Dibenzosiloles

Title Dibenzosilol Polymers, Their Preparation and Uses... 

Research Focus Preparation of dibenzosilole copolymers as light-emitting devices with enhanced oxidative degradalive resistance. 

Originality Solvent-soluble dibenzosiloles having enhanced oxidative resistance are... 

Observation A novel class of solvent-soluble conjugated dibenzosilole copolymers have... 

The reaction of MeLi and n-BuLi with various dibenzosiloles produces 1,1-dialkyl-dibenzosiloles in quantitative yields, while PhLi, and particularly t-BuLi, are less reactive95 (equation 32). 

The optical absorption spectra of dibenzosilole-based polymers 618-620 have absorption maxima that are blue shifted compared to those of dithienylsilole 615-617. The absorption maximum for homopolymer 618 occurs at 377 nm, corresponding to the largest n—n transition energy in the silole-based polymer series. 

Polymers 619 and 620 exhibit two absorption maxima located at 444 and 471 nm for 619 and 473 and 503 nm for 620. These transitions likely correspond to predominantly local transitions of the constituent dibenzosilole and mono/bithiophene copolymer building blocks. The absorption maxima of polymer 618-620 cast as thin films are 401 nm (618), 484 nm (619), and 493 nm (620). The fluorine-based copolymers exhibit maxima at 427 nm in solution and 440 nm as a thin film for 621 and 456 nm in solution (with strong shoulder at 502 nm) and 460 nm as a thin film for 622. Dibenzosilole-based copolymers 619 and 620 exhibit significant bathochromic shifts of ca. 30-50 nm compared to the fluorine-based polymers 621 and 622. 

When tetraethoxy-l,2-dimethyldisilane is treated with tetramethylene-dimagnesium dibromide in tetrahydrofuran, bis(l-methyl-1-silacyclo-pentyl) (XV), and not its isomeric bicyclodisilane (XVI), is formed in 62% yield (110). Analogously, the reaction with 2,2 -dilithiodiphenyl produces 5,5 -dimethyl-5,5 -bis(dibenzosilole) (127). 

Polydibenzosiloles are wide band gap SCPs. Poly(3,6-dibenzosilole)s 5 (Fig. 3) show absorptions peaks (Aab) at 283 nm and absorption edges at 310 nm in solutions as well as in thin films.22 The calculated optical band gaps of poly(3,6-dibenzosilole)s are 4.0 eV, which is the widest band gap so far... 

Because they have the largest band gap among SCPs, poly(3,6-dibenzosilole) s 5 are expected to show fluorescence at the shortest wavelength. Under... 

Highly efficient green photoluminescence has also been realized from SCPs. Copolymers 11 (Fig. 5) derived from 2,7-fluorene and 2,3,4,5-tetraphenylsilole show absolute PL quantum yields up to 84%.28 A well-defined alternating copolymer 12 with a repeating unit made up of ter-(2,7-fluorene) and 2,5-silole possesses an absolute PL quantum yield >80%.29 SCPs 13 with a main chain structure of 3,6-carbazole-2,7-fluorene-2,5-silole also show absolute PL quantum yields up to 86%.30 An energy transfer copolymer 14 of 2,7-dibenzosilole and... 

Copolymers 15 (Fig. 6) derived from 2,7-fluorene and 2,5-dithienylsilole show red fluorescence via an energy transfer process.31 The APl could be 591 nm for copolymers with higher contents of 2,5-dithienylsilole. The absolute PL quantum yields (<30%) of the copolymers are somewhat lower than the green fluorescent SCPs. A copolymer 16 derived from 2,7-dibenzosilole and 4,7-dithienyl-2,l,3-benzothiadiazole show a better red fluorescence.26 The APL of the copolymer is at 629 nm, with an absolute PL quantum yield of 53%. 

Due to the large band gap and high triplet energy level of the poly(3, 6-dibenzosilole) 5, the copolymer is an excellent host for the fabrication of blue polymer phosphorescent light-emitting diodes. A high external quantum efficiency (t/el) of 4.8% and a luminance efficiency of 7.2 cd/A at 644 cd/m2 have been achieved for blue phosphorescence devices (emission peak (AEL) at 462 nm, CIE coordinates x = 0.15,y = 0.26). The performances of the devices are much better than those reported for blue phosphorescent devices with poly(A--viny 1 cabarzo 1 e) (PVK) as the host.32... 

Copolymers 10 derived from 3,6-dibenzosilole and 2,7-fluorene are blue electroluminescent SCPs.27 When the copolymers are used as the emissive layer in EL devices, highly efficient pure blue emissions with CIE coordinates of (x = 0.16, y = 0.07), a 7EL of 3.34%, and a luminance efficiency of 2.02 cd/A at 326 cd/m2 are achieved from the copolymer with 90% fluorene content. The blue color matches the NTSC blue standard (x = 0.14, y = 0.08) quite well. The EL spectral stability of the devices is quite good, even under operation at elevated temperatures. Copolymer 9 derived from 3,6- and 2,7-dibenzosiloles also exhibits high performance with a jyEE of 1.95%, a luminous efficiency of 1.69 cd/A, and a maximal brightness of 6000 cd/m2, with the CIE coordinates of (x = 0.162, y = 0.084).26... 

Green electroluminescence is also achieved from the well-defined alternating copolymer 12 with a repeating unit made up of ter-(2,7-fluorene) and 2, 5-silole.29 With its neat film as the emissive layer, the EL device shows a maximum //Kr of 0.47%, but the device performance can be largely improved to a maximum //Ki. of 1.99% when using a copolymer/PF8 blend film as the emissive layer. Copolymer 14 derived from 2,7-dibenzosilole and 2,1,3-benzothiadiazole is also an excellent green EL polymer.26 A maximum 7el of 3.81% can be realized in EL devices. 

Alternating copolymer 20 derived from 2,7-dibenzosilole and 4,7-dithienyl-2,1,3-benzothiadiazole is an outstanding polymeric electron donor in photovoltaic cells.37 With an active layer made up of copolymer to PCBM in a 1 2 ratio, the solar cell displays a high short-circuit current of 9.5 mA/cm2, an open-circuit voltage of 0.9 V, and a fill factor of 50.7%, under illumination of an AM 1.5 solar simulator at 80 mW/cm2. The calculated energy conversion efficiency is 5.4%, which is one of the highest efficiencies so far reported for polymeric photovoltaic cells. 

The reaction of the arylsilane 126 with MeLi gives the pentaorganosilyl anion 127 which can be further converted to the dibenzosilole 128 <1996AGE1127>. 

Benzosilole anions have been obtained in three ways, as summarized in Scheme 16. 5-Lithio-5-methyldibenzosilole is thus obtained from bis(di-benzosilole) with lithium in THF, which affords 5,5-dimethyldibenzosilole (54% yield) after trapping by dimethyl sulfate (66). The same species is also formed from 5-methyl-5-(trimethylsilyl)dibenzosilole via the Si-Si bond cleavage on treatment with PhjMeSiLi in THF at -78°C, together with tetramethyldiphenyldisilane (67). The potassium analog can be prepared from the I-hydridobenzosilole by treatment with strong nonnucleo-philic bases such as KH (14). 

The H NMR spectrum of the lithium species reveals a sharp resonance at 8 0.24 ppm due to the CH3-S1 protons. The UV spectrum of the anion in THF exhibits characteristic absorptions in the longer-wavelength region at 377 nm (e 5700) and 546 nm (e 1200), in comparison with the neutral dibenzosilole, which has several absorption maxima up to 318 nm (e 170). 

Abstract Poly(dibenzosilole)s are an emerging class of polymers with similar optoelectronic properties to polyfluorenes. With increased stability towards oxidation, several poly(dibenzosilole)-based devices, such as light emitting diodes, have shown improved performance over their polyfluorene counterparts. As a consequence of reduced conjugation in the polymer chain, some poly(dibenzosilole)s have high triplet excited state energies, which make them suitable hosts for blue triplet emitters in electrophosphores-cent devices. 

Keywords Conjugated polymer Dibenzosilole Light emitting diode ... 

Fig. 1 Structural illustrations of fluorene 1, carbazole 2, dibenzophosphole oxide 3, diben-zothiophene dioxide 4 and dibenzosilole 5 with ring numbering...

Scheme 1 Synthesis of poly(2,7-dibenzosilole) [23]. Reagents and conditions (a) Cu, DMF, 125 °C, 88% (b) Sn, HCl, EtOH, 110 °C (bath temp), 72% (c) nitrosylsulfuric acid, concentrated H2SO4, 0°C, then aq. KI, - 10 to 50 °C, 30% (d) f-BuLi (4 equiv), THF, -90 to -78°C, then dichlorodihexylsilane or dichlorodioctylsilane, 25 °C, 52% (e) t-BuLi, diethyl ether, - 78 °C, then 2-isopropoxy-4,4/,5,5/-tetramethyl-l,3,2-dioxaboralane, 25 °C,...

Scheme 2 Synthesis of poly(2,7-fluorene-co-2,7-dibenzosilole) [36]. Reagents and conditions (a) NaN02, CuBr, HBr (40%), 0°C (b) I2, KI03, H2S04, AcOH, 90 °C (c) 2 equiv n-BuLi, 2 equiv dichlorodimethylsilane, THF, -100°C (d) Pd(PPh3)4 (2 mol %), toluene/2 M K2C03, 90 °C...

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