Synthesis Of Phenanthroimidazole Derivatives And Their Applications In Organic Light Emitting Devices

In recent years, along with the great potential in full-color displays and solid-statelighting sources, organic light-emitting devices (OLEDs) received much attention fromboth scientific and practical points. Organic small molecules are easily modified,well-organized, and of easy purification, therefore, they are widely used to develophigh-performance materials. To date, the20%theoretical external quantum efficiency hasalready been achieved in red and green PhOLEDs, however, highly efficient stabledeep-blue fluorescent emitter is still under developed. Excellent blue emitters can not onlyeffectively reduce the power consumption of a full-color OLED but also be utilized togenerate light of other colors by energy cascade to a lower-energy fluorescent orphosphorescent dopant. On the other hand, the phosphorescent emitter is usually embeddedin a suitable host to avoid self-quenching and triplet-triplet annihilation, so theperformance and cost of host materials significantly influences the efficiency anddevelopment of phosphorescent devices. Therefore, the development of efficient bluefluorescent and host materials are very important for the industrialization and practical useof OLEDs. Phenanthroimidazole derivatives have rigid planar skeleton, good thermalstability, balanced carrier injection/transport ability and high fluorescence quantum yield innear-ultraviolet region, and they are easily synthesized with simple purification and highyields. In this regard, we designed and synthesized four series of optoelectronic moleculesbased on phenanthroimidazole moiety, investigated their crystal structures, thermalproperties, photophysical properties, electrochemical properties and electroluminescent properties, and finally discussed the relationship between the structures and properties.1、 In chapter II, two phenanthroimidazole derivatives (m-DPPI and m-PPPI)connected at the para-position of the phenyl ring have been designed and synthesizedthrough simple synthetic procedure. There is no interfacial π···π interaction in the packingmode of the crystal of compound m-DPPI, it stacks mainly by C–H···π intermolecularinteractions. Both the compounds exhibit good thermal stability. The photophysicalmeasurement indicates that the introduction of1,10-phenanthroline leads to intramolecularcharge transfer within m-PPPI, but it still keeps the high triplet energy of PPI core.Compared with m-DPPI, m-PPPI possesses lower HOMO and LUMO energy levels. Thesingle-carrier devices of undoped films and guest molecules doped films for both of themindicate that the phosphorescent dyes decrease the hole current density owing to chargetrapping sites; in contrast, the electron currents are slightly increased, which should arisefrom that doped molecules creates an additional electron transporting channel.High-performance green and red phosphorescent devices using them as host materials werefabricated and they exhibited low efficiencies roll-off.2、In chapter III, the electron transfer group dimesitylboryl group was attached to thepara-postion or meta-postion of PPI core. Intramolecular charge transfer is suppressed bythe meta linkage, leading to a similar triplet energy level (2.60eV) of m-BPPI as the corePPI (2.61eV). The triplet energy of p-BPPI is2.39eV. They both undergo reversibleoxidation and reduction process to permit the formation of stable cation and anion radials.The single-carrier devices exhibite that the electron current densities for both of them aregreatly increased compared to PPI, indicating that the appending Mes2B moiety improvedthe electron injection/transport properties. Both the hole and electron current densities aregreatly decreased after doping the phosphorescent dyes, implying the doped iridiumcompound act as carrier trapping sites. The hole and electron current of p-BPPI baseddoping films are slightly higher than m-BPPI. High-performance yellow and redphosphorescent devices using p-BPPI as host material were fabricated andhigh-performance green, yellow and red phosphorescent devices using m-BPPI as host material were fabricated. The devices based on p-BPPI show better performance,indicating that on the premise of the triplet energy is large enough, the ability and balanceof carrier injection/transport play a greater role on the devices performance.3、In chapter IV, two novel electron-accepting phosphine oxide moiety attachingphenanthroimidazole hybrids have been designed and synthesized, namely DPO-PPI andDPO-2PPI. The P=O group is used as a point of saturation between PPI moiety and theouter phenyl groups, so the high triplet energy of PPI is almost preserved. The strongintermolecular interactions owing to P–O···H–C and C–H···π bonds red shift thefluorescence of films to deep-blue emission with high quantum yields (0.57for DPO-PPIand0.55for DPO-2PPI). The thermal stability and carrier injection/transport ability havebeen greatly improved after appending the DPO moiety. Moreover, they exhibit balancedcarrier injection/transport ability in non-doped films and phosphorescent emitters-dopedfilms. Compared to DPO-PPI, an additional PPI moiety in DPO-2PPI increases its electrontransporting ability. The multifunction enables adaptation of several simplified deviceconfigurations. The non-doped deep-blue fluorescent device exhibits external quantumefficiency of2.24%with CIE of (0.16,0.08), very close to NTSC blue standard CIE (0.14,0.08). High-performance green (65.4cd A-1,73.3lm W-1and18.0%) and red (19.0cd A-1,21.3lm W-1and13.5%) phosphorescent devices using them as hosts have been achieved.The relationship between molecular structures and optoelectronic properties are discussedexperimentally and theoretically.4、In chapter V, two coordination complex emitters as well as host materials Be(PPI)2and Zn(PPI)2are designed, synthesized and characterized. The incorporation of the metalatom leads to a twisted conformation and rigid molecular structure, which improve thethermal stability of Be(PPI)2and Zn(PPI)2with high decomposition and glass transitiontemperature at around475°C and217°C. The introduction of the electron-donating phenolgroup results in the emission color shifting to the deep-blue region and the emissionmaximum appears at around429nm with narrow full-width at half maximum (FWHM).Thus, then have small singlet-treplet splitting of0.35eV for Be(PPI)2and0.21eV for Zn(PPI)2. The theoretical calculations and single-carrier devices indicate that bothcompounds have ambipolar transport property and pure Be(PPI)2and Be(PPI)2doped withphosphorescent dyes possess a more balanced charge transport ability then Zn(PPI)2. Anundoped deep-blue fluorescent device using Be(PPI)2as emitter exhibits maximum powerefficiency of2.5lm W-1with CIE coordinates of (0.15,0.09). Green and redphosphorescent OLEDs using them as host materials show high performance. The highestpower efficiencies of67.5lm W-1for green PhOLEDs and21.7lm W-1for red PhOLEDsare achieved. In addition, the Be(PPI)2-based devices show low efficiency roll-off behavior,which is attributed to the more balanced carrier injection/transport property of Be(PPI)2.In summary, we have designed and synthesized four series of organic optoelectronicmolecules based on PPI core. The relationship between structures and properties have beencarefully investigated. At the same time, we got some materials with excellentperformance.