A-Bithiophenes

The sensor covalently joined a bithiophene unit with a crown ether macrocycle as the monomeric unit for polymerization (Scheme 1). The spatial distribution of oxygen coordination sites around a metal ion causes planarization of the backbone in the bithiophene, eliciting a red-shift upon metal coordination. They expanded upon this bithiophene structure by replacing the crown ether macrocycle with a calixarene-based ion receptor, and worked with both a monomeric model and a polymeric version to compare ion-binding specificity and behavior [13]. The monomer exhibited less specificity for Na+ than the polymer. However, with the gradual addition of Na+, the monomer underwent a steady blue shift in fluorescence emission whereas the polymer appeared to reach a critical concentration where the spectra rapidly transitioned to a shorter wavelength. Scheme 2 illustrates the proposed explanation for blue shift with increasing ion concentration. 

In a second example, anthracene moieties were substituted on a bithiophene core to give molecule lh.[55] OTFTs employing this compound had a mobility of 0.12... 

Scheme 7.13 Chain growth polymerization of a bithiophene monomer by Ni ring walk across a nonconjugated monomer.

Sannicolo et al. first reported the electropolymerization of a Ceo-derivatized bridged bithiophenic precursor (235) [431]. Ferraris et al. have described the synthesis and the electropolymerization of a bithiophene with a Cgo group attached at P-position by an alkyl spacer (236), whereas processable PT copolymers were postfunctionalized with Cgo to give polymers [432]. Recently, PT with Cgo groups connected to the rr-conjugated polymer chain by a phenyl polyether linker has been prepared by electropolymerization of bithiophene 237 [427,433]. While the donor backbone and the acceptor moiety do not interact in the ground state, a photoinduced electron transfer between the PT chain and the attached Cgo group has been demonstrated by ESR measurements. 

The same group further prepared dendrimer 5.63 by inserting a bithiophene unit between the core of 5.60 and the periphery (Chart 1.76). Despite its relatively large optical bandgap of 2.1 eV, fabrication of OSCs based on a blend of 5.63 and PCBM (1 4) revealed a power conversion efficiency of 1.3 % under simulated AMI.5 illumination [506]. 

Other polyrotaxanes containing a thiophene-based conjugated backbone have been synthesized from the Cu(I)- or Zn(II)-driven assembly between a macrocyclic phenanthroline and a bithiophene-substituted phenanthroline [322] or bipyridine [323, 324]. Swager and co-workers have demonstrated that the contribution of the metal ion to the electronic properties of the polyrotaxane was possible when more electron-rich 3,4-(ethylenedioxy)thio-phene groups were used in place of thiophenes in the polymer backbone [324]. So a 10 -10 -fold increase in the polymer s conductivity was observed after poly(24) was treated with Cu(II) solution. This result was ascribed to the oxidation of the poly(24) backbone by Cu " " ions to generate poly(24,Cu) with charge carriers in the polymer backbone. 

FIGURE 13.23 Dimesitylborane connected via a bithiophene bridge to donor moieties. (Adapted from Branger, C. et al., J. Mater. Chem. 1996, 6,4, 555.)... 

Finally, the third and current era of ohgothiophenes was initiated in 1982 with the first synthesis of polythiophene PI as an electrically conductive polymer [8]. Somewhat later, a-bithiophene 2 and a-terthiophene 3 were used as monomers, instead of thiophene 1 itself, as substrates for electropolymerization in order to control the structure of the polymer [Eq. (3)] [28]. Owing to their lower oxidation potential. 

Table 3. Preparation and physical properties of / -substituted a-bithiophenes. 

Finally, the structure of various other substituted 2,2 -bithiophenes have also been investigated recently. This is particularly the case of 5,5 -dimethyl-2,2 -bithiophene and 5,5 -bis(trimethylsilyl)-2,2 -bithiophene [76], (3, 4,4, 5 -tetramethyl-2,2 -bithiophene-5-yl)-(3, 4,4, 5,5 -pentamethyl-2,2 -bithiophene-3-yl)methane [77], the solvatochromic 5-dimethylamino-5 -nitro-2,2 -bithiophene [78], and a bithiophene-derived annulene [79]. 

Composites having electrical conductivities up to about 5 Scm" can be synthesized starting from porous cross-linked polystyrene imbibed with a bithiophene solution after partial drying of the saturated host polymer imbibing with a FeCla solution PT is produced by oxidative polymerization, and the PT network is additionally doped by FeCls [25, 48]. 

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