Computational Studies On The Phosphorescent Properties Of Transition-metal Complexes And The Structural Design Of Efficient Blue-emitting Materials

Transition-metal complexes have usually been applied in flat-panel displays and solid state lighting devices,owing to their ideal phosphorescent properties.To further tune the emission wavelength and improve the phosphorescence quantum efficiency of organometallic complexes,density functional theory(DFT)and time-dependent density functional theory(TD-DFT)were employed to simulate the molecular structures and the experimental conditions,aiming to reproduce the experimental phenomenon.Meanwhile,modifications were conducted on the structures of the molecule,in order to perfect their properties.To tune the emission wavelength towards blue,we designed a series of cyclometalated Ir and Pt complexes through incorporation of substituents into the ligand,controlling π-conjugation or changing the ligand.Moreover,we employed TD-DFT with spin-orbit coupling(SOC)effect considered to calculate the radiative decay rate constant.While,the non-radiative decay was analyzed from three respects,including the vibrational coupling,spin-orbit coupling and the thermal deactivation.As demonstrated,not only are the experimental phenomena reproduced,but also the phosphorescent properties are estimated theoretically.We hope to offer judicious strategies for further design of high-performance phosphors.As a conclusion,this thesis consists of five parts,shown below:The first part,this part sets out the development of organic light emitting diode,the structure and the operating principle of the device.The research status of organometallic complexes as phosphorescent materials is also elucidated.Based on the host and the difficult subjects concerning this field,we put forward the significance of our research.The second part,the theories involved in the computation and the theories concerning the phosphorescent properties of the transition-metal complexes,including the DFT,TD-DFT,the SOC interaction between the emitting triplet excited state and the singlet excited state,the radiative decay and non-radiative decay.The third part,density functional theory(DFT)and time-dependent density functional theory(TD-DFT)were employed to explore the electronic structures and phosphorescent properties of synthesized terdentate Pt(II)complexes [Pt(CNC)X] with carboranyl as a chelating unit(1,X= triphenylphosphine;2,X= t-butylisonitrile).To understand the marked difference in phosphorescence quantum efficiency between 1 and 2,the relaxation dynamics of excited states were deeply elucidated.Aiming to formulate the radiative relaxation,the zero-field splitting(ZFS)and the radiative decay rate constant(kr)were calculated by SOC-perturbed TDDFT(pSOC-TDDFT).Meanwhile,the temperature-independent non-radiative relaxation was analyzed by calculating the Huang–Rhys factor(S),the SOC interaction between the emitting state and the ground state.While the temperature-dependent non-radiative decay mechanism was studied by depicting the thermal deactivation process via metal-centered excited 3MC state.Based on the results,1 and 2 show few differences in the temperature-independent non-radiative rate.However,the activation barrier for the population of non-emissive 3MC is greatly raised at complex 2.Therefore,the temperature-dependent non-radiative decay behavior of 2 is considerably suppressed,which ultimately leads to the substantially enhanced phosphorescence quantum efficiency at 2.To further tune the emission wavelength towards blue,four new complexes 3-6 were theoretically designed by modifying the terdentate ligand with azole groups based on the parent complex 2.As a result,complex 4 with pyrazole modified stands out with enhanced deep-blue phosphorescence located at 434 nm.The fourth part,in this study,density functional theory(DFT)and time-dependent density functional theory(TDDFT)were employed to elucidate the photo-deactivation mechanisms of(C^N)Pt(O^O)complexes 1-4(where C∧N=2-phenylpyridine derivatives,O∧O= dipivolylmethanoate).To make through understanding of the radiative decay,the singlet–triplet splitting energies ΔE(Sn–T1),transition dipole moment μ(Sn)for S0–Sn transitions and the spin-orbit coupling(SOC)matrix elements 〈T1|HSOC|Sn〉were all calculated.Moreover,the spin-orbit coupling between T1 and S0〈T1|HSOC|S0〉and Huang-Rhys factors were calculated to estimate the temperature-independent non-radiative decay processes.Meanwhile,the thermal deactivation via metal-centered 3MC was described to analyze the temperature-dependent non-radiative decay processes.As a result,the effective SOC interaction between the lowest triplet state and singlet excited states successfully rationalize why complexes 1 and 3 have higher radiative decay rate constant than 2.While the larger 〈T1|HSOC|S0〉and lower energy barrier for thermal deactivation in 3 reasonably explain why 3 has larger non-radiative rate than 1 and 2.Consequently,it can be concluded that it is the 〈T1|HSOC|S0〉 and thermal population of 3MC that account for the non-emissive behavior of(C^N)Pt(O^O)complexes and controlling π-conjugation is an efficient method tuning phosphorescence properties of transition metal complexes.The fifth part,the photo-deactivation mechanism of heteroleptic Ir(III)(C^N)2(LX)complexes [(dfpypy)2Ir(pic)](1)and [(dfppy)2Ir(pic)](2)[where dfpypy=2′,6′-difluoro-2,3′-bipyridine,dfppy= 2-(2,4-difluorophenyl)pyridine,pic= 2-picolinate] were elucidated by density functional theory/time-dependent density functional theory(DFT/TD-DFT)to rationalize the nature of the significant differences in luminescence efficiency between 1 and 2.Specifically,the radiative deactivation was formulated through the calculation of zero-field splitting(ZFS)and the radiative decay rate constant(kr)based on spin-orbit coupling SOC.Meanwhile,the non-radiative deactivation was estimated by considering the structural distortion,the SOC between the emitting state and the ground state,as well as the thermal deactivation process via metal-centered excited 3MC state.Based on the results,1 has much smaller SOC matrix elements between T1 and S0 than 2,which determines its small non-radiative decay rate constant,thus one may understand why 1 has higher phosphorescence quantum efficiency than 2.To further explore the structure-property relationship of Ir(III)complexes,four other new complexes 3-6 were designed by incorporating trimethylphenyl(R1),phenyl(R2),ter-butyl(R3),diphenylamine(R4)to the pyridine rings of dfpypy ligand of 1,respectively.Through analyzing,complex 4 with larger radiative decay rate and smaller non-radiative decay rate may be considered as a potential candidate as robust blue-emitting material applied in OLEDs.