Introduction

Polyhedral oligomeric silsesquioxanes (POSS) unit is a cube-shaped nanoparticle in which the rigid silica-like inorganic core can be attached with eight organic functional groups at its silicon vertices [1, 2]. POSS molecules are regarded as organic–inorganic hybrid materials at a molecular level. They are used for the development of high performance materials in medical, aerospace, microelectronics and so on [3–7]. POSS molecules can be easily synthesized to be monofunctional or multifunctional materials for commercial applications by grafting with organic functional groups. Moreover, POSS compounds may possess a high degree of compatibility in blended polymers and can easily be covalently linked into a polymer backbone [4, 8, 9]. The incorporation of POSS derivatives into polymeric materials produces nanocomposites with improved properties such as increases in glass transition temperature, thermal stability, mechanical strength, oxidation resistance and ease of processing.

The organic–inorganic hybrid materials based on POSS have attracted great interest in the last decade due to their potential applications in optical and electronic devices [3, 6, 9]. The incorporation of POSS into organic light emitting materials has led to significant improvements in the performance of organic light emitting devises (OLEDs). Since the first use of POSS as central cores for solution processed hybrid hole transport materials in OLEDs that enhanced brightness and efficiencies as compared to the devices that used traditional hole transport materials [10], the POSS functionalized light emitting materials were widely used in OLEDs. The Heeger group pioneered the use of POSS-based light emitting polymers in which POSS cores as end-cappers were attached at the chain ends of poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) and poly(9,9ʹ-dioctylfluorene) [5]. The electroluminescent device made from the POSS-capped MEH-PPV polymer was found to exhibit higher brightness (1320 cd/m2 at 3.5 V) and external quantum efficiency (EQE) (ηext = 2.2 %) than the POSS-free MEH-PPV (230 cd/m2 at 3.5 V and ηext = 1.5 %). The enhanced device properties may be attributed to reduced aggregation caused by the bulky POSS end groups and improved adhesion of the polymer to the ITO coated glass substrate. Thereafter, some research groups reported the photoluminescence and electroluminescence properties of POSS-functionalized polyfluorene materials in which POSS units were appended to the polyfluorene backbone using an alkoxy spacer [11–13]. These POSS-functionalized polyfluorene materials showed higher fluorescence quantum yields and better thermal stability than the corresponding pristine polymers because the inorganic POSS units strongly suppressed intermolecular aggregation and/or thermal oxidation and crosslinking. Additionally, the light-emitting devices fabricated with the POSS-functionalized polyfluorene materials showed much improved brightness, maximum luminescence intensity, and quantum efficiency in comparison with devices fabricated with the corresponding pristine polymers, and showed a purer blue or deep blue emission in electroluminescent devices by preventing interchain interaction and fluorenone formation. Recently, research on phosphorescent emitters and efficient light-emitting devices based on POSS materials have also attracted tremendous interest [14–16]. Very efficient white emitting OLEDs have been prepared from POSS-based blue, green, and red emitting materials. EQEs of 8.0 %, power efficiencies of 8.1 lm/W, and CIE coordinates of (0.36, 0.39) at 1000 cd/m2 offer promise for use of solution processable POSS materials for application in white lighting.

Carbazole derivatives have high thermal stability and excellent photophysical property, they are effective hole-transporting materials and have great potential applications in many fields. Thus, the design and synthesis of new carbazole derivatives with high performance constitutes a very active area of research [17, 18]. In this work, we reported the synthesis and characterization of a new inorganic–organic hybrid material based on POSS capped with carbazolyl substituents, octakis[3-(carbazol-9-yl)propyldimethylsiloxy]-silsesquioxane (POSS-8Cz). The X-ray crystal structure, photophysical properties and thermal stablity of POSS-8Cz were investigated detailedly.

The synthetic route of POSS-8Cz was shown in Scheme 1.

Scheme 1
scheme 1

The synthetic route of POSS-8Cz

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Experimental

Materials and methods

Carbazole and allyl bromide were bought from Alfa Aesar. Platinum complex (platinum-1,3-divinyl-1,1,3,3-tetramethyldisiloxane, Pt-dvs, 2 wt.% Pt in xylene) were purchased from Aldrich, USA. Octakis(dimethylsiloxy)silsesquioxane (Q8M8 H) containing eight hydro-silane groups was purchased from the Hybrid Plastics Co., USA. Toluene was dried by distillation before use in the hydrosilylation reaction. All other chemicals were analytical grade reagent.

1H NMR spectra were obtained on Unity Varian-500 MHz. C, H, and N analyses were obtained using an Elemental Vario-EL automatic elemental analysis instrument. UV–vis absorption and photoluminescent spectra were recorded on a Shimadzu UV-2550 spectrometer and on a Perkin-Elmer LS-55 spectrometer, respectively. Melting points were measured by using an X-4 microscopic melting point apparatus made in Beijing Taike Instrument Limited Company, and the thermometer was uncorrected.

Synthesis and characterization of octakis[3-(carbazol-9-yl)propyldimethylsiloxy]-silsesquioxane (POSS-8Cz)

9-allyl-9H-carbazole (allyl-Cz)

A mixture of carbazole (10.0 g, 0.06 mol), potassium hydroxide (8.98 g, 0.16 mol) and acetone (100 mL) was stirred vigorously for 2 h at room temperature. Allyl bromide (6.6 mL, 0.072 mol) was added dropwise. After stirring for 20 h at room temperature, the reaction mixture was poured into cold water (300 mL). The mixture was extracted with CH2Cl2 (3 × 200 mL), and the combined organic phase was dried over anhydrous Na2SO4. After filtering, the filtrate was evaporated to dryness under reduced pressure. The crude was purified by chromatography on silica gel using petroleum ether as the eluent to give allyl-Cz as a white solid (9.1 g, 73.4 %). m.p.: 51–53 °C. 1H NMR (400 MHz, CDCl3, δ, ppm): 8.11 (d, J = 8.0 Hz, 2H), 7.46 (t, J = 8.8 Hz, 2H), 7.37 (d, J = 8.0 Hz, 2H), 7.25 (t, J = 8.0 Hz, 2H), 6.03–5.94 (m, 1H), 5.15 (d, J = 10.6 Hz, 1H), 5.05 (d, J = 18.3 Hz, 1H), 4.91 (d, J = 4.8 Hz, 2H). Anal. Calcd for C15H13N: C, 86.92; H, 6.32; N, 6.76; Found: C, 87.02; H, 6.29; N, 6.71.

POSS-8Cz

Q8M8 H (200 mg, 0.196 mmol) and 9-allyl-9H-carbazole (400 mg, 1.93 mmol) were placed in a round bottom flask (100 mL). The mixture was degassed for 10 min and protected with argon, then 30 mL anhydrous toluene was added. The mixture was stirred for 15 min at room temperature, then platinum-1,3-divinyltetramethyldisiloxane (Pt-dvs) (0.02 mL) was added and the reaction mixture was stirred at room temperature for 24 h under an argon atmosphere. After the reaction was complete, the solvent was evaporated off. The crude product was purified by chromatography on silica gel using ethyl acetate/petroleum ether (1:20, v/v) as the eluent to give white solid (0.45 g, 84.7 %). m.p.: 118–120 °C. 1H NMR (400 MHz, CDCl3, δ, ppm): 8.06 (d, J = 7.6 Hz, 16H, Ar-H), 7.39 (t, J = 8.0 Hz, 16H, Ar-H), 7.28 (t, J = 7.4 Hz, 16H, Ar-H), 7.18 (t, J = 7.2 Hz, 16H, Ar-H), 4.10 (t, J = 7.2 Hz, 16H, Cz-CH2-), 1.83–1.76 (m, 16H, −CH2-), 0.55 (t, J = 8.4 Hz, 16H, Si-CH2-), 0.02 (s, 48H, Si-CH3).

Crystallography

The diffraction data were collected with a Bruker Smart Apex CCD area detector with graphite-monochromatized Mo-Kα radiation (λ = 0.71073 Å) at 298(2) K. The structure was solved by using the program SHELXL and Fourier difference techniques, and refined by full-matrix least-squares method on F 2. All hydrogen atoms were added theoretically.

Results and discussion

Synthesis and characterization of POSS-8Cz

POSS-8Cz was synthesized through the hydrosilylation reaction of 9-allylcarbazole with octakis(dimethylsiloxy)silsesquioxane (Q8M8 H) in the presence of platinum(0)-1,3-divinyl-1,1,3,3-tetramethyldisiloxane (Pt-dvs) as the catalyst, as shown in Scheme 1. POSS-8Cz was soluble in common organic solvents, such as toluene, THF, chloroform, dichloromethane, and could be purified by ordinary methods. POSS-8Cz was characterized using X-ray crystallography, 1H NMR, elemental analysis and X-ray diffraction (XRD) analysis.

Suitable crystal of POSS-8Cz was obtained by the evaporation of its mixtured ethyl acetate and petroleum ether solution. The crystallographic data of POSS-8Cz is shown in Table 1.

Table 1 Crystallographic and experimental data for POSS-8Cz
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The crystal structure of POSS-8Cz is given in Fig. 1. The crystal of POSS-8Cz belongs to the monoclinic space group C2/c, a = 22.5956(18) Å, b = 24.7175(19) Å, c = 27.098(2) Å, α = γ = 90°, β = 104.007(2)o, U = 14684(2) Å3, Z = 4, Dc = 1.211 g/cm3, μ = 0.202 mm−1. In this structure, the arrangement of eight carbazole groups are not identical to each other, and all eight carbazole groups do not interact with their nearest neighbors. Two carbazole groups placed on opposite Si atoms of the Si-O cage show trans arrangement, and two planes of carbazole groups are parallel to each other. In the eight carbazole groups, it is notable that two carbazole groups placed on opposite Si atoms (Si1 and Si1ʹ) of the Si-O cage actually fold toward the Si-O cage and the POSS core is sandwiched between the planes of two carbazole groups, and the other six carbazole groups extend away from the Si-O cage.

Fig. 1
figure 1

Crystal structure of POSS-8Cz

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Figure 2 shows 1H NMR spectra of POSS-8Cz, allyl-Cz, and Q8M8 H. The resonance of Si-H protons in Q8M8 H is located at 4.7 ppm (Fig. 2c). As shown in Fig. 2b, for allyl-Cz, two doublets (with fine coupling) and the quartet resonance proton resonance peaks from the allyl group in allyl-Cz are located at 5.03, 5.15 and 5.99 ppm with a relative molar ratio of 1:1:1, corresponding to iso, trans- and substituted vinyl protons. The resonance of methylene (−CH2-Cz) is observed at 4.90 ppm. In 1H NMR spectrum of POSS-8Cz (Fig. 2a), the peaks for the vinyl group of allyl-Cz (5.03, 5.15 and 5.99 ppm) in Fig. 2b and Si-H protons (4.7 ppm) in Fig. 2c disappeared, supporting the complete hydrosilylation reaction. In addition, the resonance of methylene next to the carbazole ring is shifted from 4.90 to 4.10 ppm, and at the same time the 1H NMR spectrum of POSS-8Cz displays two resonance of methylene at 1.83 and 0.55 ppm, respectively.

Fig. 2
figure 2

1H NMR spectra of (a) POSS-8Cz (b) Cz-allyl and (c) Q8M8 H in CDCl3

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Figure 3 presents the X-ray diffraction (XRD) pattern of POSS-8Cz. POSS-8Cz exhibits several sharp XRD peaks at 5.84°, 6.76°, 8.11°, 9.87°, 11.71°, 15.97° , 17.01°, 18.30° and 20.04°, and other positions also appeared lots of weak diffraction peaks, suggesting that the synthesized POSS-8Cz presents a crystal morphology.

Fig. 3
figure 3

X-ray diffraction pattern of POSS-8Cz

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Thermal property of POSS-8Cz

The thermogravimetry (TG) and differental thermal analysis (DTA) measurements were performed in flowing drying nitrogen atmosphere at the heating rate of 10 °C/min. The result of TG and DTA measurements of POSS-8Cz is shown in Fig. 4. From Fig. 4, it can be seen that the DTA curve shows an endothermic peak at 119 °C, but without mass loss in the TG curve, indicating that the temperature is the melting point of the compound, which is in agreement with the result obtained by a melting point apparatus. With increasing temperature, POSS-8Cz shows good thermal stability until about 450 °C. The compound begins to decompose at 453 °C, at 512 °C an endothermic peak was observed in the DTA curve, which was accompanied by a sharp weight loss in the TG curve. It shows that the compound undergoes decomposition.

Fig. 4
figure 4

The therogravimetry (TG) and differential thermal analysis (DTA) of POSS-8Cz

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UV–vis absorption and photoluminescence spectra

The UV–vis absorption and photoluminescence spectra of POSS-8Cz and allyl-Cz were measured in diluted dichloromethane solutions, as shown in Fig. 5. Allyl-Cz exhibits two intense absorption bands at 332 and 347 nm, respectively, which can be attributed to the π-π* transitions of carbazole. The absorption shape of POSS-Cz is quite similar to that of allyl-Cz, but two intense absorption bands were blue-shifted by about 4 nm, which located at 328 and 343 nm. The photoluminescence spectrum of POSS-8Cz shows two peaks at 351 and 368 nm, while the photoluminescence spectrum of allyl-Cz presents two peaks at 365 and 389 nm. Compared with allyl-Cz, the hypsochromic shift of UV–vis absorption and photoluminescence spectra of POSS-8Cz occurs. The result indicated that each carbazole unit seems to be isolated by the rigid POSS core in the case of POSS-8Cz, so that the formation of excimer is prevented.

Fig. 5
figure 5

Normalized UV–vis absorption and photoluminescence spectra of POSS-8Cz and allyl-Cz in dichloromethane solutions. (C = 1 × 10−6 mol/L)

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Figure 6 illustrates photoluminescence spectra of POSS-8Cz and allyl-Cz in solid state. The photoluminescence spectrum of POSS-8Cz shows one strong emission peaks at 370 nm, while that of allyl-Cz exhibits a broad and weak emission peak at 429 nm. Due to more intermolecular interaction (π-π stacking) in the solid state, the photoluminescence spectrum of allyl-Cz is much broader than that in dichloromethane solution, and the emission peak is red-shifted to 429 nm, suggesting the formation of excimers by aggregation. For POSS-Cz, the emission shape in the solid state is different from that in dichloromethane solution, but the spectral shift is very small. It was indicated that the shape of POSS-8Cz does not strongly affect to the aggregation of carbazole moiety. This result suggests that the carbazole in POSS-8Cz is almost isolated even in the solid state, different from allyl-Cz. POSS-8Cz has eight propyl-siloxy spacers between the POSS core and carbazole groups, but the spacer length seems too short to form aggregates, and showed monomeric behavior in photoluminescence.

Fig. 6
figure 6

Photoluminescence spectra of POSS-8Cz and allyl-Cz in the solid state

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From the above results, it was found that the material POSS-8Cz has not only hight thermal stabily, but also should be a promising material for application in a ultraviolet OLEDs[19]. Further investigation on the ultraviolet OLEDs of POSS-8Cz is progressing at our laboratory.

Conclusions

A new inorganic–organic hybrid material of polyhedral oligomeric silsesquioxane containing carbazole, octakis[3-(carbazol-9-yl)propyldimethylsiloxy]-silsesquioxane (POSS-8Cz), was successfully synthesized by hydrosilylation reaction in the presence of platinum(0)-1,3-divinyl-1,1,3,3-tetramethyldisiloxane (Pt-dvs) as the catalyst. The structure of POSS-8Cz was characterized by single crystal X-ray crystallography and 1H NMR. The compound has hight thermal stabily. The result showed that the presence of the POSS core does not affect the electronic properties because each carbazole could be effectively isolated by the steric and electronic effects.