Organic light emitting diodes (OLEDs) enable the energy-efficient generation of light, and thus find application for displays or lighting. In particular, luminescent copper(I) complexes present a promising, resource- and cost-efficient class of emitting materials for OLEDs and have attracted enormous interest due to their high emission efficiencies and color tunability by ligand variation. The assessment of thermally activated delayed fluorescence (TADF) to copper(I) compounds has accelerated the development and investigation of several complex classes. Herein, novel emitting materials based on mononuclear neutral copper(I) complexes of the type (N DEGREESN)Cu(P DEGREESP)] have been developed and a deeper understanding of the structureproperty relationships was achieved by comprehensive spectroscopical studies. The investigation of a large variety of complexes by absorption and emission spectroscopy, supported by theoretical calculations and electrochemical measurements, enabled a thorough understanding of the steric and electronic effects of the ligands on the complexes' emission. Furthermore, the mechanism of thermally activated delayed fluorescence could be illustrated by means of time-resolved emission spectroscopy, and the intersystem crossing of a representative TADF complex determined in the solid state for the first time, which is essential for the design of efficient T