Organoboron copolymers containing thienothiophene and selenophenothiophene analogues: optical, electrochemical and fluoride sensing properties

Conjugated donor-acceptor (D-A) copolymers possessing alternating fused bicyclic aromatic rings thieno [3,2-b]thiophene, selenopheno[3,2-b]thiophene, thieno[2,3-b]thiophene and selenopheno[2,3-b]thiophene as donors, thiophene as a pi-conjugated bridge and mesitylboron as an acceptor were synthesized and characterized by spectroscopic methods. Large Stokes shifts of 96-166 nm were recorded and the solution quantum yields of the polymers were in the range of 5-18%. Their ionization potentials and electron affinities were investigated by cyclic voltammetry. Optical band gaps varied between 2.26 and 2.78 eV. (TD)-DFT studies were performed to unveil their electronic structures, Kohn-Sham orbitals and electronic transitions. Absorption and emission measurements revealed that these polymers have high sensitivity to fluoride anions, among which the polymer possessing cross-conjugated thienothiophene units had the best sensing properties supported by orbital composition analysis with Mulliken partition. Thus, copolymers P1-P4 could be used as colorimetric and fluorescent sensors for small fluoride anions.