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Article

Deboronation-Induced Ratiometric Emission Variations of Terphenyl-Based Closo-o-Carboranyl Compounds: Applications to Fluoride-Sensing

by
Hyunhee So
†,
Min Sik Mun
†,
Mingi Kim
†,
Jea Ho Kim
,
Ji Hye Lee
,
Hyonseok Hwang
,
Duk Keun An
* and
Kang Mun Lee
*
Department of Chemistry, Institute for Molecular Science and Fusion Technology, Kangwon National University, Chuncheon 24341, Korea
*
Authors to whom correspondence should be addressed.
†
These authors contributed equally to the work.
Molecules 2020, 25(10), 2413; https://doi.org/10.3390/molecules25102413
Submission received: 25 April 2020 / Revised: 20 May 2020 / Accepted: 21 May 2020 / Published: 21 May 2020
(This article belongs to the Special Issue Boron in Catalysis and Materials Chemistry: A Themed Issue in Honor of Professor Todd B. Marder on the Occasion of His 65th Birthday)
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Abstract

:
Closo-o-carboranyl compounds bearing the ortho-type perfectly distorted or planar terphenyl rings (closo-DT and closo-PT, respectively) and their nido-derivatives (nido-DT and nido-PT, respectively) were synthesized and fully characterized using multinuclear NMR spectroscopy and elemental analysis. Although the emission spectra of both closo-compounds exhibited intriguing emission patterns in solution at 298 and 77 K, in the film state, closo-DT mainly exhibited a π-π* local excitation (LE)-based emission in the high-energy region, whereas closo-PT produced an intense emission in the low-energy region corresponding to an intramolecular charge transfer (ICT) transition. In particular, the positive solvatochromic effect of closo-PT and theoretical calculation results at the first excited (S1) optimized structure of both closo-compounds strongly suggest that these dual-emissive bands at the high- and low-energy can be assigned to each π-π* LE and ICT transition. Interestingly, both the nido-compounds, nido-DT and nido-PT, exhibited the only LE-based emission in solution at 298 K due to the anionic character of the nido-o-carborane cages, which cannot cause the ICT transitions. The specific emissive features of nido-compounds indicate that the emissive color of closo-PT in solution at 298 K is completely different from that of nido-PT. As a result, the deboronation of closo-PT upon exposure to increasing concentrations of fluoride anion exhibits a dramatic ratiometric color change from orange to deep blue via turn-off of the ICT-based emission. Consequently, the color change response of the luminescence by the alternation of the intrinsic electronic transitions via deboronation as well as the structural feature of terphenyl rings indicates the potential of the developed closo-o-carboranyl compounds that exhibit the intense ICT-based emission, as naked-eye-detectable chemodosimeters for fluoride ion sensing.

Graphical Abstract

1. Introduction

Closo-ortho-carboranes (1,2-dicarba-closo-o-dodecaboranes, o-1,2-C2B10H12) are well-known boron-cluster components of three-dimensional (3D) icosahedral analogs. Recently, closo-ortho-carboranes have attracted significant attention as new molecular scaffolds of steric and electronic substituents for luminescent organic and organometallic compounds due to their unique photophysical properties and reasonable thermal and electrochemical stabilities originating from the o-carborane unit [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28]. These electronic features are imparted by the electron-withdrawing properties of the carbon atoms, and the high polarizability of the σ-aromaticity of the organic and organometallic luminophores that comprise the o-carborane moiety. These characteristics lead to the formation of electronic donor-acceptor dyad systems that induce intrinsic intramolecular charge transfer (ICT) transitions between the π-conjugated aromatic groups and the o-carborane cage [29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53]. Such ICT characteristics can induce unique luminescence behavior in various o-carborane-based organic luminophores [29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70]. Interestingly, such an intramolecular radiative mechanism activated by the ICT transitions in the o-carboranyl luminophores has been found amenable to modifications via variations to the structure of the o-carborane cages or appended aryl groups [32,43,44,45,46,47,48,49,50,51,52,53,70] and their molecular geometries [66,67,68,69]. Furthermore, the direct control of the ICT-based emission in the closo-o-carboranyl compounds involves the conversion of closo-o-carboranes to nido-o-species (o-1,2-C2B9H12−, one boron atom removed analog of the closo-o-carborane cage) by reaction with nucleophilic anions. This powerful process can cause dramatic changes in the inherent electronic environment because of the strong electron-donating property of nido-o-carboranes [71], leading to the alteration of their luminescent features [72,73,74,75,76,77,78]. For example, Carter et al. reported a fluorene-based dimer bearing an o-carborane, which exhibited a visible fluorescence change (orange to bright blue) by degradation to the nido-species [73]. Furthermore, Núñez et al. reported photoluminescent closo- and nido-di-carboranyl and tetra-carboranyl derivatives, which possessed intrinsic ICT electronic transitions and demonstrated differences in the emission band maxima of the two species [75]. We recently reported polyolefins bearing pendant o-carborane moieties that exhibit strong blue emissions in the solid state. Notably, the observed emissions disappeared after degradation of the carborane cage upon reaction with hydroxyl ions [76]. Additionally, 1,3,5-tris-(closo-o-carboranyl-methyl)benzene displayed ratiometric emissive color change via deboronation to the corresponding nido-o-species [77]. The degradation of the closo-o-carborane–triarylborane dyad to the nido-o-carboranyl compound exhibited a turn-on fluorescence response toward fluorides [78]. Thus, these closo-o-carboranyl derivatives exhibited great potential as polymeric or single-molecular chemodosimeters for sensing nucleophilic anions.
On the basis of inducing significant changes in the electronic properties through the conversion of closo-o-carborane to its anionic nido-o-species, we sought to investigate in detail the impact of the deboronation of the closo-o-carboranyl compounds on their ICT-based emission. For this study, we designed two simple terphenyl-based o-carboranyl compounds based on our previous results [69] (Figure 1). The first is 2′,5′-dimethyl-1,1′:4′,1′′-terphenyl-based closo-o-carborane (DT), with distorted o-terphenyl rings, which showed a weak ICT-based emission transition, and the second is 6,6,12,12-tetramethyl-6,12-dihydroindeno[1,2-b]fluorene-based closo-o-carboranyl compound (PT), with planar o-terphenyl rings, which possessed an intense emission from the ICT transition. Subsequently, the designed closo-o-compounds, as well as the nido-o-carboranyl compounds, were prepared and fully characterized. The comparison of the photophysical properties of these closo- and nido-compounds indicated that the deboronation of the o-carborane moiety and the structural feature of the appended terphenyl rings may deactivate the ICT transition and also quenching of the emission, thereby providing a novel method for fluoride sensing.

2. Results and Discussion

2.1. Synthesis and Characterization

The synthetic routes for the terphenyl-based closo-o- (closo-DT and closo-PT) and nido-o- (nido-DT and nido-PT) carboranyl compounds, where the o-carborane cages are substituted at both the ends by terphenyl moieties, are outlined in Figure 1. The Sonogashira coupling reaction between ethynyltrimethylsilane and the bromo-precursors DT1 and PT1 produced the ethynyltrimethylsilane-substituted terphenyl compounds DT2 and PT2, respectively, in high yields (62% for DT2 and 83% for PT2). The mild base (K2CO3)-mediated deprotection of the trimethylsilyl protecting groups of DT2 and PT2 furnished DT3 and PT3, respectively, which were then subjected to decaborane (B10H14)-promoted cage-forming reactions in the presence of Et2S (Figure 1) [79,80,81] to prepare the closo-o-carborane-substituted terphenyl compounds closo-DT and closo-PT, respectively. The dimethyl groups of closo-PT were introduced to achieve good solubility in a range of organic solvents. Subsequent treatment of closo-DT and closo-PT with excess n-tetrabutylammonium fluoride (NBu4F, TBAF) in THF at 60 °C led to the conversion of the closo-carboranes to the nido-species; nido-DT is the (NBu4)2-salt of the nido-form of closo-DT, and nido-PT is the (NBu4)2-salt of the nido-form of closo-PT (Figure 1).
All of the prepared closo- and nido-o-carboranyl compounds were fully characterized using multinuclear (1H{11B}, 13C, and 11B{1H}) NMR spectroscopy (Figures S1–S12 in the Supplementary Material) and elemental analysis. The 1H and 13C NMR spectra of closo-DT and closo-PT exhibited resonances corresponding to the terphenyl moieties. In addition, five broad singlet peaks were observed between −2 and −15 ppm in the 11B{1H} NMR spectra of both closo-DT and closo-PT, which confirmed the presence of the closo-o-carborane cage. Furthermore, signals were observed at ~78 and ~61 ppm in the 13C-NMR spectra, which were attributed to the two carbon atoms of the closo-o-carboranyl groups. Unlike the neutral closo-DT and closo-PT, the broad singlets (δ = −2.3 and −2.4 ppm) in the 1H{11B} NMR spectra of both nido-DT and nido-PT are characteristic of the B–H–B bridge protons of nido-o-carborane moieties. The 11B{1H} NMR signals of nido-DT and nido-PT at δ ca. −8 to −37 ppm, which are shifted significantly upfield due to the anionic character of the nido-o-carboranes, clearly confirmed the presence of the nido-o-carboranyl boron atoms.

2.2. Photophysical Properties of the Closo- and Nido-o-Carboranyl Compounds

The photophysical properties of all terphenyl-based closo- and nido-o-carboranyl were investigated using UV/Vis absorption and photoluminescence (PL) spectroscopies (Figure 2 and Table 1). The closo-o-carboranyl compounds, closo-DT and closo-PT, displayed major absorption bands at λabs = ~268 and 336 nm, respectively, with structureless vibronic features. These bands were attributed to spin-allowed π−π* LE transitions of the central terphenylene groups [82] and typical ICT transitions between the o-carborane units and the central phenyl rings (see the time-dependent density functional theory (TD-DFT) results vide infra). Indeed, these ICT-based low-energy absorption bands were not present in the spectra of nido-DT and nido-PT, due to which those absorption spectra were slightly blue-shifted (λabs = 254 and 323 nm, respectively, Table 1) compared with those of the closo-compounds. These findings imply that the deboronation of the o-carborane cages in the nido-species quenches the ICT transitions involving the o-carborane unit.
To gain insight into the intrinsic photophysical properties of all o-carboranyl compounds, the emissive properties of closo-o-compounds were examined by PL under a variety of conditions, and further, the emissions of nido-o-compounds in THF at 298 K were investigated (Figure 2 and Table 1). Although the PL spectra of both closo-DT and closo-PT in THF exhibited intriguing emission patterns in all states upon excitation at 292 and 345 nm, respectively, the closo-DT emission was focused in the high-energy region centered at λem = ~350 nm, whereas closo-PT exhibited an intense low-energy emission in the 500 to 600 nm range, which tailed off at 650 nm. With reference to the results of the TD-DFT computational study (vide infra), this high-energy emission appears to originate from the π–π* LE transitions of the central terphenyl moieties. In contrast, the low-energy emission is closely associated with ICT transitions between the o-carborane cages and the terphenyl rings. Furthermore, the emission spectrum of closo-DT in THF at 298 K exhibited an intense emission in the high-energy region at λem = 350 nm due to π−π* LE transitions based on the central phenyl rings. The fact that the high-energy emission band of closo-DT was consistently maintained in a variety of solvents of different polarities (λem = 349–350 nm, Table 1 and Figure S14a in the Supplementary Material) and that the low-energy emission of closo-PT was dramatically altered (Table 1 and Figure S14b), strongly indicates that closo-DT and closo-PT exhibit LE- and ICT-based emissive characteristics, respectively. These intriguing features are clear evidence that the planarity of the terphenyl rings plays an important role in the alternation of the intramolecular electronic transitions as well as the corresponding radiative decay mechanism [69]. Moreover, closo-DT exhibited only a trace ICT-based emission in solution (THF solution at 298 K), and the PL spectra in the rigid state (THF at 77 K and in the film state, i.e., 5 wt% doped on poly(methyl methacrylate) (PMMA)) showed an enhanced low energy emission (λem = 482 nm in THF at 77 K and λem = 492 nm in the film) that tailed to 550 nm. The emission band for closo-PT around 500 nm was also significantly increased in the rigid state (THF at 77 K and in film), indicating that the electronic transition for both closo-compounds are governed by non-radiative process in solution state at 298 K. This behavior originates from the increased efficiency of the radiative decay associated with the ICT transition in the rigid molecular state, which restricts structural fluctuations such as C–C bond variations in the o-carborane cage [9,38,66,67,68,69]. In addition, the PL spectra of the two nido-o-compounds in THF at 298 K exhibited identical emission patterns in the high-energy region (λem = 343 nm for nido-DT and 390 nm for nido-PT, respectively) alone, and each spectrum of the closo-compounds corresponded to the terphenyl-centered π−π* LE transition. Accordingly, these phenomena demonstrate that the CT-based emission can be quenched by the anionic character of nido-o-carborane as well as the distortion of the terphenyl rings, which inhibits the ICT transitions. Such features suggest that closo-o-carboranyl compounds that exhibit the intense ICT-based emission, such as closo-PT, can cause dramatic emission color changes via deboronation of the o-carborane cage, owing to the interruption of the ICT transition corresponding to the o-carborane and conservation of the LE transition. This phenomenon was verified by spectral changes in the emission of closo-PT in the presence of TBAF (vide infra).

2.3. Computational Chemistry and Orbital Analyses for Closo-o-Carboranyl Compounds

To elucidate the nature of the electronic transitions and to analyze the orbitals of closo-DT and closo-PT, their S0- and S1-optimized structures were subjected to TD-DFT calculations using the B3LYP functional (Figure 3 and Table 2). To include the effects of the THF solvent [83,84], a conductor-like polarizable continuum model was chosen. The computational data for the S0 state showed that HOMO → LUMO transitions are the major lowest-energy electronic transitions in both closo-o-carboranyl compounds. The HOMO of each compound is entirely localized on the central terphenyl group (>96%; Tables S2 and S4 in the Supplementary Material), whereas the orbital contribution of the o-carborane unit to each LUMO is slightly higher, at >16%. These results indicate that the lowest-energy absorptions of both closo-compounds are attributable to the π−π* transitions on the central terphenyl moieties, with minor contributions from the ICT transitions between the o-carborane and terphenyl groups as well. All calculated results based on the optimized S0 structures are in good agreement with the experimentally observed UV/Vis absorption spectra.
In contrast, the calculated results for the S1 states of closo-DT and closo-PT indicate that the major transitions associated with the low-energy emissions involve both HOMO → LUMO and HOMO → LUMO+1 transitions (Figure 3 and Table 2). Although the LUMO of each compound is significantly localized on the o-carborane moiety (∼80%; Tables S2 and S4), each HOMO is predominantly located on the central terphenyl group (>92%). These results strongly suggest that the experimentally observed emissions in the low-energy regions mainly originate from ICT transitions between the o-carborane and terphenyl moieties. In addition, each LUMO+1 is mainly located on the central terphenyl group (>86%; Tables S2 and S4), strongly indicating that the intense emissions observed in the high-energy region, centered at ~350 nm for closo-DT and ~370 nm for closo-PT, originate from π−π* transitions in the terphenyl moieties, i.e., LE-based emissions. Consequently, the electronic transitions that occur in each o-carboranyl compound were precisely predicted using computational methods.

2.4. Emission-Color Changes of Closo-o-Carboranyl Compounds Via Treatment of Fluoride Anion

Finally, to clarify the changes in the photoluminescence properties exhibited during the conversion of both closo-DT and closo-PT to the nido-species, we investigated the changes in the emissive patterns of both closo-compounds as a function of increasing amounts of TBAF in THF. These conversion processes of both closo-compounds to the respective nido-species by reaction with the fluoride anion occur consecutively, as clearly evidenced from the changes in the specific peaks of the 1H-NMR spectra in THF-d8 (Figure 4). The aryl protons of both closo-compounds in the region from 8.0 to 7.0 ppm shifted steadily to the upfield region upon increasing the concentration of TBAF, and finally, these peaks merged with the corresponding peaks in the spectra of each nido-compound in THF-d8, respectively. In particular, the broad singlet peaks around δ = −2.0 and −2.5 ppm, which were assigned to the B–H–B bridge protons of the nido-o-carborane, could be gradually monitored by increasing the concentration of TBAF. The results of 1H-NMR spectral changes indicate that the conversion of the closo-compounds to the nido-species almost reached full conversion to that of corresponding pure nido-compounds when 5 equivalents of TBAF was used for the deboronation process.
As illustrated in Figure 5, upon addition of incremental amounts of TBAF (0–5 equivalents) into the respective solutions of closo-DT and closo-PT, followed by heating at 60 °C for 2 h, the LE-based emission for closo-DT (λem = ~350 nm) did not change significantly, whereas the ICT-based emission for closo-PT (λem = ~550 nm) underwent gradual quenching, and eventually, a slightly enhanced LE-based emission (λem ≈ 380–410 nm) remained. In particular, the emission intensities and band shapes of each closo-compound after treatment with 5 equivalents of TBAF were mostly similar to those (Figure 5, red-solid lines) of the nido-compounds. Consequently, the conversion of closo-PT to nido-PT exhibited a vivid emission color change from orange to deep-blue (insets in Figure 5b), whereas closo-DT did not display any color changes from the emission in spite of the deboronation (insets in Figure 5a). These results demonstrate that degradation to the nido-form can not only prevent the ICT transition in the o-carboranyl compounds, but also reinforce the π-π*–LE transition, which induces the emission color changes. Consequently, the luminescence-based color change response due to the alternation of the intrinsic electronic transitions caused by the reaction with fluoride anion and the structural feature of central terphenyl groups, indicates the potential of closo-PT as a naked-eye-detectable chemodosimeter for fluoride ion sensing.

3. Materials and Methods

3.1. General Considerations

All operations were performed under an inert nitrogen atmosphere using standard Schlenk and glove-box techniques. Anhydrous solvents (toluene, trimethylamine (NEt3), and methanol; Aldrich) were dried by passing through an activated alumina column and stored over activated molecular sieves (5 Å). Spectrophotometric-grade solvents (tetrahydrofuran (THF), toluene, dichloromethane (DCM), methanol, and n-hexane) were used as received from Alfa Aesar (Ward Hill, MA, USA). Commercial reagents were used without any further purification after purchase from Sigma-Aldrich (potassium carbonate (K2CO3), magnesium sulfate (MgSO4) St. Louis, MO, USA), bis(triphenylphosphine)palladium(II) dichloride (Pd(PPh3)2Cl2), copper(I) iodide (CuI), diethyl sulfide (Et2S), ethynyltrimethylsilane, and poly(methyl methacrylate) (PMMA)). Decaborane (B10H14) was purchased from Alfa Aesar. The dibromo precursors, 4,4''-dibromo-2',5'-dimethyl-1,1':4′,1''-terphenyl (DT1) and 2,8-dibromo-6,6,12,12-tetramethyl-6,12-dihydroindeno[1,2-b]fluorene (PT1), were prepared as reported in the literature [69]. CD2Cl2 and THF-d8, purchased from Cambridge Isotope Laboratories, were dried over activated molecular sieves (5 Å). All nuclear magnetic resonance (NMR) spectra were recorded on a Bruker Avance 400 spectrometer (400.13 MHz for 1H, 100.62 MHz for 13C, and 128.38 MHz for 11B, Bruker, Billerica, MA, USA) at ambient temperature. Chemical shifts are given in ppm and are referenced against external Me4Si (1H and 13C) or BF3·Et2O (11B). Elemental analysis was performed on an EA3000 instrument (Eurovector) at the Central Laboratory of Kangwon National University. UV–Vis absorption and photoluminescence (PL) spectra were recorded on Jasco V-530 (Jasco, Easton, MD, USA) and Horiba FluoroMax-4P spectrophotometers (HORIBA, Edison, NJ, USA), respectively. Fluorescence decay lifetimes (τobs) were measured using a time-correlated single-photon counting spectrometer (FLS920, at the Central Laboratory of Kangwon National University, Edinburgh Instruments Ltd., Livingston, UK) equipped with an EPL 375 ps pulsed semiconductor diode laser as the excitation source and a microchannel plate photomultiplier tube (200–850 nm) as the detector, at 298 K. The absolute PL quantum yields (Фem) were obtained with an absolute PL quantum yield spectrophotometer (HORIBA FluoroMax-4P equipped with an FM-SPHERE 3.2-inch internal integrating sphere, HORIBA, Edison, NJ, USA) at 298 K.

3.2. Synthesis of DT2

Triethylamine (16 mL) was added via cannulation to a mixture of DT1 (0.42 g, 1.0 mmol), copper iodide (15 mg), and Pd(PPh3)2Cl2 (62 mg) at 25 °C. After stirring for 15 min, ethynyltrimethylsilane (0.55 mL, 4.0 mmol) was added, and the reaction mixture was heated at 90 °C with stirring for 24 h. After cooling to 25 °C, the volatiles were removed by rotary evaporation to afford a dark brown residue. The crude product was purified by column chromatography on silica gel (eluent: DCM/n-hexane = 1/10, v/v) to yield DT2 as a yellow solid, 0.28 g (yield = 62%). 1H-NMR (CD2Cl2): δ 7.51 (d, J = 8.3 Hz, 4H), 7.32 (d, J = 8.4 Hz, 4H), 7.13 (s, 2H), 2.25 (s, 6H, –CH3), 0.27 (s, 18H, –Si(CH3)3). 13C-NMR (CD2Cl2): δ 142.35, 140.71, 133.04, 132.05, 131.97, 129.60, 121.97, 105.26 (acetylene-C), 94.83 (acetylene-C), 20.01 (–CH3), 0.03 (–Si(CH3)3). Anal. Calcd. for C30H34Si2: C, 79.94; H, 7.60. Found: C, 79.87; H, 7.49.

3.3. Synthesis of PT2

PT2 was prepared according to a procedure analogous to that used for DT2, with PT1 (0.47 g, 1.0 mmol), copper iodide (15 mg), Pd(PPh3)2Cl2 (62 mg), and ethynyltrimethylsilane (0.55 mL, 4.0 mmol), and was isolated as a yellow solid (0.42 g; yield = 83%). 1H-NMR (CD2Cl2): δ 7.78 (s, 2H), 7.71 (d, J = 7.9 Hz, 2H), 7.55 (s, 2H), 7.45 (d, J = 7.8 Hz, 2H), 1.53 (s, 12H, –CH3), 0.27 (s, 18H, –Si(CH3)3). 13C-NMR (CD2Cl2): δ 154.55, 154.16, 140.07, 138.92, 131.40, 126.63, 121.79, 120.08, 114.99, 106.24 (acetylene-C), 94.53 (acetylene-C), 46.96 (–C(CH3)2), 27.37 (–CH3), 0.07 (–Si(CH3)3). Anal. Calcd. for C34H38Si2: C, 81.21; H, 7.62. Found: C, 80.99; H, 7.55.

3.4. Synthesis of DT3

K2CO3 (0.28 g, 2.0 mmol) was dissolved in methanol (10 mL) and added to a solution of DT2 (0.23 g, 0.5 mmol) in DCM (5 mL). After stirring for 2 h at 25 °C, the resulting mixture was treated with DCM (50 mL) and the organic layer was separated. The aqueous layer was further extracted with DCM (20 × 2 mL). The combined organic extracts were dried over MgSO4, filtered, and evaporated to dryness to afford a white residue. The crude product was purified by washing with n-hexane (10 mL) to yield DT3 as a white solid, 0.13 g (yield = 84%). 1H-NMR (CD2Cl2): δ 7.56 (d, J = 8.0 Hz, 4H), 7.34 (d, J = 8.0 Hz, 4H), 7.13 (s, 2H), 3.18 (s, 2H, –CCH), 2.26 (s, 6H, –CH3). 13C-NMR (CD2Cl2): δ 142.68, 140.68, 133.06, 132.24, 132.07, 129.66, 120.90, 83.81 (acetylene-C), 77.69 (acetylene-C), 20.01 (–CH3). Anal. Calcd. for C24H18: C, 94.08; H, 5.92. Found: C, 93.77; H, 5.62.

3.5. Synthesis of PT3

PT3 was prepared according to a procedure analogous to that used for DT3 with PT2 (0.40 g, 0.8 mmol) and K2CO3 (0.44 g, 3.2 mmol), and was isolated as a white solid (0.25 g; yield = 88%). 1H-NMR (CD2Cl2): δ 7.80 (s, 2H), 7.74 (d, J = 7.9 Hz, 2H), 7.59 (s, 2H), 7.50 (d, J = 7.8 Hz, 2H), 3.20 (s, 2H, –CCH), 1.54 (s, 12H, –CH3). 13C-NMR (CD2Cl2): δ 154.59, 154.15, 140.34, 138.91, 131.64, 126.88, 120.72, 120.15, 115.06, 84.70 (acetylene-C), 77.44 (acetylene-C), 46.98 (–C(CH3)2), 27.36 (–CH3). Anal. Calcd. for C28H22: C, 93.81; H, 6.19. Found: C, 93.77; H, 6.04.

3.6. Synthesis of closo-DT

Excess Et2S (2.5 equiv., 1.2 mmol) was added at 25 °C to a solution of decaborane (B10H14, 0.52 mmol) and DT3 (61 mg, 0.20 mmol) in toluene (20 mL). After heating to reflux, the reaction mixture was further stirred for 72 h. The solvent and volatiles were removed under vacuum and methanol (10 mL) was added. The resulting solid was filtered and redissolved in toluene. The crude product upon washing with n-hexane (15 mL), afforded closo-DT as a white solid (47 mg. Yield = 43%). 1H{11B} NMR (THF-d8): δ 7.66 (d, J = 8.2 Hz, 4H), 7.38 (d, J = 8.1 Hz, 4H), 7.12 (s, 2H), 5.13 (s, 2H, CB-CH), 2.54 (br s, 8H, CB-BH), 2.39 (br s, 3H, CB-BH), 2.30 (br s, 9H, CB-BH), 2.23 (s, 6H, –CH3). 13C-NMR (THF-d8): δ 144.05, 140.59, 133.27, 133.24, 132.34, 130.14, 128.04, 77.58 (CB-C), 61.49 (CB-C), 19.74 (–CH3). 11B{1H} NMR (THF-d8): δ −4.44 (3B), −6.57 (1B), −10.84 (5B), −12.77 (7B), −14.67 (4B). Anal. Calcd. for C28H38B20: C, 56.92; H, 6.48. Found: C, 56.79; H, 6.33.

3.7. Synthesis of closo-PT

Closo-PT was prepared according to a procedure analogous to that used for closo-DT, with decaborane (B10H14, 0.52 mmol), PT3 (78 mg, 0.20 mmol), and Et2S (2.5 equiv.). The crude product upon washing with n-hexane (15 mL), afforded closo-PT as a white solid (42 mg, Yield = 35%). 1H{11B} NMR (THF-d8): δ 7.94 (s, 2H), 7.81 (d, J = 8.1 Hz, 2H), 7.67 (s, 2H), 7.58 (d, J = 7.9 Hz, 2H), 5.14 (s, 2H, CB-CH), 2.56 (br s, 7H, CB-BH), 2.50 (br s, 1H, CB-BH), 2.39 (br s, 2H, CB-BH), 2.30 (br s, 10H, CB-BH), 1.54 (s, 12H, –CH3). 13C-NMR (THF-d8): δ 155.41, 154.80, 141.67, 138.98, 133.36, 127.39, 122.52, 120.66, 115.65, 78.45 (CB-C), 61.55 (CB-C), 47.50 (–C(CH3)2), 27.08 (–CH3). 11B{1H} NMR (THF-d8): δ −2.73 (3B), −4.64 (1B), −9.09 (5B), −10.77 (7B), −12.86 (4B). Anal. Calcd. for C32H42B20: C, 59.79; H, 6.59. Found: C, 59.87; H, 6.45.

3.8. Synthesis of nido-DT

Closo-DT (0.027 g, 0.05 mmol) was dissolved in 0.3 mL of a 0.2 M solution of n-tetrabutylammonium fluoride (TBAF) in THF at 25 °C. The reaction mixture was heated to reflux (60 °C) and stirred for 2 h. After cooling to 25 °C, the resulting mixture was treated with 50mL of distilled water and 50 mL of DCM, and the organic portion was separated. The aqueous layer was further extracted with DCM (20 mL). The combined organic portions were dried over MgSO4, filtered, and concentrated to dryness, affording a pale yellow residue. The crude product upon washing with methanol (15 mL), afforded nido-DT as a white solid (26 mg, Yield = 52%). 1H{11B} NMR: δ 7.28 (d, J = 8.0 Hz, 4H), 7.13 (d, J = 7.9 Hz, 4H), 7.07 (s, 2H), 3.11 (m, 16H, n-butyl-CH2), 2.36 (s, 2H, CB-CH), 2.25 (s, 6H, –CH3), 2.12 (br s, 4H, CB-BH), 1.88 (br s, 4H, CB-BH), 1.82 (br s, 4H, CB-BH), 1.62 (m, 16H, n-butyl-CH2), 1.43 (m, 16H, n-butyl-CH2), 1.26 (br s, 6H, CB-BH), 1.02 (t, J = 7.2 Hz, 24H, n-butyl-CH3), −2.36 (br s, 2H, B-H-B). 13C NMR (CD2Cl2): δ 144.65, 140.78, 138.40, 132.83, 132.21, 128.64, 126.71, 59.43 (n-butyl-CH2), 24.29 (n-butyl-CH2), 20.18 (–CH3), 20.10 (n-butyl-CH2), 13.76 (n-butyl-CH3). 11B{1H} NMR (CD2Cl2): δ −8.97 (3B), −10.43 (2B), −13.79 (1B), −18.28 (3B), −19.50 (1B), −23.00 (1B), −32.95 (3B), −36.10 (4B). Anal. Calcd. for C60H110B18N2: C, 68.37; H, 10.52; N, 2.66. Found: C, 68.11; H, 10.42; N, 2.54.

3.9. Synthesis of nido-PT

A procedure analogous to that for nido-DT was employed using closo-PT (0.027 g, 0.04 mmol) and 0.23 mL of a 0.2 M solution of TBAF in THF. The crude product, upon washing with methanol (15 mL), afforded nido-PT as a white solid (26 mg, Yield = 60%). 1H{11B} NMR (CD2Cl2): δ 7.65 (s, 2H), 7.49 (d, J = 7.9 Hz, 2H), 7.32 (s, 2H), 7.23 (d, J = 7.9 Hz, 2H), 3.08 (m, 16H, n-butyl-CH2), 2.39 (s, 2H, CB-CH), 2.12 (br s, 4H, CB-BH), 2.00 (br s, 1H, CB-BH), 1.89 (br s, 5H, CB-BH), 1.60 (m, 16H, n-butyl-CH2), 1.48 (s, 12H, –CH3), 1.41 (m, 16H, n-butyl-CH2), 1.31 (br s, 4H, CB-BH), 1.26 (br s, 4H, CB-BH), 1.00 (t, J = 7.2 Hz, 24H, n-butyl-CH3), −2.34 (br s, 2H, B-H-B). 13C-NMR (CD2Cl2): δ 153.80, 153.54, 145.25, 138.62, 136.55, 126.10, 121.40, 118.72, 114.00, 59.40 (n-butyl-CH2), 46.71 (–C(CH3)2), 27.74 (–CH3), 24.27 (n-butyl-CH2), 20.09 (n-butyl-CH2), 13.75 (n-butyl-CH3). 11B{1H} NMR (CD2Cl2): δ −8.91 (3B), −10.45 (2B), −13.68 (1B), −18.46 (3B), −19.47 (1B), −23.09 (1B), −32.95 (3B), −36.05 (4B). Anal. Calcd. for C64H114B18N2: C, 69.49; H, 10.39; N, 2.53. Found: C, 69.30; H, 10.16; N, 2.39.

3.10. UV/Vis Absorption and Photoluminescence (PL) Experiments

The solution-phase UV–Vis absorption and PL measurements of the closo- and nido-o-carbornyl compounds were performed in degassed organic solvents with a 1 cm quartz cuvette (3.0 × 10−5 M) at 298 K. PL measurements for the closo-compounds were also performed in THF at 77 K and in the film state (5 wt% doped in PMMA) on 1.5 × 1.5 cm quartz plates (thickness = 1 mm) at 298 K.

3.11. Computational Studies

The optimized geometries for the ground (S0) and first excited (S1) states of both closo-o-carboranyl compounds (closo-DT and closo-PT) in THF were obtained using the B3LYP/6-31G(d,p) [85] level of theory. The vertical excitation energies at the optimized S0 geometries as well as the optimized geometries of the S1 states were calculated using time-dependent density functional theory (TD-DFT) [86] at the same level of theory. Solvent effects were included using the conductor-like polarizable continuum model (CPCM) [83,84]. All geometry optimizations were performed using the Gaussian 16 program [87]. The percent contribution of a group in a molecule to each molecular orbital was calculated with the GaussSum 3.0 program [88]. Visualizations were prepared using GaussView 6 [89].

4. Conclusions

We herein reported the preparation and characterization of distorted and planar terphenyl-based closo- (closo-DT and closo-PT) and nido- (nido-DT and nido-PT) o-carboranyl compounds. Although closo-DT exhibited strong π–π* LE-based emission in THF at 298 K in the high-energy region, closo-PT demonstrated intense emission in the low-energy region that was attributable to the ICT transitions involving the o-carborane cage. Interestingly, both nido-compounds exhibited LE-based emission alone in the same condition due to the anionic character of the nido-o-carborane cages, which cannot cause the ICT transitions. Consequently, the successful deboronation of closo-PT to nido-PT upon exposure to increasing concentration of fluoride anion leads to ratiometric emission color change from orange to deep-blue in solution. Such results strongly imply that the fine-tuning of electronic and structural features, which can control the ICT-based emission, shows the potential of closo-o-carboranyl compounds as candidates for naked-eye-detectable chemodosimeters for fluoride ion-sensing.

Supplementary Materials

The following are available online. Multinuclear NMR spectra (1H, 13C, and 11B) of the closo- and nido-o-carboranyl compounds and their precursors (Figures S1–S12), and photophysical (Figures S13–S14) and computational data (Figures S15–S16 and Tables S1–S8).

Author Contributions

H.S., M.S.M., M.K., and J.H.K. synthesized the compounds and analyzed the data. D.K.A. and K.M.L. analyzed the data and wrote the paper. J.H.L. and H.H. conducted the computational study, analyzed the data, and wrote the paper. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Research Foundation of Korea (NRF) grant (NRF-2016M3A7B4909246, NRF-2017R1D1A1B03035412, NRF-2018R1D1A1B07040387, and NRF-2020R1A2C1006400) funded by the Ministry of Science and ICT and the Ministry of Education.

Acknowledgments

We thank Seonah Kim and Chan Hee Ryu (Department of Chemistry, Institute for Molecular Science and Fusion Technology, Kangwon National University) for assistance with design of synthetic routes for o-carboranyl compounds.

Conflicts of Interest

The authors declare no conflicts of interest.

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Sample Availability: Samples of the o-carboranyl compounds are available from the authors.
Molecules 25 02413 g001 550
Figure 1. Synthetic routes to the terphenyl-based closo- and nido-o-carboranyl complexes, closo-DT, closo-PT, nido-DT, and nido-PT. Reaction conditions: (i) Ethynyltrimethylsilane, CuI, Pd(PPh3)2Cl2, NEt3/toluene, r.t., 24 h. (ii) K2CO3, methanol, r.t., 2 h. (iii) B10H14, Et2S, toluene, 110 °C, 72 h. (iv) n-tetrabutylammonium fluoride (TBAF), THF, 60 °C, 2 h.
Figure 1. Synthetic routes to the terphenyl-based closo- and nido-o-carboranyl complexes, closo-DT, closo-PT, nido-DT, and nido-PT. Reaction conditions: (i) Ethynyltrimethylsilane, CuI, Pd(PPh3)2Cl2, NEt3/toluene, r.t., 24 h. (ii) K2CO3, methanol, r.t., 2 h. (iii) B10H14, Et2S, toluene, 110 °C, 72 h. (iv) n-tetrabutylammonium fluoride (TBAF), THF, 60 °C, 2 h.
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Figure 2. UV–Vis absorption and photoluminescence (PL) spectra for (a) closo- and nido-DT (λex = 292 nm) and (b) closo- and nido-PT (λex = 345 nm). Black-solid: absorption spectra in THF (30 μM) for closo-species. Black-dash: absorption spectra in THF (30 μM) for nido-species. Blue-solid: PL spectra in THF (30 μM) at 298 K for closo-species. Blue-dash: PL spectra in THF (30 μM) at 298 K for nido-species. Green-solid: PL spectra in THF (30 μM) at 77 K for closo-species. Orange-solid: PL spectra of the films (5 wt% doped on PMMA) at 298 K for closo-species. Inset figures show the emission color in each state of closo-species under irradiation by a hand-held UV lamp (λex = 295 nm for closo-DT and 365 nm for closo-PT).
Figure 2. UV–Vis absorption and photoluminescence (PL) spectra for (a) closo- and nido-DT (λex = 292 nm) and (b) closo- and nido-PT (λex = 345 nm). Black-solid: absorption spectra in THF (30 μM) for closo-species. Black-dash: absorption spectra in THF (30 μM) for nido-species. Blue-solid: PL spectra in THF (30 μM) at 298 K for closo-species. Blue-dash: PL spectra in THF (30 μM) at 298 K for nido-species. Green-solid: PL spectra in THF (30 μM) at 77 K for closo-species. Orange-solid: PL spectra of the films (5 wt% doped on PMMA) at 298 K for closo-species. Inset figures show the emission color in each state of closo-species under irradiation by a hand-held UV lamp (λex = 295 nm for closo-DT and 365 nm for closo-PT).
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Figure 3. Frontier molecular orbitals of closo-DT and closo-PT in their ground states (S0) and first excited singlet states (S1), and their relative energies calculated by DFT (isovalue = 0.04). The transition energy (in nm) was calculated using the TD-B3LYP/6-31G(d) level of theory.
Figure 3. Frontier molecular orbitals of closo-DT and closo-PT in their ground states (S0) and first excited singlet states (S1), and their relative energies calculated by DFT (isovalue = 0.04). The transition energy (in nm) was calculated using the TD-B3LYP/6-31G(d) level of theory.
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Figure 4. 1H-NMR spectral changes of (a) closo-DT and (b) closo-PT upon increasing the amount of added fluoride anions and comparison with those of nido-DT and nido-PT (∗ from residual THF in THF-d8, † from n-butyl group of excess TBAF, and + from n-butyl group for each nido-compound).
Figure 4. 1H-NMR spectral changes of (a) closo-DT and (b) closo-PT upon increasing the amount of added fluoride anions and comparison with those of nido-DT and nido-PT (∗ from residual THF in THF-d8, † from n-butyl group of excess TBAF, and + from n-butyl group for each nido-compound).
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Figure 5. Spectral changes in the emission of (a) closo-DT (3.0 × 10−5 M, λex = 292 nm) and (b) closo-PT (3.0 × 10−5 M, λex = 345 nm) in THF in the presence of different amounts of TBAF, upon heating at 60 °C for 2 h. Insets are photographs of each closo- and nido-type (3.0 × 10−5 M in THF) under a UV lamp (λex = 295 nm for DT derivatives and 365 nm for PT derivatives).
Figure 5. Spectral changes in the emission of (a) closo-DT (3.0 × 10−5 M, λex = 292 nm) and (b) closo-PT (3.0 × 10−5 M, λex = 345 nm) in THF in the presence of different amounts of TBAF, upon heating at 60 °C for 2 h. Insets are photographs of each closo- and nido-type (3.0 × 10−5 M in THF) under a UV lamp (λex = 295 nm for DT derivatives and 365 nm for PT derivatives).
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Table
Table 1. Absorption and emission data for terphenyl-based o-carboranyl compounds.
Table 1. Absorption and emission data for terphenyl-based o-carboranyl compounds.
Compoundλabs1/nm
(ε × 10−3 M−1 cm−1)		λex/nmλem/nm
Tol 2THF 2DCM 277 K 1Film 3
closo-DT268 (42.6)292349350349343, 482345, 492(sh)
nido-DT254 (37.8)292-343---
closo-PT336 (84.8)345376, 521374, 549375, 560374, 514375, 524
nido-PT323 (38.2)345-390---
1c = 30 μM in THF. 2 c = 30 μM, observed at 298 K. 3 Measured in the film state (5 wt% doped on PMMA) at 298 K.
Table
Table 2. Major low-energy electronic transitions in closo-DT and closo-PT involving their ground states (S0) and first excited singlet states (S1) calculated using the TD-B3LYP/6-31G(d) level of theory 1.
Table 2. Major low-energy electronic transitions in closo-DT and closo-PT involving their ground states (S0) and first excited singlet states (S1) calculated using the TD-B3LYP/6-31G(d) level of theory 1.
Stateλcalc/nmfcalcAssignment
closo-DTS0285.71.2315HOMO → LUMO (98.0%)
S1509.370.592HOMO → LUMO (99.6%)
359.140.2721HOMO → LUMO+1 (87.7%)
closo-PTS0348.171.7222HOMO → LUMO (98.8%)
S1554.440.9435HOMO → LUMO (99.7%)
371.340.4002HOMO → LUMO+1 (78.9%)
1 Singlet energies for vertical transitions were calculated using optimized S1 geometries.

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So, H.; Mun, M.S.; Kim, M.; Kim, J.H.; Lee, J.H.; Hwang, H.; An, D.K.; Lee, K.M. Deboronation-Induced Ratiometric Emission Variations of Terphenyl-Based Closo-o-Carboranyl Compounds: Applications to Fluoride-Sensing. Molecules 2020, 25, 2413. https://doi.org/10.3390/molecules25102413

AMA Style

So H, Mun MS, Kim M, Kim JH, Lee JH, Hwang H, An DK, Lee KM. Deboronation-Induced Ratiometric Emission Variations of Terphenyl-Based Closo-o-Carboranyl Compounds: Applications to Fluoride-Sensing. Molecules. 2020; 25(10):2413. https://doi.org/10.3390/molecules25102413

Chicago/Turabian Style

So, Hyunhee, Min Sik Mun, Mingi Kim, Jea Ho Kim, Ji Hye Lee, Hyonseok Hwang, Duk Keun An, and Kang Mun Lee. 2020. "Deboronation-Induced Ratiometric Emission Variations of Terphenyl-Based Closo-o-Carboranyl Compounds: Applications to Fluoride-Sensing" Molecules 25, no. 10: 2413. https://doi.org/10.3390/molecules25102413

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