Luminescent Organic–Inorganic Hybrids of Functionalized Mesoporous Silica SBA-15 by Thio-Salicylidene Schiff Base
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Novel organic–inorganic mesoporous luminescent hybrid material N, N′-bis(salicylidene)-thiocarbohydrazide (BSTC-SBA-15) has been obtained by co-condensation of tetraethyl orthosilicate and the organosilane in the presence of Pluronic P123 surfactant as a template. N,N′-bis(salicylidene)-thiocarbohydrazide (BSTC) grafted to the coupling agent 3-(triethoxysilyl)-propyl isocyanate (TESPIC) was used as the precursor for the preparation of mesoporous materials. In addition, for comparison, SBA-15 doped with organic ligand BSTC was also synthesized, denoted as BSTC/SBA-15. This organic–inorganic hybrid material was well-characterized by X-ray diffraction, Fourier transform infrared spectroscopy, transmission electron microscopy (HRTEM), and photoluminescence spectra, which reveals that they all have high surface area, uniformity in the mesostructure. The resulting materials (BSTC-SBA-15 and BSTC/SBA-15) exhibit regular uniform microstructures, and no phase separation happened for the organic and the inorganic compounds was covalently linked through Si–O bonds via a self-assemble process. Furthermore, the two materials have different luminescence range: BSTC/SBA-15 presents the strong dominant green luminescence, while BSTC-functionalized material BSTC-SBA-15 shows the dominant blue emission.
KeywordsOrganic–inorganic hybrid material Functionalized mesoporous silica Photoluminescence Schiff-base derivative
Organic–inorganic hybrid materials have been subjected to more intense development in the field of materials science since they not only combine the respective beneficial characters of organic and inorganic components but also often exhibit exceptional properties that exceed what would be excepted for a simple mixture of the components [1, 2, 3, 4]. The hybrid materials enable both inorganic and organic dopants to be incorporated with relatively high thermal stability [5, 6, 7]. According to the chemical nature or different synergy between components, hybrids can be categorized into two main classes. Class I concerns all systems where no covalent bond is present between organic and inorganic parts but only weak interactions (such as hydrogen bonding, van der waals forces or electrostatic forces) exist between organic and inorganic moieties [8, 9]; the corresponding conventional doping methods seems hard to prohibit the problem of quenching effect on luminescent centers due to the high vibration energy of the surrounding hydroxyl groups. Class II materials belong to the molecular-based composite materials in which the organic and inorganic phases are linked together through strong chemical bonds (covalent, ion-covalent, or coordination bonds). Through the combination of chemical bonding within the different components in a single material, this kind of materials can realize the possibility of tailoring the complementary properties of both components to obtain novel multifunctional materials with attractive performances such as mechanical, thermal, and other physical and chemical properties [10, 11, 12]. As a result, a few studies in terms of the covalently bonded hybrids  have appeared and the as-derived molecular-based materials exhibit monophasic appearance [14, 15, 16, 17, 18], besides, the reinforcement of thermal and mechanical resistances has been clearly established. Our research group is concentrated on covalently grafting the ligands to the inorganic networks in which organic groups are bonded with a siloxane matrix through Si–O linkage using different modified routes, including the modification of active amino group, hydroxyl groups, and carboxyl groups with coupling agent, etc [19, 20, 21, 22].
Recently, organic-functionalized mesoporous siliceous materials have generated a great deal of interest in the fields of catalysis, adsorption [23, 24, 25, 26, 27, 28], and sensors  due to their high surface areas and large ordered pores ranging from 2 to 50 nm with narrow size distributions. Mesoporous materials are a special type of nonmaterials with ordered arrays of uniform nanochannels. Among them, SBA-15 is by far the largest pore size mesochannels, with thick walls, adjustable pore size from 3 to 30 nm, and high hydrothermal and thermal stability [30, 31, 32, 33]. These properties together with the thermal and mechanical stabilities make it as an ideal host for incorporation of active molecules, and some work has already been devoted on this field. The design and synthesis of innovative functionalized hybrid mesoporous materials are of considerable interest and open up an extraordinary field of investigation [34, 35, 36]. Many research efforts, which have focused on preparing the organic–inorganic hybrids through functionalization of the exterior and/or interior surfaces, prompted the utilization of mesoporous SBA-15 in many areas. However, the synthesis and luminescence properties of SBA-15 mesoporous materials covalently bonded with organic schiff-base ligands have hardly been explored to date . Schiff-base compounds were reported to possess cytotoxic, anticonvulsant, antiproliferative, and anticancer activities; some Schiff-base ligands have potential applications in organic synthesis, catalysis, medicinal chemistry, and biotechnology.
Here, we report on the synthesis and characterization of N, N′-bis(salicylidene) –thiocarbohydrazide (BSTC) functionalized SBA-15 mesoporous hybrid material (denoted as BSTC-SBA-15), in which BSTC was covalently bonded to the framework of SBA-15 by co-condensation of the modified BSTC (denoted as BSTC-Si) and the tetraethoxysilane (TEOS) using the Pluronic P123 surfactant as template. N,N’-bis(salicylidene) –thiocarbohydrazide (BSTC) was first synthesized by the reaction of salicylaldehyde with thiocarbohydrazide. Because the organic compounds are covalently bonded to the silica network through Si–C bonds, the organic groups become an integral part of the materials and thus homogeneous complicated huge molecule systems with regular uniform microstructures were obtained. In addition, for comparison, SBA-15 doped with BSTC was also synthesized, denoted as BSTC/SBA-15. Full characterization and detail studies of luminescence properties of all these synthesized materials were investigated in relation to guest–host interactions between the organic complex and the silica matrix.
Pluronic P123 (EO20PO70EO20, Aldrich), Tetraethoxysilane (TEOS, Aldrich) was distilled and stored under nitrogen atmosphere and 3-(triethoxysiyl)-propyl isocyanate (TEPIC, Lancaster) was used. All the other reagents are analytically pure.
Synthesis of BSTC-Functionalized SBA-15 Mesoporous Material by Covalent Bond (Denoted as BSTC-SBA-15)
Synthesis of Thiocarbohydrazide
Twenty microliter of 85% hydrazine hydrate was dissolved in 60 mL of water, and then 6 mL CS2 was added dropwise, the reaction mixture was kept under room temperature for 1 h and then heated to 90°C for an additional 8 h. After cooling, the precipitate was filtered off, washed with water, and dried. The crude product was purified by recrystallization from water and finally obtained as white needles, yield 6.02 g (72%). m.p.172–173°C. 1H NMR (DMSO): δ 4.48 (d, 4H, NH2), 8.69 (t, 2H, NH).
Synthesis of SBA-15
SBA-15 host structure was synthesized according to the reported procedure using Pluronic P123 as a structure-directing agent and tetraethyl orthosilicate (TEOS) as a silica source under acidic conditions . Typically, 1.0 g of P123 was dissolved in 7.5 g of H2O and 30 g of dilute HCl solution (2.0 M) with stirring at 35°C. Then, 2.08 g of TEOS was added dropwise to the solution with stirring for 24 h and transferred into a Teflon bottle sealed in an autoclave, which was heated at 100°C for 24 h. The solid product was filtered, washed thoroughly with deionized water, and dried at 60°C. The as-synthesized material was calcined from room temperature to 550°C at a heating rate of 2–6°C/min for 5 h to remove the templates and obtained fine mesoporous SBA-15.
Synthesis of BSTC
N, N′-bis(salicylidene)-thiocarbohydrazide (BSTC) was prepared as follows: 0.493 g (4.4 mmol) salicylaldehyde was dissolved in 20 mL of absolute ethanol, and then 2 mmol thiocarbohydrazide was dissolved in 10 mL of absolute ethanol was added dropwise. The resulting mixture was heated under reflux for 3 h. After cooling, the precipitate was filtered off. The crude product was purified by recrystallization from absolute ethanol and finally obtained as white crystals. 1H NMR (CDC13, 500 MHz): δ 2.60 (s, 2H, NH); 6.89, 6.96, 7.32, 7.70 (8H, Ar); 8.52 (s, 2H, Ar CH); 11.75 (s, 2H, OH). 13C NMR (CDC13, 100 MHz): δ 157.2, 132.2, 127.5, 121.4, 118.8, 116.2 (Ar); 156.4 (Ar CH); 184.2 (C=S).
Synthesis of BSTC-Si
A typical procedure for the preparation of the modified precursor BSTC-Si was as follows: N, N′-bis (salicylidene)-thiocarbohydrazide (BSTC) (1 mmol) was first dissolved in 20 mL of pyridine with stirring, and 3-(triethoxysilyl)-propyl isocyanate (TESPIC) (2.2 mmol) dissolved in 10 ml of pyridine was added dropwise with stirring; the mixture was warmed at 80°C for approximately 12 h in a covered flask at the nitrogen atmosphere. After isolation and purification, a yellow oil sample BSTC-Si was obtained. 1H NMR (CDC13, 500 MHz): δ 0.58 (t, 4H, CH2Si); 1.20 (t, 18H, CH3CH2); 1.65(m, 4H,NHCH2CH2CH2Si);1.80 (m, 2H, NCH2CH2CH2N); 3.18(m, 4H,NH CH2); 3.65(q, 12H, SiOCH2); 3.70 (t, 4H, NCH2CH2); 6.75, 6.90, 7.30, 7.42 (8H, Ar); 8.01 (t, 2H,NH); 8.50 (s, 2H,Ar CH). 13C NMR (CDC13, 100 MHz): δ 13.7 (CH2Si); 18.4(CH3CH2O); 25.5 (NH CH2CH2CH2Si); 31.6 (NCH2CH2CH2N); 43.2 (NHCH2); 56.8 (NCH2CH2); 58.4(CH3CH2O); 152.1, 131.2, 131.0, 130.3, 124.6, 119.7, (Ar); 155.4 (C=O), 157.5(Ar CH).
Synthesis of SBA-15 Doped With N, N′-bis (salicylidene)-thiocarbohydrazide (Denoted as BSTC/SBA-15)
The encapsulation method of the mesoporous silica SBA-15 doped with N, N′-bis (salicylidene)-thiocarbohydrazide (BSTC) in ethanol solution together with SBA-15. The mixture was stirred at room temperature for 12 h, and the resulting samples were filtered, washed with ethanol, dried at 60°C under vacuum overnight. The mesoporous materials were denoted as BSTC/SBA-15 (Fig. 1).
IR spectra were measured within the 4,000–400 cm−1 region on an infrared spectrophotometer using the KBr pellet technique. 1H NMR spectra were recorded in CDCl3 on a Bruker AVANCE-500 spectrometer with tetramethylsilane (TMS) as internal reference. The ultraviolet absorption spectra were taken with an Agilent 8453 spectrophotometer. X-ray powder diffraction patterns were recorded on a Rigaku D/max-rB diffractometer equipped with a Cu anode in a 2θ range from 0.6° to 6°. Nitrogen adsorption/desorption isotherms were measured at the liquid nitrogen temperature, using a Nova 1000 analyzer. Surface areas were calculated by the Brunauer-Emmett-Teller (BET) method, and pore size distributions were evaluated from the desorption branches of the nitrogen isotherms using the Barrett-Joyner-Halenda (BJH) model. The fluorescence excitation and emission spectra were obtained on a Perkin-Elmer LS-55 spectrophotometer. Luminescence lifetime measurements were taken on an Edinburgh FLS920 phosphorimeter using a 450-w xenon lamp as excitation source. Transmission electron microscope (TEM) experiments were conducted on a JEOL2011 microscope operated at 200 kV or on a JEM-4000EX microscope operated at 400 kV.
Results and Discussion
As detailed in the experimental section, 1H as well as 13C spectra relative to BSTC, the silylated precursors, BSTC-Si, are in full agreement with the proposed structure. The 1H NMR chemical shift relative to -OH bond can be observed in Schiff-base compound BSTC and is disappeared in the corresponding silylated precursor BSTC-Si, which indicates that the accomplishment of the hydrogen transfer reaction between OH and the TESPIC. Integration of the 1H NMR and 13C signals corresponding to ethoxy groups shows that no hydrolysis of the precursors occurred during the grafting reaction.
The maximal ultraviolet absorption wavelengths of BSTC
Textural data of SBA-15, BSTC/SBA-15 and BSTC-SBA-15
On the basis of the earlier discussion, the quantum efficiencies of the mesoporous hybrid materials can be determined. Quantum yields of BSTC and BSTC/SBA-15 (Ex at 396 nm, 300–450 nm, emission quantum yield are 0.64% and 0.57%, respectively) and the material BSTC-SBA-15 (Ex at 354 nm, 500–750 nm, emission quantum yield is 0.53%).
In summary, the organic Schiff-base has been successfully covalently immobilized in the ordered SBA-15 mesoporous material by the modification of N, N′-bis(salicylidene)–thiocarb-ohydrazide (BSTC) with 3-(triethoxysilyl)-propyl isocyanate (TESPIC) using a co-condensation method. New inorganic–organic hybrid mesoporous SBA-15-type material has been synthesized by co-condensation of TEOS and chelate ligand in the presence of P123 template. The synthesis of BSTC-SBA-15 provides a convenient approach of tailoring the surface properties of mesoporous silicates by organic functionalization, and the resulting materials all retain the ordered mesoporous structures. Further investigation into the luminescence properties of the mesoporous hybrids shows that all of the hybrids exhibit an excellent ability to absorb energy in ultraviolet–visible extent and have strong fluorescence emission intensities in blue or green range. This method allows the introduction of organic fluorophore with the preservation of uniform mesoscale channels, high specific, surface areas, and large pore volumes. As the synthesis process can be easily applied to other organic ligands and to different alkoxysilanes, we may expect to obtain stable and luminescent efficient hybrid materials in optical or catalysis areas.
This work was supported by the National Natural Science Foundation of China (20971100) and Program for New Century Excellent Talents in University (NCET-08-0398).
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