Theoretical Study Of The Ligand Isomerization Effect On The Photophysical Properties For A Series Of Ir(?) And Pt(?)Complexes

The spin orbit coupling(SOC)effect of transition metal complexes enable the radiative transition from T1 to S0 state to occur,and to increase the rapid intersystem crossing(ISC)rate between Sn and Tm states,to lead to high phosphorescence quantum efficiency(?).Among these complexes,Ir(?)and Pt(?)complexes as efficient triplet dopant emitters in organic light-emitting diode(OLED)are attracting great attentions due to their good photophysical and electrochemical properties.However,at present,the relationship between structure and properties and the essence of luminescence are still a great challenge for designing high efficiency materials.Theoretical calculations are playing a more and more important role in the molecular design of organic optoelectronic materials,and theoretical calculations can give a better understanding of the luminescence process and charge transport mechanism,which is feasible for designing molecular and predicting the luminescence mechanism.In this work,we investigated the ligand isomerization effect on the photophysical properties for Ir(?)and Pt(?)complexes(mainly focus on the phosphorescence efficiency)by DFT/TDDFT method.Two isomeric modes are discussed.Firstly,in benzopyridine ring,one-CH was replaced by N atom to form a naphthyridine ring ligand,the influence of the positional isomerism of the nitrogen atoms in the naphthyridine ring on the photophysical properties of Ir(?)complexes were discussed.Secondly,the effect of substituting positions on the photophysical properties of Ir(?)and Pt(?)complexes were explored by replacing the H atoms on the phenylpyridine ligand with B(Mes)2.We calculated the spin orbital coupling(SOC)as well as zero-field-splitting(ZFS)parameters.In addition,the Huang-Rhys factor,the degree of vibronic coupling between S0 and T1 states were also computed to describe the non-radiative decay process.It is hoped that it can provide valuable reference for improving and designing new and efficient luminescent materials experimentally.The main studies are summarized as follows:1.An Isomeric Strategy for Enhancing Phosphorescence Efficiency of Iridium(?)Complexes with N-heterocyclic Naphthyridine Ligands:A Theoretical StudyThe electronic structure and photophysical properties of a series of Ir(?)complexes with different N-heterocyclic naphthyridine ligands were investigated.As the results,the Esn,the ETm and the energy splitting can be regulated by the position of two nitrogen atoms in naphthyridine ring for studied complexes.Moreover,Ir(?)complex inclusive of quinoxaline heterocyclic ring presents large kr and knr,so its phosphorescence quantum efficiency is difficult to be high.While two Ir(?)complexes bound to quinazoline heterocyclic ring show weakly emissive because of large knr.Notablely,the presence of the cinnoline heterocyclic ring in the Ir(?)complex possesses fast singlet-triplet intersystem(ISC)rate and k,but low knr,then leads to its high phosphorescence quantum efficiency.2.Theoretical Studies on Photophysical Properties of Isomeric Iridium(?)Complexes Ir(ppy)2(acac)Containing Dimesitylboron MoietyThe phosphorescent photophysical properties for a series of Ir(?)complexes containing dimesitylboryl moiety were investigated.The results reveal that the introduction of B(Mes)2 group to the pyridine ring of the phenylpyridine ligand can strengthen the interactions between the metal and the acetylacetone ligand,reduce the structure relaxation from the ground state to the excited triplet state,and maintain the structures of octahedral field,which is conducive to restricted non-radiative transition.In addition,compared with the substitution at the pyridinyl,modifying phenyl group with B(Mes)2 could induce larger structural changes from S0 to T1 state and enhance the<S0?HSOC?T1>value.The variety of substitution position of B(Mes)2 group leads to different-splitting,different spin-orbital coupling effect in the x,y or z direction,induces the changes of zero field splitting energy and the inequality of radiative transition rates in the three substates(namely,Tx,Ty,and Tz).3.Theoretical Investigation on the Phosphorescence Quantum Yields of Cyclometalated(C?N*)Pt(?)(acac)Complexes:The impact of the position of the B(Mes)2 Moiety on the Radiative and Nonradiative Decay ProcessesThe radiative and non-radiative decay processes of four cyclometalated(C?N*)Pt(?)(acac)complexes were investigated.The results show that,the fatal factor to the low phosphorescence quantum yields in complex Pt(ppy)(acac)is a fast non-radiative decay caused by a large Huang-Rhys factor(Sm)and the maximum reorganization energy appearing in the high frequency range.However,the introduction of the B(Mes)2 moiety,a bulky and featuring strong electron-withdrawing character and vacant p-orbital of B atom,could decrease the values of Huang-Rhys factor by suppressing the C-C stretching vibrations in the ppy ligand,inhibiting the non-radiative decay process.Besides,the introduction of the B(Mes)2 group to the,meta-position of the pyridine ring in(C?N)chelate,added the?(aryl)-p(B)delocalization interaction,resulting in a large ZFS and kr,a small<S0?HSOC?T1>2,thus leading to a large phosphorescence efficiency.