Investigation of Fluorine Atom Effect on Benzothiadiazole Acceptor Unit in Donor Acceptor Donor Systems

Synthesis of two new π-conjugated monomers containing fluorine substituted benzothiadiazoles as acceptor units and 3,4-ethylenedioxythiophene (EDOT) as donor unit was accomplished by facilitating Stille cross-coupling reactions (4,7-bis(2,3-dihydrothieno[3,4- b ][1,4]dioxin-5-yl)-5-fluorobenzo[ c ][1,2,5]thiadiazole ( E 2 BTD-F ) and 4,7-bis(2,3-dihydrothieno[3,4- b ][1,4]dioxin-5-yl)-5,6-difluorobenzo[ c ][1,2,5] thiadiazole ( E 2 BTD-2F )). NMR and HRMS spectra of newly synthesized compounds are in agreement with the structures. To compare fluorine substitution effects, non-fluorinated analogue (4,7-bis(2,3-dihydrothieno[3,4- b ][1,4]dioxin-5-yl)benzo[ c ][1,2,5]thiadiazole ( E 2 BTD )) was also synthesized. Besides a blue shift in their absorption spectra, an anodic shift was also noted in the oxidation potentials of both monomers with fluorine atom substitution on the acceptor unit. This was further confirmed by DFT calculations such that upon fluorine substitution EDOT donor units deviate from planarity. Electropolymerization of the monomers were achieved successfully via potentiodynamic polymerization method. A similar trend in optoelectronic properties was also observed for the polymer films as for the monomers.