Design, Synthesis And Properties Of Functional Phosphorescent Iridium Dendrimers

Organic light-emitting diodes (OLEDs) have been attracting considerable interestsin the past decades due to their potential applications in the next generation flat-paneldisplays and solid-state illumination sources. As critical component of OLEDs,electroluminescent materials, especially electrophosphors, usually suffer frominter/intramolecular interactions which could cause severe concentration quenching.In this thesis, to address this issue, a series of novel luminescent materials have beendesigned and synthesized. The structure-property relationships have beeninvestigated.The main contents and results are described as follows:In Chapter1, a brief introduction of the concept of OLEDs including the operatingmechanism, device structures and functional materials therein is given at first. Then,strategies and approaches of developing highly efficient phosphorescent materials foruse in polymer light emitting diodes (PLEDs) are illustrated. Finally, the designstrategies and main contents of this thesis are outlined.In Chapter2, three new triphenylamine-based homoleptic iridium(III) complexes,namely G-Ir, O-Ir and R-Ir, are designed and synthezied by simply altering theligation positions of triphenylamine units. The peripheral triphenylamine units canenhance the thermal stabilities of the complexes and suppress the intermolecularinteractions among the emissive cores. The theoretical calculations reveal that thedifference in the ligation position has significant influence on the optical andelectronic properties of the complexes. Through dispersing the phosphors into apolymer matrix, the green/orange/red phosphorescent devices achieve the maximumexternal quantum efficiencies of15.1%/18.9%/15.3%, respectively. Single-layer whitelight-emitting device is fabricated by doping sky-blue emitter iridium(III)bis(2-(4,6-difluorophenyl)-pyridinato-N,C2)picolinate (FIrpic), G-Ir, O-Ir and R-Irinto a general polymer matrix, with the maximum current/power/external quantumefficiencies of23.5cd/A/12.0lm/W/8.6%.In Chapter3, triphenylamine-dendronized orange-emitting homoleptic Ir(III)complexes, namely O-Ir1, O-Ir2and O-Ir3, with six, eighteen and up to forty-twotriphenylamine units, respectively, are designed and efficiently synthesized throughconvergent strategy. Both linear enlargement of the dendritic arms and the “double-dendron” strategy are applied to maximize the degree of site-isolation of theemissive center. The relationship between the dendritic structures and theirphotophysical, electrochemical and electrophosphorescent performances isinvestigated. Phosphorescent organic light-emitting diodes (PhOLEDs) employing thedendrimers as solution-processed emitters are fabricated. The nondoped devices basedO-Ir1and O-Ir2exhibit almost equal performance with external quantum efficiencyover15.0%. The device with O-Ir3as emission layer suffered severe drop inperformances due to the overlarge size and much annihilation of triplet excitons ontriphenylamine units. The device results also indicate that the first generation ofdouble-dendron Ir dendritic array is structurally enough for solution-processablehost-free phosphors. Highly efficient all-phosphor White PLEDs is obtained bydouble-doping FIrpic and O-Ir2.In Chapter4, four iridium dendrimers featured with electron-transportingphosphine oxide groups and/or hole-transporting arylamine units are prepared andtheir applications as nondoped emitters for solution-processed electro-phosphorescence are investigated. Different charge-transporting moieties areintegrated onto the emissive core to manipulate the charge balance and suppress theconcentration quenching and triplet-triplet annihilation. The device based on POIr2achieves the maximum current efficiency of12.4cd/A and maximum externalquantum efficiency of8.8%at a luminance of250cd/m2, which are comparable to thebest performances of the solution-processed orange-red light-emitting PhOLEDs.In Chapter5, four iridium dendrimers based on1-phenylisoquinoline ligand aredesigned and synthesized. Through the incorporation of electron-transportingphosphine oxide groups and/or hole-transporting triphenylamine units, thecharge-balance of the complexes in electroluminescent devices can be tuned. Therigid charge-transporting groups also can protect the deep-red emitting core fromtroublesome intermolecular interactions. Device based on PIQIr4emits deep-redlight centered at694nm with maximum external quantum efficiency of3.7%.In Chapter6, three phosphorescent dendrimers IrNOH comprisingtriphenylamine-featured iridium core and surfactant-like surface group are designedand synthesized. The bis(2-hydroxyethyl)amine groups are included as surface groupsto render the dendrimers soluble in alcohol and facilitate electron-injection in thedevices. Using the dendrimers as emission layer, single-layer phosphorescent devices with different cathode have been fabricated. With the density of the surface groupincreases, the performances of devices made from CsF/Al cathode improve graduallydue to the enhanced electron-injection. When Ir-NOH3is used as emission layer, thedevice with Al cathode shows almost equal performance with that using CsF/Alcathode. Device results show that the more surface groups, the betterelectron-injection ability.In Chapter7, two photo-crosslinkable iridium dendrimers are designed andsynthesized aimed at the fabrication of efficient dual-layer complementary-colorall-phosphor white PLEDs. Through attaching oxetane groups onto the dendrons assurface groups, thin layer of the so-formed dendrimers can be crosslinked byundergoing photoinitiated cationic ring-opening polymerization (CROP), withoutdegrading their electrical properties. After the procedure, Ir-6CL with six oxetanegroups can be totally insoluble. Using the dendrimer as self-host orange emitter andhole-transporting material, efficient two-layer white PLEDs have been fabricated bysolution processing blue phosphorescent emission layer ontop of orange emissionlayer. The devices achieve moderate performance since the oxonium cation formedduring CROP would quench the electroluminescence.In Chapter8, eight blue fluorescent host emitters with bifunctional chargetransport groups appended to the9-and10-positions of anthracene core have beendesigned and synthesized. By the introduction of peripheral bulky aryl-substitutiongroups to the emissive core, the compounds show a decreased tendency to crystalline.The theoretical calculations reveal that the blue emitters possess noncoplanar structureto suppress the intermolecular interaction in films. Meanwhile, the amorphouscompounds exhibit strong blue emission both in solution and solid state. Withdifferent end-capping groups, the photophysical and electrochemical properties aretuned to produce efficient deep-blue performance with simple device architecture.Nodoped dvice featuring B4as the emitter achieves a maximum power efficiency of2.0lm/W with Commisssion Internationale de L’Eclairage (CIE) coordinates of (0.16,0.10). Solution-processed nondoped devices based on the fluorene-bridged anthracenederivatives achieves a maximum current efficiency of2.0cd/A with CIE coordinatesof (0.15,0.13).