The Color Adjuestment And Performance Study Of Deep-red And Near-infrared Phosphorescent Iridium(?) Complexes

Deep red and Near infrared(DR-NIR,700?2500 nm)materials have attracted growing attention from the academic and industry because of its wide application prospects in the fields of night vision,sensor,detection,imaging,energy,face recognition and displaying.Near infrared luminescent materials with properties of high quantum yield,short phosphorescent lifetime,good thermal stability and balanced carrier transporting are very important for the fabrication of high-performance DR-NIR OLEDs.Organometallic complexes based on the heavy nuclear atoms(such as iridium,platinum,etc.)as the core fill the lowest triplet energy state(T₁)through rapid intersystem crossing due to strong spin-orbital coupling(SOC),thus can utilize all the excitons,including singlet excitons(S,25%)and triplet excitons(T,75%),formed by recombination of carriers during electroluminescence.Iridium(?)complexes have become one of the ideal luminescent materials for the preparing the high efficiency OLED due to its basic properties of simple synthetic method and facile color adjustment,high efficiency and short lifetime of excited state.The phosphorescent emission of iridium complexes originate from spin-forbidden?-?*and d-?*transitions,which means it has the hybrid characteristic of ³LC and ³MLCT,and it is usually dominated by one of the transitions.However,according to the�energy gap rule',the radiative transition rate is closely related to the photon energy,that is,inversely proportional to the cubic power of the transition energy.In the low energy region of the visible spectrum,iridium complex still has much room for improvement.Therefore,how to obtain DR-NIR emitting materials with high efficiency and different emission colors through molecular design is still a very necessary and promising research field.In this paper,1-phenylisoquinoline(piq)is used as the basic unit of cyclometalating ligand(C^N),and its benzene ring and isoquinoline ring are replaced and modified by other aromatic rings or different substituent groups at different sites,respectively.Amidine derivatives(N=C(C)=N)and guanidine derivatives(N=C(N)=N)are used as ancillary ligands(LX),thus a series of(C^N)₂Ir(LX)type Ir(?)phosphorescent complexes with DR-NIR emission based on iridium metal central ions were constructed.Through the detailed test and research methods in terms of thermal stability,photophysics,cyclic voltammetry electrochemistry,DFT calculation,OLED preparation and performance characterization,the effects and basic laws of different molecular structures on the photoelectric properties of corresponding DR-NIR phosphorescent complexes,such as luminescence color,half peak width,phosphorescent quantum yield,excited state lifetime and electroluminescence(EL)were systematically discussed.The details are as follows:1.Two deep red Ir(?)phosphorescent complexes TIQBA and TIQPG were synthesized by replacing the benzene ring in the basic unit of 1-phenylisoquinoline(piq)ligand with thiophene ring to obtain 1-thiophenyl isoquinoline as cyclometalating ligand and amidine and guanidine groups as ancillary ligands.The peak positions are 678 nm and 686nm respectively,the full width half maximum(FWHM)is less than 60 nm,and the phosphorescent life is less than 0.5?s,the PLQY is between 0.2?0.25.Through the analysis of comparative experimental and theoretical calculation results,the red shift of the above two phosphorescent molecules relative to the original Ir(piq)₃ spectrum is due to the electron rich characteristics of the new ligand,which induces the increase of HOMO energy level of the complex,resulting in the decrease of HOMO-LUMO energy gap.The decrease in the FWHM is due to the better rigidity of the four membered ring configuration formed by thiophene and ancillary ligands and central metal ions,which largely avoids the vibration at the molecular level.At the same time,this rigid and compact molecular structure can inhibit the non radiative transition and improve the luminescence quantum yield.The OLEDs based on TIQBA and TIQPG as emitting dopants have low turn-on and driving voltages,and the operating voltage is between 2.5?8.5 V.Their maximum external quantum efficiency(EQE)are 15.3%and 14.7%,and the CIE color coordinates are(0.67,0.28),It is the highest level of OLED based on phosphorescent Ir(?)complexes with approximate luminescence wavelength.2.Six Ir(?)phosphorescent complexes with DR-NIR emission o-t Fpiq BA,o-t Fpiq PG,m-t Fpiq BA,m-t Fpiq PG,p-t Fpiq BA and p-t Fpiq PG were synthesized by trifluoromethyl substitution at the o-,m-and p-positions of the benzene ring of 1-phenylisoquinoline(piq)and using amidine and guanidine groups as ancillary ligands.The maximum emission peaks were in the range of 663?697 nm.The results show that the emission wavelength of Ir(?)complex can be finely and precisely tuned by substituting or replacing the electron absorbing group trifluoromethyl at different positions of cyclometalating ligand.It is worth mentioning that electron deficient trifluoromethyl has a lowering effect on the HOMO and LUMO,but the affecting degree is different,resulting in the red shift or blue shift of the spectrum.The emission wavelengths of this series of deep red to near-infrared OLEDs are between 664 nm and 704 nm,and with the red shift of the emission spectrum,the maximum EQE of the OLEDs were gradually decreases from 15.5%to 4.0%.3.Four Ir(?)complexes with DR-NIR emission color Fpiq BA,Fpiq PG,OFpiq BA and OFpiq PG,were synthesized by introducing methoxy and fluorine atoms into the benzene and isoquinoline rings of 1-phenylisoquinoline(piq),and by using amidine and guanidine groups as ancillary ligands.Among them,the mechanism of spectral red shift caused by two kinds of substitution is different through electrochemical test and theoretical calculation.The electron absorption characteristics of fluorine atom on isoquinoline induce the decrease of LUMO energy level,and the strong?-donor characteristics of nitrogen-containing four-member ring structure formed by ancillary ligand and metal ions induce the increase of HOMO energy level,resulting in the decrease of HOMO-LUMO energy gap.Therefore,the luminescence of Fpiq BA and Fpiq PG are at 684 nm and 695 nm,respectively.The methoxy substituted OFpiq BA and OFpiq PG,due to the increase of the electron density of the C chelating fragment in the chelating ligand,the HOMO energy level of the molecule is improved,and the luminescence is further red shifted to 723 nm and 734 nm,belonging to near-infrared emission.Based on the above four molecules as doped emitters,two series of doped OLEDs were prepared.When the concentration of 5 wt%was doped,FBA5(Fpiq BA)achieved the maximum emission peak at 688 nm,and the maximum EQE was 12.9%;FPG5(Fpiq PG)achieved the maximum luminescence peak at 696 nm,and the maximum EQE was9.5%;OFBA5(OFpiq BA)was 736 nm and the maximum EQE was 4.84%;OFPG5(OFpiq PG)was 744 nm and the maximum EQE was 3.98%,and the efficiency roll off is not obvious..4.Two new cyclometalating ligands were obtained by replacing the isoquinoline ring in 1-phenylisoquinoline(piq)and 1-(3-methoxyphenyl)isoquinoline with quinazoline.Using amidine and guanidine groups as ancillary ligands,three phosphors PQZBA,PQZPG and OPQZPG were synthesized.Their emission peaks were near-infrared light emission between709?730 nm.The red shift mechanism of light color is due to the strong?-donor nature of ancillary ligands and metal central ions and the strong electronegativity of newly introduced N atoms in quinazoline.While improving the HOMO energy level of complexes,it will also reduce the LUMO energy level to a certain degree,resulting in the narrowing of band gap and red shift of luminescence.The emission peak of the prepared OLED is between724?756 nm.Among them,the device with PQZBA as the dopant emitter shows near-infrared emission of 724 nm,and the maximum external quantum efficiency is 2.4%.In conclusion,to shift the luminescence color of Ir(?)phosphorescent complex based on 1-phenylisoquinoline(piq)as the basic cyclometalating ligand to the near-infrared region,the following effective strategies are adopted:1)increase the electron cloud density of C-chelating fragment and promote the increase of molecular HOMO energy level;2)decreasing the electron cloud density of N-chelated fragment induces the decrease of molecular LUMO level;3)the strong?-donor structure with electron rich properties is used as an ancillary ligand,resulting in the increase of molecular HOMO energy level.In the above optimization process,the following laws are summarized.Reducing the electron cloud density of chelating ligand is equivalent to increasing the conjugation degree of cyclometalating ligand.Its excited state properties are mainly dominated by ³LC centered on ligand,weakening the spin orbit coupling,resulting in low PLQY.On the other hand,ancillary ligands with electron donor properties or electron clouds with increased C-chelated fragments will directly or indirectly lead to the increase of metal t_2g orbital and increase the characteristics of ³MLCT,which is conducive to improving the radiation transition rate.