Synthesis And Photophysical Study Of 2-(2'-hydroxyphenyl) Benzothiazole Derivatives

Organic light-emitting diodes (OLEDs) have attracted much attention due to potential application in flat-panel display and lighting. The last decade has seen great progresses in both material development and device techniques. However, the luminous efficiency and device stability are still behind what the practical applications demand. The extra low electron transporting mobility of the present n-type organic semiconductors and the consequent imbalanced transportation of the positive and negative charges in OLED have been proven to be one of the key reasons to limit the OLED efficiency and lifetime. Therefore, it is necessary to develop novel electron transporting materials with improved electron mobility.Zinc 2-(2'-hydroxyphenyl)benzothiazolate (Zn(BTZ)2) is one famous electron transporting materials used in OLEDs having comparable electron transporting ability with the well-known Alq3. However, the lowest unoccupied molecular orbit (LUMO) level of Zn(BTZ)2 can not match the neighbouring organic function layers and the metal cathode. Furthermore, similar to other electron transporting material, the electron mobility of Zn(BTZ)2 is several orders of magnitude lower than the hole mobility of most organic p-type materials.In order to study and improve the electron transporting ability, groups of 2-(2-hydroxyphenyl)benzothiazole derivatives and their zinc complexes were designed and synthesized in this thesis. The electron-withdrawing groups such as F, CF3 or electron-donating groups such as CH3, OCH3 were introduced to the phenyl ring on hydroxyphenyl frame, with the expectation that the substituents will influence the electronic state and consequently the luminous properties, frontier energy levels, and electron transporting mobility of the 2-(2'-hydroxyphenyl)benzothiazole derivatives and their Zinc (â…¡) complexes. The target 2-(2'-hydroxyphenyl)benzothiazole derivatives were purified by recrystallization and column chromatography, and characterized by 1H NMR and MS. The corresponding Zinc (â…¡) complexes are not well dissolved in common organic solvents, therefore were purified by vacuum train sublimation. Elemental analysis data confirmed the chemical structures and purities of these complexes.In addition, the 2-(2'-hydroxyphenyl)benzothiazole derivatives have intramolecular hydrogen bonding and are typical substances exhibiting the excited state intramolecular proton transfer (ESIPT) behaviors. The ESIPT properties of these compounds were studied by means of UV-Vis absorption and steady-state fluorescence in different solutions. The solvent effects demonstrate that polar and protic solvent inhibits ESIPT process and favor the short-wavelength normal emission. While the substituent effect reveal that the electron-withdrawing groups such as CF3 or CN facilitate the ESIPT process and favor the long-wavelength tautomer emission. Furthermore, three fluorescence bands, resulting from normal rotamer. tautomer and phenolate anion respectively, were detected simultaneously in protic solvents for those 2-(2'-hydroxyphenyl)benzothiazole derivatives with strong electron-withdrawing groups such as CF3 or CN. It is quite valuable that some of the present 2-(2'-hydroxyphenyl)benzothiazole derivatives with such simple chemical structures possess tri-fluorescence merit, suggesting that they may have potential applications to construct white light emitting diodes or use as media probes.