Optical gap energy study of poly(thienylene-2,5-dialkoxyphenylene) in solid-state films

Optical gap energy (E_gap) in luminescent π-conjugated polymers presents several difficulties in its determination, particularly when using CW conventional optical spectroscopy, absorption and emission. This happens due to several physicochemical parameter's dependence. Among others, the molecular conformation, intramolecular interactions, structural defects, polymer processability and solvent interaction stand out. In addition, there is a distribution of conjugated segments along the polymeric main chains that differentiate optical absorption transition from emission processes. In other words, these processes do not necessarily occur in the same conjugated segment owing to the very efficient ratios of energy transfer or charge migration in these materials. In this work we present a systematic study of the determination of E_gap for the polymer poly(thienylene-2,5-dialkoxyphenylene). We present a comparison between the solution and solid-state film, clearly showing the presence of a polymer-polymer interaction as aggregate species. The goal of this paper is to isolate and aggregate the contribution determination of each species through systematic analysis of optical spectra, as well as to obtain, even on film, the E_gap of the isolated polymer which is very similar to the polymer solution at about 2.37 eV. The intersection theory and the voltammetry methods corroborate the experiment and the discussion of the results obtained.