Thiophene units

Polythiophene can be synthesized by electrochemical polymerization or chemical oxidation of the monomer. A large number of substituted polythiophenes have been prepared, with the properties of the polymer depending on the nature of the substituent group. Oligomers of polythiophene such as (a-sexithienyl thiophene) can be prepared by oxidative linking of smaller thiophene units (33). These oligomers can be sublimed in vacuum to create polymer thin films for use in organic-based transistors. 

Polyacetylene is considered to be the prototypical low band-gap polymer, but its potential uses in device applications have been hampered by its sensitivity to both oxygen and moisture in its pristine and doped states. Poly(thienylene vinylene) 2 has been extensively studied because it shares many of the useful attributes of polyacetylene but shows considerably improved environmental stability. The low band gap of PTV and its derivatives lends itself to potential applications in both its pristine and highly conductive doped state. Furthermore, the vinylene spacers between thiophene units allow substitution on the thiophene ring without disrupting the conjugation along the polymer backbone. 

The chemical scheme of oligothiophenes (wT, where rt stands for the number of thiophene units) is shown in Figure 14-14. 

Thus, the combined experimental and theoretical results indicate that the chemical shift observed for the S(2p) core level, of about 1.6 eV, should be due to a secondary effect from the attachment of Al atoms to the adjacent carbon atoms. Indeed, this is fully consistent with tib initio Hartree-Fock ASCF calculations of the chemical shifts in aluminum-oligolhiophene complexes 187], From calculations on a AI2/a-3T complex, where the two AI atoms are attached to the a-car-bons on the central thiophene unit, the chemical shift of the S(2p) level for the central sulfur atom is found to be 1.65 eV, which is in close agreement with the experimental value of about 1.6 eV [84]. It should be pointed out that although several different Al-lhiophene complexes were tested in the ASCF calculations, no stable structure, where an Al atom binds directly to a S atom, was found [87]. 

Figure 10 The -conjugated polythiophene (10) containing thiophene units and boron atoms in its backbone. (Adapted from ref. 31.)...

Another PHOX analogue has the aryl ring of the PHOX catalyst replaced by a thiophene unit 16 (Fig. 29.4) [15]. The synthesis is similar to that of the PHOX catalysts, starting with oriho-nu lallaliori of the thiophene. The catalysts showed similar selectivity to PHOX, and were used to hydrogenate substrates 1 and 2 with maximum enantioselectivities of 99% and 94%, respectively. 

Ag+/Ag, in the range of the oxidation potential of 3-methylthiophene, (14.) due to the irreversible oxidation of the monomeric thiophene unit. Figure 1 shows the typical cyclic voltammetry upon repeatedly scanning the potential of a Pt electrode between 0.0 V and 1.5 V vs. Ag+/Ag in a solution of 0.2 M 1. 

An increase in the PL QE of the fluorene-thiophene copolymers can be achieved by introduction of -oxidized thiophene units (although no efficient EL from such materials was reported). This aspect and the chemical structures of thiophene-iS,5 -dioxide-fluorene copolymers are discussed in more detail in Section 2.4. 

The presence of HH coupling in irregular PTs causes an increased twisting of thiophene units (due to steric repulsion) with concomitant loss of conjugation. This results in an increased band gap (blue shift in an absorption and luminescence), decreased conductivity, and other undesirable changes in electronic properties. As it will be shown below, regioregu-larity also plays an important role in luminescence properties of PTs and is used as a tool to tune the properties of PT-based LEDs. 

The combination of thiophene and thiophene-51,S -dioxide units in a copolymer allows tuning the emission color from green to pure red [407,549], However, the PLEDs fabricated with these materials showed a rather low 4>el< 0.01% that further decreased with an increasing number of thiophene units. Similar results (significant decrease of the PL QE) were observed for thiophene thiophene-5,5-dioxide copolymers containing 3,6-dimethoxyfluorene (449 [303]) and carbazole units (450 [550]) (d>PL = 20-25% in solution). 

V. Saxena and V.S. Shirodkar, A study of light-emitting diodes constructed with copolymers having cyclohexyl thiophene and hexyl thiophene units, J. Appl. Polym. Sci., 77 1051-1055, 2000. 

In order to quantify the transition metal ion concentration, Jones et al. [107] developed a highly sensitive fluorescent chemosensor in the form of dialkoxy-phenyleneethynylene-thiophene copolymers 68/69. The PAEs were functionalized on the thiophene unit with terpyridine (68), and included 2,2 -bipyridine (69) as a Lewis acid receptor. The terpyridine polymers [108] were found to respond quantitatively to transition metal ions at concentrations as low as 4x10 M (NP, Hg, Cr ", and Co " ). The additionally used bpy-PAE demonstrates that variation in the chelation at the receptor site is an important variable in tuning selectivity. The observed dynamic quenching mechanism, combined with the solubility of this material, provides the opportunity to extend these initial investigations to thin solid films for use in real-time monitoring applications. 

The electronic properties of n-conjugated polymers reflect well the basic electron-withdrawing or electron-donating properties of the components of the Ti-conjugated polymer [62]. In view of the electrochemical reduction potential, the thiophene unit and tetrathiafulvalene unit (Nos. 8 and 9 in Table 1) have a similar electronic effect in PAEs. It is reported that poly(arylenevinylene)s are also susceptible to electrochemical reduction [63, 64]. Due to the electron-accepting properties, PAEs are usually inert in electrochemical and chemical (e.g.,by I2 [54]) oxidation. 

A main focus of preparing metal dithiolenes functionalized with thiophene units has been for their incorporation into conjugated organic materials. On first glance, it would seem that the combination of metal dithiolenes and conjugated polymers could produce attractive new materials for use in such applications as field effect transistors and NIR optical materials. However, several challenges remain before these materials can be applied to useful... 

Starting from 3,6-dimethylthieno[3,2-3]thiophene 71a and 3,6-dimethylselenolo[3,2-/ ]selenophene 71b <1994H(38)143>, their dimers, trimers, and tetramers, in which each thieno[3,2-3]thiophene unit and selenolosele-nophene unit was regularly connected at their a-positions, were satisfactorily synthesized (Scheme 4) <1996X471 >. 

Numerous methods are available for the synthesis of each of the common heteroaromatics, many of which have been incorporated into phane structures by the utilization of a functionalized macroring, such as cyclododecanone. The synthetic particulars can be found in most modern heterocyclic texts (B-82MI52201). The syntheses of heterophanes containing pyridine, furan and thiophene units have been reviewed (77CRV513). 

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