Alternating donor/acceptor repeat units in polythiophenes. Intramolecular charge transfer for reducing band gaps in fully substituted conjugated polymers

This paper describes a method to limit the band gap widening that occurs in fully substituted conjugated polymers. This is done by constructing step growth [AB] polymers where the A-units are electron rich and the B-units are electron deficient. The thiophene-based polymers were prepared by modified Stille polymerizations using Pd(0)/CuI catalyst systems in which aryldibromides were coupled with aryldistannanes. The donor units were N,N'-(bis-tert-butoxycarbonyl)-3,4-diaminothiophene N,N'-(bis-tert-butoxycarbonyl)N,N'-(dimethyl)- 3,4-diaminothiophene, 3,4-diaminothiophene or 3,4-dialkoxythiophenes while the acceptor units were 3,4-dinitrothiophene, 3,4-(N-n-butylimido)thiophene, or 3,4-diketone-containing thiophenes. The optical spectra showed lambda(max) values ranging from 400 to 676 nm (solution) and 400-768 (film) for these fully substituted polythiophenes, consistent with significant decreases in the band gaps. Intramolecular charge transfer character between the consecutive units explained the lowering of the band gaps. Two polymer systems based solely on electron deficient thiophenes were prepared via an Ullmann coupling which had optical absorption maxima that were in the range of 300-340 nm; considerably smaller than the lambda(max) values for the donor/acceptor systems. Several model trimers were prepared which had significantly shorter wavelength optical absorptions than their corresponding polymers, thus confirming the need for the extended polymeric backbones.