Not the sum of their parts : understanding multi-donor interactions in symmetric and asymmetric TADF emitters

A pair of thermally activated delayed fluorescence (TADF) emitters with symmetric and asymmetric D-A-D structure are investigated. Despite displaying near-identical photoluminescence spectrum and quantum yields, the symmetric material possesses significantly better delayed fluorescence characteristics and OLED performance. Building on a previous study of analogous D-A materials we are able to explain these differences in terms of different strengths of electronic interactions between the two donor units. This interaction lowers the energy of the TADF-active triplet state in the asymmetric molecule, increasing its singlet-triplet energy gap and leading to worse performance. This result therefore demonstrates a new strategy to selectively control the triplet states of TADF molecules, in contrast to established control of singlet states using host environment. These results also show that multi-donor TADF emitters cannot be understood simply as the sum of their isolated parts; these parts have different electronic interactions depending on their relative positions, even when there is no scope for steric interaction.