Design, Synthesis and Properties of Isophorone-based Light-Emitting Materials
|Keywords||isophorone multrbranched compound fluorescence quenching effect solvent effect two-photon electroluminescence|
In recently years, organic light-emitting diodes (OLEDs) have become an active research area of flat panel display all over the world due to the following properties: self-luminous, high-brightness, wide viewing angle, high contrast, thinness, low power consumption, fast response time and flexiblility, and have the potential! applications on mobile phones, digital cameras, vehicle display, notebook computers, televisions. In the past decades, great progress in the design of materials and device fabrication techniques have been achieved, moreover, some materials can be practicality applied. However, there are still many limitations of materials and devices, such as short working life, the low efficiency in the solid state, the poor color purity, immature production technics. It restricts the development of OLEDs. Red-green-blue materials with good color purity, high efficiency, sufficient brightness and nice thermal stability are necessary for full-color display. Compared with the other two colors, the development of red emitters is far behind in terms of both color purity and efficiency. The red-emitting materials with excellent quality are still exiguous, which is one of the restriction for the development of OLEDs. In order to get excellent red electroluminescent (EL) materials and find out the relationship between the molecular structure and optical properties, some isophorone-based EL materials have been synthesized, and their relationship between structure and properties are discussed in the dissertation.The well-known pyran-containing laser dyes have been widely used and regarded as the classicial red emitters for OLEDs, because of high fluorescent quantum efficiency. However, their performances in OLEDs share some typical problems including broad peak width at half height and poor color purity. Futhermore, their synthesis are severely complicated, and the purification is difficult, because the desired products are inevitably accompanied by double-condensed byproducts. To avoid these shortcomings, isophorone is chosen as key skeleton instead of pyran to form a series of multibranched compounds with triphenylamine (carbazole) as electron donor and malononitrile as electron acceptor. Because the isophorone has one active methyl to perform condensation reaction, the isophorone-based emitters are simply synthesized and easily purified, which is a significant advantage for large-scale commercial applications.The crystal structure is very important for the study of the relationship between the compounds and optical properties. Three new crystals are obtained by solvent-evaporating method, and their crystal structures were investigated by x-ray diffraction. The results confirmed that these compounds with intramolecular electron transfer properties possessπ-conjugated system with better electron delocalization. The cyclohexene rings are nonplanar, moreover, their two methyls are vertical with conjugated system, which is benefitable to hold up the molecular congregation. Compared to A1-1, A1-3 has bigger conjugated system and more intermolecular interaction. A2-1 has a larger torsion angle between carbazole moiety and neighboring benzene ring.It is important to learn the photoluminescence for electroluminescence. The linear optical properties of those compounds were investigated in detail, and the relationship between the initiator structures and their properties was discussed. The result shows that these compounds have obvious solvent effect. With increasing of solvent polarity, the interaction between compounds and solvents enhances, one-photon fluorescence shows obvious red-shift, and the fluorescence quantum yields of multibranched derivatives dramatically decrease. With increasing of branches, the optical properties of these materials show obvious change. The change was listed as following: (1) the linear absorption apexes show obvious red-shift due to the largeπconjugated system of multibranched materials, (2) the one-photon emitting wavelengths do not shifted so much, because the molecular structure at excited state changed and the largeπconjugated system was destroyed. With increasing of electron-donating ability, the linear absorption, one- and two-photon fluorescence peaks show red shift.Redox behaviors of these compounds were investigated to analyze the electron transfer and light-emitting properties by cyclic voltammetry. The difference of LUMO or HOMO level of these compounds is very little. Therefore, we can prepare the devices with the similar structure to study the relationship between the molecular structure and the EL properties. The OLEDs were prepared with these compounds as non-doped light-emitting layers, and the electrooptical properties of these devices were measured. With increasing of branches, the EL peaks are red shifted, however, the luminance and efficiency are decreased, which can be attributed to the narrow energy gap and strong fluorescence quenching of the multibranched compounds. With increasing of branches, the turn-on voltages enhance. A2-1 has weak elecron-donating ability due to the large torsion between carbazole and neighboring benzene, therefore, the electroluminescent peak show is blue shifted, the luminance and the efficiency decrease, and the turn-on voltage increases. The compounds with strongly electron-donating group have longer EL peak, higher luminance efficiency. Compounds with few branches have low turn-on voltage, strong luminance and high efficiency.Isophorone-based compounds probably have shrong two-photon absorption (TPA) effect, because of their large conjugated system and donor-π-acceptor structure. The results show that these compounds have large TPA cross-section, and the largest cross-section is 1260 GM. The TPA of the compounds are significantly influenced by their structure. With increasing of branches, TPA cross-section enhanced dramatically. Compounds with good plane have large TPA cross-sections.In summary, the isophorone-based materials were used as light-eimtting layers without doping, and three red-emitting materials with potential applications for OLEDs are obtained. The results show single-branched compounds are good light-emitting materials due to their low turn-on voltage, strong luminance and high efficiency. The two-photon absorption properties of the materials were measured. The results show the interaction of branches would enhance TPA cross section.