Cataloged from PDF version of article.A silyloxy-functionalized tetraphenylethylene (TPE) derivative shows a remarkable change in the absorption spectrum on deprotection with fluoride ions. The reaction process is highly selective for fluoride and the resulting charge transfer band results in a bright green solution. A simple selective visual assay of aqueous fluoride ions was also obtained by the impregnation of cellulose strips with the TPE derivative

Pir, F.

Simsek, I.

Sozmen, F.

Tetrahedron Letters

Cataloged from PDF version of article.A silyloxy-functionalized tetraphenylethylene (TPE) derivative shows a remarkable change in the absorption
spectrum on deprotection with fluoride ions. The reaction process is highly selective for fluoride and
the resulting charge transfer band results in a bright green solution. A simple selective visual assay of
aqueous fluoride ions was also obtained by the impregnation of cellulose strips with the TPE derivativ

Bilkent University Institutional Repository

Tetrahedron Letters 55 (2014) 456–459Contents lists available at ScienceDirectTetrahedron Lettersjournal homepage: www.elsevier .com/ locate / tet letHighly selective ﬂuoride sensing via chromogenic aggregationof a silyloxy-functionalized tetraphenylethylene (TPE) derivative0040-4039/$ - see front matter  2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.tetlet.2013.11.059⇑ Corresponding author. Tel.: +90 312 290 3568; fax: +90 312 266 4068.E-mail address: sozmen@unam.bilkent.edu.tr (F. Sozmen).Ilke Simsek Turan a, Fatma Pir Cakmak b, Fazli Sozmen a,⇑aUNAM-National Nanotechnology Research Center, Bilkent University, Ankara 06800, TurkeybDepartment of Chemistry, Bilkent University, Ankara 06800, Turkeya r t i c l e i n f o a b s t r a c tArticle history:Received 25 July 2013Revised 24 October 2013Accepted 14 November 2013Available online 23 November 2013Keywords:Fluoride sensingTPEChromogenic aggregationFluoride ionsSelectivityA silyloxy-functionalized tetraphenylethylene (TPE) derivative shows a remarkable change in the absorp-tion spectrum on deprotection with ﬂuoride ions. The reaction process is highly selective for ﬂuoride andthe resulting charge transfer band results in a bright green solution. A simple selective visual assay ofaqueous ﬂuoride ions was also obtained by the impregnation of cellulose strips with the TPE derivative. 2013 Elsevier Ltd. All rights reserved.Tetraphenylethylene (TPE) and its derivatives have attractedremarkable attention due to their well-known aggregation-in-duced emission (AIE)1 properties, which make this small moleculean indispensible building block in several areas such as OLEDs,chemosensors, and bioprobes.2 The outstanding electrochemical,physical, and excited state properties, especially in the solid state,are a result of the extended p-system found in TPE.3Fluoride ion sensing is an important topic because of the biolog-ical signiﬁcance of this anion, especially in relation to dental care,and in the treatment of osteoporosis.4 However, ﬂuoride ion sens-ing is a highly challenging task due to the high electronegativity ofﬂuorine, its strong hydrogen bonding ability and its size (smallestanion).In addition, real time monitoring of ﬂuoride ions is extremelydifﬁcult in aqueous media, especially via chromogenic chemosen-sors due to competitive hydrogen bonding between ﬂuoride andwater molecules.5 A reaction-based sensor could have some advan-tages considering the simplicity of the analytical procedure. In thisrespect, naked eye anion detection with chemosensors capable ofdetecting anions is becoming increasingly important for rapidon-site analysis, on a real time basis. Current chromogenic anionsensors typically have hydrogen bonding receptor sites, some withselectivity for ﬂuoride, but are less likely to work in aqueousmedium.6The high chemical afﬁnity between ﬂuoride and silicon hasbeen widely used in previous studies to sense ﬂuoride anions,however, there are only a few which work in aqueous solutions.7Herein, we report the design and synthesis of a highly selectiveand sensitive chromogenic ﬂuoride anion sensor, TPE 5. The syn-thesis of target compound 5 started with the reaction between 4-methoxyphenyllithium (from 1-bromo-4-methoxybenzene andbutyllithium) and 4-methoxybenzaldehyde to produce alcohol 1,which was oxidized to compound 2 using manganese(IV) oxidein dichloromethane. Compound 3 was then obtained through aMcMurry coupling of compound 2. This was followed by demeth-ylation to give 4. Further reaction of compound 4with chlorotriiso-propylsilane in the presence of imidazole yielded the targetﬂuoride anion sensor 5 (Scheme 1).Fluoride sensing of TPE 5 was accomplished by deprotection ofthe silyl groups from the TPE core generating four phenoxide ionsin full conjugation with the TPE moiety, which leads to very strongintramolecular charge transfer (ICT).8This strong ICT resulted not only in the formation of new chargetransfer bands in the absorbance spectrum corresponding to abright green solution, but also led to quenching of the emissionin the ﬂuorescence spectrum.In the electronic absorption spectrum, a maximum was ob-served at 340 nm for compound 5 in DMF. Next, the solution ofcompound 5 (50 lM in DMF) was treated with ﬂuoride ions inthe form of a tetrabutylammonium salt (TBAF). The absorptionspectra during titration showed new bands appearing at 452,510, 600 and 680 nm, respectively. The band at 600 nm was theScheme 1. Synthesis of ﬂuoride sensor 5.Figure 2. The digital photographs show the appearance of the solutions under ahand-held UV lamp (360 nm) (top) and under ambient light (bottom). Probeconcentrations were 50 lM and the anions were added at 0.4 mM concentrations,all in DMF.I. Simsek Turan et al. / Tetrahedron Letters 55 (2014) 456–459 457most intense, resulting in a bright green color of the solution(Fig. 1).The electronic absorption spectra of 5 were also investigatedupon the addition of eight equivalents of the TBA salts of HSO4 ,H2PO4 , AcO, NO3 , CN, I, Br, and Cl to DMF solutions of 5.The addition of these anions did not result in any changes in theabsorption spectrum (Fig. S1), or the emission spectrum.Figure 2 (bottom) shows the color change after the addition ofeight equivalents of the anions as TBA salts to the DMF solutionFigure 1. Absorbance spectra of compound 5 + F in the presence of increasing Fconcentrations. The inset is a digital photograph of the probe (left) and probefollowing the addition of 8 equivalents of F (right). Probe concentration is 50 lM inDMF.of 5. Clearly, no color change was observed upon addition of theother anions; only ﬂuoride ions led to a color change.The emission spectra of compound 5 (50 lM in DMF) showeddrastic quenching upon addition of ﬂuoride anions in the form ofTBA salts (Fig. 3). The strong charge transfer character of theformed phenoxide moieties leads to this quenching. As expected,no emission quenching was observed upon addition of other ionsas TBA salts as shown in Figure 4 (top). The spectacular differencebetween ﬂuoride anions and the other anions in terms of emissionintensity is clearly shown in the bar graphs (Fig. 4, bottom). Fur-thermore, selective quenching of ﬂuoride is shown in the digitalphotographs of DMF solutions under the indicated conditions(Fig. 2, top).Figure 3. Emission spectra of compound 5 + F in the presence of increasing Fconcentrations. Probe concentration is 50 lM in DMF. Excitation wavelength is340 nm.Figure 4. Emission spectra of compound 5 upon the addition of 8 equivalents ofCl, Br, I, HSO4 , AcO, NO3 , H2PO4 , and CN. The F and probe concentrations areboth 50 lM, all in DMF. The bar graph shows the emission ratios (I/Io).Figure 5. The color changes of the treated cellulose strips prepared for thedetection of ﬂuoride anions in aqueous environments, on exposure to differentﬂuoride concentrations.458 I. Simsek Turan et al. / Tetrahedron Letters 55 (2014) 456–459In order to explore whether compound 5 could respond to ﬂuo-ride anions in solid state via chemical reactions, we decided to pre-pare a test paper for detecting ﬂuoride in an aqueous environmentby dipping a cellulose strip (3.0  0.5 cm2) into a tetrahydrofuransolution of 5 (4.0 mM). After drying the cellulose strip under ambi-ent conditions, it was immersed in an aqueous ﬂuoride solution(water/tetrahydrofuran, 2:8) for several seconds followed by ashort heat treatment.The color of the cellulose strip was clearly and reproduciblychanged from white to green (Fig. 5). However the color changewas not uniform throughout the test strip. The underlying reasonfor this maybe that the sensing process was achieved by a chemicalreaction taking place on the cellulose ﬁbers, introducing some het-erogeneity in the solid state.In conclusion, we have reported a new chemical reaction basedon a chromogenic ﬂuoride anion probe operating via deprotectionof the silyl groups from TPE derivative 5. The deprotection of foursilyl groups results in four donor phenoxide groups causing strongintramolecular charge transfer. This strong ICT also induces consid-erable ﬂuorescence quenching and the occurrence of new absor-bance bands in the visible region resulting in a bright greensolution. This highly selective sensing was also, demonstrated inaqueous environments using a stained cellulose strip. Thus, anaked-eye chromogenic chemosensor 5 has been described fordetection of ﬂuoride anions and this dye impregnated cellulosestrip could be a prototype material for the easy and fast chemicaldetection of ﬂuoride.AcknowledgmentThe authors thank Professor Dr. Engin U. Akkaya for fruitful dis-cussions. I.S.T. acknowledges support from TUBITAK-BIDEB in theform of a scholarship.Supplementary dataSupplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.tetlet.201311.059.References and notes1. (a) Luo, J.; Xie, Z.; Lam, J.; Cheng, L.; Chen, H.; Qiu, C.; Kwok, H. S.; Zhan, X.; Liu,Y.; Zhu, D.; Tang, B. Z. Chem. Commun. 2001, 1740–1741; (b) An, B. K.; Kwon, S.K.; Jung, S. D.; Park, S. Y. J. Am. Chem. Soc. 2002, 124, 14410–14415; (c) Chen, J.;Law, C. W.; Lam, J.; Dong, Y.; Lo, S.; Williams, I. D.; Zhu, D.; Tang, B. Z. 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Highly selective fluoride sensing via chromogenic aggregation of a silyloxy-functionalized tetraphenylethylene (TPE) derivative

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Bilkent University Institutional Repository

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