Isomeric spiro-[acridine-9,9′-fluorene]-2,6-dipyridylpyrimidine based TADF emitters : insights into photophysical behaviors and OLED performances

TADF molecules with a higher horizontal-dipole ratio have recently been realized to show a large conversion efficiency in organic light-emitting diodes (OLEDs) and hence great promise for their application in display and solid state lighting sources. The current work focuses on fine-tuning the structure of the parent 2,6-diphenylpyrimidinyl compound PhPMAF and its derivatives with a series of 2,6-dipyridylpyrimidine acceptors; namely: 2NPMAF, 3NPMAF and 4NPMAF, among which the intramolecular CH⋯N bonding interaction was confirmed for 2NPMAF, showing good agreement with the NMR results. The solvatochromism and TADF behaviors, together with TD-DFT computations, of the titled compounds were explored to gain insights into the structure-photophysics relationship. Eventually, these functional 2,6-dipyridylpyrimidine acceptor based TADF compounds exhibit electroluminescence ranging from deep-blue to sky-blue, among which an external quantum efficiency (EQE) of 24.9% was achieved for the derivative 3NPMAF. The high horizontal-dipole ratios (∼0.86-0.91) obtained for this class of compounds elucidates the superior OLED performance. Moreover, the non-doped OLED architecture achieves an EQE of 14.1% for 3NPMAF, which is superior to that of PhPMAF with an EQE of 5.1%, demonstrating the significance of dipyridylpyrimidine as an acceptor for the future structural design of TADF emitters.