Molecular Design And Luminescent Properties Modulation Of A Novel Tetra-coordinated N-heterocyclic Carbene Copper Complexes

Copper complexes have better application prospects due to their wide range of central metal copper sources and environmentally friendly characteristics,which can replace precious metal complexes.At present,the photophysical properties of copper complexes are mainly modified by ligands,and compared with traditional organic ligands,nitrogen heterocyclic carbene（NHC）ligands are more suitable for modifying metal-copper complexes because of their strongσelectron giving ability and strong stability.At present,it has been reported that there are many dicoordination and tricoordination Cu（Ⅰ）-NHC complexes with effective thermally activated delayed fluorescence（TADF）or phosphorescence,and their ligands are mainly concentrated on imidazole and imidazole derivatives,and the form of the ligand is single to limit the regulation of the complex.The ligand structure of the tetracoordination is varied,easy to control,and also has excellent photophysical properties.Therefore,tetracoordinated nitrogen heterocyclic carbene copper complexes have broader application prospects.In this paper,eleven tetracoordinated nitrogen heterocyclic carbene copper complexes were designed and synthesized,and the optical properties of the HOMO-LUMO orbital could be changed by changing the structure of the NHC ligand and the bisphosphine ligand.First,four four-coordinated NHC-Cu（Ⅰ）complexes with different electronic property groups（-H,-CH₃,-OCH₃and-CF₃）were synthesized and characterized.The luminescence characteristics of the complex are regulated by changing the electronic properties of the substituents on the nitrogen heterocyclic carbene ligand.The emission color of P1-P4 complexes is regulated from green to yellow in the range of543-557 nm.This series has excellent optical properties,high quantum yield（38.8-66.2%）and long excited state lifetime（35.4-83.3μs）,which is a more suitable luminescent material.Secondly,four tetracoordinated NHC-Cu（Ⅰ）complexes containing different polycyclic groups（naphthyl,phenanthretic,carbazole and 9-phenylcarbazole）were synthesized and characterized.The luminescence characteristics of the complex were regulated by changing the conjugation and spatial structure on the nitrogen heterocyclic carbene ligand.In the spectral range of 537-555 nm,the emission color of the corresponding complex can change from yellow-green to yellow.The quantum yield varied from 2.8%to 36.0%,and the excited state lifetime was 76.1-97.6μs in solid.Finally,three tetracoordinated NHC-Cu（Ⅰ）complexes with different bisphosphine ligands（naphphos,cyc-xantphos and xantphos）are synthesized and characterized.The luminescence characteristics of complexes are regulated by changing the spatial structure of bisphosphine ligands.In the spectral range of525-555 nm,the emission color of the corresponding complex can change from green to yellow,the quantum yield varies from 5.50%to 39.5%,and the excited state lifetime is 54.5-237μs in solid.Density functional theory（DFT）and time-bearing density functional theory（TDDFT）were used to explain the difference between the photophysical properties of the resulting complexes.