Chalcomer assembly of optical chemosensors for selective Cu²⁺ and Ni²⁺ ion recognition

Abstract

The o-, m- and p-isomeric units of chalconyl triazole-based, caged organosilicon complexes were efficiently synthesized and explored for their cationic chemosensing activities. The UV-vis spectral studies performed show considerable variations in absorption spectra and molar absorptivity constant. The recognition studies display efficient sensing for the o-isomer of chalcone-linked 1,2,3-triazole silatrane (CTSI) 1–3, which act as dual-ion fluorescent sensors towards Cu²⁺ and Ni²⁺ ions. This preference of o-isomers (CTSI 1–3) over m- and p-isomers (CTSI 4–9) in quenching is due to specific ‘fitting in’ of the coordination sphere available for ion binding. Further, the exceptional activity of CTSI 8 to exclusively sense Ni²⁺ ions differs from the other studied quenching response patterns, acting via a ‘turn-on’ fluorescence response. The variation of pH and temperature on the chemosensing behavior of CTSI 1–3 led us to optimize conditions for quenching studies. Moreover, competitive quenching studies confirm the feebly enhanced selectivity for Cu²⁺ over Ni²⁺ ions. Stern–Volmer constant (K_SV) for all active isomers show comparative quenching response towards both cationic species. This is the first the time that organosilicon complexes are used to actively sense Cu²⁺ and Ni²⁺ ions using water as part of the solvent mixture.

---

Introduction

Isomers continue to be an area of keen interest and play a fundamental role in the efficient functioning of living systems.¹ The variation in chemical properties of isomeric units is regulated by the difference in their reactivity and interaction with substrate entities. This effect can be best illustrated in this scenario: one isomer of a dietary ingredient acts as an active and medicinally essential pharmacophore, while its isomeric forms may be potentially fatal.² This utility of isomers draws a parallel analogy to biomolecules that operate by ‘lock and key mechanism’ and via ‘on–off’ activity.³ The analytical challenge imposed by the difference in activity of isomeric units presents a major complication in developing biological applications. Moreover, the importance of isomers in qualitative and quantitative detection of excess metal ions by fluorescent recognition studies is little explored and forms an area of increasing concern in ion detection therapeutic studies.^4,5

Chemosensors form robust and consistent molecular tools. They are extensively used in the detection of environmental pollutants such as toxic metals and act as biological markers in medical diagnostic studies.^6–8 The ion-detecting property of a fluorescent chemosensor is based upon host–guest relationship, in the way the receptor is linked to the fluorophore in the recognition process.⁹ This ‘host’ component is covalently or coordinate-covalently linked to the ‘guest’ module, and complexation with the target metal ion results in the variation of either intensity or position of the emission band for the chem–fluorophore.¹⁰ The specificity and selectivity of a receptor in binding targets impart singularity to each sensor.¹¹ The selective recognition of target multivalent transition metals by chemosensors has attracted considerable attention due to increasing concerns on human health and environmental safety.^12,13 Heavy metal ions like Cd²⁺, Cu²⁺, Hg²⁺, Pb²⁺, Ni²⁺, Fe²⁺, Al³⁺ and Zn²⁺ form a major component of environmental pollutants that indirectly affect organisms with serious health disorders. Therefore, the qualitative and quantitative detection of these toxic elements is a must for proper treatment of these toxic contaminants.¹⁴

Copper and nickel are among the crucial transition metal ions for living systems, helping in proper enzyme functioning and facilitating effective iron absorption.¹⁵ Cu(II) acts as cofactor for numerous metalloenzymes, whereas Ni(II) assists in iron uptake from the intestine, thus regulating cellular energy level and maintaining metabolic homeostasis. The exposure to a slight excess of these cations can cause major syndromes such as Menke's disease, Wilson's disease, Alzheimer's disease, and gastrointestinal disorder leading to liver or kidney damage.¹⁶ The design of new fluorogenic chemosensors for the active recognition of these excess cations with good emission wavelength and optical stability is a must to regulate the toxicity levels. Many systems have been reported, citing the fluorescence sensing of Cu²⁺ and Ni²⁺ ions independently, which is based upon the binding capability of various chelating agents.^17–19 Recently, there has been a great interest in the chemosensing of transition metal ions, but limited literature is available for dual sensing for Cu²⁺ and Ni²⁺ ions.^20–23 As a continuous work, nine isomers of the triazolyl-substituted α,β-unsaturated chalcone (chalcomer) backbone are explored for their fluorogenic receptor properties as a dual chemosensor for both Cu²⁺ and Ni²⁺ ions.

Chalcones are α,β-unsaturated keto products synthesized by the Claisen–Schmidt condensation reaction of aldehydes and ketones. The significant attention on chalcones is due to their wide applicability in medicine, with their antimalarial, antipathogenic, antitumorigenic, anti-inflammatory, antioxidant, antituberculosis, and anti-HIV activities.^24–29 Besides therapeutic effects, chalcones can also act as good chemosensors for various cations due to their conjugate π-electronic system.³⁰ ‘Click chemistry’ can be exploited to generate active podants that can act as fluorescent potential chemosensors for the detection of various transition metal ions above their inhibitory toxic concentration level.^31,32 Moreover, the five-membered triazolyl heterocycle is associated with vast applications such as antimycobacterial, antituberculosis, antiangiogenic, antiviral, anticancerous and anti-HIV activity.^33–36 Silicon capping results in a stable product that increases the applicability of chalcone triazolyl silicon compounds to nanoparticles and biologically active entities.^37–43 In particular, our focus lies on readily available substrates that can undergo transformations to generate chalcone 1,2,3-triazole silatranes isomers CTSI 1–9 (Scheme 1).

Scheme 1 Entities for generation of chalcomers 1–9.

Experimental

Caution! Azide compounds are explosive to heat and shock. Great care and protection is required for handling of these compounds.

General material and methods

All the syntheses were carried out under dry nitrogen atmosphere using vacuum glass line. The organic solvents used were dried and purified according to the standard procedure and stored under dry nitrogen atmosphere. Bromotris(triphenylphosphine)copper(I) (Aldrich), γ-chloropropyltriethoxysilane (ClPTES) (Aldrich), propargyl bromide (80 wt% solution in toluene) (Aldrich), sodium azide (SDFCL), potassium carbonate (Thomas Baker), 2-hydroxyacetophenone (SDFCL), 3-hydroxyacetophenone (SDFCL), 4-hydroxyacetophenone (SDFCL), salicylaldehyde (Aldrich), 3-hydroxybenzaldehyde (SDFCL), 4-hydroxybenzaldehyde (SDFCL) and triethanolamine (SDFCL) were used as supplied for synthesis of compounds CTSI 1–9. γ-Azidopropyltriethoxysilane (AzPTES) was synthesized according to the procedure known in literature.⁴⁴

Melting points were uncorrected and measured in a Mel Temp II device using sealed capillaries. Infrared spectrum was obtained neat on a Thermo Scientific Fischer spectrometer. Multinuclear NMR (¹H, ¹³C) spectra were recorded on a Bruker advance II 400 NMR spectrometer (in CDCl₃) at 298 K. Mass spectroscopy data of synthesized compounds CTSI 1–9 were recorded on a Waters QQ-TOF micro mass spectrometer. UV-vis spectra were recorded on a JASCO V-530 UV-vis spectrophotometer. CHN analyses were obtained on a Perkin-Elmer Model 2400 CHNS elemental analyser. Fluorescence spectroscopy study was performed on a PerkinElmer LS55 fluorescence spectrophotometer.

Synthesis of compounds CTSI 1–9

To the uniformly stirred solution of triethanolamine (1.0 equiv) and potassium hydroxide (cat. amt.) in toluene, chalcone 1,2,3-triazole triethoxysilane (C 1–9) (see ESI†) was slowly added dropwise within 2 min. The mixture was refluxed for 5 h, and after completion, the reaction mixture was cooled to room temperature. The solvent volume in mixture was reduced to 3 mL by vacuum evaporation, and the addition of 10 mL of n-pentane resulted in the separation of a slightly coloured solid product. The solid obtained was stirred for 48 h, filtered and washed twice with 5 mL n-pentane to yield CTSI 1–9.

Spectroscopic data for compounds CTSI 1–9

1,3-Bis(2-(-(1-(3-(silatranyl)propyl)-1H-1,2,3-triazol-4-yl)methoxy)phenyl)prop-2-en-1-one (CTSI 1). Yield: 72%; m.p. = 130–131 °C (decom.); empirical formula: C₃₉H₅₂N₈O₉Si₂: anal. calcd: C, 56.2; H, 6.3; N, 13.5; found: C, 56.3; H, 6.2; N, 13.5; IR (neat, cm⁻¹): 2946, 2873, 2807, 1596, 1483, 1448, 1381, 1331, 1237, 1164, 1122, 1096, 989, 909, 850, 753, 681, 616, 583, 541. ¹H NMR (400 MHz, CDCl₃) δ = 8.12–7.99 (m, 1H), 7.88 (d, ³J = 25.6 Hz, 1H), 7.80 (dd, ³J = 18.6, 6.8 Hz, 1H), 7.70–7.60 (m, 1H), 7.60–7.50 (m, 2H), 7.44 (dd, ³J = 15.1, 10.4 Hz, 1H), 7.27 (d, ³J = 8.5 Hz, 1H), 7.11–6.99 (m, 2H), 6.89–6.56 (m, 2H), 5.25–5.23 (m, 2H), 4.46–3.97 (m, 4H), 3.73–3.63 (m, 4H), 3.47 (t, ³J = 5.6 Hz, 14H) [2H extra], 2.85–2.76 (m, 12H), 2.67–2.42 (m, 6H), 1.90–1.74 (m, 4H), 0.24–0.12 (m, 4H). ¹³C NMR (101 MHz, CDCl₃) δ = 189.37, 164.64, 163.92, 150.45, 144.09, 138.84, 136.04, 135.98, 134.63, 128.54, 127.71, 123.09, 122.90, 120.56, 68.15, 61.15, 58.52, 51.52, 30.19, 13.43. MS (ES⁺) calcd for [M + Na]⁺ 855.3; found 855.5.

1-(2-(-(1-(3-(Silatranyl)propyl)-1H-1,2,3-triazol-4-yl)methoxy)phenyl)-3-(3-(-(1-(3-(silatranyl)propyl)-1H-1,2,3-triazol-4-yl)methoxy)phenyl)prop-2-en-1-one (CTSI 2). Yield: 76%; m.p. = 159–160 °C (decom.); Empirical formula: C₃₉H₅₂N₈O₉Si₂: anal. calcd: C, 56.2; H, 6.3; N, 13.5; found: C, 56.2; H, 6.2; N, 13.4; IR (neat, cm⁻¹): 2942, 2873, 2815, 1596, 1482, 1447, 1350, 1236, 1150, 1122, 1095, 973, 909, 851, 751, 680, 616, 580, 540. ¹H NMR (400 MHz, CDCl₃) δ = 7.58 (dd, ³J = 13.8, 7.4 Hz, 2H), 7.48 (d, ³J = 15.9 Hz, 1H), 7.39 (dd, ³J = 19.2, 11.6 Hz, 2H), 7.32–7.13 (m, 2H), 7.05 (dd, ³J = 12.8, 8.3 Hz, 2H), 7.01–6.87 (m, 2H), 6.81 (s, 1H), 5.24 (s, 2H), 5.14 (s, 2H), 4.28 (t, ³J = 7.1 Hz, 2H), 4.04 (t, ³J = 7.1 Hz, 2H), 3.68 (t, ³J = 5.8 Hz, 6H), 3.66 (t, ³J = 5.8 Hz, 6H), 2.72 (dt, ³J = 5.8 Hz, 12H), 1.74–1.47 (m, 4H), 0.34–0.30 (m, 2H), 0.22–0.18 (m, 2H). ¹³C NMR (101 MHz, CDCl₃) δ = 198.29, 164.68, 162.91, 148.22, 142.48, 139.22, 138.10, 137.96, 136.62, 135.47, 134.56, 134.44, 133.71, 128.94, 127.50, 127.42, 122.87, 120.18, 119.05, 68.96, 58.38, 51.51, 30.21, 13.36.

1-(2-(-(1-(3-(Silatranyl)propyl)-1H-1,2,3-triazol-4-yl)methoxy)phenyl)-3-(-(4-(1-(3-(silatranyl)propyl)-1H-1,2,3-triazol-4-yl)methoxy)phenyl)prop-2-en-1-one (CTSI 3). Yield: 73%; m.p. = 91–92 °C (decom.); empirical formula: C₃₉H₅₂N₈O₉Si₂: anal. calcd: C, 56.2; H, 6.3; N, 13.5; found: C, 56.0; H, 6.3; N, 13.4; IR (neat, cm⁻¹): 2938, 2873, 2807, 1596, 1483, 1447, 1380, 1330, 1237, 1150, 1122, 1096, 988, 909, 850, 752, 680, 617, 581, 541. ¹H NMR (400 MHz, CDCl₃) δ = 7.74 (d, ³J = 8.7 Hz, 2H), 7.61 (d, ³J = 8.8 Hz, 1H), 7.42 (d, ³J = 15.4 Hz, 2H), 7.35 (d, ³J = 16.2 Hz, 2H), 7.24 (s, 1H), 7.18 (dd, ³J = 9.1, 6.4 Hz, 2H), 7.01 (t, ³J = 6.6 Hz, 2H), 6.92 (d, ³J = 7.8 Hz, 2H), 6.78 (d, ³J = 8.7 Hz, H), 6.74–6.67 (m, 3H), 5.01 (d, 2H), 4.98 (d, 2H), 4.49 (t, ³J = 7.1 Hz, 4H), 3.62 (t, ³J = 5.8 Hz, 12H), 2.80 (t, ³J = 5.8 Hz, 12H), 1.79–1.72 (m, 4H), 0.30–0.26 (m, 4H). ¹³C NMR (101 MHz, CDCl₃) δ = 191.13, 165.74, 162.49, 147.32, 140.50, 136.02, 135.56, 134.65, 133.70, 132.30, 125.90, 120.60, 118.42, 117.86, 66.06, 57.31, 56.11, 51.11, 28.10, 11.33.

3-(2-(-(1-(3-(Silatranyl)propyl)-1H-1,2,3-triazol-4-yl)methoxy)phenyl)-1-(3-(-(1-(3-(silatranyl)propyl)-1H-1,2,3-triazol-4-yl)methoxy)phenyl)prop-2-en-1-one (CTSI 4). Yield: 77%; m.p. = 211–213 °C; empirical formula: C₃₉H₅₂N₈O₉Si₂: anal. calcd: C, 56.2; H, 6.3; N, 13.5; found: C, 56.2; H, 6.4; N, 13.3; IR (neat, cm⁻¹): 2938, 2873, 2811, 1596, 1482, 1447, 1381, 1350, 1236, 1150, 1122, 1096, 974, 909, 851, 751, 680, 615, 582, 540. ¹H NMR (400 MHz, CDCl₃) δ = 8.01 (dd, ³J = 13.7, 6.7 Hz, 1H), 7.64–7.57 (m, 2H), 7.57–7.49 (m, 2H), 7.49–7.43 (m, 2H), 7.37 (ddd, ³J = 25.9, 7.3, 2.6 Hz, 2H), 7.14 (dt, ²J = 4.6, 3.9 Hz, 1H), 7.09–6.91 (m, 2H), 5.24 (s, 2H), 5.17 (s, 2H), 4.29–4.24 (m, 4H), 3.69–3.65 (m, 12H), 2.73 (q, ³J = 5.8 Hz, 12H), 1.97–1.88 (m, 4H), 0.37–0.31 (m, 4H). ¹³C NMR (101 MHz, CDCl₃) δ = 191.59, 166.47, 161.56, 146.72, 135.12, 134.00, 133.84, 131.45, 125.84, 124.66, 119.66, 117.27, 116.86, 65.30, 58.49, 58.24, 51.43, 27.40, 10.43.

1-(3-(-(1-(3-(Silatranyl)propyl)-1H-1,2,3-triazol-4-yl)methoxy)phenyl)-3-(3-(-(1-(3-(silatranyl)propyl)-1H-1,2,3-triazol-4-yl)methoxy)phenyl)prop-2-en-1-one (CTSI 5). Yield: 77%; m.p. > 220 °C; empirical formula: C₃₉H₅₂N₈O₉Si₂: anal. calcd: C, 56.2; H, 6.3; N, 13.5; found: C, 56.1; H, 6.2; N, 13.4; IR (neat, cm⁻¹): 2929, 2873, 2807, 1660, 1578, 1482, 1438, 1381, 1249, 1169, 1122, 1097, 1012, 938, 909, 849, 772, 682, 616, 582, 541. ¹H NMR (400 MHz, DMSO) δ = 8.13 (s, 1H), 7.96–7.87 (m, 1H), 7.83–7.76 (m, 1H), 7.74 (t, ²J = 4.1 Hz, 1H), 7.69 (d, ²J = 4.7 Hz, 1H), 7.65–7.59 (m, 2H), 7.54 (dd, ³J = 7.0, 2.3 Hz, 1H), 7.51–7.45 (m, 1H), 7.43 (d, ³J = 9.3 Hz, 1H), 7.09 (dd, ³J = 18.2, 6.5 Hz, 2H), 5.24 (s, 1H), 5.21 (s, 1H), 4.89 (s, 1H), 4.86 (s, 1H), 4.24 (dd, ³J = 9.4, 4.5 Hz, 4H), 3.62 (t, ³J = 5.3 Hz, 11H) [1H missing], 2.80 (t, ³J = 5.8 Hz, 11H) [1H missing], 1.86–1.81 (m, 4H), 0.16–0.12 (dd, ³J = 9.2, 6.8 Hz, 4H). ¹³C NMR (101 MHz, CDCl₃) δ = 188.34, 163.30, 162.47, 145.23, 144.83, 137.12, 136.85, 134.68, 134.45, 129.17, 127.78, 126.52, 126.38, 119.09, 117.78, 67.63, 57.36, 56.51, 50.56, 29.19, 12.42.

1-(3-(-(1-(3-(Silatranyl)propyl)-1H-1,2,3-triazol-4-yl)methoxy)phenyl)-3-(4-(-(1-(3-(silatranyl)propyl)-1H-1,2,3-triazol-4-yl)methoxy)phenyl)prop-2-en-1-one (CTSI 6). Yield: 81%; m.p. < 220 °C; empirical formula: C₃₉H₅₂N₈O₉Si₂: anal. calcd: C, 56.2; H, 6.3; N, 13.5; found: C, 56.3; H, 6.2; N, 13.5; IR (neat, cm⁻¹): 2933, 2872, 2807, 1655, 1596, 1508, 1480, 1438, 1381, 1350, 1303, 1244, 1172, 1122, 1096, 989, 938, 909, 831, 762, 680, 616, 583, 542. ¹H NMR (400 MHz, CDCl₃) δ = 7.69–7.61 (m, 2H), 7.61–7.56 (m, 2H), 7.55–7.52 (m, 1H), 7.52–7.43 (m, 2H), 7.26 (d, ¹J = 1.3 Hz, 2H), 7.22 (dd, ²J = 5.0, 3.9 Hz, 1H), 7.14 (ddd, ³J = 13.1, 10.6, 5.0 Hz, 2H), 5.16 (m, 3H) [1H missing], 4.70 (t, ³J = 6.5 Hz, 1H), 4.24 (t, ³J = 14.9 Hz, 4H), 3.65 (dd, ³J = 12.1, 7.5 Hz, 11H) [1H missing], 2.75–2.68 (m, 12H), 1.92–1.86 (m, 4H), 0.31–0.28 (m, 4H). ¹³C NMR (101 MHz, CDCl₃) δ = 186.13, 165.74, 162.49, 147.32, 140.50, 136.02, 133.56, 133.38, 134.65, 133.70, 132.30, 125.90, 120.60, 118.42, 117.86, 66.06, 57.31, 56.11, 51.11, 28.10, 11.33.

3-(2-(-(1-(3-(Silatranyl)propyl)-1H-1,2,3-triazol-4-yl)methoxy)phenyl)-1-(4-(-(1-(3-(silatranyl)propyl)-1H-1,2,3-triazol-4-yl)methoxy)phenyl)prop-2-en-1-one (CTSI 7). Yield: 80%; m.p. = 156–158 °C; empirical formula: C₃₉H₅₂N₈O₉Si₂: anal. calcd: C, 56.2; H, 6.3; N, 13.5; found: C, 56.1; H, 6.1; N, 13.6; IR (neat, cm⁻¹): 2946, 2873, 2807, 1656, 1597, 1507, 1454, 1338, 1303, 1275, 1245, 1213, 1168, 1122, 1095, 987, 938, 909, 933, 759, 678, 616, 584, 572. ¹H NMR (400 MHz, DMSO) δ = 8.26 (s, 2H), 7.72 (s, 2H), 7.59 (dt, ³J = 15.5, 7.1 Hz, 2H), 7.51–7.46 (m, 2H), 7.08 (d, ³J = 8.9 Hz, 2H), 7.04–6.97 (m, 2H), 5.24 (s, 2H), 5.21 (s, 2H), 4.27 (dd, ³J = 18.4, 7.2 Hz, 4H), 3.64 (dd, ³J = 12.4, 6.0 Hz, 12H), 2.79 (dd, ³J = 12.3, 6.0 Hz, 12H), 1.94–1.84 (m, 4H), 0.24–0.17 (m, 4H). ¹³C NMR (101 MHz, CDCl₃) δ = 187.96, 165.23, 161.81, 148.00, 147.71, 143.81, 142.57, 136.12, 135.21, 135.02, 134.87, 133.70, 126.12, 124.63, 118.93, 117.83, 117.17, 116.12, 65.45, 57.50, 59.95, 51.55, 28.60, 11.84.

3-(3-(-(1-(3-(Silatranyl)propyl)-1H-1,2,3-triazol-4-yl)methoxy)phenyl)-1-(4-(-(1-(3-(silatranyl)propyl)-1H-1,2,3-triazol-4-yl)methoxy)phenyl)prop-2-en-1-one (CTSI 8). Yield: 78%; m.p. = 165–167 °C; empirical formula: C₃₉H₅₂N₈O₉Si₂: anal. calcd: C, 56.2; H, 6.3; N, 13.5; found: C, 56.2; H, 6.3; N, 13.5; IR (neat, cm⁻¹): 2950, 2875, 2811, 1652, 1597, 1421, 1349, 1305, 1248, 1216, 1169, 1093, 1011, 910, 777, 721, 684, 541. ¹H NMR (400 MHz, CDCl₃) δ = 8.00–7.94 (m, 2H), 7.68 (d, ³J = 15.6 Hz, 1H), 7.63–7.59 (m, 2H), 7.49–7.45 (m, 1H), 7.45–7.37 (m, 1H), 7.25 (d, ³J = 8.0 Hz, 1H), 7.16 (d, ³J = 7.6 Hz, 1H), 7.03 (s, 1H), 7.01–6.96 (m, 2H), 5.21 (s, 2H), 5.17 (s, 2H), 4.27 (t, ³J = 7.5 Hz, 4H), 3.68 (t, ³J = 5.8 Hz, 12H), 2.74 (t, ³J = 5.8 Hz, 12H), 1.96–1.89 (m, 4H), 0.37–0.35 (m, 4H). ¹³C NMR (101 MHz, CDCl₃) δ = 188.68, 162.23, 158.75, 143.74, 143.19, 142.72, 136.53, 132.10, 131.37, 130.86, 129.96, 128.51, 122.80, 122.28, 121.76, 114.72, 114.33, 62.32, 57.47, 53.48, 51.01, 26.34, 13.21.

1,3-Bis(4-(-(1-(3-(silatranyl)propyl)-1H-1,2,3-triazol-4-yl)methoxy)phenyl)prop-2-en-1-one (CTSI 9). Yield: 83%; m.p. = 173–175 °C; empirical formula: C₃₉H₅₂N₈O₉Si₂: anal. calcd: C, 56.2; H, 6.3; N, 13.5; found: C, 56.3; H, 6.2; N, 13.3; IR (neat, cm⁻¹): 2949, 2876, 2811, 1655, 1597, 1508, 1438, 1350, 1305, 1248, 1215, 1168, 1096, 984, 909, 770, 721, 684, 645, 540. ¹H NMR (400 MHz, CDCl₃) δ = 7.65 (d, ³J = 8.4 Hz, 1H) (1H missing), 7.55 (d, ³J = 13.8 Hz, 2H), 7.46 (d, ³J = 7.8 Hz, 2H), 7.31 (d, ³J = 15.7 Hz, 2H), 6.92 (d, ³J = 8.0 Hz, 2H), 6.47 (d, ³J = 8.3 Hz, 1H), 6.41 (s, 1H), 5.16 (s, 2H), 4.07 (s, 2H), 3.65 (t, ³J = 6.2 Hz, 12H), 2.75 (t, ³J = 6.2 Hz, 12H), 1.77–1.73 (m, 4H), 0.34–0.30 (m, 4H). ¹³C NMR (101 MHz, CDCl₃) δ = 195.53, 169.02, 165.28, 164.83, 148.54, 146.80, 137.73, 137.07, 134.97, 133.64, 130.28, 127.85, 127.38, 120.11, 110.16, 67.10, 60.74, 58.26, 57.52, 51.52, 29.19, 12.43.

Results and discussion

Synthesis of chalcone-linked 1,2,3-triazole silatranes (CTSI 1–9)

The synthetic procedure followed for the generation of chemosensors CTSI 1–9 is summarized in Scheme 2. The final product generated in this four-step route involves Claisen–Schmidt condensation followed by CuAAC reaction as intermediary steps to generate organotriethoxysilanes (OTES). The final step proceeds by transetherification of OTES (1.0 equiv) with triethanolamine (1.0 equiv) using toluene as solvent and KOH as catalyst, resulting in caged silicon compounds CTSI 1–9, which are further explored for chemosensing studies. The resulting silatranes lock up silicon into a position difficult for water molecules⁴⁵ to disrupt the silicon compounds; thus the structural properties of positional isomers are retained.

Scheme 2 Reaction methodology for synthesis of CTSI 1–9; reagents and conditions: (a) DMF, propargyl bromide, K₂CO₃, rt, st, 14 h; (b) EtOH, KOH, rt, st, 4 h; (c) THF : TEA (1 : 1), [Cu(PPh₃)₃Br], 65 °C, 3 h; and (d) toluene, triethanolamine, KOH (cat.) 110 °C, 5 h.

Photophysical aspects

Procedure for UV-vis/fluorescence study. UV-vis spectroscopic data was collected using MeOH : H₂O (8 : 2) mixture as solvent system at 298 K within the concentration range of 0.1–1 mM. Fluorescence properties were explored for CTSI 1–9 under similar conditions, with 10 nm slit width and excitation wavelength of 307 and 311 nm, and at concentration of 10 μM. UV-vis and fluorescence studies were performed in methanolic–water solution of the different isomers in a quartz cell with path length of 1 cm. The quenching of different metal ions was explored stepwise using a micro-syringe at different pH values and physiological temperature range.

Choice of solvent system. The chemosensing activity of an analyte in a particular solvent system is of primary importance, as it determines its applicability in natural systems. The use of methanol as solvent was very effective for chemosensing activity, but the aqueous environment was necessary to increase implementation in ion detection studies. The choice of ‘perfect solvent system’ for chemosensing activity was explored using different permutations and combinations of methanol and water (v/v) system in the following ratios: 9 : 1, 8 : 2, 7 : 3, 6 : 4, 5 : 5 and 4 : 6. The different combinations were tested using fluorescence spectral response for CTSI 1 and are shown in Fig. 1. As water content in the solvent system increases, the solubility of silatranes decreases, which precipitates out the analyte (silatrane) from the solution. This is evident from the change in coloration of the analyte for different solvent systems as shown in Fig. 1. The solution becomes progressively dilute for a given concentration, which is remarkably displayed by the decrease in emission intensity of fluorescence spectra for an analyte. After all these considerations, the best fit solvent system for effective chemosensing activity is 8 : 2 MeOH : H₂O combination.

Fig. 1 Sampling of CTSI 1 in MeOH : H₂O (v/v) ratio of 10 : 0, 9 : 1, 8 : 2, 7 : 3, 6 : 4, 5 : 5 and 4 : 6; fluorescence spectral response using 20 μM stock solution with λ_ex = 311 nm at 25 °C.

UV-vis study. UV-vis spectroscopic data recorded for 0.1–1.3 mM solutions of CTSI 1–9 are presented in Fig. 2. UV-vis spectra obtained after analysis displayed significant differences in absorption values for o-, m- and p-isomers. All CTS positional isomers exhibited a major absorption band in the 300–340 nm region. The changes in λ_max value of the UV-vis spectra for different positional isomeric units impart the uniqueness of each CTS isomer, as summarized in Table 1. It is noteworthy that with the change in the position of the aldehyde substituent unit relevant to substituted acetophenones, a significant red shift was observed on moving from o → m → p isomer position. The intensification in colour can be the result of variation in auxochrome position, which significantly affects the chromophore spectra. The change in polarity caused by –C=O auxochrome and decrease in steric hindrance resulted in a red shift by 5–25 nm, with a shift in position from o → m → p. Moreover, the conjugated electronic system absorbs at maximum intensity when in planar configuration and tends to have higher values of λ_max and molar absorptivity co-efficient ε_max.

Fig. 2 UV-vis spectra of chalcomers 1–9 (0.5–0.6 mM for CTSI 1–3; 1.2–1.3 mM for CTSI 4–6 and 1.0–1.2 mM for CTSI 7–9) in MeOH : H₂O (8 : 2) (v/v) solvent system at 25 °C.

Table 1 Molar absorptivity co-efficient (ε_max) (mol dm⁻³) of chalcomers 1–9 calculated using the path length of 1 cm and solution concentration in the range 0.1–1.3 mM in MeOH : H₂O (8 : 2) (v/v) solvent system at 25 °C

CTS isomer	1	2	3	4	5	6	7	8	9
λmax (nm)	297	310	331	303	305	339	309	330	333
εmax (×10^4)	5.94	5.17	6.02	2.53	2.34	2.61	2.81	2.75	2.78

---

Molar absorptivity. UV-vis spectra recorded in 8 : 2 (v/v) methanol–water solution at the concentration of 5–6 × 10⁻⁴ M for CTSI 1–3, 12–13 × 10⁻⁴ M for CTSI 4–6 and 10–12 × 10⁻⁴ M for CTSI 7–9 show large variations in molar absorptivity coefficient (ε_max). The calculations show the difference in activity of each chalcone-linked 1,2,3-triazolyl positional isomer. The o-isomer of the aldehyde shows a large value of molar extinction coefficient as compared to its m- and p- positional isomers (Table 1). This phenomenon is observed due to the localized elongation in the π-electronic conjugate system.

Concentration variation in UV-vis spectra. The influence of concentration change on λ_max value of the absorption spectra for CTSI 9 is represented in Fig. 3. The spectra revealed a strong change in absorption intensity arising at 269 nm, with a moderate change in peak intensity at 333 nm. The increase in CTSI 9 concentration is associated with a hypsochromic shift of 7 nm, from 340 to 333 nm, while no peak shift at 269 nm was observed. This absorption maxima resulted from increased conjugation, which causes blue shift in absorption maxima due to lowering of energy levels for both excited and ground states. There was minimal change in absorption spectra with variation in concentration for other chalcomers.

Fig. 3 Effect of concentration variation on CTSI 9 (1–10 equiv) in MeOH : H₂O (8 : 2) (v/v) system at 25 °C.

Fluorescence spectral study

Fluorescence spectral measurements. In this report, we describe the synthesis and applicability of a new type of fluorescent isomeric chemosensor that provides selectivity in fluorescence quenching. Among the nine chalcone isomers used in this study, only three o-positional isomers were found to be active in fluorescence quenching studies. CTSI 1–3 exhibited excellent fluorescence maxima at a concentration of 10 μM and show considerable activity until a threshold concentration level of 50 nM. The remaining CTSI 4–9 were inactive to fluorescence spectral studies (Fig. 4). In previous reports regarding the independent sensing of Cu²⁺ and Ni²⁺, the minimum analyte concentration required for efficient sensing lay between 20–50 μM.^19–24 This positional isomeric effect on emission spectra is a consequence of the O,O,N and N,O, –C=C– coordinating sphere, which can effectively bind Cu²⁺ and Ni²⁺ ions in o-isomers (CTSI 1–3) (Scheme 3). This spatial environment of the rigid donor atom is lacking in the case of m- and p-isomers (CTSI 4–9).

Fig. 4 Fluorescence emission spectra of CTSI 1–9 in MeOH : H₂O (8 : 2) (v/v) system at a concentration of 10 μM and λ_ex = 311 nm at 25 °C.

Scheme 3 Proposed binding modes for CTSI 1–3 for interactions with Cu²⁺ and Ni²⁺ ions.

Selectivity over various cations. The ability of conjugated systems to complex with transition elements has provided the basis for spectro-chemical analysis using fluorescence technique. The stock solutions were prepared by dissolving CTSI 1–3 in (8 : 2) methanolic–water solution at a concentration of 20 μM. The cationic stocks were also prepared in MeOH : H₂O (8 : 2) at a concentration of 10 μM. Fluorescence quenching was studied using chloride ion salts of Hg²⁺, Cd²⁺, Cr²⁺, Cu²⁺, Ca²⁺, Na⁺, Mg²⁺, Zn²⁺, Ni²⁺, Rb⁺, Ba²⁺, Ag⁺, Fe²⁺ and Fe³⁺ (Fig. 5). The binding affinities towards these cations show unrecognizable changes in spectral intensity, with the exception of Cu²⁺ and Ni²⁺, which show significant quenching in fluorescence emission spectral intensity.

Fig. 5 Chemosensing activity of probe CTSI 1 towards different cationic species in MeOH : H₂O (8 : 2) (v/v) solution after the addition of 5.0 equiv of Hg²⁺, Cd²⁺, Cr²⁺, Cu²⁺, Ca²⁺, Na⁺, Mg²⁺, Zn²⁺, Ni²⁺, Rb⁺, Ba²⁺, Ag⁺, Fe²⁺ and Fe³⁺, with λ_ex = 311 nm at 25 °C.

Detection limits. Fluorescence spectroscopic measurements were recorded in the range 390–420 nm, with the excitation wavelength for absorption maxima lying in the region of 307–311 nm. The emission value in this region is popularly referred to as the Soret band. Titration studies were performed for CTSI 1–3 with Cu²⁺ and Ni²⁺ ions at a concentration level of 15 μmol L⁻¹ and 20 μmol L⁻¹, respectively. Recognition studies were performed to evaluate threshold concentration values for the chemosensor and the minimum detection limits for these cationic quenchers.

Quenching study. The detection of metal ions by a fluorescent chemosensor is based upon two techniques that involve either quenching of emission maxima or enhancement of fluorescence intensity. The decrease in intensity on complexation with metal ions is due to the decrease in rigidity of conjugated π-electrons and the sharing of electron cloud with metal ions, while the increase in intensity of fluorescence maxima is due to a more stable and rigid system that results in increased quantum yield. The development of spectrometric studies is due to the movement of electrons between different energy levels of organometallic complexes by the energy provided by UV or visible light.⁷

Cu²⁺ and Ni²⁺ ions show significant quenching in fluorescence spectra of CTSI 1–3, while other metal ions show negligible changes. The emission intensity decreased rapidly with the addition of 10–25 equivalents of both Cu²⁺ (Fig. 6–8) and Ni²⁺ (Fig. 9–11) cationic binders. Thereafter, no decrease in emission was evident, which indicated the saturation of active fluorophore sites by quencher cationic species. Thus, ratiometric sensing of Cu²⁺ and Ni²⁺ ions fall within the physiologically relevant concentration range.

Fig. 6 Fluorescence emission spectra and variation of fluorescence quenching (inset) recorded using the Stern–Volmer plot for CTSI 1 (10 μM) upon gradual addition of Cu²⁺ (0–20 equiv) in MeOH : H₂O (v/v) (8 : 2) with λ_ex = 311 nm.

Fig. 7 Fluorescence emission spectra and variation of fluorescence quenching (inset) recorded using the Stern–Volmer plot for CTSI 2 (10 μM) upon gradual addition of Cu²⁺ (0–20 equiv) in MeOH : H₂O (v/v) (8 : 2) with λ_ex = 311 nm.

Fig. 8 Fluorescence emission spectra and variation of fluorescence quenching (inset) recorded using the Stern–Volmer plot for CTSI 3 (10 μM) upon gradual addition of Cu²⁺ (0–20 equiv) in MeOH : H₂O (v/v) (8 : 2) with λ_ex = 311 nm.

Fig. 9 Fluorescence emission spectra and variation of fluorescence quenching (inset) recorded using the Stern–Volmer plot for CTSI 1 (10 μM) upon gradual addition of Ni²⁺ (0–30 equiv) in MeOH : H₂O (v/v) (8 : 2) with λ_ex = 307 nm.

Fig. 10 Fluorescence emission spectra and variation of fluorescence quenching (inset) recorded using the Stern–Volmer plot for CTSI 2 (10 μM) upon gradual addition of Ni²⁺ (0–30 equiv) in MeOH : H₂O (v/v) (8 : 2) with λ_ex = 307 nm.

Fig. 11 Fluorescence emission spectra and variation of fluorescence quenching (inset) recorded using the Stern–Volmer plot for CTSI 3 (10 μM) upon gradual addition of Ni²⁺ (0–30 equiv) in MeOH : H₂O (v/v) (8 : 2) with λ_ex = 307 nm.

However, a straight turn-on fluorescent sensor is much harder to obtain than a turn-off sensor. Interestingly, CTSI 8 shows exceptional ability to act as a ‘turn on’ fluorescent sensor for Ni²⁺ ions, irrelevant to presence of other cationic species. There was increase in fluorescence intensity upon addition of 15 μM methanolic solution of Ni²⁺ ions. This prototype for a ‘turn-on’ fluorescence probe with Ni²⁺ ions was distinctive among the other chalcomers studied (CTSI 1–3) (Fig. 12).

Fig. 12 Fluorescence emission spectra and variation of fluorescence quenching (inset) recorded using the Stern–Volmer plot for CTSI 8 (30 μM) upon gradual addition of Ni²⁺ (0–50 equiv) in MeOH : H₂O (v/v) (8 : 2) with λ_ex = 311 nm.

It is worth noting that the behavior observed for Cu²⁺ and Ni²⁺ ion sensing is similar to that reported in previous studies.^20,46 However, in contrast with known literature, the chalcone-based sensor system is more efficient in fluorescent detection of Cu²⁺ and Ni²⁺ ions, with a threshold sensing concentration as low as 10 μM. Moreover, there is no single system, to the best of our knowledge, that can competently sense both Cu²⁺ and Ni²⁺ ions in 20% aqueous media (8 : 2, MeOH : H₂O). Further, with the evidence of considerable quenching observed for CTSI 1–3 in the case of Cu²⁺ and Ni²⁺ ions, CTSI 8 resulted in the enhancement of fluorescence maxima with the addition of Ni²⁺ ion solution showing no effect with Cu²⁺ ions, which is unique for a mono-analyte sensor system.

Stern–Volmer constant. The Stern–Volmer plot for quencher concentration of all fluorescence active chalcomers is shown in Fig. 13. The non-linearity in plots shows that the quenching is due to heterogeneous dispersion of the chromophore in the solution matrix. Moreover, the binding of transition metal ions occur in the ground state by a process called static quenching. The graph shows a higher Stern–Volmer constant (K_SV) for the o-isomer in comparison to other isomers, which demonstrates a higher quenching ability of the o-isomer as compared to its m- and p- substituted isomers in the case of both Ni²⁺ and Cu²⁺ ions.

Fig. 13 Stern–Volmer plots for different chalcomers in response to Ni²⁺ and Cu²⁺ ions in the concentration range of 10–50 μM at 25 °C.

pH effect. The effect of pH change on emission maxima is illustrated in the variation in fluorescence intensity recorded in the range of pH 5–13 (Fig. S1†). So far, no literature has cited the effect of pH on the sensing of Cu²⁺ and Ni²⁺ ions, as the studies were performed using buffer solutions. The gradual change in pH from acidic to basic (5–13) resulted in a steady increase in emission maxima. The spectral maxima become constant after pH 11, and any further increase in pH results in decreased emission wavelength (λ_ems) value. Therefore, the pH for fluorescence study lies in the range of 5–9.

A series of fluorescence titrations were performed on CTSI 1 to study the effect of pH on quenching activity of the probe towards Cu²⁺ and Ni²⁺ ions. This is illustrated by the increase in fluorescence emission maxima that arises due to an increase in hydration shells of Cu²⁺ and Ni²⁺ ions (Fig. S2†). Starting at pH 5, emission intensity was obtained at 430 nm, with λ_ext at 311 nm. With the rise in pH to 7, a slight increase in fluorescence intensity pointed towards a decrease in sensing of quencher cations by the probe. Upon raising the solution pH to 11, there was decrease in emission intensity, which points to the alteration in probe–sensor activity of the binding module.

Temperature effect. The ion recognition studies for CTSI 1 show a significant effect of temperature change on the quenching phenomenon. The chemosensing activity of the fluorophore was explored within the temperature range of 298 K to 328 K (Fig. S3†), which falls within the physiological temperature range. Upon raising the solution temperature from 298 K to 308 K, there is a noticeable increase in quenching of the emission maxima. With a further 10° rise in temperature, there was still an increase in quenching for both cationic species. On further raising the temperature by 10°, considerable change in quenching was evident. On lowering thermal activity with a decrease in solution temperature from 298 K to 288 K, a marginal dip in quenched maxima was observed. Thus, the optimal thermal condition for fluorescence titration studies lies within the physiological temperature range of 298 K to 318 K.

Conclusion

The efficient synthesis of chalcomers linked to capped silicon via the 1,2,3-triazolyl propyl chain is reported for the first time. Higher values of molar extinction coefficient in the case of all chalcone isomeric units show high absorption in UV-vis spectra. The exceptional sensing ability of o-isomer of CTS over their m- and p-isomeric analogues towards Cu²⁺ and Ni²⁺ ions shows a key difference in photophysical properties arising due to positional isomerism. This dual sensing behaviour resulted in the quenching of emission spectral maxima due to Cu²⁺ and Ni²⁺ ions. In contrast, CTSI 8 has a remarkable property to sense Ni²⁺ ions as a ‘turn on’ fluorescent sensor. The optimal conditions for fluorescent chemosensing lie within physiologically relevant conditions, with pH range of 5–9 and temperature variation of 288–318 K. Moreover, the assembled chalcomer silatranes can extend their scope to medicine with further advancement as Cu²⁺ and Ni²⁺ cationic sensors.

Conflict of interest

The authors declare no competing financial interest.

Acknowledgements

We thank Mr Avtar Singh and Mr Manish Kumar for the NMR studies (SAIF, Panjab University, Chandigarh). One of the authors, Jandeep Singh, thanks the Council of Scientific and Industrial Research (CSIR), India, for providing financial support in the form of CSIR-SRF (NET) fellowship.

References

1. T. Daniel Thangadurai, G. Chung, O. Kwon, D. Jin and Y. I. Lee, Tetrahedron Lett., 2011, 52, 6465–6469.
2. S. Kubow, J. Nutr. Biochem., 1996, 7, 530–541.
3. E. Schmitt, I. C. Tanrikulu, T. H. Yoo, M. Panvert, D. A. Tirrell and Y. Mechulam, J. Mol. Biol., 2009, 394, 843–851.
4. K. P. Carter, A. M. Young and A. E. Palmer, Chem. Rev., 2014, 114, 4564–4601.
5. T. Slezak, Z. M. Smith, J. L. Adcock, C. M. Hindson, N. W. Barnett, P. N. Nesterenko and P. S. Francis, Anal. Chim. Acta, 2011, 707, 121–127.
6. M. Li, H. Gou, I. Al-ogaidi and N. Wu, ACS Sustainable Chem Eng., 2013, 1, 713–723.
7. M. Dutta and D. Das, TrAC, Trends Anal. Chem., 2012, 32, 113–132.
8. M. Y. Berezin and S. Achilefu, Chem. Rev., 2010, 110, 2641–2684.
9. M. A Saeed, H. T. M. Le and O. Š. Miljanić, Acc. Chem. Res., 2014, 47, 2074–2083.
10. H. Kim and H. Choi, Talanta, 2001, 55, 163–169.
11. H. N. Kim, Z. Guo, W. Zhu, J. Yoon and H. Tian, Chem. Soc. Rev., 2011, 40, 79–93.
12. Z. Liu, W. He and Z. Guo, Chem. Soc. Rev., 2013, 42, 1568–1600.
13. M. Swierczewska, S. Lee and X. Chen, Phys. Chem. Chem. Phys., 2011, 13, 9929–9941.
14. N. Boens, V. Leen and W. Dehaen, Chem. Soc. Rev., 2012, 41, 1130–1172.
15. E. Denkhaus and K. Salnikow, Crit. Rev. Oncol. Hematol., 2002, 42, 35–56.
16. D. E. Kang, C. S. Lim, J. Y. Kim, E. S. Kim, H. J. Chun and B. R. Cho, Anal. Chem., 2014, 86, 5353–5359.
17. Y. Rahimi, A. Goulding, S. Shrestha, S. Mirpuri and S. K. Deo, Biochem. Biophys. Res. Commun., 2008, 370, 57–61.
18. J. Ding, L. Yuan, L. Gao and J. Chen, J. Lumin., 2012, 132, 1987–1993.
19. S. Ramakrishnan, E. Suresh, A. Riyasdeen, M. A. Akbarsha and M. Palaniandavar, Dalton Trans., 2011, 40, 3245–3256.
20. P. Krishnamoorthy, P. Sathyadevi, R. R. Butorac, A. H. Cowley, N. S. P. Bhuvanesh and N. Dharmaraj, Dalton Trans., 2012, 41, 4423–4436.
21. J. Tan, L. Zhu and B. Wang, Dalton Trans., 2009, 2, 4722–4728.
22. H. Wang, D. Wang, Q. Wang, X. Li and C. A Schalley, Org. Biomol. Chem., 2010, 8, 1017–1026.
23. G. Singh, J. Singh, S. S. Mangat and A. Arora, Tetrahedron Lett., 2014, 55, 2551–2558.
24. J. Tatsuzaki, K. F. Bastow, K. Nakagawa-Goto, S. Nakamura, H. Itokawa and K.-H. Lee, J. Nat. Prod., 2006, 69, 1445–1449.
25. F. Chimenti, R. Fioravanti, A. Bolasco, P. Chimenti, D. Secci, F. Rossi, M. Yáñez, F. Orallo, F. Ortuso and S. Alcaro, J. Med. Chem., 2009, 52, 2818–2824.
26. M. N. Clifford and F. A. Toma, J. Sci. Food Agric., 2000, 1080, 1073–1080.
27. A. Sharma, B. Chakravarti, M. P. Gupt, J. a. Siddiqui, R. Konwar and R. P. Tripathi, Bioorg. Med. Chem., 2010, 18, 4711–4720.
28. H. Sharma, S. Patil, T. W. Sanchez, N. Neamati, R. F. Schinazi and J. K. Buolamwini, Bioorg. Med. Chem., 2011, 19, 2030–2045.
29. Z. Nowakowska, Eur. J. Med. Chem., 2007, 42, 125–137.
30. H. Jin, X. Li, T. Tan, S. Wang, Y. Xiao and J. Tian, Dyes Pigm., 2014, 106, 154–160.
31. K. Namba, A. Osawa, S. Ishizaka, N. Kitamura and K. Tanino, J. Am. Chem. Soc., 2011, 133, 11466–11469.
32. Q. Shen, L. Zhou, Y. Yuan, Y. Huang, B. Xiang, C. Chen, Z. Nie and S. Yao, Biosens. Bioelectron., 2014, 55, 187–194.
33. M. Meldal and C. W. Tornøe, Chem. Rev., 2008, 108, 2952–3015.
34. N. Boechat, V. F. Ferreira, S. B. Ferreira, M. de Lourdes G Ferreira, F. de C da Silva, M. M. Bastos, M. Dos S Costa, M. C. S. Lourenço, A. C. Pinto, A. U. Krettli, A. C. Aguiar, B. M. Teixeira, N. V da Silva, P. R. C. Martins, F. A. F. M. Bezerra, A. L. S. Camilo, G. P. da Silva and C. C. P. Costa, J. Med. Chem., 2011, 54, 5988–5999.
35. G. Acquaah-harrison, S. Zhou, J. V Hines and S. C. Bergmeier, J. Comb. Chem., 2010, 491–496.
36. S. B. Ferreira, A. C. R. Sodero, M. F. C. Cardoso, E. S. Lima, C. R. Kaiser, F. P. Silva and V. F. Ferreira, J. Med. Chem., 2010, 53, 2364–2375.
37. G. Singh, S. S. Mangat, H. Sharma, J. Singh, A. Arora, A. P. Singh Pannu and N. Singh, RSC Adv., 2014, 4, 36834–36844.
38. G. Singh, S. S. Mangat, J. Singh, A. Arora and R. K. Sharma, Tetrahedron Lett., 2014, 55, 903–909.
39. G. Singh, S. S. Mangat, J. Singh, A. Arora and M. Garg, J. Organomet. Chem., 2014, 769, 124–129.
40. G. Singh, A. Arora, S. S. Mangat, J. Singh, S. Chaudhary, N. Kaur and D. Choquesillo-Lazarte, J. Mol. Struct., 2015, 1079, 173–181.
41. G. Singh, J. Singh, S. S. Mangat and A. Arora, RSC Adv., 2014, 4, 60853–60865.
42. A. T. Dickschat, F. Behrends, M. Bühner, J. Ren, M. Weiss, H. Eckert and A. Studer, Chem.–Eur. J., 2012, 18, 16689–16697.
43. K. W. Huang, C. W. Hsieh, H. C. Kan, M. L. Hsieh, S. Hsieh, L. K. Chau, T. E. Cheng and W. T. Li, Sens. Actuators, B, 2012, 163, 207–215.
44. A. Bianco, M. Maggini, M. Nogarole and G. Scorrano, Eur. J. Org. Chem., 2006, 2934–2941.
45. (a) H. F. Sore, W. R. J. D. Galloway and D. R. Spring, Chem. Soc. Rev., 2012, 41, 1845–1866; (b) L. Huerta, J. El Haskouri, D. Vie, M. Comes, J. Latorre, C. Guillem, M. D. Marcos, D. Beltra and P. Amoro, Chem. Mater., 2007, 19, 1082–1088; (c) K. Ladomenou, T. N. Kitsopoulos, G. D. Sharma and a. G. Coutsolelos, RSC Adv., 2014, 4, 21379.
46. T. Mukherjee, J. C. Pessoa, A. Kumar and A. R. Sarkar, Dalton Trans., 2013, 42, 2594–2607.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra14329a

---