Interplay of α,α- versus α,β-Conjugation in the Excited States and Charged Defects of Branched Oligothiophenes as Models for Dendrimeric Materials

This article investigates the excited and charged states of three branched oligothiophenes with methyl–thienyl side groups as models to promote 3D arrangements. A comparison with the properties of the parent systems, linear all‐α,α‐oligothiophenes, is proposed. A wide variety of spectroscopic methods (i.e., absorption, emission, triplet–triplet transient absorption, and spectroelectrochemistry) in combination with DFT calculations have been used for this purpose. Whereas the absorption spectra are slightly blueshifted upon branching, both the emission spectra and triplet–triplet absorption spectra are moderately redshifted; this indicates a larger contribution of the β‐linked thienyl groups in the delocalization of the S1 and T1 states rather than into the S0 state. The delocalization through the α,β‐conjugated path was found to be crucial for the stabilization of the trication species in the larger branched systems, whereas the linear sexithiophene homologue can only be stabilized up to the dication species. Branching out: Electronic spectroscopies and DFT calculations were used to investigate the excited‐state properties and charge delocalization in branched oligothiophenes. A loss of molecular conjugation was found for S0 ground states, whereas more delocalized quinoidal S1 and T1 excited states are formed upon branching (see figure). Delocalization through the α,β‐conjugated path is crucial in stabilizing the trication species.