The Structure And Properties Of Long-wavelength Luminescent Aromatic Phosphine Oxide Rare Earth Complexes

Lanthanide complexes are attractive as optoelectronic materials owing to their features of high luminance and color purity.Although this kind of materials have achieved significant progresses,there are still some key problems should be solved,e.g.host-guest compatibility,stability,phosphorescence quenching effects and carrier transportation.In this thesis,we utilized chelate phosphine oxide neutral ligands with self-host feature to form host-guest integration structures and improve compatibility and stability,compared to doping systems.Through functionalization of neutral ligands,the singlet(S,)and triplet(T1)excited energy levels,as well as intramolecular energy transfer efficiency,were feasibly modulated at the same time of modifying carrier transportation performance.The details were as followed:1.Lanthanide complexes with unity DBFDPO,DBTDPO,DPEPO,FDPO and XPO as neutral ligands and 2-thenoyltrifluoroacetonate(TTA),1,3-diphenyl-1,3-propanedione(DBM)and 2-dibenzothiophene-trifluoroacetonate(DBTTA)as anion ligands,respectively,were synthesized.The investigation showed that the additional oxygen atoms in the chromphor cores of neutral ligands can further saturate coordination number of lanthanide ions,effectively enhance coordination field and suppress the influence of structure relaxation by solvent molecules for more efficient energy transfer from ligand to central ions and subsequently improved photoluminescent quantum yields(PLQY),at the same time of increasing thermal stability.Among them,PLQY of Eu(TTA)3(DBFDPO)is as high as 96%,while those of Yb(DBM)3(FDPO)and Nd(DBTTA)3(XPO)can also reach to 9.76%and 0.70%.All of complexes showed the excellent thermal stability with the temperature of decomposition(Td)over 240?,among which Td of Eu(DBM)3(DBFDPO)was 3250C as the highest.Through solvent diffusion approach,single crystals of Er(DBM)3(XPO),Yb(DBM)3(XPO)and Eu(DBM)3(FDPO)were obtained and used to confirm their molecular structures.It is indicated that the molecules of Er(DBM)3(XPO)and Yb(DBM)3(XPO)consist of single lanthanide ions,while that of Eu(DBM)3(FDPO)is one-dimensional chain structure formed by FDPO bridged two Eu3+ ions.This structure increased the degree of freedom of the complex and resulted in the low PLQY and Td of FDPO-based complexes.By measuring the ultraviolet absorption of Gd coordination compound as neutral ligand and phosphorescence spectrum to calculate the singlet and triplet excited state levels of those neutral ligands in rare earth coordination compound,it is found that the hole and electronic injection capacity of the coordination compound could be adjusted by selecting suitable ligands.From the perspective of the distribution of ligand and central ion,there possibly exist two energy transfer routes of transfer itself and transfer step by step in coordination compounds,and besides ligands match with the energy level of Eu3+,so there is relatively high photoluminescence quantum efficiency.However,there is a relatively large energy level difference between Yb3+,Nd3+and Er3+and ligands,so the energy loss is serious in the process of energy transfer,existing the effect of cross relaxation between molecular,which makes the luminous efficiency of Yb3+,Nd3+and Er3 coordination compounds low.In order to verify the application of Eu3+ coordination compounds in electroluminescence,five electroluminescent devices with different structures are developed,namely,(A)ITO/PEDOT:PSS/PVK:PBD:Eu complex(65:30:5)/Ca/Al;(B)ITO/PEDOT:PSS/PVK:PO:Eu complex(65:30:5)/Ca/Al;(C)ITO/PEDOT:PSS/PVK:PBD:Eu complex(65:30:5)/B3PYMPM/Ca/Al(D)ITO/PEDOT:PSS/PVK:PO:Eu complex(65:30:5)/B3PYMPM/Ca/Al(E)ITO/PEDOT:PSS/PVK/PO:Eu complex(95:5)/B3PYMPM/Ca/Al.Among them,the device(B)with coordination compound Eu(DBTTA)3(FDPO)as its luminescent layer possesses the highest current efficiency of 0.66 cd A-1,power efficiency of 0.26 lm W-1,external quantum efficiency of 0.36%and maximum brightness of 449 cd m-2,and the device(E)with coordination compound Eu(TTA)3(DBFDPO)as its luminescent layer possesses the lowest onset voltages of 4.3 V.2.Functionalized DPEPO as the neutral ligands and TTA and DBM as the anion ligands coordinated with Eu3+ respectively to get the corresponding complexes.Compared with parent DPEPO complexes,the solubility of the functionalized DPEPO Eu3+complexes modified by hole-transporting groups has been significantly improved,making these materials feasible to form the compact thin films by solution process.At the same time,the volume increase of neutral ligands enhanced the rigidity of complexes and subsequently thermal stability of the materials.The thermal decomposition temperatures of these complexes were above 268?,among which the temperature of Eu(DBM)3(DPEPODPNA2)reached to 383?.Although the volume expansion of neutral ligands after functionalization increased the distance between ligand and central ion,which reduced the Dexter energy transfer efficiency,PLQY of Eu3+complexes based on TTA still exceeds 39%.Among them,the photoluminescence efficiency of Eu(TTA)3(DPEPOCz)even reached to 85%.Moreover,PLQY of Eu3+complexes based on DBM also were around 2%,without a remarkable drop when comparing with their parent complex.It is indicated that the stronger photo-sensitive characteristic and more suitable excited state levels of these functionalized ligands were conducive to improve the intramolecular energy transfer efficiency.According to ultraviolet absorption and phosphorescence spectra of Gd3+complexes based on functionalized DPEPO derivatives,the singlet and triplet excited energy levels of these neutral ligands in their rare earth complexes were estimated.It was found that the excited energy levels can be facily adjusted and controlled by changing donor groups in these neutral ligands,and consequently influence energy transfer procedures in the complex molecules.According to the relationship to anion ligands,energy transfer processes can be divided into three possible types,namely,section energy transfer,stepwise energy transfer,and independent energy transfer.On the basis of density function theory(DFT)simulation,it was indicated that TTA and DBM provided the majority of the lowest triplet state.Therefore,the two models of section energy transfer and stepwise energy transfer are more favorable for achieving high PLQY.Simultaneously,it was showed that hole-transporting groups in the neutral ligands has a markedly contribution to the frontier molecular orbitals of the complexes,so it can facilitate carrier injection and transportation.Cyclic voltammetry experiment also indicated that the oxidation behavior of complexes has a direct relevance with the donor moieties in the complex,so the overall hole injection and transfer capability of complex can be adjusted and controlled conveniently through utilizing the functional group with different ionization energies.Owing to their good solubility of the complexes,electroluminescence devices were fabricated through spin coating technique.Except Eu(DBM)3(DPEPODEF2),whose electroluminescent spectrum showed obvious ligand-attributed phosphorescent emission peak,electroluminescent spectra of other complexes showed typical Eu3+characteristic emission with the main peak of 614 nm,which indicates efficient energy transfers from ligands to central ions.Compared with the reported electroluminescence devices of rare earth complexes produced by other wet processes,the devices in this work achieved dramatically lower driving voltages,and among them,the onset voltages of devices based on Eu(DBM)3(DPEPODEF2),Eu(TTA)3(DPEPODEF)and Eu(DBM)3(DPEPODPNA2)were the lowest as 6 V,10 V lower than those reported in literatures about electroluminescent devices of polymer matrix Eu3+complexes.This improvement can be attributed to the good electrical properties supported by the functionalized neutral ligands.The devices of Eu(DBM)3(DPEPODPNA2)showed the best performances,including the lowest onset voltage,the highest brightness(>80 cd m-2),and the biggest current efficiency(0.09 cd A-1)and external quantum efficiency(0.04%),which were much improved for 30%compared with those of commercially available Eu(DBM)3phen.3.Functionalized DBFDPO as neutral ligand and TTA and DBM as anionic ligands are used to coordinate with Eu3+ in order to get the corresponding coordination compounds.The introduction of ligand DBFDPOArn enhances the rigidity of coordination compound,which makes it possess better thermal properties than that of Eu(DBM)3(DPEPOArn),Eu(TTA)3(DPEPOArn),Eu(DBM)3(DBFDPO),and Eu(TTA)3(DBFDPO),as well as Tg>280?.Among them Eu(DBM)3(DBFDPODPNA)holds the highest decomposition temperature of 398?.At the aspect of physical properties,though there exists solvent effect,the existence of O atom with a certain coordination capacity in the chromophore connecting two diphenyl phosphine oxide groups in neutral ligands is in favor of preventing the excited state structure slackness caused by solvent effect,so the efficiency of Eu(TTA)3(DBFDPOCz)still can reach 78.73%.4.Functionalized DBTDPO as neutral ligand and TTA and DBM as anionic ligands are used to coordinate with Eu3+ in order to get the corresponding coordination compounds.The thermal properties of the compounds obtained are much enhanced compared with that of Eu(DBM)3(DBTDPO)and Eu(TTA)3(DBTDPO),and Tg>290 0C,and among them Eu(DBM)3(DBTDPOPhCz2)has the highest decomposition temperature(405?).Besides,the relative luminous efficiency of Eu(TTA)3(DBTDPOArn)is much higher than that of Eu(TTA)3(DBTDPO).Among them the efficiency of Eu(TTA)3(DBTDPOCz)has reached 34.20%.It shows that the functional modification of neutral ligand can not only enhance the thermal property of coordination compound,but also have the sensitization on the luminescence of coordination compound.