Prior to investigating the photophysical in the solid state, first studied the X-ray single crystal structures of PhCz-O-DiKTa and PhCz-DiKTa to elucidate what influence if any intermolecular interactions would have on the photophysics; interactions that would be present at high doping concentration in the film state. From Figure , PhCz-O-DiKTa exhibits a larger interplanar angle (83.09°) between the PhCz and DiKTa subunits than PhCz-DiKTa (32.02°) due to the presence of the sp 3 -hybridized oxygen atom (Figure ). As a result of the more twisted geometry, PhCz-O-DiKTa exhibits weaker p×××p stacking interactions than PhCz-DiKTa in the dimer (Figure ), although they do act in a mutually supportive manner. PhCz-O-DiKTa packs in a head-to-tail manner, but each end of the molecule interacts with a separate neighboring molecule, leading to the formation of weakly interacting chains. The more planar PhCz-DiKTa also displays a head-to-tail intermolecular stacking pattern, but with both head and tail of the molecule interacting with the same neighbor, and thus strengthens molecular electronic coupling between adjacent emitter molecules, and broadens the PL. The reduced density gradient (RDG) analysis using the Multiwfn program corroborates the existence of intermolecular Van de Waals interactions as well as weakly repulsion steric interactions (Figure ), which are beneficial to restricting molecular motions.