Synthesis of selenopheno[3,2-c]thiophene derivatives and (opto)electrochemical properties of new low bandgap conjugated polymers

Various selenopheno[3,2-c]thiophene (STh) derivatives functionalized by 4-n-butylphenyl, 4-n-pentylphenyl, 4-tert-butylphenyl, and n-octyl were newly synthesized in a concise and efficient way. Electrochemical and optical properties of polymers were examined by cyclic voltammetry and visible-near infrared (Vis-NIR) spectrophotometry. When compared with polymers of thieno[3,4-b]thiophene, poly(selenopheno[3,2-c]thiophene) (PSTh) was more easily oxidized by ca. 0.2 V (higher HOMO). On the other hand, the bandgap of electrochemically prepared PSTh varied with substituents, but showed similar values to those of poly(thieno[3,4-b]thiophene). While the resonance effect of phenyl substituents slightly lowered HOMO, the combination of resonance and inductive effects decreased LUMO more effectively, resulting in the lowest bandgap of 0.91 eV for 4-tert-butylphenyl functionalized PSTh.