Control of Intrachromophore Excitonic Coherence in Electroluminescent Conjugated Dendrimers

We present coupled electronic oscillator calculations on conjugated dendrimers that can be used in organic light-emitting diodes. The results indicate that the nature of the excited state in these materials can be strongly influenced by electronic coherences. We use the collective electronic oscillator model approach to compare and contrast primary excitations of two families of dendrimers consisting of tris(distyrylbenzenyl) chromophores attached to either nitrogen or benzene in the 1, 3, and 5 positions. The choice of the central moiety of the dendrimer provides a direct control of the coherence between the conjugated segments, which form the emissive region of the hyperbranched molecular structure. For the case of the amine-centered dendrimer, we find that excitations are delocalized across the amine unit, whereas delocalization between distyrylbenzene units is effectively blocked by the use of a benzene core unit. The calculations provide an accurate description of the measured absorption data and an understanding of the fluorescence dynamics observed. In particular, they suggest a microscopic route to controlling the polarization anisotropy of dendrimers through choice of the central moiety. Additionally, we are able to develop a qualitative reasoning for the dependence of the solid-state emission on dendrimer generation using the results from the calculation. The level of intermolecular interactions is found to depend sensitively on the location of emissive dipoles in the dendrimer cores.