Carbazolyl-Substituted Benzoic Anhydrides as Efficient Triplet Harvesters for High-Brightness Blue Organic Light-Emitting Diodes

A simple and efficient synthetic strategy for the preparation of carbazole-basedC2-symmetric anhydrides with donor-acceptor-donor (D-A-D) architecture is presented. Four benzoic anhydrides incorporating carbazole or 3,6-di-tert-butylcarbazole donor units, linked to the anhydride acceptor moiety through either para- or ortho-positions, are rationally designed, synthesized, and systematically investigated. Photophysical studies reveal a strong dependence of photophysical properties on the linkage topology. While para-linked derivatives exhibit neither delayed fluorescence nor phosphorescence, ortho-substituted anhydrides display small singlet-triplet energy gaps and consequently show thermally activated delayed fluorescence (TADF), as well as unexpected room-temperature phosphorescence (RTP). Therefore, compounds bearing ortho-linked carbazole donor moieties are successfully employed as triplet-harvesting emitters in sky-blue organic light-emitting diodes (OLEDs), delivering electroluminescence maxima at 475 and 481 nm, maximum external quantum efficiencies of up to 21.0%, high luminance exceeding 62 000 cd/m2, and low efficiency roll-off. These results demonstrate that the formation of anhydrides from donor-substituted aromatic acids represents a versatile and effective design strategy for highly emissive TADF/RTP materials and provides a promising platform for the development of efficient multicolor OLED emitters through rational molecular engineering.