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Synthesis of dithienogermole-containing oligoand polysilsesquioxanes as luminescent materials† Joji Ohshita,*a Masashi Nakamura,a Kazuki Yamamoto,a Seiji Wataseb and Kimihiro Matsukawab Dithienogermole (DTG)-containing oligo- and polysilsesquioxanes were prepared by hydrolysis/condensation of DTGs bearing one (DTG1) or two trialkoxysilyl group(s) (DTG2). The reaction of DTG1 gave a cage-type octasilsesquioxane with eight DTG groups at the edges (DTG1-POSS) as a viscous oil, whereas the reaction of DTG2 yielded a network polymer (DTG2-PSQ) as a self-standing film. DTG1-POSS showed a photoluminescence (PL) quantum yield (Φ) of 56% in THF. This value was as high as that of DTG1 (Φ = 58%), in spite of the accumulation of DTG units in the molecule, as characteristics of the POSS structure. The PL of DTG1-POSS in THF was suppressed by contact with nitrobenzene, showing the potential of DTG1-POSS for sensing nitroaromatic explosives. Polymer DTG2-PSQ exhibited a relatively low Φ of 2% as a film, but Φ was improved to 38% by copolymerization with trimethoxymethylsilane. DTG2 was also copolymerized with a trimethoxysilyl-substituted carbazole derivative (CzS) to provide
Received 23rd February 2015, Accepted 17th March 2015
polysilsesquioxanes with DTG and carbazole units, which showed efficient photo-energy transfer from
DOI: 10.1039/c5dt00777a
carbazole to DTG in the films. Similar copolymerization of DTG2 with CzS in the presence of poly(9-vinylcarbazole) provided a composite material with hole-transporting electroluminescence properties, appli-
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cable in multi-layered organic light emitting diodes.
Introduction Oligo- and polysilsesquioxanes have received a great deal of attention as organic–inorganic hybrid materials.1,2 They are readily accessible, typically by the acid- or base-catalyzed hydrolysis/condensation of trialkoxy- or trihalosilanes with an organic group, and the resulting three-dimensional network structures composed of Si–O bonds usually provide good filmforming properties and thermal stability to the materials. The properties of oligo- and polysilsesquioxanes are significantly affected by the organic group, and it has been demonstrated that the introduction of π-conjugated units confers optoelectronic functionalities to silsesquioxanes, such as carriertransporting and emissive properties, rendering the silsesquioxanes applicable in organic optoelectronic devices.3 On the other hand, group 14 metalloles condensed with biaryls,4 such as biphenyl,5 bithiophene,6 and bipyridyl,7 have a Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University, Higashi-Hiroshima 739-8527, Japan. E-mail: [email protected] b Osaka Municipal Technical Research Institute, 1-6-50 Morinomiya, Joto-ku, Osaka 536-8553, Japan † Electronic supplementary information (ESI) available: 1H and 13C NMR, and FD-mass spectra of DTG1-POSS, IR spectra and TG traces of DTG2-MeS and CzS, 1 H NMR spectra, GPC traces, and UV absorption spectra of DTG2-CzS. See DOI: 10.1039/c5dt00777a
8214 | Dalton Trans., 2015, 44, 8214–8220
been extensively studied as a new class of chromophores with interesting electronic states. The highly planar tricyclic structures of these compounds enhance the conjugation. Furthermore, in-phase interactions between the metal σ* and biaryl π* orbitals lower the LUMO energy levels, leading to even smaller HOMO–LUMO gaps. Of these, bithiophene-condensed silole and germole, namely, dithienosilole6,8,9 and dithienogermole10 (DTS and DTG in Chart 1), are of particular interest as building units of functional π-conjugated materials, and their applications in organic electronic materials, such as donor polymers for organic solar cells8 and carrier transports for organic light emitting diodes (OLEDs),9 have been explored. It has also been demonstrated that these compounds often exhibit high photoluminescence (PL) properties both in
Chart 1 General structures of DTS and DTG with HOMO and LUMO energy levels in eV in parentheses, derived from DFT calculations for those with methyls on the bridge at B3LYP/6-31G(d,p) (aref. 6, bpresent study, cref. 15).
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solution as well as in the solid state,11,12 the primary reason being that the rigid tricyclic system minimizes the molecular vibration, thereby suppressing the non-radiative decay. In addition, the two additional substituents on the metal sterically cover the metallole chromophore to avoid concentration quenching even in the solid state. On the basis of the consideration mentioned above, we designed and prepared oligo- and polysilsesquioxanes containing DTG units for the first time, in hopes of developing new emissive materials.13 DTG is more stable than DTS towards hydrolysis, and is thus more suitable for the process. Copolymerization with a carbazole-containing monomer was also performed to furnish copolymers that showed efficient photoenergy transfer from carbazole to DTG in the films. Preliminary experiments revealing the potential applications of the resulting polysilsesquioxane containing DTG and carbazole units as electroluminescence (EL) materials for multi-layered OLEDs were also carried out. Although dibenzosilole (DBS in Chart 1) has been studied as EL materials,14 no oligo- and polysilsesquioxanes containing DBS units have been reported. Only silica attached by DBS has been studied for the separation and detection of nitroaromatic explosives.15 Because DTS and DTG have the lower and higher lying LUMO and HOMO, respectively, than those of DBS as predicted by DFT calculations,6,16 better carrier transporting properties are expected for DTS and DTG, thus rendering them more suitable as EL materials.
Experimental General All reactions were carried out in dry argon. THF, toluene, and diethyl ether, which were used as the reaction solvents, were distilled from CaH2 and stored over activated molecular sieves until use. Starting compounds 1, 2, and CzS were prepared as reported in the literature.17,18 NMR spectra were recorded on Varian 400-MR and System500 spectrometers. EI-mass spectra were recorded on a Shimadzu QP-2020A spectrometer, and ESI- and FD-mass spectra were obtained by using Thermo Fisher Scientific LTQ Orbitrap XL and JEOL JMS-T100 GCV 4G spectrometers, respectively, at N-BARD, Hiroshima University. UV–vis absorption was measured and PL spectra were recorded on Hitachi U-3210 and HORIBA FluoroMax-4 spectrophotometers, respectively. The emission quantum yields excited at 350 nm were determined using a JASCO F-6300-H spectrometer connected to a JASCO ILF-533 integrating sphere unit (ϕ = 100 mm). Thermogravimetric analysis (TGA) was carried out on a SII TG/DTA-6200 analyzer under a gentle nitrogen flow (30 mL min−1) at a heating rate of 10 °C min−1. The usual work-up described below involves hydrolysis of the reaction mixture with water, separation of the organic layer, extraction of the aqueous layer with hexane, drying the combined organic layer and extracts with anhydrous magnesium sulfate, and evaporation of the solvents, in this order.
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Preparation of hydrosilanes 3 and 4 To a solution of 5.42 g (8.83 mmol) of compound 1 in 20 mL of ether was added 5.6 mL (9.0 mmol) of a 1.6 M n-butyllithium solution in hexane at −80 °C. After stirring the reaction mixture for 30 min at that temperature, 1.0 mL (8.9 mmol) of chlorodimethylsilane was added, and the reaction mixture was further stirred at room temperature for 3 h. After the usual work-up described above (see the General subsection), the residue was subjected to silica gel column chromatography to give 4.59 g (88% yield) of 3: EI-MS: m/z = 594 [M+]. 1H NMR (in CDCl3, 400 MHz): δ = 0.32 (9H, s, Me– Si), 0.40 (6H, d, J = 4.6 Hz, Me–Si), 0.77–0.88 (12H, m, 2-ethylhexyl), 1.15–1.32 (20H, m, 2-ethylhexyl), 1.47 (2H, m, 2-ethylhexyl), 4.57 (1H, sept, J = 4.6 Hz, H–Si), 7.12 (1H, s, thiophene ring proton), 7.17 (1H, s, thiophene ring proton). 13C NMR (in CDCl3, room temperature, 100 MHz): δ = −2.51, 0.23, 11.03, 14.33, 20.68, 20.71, 23.16, 28.90, 29.02, 29.05, 35.56, 35.58, 37.07, 37.09, 136.68, 136.69, 136.89, 136.91, 138.05, 138.06, 140.99, 141.02, 146.01, 146.11, 146.21, 146.23, 146.28, 146.37, 146.38, 151.66, 151.68, 152.48, 152.53, 152.56. 13C NMR (in CDCl3, 50 °C, 125 MHz): δ = −2.51, 0.24, 11.03, 14.21, 21.06, 21.09, 23.15, 29. 05, 29.19, 29.20, 35.77, 35.78, 37.27, 37.28, 136.90, 136.92, 136.94, 138.07, 138.08, 138.09, 141.22, 146.25, 146.33, 146.36, 146.41, 151.85, 152.71 (the 13C NMR spectrum at room temperature showed broad and multiple sp2 carbon signals likely due to the restricted rotation of the 2-ethylhexyl groups. The signals became simpler by elevating the measurement temperature, but some of them were still multiple and broad to an extent). Anal. Calcd for C29H52GeS2Si2: C, 58.67; H, 8.83. Found: C, 58.81; H, 9.07. Compound 4 was prepared in 84% yield in a fashion similar to that above from compound 2 and two equiv. each of n-butyllithium and chlorodimethylsilane: EI-MS: m/z = 580 [M+]. 1H NMR (in CDCl3, 400 MHz): δ = 0.40 (12H, d, J = 4.6 Hz, Me–Si), 0.77–0.83 (12H, m, 2-ethylhexyl), 1.14–1.33 (20H, m, 2-ethylhexyl), 1.46 (2H, m, 2-ethylhexyl), 4.57 (2H, sept, J = 4.6 Hz, H–Si), 7.18 (2H, s, thiophene ring proton). 13C NMR (in CDCl3, 100 MHz): δ = −2.51, 11.07, 14.34, 20.74, 23.18, 28.95, 29.06, 35.59, 37.12 136.96, 136.99, 137.01, 138.03, 146.23, 146.34, 146.45, 152.34, 152.38, 152.42. Hydrosilylation of 3 and 4 with trialkoxyvinylsilane To a solution of 4.51 g (7.6 mmol) of 3 and 1.8 mL (8.4 mmol) of triethoxyvinylsilane in 40 mL of toluene were added a few drops of Speier’s catalyst (H2PtCl6·6H2O in i-propyl alcohol), and the reaction mixture was stirred for 12 h at room temperature. After the evaporation of the solvent, the residue was subjected to silica gel column chromatography to give 3.12 g (52% yield) of DTG1: EI-MS: m/z = 784 [M+]. 1H NMR (in CDCl3, 400 MHz): δ = 0.31 (6H, s, Me–Si), 0.33 (9H, s, Me–Si), 0.60–0.64 (2H, m, SiCH2CH2SiAr), 0.77–0.83 (12H, m, 2-ethylhexyl), 1.16–1.32 (31H, m, 2-ethylhexyl, OCH2CH3, SiCH2CH2SiAr), 1.47 (2H, m, 2-ethylhexyl), 3.81 (6H, q, J = 7.0 Hz, OCH2CH3), 7.12 (2H, s, thiophene ring proton). 13C NMR (in CDCl3, 100 MHz): δ = −2.30, 0.22, 2.63, 8.10, 11.02, 14.32,
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15.82, 18.45, 20.66, 20.70, 23.15, 28.87, 28.89, 29.03, 29.06, 35.57, 37.06, 37.10, 58.54, 136.89, 137.28, 139.10, 139.12, 140.71, 140.73, 145.83, 145.95, 146.01, 146.06, 146.07, 146.11, 151.82, 151.89, 151.94, 152.00. Anal. Calcd for C37H70GeO3S2Si3: C, 56.68; H, 9.00. Found: C, 56.63; H, 9.18. DTG2 was obtained from compound 4 in a fashion similar to that above in 41% yield: EI-MS: m/z = 876 [M+]. 1H NMR (in CDCl3, 400 MHz): δ = 0.31 (12H, s, Me–Si), 0.60–0.65 (4H, m, SiCH2CH2SiAr), 0.76–0.83 (12H, m, 2-ethylhexyl), 1.15–1.29 (24H, m, 2-ethylhexyl, SiCH2CH2SiAr), 1.46 (2H, m, 2-ethylhexyl), 3.56 (18H, s, Me–O), 7.12 (2H, s, thiophene ring proton). 13C NMR (in CDCl3, 100 MHz): δ = −2.38, 1.43, 7.95, 10.97, 14.27, 20.65, 23.12, 28.83, 29.02, 35.54, 37.07, 50.69, 137.30, 138.91, 146.02, 151.91. Exact ESI-MS Calcd for C38H74GeO6NaS2Si4 [M + Na]+: 899.31079. Found: 899.30957. Hydrolysis/condensation of DTG1 A mixture of 0.16 g (0.20 mmol) of DTG1, 0.102 mL of 10% NaOH aqueous solution, and 0.6 mL of THF was stirred at room temperature for 12 h. After the usual work-up as above, the residue was subjected to preparative GPC with toluene as the eluent to give 0.080 g (59% yield) of DTG1-POSS as a yellow viscous oil: FD-MS m/z 5381.8 (M+). 1H NMR (in CDCl3, 400 MHz): δ = 0.28–0.32 (120H, s, Me–Si), 0.60 (16H, m, SiCH2CH2SiAr), 0.74–0.79 (96H, m, 2-ethylhexyl), 1.12–1.26 (176H, m, 2-ethylhexyl, SiCH2CH2SiAr), 1.42–1.43 (16H, m, 2-ethylhexyl), 7.11 (16H, s, thiophene ring proton). 13C NMR (in CDCl3, 100 MHz): δ = −2.08, −2.06, 2.23, 4.67, 8.40, 11.08, 11.10, 14.38, 20.55, 20.57, 20.65, 20.67, 23.09, 23.15, 28.89, 29.03, 29.05, 35.56, 37.02, 37.04, 37.07, 136.80 (br), 137.24 (br), 145.87 (br), 151.97 (br). 1H and 13C NMR, and FD-MS spectra of DTG1-POSS are given in Fig. S-1 and S-2,† respectively. Hydrolysis/condensation of DTG2 A mixture of 0.13 g (0.15 mmol) of DTG2, 5 μL of 28% aqueous ammonia solution, 20 μL of water, and 0.5 mL of THF was stirred at room temperature for 5 h. The resulting mixture was heated at 70 °C for 3 h, and then at 100 °C for 12 h to give 0.10 g (90% yield) of DTG2-PSQ as an insoluble yellow solid. Copolymerization of DTG2 with MeS or CzS was carried out in a fashion similar to that above. Inclusion of carbazole units in DTG2-CzS was confirmed by the NMR spectra (Fig. S-5†). Data for the carbazole unit: 1H NMR (in CDCl3, 400 MHz): δ = 0.63 (2H, br), 1.57 (2H, br), 2.36–2.42 (2H, br), 2.73 (2H, br), 4.31 (2H, br), 7.17–7.22 (3H, br, ring proton), 7.29–7.42 (3H, br, ring proton), 8.00–8.01 (2H, br, ring proton). 13C NMR (in CDCl3, 100 MHz): δ = 11.75–12.19, 23.00–23.43, 30.39–30.50, 35.11–35.53, 43.14–43.29, 108.55, 119.31, 120.60, 123.05, 125.87, 140.07. DTG2-MeS was insoluble in organic solvents and could not be analyzed by NMR, but the IR spectra indicated that the copolymerization proceeded smoothly (Fig. S-3†).
Dalton Transactions
PVK in a weight ratio of 1 : 1 at 1500 rpm and the resulting substrate was dried in vacuo. BCP and an aluminum electrode were vapor-deposited on the DTG2-CzS-1 surface with the thickness of 40 nm and 30 nm, respectively. The active area was 5 mm2.
Results and discussion Synthesis and hydrolysis/condensation of monomers Monomers DTG1 and DTG2 were prepared from bromo(trimethylsilyl)dithienogermole (1) and dibromodithienogermole (2), respectively, as presented in Scheme 1. Lithiation of DTG1 and DTG2, followed by silylation with dimethylchlorosilane provided (dimethylsilyl)(trimethylsilyl)dithienogermole (3) and bis(dimethylsilyl)dithienogermole (4) in 88% and 84% yields, respectively. Those two compounds were then subjected to hydrosilylation with trialkoxyvinylsilane in the presence of Speier’s catalyst to give DTG1 and DTG2 as yellow viscous oils, in 52% and 41% yields, respectively. These DTGs, 1–4, DTG1, and DTG2 showed optical properties similar to those of simple DTGs reported previously.10,11 Hydrolysis/condensation of DTG1 and DTG2 was performed under basic conditions using NaOH or NH3 as the catalyst (Scheme 2). Attempted hydrochloric-acid-catalyzed reactions resulted in the cleavage of the Si–C bonds to produce compound 5. Synthesis and PL properties of DTG-containing polyhedral oligosilsesquioxane (POSS) Hydrolysis/condensation of DTG1 gave the cage compound DTG1-POSS as a viscous oil in 59% yield after purification by preparative GPC (gel permeation chromatography) with toluene as the eluent. Fig. 1 presents the 29Si NMR spectra of DTG1 and DTG1-POSS. The spectra indicate that the original T0 (RSi(OEt)3) group was converted into a T3 (RSi(OSi)3) unit cleanly, whereas silicon signals of the arms were little affected by the hydrolysis/condensation. The FD-mass spectrum of DTG1-POSS revealed a signal at m/z = 5381.8, corresponding to
Fabrication of OLEDs On an ITO glass substrate coated with PEDOT:PSS was spincoated a 1 wt% THF solution of a mixture of DTG2-CzS-1 and
8216 | Dalton Trans., 2015, 44, 8214–8220
Scheme 1
Synthesis of monomers.
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Scheme 2
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Synthesis of DTG-containing silsesquioxanes.
Fig. 2 UV absorption and PL spectra of DTG1-POSS in THF (a) and PL spectral changes upon addition of nitrobenzene for [DTG1-POSS] = 8.2 × 10−7 M (b).
Fig. 1 CDCl3.
29
Si NMR spectra of DTG1 (top) and DTG1-POSS (bottom) in Table 1
Optical data of DTG1 and DTG1-POSS in THF
UV abs
the molecular ion peak (Fig. S-2†). From its GPC analysis, a single peak that corresponded to a molecular weight of Mn = 4781 (Mw/Mn = 1.1), relative to polystyrene standards, was observed clearly indicating that DTG1-POSS possessed a single component without other lower or higher molecular weight fractions. TGA of DTG1-POSS was performed in nitrogen and 5% weight loss temperature (Td5) of 359 °C was noted, indicating its good thermal stability. The UV absorption and PL spectra of DTG1-POSS are depicted in Fig. 2a and the data are summarized in Table 1, together with those of monomeric DTGs DTG1, DTG2, 3, and 4. As can be seen in Table 1, the spectral profiles of these compounds in THF are nearly the same with a much higher molar extinction coefficient for DTG1-POSS by approximately 9.5 times, reflecting the DTG-accumulated structure. It is likely that no significant interaction among the arms occurs in DTG1-POSS and the DTG chromophores are electronically isolated, as generally observed for POSS derivatives.3 Interestingly, DTG1-POSS responded to nitrobenzene to decrease the PL intensity, as presented in Fig. 2b, indicative of the potential application of this compound as sensing materials for nitroaromatic explosives. This is likely due to the electron-
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PL
Comp.
λmax/nm
ε/L(mol cm)
DTG1 DTG2 DTG1-POSS 3 4
347 338 346 346 346
2.0 × 104 1.9 × 104 1.9 × 105 2.0 × 104 2.0 × 104
−1
λem/nm
Φ/%
403 405 403 402 402
58 55 56 76 82
transfer from a photo-excited DTG unit to nitrobenzene. Similar PL quenching of poly(tetraphenylsilole-1,1-diyl) by the interaction with nitroaromatics has been reported.19
Synthesis, and PL and EL properties of DTG-containing polysilsesquioxane (PSQ) Similar treatment of DTG2 with an aqueous ammonia solution in THF at room temperature followed by aging at 70 °C for 3 h and then at 100 °C for 12 h, provided a yellow insoluble solid of polysilsesquioxane DTG2-PSQ, as shown in Scheme 2. Carrying out the condensation in a glass vessel as the template provided a self-standing film of DTG2-PSQ, as shown in Fig. 3b. As DTG2-PSQ was insoluble in organic solvents, we could not
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Scheme 3
Fig. 3 PL spectra of DTG2 in THF, and DTG2-PSQ and DTG2-MeS-x (x = 10, 5, 3, 2, and 1) as films (a). Photographs of the self-standing DTG2-PSQ film (b), and DTS2-MeS-1 film under room light (c) and UV irradiation at 365 nm (d).
carry out the NMR spectrometric analysis. However, the IR spectrum showed the absorptions characteristic of thiophene stretching vibrations, clearly (Fig. S-3†). The thermal properties were examined with respect to TGA in nitrogen, showing good stability with Td5 = 312 °C similar to DTG1-POSS. DSC analysis of DTG2-PSQ was also carried out from −80 °C to 150 °C. However, no evident transition was observed. This is the same as that of other DTG2-based polysilsesquioxanes described below. Fig. 3a presents the PL spectra of DTG2 in THF and DTG2PSQ as a film. In contrast to DTG1-POSS that showed nearly the same PL spectrum as that of monomeric DTGs DTG1, 3, and 4, the PL band of DTG2-PSQ was red-shifted from that of DTG2, and a shoulder appeared in the low energy region. In addition, the PL quantum yield (Φ) of the DTG2-PSQ film was only 2%, and was much smaller than that of DTG2 in THF (Φ = 55%). These are indicative of the aggregation of DTG chromophores in the DTG2-PSQ film, causing concentration quenching. To enhance the PL efficiency, we carried out the copolymerization of DTG2 with trimethoxymethylsilane (MeS), as shown in Scheme 3. As summarized in Table 2, the copolymerization of DTG2 with MeS proceeded smoothly to afford good yields of copolymers DTG2-MeS-x (x represents the loading mol% of DTG2 for the polymerization) as solids that were insoluble in organic solvents, similarly to DTG2-PSQ. The copolymers were analyzed with respect to their IR spectra, showing increased incorporation of the DTG units as the loading ratio of DTG2/MeS (x) increased (Fig. S-3†). The copolymers could also form self-standing films (Fig. 3c). The intensity of the PL shoulder was minimized and Φ was increased as the loading mol% of DTG2 (x%) was decreased, as shown in Fig. 3a and d. When x = 1, the shoulder nearly dis-
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Table 2
Copolymerization of DTG2 with MeS and CzS.
Synthesis and properties of PSQ from DTG2
DTG2/mg (μmol)
Comonomer
/mg (mmol)
Polymerb
Yield/g
Φ (film)/%
160 6 (7) 9 (1) 14 (1.6) 20 (2.3) 81 (9.2) 3 (3) 8 (9.1) 12 (14) 10 (11) 51 (58)
None MeS
— (−) 95 (0.70) 65 (0.48) 69 (0.51) 59 (0.43) 110 (0.81) 145 (0.37) 163 (0.42) 165 (0.42) 85 (0.22) 210 (0.54)
DTG2-PSQ DTG2-MeS-1 DTG2-MeS-2 DTG2-MeS-3 DTG2-MeS-5 DTG2-MeS-10 DTG2-CzS-1 DTG2-CzS-2 DTG2-CzS-3 DTG2-CzS-5 DTG2-CzS-10
100a 31 29 38 35 96 62 78 89 45 191
2 38 17 11 5 7 48 30 33 45 20
CzS
a
Yield based on the complete conversion of MeO–Si groups into Si–O– Si bonds is 90%. b Number indicates the loading molar ratio (%) of DTG2 for copolymerization.
appeared and Φ reached 38%, close to that of DTG2 in THF, although the PL band was still slightly red-shifted, suggesting a chromophore–chromophore interaction in the resulting polymer solids to a certain extent even for x = 1. TGA of DTG2 and DTG2-MeS under a nitrogen atmosphere showed their enhanced thermal stability from Td5 = 383 to 430 °C, as x decreased from 10 to 1 (Fig. S-4†). One might consider the possibility of solvent inclusion in the polymer networks. However no apparent weight loss below 200 °C was observed, indicating that the effects of solvent inclusion on the polymer properties were negligible, even if present. Next, we examined the copolymerization of DTG2 with carbazole-containing trimethoxysilane (CzS) as the hole carrier with different molar ratios, in hopes of obtaining electroluminescent (EL) films (Scheme 3). The results are also summarized in Table 2. Again, the copolymerization proceeded smoothly to provide polysilsesquioxanes DTG2-CzS-x as solids in good yields (x = mol% of DTG2 used for the copolymerization). In these reactions, the resulting films were nearly
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soluble in THF and chloroform, in contrast to DTG2-PSQ and DTG2-MS, although they contain small amounts of insoluble parts. The 1H NMR spectra of the soluble parts showed the increased incorporation ratios of DTG units in the polymers as the loading ratios (x) increased (Fig. S-5†). It was also found that DTG2-CzS contained THF used as the reaction solvent. However, the amount seemed to be too small to exert any significant influence on the polymer optical properties. The molecular weights of the copolymers were determined by GPC, which showed major peaks at Mn = 2300–2400, together with small shoulders or minor peaks (Fig. S-6†). The TG analysis of DTG2-CzS in nitrogen indicated the lower Td5 values (240–290 °C) than those of DTG2-MeS (Fig. S-4†). This may be due to evaporation of THF and lower thermal stability of the DTG2-CzS framework, reflecting the existence of a larger number of organic bonds. The UV absorption spectra of DTG2CzS showed bands ascribed to both the DTG and carbazole chromophore (Fig. S-7†). The extinction coefficients of the polymers revealed no clear relationship with the DTG incorporation ratio (x), likely due to the partial insolubility of the polymers. These polysilsesquioxanes could be spin-coated to give solid films. Similarly to DTG2-MeS, PL efficiencies of the resulting polymers (DTG2-CzS-x) increased as the DTG2/CzS loading mol% (x) decreased. Interestingly, as illustrated in Fig. 4, DTG2-CzS exhibits different PL properties depending on the state; i.e., in solution or film. PL bands ascribed to the carbazole emission appeared as the major peak together with a shoulder that was likely due to the DTG emission in THF, whereas films of DTG2-CzS showed only emission from the DTG unit, indicating facile photo-energy transfer from carbazole to DTG in the film. As polysilsesquioxane films of CzS have been proven to possess semi-conducting properties,17 these results suggest the potential application of the DTG2-CzS films as OLED active materials, in which energy transfer from the exciton generated by carrier conduction through the condensed carbazole units to DTG would cause blue emission. To elucidate the
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Fig. 5 EL spectra of devices I, II, and III, based on DTG2-CzS-1, CzS, and DTG2.
spin-coated film of DTG2-CzS as an EL material, we carried out preliminary experiments. An OLED was fabricated with the structure of ITO/PEDOT:PSS/DTG2-CzS-1/BCP/Al (device I), in which the spin-coated film of PEDOT:PSS ( poly(3,4ethylenedioxythiophene):poly(styrene sulfonate) and the vapordeposited film of BCP (2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline) were the hole- and electron-injecting layers, respectively. The EL spectrum of the device shows a broad band at λmax = 540 nm, which appears at a lower energy than the PL band of the DTG2-CzS-1 film (Fig. 5). Furthermore, the spectral profile is comparable to that of a similar device prepared with a spin-coated layer of polysilsesquioxane CzS-PSQ11 which had been prepared by the hydrolysis/condensation of CzS (ITO/ PEDOT:PSS/CzS-PSQ/BCP/Al, device II). This indicates that the emission from device I is likely ascribed to the interaction between carbazole and BCP units at the interface. To enhance the hole-transporting properties of DTG2-CzS, we prepared a composite film of DTG2-CzS-1 and poly(vinylcarbazole) (PVK) in a weight ratio of 1 : 1, and applied its spin-coated film onto an OLED (ITO/PEDOT:PSS/CzS-PSQ + PVK/BCP/Al, device III). As expected, device III gave an emission band approximately at λmax = 430 nm, similar to that of DTG2-CzS-1 in the film, although a broad emission band centered at approximately 640 nm could still be seen. However, the OLED efficiency was low (maximal luminance = 0.22 cd m−2) and the further studies to optimize the device structure seem to be required.
Conclusions
Fig. 4 film.
PL spectra of DTG2 and CzS in THF, and DTG2-CzS-1 in THF and
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In summary, we prepared DTG-containing oligo- and polysilsesquioxanes by hydrolysis/condensation of DTG1 and DTG2. Polyhedral oligosilsesquioxane DTG1-POSS showed good PL properties, reflecting the structural characteristics of the POSS core. On the other hand, polysilsesquioxanes based on DTG2 were found to be potentially useful as PL and EL materials with good thermal stability and processability.20 The emissive properties are tunable by copolymerization with other alkoxy-
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silanes as well as the composite formation, providing the desired functionalities. 10
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Acknowledgements This work was supported by a Grant-in-Aid for Scientific Research on Innovative Areas “New Polymeric Materials Based on Element–Blocks (no. 2401)” from The Ministry of Education, Culture, Sports, Science, and Technology, Japan. 11
Notes and references 1 M. A. Brook, Silicon in Organic, Organometallic, and Polymer Chemistry, Wiley, 2000. 2 A. Shimojima and K. Kuroda, Chem. Rec., 2006, 6, 53. 3 H.-J. Cho, D.-H. Hwang, J.-I. Lee, Y.-K. Jung, J.-H. Park, J. Lee, S.-K. Lee and H.-K. Shim, Chem. Mater., 2006, 18, 3780; J. D. Froelich, R. Young, T. Nakamura, Y. Ohmori, S. Li, A. Mochizuki, M. Lauters and G. E. Jabbour, Chem. Mater., 2007, 19, 4991; Y.-L. Chu, C.-C. Cheng, Y.-P. Chen, Y.-C. Yen and F.-C. Chang, J. Mater. Chem., 2012, 22, 9285; B. A. Kamino and T. P. Bender, Chem. Soc. Rev., 2013, 42, 5119; D. Sun, Q. Fu, Z. Ren, W. Li, H. Li, D. Ma and S. Yan, J. Mater. Chem. C, 2013, 1, 5344. 4 X. Zhan, S. Barlow and S. R. Marder, Chem. Commun., 2009, 1948. 5 K. L. Chan, M. J. McKieman, C. R. Towns and A. B. Holmes, J. Am. Chem. Soc., 2005, 127, 7662; E. Wang, L. Wang, L. Lan, W. Zhuang, J. Peng and Y. Cao, Appl. Phys. Lett., 2008, 92, 033307. 6 J. Chen and Y. Cao, Macromol. Rapid Commun., 2007, 28, 1714; J. Ohshita, Macromol. Chem. Phys., 2009, 210, 1360. 7 J. Ohshita, K. Murakami, D. Tanaka, Y. Ooyama, T. Mizumo, N. Kobayashi, H. Higashimura, T. Nakanishi and Y. Hasegawa, Organometallics, 2014, 33, 517. 8 J. Hou, H.-Z. Chen, S. Zhang, G. Li and Y. Yang, J. Am. Chem. Soc., 2008, 130, 16144; T.-Y. Chu, J. Lu, S. Beaupre, Y. Zhang, J.-R. Pouliot, S. Wakim, J. Zhou, M. Leclerc, Z. Li, J. Ding and Y. Tao, J. Am. Chem. Soc., 2011, 133, 4250; X. Guo, N. Zhou, S. J. Lou, J. W. Hennek, R. P. Ortiz, M. R. Butler, P.-L. T. Boudreault, J. Strzalka, P.-O. Morin, M. Leclerc, J. T. L. Navarrete, M. A. Ratner, L. X. Chen, P. H. Chang, A. Facchetti and T. J. Marks, J. Am. Chem. Soc., 2012, 134, 18427; Y.-F. Li, Acc. Chem. Res., 2012, 45, 723. 9 J. Ohshita, M. Nodono, H. Kai, T. Watanabe, A. Kunai, K. Komaguchi, M. Shiotani, A. Adachi, K. Okita, Y. Harima, K. Yamashita and M. Ishikawa, Organometallics, 1999, 18, 1453; J. Ohshita, H. Kai, A. Takata, T. Iida, A. Kunai, N. Ohta, K. Komaguchi, M. Shiotani, A. Adachi,
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K. Sakamaki and K. Okita, Organometallics, 2001, 20, 4800; H. Jung, H. Hwang, K.-M. Park, J. Kim, D.-H. Kim and Y. Kang, Organometallics, 2010, 29, 2715. J. Ohshita, Y.-M. Hwang, T. Mizumo, H. Yoshida, Y. Ooyama, Y. Harima and Y. Kunugi, Organometallics, 2011, 30, 3233; C. M. Amb, S. Chen, K. R. Graham, J. Subbiah, C. E. Small, F. So and J. R. Reynolds, J. Am. Chem. Soc., 2011, 133, 10062; C. E. Small, S. Chen, J. Subbiah, C. M. Amb, S. W. Tsang, T. H. Lai, J. R. Reynolds and F. So, Nat. Photonics, 2012, 6, 115. Y. Yabusaki, N. Ohshima, H. Kondo, T. Kusamoto, Y. Yamanoi and H. Nishihara, Chem. – Eur. J., 2010, 16, 5581; H.-K. Lee, J. Ohshita, D. Tanaka, Y. Tominaga and A. Kunai, J. Organomet. Chem., 2012, 710, 53; J. Ohshita, Y. Tominaga, D. Tanaka, Y. Ooyama, T. Mizumo, N. Kobayashi and H. Higashimura, Dalton Trans., 2013, 42, 3646. T. Lee, I. Jung, K. H. Song, H. Lee, J. Choi, K. Lee, B. J. Lee, J. Pak, C. Lee, S. O. Kang and J. Ko, Organometallics, 2004, 23, 5280; J. Ohshita, D. Hamamoto, K. Kimura and A. Kunai, J. Organomet. Chem., 2005, 690, 3027; J. Ohshita, Y. Kurushima, K.-H. Lee, Y. Ooyama and Y. Harima, Organometallics, 2007, 26, 6591. Silole-containing polysilsesquioxane as a biomaterial has been reported. M. Li, J. W. Y. Lam, F. Mahtab, S. Chen, W. Zhang, Y. Hong, J. Xiong, Q. Zheng and B. Z. Tang, J. Mater. Chem. B, 2013, 1, 676; M. Mahtab, Y. Tu, J. W. Y. Lam, J. Liu, B. Zhang, P. Lu, X. Zhang and B. Z. Tang, Adv. Funct. Mater., 2011, 21, 1733; M. Faisal, Y. Hong, J. Liu, Y. Yu, J. W. Y. Lam, A. Qin, P. Lu and B. Z. Tang, Chem. – Eur. J., 2010, 16, 4266. L. Chan, J. G. Labram, T. D. Anthopoulos, S. E. Watkins, M. McKiernan, A. J. P. White, A. B. Holmes and C. K. Williams, J. Mater. Chem. C, 2011, 21, 11800. H. P. Martinez, C. D. Grant, J. G. Reynolds and W. C. Trogler, J. Mater. Chem., 2012, 22, 2908; J. Yang, S. Aschemeyer, H. P. Martinez and W. C. Trogler, Chem. Commun., 2010, 46, 6804. H. Kai, J. Ohshita, S. Ohara, N. Nakayama, A. Kunai, I.-S. Lee and Y.-W. Kwak, J. Organomet. Chem., 2008, 693, 3490. S. Watase, D. Fujisaki, M. Watanabe, K. Mitamura, N. Nishioka and K. Matsukawa, Chem. – Eur. J., 2014, 20, 12773. J. Ohshita, M. Miyazaki, D. Tanaka, Y. Morihara, Y. Fujita and Y. Kunugi, Polym. Chem., 2013, 4, 3116. H. Sohn, M. J. Sailor, D. Magde and W. C. Trogler, J. Am. Chem. Soc., 2003, 125, 3821. Silole-containing silsesquioxanes for PL imaging have been reported J. Liu, J. W. Y. Lim and B. Z. Tang, J. Inorg. Organomet. Polym., 2009, 19, 249.
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Synthesis of dithienogermole-containing oligoand polysilsesquioxanes as luminescent materials† Joji Ohshita,*a Masashi Nakamura,a Kazuki Yamamoto,a Seiji Wataseb and Kimihiro Matsukawab Dithienogermole (DTG)-containing oligo- and polysilsesquioxanes were prepared by hydrolysis/condensation of DTGs bearing one (DTG1) or two trialkoxysilyl group(s) (DTG2). The reaction of DTG1 gave a cage-type octasilsesquioxane with eight DTG groups at the edges (DTG1-POSS) as a viscous oil, whereas the reaction of DTG2 yielded a network polymer (DTG2-PSQ) as a self-standing film. DTG1-POSS showed a photoluminescence (PL) quantum yield (Φ) of 56% in THF. This value was as high as that of DTG1 (Φ = 58%), in spite of the accumulation of DTG units in the molecule, as characteristics of the POSS structure. The PL of DTG1-POSS in THF was suppressed by contact with nitrobenzene, showing the potential of DTG1-POSS for sensing nitroaromatic explosives. Polymer DTG2-PSQ exhibited a relatively low Φ of 2% as a film, but Φ was improved to 38% by copolymerization with trimethoxymethylsilane. DTG2 was also copolymerized with a trimethoxysilyl-substituted carbazole derivative (CzS) to provide
Received 23rd February 2015, Accepted 17th March 2015
polysilsesquioxanes with DTG and carbazole units, which showed efficient photo-energy transfer from
DOI: 10.1039/c5dt00777a
carbazole to DTG in the films. Similar copolymerization of DTG2 with CzS in the presence of poly(9-vinylcarbazole) provided a composite material with hole-transporting electroluminescence properties, appli-
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cable in multi-layered organic light emitting diodes.
Introduction Oligo- and polysilsesquioxanes have received a great deal of attention as organic–inorganic hybrid materials.1,2 They are readily accessible, typically by the acid- or base-catalyzed hydrolysis/condensation of trialkoxy- or trihalosilanes with an organic group, and the resulting three-dimensional network structures composed of Si–O bonds usually provide good filmforming properties and thermal stability to the materials. The properties of oligo- and polysilsesquioxanes are significantly affected by the organic group, and it has been demonstrated that the introduction of π-conjugated units confers optoelectronic functionalities to silsesquioxanes, such as carriertransporting and emissive properties, rendering the silsesquioxanes applicable in organic optoelectronic devices.3 On the other hand, group 14 metalloles condensed with biaryls,4 such as biphenyl,5 bithiophene,6 and bipyridyl,7 have a Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University, Higashi-Hiroshima 739-8527, Japan. E-mail: [email protected] b Osaka Municipal Technical Research Institute, 1-6-50 Morinomiya, Joto-ku, Osaka 536-8553, Japan † Electronic supplementary information (ESI) available: 1H and 13C NMR, and FD-mass spectra of DTG1-POSS, IR spectra and TG traces of DTG2-MeS and CzS, 1 H NMR spectra, GPC traces, and UV absorption spectra of DTG2-CzS. See DOI: 10.1039/c5dt00777a
8214 | Dalton Trans., 2015, 44, 8214–8220
been extensively studied as a new class of chromophores with interesting electronic states. The highly planar tricyclic structures of these compounds enhance the conjugation. Furthermore, in-phase interactions between the metal σ* and biaryl π* orbitals lower the LUMO energy levels, leading to even smaller HOMO–LUMO gaps. Of these, bithiophene-condensed silole and germole, namely, dithienosilole6,8,9 and dithienogermole10 (DTS and DTG in Chart 1), are of particular interest as building units of functional π-conjugated materials, and their applications in organic electronic materials, such as donor polymers for organic solar cells8 and carrier transports for organic light emitting diodes (OLEDs),9 have been explored. It has also been demonstrated that these compounds often exhibit high photoluminescence (PL) properties both in
Chart 1 General structures of DTS and DTG with HOMO and LUMO energy levels in eV in parentheses, derived from DFT calculations for those with methyls on the bridge at B3LYP/6-31G(d,p) (aref. 6, bpresent study, cref. 15).
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solution as well as in the solid state,11,12 the primary reason being that the rigid tricyclic system minimizes the molecular vibration, thereby suppressing the non-radiative decay. In addition, the two additional substituents on the metal sterically cover the metallole chromophore to avoid concentration quenching even in the solid state. On the basis of the consideration mentioned above, we designed and prepared oligo- and polysilsesquioxanes containing DTG units for the first time, in hopes of developing new emissive materials.13 DTG is more stable than DTS towards hydrolysis, and is thus more suitable for the process. Copolymerization with a carbazole-containing monomer was also performed to furnish copolymers that showed efficient photoenergy transfer from carbazole to DTG in the films. Preliminary experiments revealing the potential applications of the resulting polysilsesquioxane containing DTG and carbazole units as electroluminescence (EL) materials for multi-layered OLEDs were also carried out. Although dibenzosilole (DBS in Chart 1) has been studied as EL materials,14 no oligo- and polysilsesquioxanes containing DBS units have been reported. Only silica attached by DBS has been studied for the separation and detection of nitroaromatic explosives.15 Because DTS and DTG have the lower and higher lying LUMO and HOMO, respectively, than those of DBS as predicted by DFT calculations,6,16 better carrier transporting properties are expected for DTS and DTG, thus rendering them more suitable as EL materials.
Experimental General All reactions were carried out in dry argon. THF, toluene, and diethyl ether, which were used as the reaction solvents, were distilled from CaH2 and stored over activated molecular sieves until use. Starting compounds 1, 2, and CzS were prepared as reported in the literature.17,18 NMR spectra were recorded on Varian 400-MR and System500 spectrometers. EI-mass spectra were recorded on a Shimadzu QP-2020A spectrometer, and ESI- and FD-mass spectra were obtained by using Thermo Fisher Scientific LTQ Orbitrap XL and JEOL JMS-T100 GCV 4G spectrometers, respectively, at N-BARD, Hiroshima University. UV–vis absorption was measured and PL spectra were recorded on Hitachi U-3210 and HORIBA FluoroMax-4 spectrophotometers, respectively. The emission quantum yields excited at 350 nm were determined using a JASCO F-6300-H spectrometer connected to a JASCO ILF-533 integrating sphere unit (ϕ = 100 mm). Thermogravimetric analysis (TGA) was carried out on a SII TG/DTA-6200 analyzer under a gentle nitrogen flow (30 mL min−1) at a heating rate of 10 °C min−1. The usual work-up described below involves hydrolysis of the reaction mixture with water, separation of the organic layer, extraction of the aqueous layer with hexane, drying the combined organic layer and extracts with anhydrous magnesium sulfate, and evaporation of the solvents, in this order.
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Paper
Preparation of hydrosilanes 3 and 4 To a solution of 5.42 g (8.83 mmol) of compound 1 in 20 mL of ether was added 5.6 mL (9.0 mmol) of a 1.6 M n-butyllithium solution in hexane at −80 °C. After stirring the reaction mixture for 30 min at that temperature, 1.0 mL (8.9 mmol) of chlorodimethylsilane was added, and the reaction mixture was further stirred at room temperature for 3 h. After the usual work-up described above (see the General subsection), the residue was subjected to silica gel column chromatography to give 4.59 g (88% yield) of 3: EI-MS: m/z = 594 [M+]. 1H NMR (in CDCl3, 400 MHz): δ = 0.32 (9H, s, Me– Si), 0.40 (6H, d, J = 4.6 Hz, Me–Si), 0.77–0.88 (12H, m, 2-ethylhexyl), 1.15–1.32 (20H, m, 2-ethylhexyl), 1.47 (2H, m, 2-ethylhexyl), 4.57 (1H, sept, J = 4.6 Hz, H–Si), 7.12 (1H, s, thiophene ring proton), 7.17 (1H, s, thiophene ring proton). 13C NMR (in CDCl3, room temperature, 100 MHz): δ = −2.51, 0.23, 11.03, 14.33, 20.68, 20.71, 23.16, 28.90, 29.02, 29.05, 35.56, 35.58, 37.07, 37.09, 136.68, 136.69, 136.89, 136.91, 138.05, 138.06, 140.99, 141.02, 146.01, 146.11, 146.21, 146.23, 146.28, 146.37, 146.38, 151.66, 151.68, 152.48, 152.53, 152.56. 13C NMR (in CDCl3, 50 °C, 125 MHz): δ = −2.51, 0.24, 11.03, 14.21, 21.06, 21.09, 23.15, 29. 05, 29.19, 29.20, 35.77, 35.78, 37.27, 37.28, 136.90, 136.92, 136.94, 138.07, 138.08, 138.09, 141.22, 146.25, 146.33, 146.36, 146.41, 151.85, 152.71 (the 13C NMR spectrum at room temperature showed broad and multiple sp2 carbon signals likely due to the restricted rotation of the 2-ethylhexyl groups. The signals became simpler by elevating the measurement temperature, but some of them were still multiple and broad to an extent). Anal. Calcd for C29H52GeS2Si2: C, 58.67; H, 8.83. Found: C, 58.81; H, 9.07. Compound 4 was prepared in 84% yield in a fashion similar to that above from compound 2 and two equiv. each of n-butyllithium and chlorodimethylsilane: EI-MS: m/z = 580 [M+]. 1H NMR (in CDCl3, 400 MHz): δ = 0.40 (12H, d, J = 4.6 Hz, Me–Si), 0.77–0.83 (12H, m, 2-ethylhexyl), 1.14–1.33 (20H, m, 2-ethylhexyl), 1.46 (2H, m, 2-ethylhexyl), 4.57 (2H, sept, J = 4.6 Hz, H–Si), 7.18 (2H, s, thiophene ring proton). 13C NMR (in CDCl3, 100 MHz): δ = −2.51, 11.07, 14.34, 20.74, 23.18, 28.95, 29.06, 35.59, 37.12 136.96, 136.99, 137.01, 138.03, 146.23, 146.34, 146.45, 152.34, 152.38, 152.42. Hydrosilylation of 3 and 4 with trialkoxyvinylsilane To a solution of 4.51 g (7.6 mmol) of 3 and 1.8 mL (8.4 mmol) of triethoxyvinylsilane in 40 mL of toluene were added a few drops of Speier’s catalyst (H2PtCl6·6H2O in i-propyl alcohol), and the reaction mixture was stirred for 12 h at room temperature. After the evaporation of the solvent, the residue was subjected to silica gel column chromatography to give 3.12 g (52% yield) of DTG1: EI-MS: m/z = 784 [M+]. 1H NMR (in CDCl3, 400 MHz): δ = 0.31 (6H, s, Me–Si), 0.33 (9H, s, Me–Si), 0.60–0.64 (2H, m, SiCH2CH2SiAr), 0.77–0.83 (12H, m, 2-ethylhexyl), 1.16–1.32 (31H, m, 2-ethylhexyl, OCH2CH3, SiCH2CH2SiAr), 1.47 (2H, m, 2-ethylhexyl), 3.81 (6H, q, J = 7.0 Hz, OCH2CH3), 7.12 (2H, s, thiophene ring proton). 13C NMR (in CDCl3, 100 MHz): δ = −2.30, 0.22, 2.63, 8.10, 11.02, 14.32,
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15.82, 18.45, 20.66, 20.70, 23.15, 28.87, 28.89, 29.03, 29.06, 35.57, 37.06, 37.10, 58.54, 136.89, 137.28, 139.10, 139.12, 140.71, 140.73, 145.83, 145.95, 146.01, 146.06, 146.07, 146.11, 151.82, 151.89, 151.94, 152.00. Anal. Calcd for C37H70GeO3S2Si3: C, 56.68; H, 9.00. Found: C, 56.63; H, 9.18. DTG2 was obtained from compound 4 in a fashion similar to that above in 41% yield: EI-MS: m/z = 876 [M+]. 1H NMR (in CDCl3, 400 MHz): δ = 0.31 (12H, s, Me–Si), 0.60–0.65 (4H, m, SiCH2CH2SiAr), 0.76–0.83 (12H, m, 2-ethylhexyl), 1.15–1.29 (24H, m, 2-ethylhexyl, SiCH2CH2SiAr), 1.46 (2H, m, 2-ethylhexyl), 3.56 (18H, s, Me–O), 7.12 (2H, s, thiophene ring proton). 13C NMR (in CDCl3, 100 MHz): δ = −2.38, 1.43, 7.95, 10.97, 14.27, 20.65, 23.12, 28.83, 29.02, 35.54, 37.07, 50.69, 137.30, 138.91, 146.02, 151.91. Exact ESI-MS Calcd for C38H74GeO6NaS2Si4 [M + Na]+: 899.31079. Found: 899.30957. Hydrolysis/condensation of DTG1 A mixture of 0.16 g (0.20 mmol) of DTG1, 0.102 mL of 10% NaOH aqueous solution, and 0.6 mL of THF was stirred at room temperature for 12 h. After the usual work-up as above, the residue was subjected to preparative GPC with toluene as the eluent to give 0.080 g (59% yield) of DTG1-POSS as a yellow viscous oil: FD-MS m/z 5381.8 (M+). 1H NMR (in CDCl3, 400 MHz): δ = 0.28–0.32 (120H, s, Me–Si), 0.60 (16H, m, SiCH2CH2SiAr), 0.74–0.79 (96H, m, 2-ethylhexyl), 1.12–1.26 (176H, m, 2-ethylhexyl, SiCH2CH2SiAr), 1.42–1.43 (16H, m, 2-ethylhexyl), 7.11 (16H, s, thiophene ring proton). 13C NMR (in CDCl3, 100 MHz): δ = −2.08, −2.06, 2.23, 4.67, 8.40, 11.08, 11.10, 14.38, 20.55, 20.57, 20.65, 20.67, 23.09, 23.15, 28.89, 29.03, 29.05, 35.56, 37.02, 37.04, 37.07, 136.80 (br), 137.24 (br), 145.87 (br), 151.97 (br). 1H and 13C NMR, and FD-MS spectra of DTG1-POSS are given in Fig. S-1 and S-2,† respectively. Hydrolysis/condensation of DTG2 A mixture of 0.13 g (0.15 mmol) of DTG2, 5 μL of 28% aqueous ammonia solution, 20 μL of water, and 0.5 mL of THF was stirred at room temperature for 5 h. The resulting mixture was heated at 70 °C for 3 h, and then at 100 °C for 12 h to give 0.10 g (90% yield) of DTG2-PSQ as an insoluble yellow solid. Copolymerization of DTG2 with MeS or CzS was carried out in a fashion similar to that above. Inclusion of carbazole units in DTG2-CzS was confirmed by the NMR spectra (Fig. S-5†). Data for the carbazole unit: 1H NMR (in CDCl3, 400 MHz): δ = 0.63 (2H, br), 1.57 (2H, br), 2.36–2.42 (2H, br), 2.73 (2H, br), 4.31 (2H, br), 7.17–7.22 (3H, br, ring proton), 7.29–7.42 (3H, br, ring proton), 8.00–8.01 (2H, br, ring proton). 13C NMR (in CDCl3, 100 MHz): δ = 11.75–12.19, 23.00–23.43, 30.39–30.50, 35.11–35.53, 43.14–43.29, 108.55, 119.31, 120.60, 123.05, 125.87, 140.07. DTG2-MeS was insoluble in organic solvents and could not be analyzed by NMR, but the IR spectra indicated that the copolymerization proceeded smoothly (Fig. S-3†).
Dalton Transactions
PVK in a weight ratio of 1 : 1 at 1500 rpm and the resulting substrate was dried in vacuo. BCP and an aluminum electrode were vapor-deposited on the DTG2-CzS-1 surface with the thickness of 40 nm and 30 nm, respectively. The active area was 5 mm2.
Results and discussion Synthesis and hydrolysis/condensation of monomers Monomers DTG1 and DTG2 were prepared from bromo(trimethylsilyl)dithienogermole (1) and dibromodithienogermole (2), respectively, as presented in Scheme 1. Lithiation of DTG1 and DTG2, followed by silylation with dimethylchlorosilane provided (dimethylsilyl)(trimethylsilyl)dithienogermole (3) and bis(dimethylsilyl)dithienogermole (4) in 88% and 84% yields, respectively. Those two compounds were then subjected to hydrosilylation with trialkoxyvinylsilane in the presence of Speier’s catalyst to give DTG1 and DTG2 as yellow viscous oils, in 52% and 41% yields, respectively. These DTGs, 1–4, DTG1, and DTG2 showed optical properties similar to those of simple DTGs reported previously.10,11 Hydrolysis/condensation of DTG1 and DTG2 was performed under basic conditions using NaOH or NH3 as the catalyst (Scheme 2). Attempted hydrochloric-acid-catalyzed reactions resulted in the cleavage of the Si–C bonds to produce compound 5. Synthesis and PL properties of DTG-containing polyhedral oligosilsesquioxane (POSS) Hydrolysis/condensation of DTG1 gave the cage compound DTG1-POSS as a viscous oil in 59% yield after purification by preparative GPC (gel permeation chromatography) with toluene as the eluent. Fig. 1 presents the 29Si NMR spectra of DTG1 and DTG1-POSS. The spectra indicate that the original T0 (RSi(OEt)3) group was converted into a T3 (RSi(OSi)3) unit cleanly, whereas silicon signals of the arms were little affected by the hydrolysis/condensation. The FD-mass spectrum of DTG1-POSS revealed a signal at m/z = 5381.8, corresponding to
Fabrication of OLEDs On an ITO glass substrate coated with PEDOT:PSS was spincoated a 1 wt% THF solution of a mixture of DTG2-CzS-1 and
8216 | Dalton Trans., 2015, 44, 8214–8220
Scheme 1
Synthesis of monomers.
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Scheme 2
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Synthesis of DTG-containing silsesquioxanes.
Fig. 2 UV absorption and PL spectra of DTG1-POSS in THF (a) and PL spectral changes upon addition of nitrobenzene for [DTG1-POSS] = 8.2 × 10−7 M (b).
Fig. 1 CDCl3.
29
Si NMR spectra of DTG1 (top) and DTG1-POSS (bottom) in Table 1
Optical data of DTG1 and DTG1-POSS in THF
UV abs
the molecular ion peak (Fig. S-2†). From its GPC analysis, a single peak that corresponded to a molecular weight of Mn = 4781 (Mw/Mn = 1.1), relative to polystyrene standards, was observed clearly indicating that DTG1-POSS possessed a single component without other lower or higher molecular weight fractions. TGA of DTG1-POSS was performed in nitrogen and 5% weight loss temperature (Td5) of 359 °C was noted, indicating its good thermal stability. The UV absorption and PL spectra of DTG1-POSS are depicted in Fig. 2a and the data are summarized in Table 1, together with those of monomeric DTGs DTG1, DTG2, 3, and 4. As can be seen in Table 1, the spectral profiles of these compounds in THF are nearly the same with a much higher molar extinction coefficient for DTG1-POSS by approximately 9.5 times, reflecting the DTG-accumulated structure. It is likely that no significant interaction among the arms occurs in DTG1-POSS and the DTG chromophores are electronically isolated, as generally observed for POSS derivatives.3 Interestingly, DTG1-POSS responded to nitrobenzene to decrease the PL intensity, as presented in Fig. 2b, indicative of the potential application of this compound as sensing materials for nitroaromatic explosives. This is likely due to the electron-
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PL
Comp.
λmax/nm
ε/L(mol cm)
DTG1 DTG2 DTG1-POSS 3 4
347 338 346 346 346
2.0 × 104 1.9 × 104 1.9 × 105 2.0 × 104 2.0 × 104
−1
λem/nm
Φ/%
403 405 403 402 402
58 55 56 76 82
transfer from a photo-excited DTG unit to nitrobenzene. Similar PL quenching of poly(tetraphenylsilole-1,1-diyl) by the interaction with nitroaromatics has been reported.19
Synthesis, and PL and EL properties of DTG-containing polysilsesquioxane (PSQ) Similar treatment of DTG2 with an aqueous ammonia solution in THF at room temperature followed by aging at 70 °C for 3 h and then at 100 °C for 12 h, provided a yellow insoluble solid of polysilsesquioxane DTG2-PSQ, as shown in Scheme 2. Carrying out the condensation in a glass vessel as the template provided a self-standing film of DTG2-PSQ, as shown in Fig. 3b. As DTG2-PSQ was insoluble in organic solvents, we could not
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Scheme 3
Fig. 3 PL spectra of DTG2 in THF, and DTG2-PSQ and DTG2-MeS-x (x = 10, 5, 3, 2, and 1) as films (a). Photographs of the self-standing DTG2-PSQ film (b), and DTS2-MeS-1 film under room light (c) and UV irradiation at 365 nm (d).
carry out the NMR spectrometric analysis. However, the IR spectrum showed the absorptions characteristic of thiophene stretching vibrations, clearly (Fig. S-3†). The thermal properties were examined with respect to TGA in nitrogen, showing good stability with Td5 = 312 °C similar to DTG1-POSS. DSC analysis of DTG2-PSQ was also carried out from −80 °C to 150 °C. However, no evident transition was observed. This is the same as that of other DTG2-based polysilsesquioxanes described below. Fig. 3a presents the PL spectra of DTG2 in THF and DTG2PSQ as a film. In contrast to DTG1-POSS that showed nearly the same PL spectrum as that of monomeric DTGs DTG1, 3, and 4, the PL band of DTG2-PSQ was red-shifted from that of DTG2, and a shoulder appeared in the low energy region. In addition, the PL quantum yield (Φ) of the DTG2-PSQ film was only 2%, and was much smaller than that of DTG2 in THF (Φ = 55%). These are indicative of the aggregation of DTG chromophores in the DTG2-PSQ film, causing concentration quenching. To enhance the PL efficiency, we carried out the copolymerization of DTG2 with trimethoxymethylsilane (MeS), as shown in Scheme 3. As summarized in Table 2, the copolymerization of DTG2 with MeS proceeded smoothly to afford good yields of copolymers DTG2-MeS-x (x represents the loading mol% of DTG2 for the polymerization) as solids that were insoluble in organic solvents, similarly to DTG2-PSQ. The copolymers were analyzed with respect to their IR spectra, showing increased incorporation of the DTG units as the loading ratio of DTG2/MeS (x) increased (Fig. S-3†). The copolymers could also form self-standing films (Fig. 3c). The intensity of the PL shoulder was minimized and Φ was increased as the loading mol% of DTG2 (x%) was decreased, as shown in Fig. 3a and d. When x = 1, the shoulder nearly dis-
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Table 2
Copolymerization of DTG2 with MeS and CzS.
Synthesis and properties of PSQ from DTG2
DTG2/mg (μmol)
Comonomer
/mg (mmol)
Polymerb
Yield/g
Φ (film)/%
160 6 (7) 9 (1) 14 (1.6) 20 (2.3) 81 (9.2) 3 (3) 8 (9.1) 12 (14) 10 (11) 51 (58)
None MeS
— (−) 95 (0.70) 65 (0.48) 69 (0.51) 59 (0.43) 110 (0.81) 145 (0.37) 163 (0.42) 165 (0.42) 85 (0.22) 210 (0.54)
DTG2-PSQ DTG2-MeS-1 DTG2-MeS-2 DTG2-MeS-3 DTG2-MeS-5 DTG2-MeS-10 DTG2-CzS-1 DTG2-CzS-2 DTG2-CzS-3 DTG2-CzS-5 DTG2-CzS-10
100a 31 29 38 35 96 62 78 89 45 191
2 38 17 11 5 7 48 30 33 45 20
CzS
a
Yield based on the complete conversion of MeO–Si groups into Si–O– Si bonds is 90%. b Number indicates the loading molar ratio (%) of DTG2 for copolymerization.
appeared and Φ reached 38%, close to that of DTG2 in THF, although the PL band was still slightly red-shifted, suggesting a chromophore–chromophore interaction in the resulting polymer solids to a certain extent even for x = 1. TGA of DTG2 and DTG2-MeS under a nitrogen atmosphere showed their enhanced thermal stability from Td5 = 383 to 430 °C, as x decreased from 10 to 1 (Fig. S-4†). One might consider the possibility of solvent inclusion in the polymer networks. However no apparent weight loss below 200 °C was observed, indicating that the effects of solvent inclusion on the polymer properties were negligible, even if present. Next, we examined the copolymerization of DTG2 with carbazole-containing trimethoxysilane (CzS) as the hole carrier with different molar ratios, in hopes of obtaining electroluminescent (EL) films (Scheme 3). The results are also summarized in Table 2. Again, the copolymerization proceeded smoothly to provide polysilsesquioxanes DTG2-CzS-x as solids in good yields (x = mol% of DTG2 used for the copolymerization). In these reactions, the resulting films were nearly
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soluble in THF and chloroform, in contrast to DTG2-PSQ and DTG2-MS, although they contain small amounts of insoluble parts. The 1H NMR spectra of the soluble parts showed the increased incorporation ratios of DTG units in the polymers as the loading ratios (x) increased (Fig. S-5†). It was also found that DTG2-CzS contained THF used as the reaction solvent. However, the amount seemed to be too small to exert any significant influence on the polymer optical properties. The molecular weights of the copolymers were determined by GPC, which showed major peaks at Mn = 2300–2400, together with small shoulders or minor peaks (Fig. S-6†). The TG analysis of DTG2-CzS in nitrogen indicated the lower Td5 values (240–290 °C) than those of DTG2-MeS (Fig. S-4†). This may be due to evaporation of THF and lower thermal stability of the DTG2-CzS framework, reflecting the existence of a larger number of organic bonds. The UV absorption spectra of DTG2CzS showed bands ascribed to both the DTG and carbazole chromophore (Fig. S-7†). The extinction coefficients of the polymers revealed no clear relationship with the DTG incorporation ratio (x), likely due to the partial insolubility of the polymers. These polysilsesquioxanes could be spin-coated to give solid films. Similarly to DTG2-MeS, PL efficiencies of the resulting polymers (DTG2-CzS-x) increased as the DTG2/CzS loading mol% (x) decreased. Interestingly, as illustrated in Fig. 4, DTG2-CzS exhibits different PL properties depending on the state; i.e., in solution or film. PL bands ascribed to the carbazole emission appeared as the major peak together with a shoulder that was likely due to the DTG emission in THF, whereas films of DTG2-CzS showed only emission from the DTG unit, indicating facile photo-energy transfer from carbazole to DTG in the film. As polysilsesquioxane films of CzS have been proven to possess semi-conducting properties,17 these results suggest the potential application of the DTG2-CzS films as OLED active materials, in which energy transfer from the exciton generated by carrier conduction through the condensed carbazole units to DTG would cause blue emission. To elucidate the
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Fig. 5 EL spectra of devices I, II, and III, based on DTG2-CzS-1, CzS, and DTG2.
spin-coated film of DTG2-CzS as an EL material, we carried out preliminary experiments. An OLED was fabricated with the structure of ITO/PEDOT:PSS/DTG2-CzS-1/BCP/Al (device I), in which the spin-coated film of PEDOT:PSS ( poly(3,4ethylenedioxythiophene):poly(styrene sulfonate) and the vapordeposited film of BCP (2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline) were the hole- and electron-injecting layers, respectively. The EL spectrum of the device shows a broad band at λmax = 540 nm, which appears at a lower energy than the PL band of the DTG2-CzS-1 film (Fig. 5). Furthermore, the spectral profile is comparable to that of a similar device prepared with a spin-coated layer of polysilsesquioxane CzS-PSQ11 which had been prepared by the hydrolysis/condensation of CzS (ITO/ PEDOT:PSS/CzS-PSQ/BCP/Al, device II). This indicates that the emission from device I is likely ascribed to the interaction between carbazole and BCP units at the interface. To enhance the hole-transporting properties of DTG2-CzS, we prepared a composite film of DTG2-CzS-1 and poly(vinylcarbazole) (PVK) in a weight ratio of 1 : 1, and applied its spin-coated film onto an OLED (ITO/PEDOT:PSS/CzS-PSQ + PVK/BCP/Al, device III). As expected, device III gave an emission band approximately at λmax = 430 nm, similar to that of DTG2-CzS-1 in the film, although a broad emission band centered at approximately 640 nm could still be seen. However, the OLED efficiency was low (maximal luminance = 0.22 cd m−2) and the further studies to optimize the device structure seem to be required.
Conclusions
Fig. 4 film.
PL spectra of DTG2 and CzS in THF, and DTG2-CzS-1 in THF and
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In summary, we prepared DTG-containing oligo- and polysilsesquioxanes by hydrolysis/condensation of DTG1 and DTG2. Polyhedral oligosilsesquioxane DTG1-POSS showed good PL properties, reflecting the structural characteristics of the POSS core. On the other hand, polysilsesquioxanes based on DTG2 were found to be potentially useful as PL and EL materials with good thermal stability and processability.20 The emissive properties are tunable by copolymerization with other alkoxy-
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silanes as well as the composite formation, providing the desired functionalities. 10
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Acknowledgements This work was supported by a Grant-in-Aid for Scientific Research on Innovative Areas “New Polymeric Materials Based on Element–Blocks (no. 2401)” from The Ministry of Education, Culture, Sports, Science, and Technology, Japan. 11
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