D–A–D’ TADF emitters for LEEC fabrication: Meta vs ortho-linking controls color and efficiency

• Two novel D–A–D′ type TADF emitters based on isophthalonitrile and phthalonitrile cores were designed and synthesized. • Both emitters exhibited AIE and mechanochromic luminescence behavior. • The ΔE ST values were below 0.11 eV, enabling efficient RISC. • LEECs based on meta- and ortho-substituted emitters were fabricated via a solution process. • LEECs doped with 50 wt.% [Ir(buoppy) 2 (dmapzpy)][PF 6 ] achieved EQE value up to 6.99%. Dual-emissive cores based on isophthalonitrile and phthalonitrile in the meta and ortho positions, respectively, were designed and developed as thermally activated delayed fluorescence (TADF) emitters. The incorporation of 50 wt.% of the ionic complex [Ir(buoppy) 2 (dmapzpy)][PF 6 ] into the blue emitting meta-substituted dye 4-(9H-carbazol-9-yl)-6-(3,6-di-tert-butyl-9H-carbazol-9-yl)isophthalonitrile , and the green emitting ortho-substituted dye 3-(9H-carbazol-9-yl)-6-(3,6-di-tert-butyl-9H-carbazol-9-yl)phthalonitrile significantly enhanced their photoluminescence quantum yields (PLQYs), reaching up to 79%, especially for the doped meta-emitter. Tuning of the emission color from blue to green was achieved by substituting the acceptor core position, and a mechanochromic response was observed. The meta-substituted emitter exhibited a more rigid structure than its ortho counterpart, as evidenced by a higher glass transition temperature (T g ), due to the reduced conformational twisting between the core and the various carbazole donor derivatives. This rigidity of meta limited the spatial separation of highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), facilitating enhanced overlap. Both emitters featured small singlet–triplet energy gaps (ΔE ST < 0.11 eV), enabling efficient reverse intersystem crossing (RISC) and moderate to long delayed fluorescence lifetimes. Solution-processed light emitting electrochemical cells (LEECs) fabricated with these emitters achieved a maximum external quantum efficiency (EQE max ) of 6.99%, with stable blue emission corresponding to CIE coordinates of (0.190, 0.382).