Tailoring the Fluorescence of AIE Active Metal- Organic Frameworks

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Tailoring the Fluorescence of AIE Active MetalOrganic Frameworks for Aqueous Sensing of Metal Ions Qiyang Li, Xiuju Wu, Xiaoli Huang, Yangjun Deng, Nanjian Chen, Dandan Jiang, Lili Zhao, Zhihua Lin, and Yonggang Zhao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 11 Jan 2018 Downloaded from http://pubs.acs.org on January 11, 2018

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ACS Applied Materials & Interfaces

Tailoring the Fluorescence of AIE Active MetalOrganic Frameworks for Aqueous Sensing of Metal Ions Qiyang Li,a Xiuju Wu,a Xiaoli Huang,a Yangjun Deng,a Nanjian Chen,b Dandan Jiang,b,c Lili Zhao,*b,c Zhihua Lin,*b Yonggang Zhao*a,d a

Key Laboratory of Flexible Electronics (KLOFE), Institute of Advanced Materials (IAM),

Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University, Nanjing 211816, PR China b

College of Chemistry and Molecular Engineering, Nanjing Tech University, Nanjing 211816, PR China

c

d

Institute of Advanced Synthesis (IAS), Nanjing Tech University, Nanjing 211816, PR China

State Key laboratory of Coordination Chemistry, Nanjing University 210093, PR China

KEYWORDS: Metal-Organic Frameworks, Aggregation-Induced Emission, Fluorescence, Sensing, Aluminum Ion.

ACS Paragon Plus Environment

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ABSTRACT: A hydroxyl functionalized ligand was designed for the construction of MOF materials with aggregation-induced emission (AIE) feature, in which the fluorescence can be deliberately tailored: quenching the fluorescence to an “off” state by the decoration with heterocyclic auxiliary ligand 4,4’-bypyridine (Bpy) in the framework as quenching agent; triggering the enhanced fluorescence to an “on” state by removal of Bpy through the metal competitive coordination substitution strategy. Our study shows that the occurrence of exciton migration between the AIE linker and the conjugated auxiliary ligand Bpy cause the fluorescence quenching. Time-dependent density functional theory (TD-DFT) was employed to understand the photo induced electron transfer process, and explained the origins of fluorescence quenching. Using this strategy, the prepared MOF material can perform as fluorescence “off-on” probe for highly sensitive detection of Al3+ in aqueous media. The hydroxyl group play crucial role in the sensing as it can selectively chelate with Al3+, which directly relate to the dissociation of nonfluorescent MOF and consequent activation of AIE process. INTRODUCTION Metal–organic frameworks (MOFs), as a new type of inorganic organic hybrid materials, not only display versatile structural tunability and functionality,1-4 but also exhibit a wide range of luminescent behaviours originating from the multifaceted nature of their structure.5-6 The luminescence properties of MOFs can be systematically tuned by varying the organic linkers, framework metal ions, the structural characteristics, their interactions with guest species, et al. The wide variety of luminescence makes MOF materials ideal for sensing applications.7-9 Recent studies have shown that luminescent MOFs can be very promising candidates as selective and sensitive chemosensors for detection of cations,10-12 anions,13-14 small molecules,15 gas16 and explosives.17-20


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Aggregation-induced emission (AIE) is a fascinating photophysical phenomenon with emission enhancement by aggregate formation, and has gained considerable interests in the past decade.21-25 Some pioneering studies have shown that the introduction of organic linkers with AIE characteristics within MOFs can bring bright fluorescence.26-32 According to the restriction of intramolecular rotation (RIR) principle,33 rigidifying linkers in MOFs could efficiently and substantially tune the electronic transition energies and enhance the fluorescence, thus offer great opportunities for AIE MOFs to become fluorescence turn-on probes.34-35 Besides the RIR induced fluorescence enhancement, the AIE systems can also work in a turn-off mode when involving other non-radiative decay processes such as photo-induced electron transfer (PET) or energy transfer (ET),36-38 demonstrating the flexibility and tunability of the AIE-based luminescent behaviours. Recently, Wang group succeeded in design of MOF probe for turn-on detection of energetic heterocyclic compounds with using AIE active fluorophore 4,4’-((Z,Z)-1,4-diphenylbuta-1,3diene-1,4-diyl)dibenzoic acid (TABDC), in which the paramagnetic meta nodes Co(II) and Ni(II) were employed for tuning off the fluorescence and allowing subsequent turn-on detection of guest molecules.39 Enlightened by above design concept, in this research, we sought to synthesize AIE active MOFs by combining the hydroxyl functionalized TABDC ligand 4,4’((Z,Z)-1,4-diphenylbuta-1,3-diene-1,4-diyl)bis(2-hydroxybenzoic acid) (HTABDC) and Zn(II) of d10 electron configuration. Besides, the fluorescence of MOF containing AIE organic linker can be tuned with using auxiliary ligand 4,4’-bypyridine (Bpy) as a fluorescence quenching agent, to an “off” state. The resulting non-fluorescent material then exhibits an “off-on” switching in the presence of trace Al3+. EXPERIMENTAL SECTION


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Materials and Methods: Unless specifically mentioned, all of solvents and reagents used were purchased from Aldrich，Alfa Aesar or TCI without further purification. 1H NMR and 13C NMR spectra were measured on a Bruker AV 400 spectrometer at 298 K. Thermogravimetric analysis (TGA) was carried out using a Mettler Toledo TGA at heating rate of 10 °C per min under nitrogen. Powder X-ray diffraction patterns (PXRD) were collected on a Bruker D8 Advance rotation anode X-ray diffractometer equipped with graphite monochromatized Cu Kα radiation (λ = 1.54 Å). Scanning electron microscopy (SEM, Hitachi S-4800) was used for the morphology analysis. Fluorescent emission spectra were obtained on a F-4600 fluorescence spectrometer. Fluorescence quantum yield measurements were performed on an Edinburgh Instruments FLS980 analytical spectrometer equipped with an integrating sphere using BaSO4 as a white standard. Single crystal X-ray diffraction data were collected at 296 K on a Bruker ApexII CCD diffractometer, operating at 50 kV and 30 mA and sealed tube X-ray source with Mo-Kα radiation (λ =0.71073 Å). Elemental analyses of C, H and N were carried out on a Vario EL Cube elemental analyzer. ESI-MS experiments were carried out on an Agilent 1100 LC/DAD/MSD LC-MS. HRMS spectra were recorded on a Waters Q–TOF Permier Spectrometer. Synthesis of Zn(HTABDC)(DMF)2 (MOF 1): HTABDC (0.035g, 0.073mmol) and Zn(NO3)2·6H2O (0.029 g, 0.1 mmol) were dissolved in 5 mL DMF in a 30ml Pyrex vial. The mixture was stirred under sonication for 20 minutes, and then 5 mL water were added. The sealed vial was then placed in an oven and heated at 90 °C for 24 hours. The solid product was then filtered and washed with DMF. Colorless flaky crystals of 1 were collected after drying in air (yield: 73% based on HTABDC). Compound 1 has a formula of Zn(HTABDC)(DMF)2,


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which was derived from crystal data and elemental analysis. Anal. Calcd (%) for ZnC36H34N2O8: C 62.84, H 4.98, N 4.07; found: C 62.54, H 4.85, N 3.79. Synthesis of Zn(HTABDC)(Bpy)·DMF (MOF 2): HTABDC (0.02g, 0.042mmol) and Zn(NO3)2·6H2O (0.029 g, 0.1 mmol) and 4,4-bipyridine (0.013 g, 0.084 mmol) were dissolved in 6 mL DMF in a 30 mL Pyrex vial. The mixture was stirred under sonication for 20 minutes in order to obtain a clear solution, and then 3 mL EtOH and 3 mL water were added. The sealed vial was placed in an oven and heated at 65 °C for 72 hours. The solid product was then filtered and washed with DMF. Light yellow block crystals of 2 were collected after drying in air (yield: 62% based on HTABDC). Compound 2 has a formula of Zn(HTABDC)(Bpy)·DMF, which was derived from crystal data and elemental analysis. Anal. Calcd (%) for ZnC43H35O7N3: C 66.97, H 4.58, N 5.45; found: C 67.13, H 4.35, N 5.14. Crystal Structure Determination: The single crystal X-ray diffraction measurements were experimented on a Bruker Apex2 CCD diffractometer, operating at 50 kV and 30 mA and sealed tube X-ray source with Mo-Kα radiation (λ =0.71073 Å). The structures of HTABDC, 1 and 2 were solved by direct methods and refined anisotropically on F2 by a full-matrix least-squares refinement with the SHELXTL program.40 The diffraction data were corrected for empirical absorption based on multi-scan. Anisotropic thermal parameters were applied to non-hydrogen atoms and all hydrogen atoms of organic ligands were calculated and added at ideal positions. The crystallographic data of three complexes are given in Table 1, and selected bond lengths and angles are listed in Table S3 and Table S4. CCDC numbers are 1538926 for 1, 1538927 for 2 and 1538929 for HTABDC, respectively. Table 1. Crystallographic Data for 1, 2 and HTABDC. compound

1

2

HTABDC


5


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chem formula

C36 H34 N2 O8 Zn

C43 H35 N3 O7 Zn

C36 H36 N2 O8

fw

688.02

771.13

624.67

crystsyst

Monoclinic

Triclinic

Triclinic

space group

I 2/a

P -1

P -1

a (Å)

10.875(4)

7.528(2)

7.488(4)

b (Å)

9.029(3)

16.408(4)

8.510(4)

c (Å)

33.420(11)

16.481(4)

13.655(7)

α(deg)

90

109.521(4)

90.845(5)

β(deg)

96.686(11)

101.992(4)

103.081(5)

γ(deg)

90

90.795(4)

100.918(6)

V (Å )

3259.2(19)

1869.2(8)

830.8(7)

temp (K)

296(2)

296(2)

296(2)

Z

4

2

1

1.402 0.809 1432 2.34 –25.00

1.370 0.713 800 1.32 –25.00

1.249 0.089 330 1.53–26.66

11816 /2855

13296 / 6502

6058 / 3125

3

−3

Dcalcd (g·cm ) -1

µ [mm ] F [000] θ [º] Reflections collected/unique Goodness-of-fit on F2 R1a, wR2b [I > 2σ(I)] Rint a R1 = Σ||F0| − |Fc||/Σ|F0|.

0.937 0.998 0.0535, 0.1662 0.0546, 0.1167 0.0545 0.0452 b 2 2 2 2 2 1/2 wR2 = [Σw(F0 − Fc ) /Σw(F0 ) ]

1.130 0.0657, 0.1844 0.0304

Fluorescence Titration Experiments. In a typical experimental setup, the as-synthesized MOF sample was carefully grounded and sieved to a smaller particle size (≈1µm). The finely grounded sample (5 mg) was placed in deionized water (5 mL) and treated by ultrasonication for one hour. To obtain a stable suspension, the resulting suspension was kept undisturbed for 8 hours before the fluorescence titration experiments. The fluorescence titration experiments were performed by the incremental addition of freshly prepared 1mM stock aqueous solutions of nitrate salts of various metal ions to an aqueous suspension of 2 (2 mL)


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Computational Section. Time-dependent density functional theory (TD-DFT)41-42 was employed to understand the photo induced electron transfer process. We used cluster models that consist of two HTABDC and two Bpy ligands truncated from the crystallographic structures of 2, which contains the repeated unit proposed by experiment. This holds for the cluster model for 1, which includes two HTABDC and two DMF ligands. In each cluster model, the four terminating oxygen centers in the edge were capped with hydrogen atoms, to prevent dangling bonds. All calculations were performed by using the TD-DFT(i.e., TD-B3LYP43-44) functional with 6311G(d,p)45 basis set (termed as TD-B3LYP/6-311G(d,p)) with Gaussian 0946 program. RESULTS AND DISCUSSION Solvothermal reaction of HTABDC with Zn(NO3)2 in N,N-dimethyl-formamide (DMF) give a one-dimensional (1D) structure of 1. In 1, the deprotonated HTABDC ligands adopt a bis(bidentate) mode to bridge mononuclear Zn(II) ions to form 1D zigzag chain (Figure 1a). Different from the reported 1D MOFs in ref 36, the coordination sphere of the Zn center in 1 is completed by two DMF molecules in the cis orientation, whereas the trans disposition is observed in 1D structure of TABD-MOFs (Co/Ni). With introduction of co-ligand Bpy, threedimensional structure of 2 was prepared. Interesting, compound 1 can be considered as the precursor of 2, as the co-ligand Bpy can compete with coordinated DMF molecules in the cis orientation and bond to metal centers, bridging 1D zigzag chains to form a 4-connected threedimensional (3D) structure (Figure 1b). It is worth noting that additional independent equivalent frameworks are present in 2 in order to stabilize the whole structure, affording a four-fold interpenetrating network.


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Figure 1. (a) Depiction of 1D zigzag chains of 1. (b) Depiction of the 3D structure of 2. (c) Schematic representation of four-fold interpenetrating network of 2. C, gray; O, red; N, blue; Zn, turquoise. H atoms were omitted for clarity. Photophysical study shows the remarkable enhancement of fluorescent emission at 474 nm for HTABDC in solid state with the absolute quantum yield (Φa,f) of 17%, compared with that of 4.5% in tetrahydrofuran (THF) solution, featuring the ligand typical AIE characteristic. AIE feature of the ligand was also confirmed by the significant increase in fluorescence intensity upon the enhancement of hexane fraction in its THF-hexane mixtures (Figure S6). Solid state fluorescence measurement shows that the maxima emission of 1 is at 457 nm, with 17 nm blue shift from the emission of HTABDC (Figure 2). The Φa,f for 1 in the solid state is as high as 50%, a three-fold higher than that of the ligand, indicating the AIE ligand can be immobilized in a rigid matrix through coordination bonds as well as closely packing observed in the structure. Consequently, the rotation of the phenyl rings in 1 will be restricted, with the non-radiative pathway being blocked, thus the fluorescence efficiency and quantum yield can be effectively enhanced.


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Surprisingly, despite of the close structural correlation with 1, compound 2 is almost nonemissive with Φa,f being dramatically decreased to 0.99%.

Figure 2. Solid-state emission spectra of HTABDC, 1 (λex=350nm) and 2 (λex=370nm). Here, we proposed three possible mechanisms for fluorescence quenching of 2, including: (1) the partial vibrational quenching by the guest molecules, (2) the rotation of the phenyl rings is not completely shut off despite the immobilization of HTABDC in 2, (3) the existence of nonradiative deactivation processes such as photo-induced electron transfer (PET) or energy transfer (ET). After heating to 200℃, the emission and PXRD pattern of 2 were essentially unaffected, which excludes the vibrational quenching by solvent molecules as the major quenching mechanism (Figure S4). In general, the close C−H…π interactions and short phenyl…phenyl contacts in MOFs help to restrict the rotation of the phenyl rings and thus block the non-radiative pathway. As revealed by the structural analysis, the shortest distances of the C−H … π interactions and H…H contacts between the nearest HTABDC neighbours are 3.16 Å and 2.41 Å for compound 2, which are comparable with that of highly fluorescent compound 1 and the ligand HTABDC (3.04 Å and 2.65 Å for 1, 2.84 Å and 2.67 Å for HTABDC) (Figure 3). The


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close C−H…π interactions and short phenyl…phenyl contacts in 2 can be contributed to the high degree of interpenetration of 2. Accordingly, the uncomplete shut-off of the rotation of the phenyl rings is also excluded as one major quenching mechanism. We surmise instead that the existence of electrostatic interaction between the AIE ligand HTABDC and the auxiliary ligand Bpy through the exciton migration could possibly cause the quenching of fluorescence. It is noteworthy that the Bpy ligand in 2 adopts a nearly coplanar conformation, and the dihedral angle of two pyridine rings is only 0.20°. The conjugated Bpy with enlarged π-delocalization may lower its HOMO energy level, which is verified by the quantum chemical calculations. As compared in Figure 4 and Figure 5, The energy HOMO of 2 is predicted to be -5.325 eV, which is slightly lower than that of 1 (-5.227 eV). On the other hand, the larger π-delocalization in Bpy ligand can make it as the energetic lower LUMO, rather than the higher LUMO+1, which is totally different from 1 where the LUMO+1 is localized on the DMF ligands. The conjugated Bpy with lager π-delocalization can thus facilitate the electron transfer from the excited state of HTABDC to Bpy, then to the ground state via the non-radiative deactivation pathway. Interestingly, we recently reported a two-dimensional (2D) MOF material with using AIE active TPE derivative as linker, as well as Bpy as auxiliary ligand,47 in which the Bpy ligand adopted a nonplanar conformation with the dihedral angles of two pyridine rings being 62.97° and 6.43°, respectively. In contrast with the non-fluorescent compound 2 in this study, that 2D MOF material exhibits remarkable fluorescent properties with the absolute quantum yield as high as 43%. Hence the PET mechanism could be the major mechanism for the fluorescence quenching observed in 2.


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Figure 3. Views of the closest C−H···π interactions (pink) and the shortest H···H distances (blue) in the crystal structures of HTABDC (a), 1 (b) and 2 (c). To better understand the proposed mechanism, TD-DFT calculations at the TD-B3LYP/6311G(d,p)) level were employed to understand the photo induced electron transfer process. Figure 4 shows the most important molecular orbitals (MOs) and for the cluster model of 2, as well as the diagram of photo induced electron transfer from the ligands HTABDC to Bpy inside the molecule 2. The degenerated HOMO (-5.325 eV) and LUMO+1 (-1.529 eV) correlate with the two HTABDC ligands with similar symmetry-allowed MO shapes (see Figure 4), which allows the electron transfer from HOMO to LUMO+1 generating the excited state. However, the excited electrons at LUMO+1 are quite unstable, and thus can easily be released to the lowerenergy LUMO (i.e., -2.919 eV) which mainly localized at the two Bpy ligands. The facile electron transfer from LUMO+1 to LUMO, rather than LUMO to HOMO explained the origin of tuning off the fluorescence state, which is also verified by the very small oscillator strength of 0.001.


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Figure 4. The diagram of photo induced electron transfer from the ligands HTABDC to Bpy in compound 2. Nonetheless, the diagram of photo induced electron transfer from the ligands HTABDC to DMF in compound 1 is totally different, as compared in Figure 5. The small HOMO-LUMO gap (3.796 eV) and symmetry allowed orbitals suggest that the electrons can be readily transferred from HOMO to LUMO upon the excitation, and the electron transition from the excited state can then be easily released to the ground state via the radiative pathway emitting the fluorescence. Note that the orbital localized on DMF ligands becomes the higher LUMO+1, which cannot quench the fluorescence to the “off” state. It is significantly different from 2, in which the orbital localized on Bpy ligands becomes the lower-energy LUMO. On the basis of above considerations, the almost coplanar Bpy ligands with enlarger π-delocalization lower the orbital energy levels and even switch the orders of LUMO and LUMO+1 in 2, which plays a crucial role in tuning the fluorescence of 2 to the “off” state.


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Figure 5. The diagram of photo induced electron transfer from the ligands HTABDC to Bpy in compound 1. Although highly fluorescent MOFs are mostly desired, the fluorescence quenching observed in 2 can offer an opportunity to use it as a fluorescent probe for sensing applications in a “turn-on” fashion. The O-rich framework of 2 including uncoordinated -OH groups, as well as the good water stability (Figure S2) encourage us to employ it to detect strongly oxyphilic metal ions, particularly Al3+. As is well-known, excessive intake of aluminium has severe toxicity to human health, and can induce many serious diseases, including Alzheimer's disease, osteoporosis, rickets, anemia, softening of the bones, etc.48-49 Due to wide dispersion and the frequent use of aluminum in the environment around us, the risk of the aluminium contamination is threatening human health. Hence, the development of efficient Al3+ chemosensors capable of being used in real-life sensing applications is always desirable. To investigate the binding properties of 2 with Al3+, and different kinds of cations in aqueous media, the titration experiments were performed with the incremental addition of freshly prepared 1mM stock aqueous solutions containing nitrate salts of Na+, K+, Ca2+, Mg2+, Zn2+, Cu2+, Ag+, Cd2+, Ni2+, Co2+, Cr3+, Hg2+, Al3+, Pb2+ into


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the suspensions of 2 in water. As shown in Figure 6a, very efficient fluorescence enhancement accompanying a prominent red shift of the fluorescence maximum of about 26 nm were observed upon the gradual addition of Al3+. But for all of the other metal ions (Figure S7-S19), only minor changes in fluorescence intensity were detected, even upon the addition of 10-fold of other metal ions, which clearly demonstrates the high selectivity of 2 towards Al3+ over other selected metal ions (Figure 6b).12, 50-53 Moreover, the “turn-on” fluorescence of 2 induced by Al3+ can be easily distinguished by the naked eye under the irradiation of a handheld UV lamp (λmax= 365 nm). Especially, 2 exhibits the superior sensitivity for Al3+ with the detection limit as low as 3.73 ppb, which is significantly lower than the higher limit of U.S. Environmental Protection Agency (EPA) recommendation of Al3+ ion for drinking water (200 ppb).54 The fluorescence “turn-on” mechanism can be proposed to the complexation of HTABDC with Al3+ through the chelation of oxygen atoms from carboxylate and uncoordinated hydroxyl groups to Al3+, which results in the dissociation of 2 with eliminating the PET process from HTABDC to Bpy, hence the AIE process is reactivated. The mechanism can be evidenced by the red shift of fluorescence maxima from 460 to 486 nm, which could be ascribed to the emission of new HTABDC -Al3+ complex in origin. The titration experiments for the ligand HTABDC and compound 1 with addition of Al3+ and other metal ions were also performed. But both substrates did not exhibit any remarkable and exclusive response in emission toward Al3+ or any other selected metal ions (Figure S21 and S22).


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Figure 6. (a) Emission spectra of 2 and 2+Al3+ in water (λex=370nm). The inset represents photographs of suspensions of 2 (left), 2+Al3+ (right) taken under illumination of a UV lamp. (b) Fluorescence intensity change profiles of 2 in the presence of selected metal ions (10-fold excess, blue); 2+Al3+ upon the addition of competing metal ions (5-fold excess, brick-red) in water. The excitation wavelength was 370 nm. Encouraged by these results, competitive experiments were conducted to exclude the possibility of interference from other metal ions. The competition measurements were firstly carried out by the subsequent addition of other metal ions (5-fold excess), including Na+, K+, Ca2+, Mg2+, Zn2+, Cu2+, Ag+, Cd2+, Ni2+, Co2+, Cr3+, Hg2+ and Pb2+, to the solution of 2-Al3+ complex. The results show that the introduced other metal ions could hardly interfere 2 in detection of Al3+ (Figure 6b). Furthermore, upon the addition of Al3+ to the suspension of 2 in water in the presence of other metal ions (5-fold excess), there is no interference for the binding of Al3+ with 2 and would not disrupt the detection of 2 for Al3+. These results clearly demonstrate that 2 displays an excellent selectivity for Al3+.


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Figure 7. (a) 1H NMR spectra of HTABDC, BPY, 2 and 2 in the presence of Al3+. (b) ESI-MS spectra of 2 upon addition of Al3+. To provide more evidences in support of the proposed mechanism, 1H NMR and electrospray ionization mass spectrometry (ESI-MS) studies were performed. In the absence of Al3+, no proton signal was monitored in 1H NMR spectrum of 2 in DMSO-d6 due to the poor solubility of MOF material. Upon the addition of Al3+, the cloudy DMSO-d6 solution turned to clear with the appearance of proton signals of HTABDC and Bpy, indicating the dissociation of 2, with the formation of soluable HTABDC-Al3+ complex and release of Bpy. Besides, the signal at δH= 11.41 ppm belonging to -OH group disappeared, strongly suggesting the occurrence of chelation between the -OH group and Al3+, and the deprotonation of the phenolic hydroxyl group during


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the complexation (Figure 7a). Here, the -OH group is crucial for Al3+ sensing as it can selectively chelate to Al3+ over other metal ions, resulting in the dissociation of 2 and recovery of the AIE fluorescence. The observed shift and broadening of NMR signals of the HTABDC ligand also indicate the occurrence of complexation. Furthermore, ESI-HRMS spectra also confirmed the decomposition of 2 and the complexation of HTABDC with Al3+ (Figure 7b). Upon the addition of Al3+ to 2, the mass spectrum exhibited one intense peak at m/z 501.1 and three weak peaks at m/z 388.0, 519.1, 564.1, corresponding to the complexes: [AlL]-, [Al2L(NO3)4]2-, [AlL(H2O)]-, [Al(HL)(NO3)]-, respectively, where L represents the deprotonated HTABDC ligand. Based on the results above, the fluorescence off-on mechanism can be ascribed to the competitive metal ion complexation (Figure 8): the addition of Al3+ ions induce the dissociation of 2 with removal of fluorescent quencher Bpy and reactivating the AIE process, with the bright fluorescence being achieved.


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Figure 8. (a) Schematic representation of the “off-on” mechanism. (b) Schematic representation of metal competitive coordination substitution strategy. Conclusions In summary, two MOFs with tunable fluorescence properties were prepared by using hydroxyl functionalized AIE ligand, of which 1 is highly emissive and 2 with auxiliary ligand Bpy as quenching agent is almost nonemissive. The fluorescence quenching mechanism based on photoinduced electron transfer from the excited state of linker HTABDC to Bpy is proposed to interpret the fluorescence “off” state of 2, which can also be verified by TD-DFT calculations. Further studies show that 2 could be used as a fluorescence “off-on” probe for detection of Al3+ in water based on metal competitive coordination substitution. In this process, the chelation to Al3+ by both hydroxyl and carboxylate groups can induce the dissociation of Bpy from 2 with fluorescence being tuned to “on” state as a result of the activated AIE process. The strategy described in this work may provide a facile route to design fluorescence tunable AIE MOFs with potential applications in sensing. ASSOCIATED CONTENT Supporting Information The

Supporting

Information

is

available

free

of

charge

via

the

Internet

at

http://pubs.acs.org.on. Synthetic routes to the ligand, PXRD, TGA, PL spectra, computational details, HRMS spectra and Crystal data etc. AUTHOR INFORMATION Corresponding Author *Email: [email protected] (Z.Y.G.).


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*Email: [email protected] (L.Z.H.). *Email: [email protected] (Z.L.L.). ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (21471079 and 21501092), the NSF of Jiangsu Province for Youth (BK20140928), the NSF of Jiangsu Province for colleges and universities (13KJB150017), and Start-Up Fund of Nanjing Tech University. REFERENCES (1) Tranchemontagne, D. J.; Mendoza-Cortes, J. L.; O'Keeffe, M.; Yaghi, O. M. Secondary Building Units, Nets and Bonding in the Chemistry of Metal-Organic Frameworks. Chem. Soc. Rev. 2009, 38, 1257−1283. (2) Furukawa, H.; Cordova, K. E.; O'Keeffe, M.; Yaghi, O. M. The Chemistry and Applications of Metal-Organic Frameworks. Science 2013, 341, 1230444. (3) Zhou, H. C.; Kitagawa, S. Metal-Organic Frameworks (MOFs). Chem. Soc. Rev. 2014, 43, 5415−5418. (4) Cui, Y.; Li, B.; He, H.; Zhou, W.; Chen, B.; Qian, G. Metal-Organic Frameworks as Platforms for Functional Materials. Acc. Chem. Res. 2016, 49, 483−493. (5) Allendorf, M. D.; Bauer, C. A.; Bhakta, R. K.; Houk, R. J. Luminescent Metal-Organic Frameworks. Chem. Soc. Rev. 2009, 38, 1330−1352.


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Table of Contents Only

Fluorescence of MOFs with using AIE ligand as linker can be deliberately tailored, from which the prepared AIE MOF material can perform as a fluorescence “off-on” probe for highly sensitive detection of Al3+ in aqueous media.


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Tailoring the Fluorescence of AIE Active Metal- Organic Frameworks

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