Effects of the CN and NH2 substitutions on the geometrical and optical properties of model vinylfluorenes, based on DFT calculations

A systematic study on the structural and photo-physical properties of model bifluorenevinylene compounds based on the density functional theory (DFT) and its time-dependent (TD-DFT) version is presented. The main aim of this work is to investigate the influence of substitution on bifluorenevinylene using strong electron acceptor CN or electron donor NH2 groups on: (a) the optimal geometry, (b) torsional potentials and (c) photo-physical properties. Our results indicate that the substitution on the vinylene bridge, leads to the twisting of molecular fragment on the side of added group and are in good overall agreement with experiment. In the case of the amino mono-substituted bifluorenevinylene, the amino group leads to non-planarity at the non-substituted portion of the molecule. The chemical modification also have a pronounced impact on the electronic properties. The shape of the potential energy curves evaluated for the lowest vertically excited states is heavily dependent on the molecular conformation. Finally, we discuss how the structural and electronic information presented here can be useful in designing of novel optical materials as well as understanding of excitation-relaxation phenomena which may occur in various time-dependent optical experiments.