Theoretical Studieson Photophysical Propertiesand Phosphorescence Efficiencies Of The Platinum(Ⅱ) And Iridium(Ⅲ) Complexes

Transition-metalcomplex can effectively induce spin-orbit coupling (SOC),which makes singlet-triplet excited states sufficiently mixed and in most casesfacilitate intersystem crossing (ISC), and the photoluminescence quantum effciency(ΦPL) may be almost100%. Among these complexes, Pt(II) and Ir(III) complexes asefficienttriplet dopant emitters in organic light-emitting diode (OLED) andlight-emitting electrochemical cell (LEC) are receiving great attentions due to theirgood photophysical and electrochemical properties. However, the experimental designand synthesis of emitters for highly efficient light-emitting devices is still a grandchallenge because of the complicated charge transportation and emission process. Itshould be emphasized that quantum chemical calculations have been become animportant aid to understand charge transportation, photophysical property andΦPL,and thus reveal the structure-property relationships.In this paper, we employ density functional theory (DFT) and time-dependentDFT (TD-DFT) including SOC to explain and predict the luminescenceeffciency andmechanism of Pt(II) and Ir(III) complexes based on the analysis of the geometries,electronic structures, absorption and emission properties, and thus to providetheoretical guidelines for the development of high efficient triplet dopant emitters.The relative studies are described as follows.1. Considering the lack of high efficient blue phosphorescenct complex, theeffects of different N-heterocyclic carbene (NHC) ligands on the emission wavelengthandΦPLof Pt(II) complexes were investigated.Non-radiative and radiative decay rateconstants (knrand kr) were determined for each system through analyses of thegeometric relaxations, d-orbital splitting and SOC at the optimized S0and T1geometries. Our calculate results show that three Pt-systems bound to twoN-heterocyclic carbenes were shown to be nonemissive, while the6pz(Pt) of LUMOfor tetra-carbene complexes can induce strong SOC and faster kr. In addition, theresults highlight the coupling of ligand-identity to photophysical properties and moreimportantly, the potential for rational optimization and tuning of emissionwavelengths and phosphorescent efficiencies. Encouragingly, two of thetetra-N-heterocyclic carbene ligated systems show strong potential to serve ashighly-efficient blue and green light emitting materials, respectively.2. DFT and TD-DFT including SOC calculations were systematically carried out to rationalize the charge carrier injection/transportation and phosphorescenceefficiencies of recently synthesized two triarylboron functionalized phosphorescentPt(II) complexes. To gain high efficient multifunctional phosphorescent OLEDemitters, according to the ancillary ligand modification and variation, we designedfour complexesbased on the two experimental structures. It is found that onlymodifying the ancillary ligand of the trifunctional Pt(II) complex has little effect onthe emission nature and ΦPL, whereas5-6with the altering electron-donating ligandshave the balanced charge carrier injection/transport features. Combined withcharacterization of zero-field splitting, phosphorescent lifetimes and geometryrelaxation,5and6can server as promising candidate to be efficient multifunctionalphosphorescent OLED emitters.3. To deeply understand the aggregated-induced emission (AIE)mechanism ofcationic Ir(III) complexes, three structurally similar Ir(III) complexes1-3weredesigned and synthesized. Complexes2and3are AIE-active, whereas1is not. Weperformed detail DFT calculations on the thermal deactivation pathway of3MCandthe nature of T1emitting states as well as geometry relaxationbetween S0and T1statesfor AIE Ir(III) systems to decipher their AIE mechanism. Thecombinationexperimental and theoretical results suggest that the deactivationprocesses of them are similar, while the larger structural relaxations as well as weak3ILCT emissive excited-states characters arethe main reason for their non-emission insolution. The enhanced ΦPLin solid states is attributed to restriction of non-radiativepathways to some extent.