Photoinduced step‐growthpolymerization of thieno[3,4‐b] thiophene derivatives. The substitution effect on the reactivity and electrochemical properties

We herein report photoinduced step‐growth polymerization of highly conjugated 2,5‐dithiophenyl (thieno[3,4‐b] thiophene) (TTs) derivatives possessing 4‐cyanophenyl or 4‐methoxyphenyl or 3‐(4‐fluorophenyl) groups substituted at the terminal position. Upon irradiation at 350 nm, the excited state of these molecules forms exciplex with diphenyliodonium hexafluorophosphate (DPI) that undergoes electron transfer reaction forming radical cations. Successive proton release and radical coupling reactions yield corresponding oligothienothiophenes with overall yields varying between 19–74%. Structural and molecular weight characteristics of the oligomers thus formed were investigated by Ultraviolet, fluorescence, NMR (Nuclear Magnetic Resonance) and infrared (IR) spectroscopic methods, and GPC (Gel Permeation Chromatography), respectively. The effect of substitution and dithiophene side groups on the reactivity of the monomer and band gap of the oligomers formed was evaluated by using cyclic voltammetry. In this article, the photoinduced polymerization of thieno[3,4‐b]thiophenes having electronically different substituents is reported via the step‐growth mechanism. Upon irradiation at 350 nm of these monomers in the presence of diphenyl iodonium salt as photo‐oxidant successive electron transfer, proton release resulted in the formation of the corresponding oligomers.