Synthesis, Structures, And Optoelectronic Properties Of Four-coordinate Organoboron Compounds

The multi-layer low-driving-voltage organic light-emitting diodes(OLEDs) have been fabricated by C. W. Tang in 1987. Since then, great efforts have been devoted to the exploration of high performance electroluminescence(EL) materials and the development of the mechanism and technology for the fabrication of the devices. It is inspired by OLEDs’ considerable industrial perspectives to develop light-weight, low-cost and large-area display devices or light sources on flexible substrates. Nowadays, high performance display panels based on OLEDs are already commercial available on cell phones, digital cameras, and TV sets. Despite this success, there is a strong demand to significantly improve the performance and durability of blue, green, red, and white OLEDs for displays and lighting applications.Four-coordinate organoboron compounds were proved to be very promising emitters in the field of OLEDs since their outstanding optoelectronic properties. In general, incorporating boron moieties into a rigid ?-system can enhance not only the fluorescence quantum yield but also the electron-transporting ability of the solid materials. The length of ?-conjugation and the substituents on the ligands are always used to tune the luminescent properties of the boron compounds. However, the categories of substituents on boron atoms are extremely limitied. Phenyl, mesityl, and fluoro are popular groups employed in most of the reported four-coordinate boron compounds reported so far. On the other hand, considering the application of OLEDs as full-color displays, three kinds of luminescent materials emitting primary colors Red(R), Green(G), and Blue(B) are generally employed. However, the blue emitters always show inferior EL performance in regard of efficiencies, lifespan, color quality, and charge-carrier injection and/or transporting etc. Therefore, the design and synthesis of deep-blue four-coordinate boron emitters are very urgent and meaningful.In our present work, the design and synthesis were performed from the following three aspects.(1) We chose the benzimidazole dervatives as the ligands to synthesize novel four-coordinate boron compounds with deep-blue emissions. We changed the ?-conjugate length to tune the luminescent colors.(2) We designed two novel boron reagents to synthesize some four-coordinate boron compounds with spiro structures.(3) We chose benzothiazole dervatives, with different bridging atoms and substituent groups, as the ligands to synthesize four-coordinate boron compounds to investigate the structure-property relationship.1. In chapter II, three four-coordinate organoboron compounds with(2-hydroxyphenyl)imidazole dervatives as the ligands were synthesized by a simple synthetic procedure. There are no intermolecular ?-? interactions in the packing structure of the single crystal due to the large steric hindrance of bulky phenyl substitutes. The compounds display bright deep-blue emissions in solutions and thin films and red-shifted emission spectra with the increasing ?-conjugate length. The highest occupied orbitals(HOMO) and the lowest unoccupied molecular orbitals(LUMO) are located in the ?-conjugate skeleton of the ligands. They have high melting points(Tm: 257-348 ?C), glass transition temperatures(Tg: 104-173 ?C) and decomposition temperatures(Td5: 349-362 ?C). They are all good glassy materials. The boron compounds were used as the emitting layers in non-doped OLEDs and the devices showed bright deep-blue EL emissions with the y-component of Commission Internationale de l’éclairage(CIE) coordinates less than 0.10, which was in accordance with the standard of deep-blue emitters defined by the National Television System Committee(NTSC). Two devices exhibit better EL performance, whose maximum brightness, current and power efficiencies are 2692 cd m-2, 2.80 cd A-1 and 1.81 lm W-1, respectively. These values improve much better than the reported deep-blue emission four-coordinate boron compounds.3. In chapter III, a novel boron reagent(bromodibenzoborole) was synthesized by lithiation and boron-exchange reactions, which was used to coordinate with the traditional ligands to synthesize four-coordinate boron compounds. The compounds have spiro structures centered by the boron atom and the diphenyl groups in the compounds are almost perpendicular to the ?-conjugate skeleton of the ligands. The compounds display bright sky-blue to blue emissions in solutions and thin films. The theoretical calculations indicate that the HOMOs are mainly located in the diphenyl groups and the LUMOs are in the ?-conjugate skeleton of the ligands. They all show very good thermal stabilities and have higher Tm(260-310 ?C) and Td5(340-384 ?C), compared to the two phenyl analogues. The devices used the compounds as the emitting layers exhibite better performance than the two phenyl analogues. The device D1 shows bright blue EL emission with the peak at 468 nm, whose turn-on voltage, maximum brightness, current and power efficiencies are 3.0 V, 4044 cd m-2, 4.95 cd A-1, and 4.52 lm W-1, respectively. The device D4 displays deep-blue EL emission with the peak at 445 nm, whose CIE coordinates, turn-on voltage, maximum brightness, current and power efficiencies are(0.16,0.08), 2.9 V, 2392 cd m-2, 3.12 cd A-1, and 3.09 lm/W, respectively. To the best of our knowledge, the device has the best perfarmance in deep-blue four-coordinate boron emitters.4. In chapter IV, 9,10-dibromo-9,10-diboraanthracene was synthesized by Grignard reagent and cross-coupling reactions and it was used to coordinate with the traditional ligands to synthesize four-coordinate boron compounds. The compounds have cross-like ?-conjugate structures with two boron atoms and two ligands are almost perpendicular to the boraanthracene core. The molecules pack loosely in the single crystal structure. The compounds display bright sky-blue to blue emissions in solutions and thin films and have high fluorescence quantum efficiencies(0.42-0.69). The HOMOs and LUMOs are separated and the compounds have bipolar charge transport properties. They have high electron and hole mobilities in the same order of magnitude 10-4 cm2 V-1 s-1, which are attributed to the special structures with electron and hole transporting channels. They show very good thermal stabilities with the Tm(370-464 ?C) and Td5(410-488 ?C). The devices used the four compounds as the emitting layers exhibite good EL performance. The EL emissions originate from the boron emitters. The device D1 shows the best performance with EL emission peak at 476 nm, whose turn-on voltage, maximum brightness, current and power efficiencies are 2.8 V, 9041 cd m-2, 7.01 cd A-1, and 7.58 lm W-1, respectively. To the best of our knowledge, the device represents the brightest and most efficient blue OLEDs based on four-coordinate boron emitters.5. In chapter V, a novel diboron-bridged ladder-type molecule was designed and synthesized by a simple synthetic procedure. The molecular structure determined by X-ray diffraction analysis demonstrated that the ladder-type molecule has a seven-ring fused ?-conjugate skeleton. The introduction of the mesityl groups can effectively keep the luminescent units apart and prevent the fluorescence to be quenched. The compound exhibit green emission in CH2Cl2 solutions and only slightly red-shift in the films. It has good thermal stability(Tm: 352 ?C; Td5: 360 ?C). The device used the compound as an emitting layer shows low EL efficiencies. Interstingly, the device used the compound as emitting and electron transporting layers shows the similar EL performance to the Alq3, whose turn-on voltage, maximum current and power efficiencies are 2.8 V, 3.65 cd A-1, and 3.58 lm W-1, respectively.6. In chapter VI, the traditional benzothiazole dervatives were selected as the ligands and eight four-coordinate organoboron compounds were synthesized by coordinating to triphenylboron and bromodibenzoborole. The para-position on the benzothiazole was introduced to different electron-denoting groups. The compound with the stronger electron-denoting groups(dimethylamine) has a wider energy gap, blue-shifted emission peak and better thermal stability than the analogue with diphenylamine. The compound with diphenyl groups has a narrower energy gap, red-shifted emission peak and better thermal stability than the analogue with two phenyl groups. The introduction of ?-conjugate groups in ligands increased the conjugated degree of the whole molecule and red shifted the emission peak. For the four N,B^N chelated compouds, the introduction of electron-denoting groups on the para-position of benzothiazole made the energy gap wider, blue shifted the emission spectra and increased the thermal stability. The synthesis and comparisons of the series gave the instructions to the following design of four-coordinate boron compounds.In summary, we designed and synthesized twenty four-coordinate orgonoboron compounds and systematically investigated their single crystal structures and photophysical, electrochemical, and thermal properties. In addition, we carried out theoretical calculations to investigate their orbitals’ attribution and energy structures. The devices used the boron compounds exhibited good EL performance. The investigation on four-coordinate compounds disclosed the relationship between molecular structrures and their optoelectronic properties and gave instrutions to the following work on the field of four-coordinate organoboron compounds.