Role of symmetry and charge delocalization in two-dimensional conjugated molecules for optoelectronic applications

Three novel conjugated phenylenevinylene (PV)-based isomers containing oxadiazole moieties have been synthesized and characterized for optoelectronic applications. These molecules are tetra-substituted at the central phenyl ring with two poly(phenylenevinylene) (PPV)-based arms and two oxadiazole-derivatized PPV (OXAPPV) arms. In the three molecules, termed p-, o-, and m-OXA-X, the OXAPPV arms are positioned in a para, ortho, or meta position, respectively, in relation to each other. Comparing these molecules, we explore the role of symmetry and charge delocalization in this class of compounds. Despite having different linear segments, they have nearly identical photophysical properties, suggesting a similar charge delocalization mechanism. However, in a LED with the molecules as emissive layers between aluminum and ITO electrodes and with PEDOT:PSS and lithium fluoride to aid in hole and electron injection, respectively, o-OXA-X exhibited the highest external quantum efficiency (EQE) of 0.46% compared to analogous devices made of p-OXA-X and m-OXA-X that showed efficiencies of 0.28% and 0.10%, respectively. We explain these differences based on the changes in the film morphology between the three molecules. Also reported are data from cyclic voltammetry and morphological data from atomic force microscopy, NMR studies, thermal characterization, and X-ray diffraction studies.