The triplet state of organo-transition metal compounds. Triplet harvesting and singlet harvesting for efficient OLEDs

Based on a very comprehensive set of experimental data and on theoretical models, an understanding of the triplet state properties of organo-transition metal compounds is worked out. Important trends and guidelines for controlling photophysical properties are revealed. In this respect, we focus on spin–orbit coupling (SOC) and its importance for radiative as well as for nonradiative transitions between the lowest triplet state and the electronic ground state. Moreover, as is discussed on the basis of an extensive data set, summarized for the first time, the efficiency of SOC also depends on the geometry of a complex. The investigations are exemplified and supported by instructive case studies, such as efficient blue and very efficient green and red emitters. Additionally, trends being important for applications of these compounds as emitters in OLEDs are worked out. In particular, the properties of the emitters are discussed with respect to the harvesting of singlet and triplet excitons that are generated in the course of the electroluminescence process. The well-known triplet harvesting effect is compared to the recently discovered singlet harvesting effect. This latter mechanism is illustrated by use of a blue light emitting Cu(I) complex, which represents an efficient fluorescent emitter at ambient temperature. By this mechanism, 100% of the generated singlet and triplet excitons can, at least in principle, be harvested by the emitting singlet state. Potentially, this new mechanism can successfully be applied in future OLED lighting with a distinctly reduced roll-off of the efficiency.