Functionalized thiophene oligomers

While electropolymerization of 3-alkoxythiophenes leads to poorly conjugated chains or soluble oligomers [47b], the grafting of alkoxy or aryloxy groups at specific -positions of an oUgothiophene can exert a strong influence on the reactivity of the radical cation and hence its aptitude to electropolymerize. 

Following this approach, crown-annelated bithiophenes 9 have been recently synthesized and their anodic oxidation has led to the efficient formation of polymers that exhibit affinity with lithium cation. The oxidation potential of the polymer decreases upon addition of lithium cations, suggesting a planarization of the chain [58]. 

Double substitution of the two -positions of one thiophene ring was also considered for the electrosynthesis of corresponding polymers [10]. In this context, the development of 3,4-ethylenedioxythiophene (EDOT) represented a breakthrough for the optimization of the electrochemical polymerization of thiophene-based n-conjugated systems. 

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Redox molecules are particularly interesting for an electrochemical approach, because they offer addressable (functional) energy states in an electrochemically accessible potential window, which can be tuned upon polarization between oxidized and reduced states. The difference in the junction conductance of the oxidized and the reduced forms of redox molecules may span several orders of magnitude. Examples of functional molecules used in these studies include porphyrins [31,153], viologens [33, 34,110,114,154,155], aniline and thiophene oligomers [113, 146, 156, 157], metal-organic terpyridine complexes [46, 158-163], carotenes [164], nitro derivatives of OPE (OPV) [165, 166], ferrocene [150, 167, 168], perylene tetracarboxylic bisimide [141, 169, 170], tetrathia-fulvalenes [155], fullerene derivatives [171], redox-active proteins [109, 172-174], and hydroxyquinones [175]. 

II Conjugation alone cannot be relied upon to significantly enhance the optical nonlinearities. The conformational effects and the role of the substituents so that understanding of the molecular structure-property relation can be improved have been studied (89JPC7916). In the case of the thiophene oligomers, a rapid increase in the y value (large microscopic nonlinearity) as a function of N is found. 

Yanagi, H., Araki, Y., Ohara, T., Hotta, S., Ichikawa, M. andTaniguchi, Y. (2003) Comparative carrier transport characteristics on organic field-effect transistors with vapor deposited thin films and epitaxially grown crystals of biphenyl-capped thiophene oligomers. Advanced Functional Materials, 13, 767-73. 

FIGURE 1.8 Evolution of inter-ring C-C bond lengths in thiophene oligomers as a function of chain length as optimized with AMl-CAS (a) before and (b) after symmetry breaking and (c) with BHandHLYP/3-21G. ... 

Moro, G., et al. 2000. On the structure of polaronic defects in thiophene oligomers A combined Hartree-Fock and density functional theory study. Synthetic Met 108 165. 

Many thiophene derivatives have been polymerized in order to obtain new materials tailored for different purposes. Roncali [596] reviewed the enormous amount of literature regarding the synthesis, functionalization and applications of polythiophenes in 1992. Beside the polymerizations of thiophene and bithiophene, polymers from several thiophene oligomers, substituted thiophenes, thiophenes with... 

The first two terms often provide reasonable fits, especially if n is limited to a small number ( 5) in the calculations (see, e.g., Ozen et al, 2007 Yang, 2007). A few representative extrapolated gap values are given in Table 7.2. The evolution of the bandgap of thiophene oligomers as a function of the inverse of the number of carbon atoms along the conjugation pathway, Nc, is illustrated in Figure 7.3. 

Yurtsever and Yurtsever [898] attempted accurate ab initio calculation for thiophene oligomers bonded through a and jS-carbons. Assuming a Monte Carlo growth scheme to predict branching as a function of temperature and chain length, it was found that linear chains dominate in PT. 

Table 6. INDO/MRD-CI SOS average and chain-axis component values (in 10 esu) of the static third-order polarizability of thiophene oligomers. We also indicate the evolution of the power law dependence of 7 as a function of chain length ((7 oc together with the...

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