Multifunctional alkoxysilanes prepared by thiol–yne “click” chemistry: their luminescence properties and modification on a silicon surface

Abstract

The photoinitiated radical-based thiol–yne click reaction provides a simple and efficient method for the formulation of diverse alkoxysilanes. Seven alkoxysilanes, namely, 1,2-bis[3-(trimethoxysilyl)propylthio]hexane (T1), 1,2-bis[3-(trimethoxysilyl)propylthio]-3-chloropropane (T2), 1,2-bis[3-(trimethoxysilyl)propylthio]-3-bromopropane (T3), trimethoxy[3-(styrylthio)propyl]silane (T4), 1,2-bis{3-[dimethoxy(methyl)silyl]propylthio}hexane (D1), 1,2-bis{3-[dimethoxy(methyl)silyl]propylthio}-3-chloropropane (D2), and 1,2-bis{3-[dimethoxy(methyl)silyl]propylthio}-3-bromopropane (D3), were synthesized by reacting alkynes with 3-mercaptopropylalkoxysilane in the presence of a photoinitiator. The thiol–yne reactions ran neatly in standard glassware under 100 W UV irradiation. The functionalized trialkoxysilanes were obtained in quantitative to near-quantitative yields with high purity. Results showed that the reaction of synthesized T4 only occurred in the first cycle, and vinyl sulfide adduct was formed with two configurations of Z and E. Moreover, the isomerization of T4 from Z to E configurations was induced under UV irradiation. T1 and D1 showed excellent photoluminescence properties. Molecular calculations were also performed to confirm the experimental results. Computational results revealed that all compounds exhibited relatively large HOMO–LUMO band gaps, making them promising candidates as host materials for emitters and hole–electron blocking materials in OLED displays. In addition, T1, T2, and T3 were selected to modify the surface properties of Si (1, 0, 0), which can then be used for further functionalization or the immobilization of polymers or biomolecules.

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1. Introduction

Multifunctional alkoxysilanes, containing an alkoxysilane moiety as an anchoring group to surfaces and a second functionality, are of great interest for numerous applications.^1,2 These compounds also have an important role in biotechnology, nanotechnology, and molecular electronics.³ The organic groups span from anionic to cationic groups and from nonpolar to polar groups as well as fluorescent dyes and electroactive moieties.⁴ Given the numerous available and described silanes, the polarity and chemical reactivity of alkoxysilanes can be tailored over a wide range.⁵ However, the relative general hydrolytic fragility of alkoxysilanes, in particular, trialkoxysilanes, has considerably restricted the range of acceptable reaction types and experimental conditions that may be harnessed for their preparation. Therefore, access to alkoxysilanes depends on a relatively limited number of approaches. Alkoxysilane with special functional groups are generally prepared via the hydrosilylation reactions in the presence of platinum or rhodium complexes. However, this reaction has suffered from several deficiencies; for example, the catalytic activity of the expensive heavy metal Pt catalyst is always disturbed by the sensitive groups.⁶ Furthermore, metallic catalysts should be avoided from the standpoint of green chemistry. Therefore, the establishment of a new general synthetic strategy to prepare functionalized alkoxysilane is necessary. Thiol chemistry has recently attracted significant interest.⁷ Thiol–ene reaction has been proposed as a new click reaction particularly for the reaction systems that can be triggered photochemically or thermally and without the oxygen inhibition associated with other radical-based systems.^8,9 Thus, thiol–ene was used to synthesize small molecules and polymers such as dendrimers and stars.^10,11 Furthermore, the synthesis of functionalized trialkoxysilane under mild and neutral reaction conditions through the specific formation of robust sulfide bridges is also an attractive target because of the convenient features of the C–S bond. The divalent S atom largely dictates the particular reactivity displayed by this class of compounds.^12,13 Moreover, C–S bonds could also be activated by transition metal compounds.^14,15

Garrell et al. synthesized a range of trialkoxysilanes via facile thiol–ene click chemistry.⁶ By contrast, the thiol–yne reaction has not been extensively examined. Alkynes are relatively easy to synthesize in a variety of structural formats and are typically stable until presented with an opportunity to react.^15,16 Hence, alkynes have been considered as ideal substrates for a wide range of material applications via the Cu(I)-catalyzed Huisgen alkyne–azide reaction.^17,18 Based on this premise, determining the mechanism by which yne chemistry can be readily extended to other types of highly efficient reactions is important. The thiol–yne reaction involves addition of a thiol to an alkyne followed by addition of another thiol to the resulting alkene, giving a fully saturated species.^19,20 Thiol–yne reactions present a distinct opportunity to expand the chemistry of thiol–ene reactions providing access to a broad range of new materials and properties.²¹ After considering these attributes, dual-functionalized alkoxysilane were fabricated by photo-induced thiol–yne reaction between mercaptopropylalkoxysilane and alkynyl molecules. The double-oxyalkylation of the alkynyl molecules can significantly affects their molecular structure and action activity more than mono-oxyalkylation. This paper reports on a new method for the synthesis of multifunctional alkoxysilanes by utilizing a thiol–yne reaction. Seven alkoxysilanes (T1–T4, and D1–D3) were synthesized by reacting either alkynes with 3-mercaptopropylalkoxysilane in the presence of photoinitiator without any solvent. Thiol–yne approach is versatile in reactions in terms of reaction conditions, and it tolerates chemical functionality to yield multifunctional alkoxysilanes. In addition, T1 and D1 showed blue light emission upon UV irradiation (365 nm). Contrary to traditional luminescent molecules containing typical luminescent groups, which may restrict their application, these novel luminophors only contain alkyl chains. They could be cured to films and used as blue-light-emitting materials and devices, which have attracted enormous interest because of their great potential for various lighting applications. Furthermore, engineering of the inorganic Si surface is the key in various applications such as in materials science and biotechnology.^22–24 Given that alkoxysilyl units bind to surface hydroxy groups, the second functionality, usually separated by an alkyl group from the anchoring group, determines the final properties of the surface. Organosilane chemistry can be applied to introduce carboxyl, amine, vinyl, or thiol groups onto the surface,^25,26 which can then be used for further functionalization or the immobilization of polymers or biomolecules. Functionalized Si surface are traditionally readily prepared using 3-mercaptopropyltrimethoxysilane (MPS) and then usually submitted for further functionalization through thermal radical addition or other processes to conjugate other compounds.^27,28 However, this post-functionalization method usually results in relatively low functionality.²⁹ In this article, obtained functionalized trialkoxysilanes (T1, T2, and T3) are used to modify the Si (1, 0, 0) surface to overcome this limitation and enhance functionality.

2. Experimental

2.1 Materials

1-Hexyne, 3-chloro-1-propyn, 3-bromo-1-propyne, phenylacetylene and 2,2-dimethoxy-2-phenylacetophenone (DMPA) were supplied by Aladdin Co. (China) and used without further purification. 3-Mercaptopropylmethyldimethoxysilane (MPDMS) and 3-mercaptopropyltrimethoxysilane (MPS) were purchased from Diamond Advanced Material of Chemical Inc. (China) and distilled before use.

2.2 Characterization and measurements

The thiol–yne reaction was irradiated by UV on a Spectroline Model SB-100P/FA lamp (365 nm, 100 w). UV intensity is 4500 μW cm⁻² at a distance of 38 cm. ¹H NMR spectra was recorded on a Bruker AVANCE 400 spectrometer at 25 °C using CDCl₃ as solvent and tetramethylsilane (TMS) as an interior label. Fourier transform infrared spectra (FT-IR) were recorded on a Bruker TENSOR27 infrared spectrophotometer with KBr pellet technique within the 4000–400 cm⁻¹ region. High-resolution mass spectra were obtained using positive mode on Agilent Technologies 6510 Q-TOF LC-MS. Luminescence (excitation and emission) spectra of the samples were determined with a Hitachi F-4500 fluorescence spectrophotometer using a monochromated Xe lamp as an excitation source; excitation and emission slits were 5 and 2.5 nm, respectively. XPS analyses were performed using a ThermoFisher Scientific Escalab 250 spectrometer with a monochromated Al Kα X-ray source at a residual pressure of 10⁻⁷ Pa. Survey and high resolution scans were performed at 100 eV pass energy with 1 eV step and 20 eV pass energy with 0.05 eV step, correspondingly. Contact angle was recorded on a Dataphysics OCA-20 contact angle analyzer with distilled water as the test liquid.

2.3 Synthesis of multifunctionalized alkoxysilanes

2.3.1 Synthesis of T1, T2, and T3. A cooled oven-dried 25 mL glass vessel was charged with 1-hexyne (0.82 g, 10 mmol), MPS (4.20 g, 22 mmol), and DMPA (1 wt%, 0.04 g), and then capped and purged briefly with Ar. The vessel was placed under a 100 W UV light (λ_max = 365 nm). The reaction mixture was irradiated for 20 min with gentle stirring to yield T1 with high conversion (97%). After the thiol–yne reaction, the product was purified by precipitation in 1-hexane to eliminate unreacted mercaptans and photoinitiator.
Data of T1. ¹H NMR (300 MHz, CDCl₃): 0.73–0.77 (m, 4H, –CH₂Si(OCH₃)₃), 0.87–0.91 (m, 3H, –CH₃), 1.69–1.73 (t, 4H, SCH₂CH₂CH₂Si(OCH₃)₃), 2.58–2.61 (m, 4H, SCH₂CH₂CH₂Si(OCH₃)₃), 2.82 (dd, J = 19.0, 5.8 Hz, 1H, S(CH₂)CH–C(H_a)H_b–S–), 2.75–2.60 (m, 2H, –S–CH(CH₂–)–C(H_a)H_b–S–), 3.50–3.60 (m, 18H, Si(OCH₃)₃), 1.53–1.11 (m, 4H, aliphatic CH₂), 1.75–1.85 (m, 2H, SCH(CH₂)CH₂S). ¹³C NMR (100.62 MHz, CDCl₃, ppm): δ = 8.43, (–CH₂Si(OCH₃)₃), 13.87, (–CH₃), 22.48, (CH₃CH₂CH₂CH₂CH(CH₂)S), 28.81, (CH₃CH₂CH₂CH₂CH(CH₂)S), 23.02, (SCH₂CH₂CH₂Si(OCH₃)₃), 33.14, (CH₃CH₂CH₂CH₂CH(CH₂)S), 33.70, (CHSCH₂CH₂), 35.86, (CH₂SCH₂CH₂), 38.25, (–S–CH(CH₂–)–CH₂–S–), 45.63, (–S–CH(CH₂–)–CH₂–S–), 50.35, (Si(OCH₃)₃). HRMS (FAB) calcd for C₁₉H₄₄O₆S₂Si₂ (MNa⁺): 497.2106, found 497.1989.

T2 was synthesized similar to that of T1 and obtained as colorless liquid, yield: 96%.

Data of T2. ¹H NMR (400 MHz, CDCl₃): 0.73–0.77 (m, 4H, –CH₂Si(OCH₃)₃), 1.69–1.73 (t, 4H, SCH₂CH₂CH₂Si(OCH₃)₃), 2.58–2.61 (m, 4H, SCH₂CH₂CH₂Si(OCH₃)₃), 2.83 (dd, J = 19.0, 5.8 Hz, 1H, –S–CH(CH₂–)–C(H_a)H_b–S–), 2.93–3.03 (m, 2H, –S–CH(CH₂–)–C(H_a)H_b–S–), 3.50–3.60 (m, 18H, Si(OCH₃)₃), 3.86 (dd, J = 4.0 Hz, 1H, –SCH–(C(H_a)H_bCl)–CH₂–S–), 3.89 (dd, J = 4.1 Hz, 1H, –SCH–(C(H_a)H_bCl)–CH₂–S–). ¹³C NMR (100.62 MHz, CDCl₃, ppm): δ = 8.52, (–CH₂Si(OCH₃)₃), 22.87, (SCH₂CH₂CH₂Si(OCH₃)₃), 34.46, (CHSCH₂CH₂), 35.95, (CH₂SCH₂CH₂), 38.38 (–S–CH(CH₂–)–CH₂–S–), 46.54, (–S–CH(CH₂–)–CH₂–S–), 47.42, (CHCH₂Cl), 50.36, (Si(OCH₃)₃). HRMS (FAB) calcd for C₁₅H₃₅ClO₆S₂Si₂ (MH⁺): 466.1105, found 466.2548.

T3 was synthesized similar to that of T1. T3 was obtained as light yellow liquid, yield: 94%.

Data of T3. ¹H NMR (400 MHz, CDCl₃):0.70–0.74 (m, 4H, –CH₂Si(OCH₃)₃), 1.66–1.70 (t, 4H, SCH₂CH₂CH₂Si(OCH₃)₃), 2.57–2.60 (m, 4H, SCH₂CH₂CH₂Si(OCH₃)₃), 2.83 (dd, J = 19.0, 5.8 Hz, 1H, –S–CH(CH₂–)–C(H_a)H_b–S–), 2.93–3.03 (m, 2H, –S–CH(CH₂–)–C(H_a)H_b–S–), 3.50–3.60 (m, 18H, Si(OCH₃)₃), 3.64 (dd, J = 4.0 Hz, 1H, –SCH–(C(H_a)H_bBr)–CH₂–S–), 3.76 (dd, J = 4.0 Hz, 1H, –SCH–(C(H_a)H_bBr)–CH₂–S–). ¹³C NMR (100.62 MHz, CDCl₃, ppm): δ = 8.07, (–CH₂Si(OCH₃)₃), 22.26, (SCH₂CH₂CH₂Si(OCH₃)₃), 34.46, (CHSCH₂CH₂), 35.95, (CH₂SCH₂CH₂), 38.38 (–S–CH(CH₂–)–CH₂–S–), 45.54, (–S–CH(CH₂–)–CH₂–S–), 49.14, (CHCH₂Br), 50.32, (Si(OCH₃)₃). HRMS (FAB) calcd for C₁₅H₃₅BrO₆S₂Si₂ (MH⁺): 512.0665, found 512.1742.

2.3.2 Synthesis of T4. A cooled oven-dried 25 mL glass vessel was charged with phenylacetylene (1.02 g, 10 mmol), MPS (2.10 g, 11 mmol), and DMPA (1 wt%, 0.02 g), and then capped and purged briefly with Ar. The reaction was neatly ran. The vessel was placed under a 100 W UV light (λ_max = 365 nm). The reaction mixture was irradiated for 20 min with gentle stirring to yield T4 with high conversion (97%). After the thiol–yne reaction, the resultant was purified by precipitation in 1-hexane to eliminate unreacted mercaptans and photoinitiator.
Data of T4. ¹H NMR (400 MHz, CDCl₃): 0.73–0.76 (m, 2H, –CH₂Si(OCH₃)₃), 1.80–1.84 (t, 2H, SCH₂CH₂CH₂Si(OCH₃)₃), 2.78–2.85 (m, 2H, SCH₂CH₂CH₂Si(OCH₃)₃), 3.47–3.57 (m, 9H, Si(OCH₃)₃), 6.22–6.46 (dd, J = 14.8 Hz =C–H proton from the Z-vinylene unit), 6.48–6.74 (m, J = 15.6 Hz, =C–H proton from the E-vinylene units), 7.50–7.00 (m, 5H, Ar–H). ¹³C NMR (100.62 MHz, CDCl₃, ppm): δ = 8.22 (–CH₂Si(OCH₃)₃), 22.42 (SCH₂CH₂CH₂Si(OCH₃)₃), 38.59 (SCH₂CH₂CH₂Si(OCH₃)₃), 50.32, (Si(OCH₃)₃), 125.21, 125.40, 126.54, 126.75, (C from the Z and E vinylene unit), 126.75, 126.85, 127.53, 128.19, 128.25, 128.31, (Ar–6C). HRMS (FAB) calcd for C₁₄H₃₃O₃SSi (MNa⁺): 321.1030, found 321.0595.

2.3.3 Synthesis of D1, D2, and D3. A cooled oven-dried 25 mL glass vessel was charged with 1-hexyne (0.82 g, 10 mmol), MPDMS (3.78 g, 22 mmol), and DMPA (1 wt%, 0.04 g), and then capped and purged briefly with Ar. The reaction was neatly ran. The vessel was placed under a 100 W, λ_max = 365 nm UV light. The reaction mixture was irradiated for 20 min with gentle stirring to yield D1 with high conversion (95%). After the thiol–yne reaction, the product was purified by precipitation in 1-hexane to eliminate unreacted mercaptans and photoinitiator.
Data of D1. ¹H NMR (300 MHz, CDCl₃): 0.13–0.14 (s, 3H, CH₂Si(CH₃)(OCH₃)₂), 0.65–0.73 (m, 4H, –CH₂Si(OCH₃)₃), 0.85–0.90 (m, 3H, –CH₃), 1.53–1.11 (m, 4H, aliphatic CH₂), 1.69–1.73 (t, 4H, SCH₂CH₂CH₂Si(OCH₃)₃), 1.75–1.85 (m, 2H, SCH(CH₂)CH₂S), 2.58–2.61 (m, 4H, SCH₂CH₂CH₂Si(OCH₃)₃), 2.82 (dd, J = 19.0, 5.8 Hz, 1H, >CH–C(H_a)H_b–S–), 2.75–2.60 (m, 2H, –S–CH(CH₂–)–C(H_a)H_b–S–), 3.50–3.60 (m, 12H, Si(OCH₃)₂). ¹³C NMR (100.62 MHz, CDCl₃, ppm): δ = −5.80, Si(CH₃), 8.43, (–CH₂Si(OCH₃)₃), 13.82, (–CH₂CH₃), 28.81, (CH₃CH₂CH₂CH₂CH(CH₂)S), 23.02, (SCH₂CH₂CH₂Si(OCH₃)₃), 23.17, (CH₃CH₂CH₂CH₂CH(CH₂)S), 33.14, (CH₃CH₂CH₂CH₂CH(CH₂)S), 33.70, (CHSCH₂CH₂), 35.86, (CH₂SCH₂CH₂), 38.25, (–S–CH(CH₂–)–CH₂–S–), 45.63, (–S–CH(CH₂–)–CH₂–S–), 49.98, (Si(OCH₃)₃). HRMS (FAB) calcd for C₁₈H₄₂O₄S₂Si₂ (MH⁺): 443.2206, found 443.2104.

D2 was synthesized similar to that of D1. D2 was obtained as colorless liquid, yield: 94%.

Data of D2. ¹H NMR (400 MHz, CDCl₃): 0.13–0.14 (s, 3H, CH₂Si(CH₃)(OCH₃)₂), 0.73–0.77 (m, 4H, –CH₂Si(OCH₃)₃), 1.69–1.73 (t, 4H, SCH₂CH₂CH₂Si(OCH₃)₃), 2.58–2.61 (m, 4H, SCH₂CH₂CH₂Si(OCH₃)₃), 2.83 (dd, J = 19.0, 5.8 Hz, 1H, –S–CH(CH₂–)–C(H_a)H_b–S–)), 2.93–3.03 (m, 2H, –S–CH(CH₂)–C(H_a)H_b–S–), 3.50–3.60 (m, 12H, Si(OCH₃)₂), 3.75 (dd, J = 4.0 Hz, 1H, –SCH–(C(H_a)H_bCl)–CH₂–S–), 3.86 (dd, J = 4.1 Hz, 1H, –SCH–(C(H_a)H_bCl)–CH₂–S–). ¹³C NMR (100.62 MHz, CDCl₃, ppm): δ = −5.80, (Si(CH₃), 8.60, (–CH₂Si(OCH₃)₃), 22.89, (SCH₂CH₂CH₂Si(OCH₃)₃), 34.35, (CHSCH₂CH₂), 35.74, (CH₂SCH₂CH₂), 38.18, (–S–CH(CH₂–)–CH₂–S–), 46.58, (–S–CH(CH₂–)–CH₂–S–), 47.51, (CHCH₂Cl), 50.11, (Si(OCH₃)₃). HRMS (FAB) calcd for C₁₅H₃₅ClO₄S₂Si₂ (MH⁺): 434.1217, found 434.5601.

D3 was synthesized similar to that of D1. D3 was obtained as light yellow liquid, yield: 93%.

Data of D3. ¹H NMR (400 MHz, CDCl₃): 0.14–0.15 (s, 3H, CH₂Si(CH₃)(OCH₃)₂), 0.70–0.72 (m, 4H, –CH₂Si(OCH₃)₃), 1.67–1.71 (t, 4H, SCH₂CH₂CH₂Si(OCH₃)₃), 2.57–2.60 (m, 4H, SCH₂CH₂CH₂Si(OCH₃)₃), 2.83 (dd, J = 19.0, 5.8 Hz, 1H, –S–CH(CH₂–)–C(H_a)H_b–S–)), 2.93–3.03 (m, 2H, –S–CH(CH₂–)–C(H_a)H_b–S–), 3.50–3.60 (m, 18H, Si(OCH₃)₃), 3.66 (dd, J = 4.0 Hz, 1H, –SCH–(C(H_a)H_bBr)–CH₂–S–), 3.77 (dd, J = 4.0 Hz, 1H, –SCH–(C(H_a)H_bBr)–CH₂–S–). ¹³C NMR (100.62 MHz, CDCl₃, ppm): δ = −5.89, (Si(CH₃), 8.07, (–CH₂Si(OCH₃)₃), 22.26, (SCH₂CH₂CH₂Si(OCH₃)₃), 34.46, (CHSCH₂CH₂), 35.95, (CH₂SCH₂CH₂), 38.38 (–S–CH(CH₂–)–CH₂–S–), 45.54, (–S–CH(CH₂–)–CH₂–S–), 49.14, (CHCH₂Br), 50.31, (Si(OCH₃)₃). HRMS (FAB) calcd for C₁₅H₃₅BrO₄S₂Si₂ (MH⁺): 478.6454, found 478.2541.

2.4 Modification on Si (1, 0, 0) surface

Si wafers were cut into pieces, cleaned by sonication in chloroform, acetone, isopropanol, and deionized water, and then dried by Ar blowing. The clean Si (1, 0, 0) wafers were dipped in HF solution to remove the oxide layer and then treated with a freshly prepared Piranha solution (H₂O₂–H₂SO₄ = 1/3) for 30 min to obtain a hydroxy-terminated Si surface. The substrates were thoroughly washed with deionized water and again dried. A monolayer was formed by immersing the freshly prepared surfaces in a stirred solution of T1, T2, or T3 in toluene (0.1 vol%) which was purged with Ar for 12 h. Unreacted silanes were removed from the surface by washing with ethanol and water. The modified Si wafers were reserved in distilled water and dried by Ar blowing before testing.

3. Results and discussion

3.1 Synthesis of T1, T2, T3, D1, D2, and D3

For a widely used catalytic synthesis system, the catalytic efficiency, functional group tolerance, and structural tunability are of the primary concerns. Contrary to the low structural selectivity of the radical- and nucleophile-initiated alkyne hydrothiolations, the DMPA-catalyzed system is expected to work efficiently for a various functional monomers. In this study, the starting point was 3-mercaptopropylalkoxysilane, which was expected to produce a series of functionalized compounds via thiol–yne reaction, and the most important point is to develop a new synthetic method for diverse alkoxysilanes. 3-Mercaptopropylalkoxysilane is versatile precursor to prepare other alkoxysilane derivatives. The thiol–yne reaction has been proven to be a valid way to prepare new molecules and materials. For its' UV induced reaction process and free solvent reaction condition which are both friendly to the environment, this method meet the need of the concept of “green chemistry”.

A series of multifunctionalized alkoxysilane was synthesized via thiol–yne click reaction. Similar synthesis routes were applied to a number of alkyne monomers with different functional groups such as aliphatic chain and aliphatic chain with chlorine and bromine groups. Considering that two kinds of 3-mercaptopropylalkoxysilanes are used for these reactions, thiol–yne reaction should occur and products as depicted in Scheme 2 should be obtained. Typically, 3-mercaptopropylalkoxysilane and alkyne monomer are mixed together without any solvent. In this study, DMPA was chosen as the photoinitiator and irradiation was provided by 100 W UV-lamp (λ = 365 nm). Photoinitiation in thiol–yne systems was achieved through the generation of radical centers. In all cases, the reaction was completed within 20 min. The reaction was monitored by FTIR spectrophotometer, the results of which indicated by the disappearance of the characteristic absorption band of thiol group (2574 cm⁻¹) and the yne group (3314 and 2125 cm⁻¹) after reaction for 20 min (Fig. 1). These compounds are generally obtained in high yield. To remove DMPA and impurity, the crude compounds need to be precipitated in n-hexane. Taking T1 as an example, the ¹H-NMR spectrums of 1-hexyne, MPS and T1 were shown in Fig. 2. The appearance of a new dd-coupling peak at 2.82 ppm is attributed to hydrogen connected with chiral carbon that is adjacent to the S atom. The standard dd splitting is due to the effect of the chiral carbon. The signal that appeared at 2.82 ppm also suggests the completion of the thiol–yne reaction. The formation of compounds T2, T3, and D1 to D3 was also confirmed by ¹H-NMR and ¹³C-NMR, and MS. The mechanism of a thiol–ene addition reaction also counts for thiol–yne additions, but an additional propagation cycle is operative. From the first cycle, a vinyl sulfide adduct is formed, which then undergoes a second thiol addition to yield the final product. Therefore, a full addition reaction consists of the consecutive addition of 2 eq. thiol to an alkyne (Scheme 1). In most of the thiol–ene reactions, the internal double bond has relatively low reactivity. Given the unique two-step sequential radical addition mechanism of thiol–yne reaction, the defect of thiol–ene reaction was overcame. To further investigate the structural property of the compounds at the molecular level, molecular orbital calculations have been performed at the B3LYP/6-31G(d) level using the Gaussian 03 suite of programs. The HOMO and LUMO energies, as well as the energy gap of these compounds, are shown in Table 1. All compounds possess relatively large band gaps that range from 6.01 eV (T1) to 6.81 eV (D2), which can be attributed to the Si atom which may endow large band gaps. This condition enables the compounds as potential candidates as host materials for emitters and hole–electron blocking layer in OLED displays.

Fig. 1 FTIR spectra of 1-hexyne, MPS, and T1.

Fig. 2 ¹H-NMR data of T1, MPS, and 1-hexyne.

Scheme 1 Typical reaction mechanism of thiol–yne reactions.

Scheme 2 Synthesis route of compounds T1, T2, T3, D1, D2, and D3.

Table 1 Physical data of compounds T1 to T4, D1 to D3, and T4 Z/E

Compound	HOMO (eV)	LUMO (eV)	Band gap (eV)
T1	−5.65371	0.361095	6.01480
T2	−5.75494	0.321911	6.07685
T3	−5.71657	0.334972	6.05154
D1	−5.66324	1.108865	6.77210
D2	−5.76501	1.050632	6.81564
D3	−5.72963	0.593481	6.32311
T4 Z	−5.27357	−0.57879	4.69478
T4 E	−5.49589	−0.40246	5.09343

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3.2 Synthesis of T4 and the isomerization phenomenon

Notably, the NMR spectrum of T4 showed the presence of vinyl proton signals at 6.3 and 5.6 ppm, which corresponded to the unreacted vinylthioether intermediate. The thiol–yne reaction was stopped in the first cycle of the completed addition process either by increasing the reaction time or adding more thiol monomer (Scheme 3). Moreover, the addition of thiol to 1-phenylalkyne was found to be regio- and stereoselective, and this process leads to the more stable aryl-substituted alkenyl radical intermediate. UV light could induce the isomerization of T4. The isomerization process was monitored by measuring the change in the ¹H NMR spectrum with time using the crude products of T4. The E–Z isomerization occurred upon UV irradiation (Fig. 3). Z-Isomer was initially predominant. With an increase in the irradiation time, the Z content of the molecule decreased. The isomerization process proceeded quickly and achieved a state of equilibrium at an E/Z ratio of 72/28 after irradiation for 50 min. The initially formed Z-vinyl sulfide was probably isomerized into the E-product through reversible addition of the thiyl radical onto the alkene.

Scheme 3 Synthesis of T4 and their isomerization under UV light.

Fig. 3 Time course study on the isomerization of T4 from 5 min to 50 min.

To fully understand this phenomenon, density functional theory (DFT) was also used to calculate the molecular orbital at the B3LYP/6-31G(d) level using the Gaussian 03 suite of programs. Table 1 lists the corresponding HOMO and LUMO energies, as well as the energy gap, of T4 Z and T4 E. The calculated molecular orbital energy levels of T4 Z and T4 E together with the molecular orbital maps of the HOMOs and LUMOs are shown in Fig. 4. The HOMO and LUMO energies of T4 E are lower than that of T4 Z. The results indicate that the product T4 E corresponds to thermodynamics, whereas the product T4 Z is a dynamic product. The speculated mechanism of the Z to E isomerization is shown in Scheme 4. At first, the addition of a thiyl radical to terminal alkynes occurs regioselectively at the less hindered position and gives a vinyl radical intermediate as a Z or E-mixture of isomers in equilibrium as expected. Then, hydrogen atom abstraction from the thiol by the resulting vinyl radical leads to the E- and Z-vinylsulfides. Eventually, the reversible addition of a thiyl radical to the vinyl sulfide may promote the isomerization of the kinetically favored Z-isomer into the thermodynamically more stable E-vinyl sulfide. Second step adducts will be unavailable because of the steric hindrance and the reversible isomerization. Steric hindrance probably prevents further addition of the thiyl radical thus the E-isomer being isolated as a dominate isomer. Subtle manipulation of the conformation may give rise to new molecule with different properties.

Fig. 4 Calculated molecular orbital energy levels of T4 Z and T4 E together with the molecular orbital maps of the HOMOs and LUMOs.

Scheme 4 Possible mechanism of UV-induced isomerization of T4.

3.3 Luminescent properties of the compounds obtained

T1, D1, 3-mercaptopropylmethyldimethoxysilane (MPDMS) and 3-mercaptopropyltrimethoxysilane (MPS) emit a blue color in solution and without solvent when irradiated by UV light, whereas others do not. T1, T2, and T3 were chosen as examples because T1 and D1, T2 and D2, and T3 and D3 have similar structures. Room temperature emission spectra of T1, T2, T3, and MPS excited by 365 nm are shown in Fig. 5(a). MPS has an emission peak with a maximum emission wavelength (λ_max) of ∼414 nm in pure substance state. Compound T1 also displays analogic fluorescence spectra with (λ_max) of ∼414 nm. However, T1 showed an enhanced luminescent intensity and narrower half peak width compared with MPS after the reaction with 1-hexyne. By contrast, the fluorescence intensity of T2 or T3 was weak and could be neglected. The emission of T1 with different volume concentrations from 10% to 100% is shown in Fig. 5(b). The luminescence intensity was enhanced with increasing volume concentration, which is consistent with most organic fluorescent molecules.^30–32 Besides, the emission of MPDMS and D1 was shown in Fig. 5(c), which illustrated a similar luminescent intensity enhancement trend just like T1. We attributed this similar luminescent properties of T1 and D1 to the analogic structures.

Fig. 5 Emission spectra (λ_ex = 365 nm) of (a) MPS, T1, T2, and T3, (b) T1 in CHCl₃ with different volume concentrations varying from 10% to 100% and (c) emission spectra of MPDMS and D1.

Compared with carbon, silicon has five empty 3d orbital that can be used as electron acceptor, facilitating the acceptance of electrons. Fluorescence was assumed to be generated by the lone pair electrons of S coordinated with Si atoms. The ligand field splitted the 3d orbital of Si atom, which was degenerated before coordination. The electrons rearranged in the split orbitals, and d–d transition occurred, which could be attributed to the luminescent properties of T1 and MPS.

Different substituent groups evidently affected the electronic orbital, which agrees with experimental results as mentioned previously. The pushing electron effect of alky caused the increasing electron density of S atom in T1, which may have strengthened the coordination effect between the S and Si atoms. Consequently, the fluorescence emission of T1 was considerably enhanced. Moreover, the electron density of S atom in T2 and T3 decreased because of the effect of the electron-withdrawal of the halogen atom. This phenomenon further hampered the coordination effect between S and Si atoms, which may account for the fluorescence emission that could be ignored by T2 or T3.

XPS can effectively characterize the coordination condition via detection of the binding energy between ligand and central atom.³³ The coordination conditions were more extensively examined by performing high-resolution XPS scans for S and Si atoms in T1 and T2, respectively. The S 2p peak appeared at 163.2 eV in T2, but shifted to low binding energy at 156.7 eV (double-bond equivalent = 5.5 eV) in T1 [Fig. 6(a)]. Fig. 6(b) displays the apparent shift of the peak of Si 2p. The Si 2p peak partly shifted from 102.2 eV to 104.7 eV in T1. The formation of coordination bonds is accompanied by electron transfer between ligands and the central atoms. Thus, the shift of the binding energy peaks of S and Si atoms further confirmed the previous conjecture. Therefore, S atom coordinated with Si atom in T1, whereas T2 and T3 did not show any coordination between Si and S atoms. Such emission from structurally simple organic molecules promises potential implementation in future applications. Of particular significance in this respect is the possibility to prepare blue-light-emitting polymeric films.³⁴

Fig. 6 XPS of S 2p spectral regions in (a) T1 and T2, and Si 2p spectral regions in (b)T1 and T2.

3.4 Surface modification on Si wafer

The monolayers prepared from organosilane compounds have attracted increasing attention because of their technological applications such as in electronic devices and biomedical materials. A simple method was adopted to produce organosilane monolayers on Si (1, 0, 0). The general procedure for construction of monolayer structures on Si wafers is outlined in Fig. 7.

Fig. 7 Modification on Si wafer [Si (1, 0, 0)] using T1, T2, and T3 and the corresponding changes in contact angles.

Modified Si surfaces were prepared by a two-step method, that is, through acid wash and silanization. The effectiveness of cleaning step was confirmed by measuring the contact angle.³⁵ The contact angle of the cleaned slide was less than 8°. The exposure of Si–OH endows Si a hydrophilic surface, which was consistent with the results from Cras et al. Three different alkoxysilanes T1, T2, and T3 were grafted onto freshly prepared Si wafer. The contact angles measured by water confirmed the surface modification. T2 exhibited the lowest contact angle (67.5°), which was attributed to the Cl group. Replacement of Cl group by Br atom, that is, T3, increased the contact angle to 84.3°. T3 showed the highest contact value (84.3°) because of its low affinity for water. The obvious change in the contact angles after reaction with different alkoxysilanes demonstrated that the proposed process achieved alkoxysilane coverage. The change of contact angle is a result of the combination of the coverage content and the species of different alkoxysilanes.

The existence of an organosilicon monolayer was supported by the elemental composition analysis via XPS. T2- and T3-modified Si surfaces were selected to show the modification result. The peaks at 201 and 270 eV corresponded to 2p and 2s binding energies of Cl atom [Fig. 8(a)]. A similar case happened to T3-modified Si, the peaks at 69 and 256 eV corresponded to 3d and 3s binding energies of Br atom, respectively [Fig. 8(b)]. The inserted figure displays the enlarged scale of 3d spectral region of Br atom in T3-modified Si wafer. The presence of the monolayers was further confirmed by XPS. Besides, the XPS element composition data of T1, T2 and T3-modified Si wafer were listed in Table 2. The data related were determined by integration of the peak areas. The C/Si ratio of the three modified Si wafer was almost same as the similarity of the three compounds used for modification process.

Fig. 8 XPS spectral of (a) T2- and (b) T3-modified Si wafer [Si (1, 0, 0)].

Table 2 XPS composition data of T1, T2, and T3 modified silicon wafer

Compound%	Si%	C%	O%	S%	Si/C
T1	14.7	34.2	36.1	15.0	0.42
T2	12.2	32.0	39.7	13.1	0.38
T3	13.7	29.7	40.5	14.7	0.46

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4. Conclusions

A flexible, atom-economical, and easily scalable approach for the synthesis of multi-functionalized alkoxysilanes is reported. Customization of generally inaccessible alkoxysilanes can now be readily tailor-made. Even more complex and sensitive function groups can also be incorporated into the products using a similar method. The isomerization property of T4 is also expected to provide a gating tool in manipulating the functional properties of the functionalized trialkoxysilanes. In addition, the functionalized trialkoxysilanes may be applied in the production of Si (1, 0, 0) surfaces with organic coatings with different end groups under mild conditions. These Si surfaces can then be used for further functionalization or the immobilization of polymers or biomolecules.

Acknowledgements

The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China (no. 21274080) and the Key Natural Science Foundation of Shandong Province of China (no. ZR2009BZ006 and no. ZR2011BZ001).

Notes and references

1. A. B. Descalzo, R. Martínez-Máñez, F. Sancenon, K. Hoffmann and K. Rurack, Angew. Chem., Int. Ed., 2006, 45, 5924–5948.
2. W. Senaratne, L. Andruzzi and C. K. Ober, Biomacromolecules, 2005, 6, 2427–2448.
3. S. Onclin, B. J. Ravoo and D. N. Reinhoudt, Angew. Chem., Int. Ed., 2005, 44, 6282–6304.
4. A. Lex, P. Pacher, O. Werzer, A. Track, Q. Shen, R. Schennach, G. Koller, G. Hlawacek, E. Zojer and R. Resel, Chem. Mater., 2008, 20, 2009–2015.
5. B. Quiclet-Sire, Y. Yanagisawa and S. Z. Zard, Chem. Commun., 2014, 50, 2324–2326.
6. A. K. Tucker-Schwartz, R. A. Farrell and R. L. Garrell, J. Am. Chem. Soc., 2011, 133, 11026–11029.
7. C. E. Hoyle and C. N. Bowman, Angew. Chem., Int. Ed., 2010, 49, 1540–1573.
8. A. B. Lowe, Polym. Chem., 2010, 1, 17–36.
9. Y. Zuo, H. Lu, L. Xue, X. Wang, L. Ning and S. Feng, J. Mater. Chem. C, 2014, 2, 2724–2734.
10. C. Rissing and D. Y. Son, Organometallics, 2009, 28, 3167–3172.
11. C. Rissing and D. Y. Son, Organometallics, 2008, 27, 5394–5397.
12. S. G. Modha, V. P. Mehta and E. V. Van der Eycken, Chem. Soc. Rev., 2013, 42, 5042–5055.
13. M. Winkler, Y. S. Raupp, L. A. Köhl, H. E. Wagner and M. A. Meier, Macromolecules, 2014, 47, 2842–2846.
14. L. Wang, W. He and Z. Yu, Chem. Soc. Rev., 2013, 42, 599–621.
15. R. Tan and D. Song, Organometallics, 2011, 30, 1637–1645.
16. Z. P. Demko and K. B. Sharpless, Angew. Chem., Int. Ed., 2002, 41, 2113–2116.
17. J. E. Hein and V. V. Fokin, Chem. Soc. Rev., 2010, 39, 1302–1315.
18. F. Gonzaga, G. Yu and M. A. Brook, Macromolecules, 2009, 42, 9220–9224.
19. A. B. Lowe, C. E. Hoyle and C. N. Bowman, J. Mater. Chem., 2010, 20, 4745–4750.
20. M. Lo Conte, S. Pacifico, A. Chambery, A. Marra and A. Dondoni, J. Org. Chem., 2010, 75, 4644–4647.
21. R. Pötzsch, B. C. Stahl, H. Komber, C. J. Hawker and B. I. Voit, Polym. Chem., 2014, 5, 2911–2921.
22. M. M. Stevens and J. H. George, Science, 2005, 310, 1135–1138.
23. T. Horlacher and P. H. Seeberger, Chem. Soc. Rev., 2008, 37, 1414–1422.
24. N. S. Bhairamadgi, S. Gangarapu, M. A. Caipa Campos, J. M. J. Paulusse, C. J. M. van Rijn and H. Zuilhof, Langmuir, 2013, 29, 4535–4542.
25. J. Zhao, M. Chen, Y. An, J. Liu and F. Yan, Appl. Surf. Sci., 2008, 255, 2295–2302.
26. A. Bertin and H. Schlaad, Chem. Mater., 2009, 21, 5698–5700.
27. J.-J. Chen, K. N. Struk and A. B. Brennan, Langmuir, 2011, 27, 13754–13761.
28. S. Park, M.-r. Lee, S.-J. Pyo and I. Shin, J. Am. Chem. Soc., 2004, 126, 4812–4819.
29. J. Cras, C. Rowe-Taitt, D. Nivens and F. Ligler, Biosens. Bioelectron., 1999, 14, 683–688.
30. M. Fujiki, J. R. Koe, K. Terao, T. Sato, A. Teramoto and J. Watanabe, Polym. J., 2003, 35, 297–344.
31. Y. Chen, H. Cui, L. Li, Z. Tian and Z. Tang, Polym. Chem., 2014, 5, 4441–4445.
32. Z. Zhaozhu, Z. Yunlong, L. Xinyu, L. Shaoqin and T. Zhiyong, Biosens. Bioelectron., 2011, 26, 3081–3085.
33. Y. Yue, Y. Liang, H. Wang, L. Feng, S. Feng and H. Lu, Photochem. Photobiol., 2013, 89, 5–13.
34. D. Ratchford, K. Dziatkowski, T. Hartsfield, X. Li, Y. Gao and Z. Tang, J. Appl. Phys., 2011, 109, 103509.
35. D. H. Flinn, D. A. Guzonas and R. H. Yoon, Colloids Surf., A, 1994, 87, 163–176.

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