Effects of Donor Rigidity on Coumarin Delayed Emission: New Tricks from Old Materials

Starting from well-studied coumarin laser dyes, a novel donor-π-acceptor emitter is designed, synthesized, and investigated using quantum chemistry and spectroscopic approaches. Altering the donor unit from a freely rotating diethylamine (Coumarin 6) or 'rigidly planar' julolidine (Coumarin 545) to a highly twisted phenoxazine in the newly reported material red-shifts the emission and enables thermally activated delayed fluorescence (TADF) at the cost of a reduction in photoluminescence quantum yield. Separately, unexpected room temperature phosphorescence (RTP) is observed for Coumarin 6 in film, and both coumarins show triplet-triplet annihilation (TTA) delayed emission in solution. These delayed emission properties are explained by the contrasting structural properties of the different donor moieties and especially the availability of a twisted intramolecular charge-transfer state for Coumarin 6. Ultimately the rotational freedom of the donor enables emission mechanisms that have been previously overlooked for these primarily fluorescent laser dyes, as well as TADF for the new emitter.