Simple One-Pot Preparation of a Rapid Response AIE Fluorescent

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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 36259−36266

Simple One-Pot Preparation of a Rapid Response AIE Fluorescent Metal−Organic Framework Farzaneh Rouhani,† Ali Morsali,*,† and Pascal Retailleau‡ †

Department of Chemistry, Faculty of Sciences, Tarbiat Modares University, P.O. Box 14115-175, Tehran, Iran CNRS UPR 2301, Institut de Chimie des Substances Naturelles, Univ. Paris-Sud, Université Paris-Saclay, 1, Avenue de la Terrasse, 91198 Gif-sur-Yvette, France

‡

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S Supporting Information *

ABSTRACT: Luminogenic materials, particularly those that have turn-on response by sensing the analytes, are highly regarded as optical instruments, sensing material, fluorescent probes, etc. 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 onepot method for in situ synthesis of the AIE ligand and the metal−organic framework (MOF) 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 the 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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design fluorescence “turn-on” probes.17−20 One good example of turn-On probes is a luminescent functionalized MOF based on Ru, that in the presence of mercury ions could be responsively take apart ions and release a large amount of luminogenic guests.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 bonding,23,24 the attachment of an AIE luminogen to other materials,25 synthesis of AIE macromolecules,26 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 formation of a rigid matrix by binding the AIE-type chromophores to metal ions is more than enough as an superseded mechanism to limit the rotation of phenyl rings.27 Several studies have been recently reported on the manufacture of metal−organic frameworks with AIE characteristics, often based on tetraphenylethene derivatives (TPE),

INTRODUCTION Since the beginning of life, light has always been very important to humans. In addition to being useful to humans, light has sparked the curiosity of humanity and efforts continue to be made since the early stages of civilization to understand and discover light production. 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 nonthermal 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 for basic understanding of the luminescence phenomenon, the results obtained in dilute solutions are often not extended to concentrated solutions because light emission is often quenched or weakened by increasing concentration due to the mechanistic formation of aggregates, 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, which worked as an ordinary system in high concentrations, was discovered.15,16 Fluorescence materials based on aggregation-induced emission (AIE) characteristics, which have poor emission in dilute solution but upon aggregation show bright fluorescence, are good candidates to © 2018 American Chemical Society

Received: July 22, 2018 Accepted: September 27, 2018 Published: September 27, 2018 36259

DOI: 10.1021/acsami.8b12404 ACS Appl. Mater. Interfaces 2018, 10, 36259−36266

Research Article

ACS Applied Materials & Interfaces

cm−1 indicates the existence of −NH in the MOF. Before the sensing process, the structures were placed under vacuum at 120 °C for at least 24 h. The crystal data of TMU-40(Zn, Co, Cd) and structural refinements are presented in Table S1. Synthesis of L* Ligand. One millimol 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 the 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 the Supporting Information (SI) as Figures S2 and S4.

leading to emissive materials because the rotation and twisting of phenyl rings and ligand are reduced.24,28−31 Metal−organic frameworks, which inherently or by encapsulation exhibit 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 a turn-on fluorescence detector based on MOF employing the 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 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)-1,2,3,4tetrahydropyrazine (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) of 38.2, 31.17, and 11.69%, respectively. Significantly, the cobalt-based TMU-40 can rapidly and effectively sense 4-aminophenol (Ksv = 2.9 × 107).

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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-yl-methylene)ethane-1,2-diamine (L) with Zn (NO3)2·4H2O (or Cd(NO3)2·4H2O or Co(NO3)2·4H2O) and H2BDC for 72 h at 120 °C, which gives a framework with [M(BDC)(L*). DMF (M = Zn, Cd, Co)] structure. The L* = 5,6-dipyridin-4-yl-1,2,3,4-tetrahydropyrazine forms in situ by the L ligand via a CC aldiminecoupling reaction and cyclization. It has been reported that CN− could act as a catalyst 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 article (Scheme S1).42 As indicated in the synthesis method, ligand L was used to prepare TMU-40-containing L* ligand. Because of the presence of free nitrogens that have a high propensity for coordination with metals and the probability of choosing this position for coordinating by metal, L* ligands deliver a product different from what we expect; the sites that are considered for future applications are occupied by metal. For this reason, the in situ synthesis method is preferable for this framework. Characterization of TMU-40. In the presence of 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 group and zinc and cadmium belong to another group in the periodic table. 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 ionic radius influences the space group chosen by the frameworks, and Cd(II), which has a larger ionic radius than the other two metals, selects a different space group. Studies of single-crystal 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 one. Overall, each primary building unit has one M2+ (M = Zn, Co, Cd) in a distorted octahedral coordination environment. The metal ion is coordinated to six atoms, which include four oxygen atoms belonging to the carboxylate groups of two different H2BDC and two N atoms belonging to the pyridine of the two L* ligands. The ORTEP plots of the asymmetric unit and the coordination environment of metal are presented in Figure S5

EXPERIMENTAL SECTION

Synthesis of N1,N2-Bis(pyridin-4-yl-methylene)ethane-1,2diamine (L). L was prepared according to the literature method.43 One equivalent of 1,2-ethylenediamine and 2 equiv of 4pyridinecarboxaldehyde were combined in EtOH solvent (15 mL) and refluxed for 3 h; the yellow powder obtained after solvent evaporation was washed three times with n-hexane and exposed to air.42 Melting point (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, dimethyl sulfoxide (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 the Supporting Information (SI) as Figures S1 and S2. 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) was heated for 72 h at 120 °C in a Teflon autoclave and a dark red crystal of TMU-40 was obtained. The crystals were isolated and washed with cold ethanol to eliminate the remaining material and dried in air.42 Yield: ∼19%. IR (KBr, cm−1) for TMU-40Zn: 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. Before adsorbing and sensing studies, the crystal placed under vacuum for 24 h at 120 °C. The synthesis of TMU-40 (Cd, Co) was performed in a completely similar manner with TMU40-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 (%) 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 (%) of TMU40(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 Figure S3. In all the cases, the presence of a sharp peak in the region of about 3280 36260


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Table 1. Average Length of the M−O and M−N Bonds and the Length of Hydrogen Bonds of TMU-40 (Co, Zn, Cd) metal node

ionic radius (pm)

space group

M−O (Ȧ )

M−N (Ȧ )

H bonding 1 (Ȧ )

H bonding 2 (Ȧ )

Co Zn Cd

70 74 95

P21/c P21/c Pbca

2.174 2.232 2.360

2.091 2.073 2.302

2.955 2.997 2.945, 3.031

2.912 2.888

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

in all three structures are shown in Table 1, which shows an increasing trend with increase in 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) (Figures 1 and S6). Powder X-ray diffraction (XRD) patterns for all three structures are well suited to those that are simulated from the single crystal X-ray diffraction (SXRD) data (Figure S7), which confirm the purity of the TMU-40. 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 the samples containing zinc, cobalt, and cadmium have a surface area (Brunauer−Emmett−Teller (BET)) of 200.5, 257.8, and 218.3 m2/g, respectively (Figure S8). Stability of TMU-40. The thermogravimetric analysis (TGA) data show 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 the TGA curve shows that the structure is stable at temperatures up to 360 °C. Similarly, the two other structures show the same heat

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 TMU-40 (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 a 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 former is formed along a chain and the latter leads to an increase in structural dimensions. In the case of TMU-40(Cd), the structure establishes one type of hydrogen bonding but with two different lengths, between the hydrogen on the tetrahydropyrazine moiety of L* in one chain and the oxygen atoms of an vicinal H2BDC in the another, and causes the connection of one-dimensional chains created along the baxis and expands the dimensions of the structure. The formation of such hydrogen-bonding interaction, which is a directional bond, can effectively prevent the rotation of L* ligand, limit the loss of nonemissive energy, and, therefore, block the nonradiative pathway.8,27,28 The average length of the M−O and M−N bonds and the length of hydrogen bonds 36261


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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 the rigid structures, which was also found in the other reported AIE-based MOFs, like PCN-94.8,22,27,28,45 The luminescence plot of cubic crystals of the MOFs in the initial values is shown in Figure S11. Also, the cubic crystals of TMU-40 are demonstrated in Figure S13. The strong luminescence of TMU-40 is observed at 405 and 435 nm, which is compatible with that of ligand molecules. TMU40(Cd) has a lower fluorescence intensity than TMU-40(Zn), which could be attributed to two reasons. First, although zinc and cadmium belong to one group of the periodic table, the difference in the radius of Zn(II) and Cd(II) ions is 25 pm, which can cause changes in the fluorescence behavior of these two structures. On the other hand, due to the fact that TMU40(Cd) chooses the Pbca space group that is more symmetrical than the P21/c space group of TMU-40(Zn), the hydrogen bonds of the structure undergo a change. This structure only provides one type of hydrogen bonding that occurs between the tetrahydropyrazine nitrogens and adjacent H2BDC oxygens. 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, 4-aminophenol, 4-chlorophenol, and 4-methylphenol by TMU-40 (Zn, Cd, and Co) was studied. As shown in the plots of Figures 3, S14, and S15, the presence of all four analytes, in contrast to what is seen for many fluorogen, greatly increased the fluorescence emission of the MOFs. The enhanced Stern−Volmer (SV) 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 a linear dependence on enhancer concentrations. For all structures, the enhancing efficiency follows the order 4-aminophenol > 4-methylphenol > phenol > 4-chlorophenol (Table 2). The Stern−Volmer plots are presented in Figures S16−S18. In addition, the detection limit of phenol derivatives by TMU-40(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 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 concentration of phenol derivatives 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 in Figure S19 and Table S2. Sensing Mechanism. The interaction of TMU-40 with phenol derivatives occurred by the π−π interaction of the

treatment, with the solvent removal occurring slowly and continuously between 25 and 250 °C and the structures remaining stable at temperatures up to 340 °C (Figure S9). For a more precise examination of the thermal stability, all three structures were placed at 350 °C for 12 h. The intact XRD pattern of these structures is compared with the simulation patterns in Figure S6. In addition, the absorbance of N2 from the heated samples was studied and less than 5% change in the BET surface area of TMU-40(Zn) confirms the porosity and thermal stability of these frameworks at 350 °C (Figure S10). The samples were immersed in water for 24 h 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 inductively coupled plasma atomic emission spectroscopy. The soaking solutions were prepared by immersing TMU-40 in water for 12 h. According to the results, the amount of Zn2+, Cd2+, and Co2+ ions in the soaking solutions were negligible, indicating that TMU-40 was rarely destroyed. Luminescence Properties. The luminescence properties of each compound and the used ligands were carefully studied at ambient temperature (Figures S11 and S12). As expected, L ligand is nonfluorescent but L* is fluorescent, having two conjugated nonplanar aromatic rings due to aldimine coupling and cyclization of a CC bond. On the other hand, the formation of a 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 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, using 9,10-diphenylanthracene 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 dramatically enhanced from 1.85 to 20.3% by increasing the ratio of water. As seen from Figure 2, ΦF of the L* in pure CH3CN was very low (ΦF ≈ 1.85) and approximately unchanged when water was added up to 30% (v/v) but increased quickly by adding water in the fraction of 90% (v/v). Also, the solid-state quantum yield of L* was measured and higher absolute solid state quantum yields (ΦF ≈ 23.6) were acquired. In comparison, the quantum yield

Figure 2. Fluorescence quantum yield versus water fraction of CH3CN/water mixtures for L* ligand. 36262


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Figure 3. Fluorescence emission spectra of TMU-40(Co) immersed in acetonitrile at various concentration of concentrations of (A) 4aminophenol, (B) 4-methylphenol, (C) phenol, and (D) 4-chlorophenol, excited at 330 nm.

Table 2. Ksv Values of TMU-40 toward Phenolic Derivatives Ksv

MOF TMU-40(Zn) TMU-40(Cd) TMU-40(Co)

band 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 the LUMO orbitals, which are a high-lying π* antibonding state that are located over the conduction band of the MOF, are transmitted to the CB of MOF, leading to fluorescence enhancement (Figure 4).

5.6 × 10 8.7 × 105 2.9 × 107 5

5.3 × 10 7.2 × 105 2.6 × 107 5

4.9 × 105 5 × 105 1.8 × 107

3.5 × 105 3.6 × 105 1.6 × 107

electron-rich ring of the phenol derivatives with the electronpoor ring of H2BDC present in the MOF and simultaneous 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. Furthermore, the hydrogen bonding between the L* with the O atom of the carbonyl group of the BDC ligand, is still present. 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. The proposed mechanism of the interactions is presented in the Supporting Information section (Figure S20). On the other hand, the main detection mechanism of sensing analytes by luminescent MOFs is often attributed to the photoinduced electron-transfer (PET) process.46−48 After excitation, the electrons from the electron-donor are first transferred from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO). Next, the excited electrons are transferred to the LUMO of the electron-acceptor and result in change in luminescence. The main driving force of the PET process is the energy gap in the LUMOs of the electron-donor and the electron-acceptor materials. About the 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, because of having the concentrated electromagnetic states, determined by narrow energy bands. In such examples, they may be taken as giant “molecules”, and the energy levels of their valence

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 measurements demonstrate that the electron-rich moieties have oxidation potentials less positive than that of TMU-40 (Table S3). Therefore, the MOF acts as an electron donor for the electronwithdrawing materials and for electron-rich electrons as an electron acceptor. After the excition, the excited electrons enter from the conduction band of the metal−organic framework to the LUMO of the analyte, so this nonradiant path leads to quenching. The solid-state UV spectra of all the 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 36263


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Figure 5. UV−vis spectra of the adsorption of 4-aminophenol by (A) TMU-40(Zn), (B) TMU-40(Cd), and (C) TMU-40(Co) in different times.

to the electron-donating and electron-accepting effects between the MOF and the analyte can be related to another factor. Absolutely, the adsorbed analyte can hinder the rotational and vibrational movements of the ligand, which results in limiting nonradiant pathways and ultimately increasing the fluorescence. The picture of MOF paste electrode is presented as Figure S22. It should be noted that, although 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 regard to the electron donation or electron acceptance of the framework, as well as their oxidation potential. Among the MOFs, the structure of 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 the three structures was measured relative to the studied analytes. The results are represented in Figure 5. As shown, probably, the greater adsorption capacity of TMU-40(Co) results in further increase in the concentration of the analyte in the vicinity of the active site of MOF and ultimately has a more significant effect on the fluorescence of the framework. 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, dichloromethane, triethylamine, hexane, tetrahydrofuran (THF), and acetone which shows that amine, carbonyl, cyanide, aromatic rings, and aliphatic chains do not interfere with the sensing results (Figure 6A). The frameworks were washed with methanol and reused for some cycles by centrifuging the dispersed solution after 4-aminophenol sensing (Figure 6B). The XRD pattern after three sensing processes and SEM images of TMU-40 are shown in Figures S23 and S24, respectively.

Figure 6. (A) 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 reuse of TMU-40 for 4-aminophenol sensing.

structures, TMU-40(Co) which have weaker fluorescence, shows a much higher sensitivity to analytes that 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 a simple and one-step synthesis of AIE materials.

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CONCLUSIONS 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 derivative. Unlike most compounds with an AIE characteristic that have complex and multistep syntheses, the MOF is synthesized simply in one reaction step. Using metal nodes with various outer-shell electron configurations led to the construction of high- and low-emissive MOFs by a simple method. The formation of rigid framework improved the main fluorescence of organic componds. These fluorogenic frameworks containing free amine nitrogens were used to sense the phenolic compounds with hydrogen-bond-forming potential. Among these three

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b12404. 36264


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ACS Applied Materials & Interfaces Characterizations, sensing, fluorescence, studies, and proposed mechanism are presented (PDF)

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Crystallographic data (CIF) (CIF) (CIF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel/Fax: +98 21 82884416. ORCID

Ali Morsali: 0000-0002-1828-7287 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Support of this investigation by Tarbiat Modares University are gratefully acknowledged. We also greatly appreciate Dr Hamed Moghanni for his precious comments in the electrochemical tests.

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REFERENCES

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Simple One-Pot Preparation of a Rapid Response AIE Fluorescent

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