Evidence of solid-state polymerization in regioregular poly-3′,4′-didodecyl-2,2′:5′,2′′-terthiophene during electrochemical cycling

The electrochemical oxidation of 3′,4′-didodecyl-2,2′:5′,2′′-terthiophene (3′4′-DDTT) at applied potential values lower than 0.8 V vs Ag/Ag+ leads to the formation of an anodic deposit that is prevalently constituted by the products of monomer coupling at positions 5 and 5′′. Upon electrochemical cycling poly-3′4′-DDTT manifests three phenomena: The cathodic shift of the main peak of poly-3′4′-DDTT oxidation, the bathochromic shift of 60 nm for the main UV-vis absorption peak, and the IR spectral evolution consisting of the progressive increase of the signals of 2,5-substituted thiophenes at expense of the signals from 2-substituted thiophenes. The observed behavior was interpreted in terms of the presence of reactive oligomers trapped in pristine as-deposited poly-3′4′-DDTT, with the oligomers undergoing further coupling in the polymeric phase upon electrochemical cycling. This combination of findings led us to conclude that poly-3′4′-DDTT manifests solid-state polymerization. After voltammogram stabilization poly-3′4′-DDTT was electrochemically stressed. The system showed a reversible electrochemical behavior up to 0.9 V vs Ag/Ag+. A depiction of the evolution of the electronic bands in poly-3′4′-DDTT when passing from the pristine state to the electrochemically cycled state has been proposed, taking into account the combination of electrochemical and optical data.