Influence of the introduction of short alkyl chains in poly(2-(2-thienyl)-1H-pyrrole) on its electrochromic behavior

We report the significant effect of the introduction of short alkyl chains on the electrochromic behavior of poly(2-(2-thienyl)-1H-pyrrole), poly(1). Hence, poly(3-ethyl-2-(2-thienyl)-1H-pyrrole), poly(2), and poly(3-n-propyl-2-(2-thienyl)-1H-pyrrole), poly(3), presented multicolor electrochromism (both cathodic and anodic) exhibiting several colors: blue-grayish (+0.8 V), light blue (+0.2 V), light brown (0 V), brown (-0.1 V), dark orange (-0.2 V) and light orange (-0.8 V). Both poly(2) and poly(3) films showed a maximum optical contrast (Delta T-max) of similar to 30% at a film thickness < 100 nm. For both polymers, the optical responses for the oxidation and reduction process were in the range of a few seconds with a coloration efficiency during the bleaching process of 107-108 cm(2)/C. The new multichromic polymers were thermally stable up to 200 degrees C and showed reduced weight loss in the temperature range from 400 to 800 degrees C. The doping level for poly(2) and poly(3) was in the range of 26-31%. AFM images illustrated that poly(2) presented a much rougher surface than poly(3). The morphology corresponding to the oxidized state for both polymers was sharp, showing a significant smoothening of the surface upon reduction. Geometry optimizations and theoretical conformational analysis performed at the B3LYP/6-311G* level pointed to an increased population of s-cis conformers in 2 and 3 that presumably will be also retained in poly(2) and poly(3).