Isomeric Fused Benzocarbazole as a Chromophore for Blue Fluorescent Organic Light-Emitting Diodes
In the present work, we report 7H-benzo[c]carbazole (BCz) based two isomeric chromophores, benzo[b]indolo[1,2,3-lm]carbazole (NI-1) and benzo[c]indolo[3,2,1-jk]carbazole (NI-2), formed by the fusion of BCz with phenyl ring. Two blue fluorescent emitters, N6,N6,N12,N12-tetraphenylbenzo[b]indolo[1,2,3-lm]carbazole-6,12-diamine (NIDPA-1) and N10,N10,N13,N13-tetraphenylbenzo[c]indolo [3,2,1-jk]carbazole-10,13-diamine (NIDPA-2), were developed by functionalizing the NI-1 and NI-2 chromophores with diphenylamine donors, respectively. The influence of the fusion position on the light-emitting performances of the emitters was investigated. The NIDPA-1 emitter fused through the naphthalene unit of BCz showed weak charge-transfer character. Whereas the NIDPA-2 emitter fused through the phenyl unit of BCz displayed a strong charge-transfer character, which red-shifted the emission spectrum of NIDPA-2. The NIDPA-1 doped blue device showed higher quantum efficiency, smaller full width at half maximum, and thus better blue color purity than the NIDPA-2 doped device. However, the NIDPA-2 device offered longer device lifetime and maximum luminance (Lmax) of above 70,000 cd/m2 than the NIDPA-1 device. However, the exceptional stability of both devices under high luminance (> 60,000 cd/m2) and current density (> 600 mA/cm2) demonstrates the significance of the NI based chromophores to develop stable blue emitters for the OLED applications.