LMET), or between different lanthanide metal ions (MMET). In fact, an excitation wavelength-dependent color-tunable luminescent complex 4 has been realized by the doping of La3+, Eu3+ and Tb3+ ions into the host frameworks, and the PXRD measure exhibit the framework structure of complex 4 is isostructural with complexes 1–3. As shown in Figure 3, when excitation at 300 nm, an orange emission can be observed with a CIE coordinates of (0.442, 0.439). Since the emission spectra of complex 1 exhibit the broad emission around at 400-550 nm, it is should be pointed, when the co-doping complex 4 was excited at 300 nm, there was not obvious broad LC emission was observed in the emission spectra of complex 4, but the sharp emission peak at 544 nm still was remained and with a stronger intensity, seems to indicate that the emission peak at 544 nm should be attributed to characteristic transitions of Tb3+ ions rather than La3+ ions. With the excitation wavelength increase, the emission intensity of blue component augments gradually along with the characteristic emission of Eu3+ and Tb3+ ions were decreased continually, and a white


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emission with a CIE coordinates of (0.330, 0.339) was observed when excitation at 305 nm. When varying the excitation wavelength to 350 nm, the blue emission with a CIE coordinates of (0. 202, 0.173) from ligand becomes dominated. Obviously, a smooth process of tunable colors from orange to white to blue has been obtained by the adjustment of excitation wavelength, implied that complex 4 is possible candidate as multifunctional luminescent sensing materials.

Figure 3. (a) Emission spectra of the suspension of complex 4 with excitation wavelengths varying from 300 to 350 nm. Inset: the corresponding luminescence photograph when excited with a Xe lamp at 300, 305 and 350 nm. (b) The corresponding CIE coordinates. Triple-Waveband Response Luminescent Sensing. In view of complex 4 shows a fluent luminescence-color change (LCC) process when the excitation wavelength vary from 300 nm to 350 nm. It inspires a series of novel ideas for LCC luminescent sensing: (1) the boring luminescence enhancement or quenching can be replaced by LCC so that the interference from environment can be avoided; (2) the design of different LCC process may meet new potential application for color-tunable luminescence materials; (3) multifunctional luminescent sensing materials will possibly be fabricated based on the multiply LCC process. Therefore, the LCC sensing processes were studied when the excitation toward 300 nm (orange), 305 nm (white) and 350 nm (blue), respectively. Firstly, the selective tests toward anion were analyzed when the suspension of complex 4 (0.2 mg/mL) was excited at 300 nm. The emission spectra of the suspension containing variable anion aqueous solutions of NaxX (X = PO43–, Br–, ClO4–, SO32–, I–, HCO3–, CO32–, NO2–, CrO42–, MoO42–, SiO32–, HSO3–, F– and HS–, x = 1–3 ) were recorded. The ratios of the 5

D4→7F5 transition intensities of Tb3+ ions (I544) and 5D0→7F2 transition intensities of



Eu3+ ions (I616) were applied to evaluate the LCC process. As shown in Figure 4 and Table S3, the luminescence spectra of the blank sample and the majority of anion ions@ complex 4 remain almost the same in terms of the ratios of I544/I616 was about 1. However, the luminescence spectra of HS–@complex 4 shown obvious LCC phenomenon compared to other Xx–@complex 4 systems with the ratios of I544/I616 closing to 1.4, exhibiting obvious LCC from orange to green. The sensing properties of complex 4 to HS– ions were further investigated by the quantitative fluorescence titration experiments. According to Figure 4b, the intensity of I544 showed negligible change when the content of HS– ions was increased gradually. However, the intensity of I616 shown evidently decrease, exhibiting obvious LCC phenomenon from orange to blue. The excellent selectivity and visible LCC phenomenon boost on the potential application on detecting HS– ions via LCC process.

Figure 4. (a) Emission spectra of Xx–@complex 4 excited at 300 nm (inset: the corresponding CIE coordinates change of HS–@complex 4). (b) Emission spectra of complex 4 toward different concentration HS– ions (inset: the corresponding luminescence photograph with HS– ions concentration from 0 to 400 µM). Secondly, the suspension of complex 4 shows a white-light emission with a CIE coordinates of (0.330, 0.339) upon the excitation at 305 nm. To further investigate the LCC sensing ability of complex 4 was excited 305 nm, it was employed for the sensing of small organic molecules (SOMs). As shown in Figure 5, the THF@complex 4 exhibit distinct LCC process compared with other SOMs. The concentration dependence analysis shown that the luminescent intensities of I544 (green) and I616 (red) gradually decrease along with the intensity of ligand-center (blue) evidently increase with the increasing THF content, which result in a LCC process from white to violet. Notably, the LCC phenomenon can be observed easily even when the content of THF is lower than 20 µL, further confirming that complex 4 can acts as highly selectivity and sensitivity LCC sensor for THF. To the best of our knowledge, complex 4 is the first example of luminescent sensor for THF based on Ln-MOFs.


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Figure 5. (a) Emission spectra of various SOMs@complex 4 excited at 305 nm (100 µL). (b) Emission spectra of complex 4 upon incremental addition of THF (insets: the corresponding CIE coordinates and photography). Thirdly, the blue emission from ligand becomes dominated when complex 4 was excited at 350 nm. Based on the previously reports, several transition metal ions can be used as the sensitizer for lanthanide ions emission, which result in a more excellent energy transfer from ligands to Ln3+ ions.44-47 In other word, the weak characteristics emission for complex 4 excited at 350 nm, will be possibly restored when certain transition metal ions were introduced into the suspension system, and a LCC process will be inspired finally. To confirm this hypothesis, various metal cation aqueous solutions of X(NO3)x (100 µM, X = Zn2+, Ca2+, Pb2+, Al3+, Hg2+, Na+, Mg2+, NH4+, Ba2+, K+, Cr3+, Ni2+, Cd2+, Ag+, Fe3+, Co2+ and Cu2+, x = 1– 3) have been added into the suspension of complex 4, respectively. As shown in Figure 6a, the LCC features for Xx+@complex 4 dependence largely on the nature of the metal cations. Several familiar quenchers, such as Fe3+, Co2+ and Cu2+ ions, exhibit drastic quenching effects, which have been frequently reported previously. In contrast, the luminescent color of Ag+@complex 4 exhibit obvious LCC from blue to green. This result imply that complex 4 can be used as a sensor to selectively detect Ag+ ions by LCC process when complex 4 was excited at 350 nm. The sensing properties of complex 4 toward Ag+ ions were further carried out by monitoring a series of emissions of complex 4 with gradually increased Ag+ ions concentration excited at 350 nm. As the increasing concentration of Ag+ ions, remarkably, the ligand-center emission of complex 4 was continuously quenched. However, the intensity of characteristic emission for Tb3+ ions was restored. It

should be noticed that there is not

evident enhancement on the characteristic emission for Eu3+ ions, which might be attributed to the energy transfer process from Tb3+ to Eu3+ was interrupted because the existence of Ag+ ions.



Figure 6. (a) Emission spectra of various Xx+@complex 4 excited at 350 nm (100 µL). (b) Emission spectra of complex 4 upon incremental addition of Ag+ ions (insets: the corresponding CIE coordinates and photography). In addition, the stability and recyclability of the complex 4 LCC sensing has been conducted through recycling experiments. Upon the suspension of complex 4 was centrifuged, filtered and washed several times with DMF, the relative intensity between LC emission, I544 and I616 in the emission spectra of the regenerative suspension was almost invariable even after being used for four repeated cycles. (Figure S12-14) The corresponding PXRD patterns support that the crystal structures are remained after the recycling experiments. (Figure S11) Mechanism of LCC Sensing. The mechanism of the LCC sensing of complex 4 was proposed according to the experiments. Firstly, the PXRD patterns of complex 4 before and after luminescent sensing HS- ions, THF and Ag+ ions were still identical with that the simulated pattern, implying that the luminescence color change not stem from the breakdown of the host framework (Figure S11). Therefore, the LCC phenomenon of complex 4 can possibly be attributed to the following reasons: certain ions may form the weak interactions with the benzene rings of ligand, such as the hydrogen bonds or π interactions; the competed photon absorption might exist between analytes and ligand; and the efficiency of intersystem crossing of S1 to T1 is likely to be promoted due to the unique properties of analytes, such as the heavy-atom effect of Ag+ ions. In order to confirm these hypotheses, some necessary experiments were carried out. The complex 4 was conducted by the in-suit doping of La3+, Eu3+ and Tb3+ ions with a molar ratio of La0.88: Eu0.02: Tb0.10. Although the contents of the Tb3+ ions is almost five times as the Eu3+ ions, the characteristic peaks of Eu3+ ions at 616 nm was stronger than Tb3+ ions at 544 nm when the complex 4 was excited at 300 nm, which can be attributed to an energy transfer from Tb3+ to Eu3+ ions (Scheme S1).19,

41, 48

Although the La3+ ions contributed a main


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contents with 88 % in complex 4, the La3+ ions are unable for transferring the energy to the Eu3+ ions due to there is no energy transfer between ligand and La3+ ions. With the gradually increased of HS– ions, the intensity of I544 shown negligible change, but the intensity of I616 shown evidently decrease, indicated that the energy transfer process from Tb3+ to Eu3+ ions was disturbed continuously.49, 50 We easily speculate that there are weak interactions (such as hydrogen bonds) existing between HS– ions and the other sections of frameworks. It may further break the energy transfer from Tb3+ to Eu3+ ions, which results in the decreasing emission of Eu3+ ions (Figure 7). In order to verify these hypotheses, the energy transfer process which located in the single-lanthanide MOF and the mixed sample of Eu-MOF and Tb-MOF (molar ratio was 1 : 5) were investigated (Figure S8). In the single phase and mixed phase system, there are almost no changes for the luminescence intensity of Tb3+ and Eu3+ ions were observed, implied that the weak interactions between HS– ions and the framework did not effected energy transfer efficiency from ligand to Ln3+ ions and the energy transfer from Tb3+ to Eu3+ ions is not going to happen in this mixed system too. So we can summarize that the energy transfer from Tb3+ to Eu3+ ions can be produced in the intra of framework only. Therefore, the in-suit doping complex 4 can be an ideal sensor for the sensing of HS– ions.

Figure 7. The schematic illustrations of the mechanism of complex 4 detect HS– ions. For the LCC sensing when complex 4 was excited at 305 and 350 nm, some underlying mechanisms were raised. For the sensing of THF, the ligands-center emission evidently increase and the characteristic of Ln3+ ions gradually decrease, indicating the antenna affect was weakened. In contrast, the luminescence spectra of complex 4 toward Ag+ ions shown that the antenna affect was amplified with the increasing Ag+ ions concentration. To further testify the underlying mechanism, the luminescence emission peaks analyses for THF@complex 4 and Ag+@ complex 4 toward various concentrations were carried out (Figure S9 and S10, Table S4 and S5). With the presence of THF, the ligand-center emission produced a blue shift, implying an increase in the energy level of the π* energy of the ligand,



leading to an disturb in the energy transfer from the π* orbits to the lanthanide ions, which results in the ligand-center emission dominated the emission spectra and the LCC from white to violet. On the contrary, a red shift of the ligand-center emission would be introduced when Ag+ ions were added into the suspension of complex 4, suggesting an enhance in the π* orbits of the ligand. The more becomingly match between the π* orbits of the ligand and the resonance energy level of the Ln3+ ions will boost up the more highly efficiency energy transfer from the ligand to the Ln3+ ions. Therefore, in the presence of Ag+ ions, complex 4 exhibited stronger characteristic emissions of Ln3+ ions and weaker ligand-center emission, which leads to a LCC process from blue to green (Scheme 1).36, 51 Although some other analytes may also form an interaction with the framework, or affect the energy level of the ligand, the properties difference of these analytes, such as the electronic absorption/ donation ability, the electronegativity, the polarity and absorbance of the solvent, etc. will affect the final luminescence emission results.52 In a word, the physical and chemical properties of the framework and analytes produce a synergistic effect on the luminescence performance of complex. Therefore, the dominating factors were discussed more detailedly in this work.

Scheme 1. The effect of THF (a) and Ag+ (b) ions on the ligand to metal energy transfer.

4. CONCLUSION Reactions of H3dcbba and Ln(NO3)3·6H2O afford a series of isostructural lanthanide metal-organic frameworks 1–3. The identical structure of complexes 1–3 provides a chance to afford isomorphic mixed La/Eu/Tb-dcbba (4) via the in-suit doping of different Ln3+ ions into the host framework. Based on the regulation of the energy transfer efficiency from ligand to metal, a novel and multifunctional sensing process was designed, which can detect HS– ions, THF and Ag+ ions via different LCC process. The remarkable color change, excellent


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selectivity and high sensitivity verify that this type of color-tunable luminescence material can be a promising sensor for anion, cation or organic small molecule. To the best of our knowledge, complex 4 is the first multifunctional material that can sense anion, cation and organic small molecule. This approach may provide potential multifunctional materials for 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 *E-mail: [email protected] *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 and 21501051) and Heilongjiang University (YJSCX2017-053HLJU).

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