Alkyl-substituted thieno[3,2-b]thiophene polymers and their dimeric subunits

A series of alkyl-substituted polythieno[3,2-b]thiophenes were synthesized by cross-coupling and oxidative coupling reactions. Their electronic properties were studied by UV-vis absorption and fluorescence spectroscopies. The poly(3-nonylthieno[3,2-b]thiophene) prepared via Stille coupling of a differentially functionalized monomer displayed a longer lambda(max) of absorption and a corresponding lower optical band gap when compared to those of regiorandom polymers prepared by the FeCl3 method or through Kumada coupling. Employing a dialkyl-substituted monomer to produce poly(3,6-dinonylthieno[3,2-b]thiophene) leads to dramatic shifting of both the absorption and emission spectra to shorter wavelength. The electrochemistry of these polymers were studied by cyclic voltammetry and revealed that mono alkyl-substituted polymers have relatively lower oxidation potentials compared to that of the dialkyl-substituted polymer. To study the conformation of monomers in the polymer backbone, dimeric subunits of these polymers were prepared. Both UV-vis spectroscopy and computations indicate that the head-to-tail linkage causes a small distortion of adjacent ring units, and head-to-head linkage results in a large decrease of conjugation.