Synthesis And Properties Characterization Of Phenanthroimidazole Incorporated Blue Fluorescent Materials

For realizing active matrix full-color OLED displays, it is essential to have the three primary colors, red, green and blue, blue materials are still a significant bottleneck for organic light-emitting diodes (OLEDs). Blue phosphorescent materials exhibit high efficiencies, but with high cost, poor CIE, serious efficiency roll-off and weak stabilities at high brightness. Blue fluorescent materials even with low CIEy value to near ultraviolet are easy to synthesis and purified, and show good stabilities and low roll-off at high current densities. Imidazole derivatives have been widely used in OLEDs. For example, benzimidazole moieties were modified as electron transport materials, electron or bipolar type host materials and fluorescent materials. These materials designed with incorporation of electron-withdrawn moiety for easier injection. However, they typically also have deep highest molecular orbital (HOMO) leading to larger hole-injection barriers at hole-transport/emitter junctions and thus higher operation voltages as well as low efficiencies. Phenanthroimidazole incorporated all aryl derivatives reveal properly high conjugation with nitrogen heterocyclic ring, which is a series of promising materials for emitting and electron transporting. A series of materials were designed and synthesized by conjugating different fluorescent moiety to the phenanthroimidazole. Meanwhile, the relationship between molecular structure and device performance was further studied. The phenanthroimidazole based materials were synthesized through controlling the linkage positions (2rd-3th Charptors), decreasing the aggregation (4th Charptors), tuning the conjugation of molecules (5nd Charptor), controlling the symmetrical structure (6nd Charptor) and using donor-linker-acceptor strategy. In conclusion, a lot of new effictive methods were developed for designing high performance blue fluorescent OLEDs.By conjugating different fluorescent moieties to2-(4-bromophenyl)-1-phenyl-lH-phenanthro[9,10-d]imidazole, high singlet energy levels, excellent quantum efficiencies and proper HOMO enengy levels of phenanthroimidazole derivatives were synthesized and farbricated to devices by vacuum deposition. All of the fluorescent materials exhibit high thermal stabilities (Td>410℃). Undoped devices using these fluorescent materials as the emitting layers (EML) showed a maximum current efficiency (ηcE,max) of4.65cd/A, a maximum power efficiency (ηPE,max) of3.18lm/W and CIE (0.1505,0.1565), respectively (2-NaCPI). Morever, when doped with sky blue dopant BUBD-1, a maximum current efficiency of12.97cd/A, a maximum power efficiency of5.97lm/W and CIE (0.153,0.329), respectively (ACPI) was achieved. All above results demonstrated that the HOMOs and LUMOs of C2-phenyl bridged fluorescent moieties overlaps well and shows high quantum efficiency, which is efficiency way to obtain high performance blue OLEDs;By conjugating different fluorescent moieties to1-(4-iodophenyl)-2-phenyl-1H-phenanthro[9,10-d]imidazole, high singlet energy levels of phenanthroimidazole derivatives were synthesized and farbricated to devices by vacuum deposition. All the fluorescent materials have the high thermal stabilities (Tg>115℃). Undoped devices using these fluorescent materials as the emitting layers (EML) showed a maximum current efficiency of4.99cd/A and CIE (0.153,0.174), respectively (PNPI). Morever, when doped with sky blue dopant BUBD-1, a maximum current efficiency of12.85cd/A, a maximum power efficiency of6.40lm/W and CIE (0.155,0.319), respectively (2-NaNPI) was achieved. The completely separation of spatial distributions of electron density of HOMOs and LUMOs induces intramolecular electron transport and low quantum efficiency. Moreover, lower HOMO levels affect higher threshold voltages for devices. All above results demonstrated that N1-phenyl bridged fluorescent moieties is not a best choice to obtain high performance blue OLEDs.Based on the1st charptor’s result, tert-butylbenzene was used to instead of benzene in the N1position, which aims to decrease intermolecular aggregation to obtain higher device efficiency without changing the electron density distribution.As CIE (0.1505,0.1565) in the1st charptor is not in deep blue area, broken the conjugation of phenanthroimidazole moiety was used to increase the steric hindrance and non-plannarity property and obtain deeper blue fluorescent materials. Undoped devices using these fluorescent materials as the emitting layers (EML) showed a maximum current efficiency of3.06cd/A and CIE (0.149,0.092), respectively (1-NaCBI). Morever, when doped with sky blue dopant BUBD-1, a maximum current efficiency of15.53cd/A, a maximum power efficiency of7.41hn/W and CIE (0.151,0.299), respectively (1-NaCBI) was achieved. The maximum external quantum efficiency8.15%exceeds5%in blue fluorescent OLEDs without enhanced optical outcoupling, which is far above the classical estimate for the maximum EQE of fluorescent OLEDs. All of the results exhibt that broken the conjugation of phenanthroimidazole moiety is a vaild mean to improve the CIE and efficiency of the device.Considering the symmetrical structure, we carry out easy, effitive and low cost secheme to synthesis and obtain good result.Considering the donor-π-acceptor strategy, carbazole and phenanthroimidazole moiety as a electron donor and acceptor moiety, respectively, bridged with benzene was synthesized.