Organic-sulfur-zinc hybrid nanoparticle for optical applications synthesized via polycondensation of trithiol and Zn(OAc)2
© Ochiai and Konta; licensee Springer. 2013
Received: 17 July 2013
Accepted: 28 August 2013
Published: 2 September 2013
Organic-sulfur-zinc hybrid materials were prepared via polycondensation of Zn(OAc)2 and trithiols bearing various alkyl groups. A soluble nanoparticle could be obtained by the polycondensation using a trithiol bearing octadecyl moieties. The good dispersing ability as nano-scaled particles was confirmed by dynamic light scattering and atomic force microscopy analyses. This hybrid nanoparticle was miscible with poly(methyl methacrylate) and served as a refractive additive to increase refractive indexes. The calculated refractive index value for the nanoparticle was 1.58.
Excellent high refractive index materials are demanded by recent rapid development of mobile devices, solar cells, and luminescent devices. Various materials have been developed by hybridization of organic and inorganic materials, complementing the properties of each component. For example, organic materials provide flexibility and easy processing, and inorganic materials provide optical and mechanical properties. Typical preparation methods for organic–inorganic hybrids are incorporation of metal oxide into polymer matrices via sol–gel methods [1–3] and mixing of polymers and nanoparticles of metal oxides [3–8] or sulfides [9, 10]. However, both of the methods contain some disadvantages. Sol–gel methods realized facile and green procedures but are typically time consuming and accompanied by shrinkage during drying processes. Mixing of nano-scaled metal compounds is advantageous by the fast process, but specific coating and precise tuning of the reaction conditions are required for the preparation of nano-scaled metal compounds.
Another approach to conquer these problems is the use of organometallic materials . Ene-thiol polyaddition of dithiols with tetravinyl-silane, germane, and tin gave polymers with high refractive indexes ranging from 1.590 to 1.703 and excellent physical properties.
Encouraged by this work, we designed new organic–inorganic hybrid materials based on sulfur as a bridge for organic and inorganic components, namely organic-sulfur-inorganic hybrid materials. The important character of sulfur for this approach is the ability to form stable linkages with both organic and inorganic structures. Another beneficial character of sulfur is its high atom refraction, by which sulfur has served as an important component for optical materials [12–17]. This bridging ability has been mostly applied for the functionalization of inorganic surfaces with organic structures such as the modification of gold surface [18–20] and quantum dots [21, 22] with thiols. Although many stable metal thiolates have been reported [23–27], these compounds have not been applied as optical materials as far as we know. As the metal for this approach, zinc was selected because of its high refractivity and low toxicity. We employed trithiols (TSHs) obtained from a trifunctional dithiocarbonate and amines, whose structures can be easily tuned by the substituent on the amines [28, 29]. Polycondensation of TSHs with Zn(OAc)2 yielded organic-sulfur-inorganic hybrid nanoparticles serving as refractive ingredients for poly(methyl methacrylate) (PMMA).
1,4-Dioxane was dried over sodium and distilled under a nitrogen atmosphere prior to use. A trifunctional cyclic dithiocarbonate, 1,3,5-tris(2-thioxo-1,3-oxathiolan-5-yl)methyl)-1,3,5-triazinane-2,4,6-trione (TDT), was prepared as reported . Other reagents were used as received.
1H and 13C nuclear magnetic resonance (NMR) spectra were measured on a JEOL ECX-400 instrument (Tokyo, Japan) using tetramethylsilane as an internal standard (400 MHz for 1H and 100 MHz for 13C). Fourier transform infrared spectra were measured on a Horiba FT-210 instrument (Kyoto, Japan). Size exclusion chromatography measurements were performed on a Tosoh HLC-8220 GPC (Tokyo, Japan) equipped with Tosoh TSK-gel superAW5000, superAW4000, and superAW3000 tandem columns using tetrahydrofuran (THF) with a flow rate of 1.0 mL/min as an eluent at 40°C. Quantitative elemental analysis was performed with a system consisting of a JEOL JSM6510A scanning electron microscope equipped with a JEOL JED2300 energy dispersive X-ray (EDX) spectrometer operated at an acceleration voltage of 20 kV. The samples were compressed as flat tablets, and the atom ratios were calculated as averages of data obtained from ten spots. Refractive indexes (nDs) were measured with an Atago DR-A1 digital Abbe refractometer (Tokyo, Japan). Dynamic light scattering (DLS) measurements were performed using a Malvern Zetasizer nano-ZS instrument (Worcestershire, UK) equipped with a 4-mW He-Ne laser (633 nm) and 12-mm square glass cuvettes at 25°C. The samples were dissolved in anhydrous THF (1.3 g/L). Atomic force microscopic (AFM) measurements were performed on an Agilent 5500 atomic force microscope (Santa Clara, CA, USA) operated in tapping mode. The samples were spin cast on freshly cleaved mica substrates from anhydrous THF solutions.
Synthesis of TSHs (typical procedure)
TSHs were prepared according to the previous report . The synthetic procedure for a trithiol bearing octadecyl chains (OTSH) is as follows. Octadecylamine (1.62 g, 6.02 mmol), TDT (1.05 g, 2.00 mmol), and THF (5.0 mL) were added to a round-bottom flask, and the mixture was stirred at room temperature for 24 h. Volatile substances were evaporated off, and the residue was purified using SiO2 gel column chromatography, eluted with EtOAc/hexane (v/v = 1/10). OTSH was obtained as a white solid (2.03 g, 1.52 mmol, 76.0%).
1H-NMR (CDCl3/CF3CO2H = 5:1, rt, % δ in ppm): 0.88 (9H, t, J = 7.0 Hz, -CH 3 ), 1.27 to 1.31 (93H, -(CH 2 )15CH3 and -SH), 1.56 to 1.65 (6H, m, -CH2CH 2 (CH2)15-), 2.92 (6H, m, -CHCH 2 SH), 3.30 to 3.41 (6H, m, -NHCH 2 CH2-), 4.11 to 4.46 (6H, m, -NCH 2 CH-), 5.75 (3H, br, -CH2CH O-), 8.06 (3H, br, -(C=S)NH CH2-). 13C-NMR (CDCl3/CF3CO2H = 5:1, δ in ppm): 13.76 (−CH2C H3), 22.64 (−(CH2)15C H2CH3), 25.90 to 27.26 (−C H2SH), 28.76 to 31.93 (−CH2(C H2)15CH2-), 44.35 (−NHC H2(CH2)15-), 45.91 (−NC H2CH-), 77.44 (−CH2C HO-), 149.29 (C=O), 188.55 (C=S). IR (KBr, cm−1): 3,320 (NH), 2,575 (SH), 1,691 (C=O), 1,165 (C=S), 1,049 (C=S).
BTSH. TSH with benzyl moieties was prepared using benzylamine (643 mg, 6.01 mmol) and TDT (1.05 g, 1.99 mmol) in a similar manner with OTSH (1.43 g, 1.68 mmol, 84.4%).
1H-NMR (CDCl3/CF3CO2H = 5:1, rt, σ in ppm): 1.32 (3H, br, -SH), 2.82 (6H, br, -CH 2 SH), 4.06 to 4.47 (6H, br, -NHCH 2 Ar), 4.47 to 4.57 (6H, br, -CH 2 CH(CH2SH)O-), 5.73 (3H, br, -CH2CH(CH2SH)O-), 7.25 to 7.36 (15H, m, Ar), 8.35 (3H, br, -NH-). 13C-NMR (CDCl3/CF3CO2H = 5:1, rt, σ in ppm): 25.98 (−C H2SH), 45.37 (−C H2CH(CH2SH)O-), 47.58 (−NHC H2Ar), 79.52 (−CH2C H(CH2SH)O-), 127.49 to 135.80 (−CH2Ar), 149.48 (C=O), 187.99 (C=S). IR (KBr, cm−1): 3,348 (NH), 2,573 (SH), 1,695 (C=O), 1,165 (C=S).
HTSH. TSH with hexyl moieties was prepared using n-hexylamine (598 mg, 5.90 mmol) and TDT (1.05 g, 1.99 mmol) in a similar manner with OTSH (1.40 g, 1.69 mmol, 84.8%).
1H-NMR (CDCl3/CF3CO2H = 5:1, rt, σ in ppm): 0.90 (9H, t, J = 16 Hz, -CH 3 ), 1.32 (18H, m, -(CH 2 )3CH3), 1.59 to 1.66 (9H, -SH and -CH 2 (CH2)3-), 2.94 (6H, br, -CH 2 SH), 3.30 to 3.41 (6H, br, -NHCH 2 CH2-), 4.11 to 4.47 (6H, br, -CH 2 CH(CH2SH)O-), 5.75 (3H, br, -CH2CH(CH2SH)O-), 8.06 (3H, br, -NH-). 13C-NMR (CDCl3/CF3CO2H = 5:1, rt, σ in ppm): 13.65 (−C H3), 22.42 (−C H2CH3), 25.91 (−C H2SH), 26.41 (−C H2CH2CH2CH3), 28.38 (−C H2CH2CH3), 31.28 (−NHCH2C H2-), 44.04 (−NHC H2-), 45.31 (−C H2CH(CH2SH)O-), 79.05 (−CH2C H(CH2SH)O-), 149.41 (C=O), 187.41 (C=S). IR (KBr, cm−1): 3,334 (NH), 2,573 (SH), 1,696 (C=O), 1,167 (C=S).
IATSH. TSH with isoamyl moieties was prepared using isoamylamine (526 mg, 6.03 mmol) and TDT (1.05 g, 1.99 mmol) in a similar manner with OTSH (644 mg, 817 μmol, 40.9%).
1H-NMR (CDCl3/CF3CO2H = 5:1, rt, σ in ppm): 0.91 to 0.95 (18H, d, J = 15 Hz, -CH(CH 3 )2), 1.43 to 1.48 (9H, -SH and -CH 2 CH(CH3)2), 1.60 to 1.63 (3H, m, -CH2CH(CH3)2), 2.91 (6H, br, -CH 2 SH), 3.19 to 3.43 (6H, br, -NHCH 2 CH2-), 4.17 to 4.47 (6H, br, -CH 2 CH(CH2SH)O-), 5.75 (3H, br, -CH2CH(CH2SH)O-), 8.03 (3H, br, -NH-). 13C-NMR (CDCl3/CF3CO2H = 5:1, rt, σ in ppm): 21.90 (−CH(C H3)2), 25.71 (−C H2SH), 26.70 (−C H(CH3)2), 37.06 (−C H2CH(CH3)2), 42.48 (−NHC H2CH2-), 45.42 (−C H2CH(CH2SH)O-), 79.10 (−CH2C H(CH2SH)O-), 149.50 (C=O), 187.50 (C=S). IR (KBr, cm−1): 3,323 (NH), 2,575 (SH) 1,696 (C=O), 1,176 (C=S).
EHTSH. TSH with 2-ethylhexyl moieties was prepared using -ethylhexylamine (773 mg, 5.99 mmol) and TDT (1.05 g, 1.99 mmol) in a similar manner with OTSH (1.43 g, 1.56 mmol, 78.2%).
1H-NMR (CDCl3/CF3CO2H = 5:1, rt, σ in ppm): 0.89 to 0.93 (18H, t, J = 18 Hz, -CH 3 ), 1.30 to 1.38 (24H, m, -CH(CH 2 CH3) (CH 2 )3CH3), 1.57 (3H, br, -SH), 1.63 to 1.67 (3H, t, J = 19 Hz, -CH(CH2CH3) (CH2)3CH3), 2.94 (6H, br, -CH 2 SH), 3.17 to 3.54 (6H, br, -NHCH 2 -), 4.18 to 4.48 (6H, br, -CH 2 CH(CH2SH)O-), 5.77 (3H, br, -CH2CH(CH2SH)O-), 8.03 (3H, br, -NH-). 13C-NMR (CDCl3/CF3CO2H = 5:1, rt, σ in ppm): 10.37 (−CH(CH2C H3)(CH2)3CH3), 13.66 (−CH(CH2CH3) (CH2)3C H3), 22.86 (−CH(CH2CH3) (CH2CH2C H2CH3)), 23.98 (−CH(C H2CH3) (CH2CH2CH2CH3)), 26.08 (−C H2SH), 28.67 (−CH(CH2CH3) (CH2C H2CH2CH3)), 30.79 (−CH(CH2CH3) (C H2CH2CH2CH3)), 38.88 (−C H(CH2CH3) (CH2CH2CH2CH3)), 45.70 (−C H2CH(CH2SH)O–), 47.17 (−NHC H2-), 79.22 (−CH2C H(CH2SH)O-), 149.37 (C=O), 187.66 (C=S). IR (KBr, cm−1): 3,326 (NH), 2,573 (SH) 1,698 (C=O), 1,172 (C=S).
Polycondensation of TSHs and Zn(OAc)2 (typical procedure)
To a flask containing OTSH (268 mg, 201 μmol), a 1,4-dioxane solution (5.0 mL) of Zn(OAc)2 (55 mg, 300 μmol) was added under a nitrogen atmosphere. The mixture was stirred at 60°C for 24 h. The mixture was poured into an excess amount of methanol, and the precipitate was collected by filtration and drying under reduced pressure after washing with cold diethyl ether (131 mg, 91.7 μmol/unit, 45.3%).
1H-NMR (CDCl3/CF3CO2H = 5:1, δ in ppm): 0.88 (9H, t, J = 7.0 Hz, -CH 3 ), 1.27 (90H, -(CH 2 )15CH3), 1.61 to 1.74 (6H, -CH2CH 2 (CH2)15-), 2.87 (6H, -CHCH 2 SH), 3.17 to 3.46 (6H, -NHCH 2 CH2-), 4.26 to 4.59 (6H, -NCH 2 CH-), 5.59 (3H, -CH2CH O-), 6.62 (3H, -(C=S)NH CH2-). 13C-NMR (CDCl3/CF3CO2H = 5:1, δ in ppm): 13.76 (−CH2C H3), 22.64 (−(CH2)15C H2CH3), 26.64 (−CHC H2S-), 29.18 to 31.98 (−CH2(C H2)15CH2-), 45.24 (−NC H2CH-), 49.75 (−NHC H2(CH2)15-), 76.54 to 77.17 (−CH2C HO-), 149.15 (C=O), 183.28 (C=S). IR (KBr, cm−1): 3,344 (NH), 1,697 (C=O), 1,160 (C=S).
BTZnS: yield = 64%, IR (KBr, cm−1): 3,393 (NH), 1,696 (C=O), 1,160 (C=S).
HTZnS: yield = 62%, IR (KBr, cm−1): 3,327 (NH), 1,696 (C=O), 1,163 (C=S).
IAZnS: yield = 68%, IR (KBr, cm−1): 3,317 (NH), 1,698 (C=O), 1,171 (C=S).
EHTZnS: yield = 62%, IR (KBr, cm−1): 3,374 (NH), 1,698 (C=O), 1,168 (C=S).
Results and discussion
Synthesis of TSH monomers
Polycondensation of TSHs and Zn(OAc)2
Polycondensation of TSH and Zn(OAc) 2
Polycondensation of OTSH and Zn(OAc) 2 under various conditions
We considered the reason for the poor solubility of the products obtained from other TSHs. The IR spectra of the soluble and insoluble products were identical as aforementioned, suggesting that the side reactions are ignorable. This polymerization is a 2 + 3-type polycondensation and potentially yields cross-linked insoluble polymers. Intermolecular coupling reactions should be adequately suppressed to obtain soluble products. We presume that longer alkyl groups are advantageous not only to increase the solubility but also to suppress intermolecular coupling reactions. As a result, OTSH, having the longest alkyl group among examined, could give soluble polymers, whereas other TSHs could not due to the shorter alkyl chains insufficient to overcome these factors. The Zn/S values of the insoluble products were higher than the theoretical values. The higher Zn content implies the self-condensation of Zn(OAc)2 to produce oligomeric ZnO , which is also responsible for the insolubility. All the reaction mixtures after the reactions were homogeneous, and we presume that the self-condensation may have occurred during the purification processes.
Refractive property of OTZnS
Refractive indexes of OTZnS/PMMA film, PMMA film, and OTSH, and calculated refractive index of OTAnS
OTZnS/PMMA (w/ w)
Calculated for OTZnS
n D a
A soluble organic-sulfur-zinc hybrid nanoparticle could be obtained by the polycondensation of OTSH and Zn(OAc)2. The resulting hybrid nanoparticle was miscible with PMMA and served as a refractive additive to increase the refractive indexes. The calculated nD value for the polymer was 1.58. This value is relatively high as a compound bearing three octadecyl chains, and we believe that further optimization of the polymerization conditions will enable the synthesis of more refractive organic-sulfur-zinc materials with higher sulfur and/or zinc contents.
BO received his Ph.D. degree in Polymer Chemistry in Tokyo Institute of Technology, Japan, in 2001. He is a professor in Yamagata University. His research activities include the development of organic-sulfur-inorganic hybrid materials, ion-conducting materials, and gene-delivery materials. HK was a Masters degree student at Yamagata University.
We thank Adaptable and Seamless Technology Transfer Program for the financial support through Target-Driven R&D (A-STEP) Feasibility Study Program by Japan Science and Technology Agency (JST) (AS221Z01415D) and JSPS KAKENHI grant number 25410208.
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