Hot exciton transition for organic light-emitting diodes: tailoring excited-state properties and electroluminescence performances of donor-spacer-acceptor molecules.

The photophysical, electrochemical and electroluminescent properties of newly synthesized blue emitters with donor-π-acceptor geometry, namely, 4'-(1-(naphthalen-1-yl)-1-phenanthro[9,10-]imidazol-2-yl)-,-diphenyl-(2-[1,1'-biphenyl]vinyl)-4-amine (NSPI-TPA), 4'-(1-(2-methylnaphthalen-1-yl)-1-phenanthro[9,10-]imidazol-2-yl)-,-diphenyl-(2-[1,1'-biphenyl]vinyl)-4-amine (MNSPI-TPA), 4-(2-(4'-(diphenylamino)-(2-[1,1'-biphenyl]vinyl)-4-yl)-1-phenanthro[9,10-]imidazol-1-yl)-1-naphthalene-1-carbonitrile (SPNCN-TPA) and 4-(2-(4-(9-carbazol-9-yl)styryl)-1-phenanthro[9,10-]imidazol-1-yl)naphthalene-1-carbonitrile (SPNCN-Cz) were analyzed. The conjugation length in the emitters is not conducive to pure emission and hence, a molecular twisting strategy was adopted in NSPI-TPA, MNSPI-TPA, SPNCN-TPA and SPNCN-Cz to enhance pure emission. The emissive state (HLCT) of twisted D-π-A molecules containing both LE and CT (HLCT) states was tuned for high PL ( ) (LE) and high exciton utilization ( ) (CT) efficiencies by replacing triphenylamine (strong donor) with carbazole (weak donor). Among strong donor compounds, namely, NSPI-TPA, MNSPI-TPA and SPNCN-TPA, the SPNCN-TPA-based device exhibited blue emission (451 nm) with CIE coordinates (0.15, 0.08), maximum current efficiency ( ) of 2.32 cd A, power efficiency ( ) of 2.01 lm W and external quantum efficiency ( ) of 3.02%. The device with SPNCN-Cz emitter exhibited higher electroluminescence efficiencies than the SPNCN-TPA-based device, with pure blue emission (443 nm, CIE: 0.15,0.07), of 3.15%, of 2.56 cd A and of 2.45 lm W.