Ratiometric Luminescent Detection of Organic Amines Due to the

Jiangsu Normal University, Xuzhou 221116, P. R. China. ‡College of Chemistry, Chemical Engineering and Materials Science, Shandong Normal University...
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Ratiometric Luminescent Detection of Organic Amines due to the Induced Lactam-Lactim Tautomerization of Organic Linker in a Metal-Organic Framework Wen-Qiang Zhang, Qiu-Yan Li, Jun-Yan Cheng, Ke Cheng, Xinyu Yang, Yinwei Li, Xinsheng Zhao, and Xiao-Jun Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b11125 • Publication Date (Web): 30 Aug 2017 Downloaded from http://pubs.acs.org on September 3, 2017

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Ratiometric Luminescent Detection of Organic Amines Due to the Induced Lactam-Lactim Tautomerization of Organic Linker in a Metal-Organic Framework Wen-Qiang Zhang,† Qiu-Yan Li,*,† Jun-Yan Cheng,‡ Ke Cheng,† Xinyu Yang,† Yinwei Li,§ Xinsheng Zhao§ and Xiao-Jun Wang*,† †

Jiangsu Key Laboratory of Green Synthetic Chemistry for Functional Materials, School of Chemistry and Materials Science, Jiangsu Normal University, Xuzhou 221116, P. R. China. ‡

College of Chemistry, Chemical Engineering and Materials Science, Shandong Normal University, Jinan 250014, P. R. China § School of Physics and Electronic Engineering, Jiangsu Normal University, Xuzhou 221116, P. R. China. KEYWORDS: luminescent metal-organic framework, linker design, lactam-lactim tautomerism, ratiometric sensor, volatile organic amine

ABSTRACT: Here we demonstrate that a fluorescent benzothiadiazole (BTD)-conjugated terphenyldicarboxylate (TPDC) linker (denoted as H2-ostpdc) has been hybridized by a quinoxaline-2,3-(1H,4H)-dione (QD) moiety possessing lactam-lactim tautomerism, which was further integrated into a robust and porous UiO-68 type zirconium metal-organic framework (MOF UiO-68-osdm) by utilizing the mixed two dicarboxylate struts with the same ligand lengths. The resultant MOF UiO-68-osdm can work as a ratiometric luminescent sensor for visual and selective detection of alkyl amines. Furthermore, it can discriminate secondary alkylamines from other type amine species.

Benefitting from their unique merits, such as high porosity, adjustable chemical composition and functionality, metal-organic frameworks (MOFs) are being explored vastly for various applications.1-5 Lately, luminescent MOF-based chemo/bio-sensors have gained tremendous and growing research attention for detecting explosives, ions, gas, pH, and small molecules, among others.6-12 However, a very large part of reported MOF-sensors work based on luminescence quenching mainly resulted from the energy or electron transfer processes between MOF host and guest analytes. These responsive “turn-off” sensors, to some extent, may still suffer from the low signal-to-noise ratio, in comparison to other type of MOF-sensors with a “turn-on” or ratiometric working mode.13-15 In particular, the ratiometric fluorescence sensing, utilizing the ratio of two emission bands as a 2D signal transduction, possesses a built-in self-calibration that can eliminate the false response originating from environmental interferences, such as excitation light intensity and concentration change of probe. Therefore, it typically exhibits the enhanced accuracy and precision in comparison to only intensity-based singlechannel detecting methods. In recent years, the sensing and detection of volatile organic amines (VOAs) has received intense interest because they are almost omnipresent analytes that play important roles in the fields ranging from quality control of food and medical diagnosis to industrial and environmental monitoring.16-20 Some conventional approaches, such as gas chromatography-mass spectrometry (GCMS) and electrochemical devices, require expensive instruments and tedious preparation for devices or samples.18 In comparison, the optical method based on various fluorescent materials can provide a quicker, simpler and less expensive way for sensing amine species both in solution and vapor phase.19,20

Among these sensory materials, porous crystalline MOFs can serve as a new promising platform for amine detections, along with expectation of enhanced selectivity and sensitivity owing to the preconcentration of analytes by the inherent porosity.6-12 In this regard, a handful of related luminescent MOFs have been successfully constructed over the past few years.21-25 A common strategy is to use electron-deficient linkers in MOF framework, such as naphthalenediimide (NDI) and pyridinium moieties, offering a charge-transfer interaction site for recognizing electron-rich amines.21-23 This always results in an emission “turn-off” process due to the electron transfer. On the other hand, analogue MOFs but with a fluorescent enhancement or “turn-on” response toward amines have also been recently reported.24,25 Although a stride has been made in the development of MOF-based sensors for detection of organic amines, however, there are no MOFs that can exhibit a ratiometric fluorescent sensing behavior for amines.

Scheme 1. The preparation for MOF UiO-68-osdm with the mixed dicarboxylate linkers.

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Scheme 2. The synthetic route for H2-ostpdc. Reagents and conditions: a) Fuming trifluoromethane sulfonic acid, Fuming nitric acid ; b) Cs2CO3, CsF, Pd(dppf)Cl2, Pd(PPh3)4, Toluene /H2O, 90 °C for 2 d; c) Fe, HAc；d) Oxalyl chloride, CH2Cl2, 0 °C for 1 h, 25 °C over night; e) KOH, DMF, H2O,CH3OH, 80 °C for 2 h; TFA, H2O,1 h, room temperature. In this work, we designed and synthesized a fluorescent 2,1,3benzothiadiazole (BTD)-conjugated terphenyldicarboxylate linker (denoted as H2-ostpdc), which has been hybridized by a quinoxaline-2,3-(1H,4H)-dione (QD) moiety possessing lactam-lactim tautomerism. Subsequently, the ligand was integrated into the robust and porous UiO-68 type zirconium MOF by using a mixedlinker approach26,27 (H2-ostpdc and H2-dmtpdc, Scheme 1). The resultant MOF UiO-68-osdm can work as a ratiometric luminescent sensor for visually and selectively detecting amine species both in solution and vapor phase, because of the induced lactamto-lactim tautomerization and its deprotonation together with the caused variation of electron density in QD moiety. Moreover, the functionalized MOF can further discriminate secondary alkylamines from other type of amines. The fluorescent organic linker 4,4'-(6,7-dioxo-5,6,7,8-tetrahydro-[1,2,5]thiadiazolo[3,4-g]quinoxaline-4,9-diyl)dibenzoic acid (H2-ostpdc) was synthesized by several typical organic reactions as depicted in Scheme 2 (for details, see SI). Briefly, the commercially available 4,7-dibromobenzo[c][1,2,5]- thiadiazole (1) was treated by the mixture of fuming HNO3 and CF3SO3H, giving a dinitro-precursor 2. Then, Suzuki coupling reaction of 2 with 4-methoxycarbonylphenylboronic acid was performed to provide methyl ester 3, which was reduced by using iron powder to generate diamine compound 4. Treatment of 4 with oxalyl chloride in anhydrous dichloromethane produced precursor 5 with the formation of a tautomeric QD moiety, which was further hydrolyzed to give the target linker H2-ostpdc in a high yield. Subsequently, a combination of organic linkers H2-ostpdc and H2dmtpdc (1:4 in molar ratio) was treated with ZrCl4 as metal source and HAc as modulator in N,N’-dimethylformamide (DMF) at 100 o C for 72 h to produce MOF UiO-68-osdm. As expected, powder X-ray diffraction (PXRD) was performed to reveal its isostructural framework with the parent MOF UiO-6828 as well as highly crystalline nature (Figure S1 in the SI). Field-emission scanning electron microscopy (FESEM) indicated its octahedral morphology with a uniform size of around 1-2 micrometers (Figure S3). The permanent porosity of UiO-68-osdm was examined by N2 sorption measurement at 77 K, indicating a type I reversible isotherm with Brunauer-Emmett-Teller (BET) surface area of 3380 m2 g-1 (Figure S5). Besides, the pore size distribution was estimated as ∼1.7 nm by using nonlocal density functional theory. Thus, the high porosity and large pore width of UiO-68-osdm will facilitate adsorption and diffusion of analytes in the framework for achieving a quick and sensitive sensing response. Firstly, we investigated the fluorescence response of the linker precursor compound 5 with a hybrid of fluorescent BTD and tautomeric QD moieties to diethylamine (DEA) as a VOA model, in order to evaluate the potential of H2-ostpdc acting as a ratiometric sensor for amines. Here the methyl ester precursor 5 was used to replace H2-ostpdc for a consideration of excluding the influence of carboxylic groups in the linker. The precursor 5 exhibited an intense emission band at 458 nm upon excitation at 370 nm in

chloroform solution, along with a weak shoulder peak at 490 nm that may be attributed to the emission from a small amount of tautomeric lactim form species of 5. It was found that the emission at 458 nm from the lactam form of 5 rapidly decreased upon addition of DEA, while a new red-shift peak at 515 nm arisen from the deprotonated species gradually appeared with an isoemission point at 493 nm (Figure S6 and Scheme S2), revealing its ratiometric fluorescent detecting behavior for DEA. And the relative ratio of emission intensities at 515 and 458 nm shows a good linear relationship with the concentration of DEA (50µM– 1 mM). These results suggest that a ratiometric MOF-based sensor toward amines could be obtained if the target organic linker H2-ostpdc was introduced into the framework. Since the MOF UiO-68-osdm has been successfully prepared, we shifted our research attention into MOF’s response to organic amines. In contrast to the emission spectra of 5, a dispersion of UiO-68-osdm in chloroform exhibited two distinct fluorescence bands at 463 and 494 nm (Figure 1), which should be attributed to emissions from tautomeric lactam (I) and lactim (II) form of organic linker in MOF framework (Figure 2), respectively. This may suggest that both lactam and lactim form of the linker are dominant in UiO-68-osdm mainly due to the immobilization of H2-ostpdc ligand in the rigid framework of MOF, probably facilitating the tautomerization of lactam and lactim.

Figure 1. Emission spectra of MOF UiO-68-osdm dispersed in chloroform (0.02 mg/mL) upon the titration of DEA (0-0.1 mM, λex = 370 nm). Inset: The relative emission ratio (I517 nm/I463 nm) varies as a function of DEA concentration. To our delight, the luminescent MOF UiO-68-osdm indeed showed a ratiometric responsive behavior to amine. Similarly, titration of UiO-68-osdm with DEA caused a quick quenching of the emission at 463 nm, as shown in Figure 1. Simultaneously, a new emission band appeared at 517 nm with a large bathochromic shift of 54 nm and an isoemission point at 501 nm. And no significant change was observed in the fluorescence spectra and the relative ratio (I517 nm/I463 nm) became constant when the concentration of DEA reaches up to 90 µM (Figure 1, inset). Besides, the

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emission ratio reveals an excellent linear proportional to the low concentration of DEA from 1 to 10 µM (R2= 0.998, Figure S7), indicating that MOF UiO-68-osdm can act as a highly sensitive chemodosimeter for quantitative detection of DEA. Furthermore, the detection limit of UiO-68-osdm toward DEA was determined to be 1.3 nM according to the equation 3σ/k, which is much lower that of linker precursor 5 (90 nM). These above results indicated that MOF UiO-68-osdm exhibited a much higher sensitivity toward DEA than that of its organic linker precursor, which could be ascribed to the well-known preconcentration effect for analyte in MOF’s framework.6-12 Also, the detecting process of UiO-68-osdm toward DEA can be clearly and easily observed by the naked eye under a portable 365 nm UV lamp (Figure 2, inset). The fluorescent color of MOF’s suspension in chloroform was instantly changed from cyan to light yellow upon addition of DEA, confirming the very fast response. Not surprisingly, this response process can be readily repeated by several cycles with addition of HAc and DEA, respectively. PXRD measurements of UiO-68-osdm after immersion in 1 mM DEA solution for 24h or after four detecting cycles revealed a good maintenance of its crystalline framework (Figure S1), mainly due to the robust framework of Zr-based MOFs.29

of QD moiety was greatly enhanced in relative to its counterpart 5 in homogeneous solution. Thus, MOF UiO-68-osdm revealed two corresponding emission bands with similar intensities at 463 and 494 nm in chloroform suspension, respectively. Upon interaction with the basic amine DEA, the tautomer of latam (I) will be quickly decreased via the deprotonation of hydroxyl groups in lactim (II), along with generation of two typical resonance forms III and IV. The new induced delocalization of electron in QD moiety (V) results in a new emission with a large red shift. In the end, such a ratiometric MOF-based fluorescent sensor can be rationally constructed by the intensity ratio of I517 nm/I463 nm. The response of MOF UiO-68-osdm toward other type of amines was further explored by using triethylamine (TEA) and npropylamine (PA) as typical tertiary and primary examples (Figure S9 and S10), respectively. As expected, UiO-68-osdm also demonstrated a ratiometric response to TEA and PA, but with a lower sensitivity in comparison with the secondary amine DEA. And the detection limits for TEA and PA were calculated to be 12 and 45 nM, respectively. These may be rationalized by the stronger basicity of secondary amine among them. Besides, the high sensitivity of UiO-68-osdm for secondary amine DEA is also further enhanced by the presence of hydrogen-bonding and electrostatic interactions between the acidic hydroxyl groups of ostpdc ligand and Lewis basic N-donor sites of amines (III, Figure 2).

Figure 3. The change for the emission ratio of UiO-68-osdm (∆I517 nm/I463 nm) in the presence of different amines (0.1 mM): 1, DEA; 2, TEA; 3, PA; 4, piperidine; 5, N-methyl piperidine; 6, cyclohexylamine; 7, piperazine; 8, N, N-dimethylpiperazine; 9, diisopropylamine; 10, N, N-diisopropylethylamine; 11, pyrrolidine; 12, N-methyl pyrrolidine; 13, aniline; 14, pyridine.

Figure 2. Tautomerism of the linker ostpdc and its resonance electron delocalization of deprotonated one in MOF UiO-68-osdm. Inset: the corresponding fluorescence photos of UiO-68-osdm in absence and presence of DEA under 365 nm light. Besides, the control MOF UiO-68S30 containing BTD conjugated TPDC linker but without QD moiety did not exhibited any response to amine DEA even in a high concentration of 1 mM (Scheme S3 and Figure S8). Another MOF UiO-68M27 that was only composed by organic linker H2-dmtpdc showed no fluorescence upon excitation at 370 nm. Based on these experimental observations, a plausible mechanism for the ratiometric response of UiO-68-osdm toward DEA was proposed and depicted in Figure 2. Since the other linker dmtpdc did not exhibit any fluorescence, this sensing behavior should be solely caused by the presence of the novel linker osptdc in UiO-68-osdm, in which tautomeric QD as the responsive site for amine and BTD as the fluorophore moiety are integrated into one single benzo-fused motif. Because the linker was rigidly immobilized into the framework of MOF, the tautomerism transformation for latam (I) to lactim (II)

Moreover, we have screened the fluorescence responses of UiO-68-osdm toward a range of various amines. It is very interesting to find that the MOF can differentiate secondary alkylamines from others via significant fluorescence change, including tertiary, primary and aromatic amines (1-14#, Figure 3 and Figure S11). As shown in Figure 3, the change of emission ratio (∆I517 nm/I463 nm = I517 nm/I463 nm – I0, 517 nm /I0, 463 nm) for secondary amines is higher than 2.5, while the value for most other amines is lower than 0.9. Also, such discrimination can be obviously observed by the naked eye under a 365 nm lamp (Figure S11), which confirms the high affinity and sensitivity of UiO-68-osdm toward secondary amines. Furthermore, the response of MOF UiO-68-osdm to other volatile organic compounds (VOCs), such methanol, benzene and acetone, was studied (15-23# in Figure S11). And no obvious variation can be detected for them, indicative of the high selectivity of UiO-68-osdm toward amine species over other VOCs. As sensing of amines in organic solutions is not feasible for practical application, thus, we fabricated a test strip with MOF UiO-68-osdm on TLC plates for promptly and conveniently detecting amines in gas phase (Figure S12). Under the light of a handheld UV 365 nm lamp, the strip emits a blue-green emission. After exposure of the strip to various amounts of DEA vapor (1-

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50 ppm) for 1 min, the fluorescence color was gradually became yellow and deeper with increase of DEA concentration, which can be easily observed by the naked eye (Figure S13). Remarkably, the test strip with UiO-68-osdm was able to give a clearly detectable response by naked eye when the concentration of DEA is as low as 10 ppm. The selectivity of the strip for amine over other VOCs was also investigated. Only remarkable change can be witnessed for amine species (Figure S14), which is well agreement with the above results in solution phase. In summary, we have rationally developed a fluorescent ligand for sensing amine by combination of BTD as the fluorophore and tautomeric QD as the responsive site into a benzofused motif, which was further successfully integrated into the robust and porous UiO-68 type Zr-MOF by utilizing the mixed-linker approach. And the MOF exhibited a ratiometric fluorescence response to organic amines but with significant enhancement of sensitivity in comparison to its ligand counterpart. Besides, the MOF revealed high selectivity toward amine species over other VOCs both in solution and gas phase. Moreover, it can discriminate secondary alkylamines from other amine species. To the best of our knowledge, this is the first example of a ratiometric fluorescent MOF-based sensor for organic amines. It is expected that MOF as a versatile platform will allow for the introduction of more novel fluorescent ligands into its framework, achieving an ideal sensor with significantly enhanced performances.

ASSOCIATED CONTENT Supporting Information Experimental procedure, additional synthesis and characterization data. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (Q.-Y. Li); [email protected] (X.-J. Wang)

ORCID Qiu-Yan Li: 0000-0001-5291-2309 Xiao-Jun Wang: 0000-0002-1461-4922

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was financially supported from NSFC (21302072 and 51403081), TAPP and PAPD of Jiangsu Higher Education Institutions.

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SYNOPSIS TOC

A ratiometric luminescent sensor based on metal-organic framework (MOF) was developed for detecting amines owing to the induced lactam-lactim tautomerization of organic linker in the framework.

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