Theoretical Study On Photophysical Process Of Iridium(?) Complexes

Organic light-emitting diodes?OLEDs?have attracted much attention due to their great potential applications and broad application prospects in flat panel displays and solid state lighting sources.It attracts a large number of researchers to be engaged in the research of luminescent materials.According to different luminescence mechanism,the luminescent materials can be divided into fluorescent materials and phosphorescent materials.Between them,phosphorescent materials can make full use of both singlet and triplet excitons,so that the theoretical internal quantum efficiency of the corresponding complexes can reach 100%.How to design and develop suitable phosphorescent materials has become the focus of academic and industrial circles.In particular,iridium???complexes are considered as one of the most promising phosphorescent materials because of their advantages such as relatively short triplet lifetimes,high photoluminescent e ciency,excellent thermal and electrochemical stability,and tunable emission colors?red,green,blue?.Quantum yield?QY?and emission coulor are important indexes of luminescent performance in OLED devices.It is assumed that if the accurate prediction or evaluation of its phosphorescence QY and emission color can be made theoretically,and then the phosphorescent materials with excellent performance can be synthesized,the research and development cycle can be greatly shortened and the research and development cost can be reduced.However,few theoretical studies have accurately predicted QY and luminescent color of iridium???complexes.So in this paper,by using the density functional theory and quadratic response time-dependent density functional theory,the phosphorescence QY of some iridium complexes were predicted by semi-quantitative and quantitative methods.In addition,the vibrationally-resolved phosphorescence spectra were also simulated and the effects of ancillary ligand and solvent effects on the photophysical processes of phosphorescent iridium???complexes were also investigated.The research work mainly includes the following three parts:1.The phosphorescent properties of two Ir???complexes with thiophene-phenylquinoline as the primary ligand were investigated.One of complexes,as a reported complex,can emit red phosphorescence,and another is a novelly designed complex with 1,3,4-oxadiazol derivertive as ancillary ligand.Calculated phosphorescent spectrum of the novelly designed complex shifts 31 nm to red compared with the known one.It is a potential deep red emitting phosphorescent complex.Then the radiative rate constant was quantitatively determined,while the nonradiative decay rate was qualitatively evaluated.It shows that both the radiative decay rate constant and the nonradiative decay rate constant of the newly designed complex are lager than those of the known complex.2.The phosphorescent properties of six Ir???complexes with 2-?4-fluoro-3-?trifluoromethyl?-phenyl?pyridine?4F-3CF3ppyH?as the cyclometalated ligand?two experimentally reported and four novelly designed?are investigated.The vibrationally resolved phosphorescence spectra were calculated to elucidate the experimental spectra.The strongest emission peak is mainly contributed by 0-0 transition.The strong shoulder peak can be ascribed as benzene ring and pyridine ring skeleton fundamental vibration transitions coupled with C-H wagging vibration transitions.It was found that all of the six compounds could emit blue phosphorescence.To obtain the quantum yields,the radiative rate constant is quantitatively determined,while the nonradiative decay rate is qualitatively studied both by temperature-independent and temperature-dependent nonradiative decay processes.It showed that five compounds have high QY exceeding 0.85.As a consequence,all of them may be promising blue-emitting Ir???complexes.3.The vibrationally resolved phosphorescence spectra of an experimental reported and four novel designed Ir???complexes with 2-?2,5,2',3',4',5',6'-heptafluoro-biphenyl-4-yl?-pyridine?HFYP?as primary ligand are investigated.Phosphorescence spectra show that there are mainly two strong peaks,which can be ascribed as low-frequency vibrational modes such as the rotation of ligand plane,and benzene ring/pyridine ring in ligand HFYP1 skeleton vibration coupled with C-H in pyridine ring in plane bending vibration.The phosphorescence quantum yields were quantitatively determined by evaluating radiative decay rate constant,intersystem crossing rate constatant and temperature-dependent nonradiative decay rate constant.The results showed that for the Ir???complex with the main ligand of HFYP,the QY was relatively high when the ancillary ligand was pyridin-2-yl-1,2,4-triazolate and 1,2-diazol-5-yl-pyridine,but it was very low when the ancillary ligand was 2'-oxyphenyl-2-oxazoline,mainly due to relatively large k_nr?T?in the latter.