Through space charge-transfer emission in lambda (Λ)-shaped triarylboranes and the use in fluorescent sensing for fluoride and cyanide ions

Abstract

A new class of Λ-shaped triarylboranes 2-(4-(N,N-dimethylamino)-8-dimesitylboryl-6H,12H-5,11-methanodibenzo[b,f][1,5] diazocine (TBBN) and 2-(4-(N,N-diphenylamino)-8-dimesitylboryl-6H,12H-5,11-methanodibenzo[b,f][1,5]diazocine (TBBN2), incorporating different electron-donating amino groups and an electron-accepting dimesitylboryl group through a rigid Λ-shaped Tröger's base linker were designed and synthesized. The compounds display twisted structures and effective intramolecular change-transfer transitions. The twisted nonplanar arrangement of the chromophores on the one hand suppresses the fluorescence quenching in the aggregated states, and on the other hand produces a through-space donor–acceptor charge transfer. As a result, dual fluorescent pathways, namely through-space charge transfer from the amino group to the dimesitylboryl group, and the π*–π transitions located on the amino groups, are observed to coexist in each molecule. The dual emissions can be selectively switched on or off by addition of fluoride or cyanide ions. Thus the dyes can be used as “switch-on” probes. The complexation of TBBN and TBBN2 with fluoride or cyanide ions induces dramatic blue shifts (about 72–140 nm) and color changes in the fluorescence, making them potential visually colorimetric and ratiometric sensors for fluoride and cyanide ions.

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Introduction

Triarylboron compounds are known as an important class of optoelectronic materials because of their intriguing electronic and optical properties, which arise from the empty p_π orbital on the boron center.¹ When combined with organic π-conjugated systems such as mesityl (2,4,6-trimethylphenyl), the boron center can act as an excellent π-electron acceptor, and further promotes highly polarized electronic transitions from the π* orbital of π-conjugated systems to the unoccupied p orbital on the boron atom.^1a–e This unique electronic property of triarylboranes has been exploited extensively for materials applications such as nonlinear optical materials,² two-photon excited emitters,³ emissive and electron-transport materials for organic light-emitting diodes (OLEDs),^3e,4 hydrogen activation and storage,⁵ and saccharide detection.⁶ In particular, owing to the steric hindrance of the large aryl substituents on the boron center, the triarylboron compounds have also been demonstrated to function as highly selective and sensitive chemosensors for the detection of fluoride and cyanide anions over other anions,⁷ both of which are small anions and play important roles in human health and environmental treatment.⁸

Most of the reported triarylboron compounds which produce intense donor–acceptor charge transfer (CT) emission through the aromatic linker generally have linear or planar conjugated geometries.^4c,9 This kind of compound typically acts as a “switch-off” fluorescent sensor when ions bind to the boron center. Very recently, Wang et al. developed a series of novel U- and V-shaped branched triarylboron compounds for use as effective “switch-on” sensors, in which the donor and acceptor groups were spatially separated by either a rigid naphthyl or non-rigid organosilicon linker with non-conjugated structures.¹⁰ As a result, dual emission pathways, namely through-space donor–acceptor intramolecular CT fluorescence and π–π* fluorescence were observed. The dual emission can be reversibly and selectively switched on or off by the addition of fluoride ions. The switchable dual emissive organoboron materials have various potential applications, especially as fluorescent sensors. To design such a charge transfer but non-conjugated triarylboron system, the selectivity of a proper linker is very important. As to this meaning, a severely twisted and nonplanar scaffold, Tröger's base, will serve as the linker. A molecular design idea is to attach the amino donor and boryl acceptor to the two sides of a Tröger's base scaffold.

Tröger's base (TB) (Fig. 1), an old compound first synthesized by Julius Tröger in 1887,¹¹ has gained much attention in recent years because of its inherent chirality, C₂ symmetry and particularly rigid, concave Λ-shaped framework.¹² In principle, this special Λ-shaped configuration will provide a nonplanar configuration to the amino donor and the boryl acceptor groups, which might further lead to an intramolecular charge transfer that occurs through space rather than through the Λ-shaped bridge. With these considerations, we designed and synthesized two donor–acceptor triarylboron molecules based on TB (namely TBBN and TBBN2, as shown in Scheme 1) incorporating an amino group and a boryl group on the two separate sides of the Λ-shape TB scaffold. Furthermore, to validate the occurrence of space charge transfer from donor to acceptor, the acceptor-only molecule TBB reported recently by our group¹³ and the donor-only Λ-shaped molecule TBNN were also included for comparison. The photophysical properties of these Λ-shaped triarylboron compounds, and their applications as fluorescent sensors were investigated. The results confirmed that upon complexation with fluoride or cyanide anions, TBBN and TBBN2 do show ratiometric fluorescence changes with significant emission shifts, while TBB behaves as a “switch-off” sensor.

Fig. 1 Molecular structure of a Tröger's base.

Scheme 1 Synthetic routes to the compounds TBB, TBBN, TBBN2 and TBNN.

Results and discussion

Synthesis

The synthesis of the two Λ-shaped triarylboron compounds TBBN and TBBN2 is depicted in Scheme 1. First, the boryl-substituted intermediate TBBr was synthesized by lithiation of 2,8-dibromo-6H,12H-5,11-methanodibenzo[b,f][1,5]diazocine (TB1) with n-BuLi followed by the addition of dimesitylfluoroborane (1.0 equivalent) at −78 °C. Following the same procedure, the addition of 2 equivalent of dimesitylfluoroborane to the lithiated TB1 prodeces TBB.¹³ Then TBBr underwent a palladium-catalyzed Suzuki coupling reaction with corresponding aryl boronic acid 4-(N,N-dimethylamino)phenylboronic acid (1) or 4-(diphenylamino) phenylboronic acid (2) to produce the two target Λ-shaped triarylboron compounds with satisfactory yields. Both TBBN and TBBN2 were fully characterized by ¹H and ¹³C NMR spectroscopy, high-resolution mass spectrometry (HRMS) and elemental analysis.

Photophysical properties in solutions

First the UV-vis absorption (Abs) and photoluminescence (PL) spectra of TBBN and TBBN2 in solution were measured in benzene. For comparison, the optical properties of the reference compounds TBB and TBNN were also recorded, as shown in Fig. 2. The corresponding data are provided in Table 1. In benzene solution, TBBN, TBBN2, TBB and TBNN display intense absorption bands with peaks at 315, 328, 332 and 301 nm, respectively. Specifically, the absorption band for TBBN and TBBN2 is presumably assigned to the π–π* transitions localized on the TB scaffold and the amino side, while the band for TBB is attributed to the intramolecular CT transition from the HOMO located on the TB scaffold to the LUMO mainly located on the boryl moiety in both of the two sides, and the band for TBNN is mainly due to the π–π* transition localized on the TB scaffold and the amino groups (for details see the Theoretical calculations section). In the PL spectrum, TBB and TBNN show intense fluorescence at 405 nm and 370 nm with high quantum yields (Φ_F = 0.88 and 0.65, respectively), while TBBN exhibits a blue emission at 445 nm, and TBBN2 shows a deep blue emission at 418 nm with relatively low fluorescence quantum yields. We have noticed that although the absorption spectrum of the triphenylamine substituted TBBN2 is red-shifted compared with that of the dimethylarylamine substituted compound TBBN, the PL spectrum of TBBN2 is nevertheless blue-shifted. Such novel photophysical behavior has been observed in other Tröger's base compounds, in which the PL blue-shift is thought to be associated with the asymmetrical alignment of the phenyl rings along the axis of the co-annular bond.¹⁴

Fig. 2 Abs and PL spectra of TBBN, TBBN2, TBB and TBNN in benzene solution. The top inset shows fluorescence photograph of benzene solutions under 365 nm irradiation.

Table 1 Optical properties of TBBN, TBBN2, TBB and TBNN

Compound	UV		FL
	λabs^a (nm)	λabs^b (nm)	λem^a (nm)	λem^b (nm)	ΦF^c
a In dilute benzene solutions (ca. 1 × 10^−6 M).b In thin film by spin-coating.c Absolute quantum yields in the film states determined by a calibrated integrating sphere system.
TBBN	315	319	445	470	0.07
TBBN2	328	335	418	424	0.05
TBB	332	333	405	405	0.04
TBNN	301	—	370	—	—

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To obtain comprehensive insight into the effect of the Λ-shaped skeleton and the substituents on the intramolecular CT transitions and excited states, we further measured the absorption and PL spectra of these four compounds in various solvents (1 × 10⁻⁶ M). Their PL spectra are shown in Fig. S1–S4,† and the corresponding data are summarized in Table S1.†

For compounds TBBN, TBBN2 and TBB, the absorption spectrum is only trivially solvent-dependent, while the PL spectrum displays significant solvatochromism (see ESI†), which are characteristic for intramolecular CT emission, indicating that these molecules should have more polarized structures in the excited state as compared to the ground state. Thus CT emission is more susceptible to the solvent polarity than absorption. In contrast to the significant solvatochromism in PL spectra for the three compounds, the donor-only molecule TBNN shows relatively weak solvatochromism in both absorption and PL spectra. To compare the degree of the polarized excited-state in these molecules, the Lippert–Mataga equation. (eqn (1)) was used,^7d,15 in which Δv is the Stokes shift, μ_e and μ_g are the dipole moments for the excited state and ground state, respectively, C is a constant, c is the speed of light, h is the Planck constant and a is the molecular radius. Δf is the solvent polarity and is described by eqn (2), in which ε is the dielectric constant, n is the refractive index of the solvent. As can be seen in Fig. 3, in all these compounds, linear relationships for the plots of Δv as a function of Δf were obtained. The slopes for compounds TBBN, TBBN2 and TBB are 11 200, 10 400 and 13 900 cm⁻¹, respectively, which are comparable to each other and much deeper than that of TBNN with 4700 cm⁻¹ (Table 2). These results suggest that the dipole moments in the excited state are significant for the first three molecules, which is reasonable considering the electron-accepting property of dimesitylboryl group. It is also clearly indicated that the distinct solvatochromism found in PL spectra of TBBN, TBBN2 and TBB originate from the highly polarized excited states induced by the intramolecular CT transition between the donor and acceptor centers. In addition, such intramolecular CT transition generally results in large Stokes shift, which coincides well with their spectral behavior and is supposed to favor hindering the fluorescence quenching in the aggregated states.

Fig. 3 The Lippert–Mataga plots for Λ-shaped compounds TBBN, TBBN2, TBB and TBNN.

Table 2 The slope of plot of Δv versus Δf for compounds TBBN, TBBN2, TBB and TBNN

Compound	k (10^4 cm^−1)
TBBN	1.12
TBBN2	1.04
TBB	1.39
TBNN	0.47

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Theoretical calculations

To gain an deep insight into the influence of the terminal substituents on the electronic structure and thus the photophysical properties, theoretical calculations on the four compounds TBBN, TBBN2, TBB and TBNN were performed by using the Gaussian 09 program.¹⁶ The geometries were optimized by using density function theory (DFT) with the B3LYP function and the 6-31G(d) basis sets. We also conducted time-dependent density-functional theory (TD-DFT) calculations of these four compounds at B3LYP/6-31G(d) theory in order to understand their intramolecular CT transitions. The diagrams of the molecular orbitals are shown in Fig. 4, and the calculated transition energies, oscillator strength, and assignments for the most-relevant singlet excited states are summarized in Table S2.†

Fig. 4 HOMO and LUMO diagrams for compounds TBBN, TBBN2, TBB and TBNN. The transition energies and oscillator strengths (f) are listed, calculated by using the B3LYP/6-31G(d) method.

According to the TD-DFT calculation results, the HOMOs of both TBBN and TBBN2 actually consist of contributions from the amino side only, whereas the LUMOs involve contributions from the dimesitylborylphenyl side only. The intense absorption band for TBBN at 315 nm is mainly due to the transition from HOMO to the LUMO + 2 located on the TB scaffold, and the band for TBBN2 at 328 nm is assignable to the HOMO → LUMO + 1 transition located on the amino side, as shown in Fig. S5 and Table S2.† It is notable that there is almost no overlap between the two frontier orbitals of these two compounds. As a consequence, the charge transfer from HOMO to LUMO occurs most likely through space rather than the Λ-shaped bridge, a result which has been suggested by the fluorescence solvatochromism; whilst the lowest electronic transitions, consisting of the HOMO → LUMO transitions, are characteristic of forbidden transitions and possess quite small oscillator strength of 0.0194 and 0.0055 for TBBN and TBBN2, respectively, which presumably make the lowest intramolecular CT transition bands too weak to be distinguished in their absorption spectra, and are further unfavorable for efficient fluorescence. As a consequence, these two compounds exhibit relatively weak emissions in solutions with low quantum yields (e.g. Φ_F = 0.09 and 0.11 in THF for TBBN and TBBN2).

In contrast to the disjoint frontier molecular orbitals in TBBN and TBBN2, the distribution of HOMO and LUMO for the donor-only molecule TBNN revealed that the delocalization of electrons were spread over the whole TB skeleton and the electron-donating dimethylaniline groups. The calculated first excited state (HOMO → LUMO transition) has an excitation energy of 3.99 eV (310 nm) with a increased oscillator strength (f = 0.5851). The acceptor-only molecule TBB has a HOMO localized mainly on the central TB bridge, and its LUMO is dominated by the boron centers and the adjacent phenyl moiety owing to the D–π–A geometry in both of two sides. Consequently, the lowest electronic transition is consistent with the charge transfer from the N centers on TB scaffold to the two boron centers with the oscillator strength of 0.2215. Probably due to the increase in the oscillator strength of the lowest electronic transitions for TBB and TBNN, they exhibit stronger emissions in solution with high quantum yields (e.g. Φ_F = 0.80 and 0.60 in THF for TBB and TBNN) as compared with TBBN and TBBN2, which are consistent well with the experimental results.

Photophysical properties in the solid state

In our experiments, all three Λ-shaped triarylboron compounds exhibit bright fluorescence when excited at 365 nm in the solid state. We thus measured their photophysical properties in thin film form. The thin films were prepared by spin-coating their benzene solutions with a concentration of ca. 3 mg mL⁻¹ onto quartz plates and then drying under vacuum at 80 °C for 2 h.

As can be seen in Fig. 5 and Table 1, the absorption spectra of the three compounds in solid state are shaped like those in benzene solutions, with a slight red shift of the peak wavelength (4 nm for TBBN, 7 nm for TBBN2, and 1 nm for TBB). The PL spectra also red-shift slightly when going from the benzene solution to the film state (25 nm for TBBN, 6 nm for TBBN2, and 0 nm for TBB). This result suggests neither the formation of close π–π stacking in the aggregated states nor excimers in the excited state by the introduction of the twisted Λ-shaped scaffold to the molecules, a finding which will be confirmed by our analysis of the crystal packing structure.

Fig. 5 Abs and PL spectra of compounds TBBN, TBBN2 and TBB in spin-coated films. The top inset shows fluorescence photograph of films under 365 nm irradiation.

Crystal structure analysis

To understand the relationship between the fluorescence behavior and intermolecular interactions in the solid state, we determined and analyzed the crystal structure of TBBN. Colorless TBBN crystals for X-ray diffraction analysis were grown by very slow evaporation of a hexane/CH₂Cl₂ solution at ambient temperature. The crystal data and intensity collection parameters are summarized in Table S3.† The molecular structure of TBBN and its packing arrangement are given in Fig. 6. There are two crystallographically independent molecules in an asymmetric unit. In each molecule, the central methylene diazocine bridge imposes an approximate C₂ symmetry on the molecule, providing a twisted Λ-shaped configuration. The dihedral angles between the two benzene rings constituting the TB scaffold are 75.72° and 83.10°. The substituted aniline group on one side shows moderate planarity to the phenyl ring adjoining the central diazocine, with twisting angles between the two adjacent phenyl rings of 11.33° and 17.62° for the two different structures. For the substituted dimesitylboryl groups, the trivalent boron center is well protected by two large mesityl groups, thus the mesityl moieties are severely twisted to each other, exhibiting a noncoplanar configuration.

Fig. 6 (a) ORTEP drawing of TBBN with 30% probability ellipsoids. (b) Molecular packing in TBBN crystal projected onto the bc and ac plane. (c) A molecular layer within the crystal structure of TBBN projected onto the bc plane. The off-set π–π interactions (black dotted line 1, red dotted line 2, blue dotted line 3) between adjacent enantiomers are indicated by dotted lines and ellipses (π-overlap extent). Hydrogen atoms have been omitted for clarity; the opposite enantiomers are colored in light green for (R,R)-A and mauve for (S,S)-A*.

In TBBN crystal, there are two enantiomers because of the chirality of Troger's base. Thus TBBN forms racemic crystals as (R,R)-enantiomers and (S,S)-enantiomers in their packing structure are equal in amount, as illustrated in Fig. 6(b). Probably due to the Λ-shaped twisted molecular configuration and the steric effect of the substituted dimesitylboryl groups, the shortest vertical distance between the phenyl planes of neighboring molecules is found to be approximately 3.69 Å with a sliding angle of 7.78° (see Fig.6(c) and Table S4†), which is larger than the normal π–π interaction distance (ca. 3.4–3.6 Å). Therefore, the molecules are far apart from each other and only a weak π–π interaction exists in the crystal. Such a structure is supposed to play an important role in suppressing excimer formation and other nonradiative pathways in the aggregated state. We speculate that TBB and TBBN2 exhibit a more twisted structure due to the presence of more steric substituents of dimesitylboryl and diphenylamine.

Fluorescent sensing of fluoride and cyanide ions

Fluoride and cyanide ions, the smallest anions, have attracted growing attention because of their important potential in both fundamental research field of supramolecular chemistry and industrial and biological applications.⁸ Thus, their recognition and detection are of growing interest. Fluorescence sensing is known to be one of the most effective and desirable methods available because of its high selectivity and sensitivity. Considering the characteristic through space intramolecular CT emission of TBBN and TBBN2, we studied their fluorescence sensing abilities for fluoride and cyanide ions.

The titration experiments of TBBN and TBBN2 toward the fluoride ion were carried out in THF solutions with n-Bu₄NF (TBAF) as the fluoride source. The specific method is as follows: the THF solution of TBBN (4.661 μM, 2 mL in a quartz cuvette) was titrated with incremental amounts of fluoride ion by addition of a concentrated TBAF solution (4.16 × 10⁻⁴ M in THF), in which TBBN was also included at its initial concentration to avoid the dilution effects. The absorption and PL spectral changes of the TBBN solution upon addition of TBAF are shown in Fig. 7. With the addition of TBAF, the fluorescence band at 518 nm decreased gradually in intensity, and simultaneously, another new fluorescence band appeared at 378 nm. That is, the combination of fluoride ions actually switched the emission of TBBN from yellow-green to blue-violet. Thus, TBBN can be used as a “switch-on” sensor for fluoride ions. As shown in the PL spectrum of TBBN in THF (Fig. S1(b)†), we can distinguish a small shoulder emission at ∼378 nm besides the main emission band at 518 nm. This shorter wavelength emission is believed to be from the π*–π transition of the amino substituted side, because the donor-only molecule TBNN with both sides of dimethylaniline substituents just shows a single emission at 377 nm (see Fig. 8). Thus in the absence of the fluoride ion, the dual fluorescent pathways that coexist in TBBN are mainly occupied by the through-space CT transition between the amino group and the boryl group, with the main emission peak at 518 nm. The binding of the fluoride ion to the boron center disrupts the through-space charge transfer transition, quenching the yellow-green emission, and simultaneously the violet emission from the π*–π transition of the dimethylaniline side becomes dominate, as shown in Scheme 2. This kind of PL behavior is similar to that of other organoboron compounds that have through space CT emission, e.g., the V-shaped ones.^10b Such a large blue shift (about 140 nm) is relatively rare in most of the triarylboron compounds when complexed with fluoride.^1g In the absorption spectrum, there is a slight blue shift (from 313 nm to 297 nm) accompanied with a decrease in intensity. From our theoretical calculations on TBBN, the intense absorption band at 313 nm is assignable to the π–π* transitions localized on the TB scaffold and the amino side. The change in absorption confirms the ignoring of the contribution from the dimesitylboryl moiety on the electronic transition. The PL change became saturated as the concentration of TBAF amounted to approximately 8.46 equiv of TBBN. It is notably that the ratio of fluorescence intensity at 378 nm to the value at 518 nm (I₃₇₈/I₅₁₈) displays a big change from 0.45 : 1 to 294 : 1, accompanied by a dramatic emission color change from yellow-green to violet, as illustrated in Fig. 7(d). Thus the compound can potentially be used as a ratiometric and colorimetric fluorescent fluoride ion sensor. For TBBN2, similar results were observed in the absorption and PL spectral changes upon addition of fluoride (Fig. S11†). The binding constants of TBBN and TBBN2 with fluoride were determined to be approximately 2.67 × 10⁴ M⁻¹ and 6.56 × 10⁴ M⁻¹, respectively, which are comparable to the values reported for other triarylboron compounds.^7j–l,9f Furthermore, the binding of TBBN and TBBN2 with fluoride in the titration experiments was also supported by ¹⁹F NMR spectroscopy (see ESI, Fig. S23 and S24†). Upon addition of incremental amounts of TBBN or TBBN2 to a TBAF solution, the free fluoride ion signal at 120 ppm is split into two distinct ¹⁹F signals, with a new one at 175 ppm assigned to the bound fluoride. From the ¹⁹F NMR titration spectrum, we noticed that the free fluoride ion signal disappeared when the amount of TBBN or TBBN2 was equal to that of the TBAF, which clearly indicated quantitative conversion of TBBN or TBBN2 to the 1 : 1 complex with the free fluoride ion.

Fig. 7 The absorption (a) and PL (b) spectral changes of TBBN (4.722 μM in THF, λ_ex = 313 nm) upon addition of TBAF solution (4.16 × 10⁻⁴ M). (c) Enlarged view of PL spectrum change at 518 nm (d) Fluorescence intensity ratio (I₃₇₈/I₅₁₈) versus concentration of F⁻. Inset in (d) photograph of THF solution of TBBN before and after addition of F⁻ with 365 nm irradiation.

Fig. 8 PL spectrum of compound TBNN in THF (λ_ex = 303 nm).

Scheme 2 Schematic representation of compounds (a) TBBN and TBBN2 used as ratiometric “switch-on” fluorescent sensors, and (b) TBB used as a “switch-off” sensor for fluoride.

The dual fluorescence pathways in TBBN and TBBN2 allow them to function as “switch-on” sensors. For comparison, titration experiments of TBB upon fluoride ions were also performed. In sharp contrast to the behavior of TBBN and TBBN2, the addition of fluoride ion to the THF solution of TBB led to complete quenching of its blue emission at 440 nm (see Fig. 9). This is accompanied by a significant decrease in intensity of the strong 330 nm band in the absorption spectrum. Thus, with both sides of boryl substituents, TBB is a “switch-off” fluorescence sensor for fluoride ion. The binding constant is determined to be 2.05 × 10⁹ M⁻², which is larger than that of TBBN and TBBN2, because there are two boron centers in one molecule. The ¹⁹F NMR titration experiments (Fig. S25†) on TBB suggested the formation of a 1 : 2 complex with two equiv of F⁻. The fluorescence behavior upon F⁻ addition is similar to that of the linear triarylboron molecules since the conjugated D–π–A geometry exists in each side of TBB. As a result, the binding of F⁻ leads to the occupation of the vacant P_π orbital of boron, and thus disrupts the intramolecular charge transfer from the N centers in TB scaffold to the two boron centers, quenching the blue emission. The results also demonstrate that the amino substituents play significant roles in the fluorescence behavior of TBBN and TBBN2 upon F⁻ addition.

Fig. 9 The absorption (a) and PL (b) spectral changes of TBB (4.372 μM in THF, λ_ex = 330 nm) upon addition of TBAF solution (4.24 × 10⁻⁴ M). Inset in (a) photograph of THF solution of TBB before and after addition of F⁻ with 365 nm irradiation; Inset in (b) Plot of fluorescence intensity change at 440 nm (I₄₄₀) versus concentration of F⁻.

The sensing selectivity of dimesitylboranes to fluoride ion over other ions such as chloride, bromide and iodide has been well established.⁷ It is well-known that these compounds are also sensitive to cyanide ions due to their small size.¹⁷ In the same way, TBBN, TBBN2 and TBB are expected to be responsive to the cyanide ion. The titration experiments on these compounds upon addition of tetrabutylammonium cyanide (TBACN) were the same as on TBAF. The addition of TBACN to a THF solution of these compounds triggers very similar changes in the fluorescence and absorption to those caused by the fluoride ion. Fig. 10 shows the absorption and PL spectra of TBBN2 in THF upon addition of TBACN. It is found that the cyanide ion quenched the blue through-space CT emission between the amino group and the boryl group at 460 nm and boosted the violet π*–π emission of the diphenylamine substituent at 388 nm. The ratios of fluorescence intensity at 388 nm and 460 nm (I₃₈₈/I₄₆₀) rise linearly nearly from 0.31 : 1 to 12 : 1 upon CN⁻ addition. Detailed studies of the response these dyes to cyanide are listed in the ESI.† To further evaluate the selectivity of these dyes, we measured their fluorescence responses toward various anions (such as Cl⁻, Br⁻, AcO⁻ and H₂PO₄⁻) as well as F⁻ and CN⁻. As shown in Fig. S26–S31,† barely no ratiometric fluorescence changes were recorded by Cl⁻, Br⁻, AcO⁻ and H₂PO₄⁻, thus the solutions displayed almost the same fluorescence intensities as that of the molecules themselves, in sharp contrast to the results on the fluoride and cyanide ions.

Fig. 10 The absorption (a) and PL (b) spectral changes of TBBN2 (2.099 μM in THF, λ_ex = 328 nm) upon addition of TBACN solution (6.64 × 10⁻⁴ M). Inset in (a) photograph of THF solution of TBBN2 before and after addition of CN⁻ with 365 nm irradiation. Inset in (b) plot of fluorescence intensity ratio (I₃₈₈/I₄₆₀) versus concentration of CN⁻.

Conclusions

In summary, we have designed and synthesized a new class of Λ-shaped triarylboron compounds TBBN and TBBN2, in which the electron-donating amino group and electron-accepting dimesitylboryl group are connected through a rigid Tröger's base linker. Both compounds exhibit twisted structures and show intense fluorescence in both dilute solution and aggregated states. The Λ-shaped Tröger's base scaffold endows the amino substituents and the dimesitylboryl group with a nonplanar framework, by virtue of which intramolecular CT can occur through space rather than through the non-conjugated bridge. As a consequence, dual fluorescent pathways, namely through-space CT from the amino substituents to the dimesitylboryl group and π*–π transitions located on the amino substituents, are observed to coexist in these molecules. The dual emission pathways can be selectively switched by addition of fluoride or cyanide ions, leading to dramatic blue shifts (about 72–140 nm) and color changes in the fluorescence. TBBN and TBBN2 can be applied as ratiometric and colorimetric “switch-on” fluorescent sensors for fluoride and cyanide ions. Thus by intruding the twisted and non-conjugated Λ-shaped TB scaffold to triarylboranes, we provide an efficient strategy to develop boryl compounds applied as visually colorimetric and ratiometric fluorescent sensors for fluoride and cyanide.

Experimental section

General

¹H, ¹³C and ¹⁹F NMR spectra were recorded with a Bruker Advance 300 or 400 spectrometer with tetramethylsilane (TMS) as internal reference. HRMS spectra were obtained on a Q-TOF 6510 spectrograph (Agilent). Elemental analyses were performed using a Vario EL III elemental analyzer. The UV-visible absorption spectra were collected on a Varian Cary 50 spectrophotometer and the fluorescence spectra were measured on a Hitachi F-4500 fluorescence spectrophotometer. All solvents used for spectroscopy experiments were spectral grade.

All reagents, unless otherwise indicated, were obtained from Aldrich or Acros chemical company and used as received. 2,8-Dibromo-6H,12H-5,11-methanodibenzo[b,f][1,5]diazocine (TB1)¹⁸ and 4-(N,N-dimethylamino)phenylboronic acid (1)¹⁹ were prepared according to the literature. Air- and moisture-sensitive reactions were performed in oven-dried glassware under argon atmosphere, and anhydrous THF was freshly distilled with sodium-sand before use. The synthesis of TBB (ref. 13) has been reported previously by our group.

Synthesis

2-Bromo-8-dimesitylboryl-6H,12H-5,11-methanodibenzo[b,f] [1,5]diazocine (TBBr). To a solution of 2,8-dibromo-6H,12H-5,11-methanodibenzo[b,f][1,5]diazocine TB1 (2.5 mmol, 0.945 g) in anhydrous THF (20 mL) was added n-BuLi (1.6 M in hexane, 1.86 mL, 2.98 mmol) slowly under argon at −78 °C. The mixture was stirred for a further 1.5 h. Dimesitylboron fluoride (3.729 mmol, 1 g in 5 mL THF) was then added dropwise by syringe and the reaction mixture was kept at −78 °C for another 1 h, then the temperature was gradually increased to room temperature and the mixture was stirred overnight. Water (30 mL) was added and the aqueous layer was extracted with CH₂Cl₂ three times. The organic layer was dried over anhydrous MgSO₄, and the solvents were removed under reduced pressure. The residue was purified by silica gel column chromatography using petroleum ether/EtOAc (10 : 1) as eluent to obtain TBBr as a white solid with a 52% yield (0.708 g, 1.29 mmol).¹H NMR (300 MHz, CDCl₃), δ (ppm): 1.95 (s, 12H), 2.29 (s, 6H), 4.05 (d, J = 16.5 Hz, 1H), 4.19–4.33 (m, 3H), 4.65 (t, J = 15.6 Hz, 2H), 6.78 (s, 4H), 6.97–7.11 (m, 4H), 7.24–7.35 (m, 2H). ¹³C NMR (75 MHz, CDCl₃), δ (ppm): 151.9, 147.2, 140.7, 138.4, 136.2, 136.1, 130.4, 130.0, 129.7, 128.1, 126.8, 126.6, 124.3, 116.4, 66.6, 58.6, 58.3, 23.5, 21.1. HRMS (ESI) calcd for C₃₃H₃₄BBrN₂: 548.1998, 550.1978; found: [M + H]⁺: 549.2120, 551.2103. Elemental analysis. Found: C, 72.61%; H, 6.73%; N, 4.76%; calcd for C₃₃H₃₄BBrN₂: C, 72.15%; H, 6.24%; N, 5.10%.

2-(4-(N,N-Dimethylamino)-8-dimesitylboryl-6H,12H-5,11-me-thanodibenzo[b,f][1,5]diazocine (TBBN). A continuously stirred mixture of 2-bromo-8-dimesitylboryl-6H,12H-5,11-methanodibenzo[b,f][1,5]diazocine TBBr (1 mmol, 0.549 g) and 4-(N,N-dimethylamino)phenylboronic acid 1 (3 mmol, 0.495 g) in degassed THF (20 mL) and aqueous K₂CO₃ (10–15 mL, 2.0 M) was bubbled with an argon flush for 20 min, and then Pd(PPh₃)₄ was quickly added. The reaction mixture was refluxed at 90 °C under argon atmosphere for 48 h. After cooling to room temperature, the mixture was extracted with CH₂Cl₂. The combined organic layer was washed with brine and dried over anhydrous MgSO₄, and the solvents were removed under reduced pressure. The residue was purified by silica gel column chromatography using petroleum ether/EtOAc (3 : 1) as eluent to attain TBBN as a pale yellow solid with a 56% yield (0.332 g, 0.56 mmol). ¹H NMR (400 MHz, DMSO-d₆), δ (ppm): 1.88 (s, 12H), 2.23 (s, 6H), 2.90 (s, 6H), 4.08 (d, J = 16.8 Hz, 1H), 4.24–4.30 (m, 3H), 4.65 (q, J = 16.0 Hz, 2H), 6.74 (d, J = 8.0 Hz, 2H), 6.78 (s, 4H), 7.04 (s, 1H), 7.09 (d, J = 8.0 Hz, 1H), 7.14–7.18 (m, 3H), 7.35 (q, J = 2.0 Hz, 1H), 7.41 (d, J = 8.0 Hz, 2H). ¹³C NMR (100 MHz, DMSO-d₆), δ (ppm): 153.6, 150.0, 146.6, 141.6, 140.2, 139.6, 138.2, 136.4, 135.9, 135.8, 128.7, 128.5, 128.0, 127.8, 127.1, 125.6, 124.9, 124.8, 124.0, 113.1, 66.5, 58.7, 58.3, 23.6, 21.2. HRMS (ESI) calcd for C₄₁H₄₄BN₃: 589.3628; found: [M + H]⁺: 590.3758. Elemental analysis. Found: C, 83.27%; H, 7.54%; N, 7.04%; calcd for C₄₁H₄₄BN₃: C, 83.52%; H, 7.52%; N, 7.13%.

2-(4-(N,N-Diphenyllamino)-8-dimesitylboryl-6H,12H-5,11-me-thanodibenzo[b,f][1,5]diazocine (TBBN2). Compound TBBN2 was prepared using the similar procedure as for TBBN by using 2-bromo-8-dimesitylboryl-6H,12H-5,11-methanodibenzo[b,f] [1,5]diazocine TBBr (1 mmol, 0.549 g), 4-(diphenylamino) phenyl-boronic acid (3 mmol, 0.867 g), degassed toluene (20 mL), aqueous K₂CO₃ (10–15 mL, 2.0 M) and Pd(PPh₃)₄. The crude product was purified by silica gel column chromatography using petroleum ether/EtOAc (5 : 1) as eluent to obtain TBBN2 as a pale yellow solid with a 51% yield (0.364 g, 0.51 mmol). ¹H NMR (400 MHz, DMSO-d₆), δ (ppm): 1.88 (s, 12H), 2.23 (s, 6H), 4.11 (d, J = 20.0 Hz, 1H), 4.23–4.31 (m, 3H), 4.58–4.74 (m, 2H), 6.78 (s, 4H), 6.98–7.07 (m, 9H), 7.13–7.16 (m, 3H), 7.24–7.32 (m, 5H), 7.41 (q, J = 2.0 Hz, 1H), 7.50 (d, J = 8.0 Hz, 2H). ¹³C NMR (100 MHz, DMSO-d₆), δ (ppm): 152.9, 147.0, 146.2, 141.1, 139.7, 138.3, 137.7, 135.7, 135.2, 134.6, 133.8, 129.4, 128.3, 127.9, 127.4, 127.1, 125.1, 124.8, 124.4, 124.3, 123.9, 123.4, 123.0, 65.9, 58.1, 57.8, 23.0, 20.6. HRMS (ESI) calcd for C₅₁H₄₈BN₃: 713.3941; found: [M + H]⁺: 714.4041. Elemental analysis. Found: C, 85.30%; H, 6.18%; N, 5.67%; calcd for C₅₁H₄₈BN₃: C, 85.82%; H, 6.78%; N, 5.89%.

2,8-Di(4-N,N-dimethylamino)-6H,12H-5,11-methanodibenzo [b,f][1,5]diazocine (TBNN). Similar procedure with compound TBBN but 2,8-dibromo-6H,12H-5,11-methanodibenzo[b,f][1,5] diazocine TB1 (1 mmol, 0.38 g) instead of 2-bromo-8-dimesitylboryl-6H,12H-5,11-methanodibenzo[b,f][1,5] diazoci-ne TBBr was used as the reactant, and 4-(N,N-dimethylamino) phenylboronic acid 1 (3 mmol, 0.50 g), degassed THF (25 mL), aqueous K₂CO₃ (15 mL, 2.0 M) and Pd(PPh₃)₄ were also included. The crude product was purified by silica gel column chromatography using CH₂Cl₂/EtOAc (3 : 1) as eluent to attain TBNN as a white solid in 73% yield (0.34 g, 0.74 mmol).¹H NMR (300 MHz, DMSO-d₆), δ (ppm): 2.89 (s, 12H), 4.12–4.26 (m, 4H), 4.67 (d, J = 16.8, 2H), 6.73 (d, J = 8.7, 4H), 7.07–7.19 (m, 4H), 7.28 (s, 2H), 7.32–7.40 (m, 4H). ¹³C NMR (75 MHz, DMSO-d₆), δ (ppm): 150.0, 146.9, 136.1, 128.8, 128.1, 127.2, 125.5, 124.8, 124.2, 113.1, 66.9, 28.8. HRMS (ESI) calcd for C₃₁H₃₂N₄: 460.2627; found: [M + H]⁺: 461.2680. Elemental analysis. Found: C, 80.51%; H, 6.70%; N, 12.07%; calcd for C₃₁H₃₂N₄: C, 80.83%; H, 7.00%; N, 12.16%.

Crystal structure determination

Single crystals of TBBN suitable for X-ray crystal analysis were obtained by evaporating a mixture of hexane/CH₂Cl₂ at ambient temperature. The X-ray diffraction intensity data were collected at 296 K on a Bruker SMART APEX-II CCD area-detector diffractometer using graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) with the ω-scan method.²⁰ Preliminary lattice parameters and orientation matrices were obtained from three sets of frames. The structure was solved in the PĪ space group by direct methods and refined by a full-matrix least-squares refinement on F² using the SHELX program.²¹ In the final stage of the least-squares refinement, all non-hydrogen atoms were refined anisotropically and all hydrogen atoms except for those of the disordered solvent molecules were placed using HFIX instructions. The crystal data, intensity collection details and refinement parameters are presented in Table S2.†

Acknowledgements

We thank the National Natural Science Foundation of China (Grant nos 51021062, 51003054, 61404097), Promotive Research Fund for Young Scientists of Shandong Province (BS2012CL020), the 973 Program of the People's Republic of China (Grant no. 2010CB630702) and the Program of Introducing Talents of Disciplines to Universities in China (111 Program no. b06017) for financial support.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Photophysical properties and theoretical calculation results for all new compounds; X-ray crystallographic data of TBBN; titration measurement of TBBN, TBBN2 and TBB with fluoride and cyanide; ¹⁹F NMR titrations of the above three compounds with fluoride; selectivity of these molecules for fluoride and cyanide; high resolution MS data of all new compounds. CCDC 905360. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra07912h

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