Toward controlled donor-acceptor interactions in noncomposite polymeric materials: synthesis and characterization of a novel polythiophene incorporating -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, E1red1/2 and E2red1/2 vs the substituents on the fluorene ring, with the sensitivity parameters ICT- 0.17-0.20 eV and CV- 0.17-0.25 V. In contrast, the oxidation process (Eox) displays very low sensitivity to the structure of the fluorene ring (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.