Synthetic approach towards ‘click’ modified chalcone based organotriethoxysilanes; UV-Vis study

Abstract

The efficient linkage of a conjugate chalcone to n-propyltriethoxysilanes (nPTES) via a 1,2,3-triazole with good yields is reported. The synthesis involves a Claisen–Schmidt condensation reaction followed by a copper(I) catalyzed azide–alkyne cycloaddition (CuAAC) reaction. Two different approaches were followed for the synthesis of organotriethoxysilanes (OTES), however, only one pathway was found to be an efficient synthetic route. The performance of the click reaction under thermal reaction conditions has been optimized using [CuBr(PPh₃)₃] as catalyst. A photoelectronic spectroscopy study in the UV-Vis region showed significant absorption maxima in the range of 300–325 nm. Moreover, the solvatochromic aspects showing the effect of solvent polarity on the absorption maxima was investigated for the first time on functionalised OTES.

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Introduction

Modification of the organic segment of hybrid silica precursors has increased their utility in the fields of drug discovery,¹ catalysis,² the surface coating of materials,^3,4 polymer formation,⁵ ion detecting fluorescent probes,^6,7 HPLC packing,⁸ and nano chemistry.⁹ The synthetic approaches leading to the generation of polyfunctional triethoxysilanes (PfTES), which are precursors to these materials, follow different pathways such as by a cross-coupling reaction,^10–12 hydrosilylation,^13,14 and transmetalation reaction.¹⁵ These reaction methodologies require the use of expensive metals, show limited functional group tolerance and result in moderate to good yields. The major shortcomings of these conventional methodologies are the purification of the ‘hydrolytically unstable’ PfTES and poor control of regioselectivity.

To override these limitations, a new technique was pioneered in 2001 by Sharpless and Meldal,^16–18 and exploited by Cattoen et al.,¹⁹ to fine tune nearly all functional groups attached to azide linked nPTES using Cu(I), thereby expanding the scope and utility of this methodology. This technique follows the CuAAC reaction of azide–alkyne fragments to 1,2,3-triazole^20,21 with efficient conversion of above 80% and 100% control over regioselectivity.²² Numerous catalytic systems have been reported for click reactions but the use of the [CuBr(PPh₃)₃]/THF–TEA system is favourable as an efficient catalytic scheme for the synthesis of functionalised OTES.^23–28 Furthermore, the 1,2,3-triazolyl heterocycle has been proven to be an important pharmacore²⁹ associated with immense medicinal importance due to antimycobacterial,^30,31 antituberculosis,³² anti-inflammatory,³³ antiangiogenic,³⁴ antiviral,³⁵ anticancerous as a histone deacetylate inhibitor^36,37 and anti-HIV activity.³⁸

The combination of smaller fragments to assemble a larger unit with enhanced pharmacological activity forms an important aspect of synthetic biochemistry and receives constant attention with increasing benefits in medicinal chemistry. The applicability of a combinatorial approach has led to the generation of substituted chalcone based moieties that act as primary precursors to flavonoids and isoflavonoids, which are abundantly distributed in edible plants and are associated with being essential cancer chemo-preventive food components.^39–41 A chalcone unit comprises two aromatic rings connected by a three carbon chain as an α,β-unsaturated carbonyl group (Fig. 1). They are considered pharmacologically relevant entities known to exert pathogenic activity⁴² along with antitumorigenic,⁴³ anti-inflammatory,⁴⁴ antiangiogenic,⁴⁵ antioxidant,⁴⁶ antituberculosis,⁴⁷ antimalarial,⁴⁸ and anti-HIV properties.⁴⁹ From this perspective, it was desirable to synthesize a material with the merged activities of both functionalities, i.e., the integration of the chalcone moiety pharmacore with the medicinally active 1,2,3-triazole.

Fig. 1 Chemical structure of a chalcone unit consisting of two aromatic rings linked by a three-carbon chain forming an α,β-unsaturated carbonyl moiety.

Experimental

General material and methods

All the syntheses were carried out under a dry nitrogen atmosphere using a glass vacuum line. The organic solvents were dried and purified according to the standard procedure and stored under a 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), and N,N-dimethylformamide (SDFCL) were used as supplied. Acetophenone (SDFCL), 2-hydroxyacetophenone (SDFCL), 3-hydroxyacetophenone (SDFCL), 4-hydroxyacetophenone (SDFCL), salicylaldehyde (Aldrich), 3-hydroxybenzaldehyde (SDFCL), 4-hydroxybenzaldehyde (SDFCL), p-methoxyacetophenone (HIMEDIA), and 2,4-dimethoxyacetophenone (HIMEDIA) were used as supplied for the synthesis of the terminal alkynes 3a–3i and 5a–5i. γ-Azidopropyltriethoxysilane (AzPTES) was synthesized according to a known literature procedure.²⁰

Synthesis of compounds 2a(i–iii) and 2b(i–iii)

To a uniformly stirred solution of 1a/1b (2 g, 16.40 mmol, 1 equiv.) in 15 ml of DMF cooled in an ice bath to −5 °C, K₂CO₃ (6.76 g, 47.2 mmol, 3 equiv.) was added. To this stirring mixture, propargyl bromide (2.15 g, 1.61 ml, 18.06 mmol, 1.1 equiv.) was slowly injected dropwise within 5 min. After the complete addition of the reactants, the temperature of the reaction mixture was slowly raised to 30 °C over 1 h and stirred at this temperature for 14 h. The reaction mixture was quenched by the addition of ice cold water and filtered to give the solid product (in the case of 2a(i–iii) and 2b(i–iii)). In the case of the low melting solid 2a(ii), ethyl acetate was used for extraction. The combined organic layers were dried over anhydrous MgSO₄ and vacuum evaporation of the solvent resulted in the formation of the desired compound. The solid alkynes were recrystallized by dissolving in a minimum amount of absolute ethanol.

Synthesis of compounds 3a–3i/5a–5i

The compounds 2a/2b (1 equiv.) and 1c(a–c) (1 equiv.) were independently dissolved in minimum amounts of absolute ethanol till clear solutions were obtained. In another round bottom flask, KOH (0.02 g, 0.36 mmol) was dissolved in ethanol and slowly added to the solutions of 2a/2b and 1c(a–c), respectively. The reaction was stirred for 4 h and the end point was monitored using TLC (hexane–ethyl acetate (8 : 2)). The aldol condensation mixture of 2a and 1c(a–c) yielded compounds 3a–3i while the mixture of 2a and 2b yielded compounds 5a–5i. On completion, the reaction was quenched by ice cold water, extracted with methylene chloride and washed twice with brine solution. The combined organic phases were dried over anhydrous MgSO₄ and vacuum evaporation of the solvent afforded the desired product.

Synthesis of compounds 4a–4i/6a–6i

In a 25 ml two neck round bottom flask, the alkynes 3a–3i/5a–5i were dissolved in a 1 : 1 solution of THF–TEA (3 ml) till a uniform solution was obtained, followed by the addition of the catalyst (0.02 mmol for 3a–3i and 0.04 mmol for 5a–5i). The slow and dropwise addition of AzPTES (1 equiv. for 3a–3i and 2 equiv. for 5a–5i) was carried out under an inert atmosphere. The temperature of the reaction mixture was raised slowly to 65 °C and stirred vigorously for 3 h. After completion of the reaction, the assembly was cooled to room temperature, the mixture containing the used Cu(I) catalyst was filtered and vacuum evaporation of the solvents resulted in the desired OTES 4a–4i/6a–6i.

Spectroscopic data for compounds 4a–4i

1-Phenyl-3-(2-((1-(3-(triethoxysilyl)propyl)-1H-1,2,3-triazol-4-yl)methoxy)phenyl)prop-2-en-1-one (4a). Yield: 91%, empirical formula: C₂₇H₃₅N₃O₅Si; anal. calcd: C, 63.6; H, 6.9; N, 8.2; found: C, 63.4; H, 6.8; N, 8.0%; IR (neat, cm⁻¹): 2965, 2929, 2876, 1659, 1597, 1574, 1485, 1448, 1332, 1315, 1258, 1213, 1162, 1073, 1015, 907, 792, 726, 691, 646. ¹H NMR (400 MHz, CDCl₃), δ = 8.00 (d, ³J = 15.8 Hz, 1H), 7.87 (d, ³J = 7.5 Hz, 1H), 7.55 (d, ³J = 6.3 Hz, 3H), 7.44 (d, ³J = 7.4 Hz, 2H), 7.38 (d, ³J = 7.7 Hz, 2H), 7.01 (d, ³J = 8.3 Hz, 1H), 6.92 (d, ³J = 8.1 Hz, 2H), 5.21 (s, 2H), 4.25 (t, ³J = 7.2 Hz, 2H), 3.69 (q, ³J = 7.0 Hz, 6H), 2.04–1.90 (m, 2H), 1.10 (t, ³J = 7.0 Hz, 9H), 0.57–0.41 (m, 2H). ¹³C NMR (101 MHz, CDCl₃) δ = 189.8, 156.4, 142.5, 139.1, 137.3, 131.6, 130.7, 128.6, 127.5, 123.2, 122.0, 120.3, 111.7, 57.4, 55.1, 51.5, 23.3, 17.3, 6.4.

1-Phenyl-3-(3-((1-(3-(triethoxysilyl)propyl)-1H-1,2,3-triazol-4-yl)methoxy)phenyl)prop-2-en-1-one (4b). Yield: 89%, empirical formula: C₂₇H₃₅N₃O₅Si; anal. calcd: C, 63.6; H, 6.9; N, 8.2; found: C, 63.3; H, 6.7; N, 8.3%; IR (neat, cm⁻¹): 2972, 2925, 2884, 1663, 1596, 1578, 1485, 1447, 1315, 1289, 1235, 1073, 1161, 1034, 1016, 955, 770, 688, 566. ¹H NMR (400 MHz, CDCl₃) δ = 8.08–7.86 (m, 2H), 7.77 (d, ³J = 15.6 Hz, 1H), 7.59–7.52 (m, 2H), 7.51–7.46 (m, 2H), 7.36–7.31 (m, 1H), 7.27 (dd, ³J = 11.4, 5.3 Hz, 2H), 7.10–6.98 (m, 2H), 5.26 (s, 2H), 4.37 (t, ³J = 7.2 Hz, 2H), 3.81 (q, ³J = 7.0 Hz, 6H), 22.04–1.90 (m, 2H), 1.21 (t, ³J = 7.0 Hz, 9H), 0.68–0.53 (m, 2H). ¹³C NMR (101 MHz, CDCl₃) δ = 189.3, 157.6, 156.8, 143.4, 137.0, 135.3, 131.8, 129.0, 127.5, 121.4, 120.7, 115.9, 113.5, 113.3, 66.9, 61.1, 57.5, 54.8, 51.5, 24.5, 23.1, 17.2, 6.4.

1-Phenyl-3-(4-((1-(3-(triethoxysilyl)propyl)-1H-1,2,3-triazol-4-yl)methoxy)phenyl)prop-2-en-1-one (4c). Yield: 90%, empirical formula: C₂₇H₃₅N₃O₅Si; anal. calcd: C, 63.6; H, 6.9; N, 8.2; found: C, 63.4; H, 7.0; N, 8.1%; IR (neat, cm⁻¹): 2972, 2933, 2885, 1659, 1595, 1572, 1507, 1447, 1422, 1389, 1335, 1291, 1213, 1172, 1072, 1015, 956, 827, 778, 691, 657, 513. ¹H NMR (400 MHz, CDCl₃) δ = 7.92 (d, ³J = 7.3 Hz, 1H), 7.69 (d, ³J = 15.6 Hz, 1H), 7.51 (dd, ³J = 8.7, 4.1 Hz, 2H), 7.40 (t, ³J = 7.5 Hz, 1H), 7.35 (d, ³J = 7.8 Hz, 2H), 7.32 (d, ³J = 3.5 Hz, 1H), 7.17 (dd, ³J = 10.1, 6.0 Hz, 2H), 7.05–7.00 (m, 1H), 6.96–6.89 (m, 2H), 5.16 (s, 2H), 4.31–4.25 (m, 2H), 3.72 (d, ³J = 7.4 Hz, 6H), 2.00–1.90 (m, 2H), 1.14 (d, ³J = 7.0 Hz, 9H), 0.56–0.42 (m, 2H). ¹³C NMR (101 MHz, CDCl₃) δ = 189.5, 158.5, 143.4, 137.4, 131.6, 129.1, 127.4, 126.8, 124.1, 121.8, 119.2, 114.3, 113.7, 61.1, 57.5, 54.8, 52.8, 23.2, 17.3, 6.4. MS (ES⁺) calcd for [M + Na]⁺ 532.2; found 532.3.

1-(4-Methoxyphenyl)-3-(2-((1-(3-(triethoxysilyl)propyl)-1H-1,2,3-triazol-4-yl)methoxy)phenyl)prop-2-en-1-one (4d). Yield: 88%, empirical formula: C₂₈H₃₇N₃O₆Si; anal. calcd: C, 62.3; H, 6.9; N, 7.8; found: C, 62.2; H, 6.8; N, 8.0%; IR (neat, cm⁻¹): 2973, 2933, 2888, 1654, 1597, 1573, 1509, 1455, 1308, 1256, 1216, 1165, 1103, 1074, 1019, 907, 753, 725, 646, 614, 587, 541. ¹H NMR (400 MHz, CDCl₃) δ = 8.05–7.97 (m, 2H), 7.91 (d, ³J = 8.8 Hz, 2H), 7.65–7.53 (m, 3H), 7.28 (t, ³J = 7.2 Hz, 1H), 7.03 (d, ³J = 8.3 Hz, 1H), 6.94 (d, ³J = 7.6 Hz, 1H), 6.88 (d, ³J = 8.8 Hz, 2H), 5.24 (s, 2H), 4.28 (t, ³J = 7.2 Hz, 2H), 3.79 (s, 3H), 3.71 (q, ³J = 6.9 Hz, 6H), 2.01–1.90 (m, 2H), 1.12 (t, ³J = 7.0 Hz, 9H), 0.63–0.39 (m, 2H). ¹³C NMR (101 MHz, CDCl₃) δ = 188.1, 162.3, 142.6, 138.3, 131.1, 130.5, 129.8, 127.4, 125.1, 123.3, 121.9, 121.6, 120.3, 112.6, 111.7, 61.5, 57.5, 54.4, 51.6, 23.2, 17.3, 6.4.

1-(4-Methoxyphenyl)-3-(3-((1-(3-(triethoxysilyl)propyl)-1H-1,2,3-triazol-4-yl)methoxy)phenyl)prop-2-en-1-one (4e). Yield: 90%, empirical formula: C₂₈H₃₇N₃O₆Si; anal. calcd: C, 62.3; H, 6.9; N, 7.8; found: C, 62.1; H, 6.7; N, 7.7%; IR (neat, cm⁻¹): 2975, 2938, 2889, 1659, 1600, 1484, 1389, 1251, 1168, 1075, 906, 831, 785, 724, 646, 616, 541. ¹H NMR (400 MHz, CDCl₃) δ = 7.97 (d, ³J = 8.8 Hz, 2H), 7.68 (d, ³J = 15.6 Hz, 1H), 7.58 (s, 1H), 7.46 (d, ³J = 15.6 Hz, 1H), 7.26 (t, ³J = 7.9 Hz, 1H), 7.19 (dd, ³J = 10.5, 4.9 Hz, 2H), 6.97 (dd, ³J = 8.1, 2.1 Hz, 2H), 6.91 (d, ³J = 8.8 Hz, 2H), 5.19 (s, 2H), 4.30 (t, ³J = 7.2 Hz, 2H), 3.81 (s, 3H), 3.73 (q, ³J = 7.0 Hz, 6H), 2.01–1.90 (m, 2H), 1.14 (t, ³J = 7.0 Hz, 9H), 0.62–0.44 (m, 2H). ¹³C NMR (101 MHz, CDCl₃) δ = 187.6, 162.5, 157.6, 142.7, 135.6, 131.1, 129.9, 127.5, 121.8, 121.3, 120.7, 115.7, 113.3, 112.9, 57.5, 54.5, 51.5, 17.3, 6.4.

1-(4-Methoxyphenyl)-3-(4-((1-(3-(triethoxysilyl)propyl)-1H-1,2,3-triazol-4-yl)methoxy)phenyl)prop-2-en-1-one (4f). Yield: 87%, empirical formula: C₂₈H₃₇N₃O₆Si; anal. calcd: C, 62.3; H, 6.9; N, 7.8; found: C, 62.4; H, 6.7; N, 7.5%; IR (neat, cm⁻¹): 2973, 2933, 2884, 1656, 1600, 1507, 1422, 1390, 1257, 1216, 1165, 1074, 958, 905, 787, 725, 646. ¹H NMR (400 MHz, CDCl₃) δ = 7.98 (d, ³J = 8.8 Hz, 2H), 7.75 (m, 2H), 7.63 (d, ³J = 4.4 Hz, 1H), 7.54 (d, ³J = 8.6 Hz, 1H), 7.42 (d, ³J = 9.6 Hz, 1H), 7.37 (s, 1H), 7.06 (d, ³J = 8.6 Hz, 1H), 6.98 (d, ³J = 8.6 Hz, 1H), 6.92 (d, ³J = 8.8 Hz, 1H), 5.22 (d, ³J = 14.5 Hz, 2H), 4.33 (t, ³J = 7.1 Hz, 2H), 3.82 (s, 3H), 3.76 (q, ³J = 7.0 Hz, 6H), 2.09–1.91 (m, 2H), 1.17 (t, ³J = 7.0 Hz, 9H), 0.61–0.50 (m, 2H). ¹³C NMR (101 MHz, CDCl₃) δ = 187.6, 162.3, 142.4, 131.1, 130.3, 129.7, 129.0, 127.7, 127.3, 122.0, 118.8, 114.1, 112.9, 61.1, 57.4, 54.5, 51.5, 23.2, 17.3, 6.4. HRMS (ES⁺) calcd for [M + Na]⁺ 562.2245; found 762.2288.

1-(2,4-Dimethoxyphenyl)-3-(2-((1-(3-(triethoxysilyl)propyl)-1H-1,2,3-triazol-4-yl)methoxy)phenyl)prop-2-en-1-one (4g). Yield: 88%, empirical formula: C₂₉H₃₉N₃O₇Si; anal. calcd: C, 61.1; H, 6.9; N, 7.4; found: C, 60.9; H, 6.7; N, 7.5%; IR (neat, cm⁻¹): 2970, 2925, 2880, 2839, 1687, 1597, 1577, 1507, 1422, 1310, 1248, 1212, 1108, 1073, 1018, 827, 759, 643. ¹H NMR (400 MHz, CDCl₃) δ = 7.84–7.64 (m, 3H), 7.64–7.53 (m, 1H), 7.53–7.44 (m, 2H), 7.43–7.15 (m, 1H), 7.07–6.99 (m, 1H), 6.96–6.89 (m, 2H), 6.55–6.34 (m, 2H), 5.22 (s, 2H), 4.30 (td, ³J = 7.2, 3.1 Hz, 2H), 3.83 (s, 3H), 3.79 (s, 3H), 3.86–3.76 (m, 3H), 3.76 (q, ³J = 7.0 Hz, 6H), 2.08–1.87 (m, 2H), 1.12 (t, ³J = 7.0 Hz, 9H), 0.60–0.42 (m, 2H). ¹³C NMR (101 MHz, CDCl₃) δ = 190.8, 163.2, 143.0, 141.8, 132.8, 131.9, 130.6, 130.3, 130.0, 122.9, 115.1, 105.1, 98.7, 62.2, 58.6, 52.6, 24.2, 18.3, 7.5.

1-(2,4-Dimethoxyphenyl)-3-(3-((1-(3-(triethoxysilyl)propyl)-1H-1,2,3-triazol-4-yl)methoxy)phenyl)prop-2-en-1-one (4h). Yield: 89%, empirical formula: C₂₉H₃₉N₃O₇Si; anal. calcd: C, 61.1; H, 6.9; N, 7.4; found: C, 61.0; H, 6.8; N, 7.2%; IR (neat, cm⁻¹): 2970, 2924, 2886, 1689, 1658, 1597, 1576, 1508, 1462, 1438, 1421, 1358, 1254, 1210, 1106, 1159, 1072, 1022, 826, 799, 721, 695, 644, 568, 541. ¹H NMR (400 MHz, CDCl₃) δ = 7.65 (d, ³J = 8.4 Hz, 1H), 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, 2H), 6.41 (s, 1H), 5.16 (s, 2H), 4.27 (t, ³J = 7.2 Hz, 2H), 3.81 (s, 3H), 3.78 (s, 3H), 3.72 (dd, ³J = 13.4, 6.6 Hz, 6H), 2.02–1.90 (m, 2H), 1.13 (t, ³J = 6.7 Hz, 9H), 0.61–0.43 (m, 2H). ¹³C NMR (101 MHz, CDCl₃) δ = 189.5, 163.0, 159.3, 158.8, 142.5, 140.8, 131.7, 131.1, 128.9, 127.6, 124.3, 121.9, 121.4, 114.1, 104.2, 61.1, 57.5, 54.7, 54.5, 51.5, 23.2, 17.3, 6.4.

1-(2,4-Dimethoxyphenyl)-3-(4-((1-(3-(triethoxysilyl)propyl)-1H-1,2,3-triazol-4-yl)methoxy)phenyl)prop-2-en-1-one (4i). Yield: 87%, empirical formula: C₂₉H₃₉N₃O₇Si; anal. calcd: C, 61.1; H, 6.9; N, 7.4; found: C, 61.3; H, 6.8; N, 7.2%; IR (neat, cm⁻¹): 2962, 2922, 2884, 1658, 1601, 1509, 1443, 1417, 1257, 1166, 1074, 1010, 864, 786, 695, 541. ¹H NMR (400 MHz, CDCl₃) δ = 7.96 (d, ³J = 16.0 Hz, 1H), 7.66 (d, ³J = 8.6 Hz, 1H), 7.55 (dd, ³J = 10.0, 2.1 Hz, 2H), 7.46 (d, ³J = 15.9 Hz, 1H), 7.42–7.28 (m, 1H), 7.24 (dd, ³J = 12.0, 5.1 Hz, 1H), 7.17 (dd, ³J = 13.9, 8.1 Hz, 1H), 7.01 (d, ³J = 8.3 Hz, 1H), 6.91 (t, ³J = 7.6 Hz, 1H), 6.47 (dd, ³J = 8.6, 2.2 Hz, 1H), 6.40 (d, ⁴J = 2.2 Hz, 1H), 5.23 (s, 2H), 4.26 (t, ³J = 7.2 Hz, 2H), 3.78 (s, 3H), 3.77 (s, 3H), 3.72 (q, ³J = 7.0 Hz, 6H), 2.04–1.92 (m, 2H), 1.12 (t, ³J = 7.0 Hz, 9H), 0.60–0.43 (m, 2H). ¹³C NMR (101 MHz, CDCl₃) δ = 189.8, 163.1, 159.4, 156.2, 142.9, 136.1, 131.8, 130.3, 127.5, 126.7, 123.8, 121.7, 121.4, 120.3, 111.9, 104.2, 97.6, 61.9, 57.5, 54.7, 54.5, 51.6, 23.2, 17.3, 6.4. HRMS (ES⁺) calcd for [M + Na]⁺ 592.2455; found 592.2365.

Spectroscopic data for compounds 6a–6i

1,3-Bis(2-((1-(3-(triethoxysilyl)propyl)-1H-1,2,3-triazol-4-yl)methoxy)phenyl)prop-2-en-1-one (6a). Yield: 90%, empirical formula: C₃₉H₅₈N₆O₉Si₂; anal. calcd: C, 57.8; H, 7.2; N, 10.4; found: C, 57.5; H, 7.1; N, 10.2%; IR (neat, cm⁻¹): 3072, 3024, 2921, 2891, 1652, 1597, 1482, 1448, 1371, 1327, 1287, 1216, 1162, 1105, 1050, 1015, 925, 872, 834, 820, 748, 676, 633, 584. ¹H NMR (400 MHz, CDCl₃) δ = 8.00 (d, ³J = 7.3 Hz, 2H), 7.67 (s, 2H), 7.60 (d, ³J = 8.6 Hz, 2H), 7.48 (d, ³J = 7.1 Hz, 2H), 7.44 (s, 2H), 7.03 (d, ³J = 8.6 Hz, 2H), 5.25 (s, 4H), 4.37 (t, ³J = 7.2 Hz, 4H), 3.81 (dd, ³J = 14.4, 7.4 Hz, 12H), 2.08–1.96 (m, 4H), 1.21 (t, ³J = 7.0 Hz, 18H), 0.65–0.53 (m, 4H). ¹³C NMR (101 MHz, CDCl₃) δ = 189.5, 159.3, 143.5, 137.4, 131.6, 130.9, 129.2, 127.5, 127.1, 127.1, 126.5 (m), 126.5, 119.1, 114.2, 61.1, 57.5, 52.7, 51.5, 23.2, 21.6, 17.3, 6.5.

1-(2-((1-(3-(Triethoxysilyl)propyl)-1H-1,2,3-triazol-4-yl)methoxy)phenyl)-3-(3-((1-(3-(triethoxysilyl)propyl)-1H-1,2,3-triazol-4-yl)methoxy)phenyl)prop-2-en-1-one (6b). Yield: 90%, empirical formula: C₃₉H₅₈N₆O₉Si₂; anal. calcd: C, 57.8; H, 7.2; N, 10.4; found: C, 57.6; H, 7.0; N, 10.5%; IR (neat, cm⁻¹): 2972, 2929, 2885, 1660, 1598, 1484, 1448, 1389, 1286, 1236, 1162, 1071, 954, 783, 757, 696, 542. 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), 3.71 (td, ³J = 13.8, 6.9 Hz, 12H), 1.94 (dd, ³J = 15.4, 7.6 Hz, 2H), 1.88–1.67 (m, 2H), 1.28–0.94 (m, 18H), 0.56–0.47 (m, 2H), 0.46–0.32 (m, 2H). ¹³C NMR (101 MHz, CDCl₃) δ = 191.3, 157.7, 155.9, 141.2, 135.5, 132.2, 131.2, 130.8, 129.6, 129.0, 128.5, 127.5, 126.7, 121.9, 120.7, 115.9, 113.2, 112.1, 61.9, 61.1, 57.5, 51.4, 23.1, 17.3, 6.4.

1-(2-((1-(3-(Triethoxysilyl)propyl)-1H-1,2,3-triazol-4-yl)methoxy)phenyl)-3-(4-((1-(3-(triethoxysilyl)propyl)-1H-1,2,3-triazol-4-yl)methoxy)phenyl)prop-2-en-1-one (6c). Yield: 89%, empirical formula: C₃₉H₅₈N₆O₉Si₂; anal. calcd: C, 57.8; H, 7.2; N, 10.4; found: C, 57.5; H, 7.1; N, 10.2%; IR (neat, cm⁻¹): 2972, 2929, 2885, 1654, 1597, 1508, 1482, 1449, 1422, 1389, 1330, 1292, 1239, 1172, 1071, 1026, 954, 858, 752, 542. ¹H NMR (400 MHz, CDCl₃) δ = 7.68–7.59 (m, 2H), 7.58–7.50 (m, 2H), 7.46 (d, ³J = 7.5 Hz, 3H), 7.29 (t, ³J = 7.9 Hz, 1H), 7.13 (d, ³J = 8.3 Hz, 1H), 7.05 (t, ³J = 7.5 Hz, 1H), 6.98 (t, ³J = 8.3 Hz, 2H), 5.30 (s, 2H), 5.23 (s, 2H), 4.36 (t, ³J = 6.7 Hz, 2H), 4.17 (t, ³J = 7.2 Hz, 2H), 3.87–3.69 (m, 12H), 2.08–1.97 (m, 2H), 1.93–1.83 (m, 2H), 1.20 (dt, ³J = 10.1, 3.5 Hz, 18H), 0.63–0.55 (m, 2H), 0.54–0.42 (m, 2H). ¹³C NMR (101 MHz, CDCl₃) δ = 192.6, 160.1, 156.7, 143.6, 143.3, 142.1, 132.8, 130.4, 130.0, 129.8, 128.1, 125.4, 122.8, 121.4, 115.2, 113.1, 67.9, 63.0, 62.1, 58.5, 52.5, 24.1, 18.2, 11.3, 7.4. MS (ES⁺) calcd for [M + Na]⁺ 833.4; found 833.5.

3-(2-((1-(3-(Triethoxysilyl)propyl)-1H-1,2,3-triazol-4-yl)methoxy)phenyl)-1-(3-((1-(3-(triethoxysilyl)propyl)-1H-1,2,3-triazol-4-yl)methoxy)phenyl)prop-2-en-1-one (6d). Yield: 87%, empirical formula: C₃₉H₅₈N₆O₉Si₂; anal. calcd: C, 57.8; H, 7.2; N, 10.4; found: C, 57.5; H, 7.0; N, 10.1%; IR (neat, cm⁻¹): 2973, 2926, 2884, 1660, 1594, 1577, 1485, 1438, 1389, 1328, 1273, 1240, 1164, 1099, 1071, 1007, 954, 867, 782, 750, 721, 696, 679, 593, 541. ¹H NMR (400 MHz, CDCl₃) δ = 7.68 (d, ³J = 15.7 Hz, 1H), 7.60 (d, ³J = 4.3 Hz, 2H), 7.55 (d, ³J = 7.2 Hz, 1H), 7.45 (s, 1H), 7.40–7.31 (m, 2H), 7.23 (dd, ³J = 9.5, 6.4 Hz, 2H), 7.15 (dd, ³J = 15.6, 7.7 Hz, 2H), 6.97 (d, ³J = 8.1 Hz, 1H), 5.26–5.04 (m, 4H), 4.29 (t, ³J = 7.1 Hz, 4H), 3.78–3.65 (m, 12H), 2.04–1.83 (m, 4H), 1.22–1.05 (m, 18H), 0.59–0.43 (m, 4H). ¹³C NMR (101 MHz, CDCl₃) δ = 188.9, 157.6, 156.9, 143.6, 142.5, 138.5, 135.3, 131.0, 128.9, 128.7, 128.4, 127.4, 121.3, 120.8, 120.5, 118.9, 116.0, 113.1, 61.1, 57.5, 51.5, 23.2, 17.3, 6.4.

1,3-Bis(3-((1-(3-(triethoxysilyl)propyl)-1H-1,2,3-triazol-4-yl)methoxy)phenyl)prop-2-en-1-one (6e). Yield: 83%, empirical formula: C₃₉H₅₈N₆O₉Si₂; anal. calcd: C, 57.8; H, 7.2; N, 10.4; found: C, 57.5; H, 7.1; N, 10.2%; IR (neat, cm⁻¹): 3019, 2977, 2881, 1651, 1593, 1521, 1484, 1390, 1325, 1214, 1161, 1075, 1033, 1015, 742, 669, 665, 627, 542. ¹H NMR (400 MHz, CDCl₃) δ = 8.05 (d, ³J = 15.8 Hz, 1H), 7.60 (s, 1H), 7.57 (s, 2H), 7.54–7.45 (m, 2H), 7.39 (dd, ³J = 8.5, 1.9 Hz, 1H), 7.36–7.27 (m, 2H), 7.13 (dd, ³J = 8.1, 2.3 Hz, 1H), 7.05 (d, ³J = 8.3 Hz, 1H), 6.95 (t, ³J = 7.5 Hz, 1H), 5.26 (s, 2H), 5.19 (s, 2H), 4.29 (q, ³J = 7.1 Hz, 4H), 3.81–3.64 (m, 12H), 2.02–1.88 (m, 4H), 1.13 (dd, ³J = 13.6, 6.9 Hz, 18H), 0.53 (dd, ³J = 16.5, 8.5 Hz, 4H). ¹³C NMR (101 MHz, CDCl₃) δ = 189.3, 157.5, 156.5, 139.2, 138.8, 131.2, 130.4, 128.7, 128.3, 127.5, 123.2, 121.8, 120.5, 118.6, 113.1, 111.8, 61.6, 61.2, 57.5, 51.6, 23.2, 17.3, 6.4.

1-(3-((1-(3-(Triethoxysilyl)propyl)-1H-1,2,3-triazol-4-yl)methoxy)phenyl)-3-(4-((1-(3-(triethoxysilyl)propyl)-1H-1,2,3-triazol-4-yl)methoxy)phenyl)prop-2-en-1-one (6f). Yield: 84%, empirical formula: C₃₉H₅₈N₆O₉Si₂; anal. calcd: C, 57.8; H, 7.2; N, 10.4; found: C, 57.9; H, 7.0; N, 10.1%; IR (neat, cm⁻¹): 2972, 2935, 2885, 1660, 1572, 1508, 1484, 1438, 1389, 1289, 1242, 1172, 1071, 1010, 954, 783, 722, 696, 542. ¹H NMR (400 MHz, CDCl₃) δ = 7.62–7.55 (m, 3H), 7.54 (d, ³J = 8.5 Hz, 2H), 7.51–7.26 (m, 3H), 7.16 (dd, ³J = 14.0, 6.4 Hz, 2H), 6.96 (d, ³J = 8.6 Hz, 2H), 5.20 (d, ³J = 8.4 Hz, 4H), 4.30 (t, ³J = 7.2 Hz, 4H), 3.73 (q, ³J = 7.0 Hz, 12H), 2.03–1.88 (m, 4H), 1.14 (t, ³J = 7.0 Hz, 18H), 0.63–0.39 (m, 4H). ¹³C NMR (101 MHz, CDCl₃) δ = 189.0, 159.3, 157.5, 143.6, 142.5, 138.9, 131.1, 129.3, 128.7, 127.5, 127.1, 121.9, 120.4, 119.0, 118.4, 114.2, 113.2, 61.2, 57.5, 51.5, 23.2, 17.3, 6.5.

3-(2-((1-(3-(Triethoxysilyl)propyl)-1H-1,2,3-triazol-4-yl)methoxy)phenyl)-1-(4-((1-(3-(triethoxysilyl)propyl)-1H-1,2,3-triazol-4-yl)methoxy)phenyl)prop-2-en-1-one (6g). Yield: 89%, m.p.: 83 °C, empirical formula: C₃₉H₅₈N₆O₉Si₂; anal. calcd: C, 57.8; H, 7.2; N, 10.4; found: C, 57.6; H, 6.9; N, 10.2%; IR (neat, cm⁻¹): 2982, 2884, 1620, 1554, 1457, 1368, 1321, 1218, 1160, 1052, 1025, 1007, 922, 820, 752, 725, 663, 618, 541. ¹H NMR (400 MHz, CDCl₃) δ = 8.08–7.99 (m, 1H), 7.98–7.88 (m, 2H), 7.63–7.54 (m, 3H), 7.39 (td, ³J = 7.4, 2.9 Hz, 1H), 7.32–7.25 (m, 1H), 7.01–6.92 (m, 4H), 5.26 (s, 2H), 5.22 (s, 2H), 4.32–4.26 (m, 4H), 3.83–3.62 (m, 12H), 1.98–1.91 (m, 4H), 1.23–1.07 (m, 18H), 0.55–0.48 (m, 4H). ¹³C NMR (101 MHz, CDCl₃) δ = 188.0, 161.9, 161.0, 160.8, 157.5, 143.5, 143.3, 139.6, 139.4, 138.3, 132.0, 131.3, 131.1, 130.5, 129.7, 129.4, 128.3, 121.8, 120.3, 114.6, 113.5, 112.9, 111.8, 62.7, 62.3, 61.6, 61.2, 58.6, 57.5, 52.7, 51.6, 24.3, 23.2, 18.4, 17.3, 7.5, 6.4.

3-(3-((1-(3-(Triethoxysilyl)propyl)-1H-1,2,3-triazol-4-yl)methoxy)phenyl)-1-(4-((1-(3-(triethoxysilyl)propyl)-1H-1,2,3-triazol-4-yl)methoxy)phenyl)prop-2-en-1-one (6h). Yield: 87%, empirical formula: C₃₉H₅₈N₆O₉Si₂; anal. calcd: C, 57.8; H, 7.2; N, 10.4; found: C, 57.5; H, 7.1; N, 10.1%; IR (neat, cm⁻¹): 3019, 2994, 2886, 1521, 1455, 1322, 1217, 1163, 1033, 929, 742, 670, 542. ¹H NMR (400 MHz, CDCl₃) δ = 7.94 (d, ³J = 8.7 Hz, 1H), 7.81 (d, ³J = 8.8 Hz, 1H), 7.62 (d, ³J = 15.4 Hz, 1H), 7.55 (d, ³J = 16.2 Hz, 1H), 7.44 (s, 1H), 7.38 (dd, ³J = 9.1, 6.4 Hz, 1H), 7.21 (t, ³J = 6.6 Hz, 1H), 7.12 (d, ³J = 7.8 Hz, 1H), 6.98 (d, ³J = 8.7 Hz, 2H), 6.94–6.87 (m, 2H), 5.20 (d, ³J = 12.1 Hz, 4H), 4.29 (t, ³J = 7.1 Hz, 4H), 3.77–3.64 (m, 12H), 2.02–1.88 (m, 4H), 1.13 (t, ³J = 7.0 Hz, 18H), 0.53–0.42 (m, 4H). ¹³C NMR (101 MHz, CDCl₃) δ = 186.1, 160.7, 157.5, 142.3, 131.0, 130.4, 130.6, 131.0, 129.6, 128.7, 127.3, 120.9, 115.6, 113.4, 112.9, 61.1, 57.3, 51.1, 23.1, 17.3, 6.3.

1,3-Bis(4-((1-(3-(triethoxysilyl)propyl)-1H-1,2,3-triazol-4-yl)methoxy)phenyl)prop-2-en-1-one (6i). Yield: 89%, empirical formula: C₃₉H₅₈N₆O₉Si₂; anal. calcd: C, 57.8; H, 7.2; N, 10.4; found: C, 57.6; H, 7.1; N, 10.2%; IR (neat, cm⁻¹): 2982, 2884, 1653, 1550, 1456, 1321, 1249, 1051, 1017, 1006, 923, 820, 750, 726, 663, 542. ¹H NMR (400 MHz, CDCl₃) δ = 7.93 (d, ³J = 8.5 Hz, 1H), 7.93 (d, ³J = 8.5 Hz, 1H), 7.83 (d, ³J = 8.7 Hz, 1H), 7.74 (d, ³J = 8.5 Hz, 1H), 7.61–7.54 (m, 1H), 7.50 (d, ³J = 8.5 Hz, 1H), 7.40 (d, ³J = 7.6 Hz, 1H), 7.35–7.19 (m, 1H), 7.21 (s, 1H), 7.04–6.92 (m, 1H), 6.97 (ddd, ³J = 13.1, 11.2, 5.5 Hz, 2H), 5.37–4.97 (m, 2H), 5.34–5.02 (m, 2H), 4.29 (t, ³J = 7.0 Hz, 2H), 4.29 (t, ³J = 7.0 Hz, 2H), 3.72 (q, ³J = 7.0 Hz, 6H), 3.72 (q, ³J = 7.0 Hz, 6H), 2.08–1.66 (m, 2H), 2.01–1.91 (m, 2H), 1.14 (t, ³J = 6.9 Hz, 9H), 1.14 (t, ³J = 6.9 Hz, 9H), 0.54–0.45 (m, 2H), 0.59–0.38 (m, 2H). ¹³C NMR (101 MHz, CDCl₃) δ = 194.6, 188.8, 186.8, 161.9, 160.9, 158.9, 142.5, 130.9, 129.5, 128.9, 127.5, 127.1, 61.2, 57.3, 51.3, 23.1, 17.3, 6.4.

Results and discussion

Synthetic approach

The synthesis of the conjugated chalcone based organotriethoxysilanes (OTES) was efficiently achieved via a three step route. The first step involved the synthesis of the propynyloxybenzaldehydes 2a(i–iii) and propynyloxyacetophenones 2b(i–iii) by reaction of the o-, m- and p-isomers of hydroxylbenzaldehyde and hydroxylacetophenone, respectively, with propargyl bromide in the presence of K₂CO₃ as a base. The strong base enables easy proton abstraction from the hydroxyl group and enhances the rate of the forwarding reaction. The second step involved the Claisen–Schmidt condensation reaction of 2a with (i) substituted acetophenones 1c(a–c) resulting in the chalcone substituted terminal alkynes 3a–3i, and (ii) propynyloxyacetophenones 2b(i–iii) resulting in the chalcone substituted terminal di-alkyne products 5a–5i. The aldol condensation reaction was carried out under strongly basic conditions provided by KOH, using ethanol as the solvent. After vigorous stirring for 4 h, the reaction progress was monitored by TLC (hexane–ethyl acetate (8 : 2)). On completion, the reaction mixture was quenched by acidic (2 N HCl) ice cold water (till a pH of 4 was attained) and the product was isolated by methylene dichloride in good yield. The third and the final step was the synthesis of mono–OTES 4a–4i and di–OTES 6a–6i via the CuAAC reaction of the chalcone based terminal alkynes with AzPTES, which proceeded with the [CuBr(PPh₃)₃]/THF–TEA system at 65 °C for 3 h under inert conditions. The final products containing the 1,2,3-triazole linking chalcone to nPTES were synthesized in good yields (Scheme 1, Table 1).

Scheme 1 Synthesis of conjugate chalcone based OTES (4a–4i) and (6a–6i).

Table 1 Synthesis of mono-OTES (4a–4i) and di-OTES (6a–6i) using ‘click silylation’ at 65 °C for 3 h

Product ID/description	Substrate alkyne	Ar	Product	Yield (%)
4a/dark red oil				91
4b/chocolate brown oil				89
4c/orange red oil				90
4d/dark brown oil				88
4e/light brown oil				90
4f/light brown oil				87
4g/thick yellow oil				88
4h/brown oil				89
4i/dark red oil				87

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Product ID/description	Substrate alkyne (A1)	Substrate alkyne (A2)	Product	Yield (%)
6a/brown oil				90
6b/dark brown oil				90
6c/dark brown solid				89
6d/brown oil				87
6e/dark red oil				83
6f/dark brown oil				84
6g/dark brown solid				89
6h/brown oil				87
6i/orange red oil				91

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We proceeded with the two possible pathways (Scheme 2) for the generation of the chalcone based triethoxysilane (4a) to investigate the affect on the product yield and the rapidity in hydrolysis of the chalcone functionalised triethoxysilanes. Pathway 1 involves the Claisen–Schmidt condensation of propynylbenzaldehyde 2 with differently substituted acetophenones, followed by a cycloaddition reaction with AzPTES resulting in OTES with good yield and high purity. In comparison, following pathway 2, the first step involves the click reaction, which efficiently gives OTES with more than 90% yield, but the second step of chalcone formation results in the hydrolysis of the triethoxysilanes as the prominent reaction, which is associated with poor yield. This confirms the inability of pathway 2 to synthesize OTES and, therefore, only pathway 1 leads to the efficient synthesis of OTES.

Scheme 2 Probable pathways followed for synthesis of OTES from a common scaffold.

The synthesized OTES ((4a–4i) and (6a–6i)) show two major shifts in the ¹H NMR spectra (i) 1.0 unit in the triplet of –N₃CH₂CH₂– after the cyclization to the heterocycle and (ii) 5.0 units in the triplet at 2.5 ppm for terminal alkynes (⁴J = 2–3 Hz) 7.5 ppm. Silanes (6a–6i) exhibit different multiplets arising due to each – CH₂ unit of the propyl chain arm in nPTES (NMR A and NMR B, Fig. 2). NMR A (6a) shows unsplit multiplets of two propyl chains while NMR B (6c) marks out the separate identity of each –CH₂ unit of the two –N₃CH₂CH₂CH₂Si– chains. In the case of the ¹³C NMR, each carbon of nPTES (6a–6i) appeared as a separate singlet giving a unique identity to each carbon atom and shows significant chemical shift. The E (trans) geometry of the chalcone double bond was evident by the large olefinic coupling constant (J = 14–16 Hz) between the relevant signals in the ¹H NMR spectrum.

Fig. 2 NMR A (6a) reveals unsplit multiplets due to two propyl chain arms; NMR B (6c) shows isolated multiplets for each propyl chain arm.

Previously reported by Cattoen et al.,¹⁹ hybrid silica precursors were synthesized under microwave conditions in excellent yield. We herein report optimized thermal reaction conditions (4a, Table 2) for the synthesis of chalcone stapled to nPTES via a 1,2,3-triazole. The effects of variation in temperature, reaction time and catalyst loading were keenly examined. On performing the reaction at room temperature with 0.01 mmol of catalyst loading, only 20% of the product was isolated. With an increase in catalyst loading to 0.02 mmol and extending the time duration, no significant improvement in product yield was observed. But raising the temperature of the reaction mixture to 65 °C and applying vigorous stirring for 3 h drastically improved the yield from 24% to 91%. Further increase of temperature under similar reaction conditions does not alter the product yield. Therefore, thermally optimized reaction conditions for the synthesis of OTES 4a–4i and 6a–6i following ‘click silylation’ are as per entry 7 in Table 2.

Table 2 Optimization of the reaction conditions under thermal environmental conditions

Entry	Catalyst (%)	Reaction conditions	Reaction duration (h)	Yield^a (%)
a Determined by ^1H NMR analysis of a crude sample.
1	1 × 10^−5	rt, st	3	20
2	1 × 10^−5	rt, st	3	24
3	1 × 10^−5	55 °C, st	3	76
4	1 × 10^−5	55 °C, st	3	81
5	1 × 10^−5	65 °C, st	1	45
6	1 × 10^−5	65 °C, st	3	86
7	1 × 10^−5	65 °C, st	3	91
8	1 × 10^−5	65 °C, st	5	91
9	1 × 10^−5	65 °C, st	3	91
10	1 × 10^−5	75 °C, st	3	85
11	1 × 10^−5	75 °C, st	3	91

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Solvatochromism

As defined by Hantzsch, solvatochromism refers to the change in the position, intensity, and shape of the absorption bands intensity maxima in UV-Vis spectroscopy with a change of solvent polarity.⁵⁰ The major factor governing this phenomenon involves the variation of solute–solvent interactions with a change in solvent polarity that affects position, shape and intensity of the absorption bands.⁵¹ The preferential solvation which takes place in the vicinity of the solute molecule alters the solvation shell arrangements, thereby shifting the maxima of the wavelengths absorbed. Negative solvatochromism corresponds to a hypsochromic shift (blue shift) whereas the reverse is referred to as positive solvatochromism or a bathochromic shift (red shift).⁵² This is the first time, to the best of our knowledge, that a solvatochromic effect has been reported for OTES.

The term ‘solvent polarity’ lacks an exact definition, although numerous attempts have been made so far, but it broadly describes all of the intermolecular interactions between a solute and a solvent. The important point concerning the so-called ‘polarity of a solvent’ is the overall solvation capability, which is the cumulative effect of all the solvent–solute interactions, excluding those such as protonation, oxidation, reduction, complexation, etc., that might lead to a chemical change of the solute.⁵³ A number of scales have been established to quantify the influence of a solvent on chemical properties. These scales are based on some physicochemical property, which could be an equilibrium constant, reaction rate constant, spectral shift using absorption spectroscopy, etc.⁵⁴ We followed the Dimroth and Reichardt polarity scale to examine the effect of solvent polarity on various PfTES.

The observed solvatochromism depends on the chemical structure and physical properties of the chromophore and their interaction with solvent molecules, which in turn determines the strength of the intermolecular solute–solvent interactions in the equilibrium ground state and the Franck–Condon excited state. Typically, a large change in dipole moment upon excitation exhibits strong solvatochromism. In addition, the ability of a solute molecule to donate or accept hydrogen bonds to or from surrounding solvent molecules in the ground and Franck–Condon excited states further determines the extent and sign of its solvatochromism. The pronounced shift in the position of the absorption bands is due to a solvent-induced change in the electronic ground state structure from a less dipolar (in less polar solvents) to a more dipolar chromophore (in highly polar solvents) with increasing solvent polarity.

Moreover, the ability of a solute to form hydrogen bonds with solvent molecules in the ground state and Franck–Condon excited state also determines the extent of solvatochromism exhibited which is a result of π–π* transitions. Typically, solvatochromic compounds can be described by extreme resonance contributing structures. The change in absorption band with solvent polarity arises from variation in the contribution of these conjugated π electronic systems.⁵⁵

To examine the solvatochromic effects on the chalcone modified OTES, an UV-Vis photoelectronic study was performed using chloroform, acetonitrile and ethanol (with relative polarities of 0.26, 0.46, 0.65 as per the Dimroth and Reichardt polarity scale) as solvent media (shown for 4b in Fig. 3, for the others, see the supporting information†) within the concentration range of 0.1–1 mM. Non-polar solvents proved to be inefficient owing to their inability to solubilize the compounds; while other polar solvents were ineffective as they have a λ_max cut off value above the absorption maxima intensity region. The effect on the absorption maxima due to different substituents and positional isomers was observed and plotted as shown in Fig. 4. With an increase in the polarity of a solvent, the shift in the λ_max values for triethoxysilanes was observed to be both bathochromic and hypsochromic. Beginning with chloroform, the absorption maxima for all OTES lie in the region between 272–331 nm (4a–4i), with maximum moieties absorbing in the region of 300 and 320 nm. With an increase in the polarity using acetonitrile, a bathochromic shift was observed for 4c, 4d, and 4h. Further increasing the polarity using ethanol, the most polar of the three solvents, red and blue shifts were observed for 4a, 4b, 4e, 4f, 4g and 4i, and 4c, 4d and 4h, respectively (Table 3).

Fig. 3 UV-Vis spectra of compound 4b in different solvents at 25 °C (1 mM).

Fig. 4 Plot depicting the effect of polarity change using CHCl₃, CH₃CN and C₂H₅OH on OTES 4a–4i.

Table 3 The absorption maxima values of 4a–4i (0.1–1 mM) in various solvents at 25 °C

Sample ID	4a	4b	4c	4d	4e	4f	4g	4h	4i
	λmax (nm)
Chloroform	302	300, 250	322	314	318	327	272	331	295
Acetonitrile	293	298, 247	333	318	314	324	271	335	294
Ethanol	295	301, 249	327	314	318	328	272	329	296

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Di-OTES 6a–6i exhibit distinct behaviors for the shifts in the λ_max value (shown for 6c in Fig. 5; for the others, see the supporting information†) compared to mono-OTES 4a–4i in these solvents. As the π electron cloud remains conjugated, the λ_max shift due to the decrease or increase in solvent polarity using chloroform, acetonitrile and ethanol, appears significantly in the absorption spectra. With the increase in solvent polarity using acetonitrile, the hypsochromic shift was observed for all compounds except 6d, 6h and 6i which exhibited a bathochromic shift (Fig. 6, Table 4). On further raising the polarity of the solvent system using ethanol, a red shift in the absorption maximum was recorded for 6b, 6c, 6f, 6g and 6i while the others significantly exhibited a hypsochromic shift in wavelength.

Fig. 5 UV-Vis spectra of compound 6c in different solvents at 25 °C (0.5 mM).

Fig. 6 Plot depicting the effect of polarity change using CHCl₃, CH₃CN and C₂H₅OH on OTES 6a–6i.

Table 4 Representation of absorption maxima values of 6a–6i (0.1 mM–1 mM) in various solvents at 25 °C

Sample ID	6a	6b	6c	6d	6e	6f	6g	6h	6i
	λmax (nm)
Chloroform	332, 298	301	297	274	265	337	301	263	274
Acetonitrile	328, 291	298	293	334	313	324	299	314	328
Ethanol	328	302	299	303	301	335	304	265	330, 275

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A solvatochromic study was performed on a series of PfTES illustrating the effects of polarity change and showing the best activity for silanes was achieved using chloroform and acetonitrile as solvent media in UV-Vis spectra. Moreover, on consideration of a different polarity scale, which alters the polarity index of the solvents used, there will be a change in the corresponding batho- and hypso-chromic shifts observed. OTES being hydrolytically unstable have limited solubility and solvent tolerance. Their dissolution in protonating solvents like ethanol may hinder the absorption spectra for some OTES which may actually be active in other aprotic polar solvents. The rapidity in the hydrolysis of OTES tends to overcome the solubility in protic solvents at high concentrations but at concentrations of 10⁻³ M or lower, we can study their photophysical properties.

Conclusions

We have synthesized 1,2,3-triazolyl chalcone linked n-propyltriethoxysilanes (4a–4i and 6a–6i) by an efficient and promising methodology. The optimized thermal reaction conditions for CuAAC, using a ‘[CuBr(PPh₃)₃]/THF–TEA’ system, prove their excellence with product formation at 65 °C in 3 h. The variation in substituent moiety and effect of positional isomerism in PfTES considerably affected the absorption spectral properties as displayed in the UV-Vis spectra of the final compounds. The solvatochromic study illustrated the dissimilarity in the λ_max values arising as a result of an increase in dipole moment, which confirmed the different behaviors of each o-, m- and p- substituted TES in the spectroscopic study. The blue shift in the absorption bands seems to be due to some strong stabilising interactions of the solvent with the substituted OTES while the red shift in the absorption band can correspond to an increased solvation of the conjugated system. The extent of the interaction of different isomers (o-, m- and p-) with the solvent system and the degree of solvation determines the band shift observed in the UV-Vis spectra.

Conflict of interest

The authors declare no competing financial interest.

Acknowledgements

We thank Mr Avtar Singh 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 a CSIR-SRF(NET) fellowship.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra08724k

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