Influence of push–pull group substitution patterns on excited state properties of donor–acceptor co-monomers and their trimers

Organic electronics form a very promising new generation of cheap, lightweight and flexible devices. Of special interest is the ability to engineer photo-physical properties of organic molecules by chemical modification. In this regard, the purpose of this research is to understand the influence of push–pull group substitution patterns on excited state properties of several donor–acceptor co-monomers an their trimers. Part of this work focuses on organic photovoltaic applications to demonstrate the practical use of the structure–property relations. In this context, the strong exciton binding energy determined by the electron–hole interaction is an important property. (Time-dependent) Density Functional Theory calculations showed a significant difference between linear- and cross-conjugated push–pull group pathways for the electron–hole interaction and the vertical exciton binding energy, which can be understood from simple Hückel theory. A linear relation between the dipole moment change upon excitation and the vertical exciton binding energy hints to a possible correlation, although this relation is less pronounced for the trimers. The overlap density between the frontier molecular orbitals alone already reveals valuable information about the relative size of the electron–hole interaction and the vertical exciton binding energy. Application of our findings in the context of organic photovoltaics results in significant support for cross-conjugated mesomeric push–pull group pathways in order to spatially separate the HOMO and LUMO.