Investigtion On Molecular Structure, Optical Electrical Properties Of Bis (2-(2-Hydroxyphenyl) Benzothiazolate) Complexes

With reaching a consensus on the development of "low-carbon economy" in the global, organic light emitting devices (OLEDs) have become preferential development technology due to its advantage of environment friendly, low power consumption and high efficiency. OLED has been called as an "intriguing" display in that it can realize flexible display as a new-type display technology and solid-state lighting. OLED lighting is at least 5 times more efficient than conventional incandescent lighting. Widespread adoption of OLED lighting could actually result in a decrease in greenhouse gas emissions.Organic electroluminescent (EL) materials play the core role in OLED. Organic metal complexes have in particular attracted considerable attentions because of offering many attractive properties such as acting as double roles of electron transport and light emission, environmental stability, ease of sublimation, excellent film-forming ability and a much greater diversity of tunable color. Blue-emitting materials are requisite in order to achieve full-color display. Therefore, a series of blue-emitting materials were synthesized in this paper by virtue of different substitutions and changing coordinated metal center. The molecular structures were confirmed by single-crystal x-ray diffraction. The relationship molecular structure, optical and electrical properties were investigated.1. It is well known that beryllium complexes have prominent performance in blue OLEDs such as Beq2, Bebq2. The blue-emitting bis(2-(2-hydroxyphenyl) benzothiazolate)beryllium (Be(BTZ)2) was synthesized and characterized by single-crystal x-ray diffraction. The crystal data for Be(BTZ)2 are monoclinic, space group C2/c. It exhibits strong blue luminescence in solution, powder and solid thin-film. Be(BTZ)2 have a better electron-transport property than that of the well-established electron transporter mer-Alq3 by using density functional theory (DFT). The maxmum peak at EL spectra of the device with single-layer Be(BTZ)2 is located at 460nm.2. Zn(3-MeBTZ)2, Zn(4-MeBTZ)2 and Zn(5-MeBTZ)2 were synthesized by introducing methyl into the ortho-, meta- and para-position of hydroxyl group on phenoxide of 2-(2-hydroxyphenyl) benzothiazolate (BTZ) ligand. The steric hindrance provided by methyl group of phenoxide ring prohibits effectively the formation of pentacoordinate complex. Zn(3-MeBTZ)2 and Zn(5-MeBTZ)2, which are the special bi-molecular structures, have distinct red shifts relative to Zn(BTZ)2 in emission spectra. Zn(4-MeBTZ)2, which is confirmed as four-coordinate monomeric structure by single-crystal x-ray diffraction, shifts to the blue light range. The order of fluorescence quantum yields of them is Zn(4-MeBTZ)2>Zn(5-MeBTZ)2>Zn(BTZ)2>Zn(3-MeBTZ)2. The fluorescence quantum yields of Zn(4-MeBTZ)2 was determined to be 1.56 times than that of Zn(BTZ)2. The results also indicated that the electroluminescent property of Zn(4-MeBTZ)2 is superior to other zinc complexes.3. Zn(4-OCH3BTZ)2, Zn(4-FBTZ)2 and Zn2(4-tfmBTZ)4 were synthesized by attaching the electron-withdrawing groups (EWGs)-F,-CF3 and electron-donating groups (EDGs)-OCH3 to the meta-position of hydroxyl group on BTZ. Zn(4-OCH3BTZ)2 and Zn(4-MeBTZ)2 exhibit four-coordinate geometric structure, and the crystal is triclinic, space group P-1. The theoretical calculations and experimental data suggested that complex Zn(4-FBTZ)2 should exist as monomer structure. Zn2(4-tfmBTZ)4 have the dimer structure and crystal is monoclinic, space group P21/n. The emission spectra of Zn(4-RBTZ)2 complexes exhibit blue-shift emission besides Zn2(4-tfmBTZ)4 compared with [Zn(BTZ)2]2. A gradual increasing red-shift order in emission peak is Zn(4-OCH3BTZ)2 < Zn(4-FBTZ)2< Zn(4-MeBTZ)2< Zn2(4-tfmBTZ)4=[Zn(BTZ)2]2. With the assistance of quantum chemical calculations, the properties of ground and excited states for all complexes were studied. The functional group fluorine behaves distinct electron-donating property due to its inductive and mesomeric effects. The EDGs of -OCH3 and -CH3 can destabilize both HOMO and LUMO energy levels. While EWG of -F group greatly stabilizes the HOMO energy and destabilizes the LUMO energy level. The EDG of -CF3 stabilizes both HOMO and LUMO energy levels.4. A series of WOLEDs were fabricated by using Zn2(4-tfmBTZ)4 as emitting layer and N,N'-diphenyl-N,N'-bis(1-naphyl)-(1,1'-biphenyl) (NPB) as hole-transport layer. The color rendering index (CRI) can reach to 87.9. With increasing the thickness of emitting layer, the emission of exciplex becomes stronger while the emisson of NPB and Zn2(4-tfmBTZ)4 become weaker. The formation of exciplexes is attributed to the high energy barriers between interface of Zn2(4-tfmBTZ)4/NPB.