5-5" -dimethyl-2, 2 :5 , 2 :5", 2 ”-quaterthiophene

As mentioned above, alkyl-chains at the a and w position (end-substitution) generally lead to films with a higher degree of order as unsubstituted molecules but similar (herringbone) structure. This is shown for dimethyl-quaterthiophene [33,49, 160, 161]. End-substitution of quaterthiophene with two amino-groups also leads to films with the molecular axis nearly parallel to the surface normal, despite the dipole moment of the substituent [162]. The dihydrochloride, however, only forms amorphous films. 

More recently, partially substituted quarterthiophenes have been investigated by Collard and co-workers who polymerized 3,3 -dioctyl-2,2 5, 2 5 ,2" -quaterthiophene.(42) Polymerization of the quaterthiophene by FeCU was inefficient even at elevated temperatures (50 C), the majority of the material recovered (73%) was the dimerized monomer (the octathiophene). However, approximately 10% of the oxidized material was polymeric and exhibited absorption maxima of 463 nm in CHCI3 and 514 nm as thin films. Although the FeCh polymerization of the dimethyl analog has also been reported without the difficulties described above, the polymerized product was completely insoluble.(75)... 

Figure 9. 0-2J9 profile for an a,a -dimethyl substituted quaterthiophene thin film in neutral form. Reprinted with permission from Ref. 106. Copyright 1991 The Royal Society of Chemistry. 

Corresponding tetramethoxy-substituted pentamers 344, 345 and a hexamer 347 were synthesized starting from 3,3"-dimethoxy-a-terthiophene 342. Pd(0)-catalyzed coupling of the bis-stannylated terthiophene with 2-iodo-3-methoxythiophene 343 results in quinquethiophene 344 (62% yield). This is subsequently lithiated and methylated in the terminal a-positions with n-BuLi/DMS to form pentamer 345 in 80% yield [Eq. (89)]. Monomethylated terthiophene 346 obtained in 81% yield from 342 with the same procedure was dimerized with the system -BuLi/Fe(acac)3 to the letramethoxy,dimethyl-substituted hexamer 347 in 70% yield [Eq. (90)]. The quaterthiophene dicarbonic acid 341 was obtained from the tetramer 335 by reaction with n-BuLi/C02 in nearly quantitative yield. 

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