Synthesis And Characterization Of Platinum(?)/Iridium(?) Complexes Containing Asymmetric Bridging Ligands Based On Pyridine

As a promising candidate for display and solid-state lighting source,organic light-emitting diodes(OLEDs) have drawn continual academic attention in the past three decades,while in the pursuit of high efficiency/long stable commercial applicable devices,luminescent materials based on third-row transition-metal complexes seem to be an especially longstanding topic.For transition-metal complexes,especially platinum(?)/iridium(?) complexes,with their electronic states undergoing strong spin-orbit coupling(SOC),formally spin-forbidden triplet excitons can be harnessed,and they usually possess phosphorescence with high quantum efficiency.However,lower energy labeled emitters(deep red/near-infrared(NIR)) are still far too few,and the performances of that are much poorer compared with the blue to orange OLEDs.Besides,in contrast to the rigorous development of mononuclear complexes,dinuclear species have been less well studied and their fundamental chemistry and applications are under-explored.However,this kind of complexes usually present special properties as a consequence of tuning the bridging ligands,the cyclometalated ligands or the two metal centers.Besides,important breakthroughs have been achieved on dinuclear species in the field of OLEDs,recently.Based on the above,in this paper,we have systemically studied several mono-/di-nuclear platinum(?)/iridium(?)complexes and analyzed the relationship between kinds of molecule structures and their photophysical performances.Solution processed OLEDs fabricated with related luminous materials performed excellent performance.Some useful information for further molecular design and device optimization were also provided.The main work of this paper includes the following parts:(1)We systemically investigated the photoelectric properties of three new deep-red quinoxaline-based iridium(?) complexes with/without 1-pyrene substituent.Large dihedral angles existed between the quinoxaline and pyrene units,rendering the lowest excited states of (c-PyMPQ)₂Ir(dpm) and (t-PyMPQ)₂Ir(dpm) possessed metal-to-ligand/intraligand charge transfer(³MLCT/³ILCT) characteristics of the non-pyrene substituted(MPQ)₂Ir(dpm) core.(c-PyMPQ)₂Ir(dpm)based device achieved a maximum external quantum efficiency(EQE) of 11.43% with a peak of 670 nm at 0.5 wt%doping.However,the easily formed pyrene-stacking configuration would most probably limit their devices performance at a higher concentration.(2)A novel iridium(?) complex grafting hole-transporting triphenylamine(TPA) unit onto cyclometalated ligand,namely(t-Bu Pyr Py TPA)₂Ir(acac),was successfully synthesized and characterized.The TPA unit contributed to the increased radiative transition as well as bathochromic shift of the spectrum.Meanwhile,by using(t-BuPyrPyTPA)₂Ir(acac) as dopant,the solution processed OLEDs presented stable NIR emission peaked at 697nm and a vibration shoulder at 764nm with a highest EQE of 0.56%.(3)Dinuclear iridium(?)complexes possessing?-conjugated bridging ligands of hole-/electron-transporting units were successfully synthesized and characterized.Dinuclear complexes exhibited similar spectral profile to that of its corresponding momonuclear counterpart.Each metal-ligand unit carried its own optical characteristic and there showed no electronic interaction between them.Solution processed OLEDs based on momonuclear complex presented stable NIR emission with a highest EQE of 1.29%.While dimetallic complex performed negligible efficiencies roll-offs at high currents.(4)Two novel mono-and dinuclear cyclometalated platinum(?) complexes of(BuPyrDPy)Pt(dpm) and(BuPyrDPy)[Pt(dpm)]₂ incorporating asymmetric pyrenyl-dipyrdine bridging ligand were synthesized and characterized.(BuPyrDPy)[Pt(dpm)]₂ presented a dual phosphorescence emissive character at room-temperature and showed an increased?than that in(BuPyrDPy)Pt(dpm).The d orbital of the second platinum ion mainly contributed to the HOMO-2 and LUMO,gave the T₁ and T₂ states a considerable ligand-to-metal charge transfer(³LMCT)nature.Solution processed OLEDs based on(BuPyrDPy)[Pt(dpm)]₂ presented stable NIR emission peaked at 695nm with a highest EQE of 0.31%.(5)Two types of asymmetric dinuclear platinum(?)complexes(Pt-1 and Pt-3)bearing bridging ligands of 2-(2,4-difluorophenyl)-5-(pyridin-2-yl)pyridine and2-(2,4-difluorophenyl)-4-(pyridin-2-yl)pyridine as well as their corresponding mononuclear counterparts(Pt-2,Pt-4,and Pt-5)were synthesized and characterized.Different chelating constructions of the second platinum(?)ions and the bridging ligands in Pt-1 and Pt-3 gave rise to two kinds of electron transition pathway during their photophysical processes.The meta-/para-carbon of nitrogen on the center pyridyl segments set different levels of ligand field strength to the second platinum(?)ions,lowering their occupied d orbital to varying degrees.Pt-1 showed an enhanced SOC that caused by the additional metal component through direct orbital hybridization at higher states,a fixed molecular skeleton induced by the additional metal-ligand bonding also helped to supress molecular distortion in excited state,ensured its high quantum yield(?,89.2% in toluene)represented among the best results in dimetallic complexes.While the second platinum(?)ion in Pt-3 seemed had no contribution to the radiative transition,only contributed to HOMO and benefit to an enlarged conjugate system.OLEDs fabricated with dimetallic Pt-1 emitter achieved superior efficiencies up to 21.38% EQE value in the kelly-green region.(6)Three novel mono-and dinuclear platinum(?) complexes containing asymmetric bridging ligand of thiophen-pyrdine-isoquinoline were synthesized and characterized.Isoquinoline was connected with the 4-bit of pyridine,this would render the bridging ligand possessed approximate ligand field strength to the two chelated metal ions.The additional metal-ligand bonding also helped to enhance the planarity of complex,suppress excited state distortion.Based on the above,dimetallic complex Pt-8 showed emission peaked at 721 nm in the NIR region,performed a 145nm bathochromic shift value compared with that of the mononuclear counterpart Pt-6.Solution processed OLEDs based on Pt-6 presented the highest EQE of 13.6%.