Silole polymers

Yamaguchi and coworkers [709] have prepared luminescent silole polymers 615a-e. The blue emission of the homopolymers 615a,b can be shifted into the red region by copolymerization with other conjugated units (but for the price of lowered PLQY). Although no device studies have been reported yet, excellent electron-transport properties are expected from such materials [710]. 

Scheme 12 Synthetic route to silole-acetylene polymers...

Table 3 lists PAEs containing Si, Fe, and B. Silole containing Si in the five-mem-bered ring [157] has a low transition energy because of its low LUMO level [158], and its homopolymers have been prepared [159, 160]. 2,5-Dili-thiosiloles, which can be prepared by the ring-closing reaction of diethynyl-silane, serve as starting materials for various 2,5-disubstituted siloles such as 2,5-dibromosiloles and 2,5-di(trialkylstannyl)siloles [161,162]. PAEs with the silole ring have been reported as shown in Nos. 1-4 in Table 3. PAE-type polymers with Si atoms in the main chain have also been prepared (Nos. 12-16), and their optical properties including photoconductivity have been revealed [155,156].

The examples of polyacetylenes whose main chain is directly bonded to heteroaromatic rings (e.g., silole, carbazole, imidazole, tetrathiafulvalene, ferrocene) are increasing in number. Such polymers are usually obtained by one of catalysts (W, Mo, and Rh). The formed polymers are expected to display interesting (opto)electronic properties such as electrochromism, cyclic voltammetry, electroluminescence, and so on. 

D. Polymers Involving only the Silicon Atom of the Silole Ring in the... 

Depending on the alkynes, the yields are variable, but they are very high in the case of disubstituted symmetrical alkynes (R = R = Me, Et, -Bu). The reaction is sometimes not specific, giving also a 1,4-disilacyclohexa-2,5-diene. It was recently used by Ishikawa and coworkers to obtain a 3,4-diethynylsilole (29) (Scheme 8) which is a precursor of polymers containing silole rings43 (see Section IV.C). 

In contrast, the electrical conductivity tends to become higher with a lower silole content, reaching up to 2.4 S cm-1 upon doping with iodine. The bulky (t-butyldimethylsiloxy)methyl side groups in the silole unit are oriented out of the n-conjugated polymer plane and hence hinder the interaction between the polymer chains, which is known to be one of the key factors for high conductivity values174. 

C. 0-71 Conjugated Polymers with an Alternating Arrangement of a Disilanylene Unit and 3,4-Diethynylene-Substituted Siloles... 

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. 

The incorporation of siloles in polymers is of interest and importance in chemistry and functionalities. Some optoelectronic properties, impossible to obtain in silole small molecules, may be realized with silole-containing polymers (SCPs). The first synthesis of SCPs was reported in 1992.21 Since then, different types of SCPs, such as main chain type 7r-conjugated SCPs catenated through the aromatic carbon of a silole, main chain type cr-conjugated SCPs catenated through the silicon atom of a silole, SCPs with silole pendants, and hyperbranched or dendritic SCPs (Fig. 2), have been synthesized.10 In this chapter, the functionalities of SCPs, such as band gap, photoluminescence, electroluminescence, bulk-heterojunction solar cells, field effect transistors, aggregation-induced emission, chemosensors, conductivity, and optical limiting, are summarized. 

Poly(2,5-silole) 7 shows a Aab of 482 nm at room temperature.24 A band gap of the polymer, if calculated with the absorption edge (650 nm), is 1.9 eV. A silole-thiophene alternating copolymer 8 can show a further decreased band gap.25 The copolymer displays a broad absorption spectrum with Aab at 648 nm in chloroform. The calculated band gap from the absorption edge is only 1.55 eV, a very small value so far reported for the synthesized SCPs. 

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. 

Poly(l,l-silole)s, SCPs catenated through the ring silicon atom, can be regarded as a new class of polysilanes. It was found that PL intensities of the toluene solution of a poly(l,l-silole) 24 (Fig. 12) could be quenched by the addition of tiny amounts of 2,4,6-trinitrotoluene (TNT), 2,4,6-trinitrophenol (picric acid), 2,4-dinitrotoluene (DNT), and nitrobenzene, demonstrating that poly(l,l-silole)s are potential chemosensors for explosives.41 TNT could also be detected using the polymer film. In an air stream containing 4 ppb TNT, 8.2% decrease of the PL intensity was found from the film. PL quenching can also be detected when the film contacts a 50 ppb TNT-water solution. 

Dehydrocoupling of 1 catalyzed by <50 mol% (i.e., M-H/Si-H < 0.5) of Red-Al yielded 2 as light yellow powders. Polymers with molecular weights (Mw) of 4600 and 4100 were isolated in 86 and 78% yields when 15 mol% and 50 mol% of Red-Al were used, respectively. Polymer yields and molecular weights when 15 mol% of Red-Al was used were higher compared to polymers obtained when 50 mol% of Red-Al was used. Products from the reaction of 1 with 15 mol% 25 mol%, and 50 mol% Red-Al were separated by preparative GPC and were characterized by NMR spectroscopy. Shorter oligomers such as silole dimer or trimer were not found in products. However, when 100 mol% of Red-Al (i.e., M-H/Si-H = 1) was used, the formation of silole dianion 333 was observed without forming 2 [Eq. (6)]. 

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