| Organic light-emitting diodes(OLEDs),as the latest generation of display devices,has the advantages of high luminous efficiency,self-luminescence,high contrast,and fast response time.In recent years,many high-end models of TV screens,computer monitors,smart phones and other flat display devices have adopted OLED screens,and their development and application in solid-state lighting has also received great attention.Organometallic complexes(such as iridium,gold,platinum,etc.)are often used as guest materials doped into the light-emitting layer of phosphorescent organic light-emitting diodes(Ph OLEDs).The heavy metal atoms in the center of these complexes have a strong spin-orbit coupling(SOC)effect,which is convenient to strengthen the intersystem crossing(ISC)process between singlet and triplet states,the theoretically highest internal quantum yield can reach 100%.Among the organometallic complexes,the platinum complex with d~6 electron arrangement is generally a planar square structure,and the coordination of its molecules can be designed into two-coordinate,three-coordinate and four-coordinate structures,and diversified coordination.Body selection and coordination structure have made it widely studied.More importantly,its square planar structure is conducive to the formation of intermolecular?-?and Pt…Pt interactions,which makes it easy to show unique aggregation luminescence behavior in solid films.At present,most of the AIE molecules are fluorescent molecules,and there are relatively few reports on materials based on new aggregation-induced phosphorescence emission of transition metal complexes.Therefore,a detailed study of the optoelectronic properties of materials with AIE properties will greatly broaden the application prospects of high-efficiency solid-state light-emitting materials.Among them,the luminescence mechanism of the aggregation state of platinum metal complexes is a frontier subject worthy of study.Here,we apply density functional theory to study the photophysical properties of a series of square planar Pt(?)complexes in both the monomeric and dimeric forms.We reveal that the four monomeric Pt(?)complexes are dominated by triplet ligand-to-ligand charge-transfer(~3LLCT),the lack of the triplet metal-to-ligand CT(~3MLCT)feature plays a weak spin orbit coupling(SOC),which leads to the limited radiative rates;meanwhile,the calculated nonradiative transition rate are one or two orders of magnitude higher than those radiative rates because a large amount of reorganization energies caused by the vibration of bipyrazol ligand can not be well suppressed in the monomeric form,so as to the four monomers exhibit a photoluminescence quenching in solution in both theoretical calculation and experiment.However,in solid state,the intense luminescence phenomenon indicates obviously distinct properties between monomer and aggregation.We carried out dimer model to interpret that the interaction of Pt…Pt induces a metal-metal-to-ligand CT(~3MMLCT)excimeric state,which brings more metal components to participate in charge transfer to enhance SOC effect.At the same time,the ligand vibration can be significantly reduced by the shortened distance and there is a strong?-?packing interaction in the dimer;then,an excellent quantum yield(QY)can be achieved in aggregation.In addition,we also disclose that introducing bulky substituents bearing electron-donating groups at the R'and R"positions have little effect on the properties of the monomers,but there is positive to restrict internal reorganization energy through the intermolecular interaction when packing in solid state.It is therefore that we can tune the substitutions to improve their properties(such as emission energy and reorganization energy).We hope that our work can shine some light on the Pt(?)emitters in fabrication of efficient OLED. |
PDF Full Text Request