Simple One-Pot Preparation of a Rapid Response AIE Fluorescent

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Functional Nanostructured Materials (including low-D carbon)

Simple One-Pot Preparation of a Rapid Response AIE Fluorescent Metal-Organic Framework Farzaneh Rouhani, Ali Morsali, and Pascal Retailleau ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b12404 • Publication Date (Web): 27 Sep 2018 Downloaded from http://pubs.acs.org on September 27, 2018

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ACS Applied Materials & Interfaces

Simple One-Pot Preparation of a Rapid Response AIE Fluorescent Metal-Organic Framework Farzaneh Rouhani1, Ali Morsali1,*, Pascal Retailleau2 1

Department of Chemistry, Faculty of Sciences, Tarbiat Modares University, P.O. Box 14115175, Tehran, Iran

2

CNRS UPR 2301, Institut de Chimie des Substances Naturelles, Univ. Paris-Sud, Université ParisSaclay,1, av. de la Terrasse, 91198 Gif-sur-Yvette, France *

To whom correspondence should be addressed, E-mail: [email protected] Tel: +98 21 82884416, Fax: +98 21 82884416

Abstract Luminogenic materials particularly which have turn-on response by sensing the analytes are highly regarded as optical instruments, sensing material, fluorescent probes, et cetera. However, most of these materials are only usable in dilute form and often show the self-quenching effect at high concentrations. The use of light emitting AIE-based materials (aggregation-induced emission) is the solution of this problem. The rigid structure of these active fluorescence ligands, which contains several aromatic rings attached to each other, does not lose its fluorescence properties by increasing the concentration. Unlike other AIE ligands, which have a complex or multistep synthetic route, here, we present a simple one-pot method for in-situ synthesis of the AIE ligand and the metal-organic framework contained therein. Presence of metal nodes having varied outer-shell electron configurations affects the fluorescence intensity of these materials and thus, both high and low emissive “turn on” MOFs were readily acquired. Based on possible interactions between the free nitrogens on the ligand and the phenolic compounds, the MOFs enable highly selective and sensitive detection of phenol derivatives in several seconds with low detection limits (less than 65 nM for 4-aminophenol and 120 nM for phenol) through turn-on emission fluorescence. Keywords: AIE characteristic, Fluorescent probe, In-situ synthesis, Metal-Organic Framework, Sensing

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Introduction Since the beginning of life, light has always been very important to humans. In addition to being useful to humans, the light has sparked the curiosity of humanity and from the early stages of civilization has effort to understand and discovery of light production has continued. Luminescence materials are a special category of materials that, after being stimulated by external stimuli, such as x-ray beam, UV light or even mechanical phenomena, emit energy in a non-thermal way.1-2 Classical luminescence studies have been generally performed in solution.3-7 Although solution studies at the molecular level have been very useful, in order to basic understanding of the luminescence phenomenon, the results obtained in dilute solutions are often not extended to the concentrated solutions; since light emission is often quenched or weakened by increasing the concentration, which caused by the mechanistic formation of the aggregates and known as “concentration quenching”8-9. This phenomenon is due to ‘‘aggregation-caused quenching’’ (ACQ). The ACQ effect is often harmful to real applications.10-14 In 2001, for the first time, an unconventional luminogen was discovered which worked as an ordinary system in high concentrations.15-16 Fluorescence materials based on AIE characteristics, which have poor emission in dilute solution but upon aggregation show bright fluorescence, are good candidates to design of fluorescence “turn-on” probes.17-20 One good example is a luminescence functionalized MOFs based on Ru, that could be responsively taken apart ions and release a large amount of luminogenic guests in the presence of mercury ions.21 It has been proven that the restriction of intramolecular rotations (RIR) in these valuable materials hinder the nonlinear pathways and opens the radiating channels.22 Many research groups have used RIR methods to develop new AIE systems. Although various methods, such as the formation of hydrogen bonding23-24, the attachment of an AIE luminogen to other materials25, 2 ACS Paragon Plus Environment

synthesis of AIE

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macromolecules26 and so on are aimed at preventing the molecular rotation, Dinca et al. showed that the tight packaging of tetraphenylethylene (TPE, an common AIE ligand) chromosomes is not necessary to illuminate fluorescence, and the forming of a rigid matrix by binding the AIEtype chromophores to metal ions is more than enough as an superseded mechanism to limit the rotation of phenyl rings.27 Several studies recently reported on the manufacture of metal-organic frameworks with AIE characteristics, often based on tetraphenylethene derivatives (TPE), lead to emissive materials; because the rotation and twisting of phenyl rings and ligand are reduced.24, 2831

Metal-Organic frameworks, which inherently or by encapsulation find the AIE property, have

extensive applications as sensor and in the catalytic reactions.32-37 In addition, it has been shown that the type of metal nodes can also affect the sensing ability of these materials.38-39 However, many of the AIEgens do not have the ability to form MOFs due to their big size or lack of suitable sites for connecting to the metal nodes, or there are problems with their synthesis.40-41 Herein, we proposed a novel method to design turn-on fluorescence detector based on MOF employing in-situ ligand fabrication with AIE characteristic and metals with different outer-shell electron configuration for use as a rapid response fluorescent probe of phenols derivatives. In addition, compared to the other similar works using d10 metal ions (such as Zn(II)) to prevent metal interference in ligand charge transfer, and given that the main purpose of forming MOF is increasing the rigidity, we utilized various metal ions with outer-shell electron configurations, with the aim of investigating the effect of metal nodes. It should be noted that despite of many reported examples of AIE-based materials, the framework is prepared in a very simple and onestep process. The coordination of 5,6-di(pyridin-4-yl)-1,2,3,4-tetrahydropyrazine (an AIE fluorophore developed by us)42 and H2BDC carboxylic acid ligand with Zn(II), Co(II) or Cd(II) leads to the preparation of strongly luminescent MOFs (TMU-40(Zn), TMU-40(Cd) and



TMU(Co)) which have fluorescent quantum yields (ΦF) 38.2%, 31.17% and 11.69% respectively. Significantly, the cobalt-based TMU-40 can rapidly and effectively sense the 4aminophenol (Ksv=2.9 × 107). Experimental section 1. Synthesizes of N1, N2-bis (pyridin-4-ylmethylene) ethane-1,2-diamine (L) L was prepared according to the literature method.43 1 eq of 1, 2-Ethylenediamine and 2 eq of 4-pyridinecarboxaldehyde were combined in EtOH solvent (15 mL) and refluxed for 3 hours; the yellow powder obtained after solvent evaporation was washed three times with n-hexane and exposed in air.42 Mp: 70 ˚C. IR (KBr, cm-1): 3037.3, 2843.5 1652.7, 1591.1, 1408.9, 1318, 1224.2, 1011.4, 813.5, 635.9, 530.1. 1H NMR (500 MHz, DMSO): d 8.6(d, 4 H, ArH), 8.3(s, 2 H, HC=N), 7.61(d, 4 H, Ar-H) and 3.9(s, 4 H, HC-C). Elemental analysis (%) calculated data: C 67.2, H 6.5, N 26.1. Found data: C 67.4, H 6.5, N 26.1. The Mass and 1H NMR spectrum of L ligand are provided in supporting information (S.I) as Figure S1 and S2. 2. Synthesizes of [M(BDC)(L*)]. DMF (TMU-40), M= Zn, Cd, Co A mixture of 1 mmol H2BDC (terephthalic acid) (0.0166 g), 1 mmol of Zn(NO3)2·4H2O (0.297 g) and 1 mmol of L (0.256 g) in DMF solvent (15 mL) heated for 72 hours at 120 °C in a Teflonautoclave and the dark red crystal of TMU-40 was obtained. The crystals were isolated and washed with cold ethanol to elimination the remaining material and dried in expose to air.42 Yield: ∼19%. IR (KBr, cm-1) for TMU-40-Zn: 3281.2, 3059.2, 2920.9, 2854.8, 1675.1, 1601.1, 1498.2, 1384.9, 1213.2, 1141.7, 1089.2, 834.5, 746.3, 624.8, 559.2, 484.8. Elemental analysis (%) calculated data: C 55.5, H 4.6, N 12.9. Found data: C 55.4, H 4.7, N 13.1. Prior to adsorbing and sensing studies, the crystal placed under vacuum for 24 hours at 120 °C. The synthesis of 4 ACS Paragon Plus Environment

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TMU-40 (Cd, Co) was performed in a completely similar manner with TMU-40-Zn, except that instead of Zn(NO3)2·4H2O, the compounds of Cd(NO3)2.4H2O and Co(NO3)2.4H2O were used, respectively. Elemental analysis (%) calculated of TMU-40(Cd): C 51.6, H 4.3, N 11.9. Found data: C 51.4, H 4.3, N 12.1. Elemental analysis (%) calculated of TMU-40(Co): C 56.2, H 4.6, N 13.1. Found data: C 55.9, H 4.6, N 13.4. IR spectrum data of the three compounds are given in the Figure S3. In all cases, the presence of a sharp peak in the region of about 3280 cm-1 indicates the existence of -NH in the MOF. Before sensing process, the structures placed under vacuum at 120 °C for at least 24 h. The crystal data of TMU-40(Zn, Co, Cd) and structure refinements are presented in Table S1. 3. Synthesis of L* ligand 1 mmol of L ligand in 15 ml of DMF solvent was heated for 5 min at 120 °C while stirring to create a clear yellow solution. After complete dissolution of the ligand, 0.1 g NaCN was added to the mixture and reacted for at least 15 min. The appearance of a dark red color indicates a completion of the reaction. Mp: 70.2 ˚C. IR (KBr, cm-1): 3301(s), 3032(m), 2843(s), 1667(m), 1595(s), 1406(s), 1318.5(w), 1218(w), 1095(w),992 (w), 823(m), 586(m). 1H NMR (500 MHz, DMSO): d 10.3 (d, 2 H, -NH), 8.3 (d, 4 H, ArH), 7.61 (d, 4 H, Ar-H) and 3.9 (s, 4 H, HC-C). Elemental analysis (%) calculated: C 67.27, H 6.6, N 26.15. Found: C 67.2, H 6.6, N 26.2.The Mass and 1H NMR spectrum of L* ligand are given in supporting information (S.I) as Figure S2 and S4. Results and discussion Synthesis. The syntheses of the three TMU-40(Zn, Cd and Co) were obtained by heating a DMF mixture of solution mixture of N1, N2-bis (pyridin-4-ylmethylene) ethane-1,2-diamine (L) with



Zn (NO3)2.4H2O (or Cd(NO3)2.4H2O or Co(NO3)2.4H2O) and H2BDC for 72 hours at 120 °C, that gives a framework with [M(BDC)(L*). DMF (M=Zn, Cd, Co)] structure. The L* =5,6dipyridin-4-yl-1,2,3,4-tetrahydropyrazine forms in-situ by the L ligand via a C=C aldimine coupling reaction and the cyclization. It has been reported that CN- could act the catalysis role for such aldimine coupling reactions.44 In the present work, the mechanism of electrophilic attack and proton-transfer process for C=C coupling is proposed, which has been fully explained in our previous paper (Scheme 1).42 As indicated in the synthesis method, ligand L was used to prepare TMU-40 containing L* ligand. Because the presence of free nitrogens on the L* ligand which have a high propensity for coordination to metals and the probability of choosing this position for coordinating by metal, use of L* leads to a product other than what we expect, and the sites that are considered for future applications are occupied by metal. For this reason, in-situ synthesis method is preferable to this framework.

Scheme 1. Offered mechanism for C=C coupling reaction and cyclization of L to L*.


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Characterization of TMU-40. In the presence of the ligands and at the same time and temperature, we succeeded in synthesizing three different structures with only a change in the type of metal nodes. Cobalt and zinc belong to one period, and on the other hand, zinc and cadmium belong to one group in the periodic table of elements. According to the ionic radii of Co(II), Zn(II) and Cd(II), which are 70, 74 and 95 pm, respectively, the crystallography results (Figure S5) show that although the general manner of the components bonding is very close, the ion radius is influential in the space group chosen by the frameworks, and Cd(II), which has a larger ion radius than the other two metals, selects a different space group. Studies of singlecrystal X-ray (Table S1) show that the two TMU-40 structures with zinc and cobalt metal nodes are crystallized in the P21/c space group, but TMU-40(Cd) selects the Pbca space group, which is more symmetrical than the P21/c. Overall, each primary building unit (PBU) has one M2+ (M = Zn, Co, Cd) in a distorted octahedral coordination environment. The metal ion is coordinates to six atoms, which include four oxygen atoms belong to the carboxylate groups of two different H2BDC and two N atoms belonging to the pyridine of the two L* ligands. ORTEP plots of asymmetric unit and coordination environment of metal are presented in in Figure S5 for all three structures. A two-dimensional plate is constructed along the a×b axes by four adjacent metal ions (M2+), which are connected by two H2BDC and two L* ligands. In the case of TMU40 (Zn, Co), these plates are connected to each other alternately in the direction of the c axis via hydrogen bonds between tetrahydropyrazine groups or N atom of L* in one chain and the oxygen atoms of an vicinal H2BDC ligand to give a supramolecular structure. It should be noted that these two structures are capable of carrying out two kinds of hydrogen bonding, the prior being formed along a chain, and the second type leads to increase in the structural dimensions. In the case of TMU-40(Cd), the structure establishes one type hydrogen bonding but with two different



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lengths, which forms between the hydrogen on the tetrahydropyrazine moiety of L* in one chain and the oxygen atoms of an vicinal H2BDC, and causes the connection of 1D chains created along the b axis and expands the dimensions of the structure. The formation of such hydrogen bonding interactions, which is a directional bond, can effectively prevent the rotation of L* ligand, limit the loss of nonemissive energy and so the nonradiative pathway blocked.8, 27-28 The average length of the M-O and M-N bonds and the length of hydrogen bonds in all three structures are shown in Table 1, which have increased according to the increase of the metal ion radius. Also, computed void spaces per unit cell for frameworks after removing the solvent are 19.6% (485.9 Å3) for TMU-40(Zn), 20.8% (531.9 Å3) for TMU-40(Co) and 20.8% (1071.6 Å3) for TMU-40(Cd) (Figure 1 and Figure S6). Powder X-ray diffraction patterns (PXRD) for all three structures are well suited to those that are simulated from SXRD data (Figure S7), which confirms the purity of the TMU-40. In order to study the porosity of the frameworks, the adsorption-desorption measurements of N2 gas at 77 K and 1 bar pressure were performed on the structures. The obtained results show that samples containing zinc, cobalt and cadmium have a surface area (BET) of 200.5, 257.8 and 218.3 m2/g, respectively (Figure S8). Table 1. The average length of the M-O and M-N bonds and the length of hydrogen bonds of TMU-40 (Co, Zn, Cd). Metal node Co

Ionic radius (pm) 70

Space group P21/c

M-O (Ȧ) 2.174

M-N (Ȧ) 2.091

H bonding1 (Ȧ) 2.955

H bonding2 (Ȧ) 2.912

Zn

74

P21/c

2.232

2.073

2.997

2.888

Cd

95

Pbca

2.360

2.302

2.945 , 3.031

̶


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Figure 1. Schematic views of the structure, pore size and hydrogen (Up) and pore distribution of TMU-40 (Zn, Cd, Co are A, B and C, respectively) (Down).

Stability of TMU-40. The studies of thermogravimetric analysis (TGA) data shows that the weight loss value of TMU-40 with Cd (II) metal node is about 13% at 200 ˚C which relates to the removal of the solvent in the pore, and based the TGA curve the structure is stable at temperatures up to 360 ˚C. Similarly, the two other structures have the same heat treatment and



solvent removal slowly and continuously occurred between 25-250 ˚C and the structures are stable at temperatures up to 340 ˚C (Figure S9). For more precise examination of the thermal stability, all three structures were placed at 350 °C for 12 hours. The intact XRD pattern of these structures is compared with the simulation patterns in Figure S6. In addition, the absorbance of N2 from heated samples was studied and less than 5% change in the BET surface area of TMU40(Zn) confirms the porosity and thermal stability of these frameworks to 350 °C (Figure S10). The samples were immersed in water for 24 hours to evaluate the stability of the frameworks in the water. The intact XRD pattern and the unchanged BET value of the frameworks, confirm their stability in water. Further evaluation of TMU-40 stability was also carried by measuring the amount of metal ions in the soaking solutions through ICP-AES. The soaking solutions were prepared by immersing TMU-40 in water for 12 hours. According to the results, the amount of Zn2+, Cd2+ and Co2+ ions in the soaking solutions were negligible, which indicate that TMU-40 was destroyed rarely. Luminescence Properties. The luminescence properties of each compound and the used ligands were carefully studied at ambient temperature (Figure S11 and Figure S12). As expected, L ligand is non-fluorescent, but L* is a fluorescent ligand having two conjugated non-planar aromatic rings due to an aldimine C=C coupling and cyclization, which are connected through a C=C bond. On the other hand, the formation of middle ring creates some degree of steric hindrance for the circulation of phenyl rings around the double bond. The luminescence of TMU-40 is ascribed to be the corresponding ligand luminescence, and the red shift of 38 nm for L* and 59 nm for H2BDC compared to the free ligands should be because of the ligand-to-metal charge transfer (LMCT) (Figure S12).20 The quantitative increment of L* emission by concentration was calculated by the fluorescence quantum yields (FFL), using 9, 1010 ACS Paragon Plus Environment

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diphenylanthracene (DPA) as the standard, in CH3CN and mixed solvents of CH3CN and water; because L* is highly soluble in CH3CN but sparingly soluble in water. It is significant that the ΦF of L* was significantly enhanced from 1.85% to 20.3% by increasing the ratio of water. As seen from Figure 2, the ΦF of the L* in pure CH3CN was very low (ΦF≈ 1.85), and was approximately unchanged when water was added up to 30% (v/v), but increase quickly by adding the water fraction to 90% (v/v). On the other hand, the solid state quantum yield of L* was measured, higher absolute solid state quantum yields than those in the solution state (ΦF ≈ 23.6) was acquired. In comparison, the quantum yield of TMU-40(Zn) is about 2 times higher than that of L* in solid state. The blue shift and ΦF enhancement of TMU-40 should be attributed to the immobilization of L* linker in rigid structures, which was also found in the other reported AIE based MOFs, like PCN-94.8, 22, 27-28, 45

Figure 2. Fluorescence quantum yield versus water fraction of CH3CN/water mixtures for L* ligand.



The luminescence plot of cubic shape crystals of the MOFs in the initial values are shown in Figure S11. Also, the cubic shape crystals of the TMU-40 are demonstrated in Figure S13. The strong luminescence of TMU-40 is observed at 405 and 435 nm, which is compatible with ligand molecules. TMU-40(Cd) has lower fluorescence intensity than TMU-40(Zn), which can be offered two reasons for it. First, although zinc and cadmium belong to one group of periodic table, the radius of Zn(II) and Cd(II) ions are 25 pm apart, and this difference in radius can cause changes in the fluorescence behavior of these two structures. On the other hand, due to the fact that TMU-40(Cd) chooses the Pbca space group that is more symmetrical than P21/c space group of TMU-40(Zn), the hydrogen bonds of the structure undergoing a change. This structure only provides one type of hydrogen bonding that occurs between the tetrahydropyrazine nitrogens and adjacent H2BDC oxygen’s. Consequently, in comparison with the zinc containing MOF, there is less hindrance effect on the rotation of L*, which results in the intensity of lower fluorescence of the structure. As expected, the quantum yield decreases dramatically after Zn(II) is replaced with Co(II) to synthesize the third framework. A clear hypochromic shift is observed in the emission spectrum of this framework (Figure S12), which indicates that LMCT transitions dominate its optical properties, and thus result in fluorescence quenching. Due to the structural characteristics of TMU-40 and the presence of amine nitrogens capable of hydrogen bond formation, the study of detection and sensitivity is concentrated on four different phenolic-based substances. Therefore, it can be used to sense the phenolic compounds. The sensing behavior of phenol, 4Aminophenol, 4-Chlorophenol and 4-methylphenol by TMU-40 (Zn, Cd, Co) were studied. As shown in plots of Figure 3, Figure S14 and Figure S15, the presence of all four analytes, in contrast to what is seen for many fluorogen, greatly increased the fluorescence emission of the MOFs.


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Figure 3. Fluorescence emission spectra of TMU-40(Co) immersed in acetonitrile at various concentration of concentrations of (A) 4-aminophenol, (B) 4-methylphenol, (C) phenol and (D) 4-chlorophenol, excited at 330 nm. The Compounds Stern−Volmer (SV) enhancing constants (Ksv) were calculated by monitoring the fluorescence intensity response of the frameworks at various concentrations of the analytes. The Stern−Volmer plots of TMU-40 show the linear dependence on enhancer concentrations. For all structures, the enhancing efficiency follows the 4-aminophenol> 4-methylphenol> phenol> 4-chlorophenol order (Table 2). The Stern−Volmer plots are presented in Figure S16, Figure S17 and Figure S18. In addition, the detection limit of phenol derivatives by TMU40(Co) was investigated for better comparison with other fluorescence probes. The detection limit was determined from the fluorescence titration data, and it should be noted that, the



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fluorescence spectrum of probe TMU-40(Co) was measured five times. The linear equation was found to be y=12.814x+199.57 (R2=0.9987), where y is the fluorescent intensity at 405 nm measured at a given phenol derivatives concentration and x represents the concentration of phenol derivatives added. So the detection limit for 4-Aminophenol was calculated to be less than 65 nM. The linear region and the TMU-40(Co) detection limit for other phenolic derivatives are also given as Figure S19 and Table S2. Table 2. Ksv Values of TMU-40 toward phenolic derivatives. Ksv MOF TMU-40(Zn)

5.6 × 105

5.3 × 105

4.9 × 105

3.5 × 105

TMU-40(Cd)

8.7 × 105

7.2 × 105

5 × 105

3.6 × 105

TMU-40(Co)

2.9 × 107

2.6 × 107

1.8 × 107

1.6 × 107

Sensing Mechanism: The interaction of the TMU-40 with phenol derivatives occurred by the ππ interaction of the rich electron ring of the phenol derivatives with the poor electron ring of H2BDC present in the MOF and simultaneously interaction of the phenolic hydrogen with the free electrons of nitrogen from L*. The proper orientation of the nitrogen-electron pair of ligands L* provides the conditions for the hydrogen bonding between the N atom of the ligand and the phenolic hydrogen of the guest species. In this condition, the hydrogen bonding between the L* with the O atom of the carbonyl group of the BDC ligand, is also preserved. In sum, these conditions (increase of supramolecular interactions due to the increase in the entry of phenols into the MOF network) increase the rigidity of the framework and thereby increase the fluorescence intensity. Proposed mechanism of the interactions presented in supporting information section (Figure S20). 14 ACS Paragon Plus Environment

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On the other hand, the main detection mechanism of sensing analytes by luminescent MOFs is often attributed to the photo-induced electron transfer (PET) process.46-48 After excitation, electrons from the electron-donor are firstly transferred from the highest occupied molecular orbital (HOMO) to its lowest unoccupied molecular orbital (LUMO). Next, the excited electrons transfer to the LUMO of electron-acceptor and result in luminescence changing. The main driving force of the PET process is the energy gap of LUMOs between the electron-donor and electron-acceptor materials. About electron-withdrawing aromatic analytes, the energy of LUMO which is a low-lying π*-type orbital, is placed under the conduction band (CB) of the MOF. Although MOFs have developed networks, they are especially those containing d10 metals often determined by narrow energy bands because of concentrated electromagnetic states. In such examples, they may be taken as giant “molecules”, and energy levels of their valence band (VB) and CB can be defined in a way similar to that for molecular orbitals (MOs).49 On the other hand, in the case of electron-rich analytes, the excited electrons in LUMO orbitals, which are a highlying π* anti-bonding state that are located over the conduction band of the MOF, are transmitted to the CB of MOF, so leading to fluorescence enhancement (Figure 4).



Figure 4. Schematic plot of the electronic structure of the fluorescence quenching (Left) and enhancing (Right) process respectively by electron deficient and electron- donating analytes.

It should be noted that the electron transfers between the analyte and the excited state can be studied through their oxidation potential. Cyclic voltammetry (CV) measurements demonstrate that electron-rich moieties have oxidation potentials less positive than that of TMU-40 (Table S3). Therefore, MOF acts as an electron donator for electron-withdrawing materials and for electron-rich electrons as electron acceptor. After the exciting, the excited electrons enter from the conduction band of metal-organic framework to LUMO of analyte, so this non-radiant path leads to quenching. The solid state UV spectrums of all three MOFs are presented in Figure S21. On the other hand, due to the AIE characteristic mentioned in the study, the enhancement of the fluorescence intensity in the presence of the analyte in addition to the electron donatingaccepting effect between the MOF and the analyte can be related to another factor. Absolutely, adsorbed analyte can hinder rotational and vibrational movements of the ligand, which results in limiting non-radiant pathways and ultimately increasing fluorescence. The picture of MOF paste electrode is presented as Figure S22. It should be noted that, although a few numbers of phenolic derivatives have been studied in this research, other phenolic derivatives can also be identified by the framework. The TMU-40 fluorescence behavior relative to the other phenol derivatives can be different in regarding to the electron donation or electron accepting of the framework, as well as their oxidation potential. Among the MOFs, the structure of the TMU-40(Co) was more sensitive to the studied phenolic analytes than the other two structures. For further investigation of the reasons of this event, the adsorption capability of all three structures was measured relative to the studied analytes. The results represent in Figure 5. As shown, probably, the greater adsorption capacity of TMU16 ACS Paragon Plus Environment

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40(Co) results in a further increase in the concentration of analyte in the vicinity of the active site of MOF and ultimately, has a more significant effect on the fluorescence of the framework.

Figure 5. UV-Vis spectra of adsorption of 4-aminophenol by (A) TMU-40(Zn), (B) TMU40(Cd) and (C) TMU-40(Co) in different times. In order to investigate the selectivity of TMU-40 for the heterocyclic compounds, fluorescence spectroscopy measurements were done. No obvious changes were obtained for common organic moieties, like toluene (PhMe), aniline (AN), dichloromethane (DCM), triethylamine (Et3N) , hexane (Hex), THF, acetone show that amine, carbonyl, cyanide, aromatic rings and aliphatic chains do not show any interference with the sensing results (Figure 6A). The frameworks were washed with methanol and reused for some cycles by centrifuging the dispersed solution after 4aminophenol sensing. (Figure 6B). The XRD pattern after three sensing process and SEM images of TMU-40 are showed in Figure S23 and Figure S24, respectively.



Figure 6. (A) The relative fluorescence response of TMU-40(Co) to 10 ppm of different organics in acetonitrile. The changes in fluorescence intensity relative to hexane, dichloromethane, acetone and THF are negligible. (B) Regeneration and reusing of TMU-40 for 4-aminophenol sensing.


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Conclusion Three new MOFs with an AIE characteristic were prepared by in-situ synthesis with tunable fluorescence properties as a rapid response fluorescent probe of phenols derivate. Unlike most compounds with an AIE characteristic that have complex and multi-step syntheses, the MOF is synthesized simply in one reaction step. Using as metal nodes with various outer-shell electron configurations led to construction of high and low emissive MOFs by a simply method. The formation of rigid framework improved the main fluorescence of organic. These fluorogenic frameworks containing free amine nitrogens were used to sense the phenolic compounds with hydrogen bond forming potential. Among these three structures, despite the fact that the MOFs with Co as metal node, have a weaker initial fluorescence, it shows a much higher sensitivity to analytes, which can be attributed to the better adsorption capacity of the analyte by the framework. Further adsorption results in an increase in the concentration of analyte in the vicinity of the framework and ultimately a higher sensitivity. Simple and one-pot synthesis method and high sensitivity of these frameworks to phenolic compounds are the characteristics of the compounds and this method can be a pathway for further research for simple and one-step synthesis of AIE materials. ASSOCIATED CONTENT Supporting Information Supplementary Figures and Table including characterizations, sensing, fluorescence, studies and proposed mechanism are presented. AUTHOR INFORMATION Corresponding Author 19 ACS Paragon Plus Environment


*

E-mail: [email protected]

Notes The authors declare no competing financial interest. Acknowledgements Support of this investigation by Tarbiat Modares University and the Iran National Science Foundation (INSF) are gratefully acknowledged. We also greatly appreciate Dr. Hamed Moghanni for his precious comments in the electrochemical tests. References

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Table of Contents (TOC)


Simple One-Pot Preparation of a Rapid Response AIE Fluorescent

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