Tetra-coordinated boron-appended zinc(II)-salen: a highly selective fluorescence-based sensor for Sm³⁺ ions via sensitization

Abstract

Selective detection of metal ions is very important from the environmental and biological points of view. In this regard, selective discrimination between lanthanide ions is very difficult and challenging owing to their similar atomic radii. Herein, we report a tetra-coordinated boron-based salen ligand that shows selective fluorescence enhancement with Zn²⁺ ions. More interestingly, isolated zinc complex 6a was able to detect Sm³⁺ ions selectively. The luminescence color of complex 6a changed from green to red upon addition of Sm³⁺ ions, which makes this design highly selective in discriminating Sm³⁺ ions in the presence of other lanthanide ions at a very low concentration of 1 × 10⁻⁵ M.

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Introduction

In recent years, the design and synthesis of tri- and tetra-coordinated boron compounds have received considerable attention due to their applications in diverse fields such as light-emitting devices, photovoltaics, sensors and organic field-effect transistors.¹ The ability to attain a planar conformation, stability due to saturation of Lewis acidity and rigidity of the basic structure makes tetra-coordinated boron-based fluorophores more favorable alternatives over three-coordinated boron compounds. Among the different tetra-coordinated boron compounds, B–N coordinated compounds have secured considerable attention because of their widespread applications.² We and others have recently shown that B–N coordinated compounds can show oxygen sensing behavior, photochromism, non-linear optical properties etc.^2a,g,i,l,m,3

Selective detection of metal ions has emerged as an important area of research due to its implications in the area of environmental, biological and clinical studies. Several probes or sensors have been reported for the detection of different metal ions employing colorimetric and/or fluorometric techniques.⁴ Fluorescence-based probes or sensors have received considerable attention owing to their simplicity, selectivity and sensitivity towards specific metal ions. Among the different metal ions, lanthanide ions constitute a family with a very similar and a non-discriminative coordination behavior. On the other hand, the use of lanthanides in industrial applications is ever burgeoning, including in magnetism, displays, components of nuclear control rods and even as catalysts to name a few.⁵ As some or many of the lanthanide ions are toxic in nature their detection is an important task. Although there are reports on the detection of different lanthanide ions,⁶ selective discrimination and detection of lanthanide ions is a major challenge in view of their similarity in terms of size and chemical behavior.

Taking advantage of the emissive and electron-transporting properties of the B–N coordinated fluorophores with the chelating ability of salen ligands, which can find use in catalysis and materials chemistry, we designed a tetra-coordinated boron-functionalized pyrazole-based salen ligand (Fig. 1). We envisioned that this molecular architecture will enable the selective detection of Zn²⁺ using pocket 1 and also further expected that pocket 2 will discriminate lanthanide ions.

Fig. 1 Structure of the ligand used in this study.

Results and discussion

Selective detection of zinc ion plays a vital role in the chemical and biological sciences owing to its important role in gene expression, apoptosis, cellular metabolism and neurotransmission.⁷ A number of analytical techniques such as UV/Vis, fluorescence, atomic absorption spectroscopy, and electrochemical methods are used for the detection of Zn²⁺ ions.⁸ Among the different methods, fluorescent sensors have shown superior sensitivity. This motivated us to investigate the coordinating ability of boron functionalized compound 5a as a luminescent probe for Zn²⁺. The synthetic route to pyrazole-based boron functionalized imine ligand 5a is outlined in Scheme 1. Compounds 1–3 were prepared according to the literature reported methods.^3g Treatment of 2 and 3a (Scheme S1, ESI†) with Na₂CO₃ and a catalytic amount of Pd(PPh₃)₄ under Suzuki coupling conditions resulted in a moderate yield (61%) of 4a. Synthesis of the required ligand 5a (Scheme 1) was achieved in an excellent yield (92%) by a simple condensation reaction of 2 equivalents of 4a with ethylene diamine in dry methanol as a solvent. Compound 5a was characterized using multi-nuclear NMR and HRMS. Furthermore, compound 5a was also analysed using ¹H–¹H COSY (Fig. S8, ESI†).

Scheme 1 Synthesis of ligands 5a–5c.

The UV/Vis absorption and fluorescence spectra of 5a (10 μM) reveal an absorption maximum at 330 nm and an emission maximum at 464 nm. To understand the sensing properties of 5a, the absorption and fluorescence spectra of 5a were recorded with different amounts of Zn²⁺ ions. Sequential addition of Zn²⁺ ions (0–2.4 eq.) to 5a resulted in a new absorption peak at 353 nm with an isobestic point at 339 nm (Fig. S2, ESI†). As shown in Fig. 2, the fluorescence intensity gradually increased with sequential addition of Zn²⁺ ions (0–2.4 eq.). Furthermore, a selectivity test was performed using various cations such as Cd²⁺, Co²⁺, Fe³⁺, Fe²⁺, Hg²⁺, K⁺, Mn²⁺, Cr³⁺, Na⁺, Ni²⁺ and Mg²⁺ to investigate the binding interaction of 5a (10 μM). As shown in Fig. 2(c), only Zn²⁺ caused an enhanced emission, while other cations were silent. This result indicates that 5a discriminates Zn²⁺ from other cations, which can be detected by green emission.

Fig. 2 (a) Fluorescence spectra (λ_ex = 330 nm) of 5a (10 μM) with 2.2 equiv. of NaOAc upon addition of 0–2.4 equiv. of Zn²⁺ in the (10 : 90) methanol/THF solvent system. (b) Linear increase of the fluorescence intensity at 497 nm of 5a (10 μM) upon addition of 0–2.4 equiv. of Zn²⁺ (λ_ex = 330 nm). (c) Fluorescence intensity profile changes of 5a in the presence of 10 μM concentration of various metal ions at room temperature (λ_ex = 330 nm).

To determine the binding mode of the zinc ion with salen, we independently synthesized (Scheme 2) the Zn-salen and characterized it using different techniques like ¹H, ¹³C, and ¹¹B. The disappearance of the OH peak at 13.61 ppm of ligand 5a and the up-field shifting of four ethylene protons from 3.94 ppm in 5a to 3.74 ppm in 6a confirms the formation of Zn²⁺ complex, 6a. To our delight X-ray quality crystals of complex 6a could be grown in the DMF/CH₃CN solvent system. The crystal structure confirms a 1 : 1 binding of 5a with Zn²⁺ (Fig. S1, ESI†). The detection limit for Zn²⁺ was calculated and found to be 72 nM. These results reveal the applicability of 5a in determining Zn²⁺ ions with high selectivity.

Scheme 2 Synthesis of zinc complexes 6a–6c and structure of the complex 6d used in this study.

Complex 6a crystallizes in a triclinic P1̄ space group. The molecular structure reveals that the zinc atom adopts a five-coordinate distorted square pyramidal geometry. The zinc atom is 0.447 Å above the plane formed by N₂O₂ coordination from the salen motif. The axial position is occupied by a water molecule with the bond length of 2.066 Å. The Zn–O/N bond distances and angles observed for compound 6a are consistent with previously reported Zn²⁺-salen complexes.⁹ The pyrazoles (Fig. 1, Pz₁ and Pz₂) and the N-phenyls (Fig. 1, Ar₁ and Ar₄) form a three cycle co-planarized system via B–N coordination. The torsional angles observed for Ar₃–Ar₄ and Ar₂–Ar₁ are 18.5° and 32.3°, respectively, which indicates that Zn-salen and the B–N coordinated pyrazole moieties have slight deviation from planarity.

With this success, we further investigated the coordination ability of pocket 2 in 6a towards lanthanide ions. To our surprise complex 6a discriminated lanthanide ions and was able to detect Sm³⁺ ions selectively using pocket 2. We examined the emission spectra of complex 6a upon addition of 1.2 equiv. of various lanthanide ions such as Sm³⁺, Ce³⁺, Dy³⁺, Er³⁺, Eu³⁺, Gd³⁺, Ho³⁺, La³⁺, Pr³⁺, Tb³⁺, Tm³⁺ and Yb³. Except for Sm³⁺ ions, all other ions showed a weak fluorescence quenching at 500 nm (Fig. 3). However, a strong fluorescence quenching at 500 nm and enhanced emission peaks at 560 nm, 602 nm and 645 nm were observed on addition of 1.2 equivalents of Sm³⁺ ions to complex 6a. This could be due to an efficient energy transfer from 6a to the Sm³⁺ center upon complexation. These are characteristic peaks of Sm³⁺ emission resulting from the deactivation of the ⁴G_5/2 excited state to the ⁶H_J ground state (J = 9/2, 7/2, and 5/2).¹⁰ To determine the capability of 6a to detect Sm³⁺ in an aqueous medium, 1.5 equivalents of Sm³⁺ ions in water were added to 6a in THF solution, which showed an emission reminiscent of the methanol/THF system. This result suggests that 6a can also be used to discriminate lanthanide ions in the THF/H₂O medium.

Fig. 3 (a) Normalized fluorescence spectra of 6a (10 μM) upon the addition of different lanthanide ions (1.2 eq.) in (10 : 90) methanol/THF (λ_ex = 357). (b) Emission spectra of 6a + Sm³⁺ in (10 : 90) methanol/THF with increasing concentrations of Sm³⁺ ions (6a concentration of 1 × 10⁻⁵ M); (insets) color changes under a UV lamp of 6a and 6a + 1.2 equiv. Sm³⁺ at 1 × 10⁻⁵ M concentration in (10 : 90) methanol/THF.

In general, the lanthanide ion sensitization process takes place from the triplet state of the sensitizer.¹¹ To understand the electronic properties of complex 6a, and the sensitization phenomenon, TD-DFT calculations were performed using the optimized ground-state geometry of 6a. The T1 triplet energy of complex 6a was predicted using the corresponding TD-DFT calculation results of 6a, which gives an energy of 1053 nm/9497 cm⁻¹ (Table S2, ESI†). The triplet state energy of 6a is lower than the emissive state of Sm³⁺ ions (558 nm/17 900 cm⁻¹), which proves that the sensitization of Sm³⁺ by 6a is not taking place from the triplet state. Previous reports have also shown that the sensitization of Ln³⁺ centers can occur from the singlet state of the sensitizers.¹² From TD-DFT calculations, it was calculated that the S1 singlet state energy of 6a (388 nm/25 773 cm⁻¹) is above the emissive state ⁴G_5/2 of Sm³⁺. Thus, complex 6a is expected to sensitize Sm³⁺ emission from its singlet S1 state.

In order to investigate the interactions between 6a and Sm³⁺, we reacted complex 6a with Sm(NO₃)₃·6H₂O in a 1 : 1.5 ratio in the CHCl₃/EtOH solvent system at room temperature and isolated pure 6a·Sm³⁺ complex as a yellow precipitate. Single crystals of 6a·Sm³⁺ were grown by slow evaporation of DMF and methanol. To our delight, single-crystal X-ray diffraction analysis revealed the formation of a 1 : 1 adduct, in which Sm³⁺ forms a nine-coordinated complex with 6a (Fig. 4). Complex 6a·Sm³⁺ crystallizes in the monoclinic system with a space group of P2₁/c. The complex 6a is bound to the samarium ion through four oxygen atoms, in which the Sm1–O4 (2.667(4) Å) and Sm1–O5 (2.770(4) Å) bond distances are significantly longer than the Sm1–O1 (2.372(4) Å) and Sm1–O2 (2.357(3) Å) bond distances. The samarium ion is further coordinated to three nitrate ions adopting a distorted square pyramidal structure achieving a coordination number of nine. The zinc ion adopts a distorted square pyramidal coordination environment in the heterometallic complex. The Sm⋯Zn distance is 3.454 Å, suggesting that there is no direct Sm⋯Zn bond.

Fig. 4 (a) Crystal structure of the 6a·Sm³⁺ complex. Thermal ellipsoids are drawn at the 50% probability level. Hydrogen atoms are removed for clarity. Solid-state emission spectra of complexes (b) 6a and (c) 6a·Sm³⁺.

To determine the solid state emission properties of the 6a·Sm³⁺ complex, we excited the complex at 350 nm. The solid state emission of 6a·Sm³⁺ shows well-resolved peaks at 562 nm, 598 nm, 646 nm and 712 nm, which correspond to the transitions, ⁴G_5/2 → ⁶H_5/2, ⁴G_5/2 → ⁶H_7/2, ⁴G_5/2 → ⁶H_9/2 and ⁴G_5/2 → ⁶H_11/2, respectively. Among the emission peaks, the most intense emission at 645 nm corresponds to the ⁴G_5/2 → ⁶H_9/2 transition (Fig. 4). The complex 6a showed green emission whereas 6a·Sm³⁺ shows red emission in the solid state (Fig. 4).

The –OMe functionality plays an important role in forming the complex with Sm³⁺ ions. To test our hypothesis, we made compound 6b (Scheme 2) which does not have any –OMe functionality. As expected, the emission of compound 6b did not alter upon addition of Sm³⁺ ions (Fig. S5, ESI†). To investigate the importance of boron coordination, we synthesized complex 6c (Scheme 2) that does not have boron in the framework and characterized it using ¹H and ¹³C spectroscopy. Complex 6c showed a blue-shifted absorption and emission with respect to 6a (Fig. S7, ESI†). This signifies that boron coordination increases the rigidity of the pyrazole moiety, which resulted in slightly higher quantum yield and higher emission intensity for 6a with respect to 6c (Table S2, ESI†). When 1.5 equivalent of Sm³⁺ was added to 6c (1 × 10⁻⁵ M), we observed very less intense Sm³⁺ characteristic peaks in the emission spectrum and some residual emission from 6c (Fig. 5(a)). When the solution was kept under a hand held UV-lamp, no red emission was observed unlike for complex 6a (Fig. 5(d)). When 1.5 equiv. of Sm³⁺ was added to 6a, we observed intense Sm³⁺ characteristic peaks in the emission spectrum with no residual emission form 6a and intense red emission under a hand held UV-lamp (Fig. 5(a and b)). We believe that non-radiative decay due to lack of rigidity in complex 6c resulted in less intense red emission with respect to 6a. The lack of rigidity in 6c also resulted in inefficient energy transfer from the tri-phenyl pyrazole moiety to the Sm³⁺ center and thus caused relatively less intense red emission.

Fig. 5 (a) Fluorescence spectra of 6a–6d with 1.5 equiv. of Sm³⁺ ions in (10 : 90) methanol/THF, concentration = 1 × 10⁻⁵ M. (b–e) Color changes under a UV lamp of 6a and 6a + 1.5 equiv. Sm³⁺; 6b and 6b + 1.5 equiv. Sm³⁺; 6c and 6c + 1.5 equiv. Sm³⁺; 6d and 6d + 1.5 equiv. Sm³⁺ at 1 × 10⁻⁵ M ion concentration in (10 : 90) methanol/THF.

Dong and co-workers^6h reported a bimetallic Zn–Sm complex derived from a Salamo-type ligand and also observed selective sensitization of the Sm³⁺ ions. Although structural characterization and emission for the Zn(II)–Sm(III) complex was presented, there is no discussion about the concentration of the solution used for this study. We synthesized compound 6d, which is analogues to the complex of Dong and co-workers, and analysed its potential to test Sm³⁺ under identical conditions that we used in our study. Use of 6d as a probe did not show any emission color change (conc. 1 × 10⁻⁵ M), when 1.5 equiv. of Sm³⁺ was added to the THF/methanol solution of 6d (Fig. 5(a and e)). At the same time, 6a made it possible to detect Sm³⁺ at 1 × 10⁻⁵ M concentration.

In order to compare the energy transfer in 6a, 6c and 6d, we increased the concentration from 1 × 10⁻⁵ M to 5 × 10⁻⁵ M and plotted I_644nm/I_Zn (I_zn = emission maximum of the respective zinc complex) for all complexes with the addition of 1.5 equiv. Sm³⁺ ions. It was observed that the I_644nm/I_Zn ratio for 6a + 1.5 equiv. Sm³⁺ is almost 100 times higher than that of 6d and 20 times higher than that of 6c at the same concentration. The inset emission spectra, shown in Fig. 6(a), reveal the presence of residual emission from 6c at 484 nm and 6d at 444 nm. This residual emission from 6c and 6d masks the red emission from Sm³⁺, which can be observed under a hand held UV-Lamp (Fig. 6(b)). From these studies we conclude that our design has the advantage to discriminate Sm³⁺ ions at a very low concentration.

Fig. 6 (a) Comparing the energy transfer process in 6a, 6c, and 6d with addition of 1.5 equiv. of Sm³⁺ at 5 × 10⁻⁵ M ion concentration in (10 : 90) methanol/THF. (b) Comparison of 6a, 6c and 6d with 1.5 equiv. Sm³⁺ at 5 × 10⁻⁵ M concentration under a hand held UV-lamp.

Conclusions

To conclude, we have successfully synthesized a tetra-coordinated boron-based salen ligand that exhibits excellent selectivity for the detection of Zn²⁺ ions using pocket 1. Furthermore, the zinc complex 6a, showed remarkable luminescence response at a very low concentration upon the addition of Sm³⁺ ions and discriminates them from other lanthanide ions. X-Ray crystallography and other studies reveal that pocket 2 is responsible for the selective binding and sensitization of Sm³⁺ ions via a cooperative use of the B–N coordinated pyrazole motif and the –OMe functionality. The sensitivity of 6a was found to be 100 fold that of 6d. The design described here will be an excellent lead for the further development of ligands for the selective discrimination of metal ions.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors thank the Department of Atomic Energy (DAE) for financial support. We thank Dr U. Louderaj for helpful discussion about DFT studies. We also thank Principal Scientist Dr R. Natarajan from IICB-Kolkata for his help with X-ray studies and Dr Mriganka Sadhukan for elemental analysis.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental procedures, spectroscopic data, and copies of the ¹H and ¹³C NMR spectra. CCDC 2152949 and 2152950. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d2ma00516f

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