3-methoxy-2-2 -bithiophene

FIGURE 5.4 Chemical structures of photo- and electroluminescent polymers employed for polarized LEDs poly(2-methoxy-5-(2 -ethyl-hexyloxy)-p-phenylene vinylene) (MEH-PPV) poly[2,5-dioctyloxy-l, 4-diethynyl-phenylene-a/t-2,5,-bis(2 -ethylhexyloxy)-l,4-phenylene] (EHO-OPPE) poly(p-phenylene), PPP poly(3-(4-octylphenyl)-2,2 -bithiophene), PTOPT poly(p-phenylene vinylene), PPV poly(3-alkylthio-phene vinylene), P3AT Acetoxy-PPY PPV-polyester, poly(9,9-dialkyl fluorene), PF. 

Studies by Heinze etal. on donor-substituted thiophenes or pyrroles [33] such as methylthio (= methylsulfonyl) or methoxy-substituted derivatives provide further clear evidence for this reaction pathway. They found, for instance, that 3-methylthiothiophene or 3-methoxythio-phene (2) undergo a fast coupling reaction. However, deposition processes or insoluble film formation could not be detected in usual experiments with these compounds, even at high concentrations. Similarly, the corresponding 3,3 -disubstituted bithiophenes (2a) do not polymerize, but the anodic oxidation of 4,4 -disubstituted bithiophenes (2c) produces excellent yields of conducting polymers. 

Smaller oligothiophenes with n < 4 show a large tendency to dimerize to molecules with chain length 2n upon oxidation. Only by inclusion into (acidic) zeolites [199,200] can the dimerization be blocked, whereas a-substitution with methyl-groups does not hinder the dimerization at room temperature but slows it down [201,202]. Trimethylsilyl- [203], methoxy- and Br-a-substitution [204] stabilizes monomeric cations for ter-and in the two latter cases even for bithiophene. 

Di Cdsare et al. studied the lowest energy structures of several bithiophenes substituted with methoxy and methyl groups in various positions using HF/3-21G calculations, and also performed a detailed conformational analysis [74]. The effect of the position of substituents on structural parameters and on torsional potentials was examined. The results of these calculations are in good agreement with available experimental data. 

N. Di Cesare, M. Belletete, M. Leclerc, G. Durocher, HE/3-21G ab initio calculations on methoxy-substituted bithiophenes, THEOCHEM, 467, 259-273 (1999). 

Polythiophene and poly(3-alkylthiopene) are not so stable in the anion-doped state as polypyrrole. Conductivity decreases rapidly during storage. This is a serious drawback to the industrial application of polythiophenes. A method to improve stability is to decrease the oxidation potential of polymer. Introduction of a methoxy group on a thiophene ring reduces the oxidation potential, thus poly(3-methoxythiophene) is stable in the doped state. Another method to improve stability is to make room for the accommodation of dopants. This is a kind of free-volume theory. Poly(3,3"-dialkyl-2,2, 5 -2"-bithiophene) is very stable in the doped state. This is due not only to the... 

Dietrich and Heinze electrochemically prepared poly(4,4 -dimethoxy-2,2 -bithiophene [56]. Since the 5 and 5 positions were reactive due to the adjacent methoxy groups and were available for polymerization, a high-quality polymer was obtained. The polymer was electrochemically stable over a wide range of applied potentials. The thin films of the polymer were deep blue in the neutral state and a light gray-blue in the doped state. The neutral form exhibited a broad absorption in the visible region with a maximum at 610 nm. The doped polymer showed no absorption peaks in the range 300-850 nm. 

Fig. 19 Common organic semiconductors used in organic solar cells a poly[2-methoxy-5-(3, 7 -dimethyloctyloxy)-l,4-phenylenevinylene] (MDMO-PPV), b perylene, c poly(triaryl amine), d poly[(9,9-di-n-octylfluorenyl-2,7-diyl) ((F8T2)), e Phenyl-C61-butyric acid methyl ester-alt-2,2 -bithiophene-5,5 -diyl)] (PCBM), f poly( 4,8-bis[(2-ethylhexyl)oxy]benzo[l,2-h 4,5-b ] dithiophene-2,6-diyl 3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-h]thiophenediyl ) (PTB7)...

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