Highly twisted tetra-N-phenylbenzidine-phenanthroimidazole based derivatives for blue organic light emitting diodes: Experimental and theoretical investigation

A series of blue phenanthroimidazole based molecules with (D-pi-A) structures are designed and synthesized by choosing a tetra-N-phenylbenzidine (TPB) as hole transporting (D) and structurally stable phenanthroimidazole (A) as an electron transporting moieties with the different functional group at the N1 position of phenanthroimidazole. All the molecules were structurally characterized by spectroscopic (H-1, C-13, F-19 NMR and FTIR) method. These molecules showed good thermal stability with high thermal decomposition temperature (T-d ). Their physical properties, including UV-Vis, photoluminescence and electrochemical, were systematically studied. The DFT calculation also carried out to know HOMO-LUMO energy level as well as excited energy level (singlet and triplet) of the molecules. The synthesized molecules were examined via solution-processed organic light emitting diodes (OLEDs). Among all fabricated OLED devices, 1 wt% 1-tert-butylbenzene decorated molecule (PITPB-3) based device showed best electroluminescent performance compare to the other molecules, with a power efficiency of 0.4 lm/W, a current efficiency of 0.7 cd/A and an external quantum efficiency of 1.7% with Commission International deL'Eclairage (CIE) coordinates of (0.15, 0.06) at the brightness 100 cd/m (2), which is precisely meeting with the standards of European Broadcasting Union (EBU) standard for flat-panel displays and solid-state lighting. These results demonstrate that TPB integrated with phenanthroimidazole plays an important role in device performance.