Synthesis of 2,2′-biimidazole-based platinum(II) polymetallaynes and tuning their fluorescent response behaviors to Cu²⁺ ions through optimizing the configuration of the organic spacers and steric effect

Abstract

Under controlled conditions, platinum(II) polymetallaynes with different 2,2′-biimidazole-based organic spacers have been synthesized readily. Their photophysical properties and fluorescent response behaviors to Cu²⁺ ions have been investigated in detail. By adjusting both the configuration of the 2,2′-biimidazole-based spacers and the steric effect, the fluorescent response behaviors of the polymetallaynes to Cu²⁺ ions can be tuned dramatically. The fluorescent signal from the polymetallayne with an optimized structure can be quenched rapidly by Cu²⁺ ions with a high Stern–Volmer constant K_SV of ca. 6.8 × 10⁴ M⁻¹ and a low detecting limit (DL) of ca. 0.99 ppm. These results not only highlight the great potential of these polymetallaynes as novel Cu²⁺ sensors, but also provide new strategies for optimizing the sensing abilities of the 2,2′-biimidazole-based ion sensors.

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1. Introduction

Despite the first report of 2,2′-biimidazole by Debus in 1858,¹ the chemistry of this biheterocycle compound had received little attention because the synthesis of the bicyclic structure is very laborious.² Furthermore, the extremely poor solubility of 2,2′-biimidazole in organic solvents has restrained its direct functionalization.² However, the 2,2′-biimidazole unit should possess great potential in areas, such as material science^3–5 or pharmacology,^6–8 based on its structural features, including nitrogen-containing heterocycle and conjugation. Therefore, the functionalization of 2,2′-biimidazole should be of great importance in view of the full tapping potential for its application in different fields. Triggered by this motivation, various strategies have been developed to functionalize 2,2′-biimidazole easily under mild conditions.^9,10 Critically, the unique structure associated with 2,2′-biimidazole can guarantee it as an ideal N,N′-bidentate ligand for constructing novel coordination complexes¹¹ with great potential in antitumour drugs,^6,7 bioinorganic chemistry,¹² catalysis,⁵ and supermolecular frameworks.¹³ Unfortunately, the biimidazole moiety has rarely been introduced into conjugated polymers, which represent a category of important optoelectronic materials. 2,2′-Biimidazole-based homopolymers have been prepared by the dehalogenative polycondensation by Yamamoto's group.¹⁴ Later, MacLean et al. successfully prepared conjugated polymers of 2,2′-biimidazole via electrochemical polymerization.¹⁵ However, these studies focused mainly on the structural and chemical aspects rather than the possible application features. Recently, Bai et al. prepared some novel pure organic conjugated polymers containing 2,2′-biimidazole units.^16,17 Importantly, these emissive conjugated polymers can serve as fluorescent ion sensors,^16,17 representing interesting applications of the 2,2′-biimidazole-based materials. Therefore, developing novel 2,2′-biimidazole-based polymers should represent important research lines in view of fully exploring the potential of 2,2′-biimidazole units for developing novel materials.

Polymetallaynes obtained by the incorporation of transition metal ions into a polymeric skeleton have been identified as an exciting and extensively investigated area in the field of conjugated polymers.¹⁸ The novel skeletons of polymetallaynes provide good scope for coupling the chemical, electronic and optical properties of metal complexes to those of the organic component, thus accessing novel polymers with new functional properties such as optical power limiting,¹⁹ electroluminescence,²⁰ and photovoltaic behavior.²¹ It has been shown that the chemical and electronic features of the organic components can have a great influence on the optoelectronic properties of polymetallaynes.¹⁸ Therefore, employing the 2,2′-biimidazole unit should be a feasible way to develop novel polymetallaynes, which has never been reported to the best of our knowledge. Bearing this in mind, polymetallaynes with the 2,2′-biimidazole unit have been developed. In addition, their fluorescent response behaviors to Cu²⁺ ions have been successfully tuned by optimizing the configuration of the 2,2′-biimidazole units and the steric effect. The results obtained will not only provide important structure–property relationship information of these polymetallaynes, but also indicate their potential in the field of ion sensors. Importantly, this contribution should provide a new strategy for optimizing the properties of the 2,2′-biimidazole-based sensors.

2. Experimental section

2.1. General information

All commercially available starting materials were used directly with no further purification. The solvents were dried carefully prior to use. 2,2′-Biimidazole was prepared using the adapted literature method.²² All reactions were monitored using thin-layer chromatography (TLC) purchased from Merck & Co., Inc. Flash column chromatography and preparative TLC were carried out using silica gel bought from Shenghai Qingdao (200–300 mesh). The ¹H NMR, ¹³C NMR and ³¹P NMR spectra were obtained in CDCl₃ on a Bruker Advance 400 MHz spectrometer and the chemical shifts were referenced to the solvent residual peak at δ 7.26 for ¹H and 77.0 for ¹³C, respectively. H₃PO₄ was used in the ³¹P NMR study as an external reference. Elemental analyses were preformed on a Flash EA 1112 elemental analyzer. The fast atom bombardment (FAB) mass spectra were obtained on a Finnigan MAT SSQ710 system. The thermogravimetric data were collected on a NETZSCH STA 409C instrument. The UV-vis absorption spectra were measured at room temperature on a Shimadzu UV-2250 spectrophotometer. The emission spectra, excitation spectra and lifetimes of these complexes were obtained on an Edinburgh Instruments Ltd. (FLSP920) fluorescence spectrophotometer using the software package provided by Edinburgh Instruments. The fluorescent quantum yields (Φ_F) were determined in THF solutions at 293 K against quinine sulfate in 1.0 M H₂SO₄ (Φ_F ca. 0.56).²³ The emission spectra of these complexes at 77 K were tested in THF matrix frozen by liquid nitrogen. The molecular weights of the polymetallaynes were determined using a Waters 2695 GPC in THF. The number-average and weight-average molecular weights were estimated using the calibration curve of polystyrene standards. The mass spectra for the species of organic ligands bonded with Cu²⁺ ions were collected using a micrOTOF-QII mass spectrometer with a mild electrospray ionization source. To the solution of the organic ligand (ca. 10⁻⁶ M), 1.0 equivalent of Cu²⁺ ions were then added. The corresponding solution (ca. 0.1 mL) was added to methanol (ca. 0.3 mL). After mixing, the solution (25 μL) was injected into the instrument to obtain the mass spectra.

2.2. Fluorescence titration

The solution of polymetallayne (2.0 mL, 5.0 × 10⁻⁵ M for the repeating unit in the polymer backbone) was placed in a quartz cell (10.0 mm width). The solution of CuCl₂ (1.0 μL, 5.0 × 10⁻³ M) in deionized water was then added to the quartz cell with a microsyringe. After gently shaking and standing for 60 seconds in the dark, the fluorescence spectrum was obtained. The other cation stock solutions were prepared by dissolving the metal salts in deionized water with a concentration of 5.0 × 10⁻³ M. For the fluorescence turn-on titration, the fluorescence spectra before and after adding 1.0 equivalent of Cu²⁺ ion were obtained first. The fluorescence spectra of the sample solution were measured after adding 0.5 and 1.0 equivalents of S²⁻ ion from Na₂S in deionized water (5.0 × 10⁻³ M). Before measuring the fluorescence spectra, the sample was shaken gently after the addition of the S²⁻ ions and kept in the dark for 5 minutes.

2.3. Computational details

Geometrical optimizations were conducted using the popular B3LYP functional theory. The basis set used for C, H, and N was 6-311G(d, p).^24,25 All calculations were carried out using the Gaussian 09 program.²⁶

2.4. Synthesis

General synthesis procedure for BIA1, BIA2 and BIA3. To a suspension of 2,2′-biimidazole in DMF, solid NaOH (2.2 equivalents) was added. After mixture was stirred for half an hour at room temperature, the corresponding haloalkane (2.2 equivalents of CH₃I for BIA1; 2.2 equivalents of 1-bromohexane for BIA2; 1.1 equivalents of 1,2-dibromoethane for BIA1) was added. After stirring for 4 h at room temperature, water was added to the reaction mixture, which was then extracted with dichloromethane (DCM). The organic phase was collected and dried over anhydrous Na₂SO₄. After concentration, the crude product was chromatographed on a silica column with mixture solvents of DCM/MeOH to give the title compounds.

BIA1 (V_DCM/V_MeOH = 20 : 1, yield 95%) ¹H NMR (400 MHz, CDCl₃): δ (ppm) 7.10 (d, J = 0.8 Hz, 2H), 6.95 (d, J = 0.4 Hz, 2H), 4.03 (s, 6H); ¹³C NMR (100 MHz, CDCl₃): δ (ppm) 138.64, 127.81, 122.59, 35.32; FAB-MS (m/z): 162 [M]⁺; anal. calcd for C₈H₁₀N₄: C, 59.24; H, 6.21; N, 34.54; found: C, 59.19; H, 6.28; N, 34.47.

BIA2 (V_DCM/V_MeOH = 20 : 1, yield 89%) ¹H NMR (400 MHz, CDCl₃): δ (ppm) 7.09 (d, J = 1.2 Hz, 2H), 6.98 (d, J = 1.2 Hz, 2H), 4.41 (t, J = 7.2 Hz, 4H), 1.70 (t, J = 6.8 Hz 4H), 1.26 (m, 12H), 0.83 (t, J = 6.8 Hz, 6H); ¹³C NMR (100 MHz, CDCl₃): δ (ppm) 138.04, 127.88, 120.95, 47.31, 31.24, 30.99, 26.13, 22.44, 13.93; FAB-MS (m/z): 302 [M]⁺; anal. calcd for C₁₈H₃₀N₄: C, 71.48; H, 10.00; N, 18.52; found: C, 71.36; H, 10.11; N, 18.43.

BIA3 (V_DCM/V_MeOH = 10 : 1, yield 83%) ¹H NMR (400 MHz, CDCl₃): δ (ppm) 7.18 (s, 2H), 6.95 (s, 2H), 4.36 (s, 4H); ¹³C NMR (100 MHz, CDCl₃): δ (ppm) 137.99, 130.21, 118.71, 43.12; FAB-MS (m/z): 160 [M]⁺; anal. calcd for C₈H₈N₄: C, 59.99; H, 5.03; N, 34.98; found: C, 59.89; H, 5.15; N, 34.88.

General synthesis procedure for BIBr1, BIBr2 and BIBr3. To a solution of BIA1/BIA2/BIA3 in chloroform, N-bromosuccinimide (2.2 equivalents) was added slowly at 0 °C. After addition, the reaction was allowed to proceed for 2 h at room temperature. The reaction mixture was extracted with DCM after the addition of an aqueous Na₂S₂O₃ solution. The organic phase was collected and dried over anhydrous Na₂SO₄. After concentration, the crude product was chromatographed on a silica column with mixture solvents of DCM/MeOH to give the title compounds.

BIBr1 (V_DCM/V_MeOH = 20 : 1, yield 63%) ¹H NMR (400 MHz, CDCl₃): δ (ppm) 7.19 (s, 2H), 3.96 (s, 6H); ¹³C NMR (100 MHz, CDCl₃): δ (ppm) 138.89, 128.51, 106.72, 33.87; FAB-MS (m/z): 320 [M]⁺; anal. calcd for C₈H₈Br₂N₄: C, 30.03; H, 2.52; N, 17.51; found: C, 29.91; H, 2.57; N, 17.46.

BIBr2 (V_DCM/V_MeOH = 20 : 1, yield 66%) ¹H NMR (400 MHz, CDCl₃): δ (ppm) 7.08 (s, 2H), 4.44 (t, J = 7.6 Hz, 4H), 1.66 (t, J = 6.8 Hz, 4H), 1.25 (m, 12H), 0.86 (t, J = 6.4 Hz, 6H); ¹³C NMR (100 MHz, CDCl₃): δ (ppm) 138.41, 128.70, 105.53, 46.09, 31.15, 30.22, 26.05, 22.48, 13.94; FAB-MS (m/z): 460 [M]⁺; anal. calcd for C₁₈H₂₈Br₂N₄: C, 46.97; H, 6.13; N, 12.17; found: C, 46.89; H, 6.21; N, 12.09.

BIBr3 (V_DCM/V_MeOH = 10 : 1, yield 60%) ¹H NMR (400 MHz, CDCl₃): δ (ppm) 7.19 (s, 2H), 4.32 (s, 4H); ¹³C NMR (100 MHz, CDCl₃): δ (ppm) 138.25, 130.72, 102.89, 41.64; FAB-MS (m/z): 318 [M]⁺; anal. calcd for C₈H₆Br₂N₄: C, 30.22; H, 1.90; N, 17.62; found: C, 30.17; H, 2.05; N, 17.49.

General synthesis procedure for BISi1, BISi2 and BISi3. Under a N₂ atmosphere, BIBr1/BIBr2/BIBr3, trimethylsilylacetylene (4.0 equivalents), PdCl₂(PPh₃)₂ (0.1 equivalent), and CuI (0.1 equivalent) were mixed in dry Et₃N. After reacting for 30 min at room temperature, the mixture was heated to 65 °C for 24 h. The solution was then cooled to room temperature and the solvent mixture was evaporated under vacuum. The crude product was purified by column chromatography on silica gel with a solvent combination of DCM/MeOH as the eluent to provide title compounds.

BISi1 (V_DCM/V_MeOH = 50 : 1, yield 75%) ¹H NMR (400 MHz, CDCl₃): δ (ppm) 7.36 (s, 2H), 4.00 (s, 6H), 0.27 (s, 18H); ¹³C NMR (100 MHz, CDCl₃): δ (ppm) 138.40, 133.75, 118.34, 103.64, 92.57, 33.59, −0.17; FAB-MS (m/z): 354 [M]⁺; anal. calcd for C₁₈H₂₆N₄Si₂: C, 60.97; H, 7.39; N, 15.80; found: C, 60.89; H, 7.32; N, 15.73.

BISi2 (V_DCM/V_MeOH = 50 : 1, yield 73%) ¹H NMR (400 MHz, CDCl₃): δ (ppm) 7.33 (s, 2H), 4.49 (t, J = 7.6 Hz, 4H), 1.70 (t, J = 6.8 Hz, 4H), 1.25 (m, 12H), 0.87–0.84 (m, 6H), 0.26 (s, 18H); ¹³C NMR (100 MHz, CDCl₃): δ (ppm) 137.77, 133.94, 117.43, 103.00, 93.04, 46.05, 31.26, 30.38, 26.20, 22.50, 13.96, −0.21; FAB-MS (m/z): 494 [M]⁺; anal. calcd for C₂₈H₄₆N₄Si₂: C, 67.96; H, 9.37; N, 11.32; found: C, 67.91; H, 9.42; N, 11.27.

BISi3 (V_DCM/V_MeOH = 30 : 1, yield 85%) ¹H NMR (400 MHz, CDCl₃): δ (ppm) 7.41 (s, 2H), 4.36 (s, 4H), 0.27 (s, 18H); ¹³C NMR (100 MHz, CDCl₃): δ (ppm) 135.96, 132.02, 128.48, 104.21, 91.49, 41.14, −0.22; FAB-MS (m/z): 352 [M]⁺; anal. calcd for C₁₈H₂₄N₄Si₂: C, 61.32; H, 6.86; N, 15.89; found: C, 61.23; H, 6.93; N, 15.79.

General synthesis procedure for L1, L2 and L3. Under N₂ atmosphere, tetrabutylammonium fluoride (2.2 equivalents) was added to the solution of BISi1/BISi2/BISi3 in DCM. The reaction mixture was stirred at room temperature for 2 h. The solvent was removed under reduced pressure. The residue was purified by column chromatography on silica gel using DCM/MeOH as the eluent to provide the title compounds.

L1 (V_DCM/V_MeOH = 20 : 1, yield 97%) ¹H NMR (400 MHz, CDCl₃): δ (ppm) 7.40 (s, 2H), 4.04 (s, 6H), 3.56 (s, 2H); ¹³C NMR (100 MHz, CDCl₃): δ (ppm) 138.50, 134.10, 117.33, 85.69, 72.15, 33.67; FAB-MS (m/z): 210 [M]⁺; anal. calcd for C₁₂H₁₀N₄: C, 68.56; H, 4.79; N, 26.65; found: C, 68.47; H, 4.84; N, 26.55.

L2 (V_DCM/V_MeOH = 20 : 1, yield 90%) ¹H NMR (400 MHz, CDCl₃): δ (ppm) 7.37 (s, 2H), 4.54 (t, J = 7.2 Hz, 4H), 3.53 (s, 2H), 1.70 (t, J = 7.2 Hz, 4H), 1.24 (m, 12H), 0.85 (t, J = 6.4 Hz, 6H); ¹³C NMR (100 MHz, CDCl₃): δ (ppm) 137.88, 134.44, 116.41, 85.16, 72.45, 46.06, 31.17, 30.43, 26.06, 22.45, 13.93; FAB-MS (m/z): 350 [M]⁺; anal. calcd for C₂₂H₃₀N₄: C, 75.39; H, 8.63; N, 15.98; found: C, 75.29; H, 8.60; N, 15.93.

L3 (V_DCM/V_MeOH = 10 : 1, yield 97%) ¹H NMR (400 MHz, CDCl₃): δ (ppm) 7.46 (s, 2H), 4.04 (s, 4H), 3.59 (s, 2H); ¹³C NMR (100 MHz, CDCl₃): δ (ppm) 136.35, 132.04, 128.43, 86.17, 71.24, 41.23; FAB-MS (m/z): 208 [M]⁺; anal. calcd for C₁₂H₈N₄: C, 69.22; H, 3.87; N, 26.91; found: C, 69.15; H, 3.92; N, 26.85.

General synthetic procedure for the polymetallaynes. Under a N₂ atmosphere, a catalytic amount of CuI was added to the solution of the corresponding organic ligand and trans-[PtCl₂(PBu₃)₂] (1.0 equivalent) in a mixture of Et₃N/DCM (v/v = 1 : 2). The mixture was stirred for 8 h at room temperature. After adding a sodium pyrophosphate aqueous solution, the reaction mixture was extracted with DCM. The organic phase was dried over anhydrous Na₂SO₄. The organic phase was filtered through a 0.45 μm PTFE syringe filter to remove the particles resulting from copolymerization. After concentration, the polymetallayne was purified twice by precipitation in methanol and dried under vacuum.

P1 (yield 87%) ¹H NMR (400 MHz, CDCl₃): δ (ppm) 7.05 (s), 3.99 (s), 2.12 (br), 1.58 (br), 1.46–1.41 (m), 0.92 (m); ³¹P NMR (161.9 MHz, CDCl₃): δ (ppm) 3.42; gel permeation chromatography (GPC): number-average molecular weight (M_n) = 8.8 × 10⁴ g mol⁻¹, polydispersity index (PDI) = 1.40 (against polystyrene standards).

P2 (yield 85%) ¹H NMR (400 MHz, CDCl₃): δ (ppm) 7.02 (s), 4.53 (s), 2.11 (br), 1.62–1.49 (br), 1.47–1.41 (m), 1.20 (br), 0.93–0.90 (m), 0.85–0.81 (m); ³¹P NMR (161.9 MHz, CDCl₃): δ (ppm) 3.64; gel permeation chromatography (GPC): number-average molecular weight (M_n) = 8.0 × 10⁴ g mol⁻¹, polydispersity index (PDI) = 1.48 (against polystyrene standards).

P3 (yield 86%) ¹H NMR (400 MHz, CDCl₃): δ (ppm) 7.10 (s), 4.26 (s), 2.09 (br), 1.58–1.57 (br), 1.47–1.41 (m), 0.95–0.91 (m); ³¹P NMR (161.9 MHz, CDCl₃): δ (ppm) 3.47; gel permeation chromatography (GPC): number-average molecular weight (M_n) = 8.8 × 10⁴ g mol⁻¹, polydispersity index (PDI) = 1.47 (against polystyrene standards).

3. Results and discussion

3.1. Synthesis and characterization

The synthesis procedures for the 2,2′-biimidazole-based polymetallaynes are shown in Scheme 1. After treating with solid NaOH in DMF at room temperature, the 2,2′-biimidazole was alkylated by adding the corresponding haloalkane to obtain BIA1–BIA3 in high yield (Scheme 1). In the ¹H NMR spectrum of BIA3, the single resonance peak at ca. 4.36 ppm can be ascribed to the ethylene unit, indicating the desired cyclization in the alkylation process of 2,2′-biimidazole. The 5,5′-dibromo-2,2′-biimidazole derivatives BIBr1–BIBr3 were obtained in high yield by the bromination of BIA1–BIA3 with N-bromosuccinimide (NBS) at room temperature. The compounds of BISi1–BISi3 were prepared by a Sonogashira cross-coupling reaction between the corresponding bromine derivative and trimethylsilylacetylene. The organic ligands L1–L3 were prepared successfully by cleaving the trimethylsilane groups with [nBu₄N]F from BISi1–BISi3. The two alkynyl groups in L1–L3 can afford their ability of copolymerization with trans-[PtCl₂(PBu₃)₂] to form the designed 2,2′-biimidazole-based polymetallaynes through the Sonogashira cross-coupling procedure provided in Scheme 1. After polymerization, the crude polymetallaynes solution was filtered through a 0.45 μm PTFE syringe filter to remove the particles resulting from copolymerization. The concentrated polymetallayne solution was added to methanol to precipitate the copolymers as off-white powders in high yield. The preparation of polymetallaynes is a very convenient way to obtain 2,2′-biimidazole-based conjugated polymers in high yield under very mild condition. Therefore, it should provide a good chance to more precisely control the content of the 2,2′-biimidazole units in the prepared polymers. This should be desirable for developing novel sensors with the 2,2′-biimidazole unit as a recognition moiety.

Scheme 1 Synthesis scheme for the biimidazole-based polymetallaynes.

3.2. Photophysical and thermal properties

In the UV-vis spectra for these polymetallaynes (Fig. 1), there is only one major absorption band located before ca. 400 nm. This can be safely assigned to the π–π* transition from both the biimidazole-based organic spacers and the polymer backbones. Clearly, the absorption maximum (ca. 383 nm) and absorption onset for P3 exhibit a bathochromic effect compared to those of P1 and P2 (Fig. 1 and Table 1). This result can be attributed to the good coplanar feature of the biimidazole units in P3 afforded by the locking effect of the ethylene unit. Therefore, it will definitely extend the conjugation of P3 and lead to a red-shifted absorption maximum and absorption onset for P3 as aforementioned. Clearly, the bulky hexyl groups in the biimidazole unit of P2 will induce a higher steric effect with the butyl moieties in PBu₃ units than the small methyl groups in P1. The steric effect will definitely impede the coplanarity of the biimidazole units and therefore extending the conjugation in P2. Therefore, the absorption maximum of P2 (ca. 377 nm) shows hypochromic effect referred to that of P1 (ca. 380 nm) (Fig. 1 and Table 1).

Fig. 1 The UV-vis and photoluminescence (PL) spectra for the 2,2′-biimidazole-based polymetallaynes in THF at 293 K.

Table 1 Photophysical and thermal-stability data for the 2,2′-biimidazole-based polymetallaynes

Polymers	Absorption λabs^a (nm) 293 K	Emission λem^a (nm) 293 K/77 K	τ^c S1/T1	ΦF^d	Eg^e (eV)	ΔT5% (°C)
a Measured in THF at a concentration of 10^−5 M for the repeating unit.b Shoulder.c The lifetime (τ) for the first singlet states (S1) was referred to the emission band at ca. 410 nm and that for the first triplet states (T1) was referred to the emission band around 510 nm. The τ for T1 was measured at 77 K. The excitation wavelength for the measure was set to 330 nm.d The fluorescent quantum yields (ΦF) were measured in THF with quinine sulfate in 1.0 M H2SO4 (ΦF ca. 0.56) as the standard. The excitation wavelength for the measure was set to 334 nm.e Calculated from the absorption onset.
P1	380	405, 420^b, 505/502, 540	1.75 ns/121.5 μs	0.10	3.08	300
P2	377	406, 420^b, 505/504, 540	1.75 ns/141.3 μs	0.12	3.09	305
P3	383	413, 427^b/518, 554	2.59 ns/131.1 μs	0.10	3.00	303

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To obtain the proper photoluminescence (PL) spectra of these polymetallaynes, their excitation spectra were measured in THF. From the excitation spectra (Fig. S1†), the most suitable excitation wavelength should be in the range from 330 nm to 350 nm. Therefore, the excitation wavelength was set to 340 nm for PL measurements of these biimidazole-based polymetallaynes. In THF solution, all the polymetallaynes exhibited an emission band at ca. 410 nm (Fig. 1 and Table 1) in their PL spectra under excitation with 340 nm light. From the absorption onset, P3 should possess the smallest band gap (E_g) (Table 1 and Fig. 1) due to the better conjugation extending ability of the biimidazole unit in P3. Therefore, it shows the longest emission wavelength (ca. 413 nm). Possessing a similar E_g, P1 and P2 display a similar emission maximum (Table 1 and Fig. 1). It seems that there is another emission band at a longer wavelength, ca. 500 nm, for P1 and P2, especially for P1. At 77 K, all the polymetallaynes showed a dominant emission band around 510 nm with a long lifetime in the order of microseconds (Table 1 and Fig. S2†), indicating its phosphorescent character. The major emission band at low temperatures can show good overlap with the weak emission band in P1 and P2 at room temperature (Fig. S2†). Therefore, this weak emission with long wavelength should be the phosphorescent signal induced by the platinum(II) centers. The phosphorescent signal at room temperature has also been observed in the fluorine-based analog. With quinine sulfate as a standard, the fluorescent quantum yields (Φ_F) for P1, P2 and P3 are 0.10, 0.12 and 0.10, respectively.

The thermal stability of the polymetallaynes was investigated by thermogravimetric analysis (TGA) under a nitrogen atmosphere. The TGA results indicate their thermal stability with the 5% weight-reduction temperature (ΔT_5%) at ca. 300 °C (Table 1). The good thermal properties will benefit their applications.

Fluorescence response in Cu²⁺ recognition. As aforementioned, 2,2′-biimidazole can be an ideal N,N′-bidentate ligand to chelate transition metal ions.^11,27,28 It has been reported that the N,N′-bidentate ligand can react with Cu²⁺ ions rapidly to form complex. Accordingly, it can be expected that the Cu²⁺ ions can chelate with 2,2′-biimidazole units in these polymetallaynes and the chelating process will have a high probability of inducing a variation of the fluorescence signal of the polymetallaynes. To evaluate the fluorescence response behavior of these polymetallaynes to the Cu²⁺ ions, the fluorescence titration experiment has been carried out for the THF solutions of the polymetallaynes with concentration ca. 5 × 10⁻⁵ M for the repeating units by adding an aqueous solution of CuCl₂ at 5 × 10⁻³ M. The experimental results are shown in Fig. 2.

Fig. 2 Fluorescence spectra of the biimidazole-based polymetallaynes upon the titration of Cu²⁺ in THF at 293 K.

From the fluorescence titration spectra for these polymetallaynes (Fig. 2), it can be seen clearly that the fluorescent signal for P1 and P3 has been efficiently quenched by Cu²⁺ ions. As shown in Fig. 2a and c, upon the addition of Cu²⁺ ions, the emission peaks at 405 nm for P1 and 413 nm for P2 decrease gradually with increasing Cu²⁺ concentration in the solution samples. After adding 1.2 equivalents of Cu²⁺, 73% and 84% of the fluorescence intensity has been quenched for P1 and P2, respectively. However, it seems that the fluorescent signal for P2 is quite insensitive to the added Cu²⁺ ions, because only 15% of the fluorescence intensity can be quenched at 1.2 equivalents of Cu²⁺ ions. The fluorescence quenching efficiency can be described by the Stern–Volmer equation, I₀/I = K_SV[A] + 1,²⁹ relating to the fluorescence intensity, I, at different concentrations of the analyte quencher, [A]. In this equation, I₀ is the intensity at [A] = 0 and K_SV is the Stern–Volmer constant. Based on the fluorescence titration results of these polymetallaynes in THF solution with Cu²⁺, K_SVs of these systems were determined to be 3.5 × 10⁴ M⁻¹ for P1, 7.9 × 10³ M⁻¹ for P2 and 6.8 × 10⁴ M⁻¹ for P3 (Fig. S3†). Accordingly, the detecting limits (DL) obtained by the literature method³⁰ were 2.0 × 10⁻⁶ M, 3.4 × 10⁻⁶ M and 9.9 × 10⁻⁷ M for P1, P2 and P3, respectively. The data indicates quantitatively a much higher sensitivity of the fluorescent signal of P1 and P3 to the external Cu²⁺ ions than that of P2. It is well known that the Cu²⁺ ions can coordinate easily with bipyridine (N,N′)-type bidentate ligands.²⁹ Therefore, the fluorescence quenching in these polymetallaynes should be ascribed to the electron-transfer interactions between the polyyne backbone and Cu²⁺ ions induced by the coordination of the 2,2′-biimidazole units with the Cu²⁺ ions. Based on the K_SV and DL data, it is clear that the coordinating ability of the 2,2′-biimidazole units in different polymetallaynes is in the order of P3 > P1 ≫ P2, indicating the critical role played by the structural features of the 2,2′-biimidazole unit in determining the fluorescence response behaviors of these polymetallaynes.

The titration plots of the emission intensity of these biimidazole-based polymetallaynes against [Cu²⁺]/[biimidazole unit] were obtained to interpret their fluorescence quenching behaviors. From the titration plots (Fig. S4†), the 1 : 1 stoichiometry for [Cu²⁺]/[biimidazole unit] can be clearly observed.³¹ Furthermore, the mass spectra also revealed that the organic ligands can form one-to-one adducts with Cu²⁺ ions as well (Fig. S5†). Based on the stoichiometry of the fluorescence quenching process, the chelating pattern of Cu²⁺ ions with the N,N′-type ligands^5,11 and the mass spectrum results, the possible mechanism involved in the fluorescence quenching of these biimidazole-based polymetallaynes is proposed in Fig. 3. According to the chelating pattern of Cu²⁺ ions with the N,N′-type bidentate ligands,^5,11 the coordinating of the 2,2′-biimidazole units with Cu²⁺ ions in P1 and P2 will involve the rotation of the imidazole rings to induce the configuration conversion of the 2,2′-biimidazole units from the low-energy optimized configuration with two alkyl groups in the trans pattern to the coplanar configuration with two alkyl groups in cis form (Fig. 3). Despite the energy difference between the two types of configurations of the 2,2′-biimidazole units in P1 and P2 being nearly identical (Fig. 4), P1 and P2 still exhibit markedly different fluorescent titration spectra (Fig. 2). Therefore, it can be concluded that energy associated with the configuration transfer of the 2,2′-biimidazole units might not be the decisive factor in the coordination process between the 2,2′-biimidazole units and Cu²⁺ ions in P1 and P2. To clarify this issue, the optimized configuration of the segment of P2 was obtained (Fig. 5). From the optimized configuration of the segment of P2, it can be seen clearly that there should be severe steric effect between the hexyl groups and the bulky tributylphosphine units chelated with the platinum(II) centers in the configuration transfer of the 2,2′-biimidazole units (Fig. 5). Obviously, the severe steric effect will greatly restrain the rotation of the imidazole rings and impede the formation of the desired configuration favorable for the coordination of the 2,2′-biimidazole units with Cu²⁺ ions (Fig. 3). Therefore, P2 shows much lower K_SV (7.9 × 10³ M⁻¹), almost an order magnitude lower, than P1 and P3 together with the highest DL of 3.4 × 10⁻⁶ M. However, there would be almost no space hindrance effect if the alkyl units attached to the 2,2′-biimidazole units are much smaller methyl groups. Therefore, the imidazole rings can easily rotate to facilitate the formation of the cis coplanar configuration of the 2,2′-biimidazole units in P1 and the coordination with the external Cu²⁺ ions to effectively quench the fluorescent signal of P1 (Fig. 3). Owing to the great tendency for the coordination of the Cu²⁺ ions with N,N′-type bidentate ligands,²⁷ the coordination between Cu²⁺ ions and the 2,2′-biimidazole units in P1 should release much more energy than that required for the configuration conversion of the 2,2′-biimidazole units. This means that the coordination between the cis 2,2′-biimidazole units and Cu²⁺ ions should be an energy-favored processed. Therefore, it will guarantee the coordination of the Cu²⁺ ions with the 2,2′-biimidazole units in P1 and effective fluorescence quenching in P1 by the added Cu²⁺ ions, as indicated by Fig. 2. Therefore, P1 exhibits a much higher K_SV of 3.5 × 10⁴ M⁻¹ as well as a lower DL of 2.0 × 10⁻⁶ M than P2.

Fig. 3 Proposed mechanism for the different fluorescence quenching behaviors in P1 (a), P2 (b) and P3 (c).

Fig. 4 Energy difference between the low-energy optimized trans configuration and the coplanar cis configuration of the different 2,2′-biimidazole units in P1 (a), P2 (b) and P3 (c).

Fig. 5 Optimized configuration for the repeating unit of the backbone of P2.

Obviously, the configuration of the 2,2′-biimidazole units in the backbone of P3 should be in great favor of their coordination with Cu²⁺ ions. The imidazole ring would need nearly no rotation to form the favored cis configuration of the 2,2′-biimidazole units in P3 (Fig. 3) because the dihedral angle between the two imidazole rings is only ca. 12° in the optimized configuration of the 2,2′-biimidazole units in P3 (Fig. 4 and S6†). Furthermore, the energy difference between the low-energy optimized configuration and the coplanar cis configuration of the 2,2′-biimidazole units in P3 is very small (Fig. 4), which will greatly facilitate the formation of the coplanar configuration. All these factors will effectively promote the coordination of Cu²⁺ ions with the 2,2′-biimidazole units in P3. Therefore, the fluorescent signal of P3 can be quenched most effectively by the added Cu²⁺ ions, which is indicated by its fluorescent titration spectra (Fig. 2) and the highest K_SV of 6.8 × 10⁴ M⁻¹ and the lowest DL of 9.9 × 10⁻⁷ M for P3 among the three polymetallaynes.

Interestingly, the quenched fluorescent signals of these polymetallaynes can be restored easily by adding S²⁻ ions (Fig. S7†). Clearly, the recovered fluorescence should be due to the very strong bonding between Cu²⁺ cations and S²⁻ anions indicated by the extremely low K_sp (ca. 1.3 × 10⁻³⁶ at 298 K) of CuS. This result also shows that the interaction between Cu²⁺ cations and 2,2′-biimidazole units should form one-to-one adducts, which leaves the one side of the Cu²⁺ cations exposed to the S²⁻ anions to facilitate their bonding process. Therefore, the result should also indicate the validity of the proposed mechanism in Fig. 3. In addition, these results should mean that the polymetallaynes might be recycled for Cu²⁺ sensing by adding S²⁻ anions.

From the aforementioned results, it can be concluded that optimizing both the configuration of the 2,2′-biimidazole units and the steric effect can play a very critical role in tuning the fluorescent response behaviors of the polymetallaynes to Cu²⁺ ions. Clearly, these results will definitely provide valuable information for optimizing the sensing properties of the 2,2′-biimidazole derivatives as Cu²⁺ sensors.

The response behavior of the fluorescent signal of these polymetallaynes to other cations with different charge has also been investigated. As shown in Fig. 6 and S8,† the experimental results clearly show that only the Cu²⁺ ions can show the most efficient quenching effect to the fluorescent signal of these polymetallaynes, especially P1 and P3 (Fig. 6). In addition, after absorption calibration, the precious metal ions, such as Ru³⁺, Ir³⁺ and Rh³⁺, will not interfere with the Cu²⁺ sensing process of P1 and P3 as well. This result shows that P1 and P3 can exhibit good selectivity, if they would be employed as Cu²⁺ sensors.

Fig. 6 Selectivity of the fluorescent signal of the P1 (a) and P3 (b) to the Cu²⁺ ions (I: the fluorescent signal intensity after adding 1.2 equivalents of different ion; I₀: the fluorescent signal intensity before adding ions). 1: Ag⁺, 2: Zn²⁺, 3: Fe³⁺, 4: Ni²⁺, 5: Na⁺, 6: Mn²⁺, 7: Mg²⁺, 8: K⁺, 9: Co²⁺, 10: Hg²⁺, 11: Cd²⁺, 12: Ca²⁺, 13: NH₄⁺, 14: Ru³⁺, 15: Ir³⁺, 16: Rh³⁺, 17: Cu²⁺.

4. Conclusion

In summary, three novel 2,2′-biimidazole-based platinum(II) polymetallaynes were successfully designed and synthesized through the Sonogashira cross-coupling procedure. Their fluorescent response behaviors to the Cu²⁺ ions were studied in detail. It was found that the chemical structures of the 2,2′-biimidazole-based spacers can play a very critical role in determining the fluorescent response behaviors of the concerned polymetallaynes to Cu²⁺ ions. By optimizing both the configuration of the 2,2′-biimidazole units and the steric effect involved, the fluorescent signal of the polymetallaynes can be quenched effectively by the Cu²⁺ ions. With the optimized structure, the polymetallayne can show a high Stern–Volmer constant K_SV of ca. 6.8 × 10⁴ M⁻¹ to Cu²⁺ ions with a detection limit (DL) of ca. 0.99 ppm. Furthermore, the fast fluorescent response behavior of the optimized polymetallaynes can show good selectivity to Cu²⁺ ions, avoiding interference from other cations. Therefore, in view of their facile preparation under very mild conditions even at room temperature, these polymetallaynes should exhibit great potential in Cu²⁺ sensing. More importantly, the obtained results will provide valuable information for the development of novel sensors based on the 2,2′-biimidazole derivatives.

Acknowledgements

This study was financially supported by Tengfei Project from Xi'an Jiaotong University, the Fundamental Research Funds for the Central Universities (cxtd2015003), the Program for New Century Excellent Talents in University, the Ministry of Education of China (NECT-09-0651), the Key Creative Scientific Research Team in Shaanxi Province (2013KCT-05), the China Postdoctoral Science Foundation (Grant no. 20130201110034), and the National Natural Science Foundation of China (no. 20902072, 21572176).

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Footnote

† Electronic supplementary information (ESI) available: Synthetic detail of 2,2′-biimidazole, PL spectra at different temperature and theoretical fitting results. See DOI: 10.1039/c5ra17853c

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