Toward Controlled Donor−Acceptor Interactions in Noncomposite Polymeric Materials: Synthesis and Characterization of a Novel Polythiophene Incorporating π-Conjugated 1,3-Dithiole-2-ylidenefluorene Units as Strong D−A Components

A novel polythiophene bearing a strong electron-accepting fluorene unit has been synthesized. Poly( 2d ) can be prepared by chemical (iron(III) chloride) or electrochemical oxidation. Intramolecular charge-transfer (ICT) within the polymer was studied by electronic absorption spectroscopy and compared to a series of model monomer derivatives. The redox properties of the materials were studied by cyclic voltammetry; polythiophene main chain in poly( 2d ) is oxidized at ca . +0.7 V vs Ag/AgCl, while the nitrofluorene repeat unit is reduced at ca. −0.5 and −0.7 V. The model compounds 3a − e show a Hammett-type correlation for ICT energies, E 1red 1/2 and E 2red 1/2 vs the substituents on the fluorene ring, with the sensitivity parameters ρ ICT - ≈ 0.17−0.20 eV and ρ CV - ≈ 0.17−0.25 V. In contrast, the oxidation process ( E ox ) displays very low sensitivity to the structure of the fluorene ring (ρ CV - ≈ 0.04 V), indicating that ICT can be substantially tuned without a noticeable effect on the redox properties of the thiophene moiety in 3; this behavior can be extended (extrapolated) to terthiophenes 2 and their polymers. Photoinduced IR spectroscopy of poly( 2d ) provides evidence of long-living photoexcited charge transfer in the polymer.