Chromophores of neutral and dicationic thiophene-based oligomers – A study by first-principle methods

Time-dependent density functional theory (TD-DFT) and semiempirical methods (ZINDO-CIS, PPP-CIS) were applied to colored neutral and dicationic thiophene-based oligomers; in addition to the parent oligothiophenes and various α,α′-end-capped derivatives, oligothiophenes with thiophene rings linked by methine, vinylene or phenylene groups were studied. Being π-chromophores, the compounds absorbed intensely in the Vis–NIR region; variation of the absorption energy with the reciprocal of the number of thiophene rings resulted in low band gap (as is known for low band gap polymers). Whilst the parent oligothiophenes displayed no singlet/singlet instability up to the octadecithiophene, the long chain oligomers were singlet/triple unstable. According to BS-UDFT calculations, biradical oligomers (two-plarons) were slightly favored over conventional dications (bipolarons). The color-determining electronic transitions of large size oligothiophenes calculated by open shell TD-DFT were little shifted in energy relative to those calculated using closed shell TD-DFT. The electronic structure of the oligomers is discussed in terms of bond length alternation and nucleus-independent chemical shifts.