Solution-processed deep-blue (y∼0.06) fluorophores based on triphenylamine-imidazole (donor-acceptor) for OLEDs: computational and experimental exploration

Abstract

Developing solution-processable deep-blue emitters for organic light-emitting diodes (OLEDs) is still a challenging task. In this context, two new solution-processable deep-blue emitters, N, N-diphenyl-4’-(1-(3-(trifluoromethyl)phenyl)-1H-phenanthro[9,10-d]imidazol-2-yl)-[1,1’-biphenyl]-4-amine(4-PIMCFTPA) and 4’-(4,5-diphenyl-1-(3-(trifluoromethyl)phenyl)-1H-imidazol-2-yl)-N,N-diphenyl-[1,1’-biphenyl]-4-amine (4-BICFTPA), were successfully designed and synthesized by incorporating phenanthroimidazole (PI)/diphenylimidazole (BI)-triphenylamine (TPA), which is functional at the N1 position of the imidazole, with  Ph-mCF3. The thermal, photophysical, and electrochemical properties of both fluorophores were systematically explored. These fluorophores showed a deep-blue emission in the solution as well as in the solid state. The highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) energy level of the fluorophores was calculated using electrochemical studies and compared with the theoretical calculation [the density functional theory (DFT)]. The asymmetrically twisted conformation of 4-PIMCFTPA between PI-TPA efficiently showed a high photoluminescence quantum yield. OLED (undoped and doped) devices were fabricated with the newly synthesized emitters, and 4-PIMCFTPA demonstrated better electroluminescence (EL) performance than the BI-based emitter. Thus, the OLED based on 4-PIMCFTPA (1 wt% in the CBP host) had the best EL performance, with a maximum external quantum efficiency 1.7% and CIE coordinates of (0.17, 0.06).