Theoretical Investigation On The Photophysical Properties Of Quinolyl-imidazole And POP Ligands Coordinated Cu(?) Complexes And Molecular Design

The application of transition metal complexes in organic lightemitting diodes?OLEDs?materials have received a lot of attention.The strong spin-orbit coupling?SOC?induced by heavy transition metal atoms can effectively promote fast intersystem crossing?ISC?and enable the harvest of both singlet and triplet excitons.To date,electrophosphorescent materials for OLED production have mostly been noble metal complexes based on Ru???,Ir???,Pt???and Au???among others.The advantages of these noble metal complexes are their high resolution,high luminous efficiencyand fast response speed.However,cost and environmental concerns prevent these complexes from large scale applications.Cu???complexes have emerged as a promising alternative to noble metal complexes due to their abundant storage,lower cost and environmental toxicity.The Cu???ion lacks a non-emissive metal-centered?MC?d-d* transition,has a d10 electronic configuration and can adopt various coordination geometries.Moreover,the emission wavelength of Cu???complexes can cover the whole spectral region.We carried out a theoretical study using density functional theory?DFT?and time-dependent DFT?TD-DFT?methods to investigate Cu???complexes with 2-?2'-pyridyl/quinolyl?imidazole and bis[2-?diphenylphosphino?phenyl]ether mixed ligands.To investigate the photophysical properties,the structures and energies of the ground state and the lowest triplet excited state,the highest occupied molecular orbital?HOMO?energy,the lowest virtual molecular orbital?LUMO?energy and energy gaps,absorption and emission spectra,spin-orbit coupling matrix elements and radiative decay rate?kr?were computed.After applying crystal field theory?CFT?and molecular orbital theory?MOT?to analyze the geometric and electronic structures,we found that the higher HOMO electronic distribution in the dx2-y2 orbital is,the shorter Cu-N₂ bond would be.This markedly shortened Cu-N₂ bond contributed much to enhancing the metal-ligand interaction,which could lead to larger kr.The designed complexes of 2a and 2b,with electron-donating alkyl substituents introduced into N^N ligand,can effectively promote the MLCT.The kr values of 2a and 2b are 1.34 and 22.70 times that of 2,respectively.While the knr values were kept as small as that of 2 with the increasing kr.Higher PLQYs were achieved eventually.We hope that this investigation on the microscopic mechanism of the photoluminescent properties of these complexes can provide useful information for experimentalists.