Bithiophene substrates

Figure 10. Postulated formation of C20 isoprenoid thiophenes and bithiophenes by incorporation of sulfur into phytenes. Different substrates lead to different thiophenes which is the major control for the thiophene ratio.

Ultraviolet photoelectron spectroscopy (UPS) has been used to study the evolution of the valence electronic states as a function of conjugation length for thiophene, bithiophene, terthiophene, and sexithiophene films deposited in vacuum on gold substrates at 130 K. 

Alkoxy-substituted bithiophenes have also been successfully polymerized, using Cu(C104)2 as oxidant.50 As with related FeCl3 oxidations of alkyl-substituted bithiophenes,51 these dimeric substrates are easier to oxidize than the corresponding substituted thiophene monomers, due to their lower oxidation potentials. 

The morphology of the poly(2,2 bithiophene) films is quite complex and appears to be a function of several factors such as the nature of the growth substrate, the growth stage, and the rate of growth as controlled by the current rate. When a platinum electrode is used as the growth substrate, a smooth thin layer several hundred angstroms thick initially covers the entire electrode surface. 

Side-chain substitution Small molecule organic semiconductors can also be rendered solution processable by attachment of flexible side chains. The substitution pattern needs to be designed carefully so that the side chains, which are needed to impart adequate solubility and film forming properties, do nof inferfere with the ability of the molecule to Jt-stack. Katz reported side-chain-substituted small molecule semiconductors such as dihexylanthradithiophene [47,48] that can be solution-deposited with mobilities of 0.01-0.02 cmWs. In bis(hexyl-bithiophene)benzene solution-cast onto a heated Si02 substrate, mobilities of up to 0.03 cmWs were reported [49]. Due to the relatively low solubility of these molecules, growth... 

Extensive data were obtained from the related compound 5,5 -bis(4-hexylphe-nyl)-2,2 -bithiophene, abbreviated 6PTTP6 [36]. Xylene was an ideal solvent for casting films of this compound, keeping the substrate 5-30° below the boiling point. 

Electrochemical deposition of polythiophene films from solution of mono- or bithiophenes is one of the commonly used methods and can also be applied to prepare oligothiophene films. The films are formed in situ during the polymerization process. Due to the oxidative polymerization only doped (oxidized) materials are obtained and appropriate counter-ions have to be added. This method can only be applied to conductive substrates. 

The selective monoarylation of unsubstituted thiophene and furan can be made by using an excess amount of the substrate [ 130]. It has been demonstrated that the use of AgF and DMSO as base and solvent, respectively, enables the reaction at 60 °C [ 135]. Various 2- or 3-substituted thiophenes andbenzothiophenes have been subjected to the catalytic arylation [8,128,130,134-139]. 2,2 -Bithiophene can be arylated at the 5 and 5 positions (Eq. 55) [137]. 

An application of the developed methodology to simple substrates for metal halogen exchange reactions but also to metallation procedures afforded reliable results. Thus, the technique was utilized for a detailed kinetic investigation of a double-sided Halogen Dance (Fig. 8, A) reaction towards 4,4 -dibromo-2,2 -bithiophene (Bobrovsky et al., 2008). 

V(bithiophene) 1.14 (26) [46]. Electrochemical polymerization of (25) and (26) was possible under conditions close to those for thiophene and (25) and (26) was possible under conditions close to those for thiophene and bithiophene, respectively. Trimethylsilyl groups permitted specific activation of the 2 and 5 positions with respect to the electrochemical oxidation and allowed the preparation of polythiophene films, starting from oligomers, which gave rise to low selectivity without silyl substituents, or monomers with electron-withdrawing substituents. Masuda et al. elec-trochemically polymerized (25), (27) and (28) [47]. Elemental analysis of the resulting films indicated that almost all silicon atoms were eliminated during polymerization. IR spectra, absorption spectra and cyclic voltammometry indicated that the films were made up from polythiophene. Roncali et al. electro-chemically oxidized (29) and obtained a polythiophene film [48]. This particular structure of (29) allows electrochemical preparation to be performed at a very low substrate concentration while the reduction of the silicon to monomer ratio to its minimum value would lead to further improvements in the quality of the polythiophene film. 

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