Theoretical Investigation Excited-state Properties And Molecular Design Of Blue Phosphorescence Metal Complexes

Organic light-emitting devices?OLEDs?have attracted much attention due to their great potential applications in the field of solid-state lighting and flat-panel display.Transition metal complexes have been widely used in OLEDs because they are capable of utilizing both singlet and triplet excitons to achieve an internal quantum efficiency up to 100%.Among the many phosphorescent transition metal complexes,Pt?II?and Ir?III?complexes have been widely researched for their relatively short phosphorescence lifetime,high quantum efficiency and thermal stability.It is well known that the full-color display can be achieved by using the three prime colors of red,green and blue.In comparison to commercialized applications of red and green phosphorescent metal complexes,room temperature blue phosphorescence with high efficiency and long-term operational stability remains a significant challenge.Due to the complexity of the phosphorescence emission process,there are few theoretical studies on it,especially for the properties of excited states.Herein,we have investigated the properties of the excited states of blue phosphorescent Pt?II?and Ir?II?complexes by quantum chemical calculations,especially the thorough and in-depth research of deactivation processes of an emissive excited state through both radiative and nonradiative decay pathways.In addition,we also theoretically studied the excited-state properties of Pd?II?complexes with metal-assisted delayed fluorescence?MADF?,and elaborated their luminescence mechanism.Based on the in-depth understanding of the excited-state properties,we have also designed a series of materials and predicted their luminescence properties,thus giving a hint for the future rational design of highly efficient and stable blue phosphorescent complexes.Our work mainly includes four parts as follows:Part 1:We performed a comprehensive computational investigation the excited-state properties of blue phosphorescent Pt?II?complexes with phenylene-bridged pincer ligands to rationalize the marked difference in the phosphorescence efficiencies.The geometrical and electronic structures,absorption and emission properties of Pt?II?complexes were carried out by means of density functional theory?DFT?and time-dependent DFT methods.The relevant excited-state deactivation processes that determine the quantum efficiencies,such as the radiative and nonradiative processes were analyzed in detail.In particular,the potential energy profiles for the nonradiative deactivation process via various pathways were also explored to reveal the effect of nonradiative decay on phosphorescence efficiencies.The calculated results indicate that the marked difference of the quantum efficiencies of the platinum complexes lies in their different thermal deactivation pathways,and the stability of the excited state is crucial for obtaining a highly efficient emitter.Therefore,increasing the rigidity of the complexes and raising the ligand-field strength could be favorable to making the dissociative metal-centered?³MC?state less accessible,which is beneficial to improving the efficiency.We hope that our work gives more in-depth insight into the nature of the emissive excited state,shielding light on a better understanding of the excited-state behavior of the structurally similar Pt?II?complexes,and gives theoretical guidelines on the design of stable and highly efficient blue phosphorescence Pt?II?complexes.Part 2:We have carried out the quantum chemistry studies on Ir?III?complexes with isomerized phenyltriazole?ptz?ligands.The isomerization of ptz ligands results in significant differences in the photophysical properties of the complexes.The reason why the isomerized ptz ligands result in such significant differences in quantum efficiency remains elusive,and deserves an in-depth investigation on a microscopic level.To reveal the coordinating effect of the isomerized ptz ligands on the phosphorescence efficiencies of Ir?III?complexes.We analyzed the geometrical and electronic structures,absorption and emission properties,and the parameters that affect the quantum efficiency.The calculated results show that the radiative rate constants are comparable and within the same order of magnitude,and thus the marked difference in the quantum efficiencies is ascribed to the different thermal deactivation pathways via the ³MC states and the nonradiative vibrational relaxation.This work provides an in-depth understanding of the structure-property relationships for the iridium complexes with different ptz and account for the luminescence properties of this family of iridium complexes,and reveals the excited-state behaviors,thus giving a hint for the future rational design of highly efficient phosphorescent iridium complexes with ptz ligands.Part 3:A series of dfppy?based iridium complexes?dfppy=2??2,4?difluorophenyl?pyridine?,have been investigated theoretically for screening the highly efficient deep?blue phosphorescent materials.To further obtain blue?shifted phosphorescence emission,we design the novel blue iridium complexes theoretically by the strategy of introducing strong electron?withdrawing groups,such as cyano?-CN?,trifluoromethyl?-CF₃?and o?carborane?-CB?and a bulky electron?donating?tert?butyl?group in dfppy ligands while keeping the ancillary ligand phenyl?pyridin?2?yl?phosphinate unchanged.The electronic structures,absorption and emission spectra,radiative and nonradiative decay processes,and redox stability and charge transport properties have been studied or predicted by means of DFT and TDDFT calculations.In addition,we also predict the properties of the designed iridium complexes.The calculated result that the introduction of electron?withdrawing groups in dfppy ligands could effectively tune the emission color to deep blue and the designed complexes are expected to have ideal quantum efficiencies.In addition,the designed complexes have better charge transport properties,and they may be potential to be an efficient deep?blue emitting phosphor with the high quantum efficiencies.This work provides an actionable design guidance on the stable and deep?blue phosphorescent iridium materials for the use in OLEDs.Part 4:MADF materials can achieve high-level emission that is beneficial to the blue-shift of emission color.We have theoretically investigated the excited state properties of Pd?II?complexes with carbazolyl-pyridine tetradentate ligands.The geometrical and electronic structures,absorption and emission properties,fluorescence and phosphorescence radiative rate,singlet-triplet energy gap,intersystem crossing and reverse intersystem crossing rates of the involved excited states were analyzed in detail by means of DFT and TDDFT methods.We aim to account for the luminescence properties of the MADF Pd?II?complexes,and revealing the nature of intersystem crossing and reverse intersystem crossing process of relevant excited states.Based on our understanding of the MADF,we also designed the novel Pd?II?complexes theoretically by the strategy of enhancing the electron-donating ability,and predicted the photophysical properties of designed Pd?II?complexes.The calculated results indicate that the designed Pd?II?complexes can also achieve thermally activated delayed fluorescence emission.This work provides more in-depth understanding the structure-property relationship of MADF Pd?II?complexes,and offers feasible theoretical guidance for the design of high-energy emission materials.