The Influences Of Electron Donating Groups On The Photophysical Properties Of NNI-R Series Molecules

Since the triplet state of organic molecules has a long emission life and a long exciton migration distance,the room temperature phosphorescent?RTP?materials are widely used in the fields of organic light-emitting diodes,photodynamic therapy,biosensors and biological imaging.So far,the design and research of traditional RTP materials mostly contain precious metal elements.The application of the organometallic complexes is limited in some extents for their high cost and potential biotoxicity and instability.In recent years,researchers have focused on the development of cheaper and more environmentally friendly pure organic room temperature phosphorescent materials.The researchers used a combination of strategies to design and synthesize organic molecules to improve the luminescence properties of molecules,and gain RTP materials with long life and high quantum yield.In this paper,N-phenyl-1,8-naphthalimide?NNI-ph?was used as the prototype molecule,a series molecules?NNI-R?were designed by introducing different donors.The density functional theory?DFT?and time-dependent density functional theory?TD-DFT?were used to study these small organic molecules.We calculated the electronic structure and some photophysical properties including energy of the ground state,singlet excited state and triplet excited state,the highest occupied molecular orbital?HOMO?energy,the lowest virtual molecular molecular orbital?LUMO?energy and energy gaps,spin-orbit coupling?SOC?matrix element,the fluorescence rate(k_FL)and the intersystem crossing rate(k_ISC);discussed the possibility of that NNI-Rs emit phosphorescence in polar dichloromethane solvent and gas phase;analyzed the effects of different transition configurations on intersystem crossing?ISC?by the EL-sayed rule and natural orbital transition?NTO?;clarified the relationship between the charge change on donor moieties and k_FLor k_ISCby visualizing the hole-electron distributions created by Multiwfn.It should be emphasized that in polar solvent?dichloromethane solvent?,the first singlet excited state?S₁?has two isomers,one has charge-transfer?CT?character,the other has localized excitation?LE?feature for NNI-Rs with-OMe or –OH.The dihedral angle between donor and receptor of LE structure is vertical,and the total energy of the structure with LE character is lower than the one of CT structure,while LE structure inhibits occurrence of the ISC process,so the room temperature phosphorescence will not generate.In the gas phase,all the first singlet excited states of NNI-Rs have only stable geometry with CT characters,which significantly inhibit the fluorescence?FL?rate and effectively promote the intersystem crossing?ISC?rate,and the k_ISCwill increases with the increase of the charge change on donor moieties.The change of electronic transition caused by the change of excited state structure makeNNI-Rs possible as RTP materials.Our work clarifies that the donor substituent modification of NNI-ph-R in gas phase can effectively promote the occurrence of ISC,which provides theoretical basis for the synthesis and characterization of pure organic RTP materials based on NNI-ph.