Article Cite This: Organometallics XXXX, XXX, XXX−XXX
OFF−ON Fluorescence Sensing of Fluoride by Donor−Antimony(V) Lewis Acids Ajay Kumar,†,§ Mengxi Yang,‡,§ Minji Kim,† François P. Gabbaï,*,‡,§ and Min Hyung Lee*,†,§ †
Department of Chemistry and EHSRC, University of Ulsan, Ulsan 44610, Republic of Korea Department of Chemistry, Texas A&M University, College Station, Texas 77843-3255, United States
‡
S Supporting Information *
ABSTRACT: A series of triarylmethylstibonium Lewis acids of general formula [Ph2MeSb-(p-(C6H4))-FLUO]+ bearing a peripheral electron-rich fluorophore (FLUO = 10H-phenoxazine ([3a]+), diphenylamine ([3b]+), and 9H-carbazole ([3c]+)) have been synthesized and investigated for the fluorescence turn-on sensing of fluoride anions. Treatment of the stibonium cations with fluoride anions leads to the corresponding fluorostiboranes (3a-F−3c-F). While the stibonium cations are almost nonemissive, the fluorostiboranes display fluorophore-centered emissions arising from the corresponding π−π* excited state. The carbazole-containing derivative [3c]+ exhibits the most intense fluorescence turn-on response. It also displays a high binding constant (K > 107 M−1) in MeCN and shows compatibility with protic media such as MeOH (K = 950(±50) M−1). Computational studies aimed at identifying the origin of the turn-on response show that the excited state of the stibonium cations is best described as charge transfer in nature with the π system of the fluorophore acting as the donor and the π*−σ* system of the stibonium unit acting as the acceptor. This π(FLUO)−π*/σ*(Ph2MeSb-(p-(C6H4))) excited state is nonemissive, making these cations dark in the absence of fluoride anions. Conversion to the fluorostiboranes occurs via donation of a fluoride lone pair into the antimonycentered σ*. Formation of this Sb−F bond modifies the electronic structure of the platform and restores the emissive π−π* excited state of the fluorophore, thus accounting for the observed OFF−ON fluorescence response.
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INTRODUCTION Fluoride anion sensing is a topic of ongoing interest because of the possible adverse effects caused by excessive intake of this anion.1−4 Lewis acidic compounds based on boron5−11 and antimony(V)12−25 have recently been exploited as fluoride sensors for their high affinity. In particular, organoantimony(V) compounds are attracting a growing interest due to their stability and high sensitivity toward fluoride in both organic and aqueous media.12−25 Additionally, when they are substituted with a fluorescent reporter, these antimony-containing compounds sometime produce a turn-on fluorescence response upon fluoride anion complexation.12−15 In previous studies,12,13 it has been demonstrated that while stibonium cations of general formula [FLUO-SbPh2R]+ (R = Ph, FLUO = 9-anthryl ([I]+), 9-phenanthryl ([II]+), 1-pyrenyl ([III]+)) are almost nonemissive, their fluorostiboranes (I-F−III-F) show intense fluorescence from the fluorophore (Chart 1). This OFF−ON mechanism rests on a structural distortion that takes place in the excited state of the stibonium cation. In this process, the antimony atom coordination geometry distorts from a tetrahedral geometry (A) to a seesaw geometry (B), resulting in an inversion of the π* and σ*(Sb−C) orbital energies. It follows that B is no longer emissive because of the π−σ*(Sb− C) rather than π−π* nature of the excited state (Figure 1, left). © XXXX American Chemical Society
Chart 1. Fluorophore−[Sb(V)] Lewis Acids and Turn-on Fluorescence Response toward Fluoride Binding
Since fluoride anion binding occurs by donation of a fluoride lone pair into a σ*(Sb−C) orbital, this dark excited state is no longer accessible after anion binding, leading to the observed turn-on response.14,26 The same mechanism has been invoked to explain the fluorescence quantum yield increase observed Received: October 11, 2017
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DOI: 10.1021/acs.organomet.7b00759 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics Scheme 1. Synthesis of Methylstiboniums and Their Fluorostiboranesa
Conditions: (i) n-BuLi, Et2O, −78 °C; (ii) Ph2SbCl, THF/ether, −78 °C; (iii) MeOTf, CH2Cl2, 30 °C; (iv) KF, MeOH, room temperature. a
Figure 1. (left) Energy diagram showing the frontier molecular orbitals of [I]+ in the excited state, before and after distortion. (right) Frontier orbital energy diagram of the compounds targeted in this study, with a σ*(Sb−C) orbital acting as the LUMO, even without distortion from the ground-state geometry.
upon anion coordination to the BODIPY-stibonium platform [IV]+.14 With the view of adding credence to this model, we have now become interested in platforms for which the σ*(Sb− C) orbital falls between the π and π* orbitals of the fluorophore, even at the ground state geometry (Figure 1, right). It occurred to us that such an electronic structure could be generated using a fluorophore whose π and π* levels are higher than those of the aromatic hydrocarbons used in system of type [I]+. These considerations have led us to focus on electron-rich moieties such as the 10H-phenoxazine, diphenylamine, and 9H-carbazole units and their incorporation as peripheral fluorophores (FLUO) in stibonium cations of general formula [Ph2MeSb-(p-(C6H4))-FLUO]+ ([3a−c]+; Chart 1).
Figure 2. Structure of [3a]+ in the crystal of [3a]OTf. Ellipsoids are drawn at the 30% probability level. Hydrogen atoms, the OTf anion, and the solvent molecules were omitted for clarity. Selected bond lengths (Å) and angles (deg): Sb(1)−C(16) 2.112(2), Sb(1)−C(19) 2.102(2), Sb(1)−C(25) 2.104(2), Sb(1)−C(31) 2.092(2); C(16)− Sb(1)−C(19) 99.73(8), C(16)−Sb(1)−C(25) 104.71(8), C(16)− Sb(1)−C(31) 109.22(8), C(19)−Sb(1)−C(25) 113.85(8), C(19)− Sb(1)−C(31) 110.95(10), C(25)−Sb(1)−C(31) 116.67(9).
the BODIPY-substituted methylstibonium [IV]+, for which a short Sb···OTf distance of 2.860(3) Å was observed in the solid state.14 This difference suggests that the antimony center of [3a]+ is not as Lewis acidic as in [IV]+, a factor assigned to the electron-donating rather than -withdrawing nature of the phenoxazine substituent. The large dihedral angle of ca. 67° formed by the plane containing the phenoxazine unit and that containing the phenylene ring originates from steric effects and suggests minimum conjugation between the phenoxazine unit and the rest of the molecule. A similar situation has been observed for complexes featuring N-phenylphenoxazine units.27,28 We were also able to isolate fluorostiboranes 3a-F− 3c-F by treating the stibonium cations with excess (3 equiv) KF in MeOH (Scheme 1). The 1H NMR spectra exhibited methyl resonances in the 2.0−2.2 ppm range shifted upfield in comparison to those of [3a]+−[3c]+, indicating the neuralization of the cationic charge. For all three fluorostiboranes, a 19F resonance was observed at −72 ppm, which is close to the chemical shift displayed by other fluorostiboranes such as I-F, II-F, and III-F (Figures S8−S10 in the Supporting Information).12,13 Photophysical Properties. The photophysical properties of the stibonium salts and the fluorostiboranes were examined by UV−vis absorption and photoluminescence (PL) spectroscopy in MeCN (Figure 3 and Table 1). All fluorostiboranes
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RESULTS AND DISCUSSION Synthesis and Characterization. In order to access the target complexes, we first prepared the known FLUO-(p(C6H4))-Br derivatives (FLUO = 10H-phenoxazine (1a), diphenylamine (1b), 9H-carbazole (1c)). Lithiation of these compounds followed by reaction with Ph2SbCl produced the corresponding stibines 2a−c in moderate yields (Scheme 1). These stibines were treated with methyl triflate (MeOTf) in CH2Cl2 to afford the corresponding methylstibonium cations [3a]+−[3c]+ as air-stable triflate salts. The stibines and the corresponding stibonium salts have been characterized by NMR spectroscopy and elemental analyses. The stibonium salts showed a methyl proton resonance in the 2.5−2.6 ppm range, confirming quaternization of the antimony atom (Figures S5− S7 in the Supporting Information). The structure of the phenoxazine derivative [3a]OTf was determined by X-ray crystallography (Figure 2). The antimony center adopts a distorted-tetrahedral geometry with no obvious short contact with the triflate counteranion (Figure S1 in the Supporting Information). This is in contrast to the structure of B
DOI: 10.1021/acs.organomet.7b00759 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics
stibonium as an acceptor. This assignment is supported by the fact that these bands are significantly quenched ([3a]+ and [3c]+) or undergo a blue shift ([3b]+) once the stibonium cation electron deficiency is neutralized by fluoride anion complexation (see also the TD-DFT results below). PL spectra of each stibonium−fluorostiborane pair were compared on excitation at the isosbestic points (Figure 3, right). While all stiboniums are poorly emissive (ΦPL ≈ 0), the corresponding fluorostiboranes showed significant emission, generating a turn-on response toward fluoride, observed in [I]+−[IV]+.12−14 In contrast to the absorption spectra, there is no distinctive difference in the emission band shapes of the fluorostiboranes and those of the free stibonium cations. Therefore, the emission band solely arises from the π−π* excited state of the peripheral donor (or fluorophore) moiety. We also concluded from these spectra that the ICT-excited state is nonemissive. Note that the PL spectrum of 3c-F exhibits the vibronic progression typically observed for carbazole derivatives.33,34,36 The emission lifetime (τ) of 3.9 ns for 3c-F confirms the singlet nature of the emission. Although all three stibonium cations exhibit turn-on fluorescence upon fluoride binding, the carbazole-based derivative 3c-F shows the most intense fluorescence and thereby displays the strongest OFF− ON fluorescence response. This finding is also consistent with the photoluminescence quantum yields (PLQYs) of the compounds: while the PLQYs of 3a-F and 3b-F are very low (ΦPL < 0.01), 3c-F is moderately fluorescent (ΦPL = 0.060). These fluorescent features of [3c]+ and 3c-F can also be observed in a rigid matrix such as a PMMA film (Figure S12 in the Supporting Information). This suggests that the nonemissive character of [3c]+ indeed originates from the aforementioned ICT process rather than from vibrational or solvent-induced nonradiative relaxations. [3c]+ was selected for further investigation. Fluoride Binding and Sensing Properties. The fluoride binding properties of [3c]+ were examined in detail using UV− vis and fluorescence titration experiments. The titration experiments were first carried out in MeCN (Figure 4). Upon addition of incremental amounts of fluoride, the lowenergy absorption band at 322 nm underwent gradual quenching. The absorption profile after the addition of 1 equiv of fluoride is identical with that of 3c-F (Figure 3c). The absence of additional changes upon addition of excess fluoride
Figure 3. UV−vis absorption (left) and PL (right) spectra of (a) [3a]OTf and 3a-F (5.0 × 10−5 M, λex 284 nm), (b) [3b]OTf and 3b-F (2.5 × 10−5 M, λex 312 nm), and (c) [3c]OTf and 3c-F (2.5 × 10−5 M, λex 301 nm) in MeCN.
Table 1. Photophysical Data of Methylstiboniums and Their Fluorostiboranes compd [3a]OTf 3a-F [3b]OTf 3b-F [3c]OTf 3c-F
λabs/nma 319, 323 287, 306 290, 291,
375 318 322, 334 323, 337
λem/nma,b
ΦPLa,c
361 362, 486 382 375 346, 361 346, 361
