Theoretical study of structural and optical properties of regioregular head-to-tail oligo (3-n-octylthiophene) and related star molecules

The quantum chemical calculations of selected nonsubstituted or regioregular head-to-tail oligo (3-n-octylthiophene) and model thiophene-based star molecules using Density Functional Theory (DFT) are presented. The influence of the alkyl-substitution on the optimal geometries and relevant optical properties was analyzed. In the case of model star compounds, the presence of alkyl groups has a leading influence on structure as well as optical transitions. The Time Dependent DFT calculations of the vertically excited states show that the presence of alkyl group at the inner β-positions with respect to the phenylene central core is responsible for the generation of the low energy optical transitions with dominant oscillator strengths (over 0.25). The molecular orbital analysis of the lowest energy transition of the largest studied model star molecule (1,3,5-tris-(sexithienyl)-phenylene) also shows that the meta-conjugation in this zero-order dendrimer allows certain interactions among arms altogether.