Synthesis And Properties Of Thermally Activated Delayed Fluorescent Materials Based On Triazine And Pyrazine

Organic light-emitting diodes (OLEDs) have been developed rapidly in recent years for potential applications in new-generation flat-panel displays and solid-state lighting. In comparision with traditional fluorescent materials and phosophorescent materials, thermally activated delayed fluorescent (TADF) materials can harvest both singlet and triplet excitons and the internal quantum efficiency of its OLEDs is possible to achieve a theoretical limit of unity.Thermally activated delayed fluorescent materials consist of organic small molecules which have stable chemical properties and do not need to coordinate with the precious metals, all of these properties can make cost significantly reduced. Therefore, developing novel TADF materials has the extremely important significance.Triazine derivatives are a kind of excellent electron transporting materials, which can provide steric hindrance to reduce the stacking of molecules.Their applications in OLEDs have been reported for many times. OLEDs using triazine derivatives as host and dopant respectively both exhibit high efficiency. Pyrazine and its derivatives usually have high fluorescence quantum yields, excellent thermal and chemical stability, and are thus widely used in OLEDs. But the utilization of cyano-decorated pyrazine to construct TADF molecules has not been reported yet. Carbazole derivatives are a class of hole transporting materials with very strong electron-donating properties. The carbazole containing compounds usually have non-planar conformation and high hole mobility and good thermal stability. In this thesis, two series of novel fluorescence materials containing N-phenylcarbazole as p-type units and triazine or cyano-decorated pyrazine as n-type units were designed and synthesized, for potential application as light-emitting materials or host materials in OLEDs.Their photophysical properties have been studied by UV-Vis absorption and steady-state and time-resolved photoluminescence spectra. And the electrochemical properties have been studied by cyclic voltammetry (CV) measurements as well. All compounds exhibit reversible reduction process in the negative potential region in CV measurements. The frontier molecular orbitals (HOMO and LUMO) distribution for compounds was calculated by Gaussian 03 package. It was observed that the HOMO and LUMO of all these compounds are almost completely separated. The nanosecond transient-absorption spectra and time-resolved fluorescence spectra measurements were used to further study the photophysical behavior of compound M5. The transient-absorption results show that M5 has a long excited state lifetime (32.2 us) at room temperature, which can be assigned to the triplet excited states. This indicates that triplet excited states can be easily formed even at room temperature for this organic molecule. At an emission wavelength of 500 nm, the fluorescence decay includs a fast (nanosecond) component followed by a slow (microsecond) component. The short-lived component of M5 belongs to prompt fluorescence (PF), and the long-lived component belongs to delayed fluorescence (DF). By laser induced emission method, the lifetime of the delayed fluorescence was determined as 52 ?s in N2-saturated toluene solution, which is almost 1000 times longer than that of traditional fluorescence.In addition, the triazole-based material Ml was used as the bipolar host material to fabricate blue and green phosphorescent OLEDs with famous FIirpic and Ir(ppy)3 as the emitting guests, respectively. These blue and green PHOLEDs exhibited excellent performances. Particularly, the green phosphorescent OLED with Ml as host delivered the maximum efficiencies of 96.9 cdA"1 and 28.7%. The results suggest that Ml is not only a potential light-emitting material, but also a promising host material with excellent performance.