Syntheses, Structure, And Properties Of Luminescent Organoboron Compounds

Organic light-emitting diodes (OLEDs) are the next-generation of display technique, possessing the outstanding features of low driving voltages, ultrathin flat panels, rich emitting colours and soft display, and this technique will provide us with more convenience in our life and work. In addition, OLEDs are very promising technique for general lighting. To promote the development of this technique, much effort has been devoted to the design and synthesis of various electroluminescent (EL) materials. Among these materials, organoboron compounds were proved to be very promising emitters. In addition, some of these compounds exhibited certain electron-transporting properties, so that they were employed to fabricate simple double-layer EL devices. However, the performance of the devices based on boron compounds was usually not satisfactory. Though boron compounds were extensively used in the devices with emitting colours ranging from blue to red, their applications in white-light EL devices are scare. High luminescent quantum yields of materials are very important for high-performance devices; however, most of the boron compounds possess a relatively low quantum yield, especially in the solid-state, which should be partly responsible for their relatively poor device performance. Considering the above mentioned several points, we synthesized some boron compounds with novel molecular structures or modified the structures of some known compounds to develop some high-performance bifunctional material and some highly emissive solids as well as to enrich the species of boron compounds that fit for white-light EL devices. At the meantime, we investigate the structure-property relationship of these materials.1. In chapter II, by using the intramolecular B-N chelation, we designed and synthesized four ladder-typeπ-conjugated boron compounds and one corresponding mono-boron compound. Their structures were characterized by NMR, elemental analyses, and mass spectra. We have systematically investigated their crystal structures and thermal, electrochemical and photophysical properties. In addition, we carried out theoretical calculations for these compounds to investigate their electronic structures and transition character. The influence of ladder structure, substituent and boron chelation on materials'properties was elucidated. Compared with the mono-boron compound, ladder compounds possess higher thermal stabilities, lower LUMO levels, enhanced reversibility of electrochemical reduction and obviously red-shifted emission. When the boron-bonded ethyl groups are replaced by the phenyl groups, materials'thermal stabilities will greatly increase and the HOMO and LUMO levels will decrease. In addition, the reversibility of their electrochemical oxidation will increase. Compared with the free ligands, the ladder compounds possess more coplanar ligand plane, red-shifted absorption, lower LUMO levels, and increased reversibility of electrochemical oxidation, due to the boron chelation. The boron-phenyl substituted compounds have very high thermal stabilities (decomposition temperatures higher than 350 oC), very low LUMO levels (about -3.0 eV), reversible redox properties and appreciate solid-state quantum yields. The device using these two compounds as both emitters and electron-transporting materials exhibited the intrinsic orange emission of boron compounds. They have quite low turn-on voltages (2.6-3.1 V), and the maximum brightness exceeded 8000 Cd/m2. These results indicate that these two compounds are very good bifunctional materials. Compared with the performance of previously reported devices based on boron compounds, the performance of these two devices was at very high level. These two ladder compounds had obviously higher EL performance than the corresponding mono-boron compound, indicating the superiority of the latter-type boron compounds as EL materials.2. In chapter III, four carbazolyl-contained phenol-pyridyl boron compounds were prepared. The influence of substituents and the conjugation length between ligand moiety and carbazolyl group on materials'properties were elucidated. There are no strong intermolecularπ???πinteractions in the crystal packing of these compounds, which should be attributed to the large steric hindrance of the boron-bonded substituents. These compounds possess extremely high thermal stabilities with the melting points between 346-374 oC and the decomposition temperature between 397-423 oC. The HOMO of these compounds is located on the carbazolyl group, while the LUMO is mainly located on the ligand part. The conjugation length between the ligand and carbazolyl group has little impact on the HOMO level, while the tert-butyl group on carbazolyl group push up the HOMO level. These compounds show blue emission both in solution and solid state, and there is no solvent effect on their emission spectra. According to the theoretical calculations, the lowest-energy absorption band of these compounds corresponds to the intraligandπ-π* transitions. The compound with methyl groups on ligand has smallest bandgap because the electron-donating properties of methyl groups. In device, these compounds could form exciplex with the hole-transporting material NPB. By adopting the yellow emission of exciplex and the blue emission of boron compounds, white-light EL devices were realized. This study enriched the species of boron compounds fit for white-light EL devices.3. In chapter IV, six fluorine-contained phenol-pyridyl boron compounds were synthesized using the reaction of unsubstituted, fluorinated and methyl substituted phenol-pyridine ligand with fluorine-contained boronic acids. Because of the introduction of fluorine, there are rich C-H???F hydrogen bonding interactions in the packing structure of these compounds. Due to the small steric hindrance of the substituents on boron,π???πinteractions between ligand parts are easily formed. The melting points of these compounds are obviously lower than those of related carbazolyl-contained compounds, and the melting samples of these compounds tend to crystallize when cooling. The HOMO and LUMO of these compounds are both located on the ligand part. Their lowest-energy absorption bands are attributed to theπ-π* transitions from HOMO to LUMO. Compared with the compounds with no substituents on ligand, the compounds bearing fluorine on ligand have lower HOMO and LUMO levels because of the electron-withdrawing properties of fluorine. The LUMO decreases a lot more than the HOMO, and thus the bandgap turns smaller, resulting in the red-shifted spectra. While the methyl substituted compounds have slightly higher LUMO level and obviously higher HOMO level than the compounds with no substituents on ligand, therefore, they have smaller bandgap and red-shifted spectra. The boron-bonded substituents only have a very small influence on the spectra of solution. In devices, these compounds could also form exciplex with NPB, resulting in the EL emission colors of yellow, orange and white.4. In chapter V, three boron compounds bearing dimethylamino-, diphenylamino- and carbazolyl-substituted phenylolbenzothiazole ligands were synthesized. We have obtained the single crystal of diphenylamino-substituted compound, and there are noπ???πinteractions in its packing structures. There was no solvent effect on the emission spectra of dimethylamino-substituted compound, while the emission spectra of diphenylamino- and carbazolyl-substituted compounds showed obvious red shift when the solvent polarity was increased. According to the calculations, both HOMO and LUMO of the dimethylamino-substituted compound are located on the whole ligand part. In contrast, for diphenylamino- and carbazolyl-substituted compounds, the HOMO is predominant by theπorbital diphenylamino or carbazolyl, while the LUMO is located on the phenylolbenzothiazole moiety. The absorption band of these compounds corresponds to the HOMO→LUMO transition; therefore, the diphenylamino- and carbazolyl-substituted compounds have an intramolecular charge-transfer excited state, which is responsible for the solvent effect on their emission spectra. All these three compounds have very high fluorescence quantum yields in low-polarity solvent. In the solid state, the quantum yield of dimethylamino-substituted compound is significantly lower than that in solution, while the solid-state quantum yield of diphenylamino- and carbazolyl-substituted compounds decreases a little compared with that in low-polarity solvent. This phenomenon should be attributed to two reasons: Firstly, the much larger steric hindrance of diphenylamino and carbazolyl effectively inhibits the interchromophoreπ-stacking; Secondly, the charge-transfer excited state provide diphenylamino- and carbazolyl-substituted compounds with larger Stockes shifts, which will reduce the decrease of solid-state quantum yield caused by self-absorption.In summary, we have obtained several series of organoboron compounds through the design and modification of molecular structures. The structures and properties of these compounds were characterized, and the relationship between structure and property was elucidated. We have obtained some high-performance bifunctional materials and some boron compounds fit for white-light EL devices. At the meantime, we have prepared some highly-emissive boron-contained organic solids.