Highly Efficient Bipolar Deep-Blue Fluorescent Emitters for Solution-Processed Non-Doped Organic Light-Emitting Diodes Based on 9,9-Dimethyl-9,10-dihydroacridine/Phenanthroimadazole Derivatives

Highly efficient, solution-processed, deep-blue fluorescent emitters are urgently required to realize inexpensive organic light-emitting diodes (OLEDs) for full-color displays and lighting applications. Herein, two new bipolar fluorescent emitters: 2-(4-(7-(9,9-dimethylacridin-10(9H)-yl)-9,9-diethyl-9H-fluoren-2-yl)phenyl)-1-phenyl-1H-phenanthro[9,10-d]imidazole (AFpPPI) and 2-(3-(7-(9,9-dimethylacridin-10(9H)-yl)-9,9-diethyl-9H-fluoren-2-yl)phenyl)-1-phenyl-1H-phenanthro[9,10-d]imidazole (AFmPPI) are rationally designed and synthesized. These two are afforded from 9,9-dimethyl-9,10-dihydroacridine (DMACR) as an electron donor and phenylphenanthroimadazole (PPI) as an electron acceptor, using 9,9-diethylfluorene as a spacer with different substitution isomers (para or meta). The photophysical, electrochemical, thermal, and charge-transport properties, as well as the electronic distribution of AFpPPI and AFmPPI are investigated and the results are well supported by density functional theory (DFT) and semi-classical Marcus theory. Interestingly, AFpPPI and AFmPPI display deep-blue emission with high fluorescence quantum yields (Φ_F). Furthermore, solution-processed, non-doped OLEDs were fabricated with AFpPPI and AFmPPI as an emitter, in which AFpPPI delivered a maximum external quantum efficiency (EQE) of 5.76% with Commission Internationale de l'Eclairage (CIE) coordinates of (0.15, 0.10) and a maximum current efficiency (CE) of 5.39 cd A⁻¹, which is the best performance for deep-blue, solution-processed fluorescent OLEDs based on non-doped bipolar emitters.