A high-emissive and stable luminescent silafluorene hybrid constructed by hydrosilylation

Introduction

Silicon-containing π-conjugated compounds are attractive and interesting molecules due to their excellent photo-electrical properties and thermal stability.^1–5 As the most typical representatives, silacyclopentadienes (siloles) have gained much attention as a new class of optoelectro-active materials with aggregation-induced emission (AIE) and/or good electron transport properties in organo-electronics.^6–15 Siloles have a relatively low-lying lowest unoccupied molecular orbital (LUMO) level due to the π*–σ* conjugation between the σ* orbital of the exocyclic Si–C bond and the π*orbital of the butadiene fragment, resulting in a high electron affinity. Moreover, silafluorene-containing compounds,^16–20 which are the analogue of fluorene, integrate the silacyclopentadiene character into the structure of fluorene; the introduction of a silicon atom in the 9-position of fluorene could retain the fluorescent properties of fluorene, such as the strong blue emission. The combination of the electrical effect of the silicon element with a silicon-bridge structure could tune the opto-electrical properties, and even improve the quantum yield of fluorescence and the thermal stability. Silafluorene derivatives are widely applied in the field of OLED, PSC, OFET and so on,^21–23 by using their unique electronic structure and photoelectric properties. However, the tendency for crystallization, the solid fluorescent yield and thermal properties are also important issues in developing and exploiting silafluorene-containing compounds.

Polyhedral oligomeric silsesquioxane (POSS) possesses a cage-like nanometer-sized structure and demonstrates high thermal stability and facile chemical modification.^24–27 In particular, the highly symmetrical and topologically ideal cube octasilsesquioxae (T₈-POSS) with a cage size of approximately 0.5–0.7 nm has drawn intensive attention as a constructive unit for fabrication of various functional organic–inorganic hybrid materials. Design and preparation of new hybrid optoelectronic materials through chromophore units covalently bounded to POSS has been an active research topic. Many POSS-based fluorescent materials have been developed by combination of POSS and various chromophores in different modes.^28–33 For example, attached mono-functionalized POSS as pendant groups onto conjugated polymers ²⁸ incorporated multi-functionalized POSS as a core in conjugated dendrimers ²⁹ and hyperbranched polymers. ³⁰ We have also developed the POSS-based organic–inorganic hybrid AIEgens. ³⁴ , ³⁵ They exhibit outstanding thermal stability, improved quantum efficiencies and excellent fluorescent detection of explosives and metal ions compared with their parent molecules without the POSS unit.

There are reports about silafluorene and tetraphenylsilole groups constructed in the oligo-siloxane chains, which show interesting photophysical properties. ³⁶ , ³⁷ In this paper, under Karstedt’s catalyst, an organic–inorganic hybrid fluorescent nano-molecular dendrimer has been synthesized by grafting silafluorene units onto the octavinylsilsesquioxane (octavinyl-POSS) core through hydrosilylation reaction. The new hybrid molecule exhibits high solid fluorescence quantum efficiency because of the nano-size and large steric hindrance of the POSS core. Thermogravimetric analysis (TGA) shows that the thermal stability of the target compound can be effectively improved by the existence of POSS. The incorporation of POSS and fluorescent units may provide a new method or route to improving the fluorescent quantum efficiency and thermal stability of the organic fluorescent molecules.

Results and discussion

Synthesis and characterization

As shown in Scheme 1, octavinylsilsesquioxane (1) was chosen as synthetic scaffold and 5-methyl-5H-dibenzo[b,d]silole (2) was chosen as chromophore. The target organic–inorganic materials were synthesized by hydrosilylation reaction in presence of Karstedt’s catalyst. First, the synthesis of compounds 1 and 2 was carried out according to the literature;^38–40 FT-IR, NMR and MALDI-TOF data confirmed that they were successfully synthesized. Using 1 and 2 as starting materials, synthesis of the target compound (3) was attempted under catalysis at room temperature in toluene, but no target was detected. However, upon raising the reaction temperature to 60 °C, 3 was obtained in 27% yield. The by-products were the mixed adducts composed of five, six or seven 5-methyl-5H-dibenzo[b,d]siloles bounded to POSS core. When the reaction was processed in refluxing toluene, the target compound was obtained in 92% yield, which means that the reaction is severely affected by the reaction temperature. The solvent tetrahydrofuran (THF) was also used under reflux and 3 was obtained in 98% yield. Thus, choice of solvent also affects the hydrosilylation reaction.

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Scheme 1.

The synthetic route to the target compound 3.

The hydrosilylation reaction results were confirmed by the nuclear magnetic resonance (NMR) analyses. Figure 1 shows the ¹H spectra of 3, POSS (1) and monomer 2. The ¹H spectrum of 1 shows that the Si–vinyl bond (6.21–5.84 ppm) is present (Figure 1(a)). The ¹H spectrum of 2 shows that the Si–H bond (4.92–4.83 ppm) and Si–CH₃ (0.56–0.50 ppm) are present (Figure 1(b)). The newly formed methylene (CH₂CH₂Si-POSS) groups were observed about 0.76–0.60 ppm (position 1) and 0.44–0.28 ppm (position 2) in the ¹H spectrum. In the NMR spectrum of Figure 1(c), the appearance of a new broad peak indicates the successful incorporation of 5-methyl-5H-dibenzo[b,d]silole into the POSS core because the two broad peaks are characteristic of the new methylene groups in the target compound. Moreover, the ¹³C NMR spectrum of POSS and 3 shows peaks for (CH₂CH₂Si(CH₃)) at 5.3 ppm and (CH₂CH₂Si-POSS) at 4.4 ppm (Figure 2(b)), providing further evidence for the hydrosilylation reaction. Compound 3 exhibits good solubility in common organic solvents, such as dichloromethane, THF and toluene.

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Figure 1.

¹ H NMR spectra of (a) POSS 1, (b) diphenylsilole 2 and (c) target compound 3.

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Figure 2.

¹³ C NMR spectra of (a) POSS 1 and (b) the target compound 3.

Thermal properties

The thermal stability of 2 and 3 was examined by TGA under N₂ and air. The temperatures at 5% weight loss (T_d) of diphenylsilole and 3 were 126 °C and 488 °C in N₂ (Figure 3(a)). T_d values of 3 were improved by 362 °C in N₂, when compared to 2. Similar thermal behaviour was shown in air; the temperatures at 5% and 10% weight loss (T_d) of 3 were about 420 °C and 490 °C (Figure 3(b)). All of these results indicated that the inorganic POSS cores could efficiently enhance the thermal stability of 3.

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Figure 3.

TGA curves of compound 2 and the target compound 3 under different conditions at a heating rate of 10 °C min⁻¹ (a) in N₂ and (b) in air.

Photophysical properties

The UV-vis absorption spectra of 3 as well as its monomer 5-methyl-5H-dibenzo[b,d]silole in THF solution are shown in Figure 4. The maximum absorption peak is about 277 nm together with a shoulder at 288 nm, ascribed to the π–π* transition of the silafluorene ring. The maximum absorption peaks of 3 coincide with the maximum absorption peaks of the corresponding small molecules untethered to the POSS core, which means the POSS has no effects in the absorption spectrum.

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Figure 4.

UV-vis spectra of compound 2 and the target compound 3 in THF solution.

The photoluminescence (PL) spectra of 3 and 2 in THF are given in Figure 5. They emit near-UV light with the emission 344 nm in THF solution. The PL of the target POSS hybrid fluorescent material as a thin film in the solid state is also about 344 nm.

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Figure 5.

PL spectra of compound 2 and the target compound 3 in THF solution.

To have a quantitative comparison, we measured the fluorescence quantum yields of the molecule and hybrid dendrimer in solution and solid state. The nano-size POSS core can improve the fluorescence quantum yield (Φ_F) of chromophore both in solution and in the solid state. Although the Φ_F values of POSS in THF are low, the Φ_F value of the 2 is about 8.3% in THF solution. The Φ_F value of 3 is about 14.5% in THF solution. The target compound 3 emits more efficiently in the solid state. The absolute Φ_F values of the thin solid films of 2 and POSS hybrid compound 3 are about 14.3% and 34.1%, respectively, which are higher than their corresponding parent 5-methyl-5H-dibenzo[b,d]silole molecules in the solid state. The introduction of POSS enhances the fluorescent intensity and quantum yield without changing in the absorption and emissive wavelengths of the chromophore.

Conclusion

The organic–inorganic hybrid fluorescent nano-molecular dendrimer (3) has been synthesized by grafting 5-methyl-5H-dibenzo[b,d]silole (2) onto a POSS core through hydrosilylation reaction. The new hybrid molecule exhibits high solid fluorescence quantum efficiency compared with 2. TGA shows that the thermal stability of 2 can be effectively improved by the existence of POSS. Since a high-emissive and stable POSS-based luminescent hybrid was successfully obtained, the introduction of POSS into fluorescent molecules may provide a general strategy in improving luminescent intensity and efficiency and thermal stability of such molecules.

Experimental section