Applications Of Double-nitrogen Heterocycle Based Compounds And Iridium(Ⅲ) Complexes In OLEDs And Chemosensors

During last thirty years, organic light-emitting diodes (OLEDs) have attracted considerableattention owing to their potential applications in displays and solid-state lighting. Now, it is widelyaccepted that efficient electron and hole injection/transport from the cathodes and anodes isessential for high efficiency OLEDs. And perfect electroluminescent (EL) materials with not onlyexcellent luminescence properties but also good electron and hole-transporting ability are required.Pyrimidine and1,3,4-oxadiazole derivatives have high electron affinities, which make them goodcandidates for electron injection and transport, high photoluminescence quantum yield, and goodthermal and chemical stabilities. These features have led to their extensive applications in OLEDs,and they have been applied in a few cases as cyclometalated ligands or ancillary ligands to enhancethe optoelectronic properties of iridium(III) complexes. Meanwhile, pyrimidine derivatives canserve as potential fluorescent probes for special metal ions, since the nitrogen atoms in thepyrimidine moiety can easily bond to special metal ions. Therefore, a novel series of pyrimidine or1,3,4-oxadiazole derivatives/iridium(III) complexes have been successfully synthesized andcharacterized, and applied as EL materials for OLEDs or as fluorescent sensors. The details areshow bellow:1. Bipolar luminescent materials containing pyrimidine terminals: synthesis,photophysical properties and theoretical studyA new series of4-monosubstituted pyrimidine bipolar materials containing carbazole (PM1-PM2) or triphenylamine (PM3-PM5) moiety as p-dope type block had been synthesized byZnCl2-catalyzed three-component coupling reaction. These4-monosubstituted pyrimidinecompounds were characterized by UV vis and fluorescence spectroscopy, cyclic voltammetry, aswell as density functional theory (DFT) calculations. These PM compounds exhibited intensefluorescence in DCM at room temperature with fluorescence quantum yields of~53%93%in theblue region. Introducing more pyrimdin-4-yl moieties onto the carbazole or triphenylaminefragment, the maximum emission peaks in DCM solution blue shifted and hardly shifted in solidstate. Both PM1and PM2had lower LUMO than those of CBP (4,4’-di(9H-carbazol-9-yl)biphenyl),meaning that both of them had better electron injection and transfer abilities. Thevariation tendencies of energy levels and absorption spectra obtained from DFT calculations wellagreed with the experiment data. The DFT study also suggested that the electronic affinities and electron reorganization energies of PM compounds could be modified effectively by introducingdifferent number of pyrimdin-4-yl onto to the p-dope type block. The incorporation ofpyrimdin-4-yl could significantly decrease the electron reorganization energies and then improvethe electron-accepting and hole-electron charge balance abilities. The narrow differences betweenhole and electron reorganization energies were found in PM2and PM5, which implied that PM2and PM5could act as excellent ambipolar materials.2. Polymer light-emitting diodes based on iridium(III) complexes containing4,6-disubsitituted pyrimidine cyclometalated ligandsA series of RGB iridium(III) complexes (called PPPIrpic, FPFIrpic, NPNIrpic, TPTIrpic,DPPIrpic, DPFIrpic, and DPTIrpic) bearing4,6-disubstituted pyrimidine and picolinic acidligands had been successfully synthesized and characterized. Complexes DPPIrpic and DPFIrpicwere additionally characterized by single crystal X-ray diffraction. The electronic properties ofthese complexes are studied by time-dependent DFT calulations. The influence of substituents onthe optical, electrochemical and electroluminescence properties of these complexes are alosinvestigated. The emission color of these complexes could be simply tuned by changing thesubstituents on the cyclometalated pyrimidine ligands. In addition, compared with the iridium(III)complexes with symmetric pyrimidine ligands (PPPIrpic, FPFIrpic, and TPTIrpic), the emissionof DPPIrpic, DPFIrpic, and DPTIrpic were effectively blue shift by introducing thediphenylamine substituent on the cyclometalated chelates, respectively. The films of thesecomplexes (5wt%doped in PMMA) exhibited high photoluminsecent quantum yield in the rangeof42%~96%. Polymer light-emitting devices with the structure of ITO/PEDOT:PSS/(70%PVK+30%PBD): x wt%DPPIrpic or DPTIrpic/TPBI/CsF/Al are fabricated. The device based onDPPIrpic (DPTIrpic) furnished a maximum external quantum efficiency of9.18%(7.96%), amaximum current efficiency of14.28cd/A(15.65cd/A) and a luminance of6025cd m-2(5270cdm-2) at2.0%doping concentration.3. A new colorimetric and fluorescent ratiometric sensor for Hg2+based on4-pyren-1-yl-pyrimidineA novel fluorescent ratiometric chemosensor based on4-pyren-1-yl-pyrimidine (PPM) had beendesigned and prepared for the detection of Hg2+with high sensitivity and selectivity. The photoexhibited fluorescence color change of PPM from blue to green without and with Hg2+, which redshifted from440nm to545nm in fluorescence emission spectra. Herein, PPM could serve as a highly selective chemodosimeter for Hg2+with ratiometric and naked-eye detection. The remarkablephotophysical properties of PPM confirmed a2:1(PPM-Hg2+) binding model and the spectralresponse toward Hg2+was established to be reversible. Moreover, the Hg2+was bound to the1-position nitrogen atom of PPM, which had been proposed on the basis of1H NMR experiments.4. Multi-functional fluorescent probe for Hg2+, Cu2+and ClO-based on a pyrimidin-4-ylphenothiazine derivativeA phenothiazine derivative PzDPM (10-ethyl-3,7-di(pyrimidin-4-yl)-10H-phenothiazine) hadbeen successfully synthesized and characterized as a new multi-functional fluorescent probe for thedetection of Hg2+, Cu2+and ClO-with satisfactory selectivity and sensitivity. The probe comprisedtwo Hg2+-specific chelating arms of pyrimidin-4-yl and an electron-donating fluorophore core of10-ethylphenothiazine. The10-ethylphenothiazine core could act as Cu2+/ClO--specific reactivemoiety as well. PzDPM in MeCN exhibited intensive green fluorescence and selectively sensedHg2+/Cu2+upon coordination/reaction in MeCN, behaved as a turn-off chemosensor or ratiometricchemodosimeter, respectively. Meanwhile, PzDPM was very weakly emissive in aqueous solutionbut exhibited as excellent turn-on chemodosimeter for ClO-in1:4(v/v) MeCN: Tris HCl (10mM,pH=7.0) with the maximum fluorescent intensity increase of over110-fold. The sensingmechanism of PzDPM for Hg2+based on the coordination between PzDPM and Hg2+, while forCu2+or ClO-relied on the oxidation of PzDPM by Cu2+in MeCN or ClO-in aqueous solution togenerate the sulfoxide derivative PzODPM. The probe PzDPM allowed the determination of Hg2+,Cu2+and ClO-at10-7M levels with satisfactory selectivity.5. Cationic Iridium (III) complexes based on1,3,4-oxadiazole-pyridine ancillary ligands:synthesis, AIPE and PCL behaviors, and polymer light-emitting devicesFive cationic iridium complexes FIrLn(n=1-5) containing2-(5-aryl-[1,3,4]oxadiazol-2-yl)-pyridine, as ancillary ligands, had been rationally designed and prepared. All complexes containedthe same cyclometalated ligand,2-(2,4-difluoro-phenyl)-pyridine, while different substitutionsfunctional ancillary ligands were used to modify their photophysical properties. Their photoghysicalproperties have been investigated and compared. FIrL2with twistable N, N-diphenylaminemodified ancillary ligand exhibited more prominent aggregation-induced phosphorescent emission(AIPE) and piezochromic luminescence (PCL) behaviors than other complexes. Both the theoreticaland experimental results suggested that the AIPE phenomenon of FIrL2might be attributed to theintramolecular rotation. And powder X-ray diffraction measurement suggested that the piezochromic luminescence (PCL) behavior was associated with the morphology transformationbetween crystalline and amorphous phase. In addition, polymer light-emitting diodes (PLEDs)based on FIrLn(n=1-3) were successfully fabricated. The optimized device with the structure ofITO/PEDOT:PSS/(70%PVK+30%PBD):1wt%FIrL3/TPBI/CsF/Al furnished a maximumexternal quantum efficiency of4.75%, a maximum current efficiency of9.28cd/A and a luminanceof4100.8cd m-2.