Theoretical Study On Room-Temperature Phosphorescent Materials Of Organic Small Molecules

Organic materials with room-temperature phosphorescence（RTP）have attracted more attention in applications such as organic optoelectronics,biomedicine,chemical sensors,and digital encryption due to their advantages of long lifetime,large Stokes shift,structural diversity,low toxicity and low cost.However,there are still some critical problems and challenges in the research of pure organic room-temperature phosphorescent materials:（1）Compared with noble metal-containing inorganic and organometallic complex materials,pure organic room-temperature phosphorescent materials due to the weak spin-orbit coupling,are not conducive to the efficient intersystem crossing process,resulting in the low luminescence efficiency of pure organic room-temperature phosphorescent materials.（2）In recent years,pure organic small molecules as room-temperature phosphorescent materials have been widely studied and applied,but there are relatively few relevant theoretical works to establish the relationship between molecular structure and room-temperature phosphorescence performance,and the mechanism about room-temperature phosphorescence emission of pure organic small molecules still needs to be studied in depth.（3）Currently,the types and quantities of pure organic room-temperature phosphorescent materials are still relatively limited,and there is an urgent need to expand new molecules with efficient room-temperature phosphorescent materials.This paper takes phenothiazine derivatives,single-bridged and double-bridged heterofluorene molecular system as the research objects,and molecular geometric structure,excited state properties,as well as the coupling interaction between singlet and triplet states have been analyzed by adopting DFT/TDDFT methods,in order to understand and study the relationship between the structure of organic small molecules and room-temperature phosphorescence performance.Moreover,the possibility of these two types of organic small molecules for room-temperature phosphorescent materials has been explored.The main research contents of this paper are as follows:1.Since the angle between the p-orbital where the lone pair electron is located and theπ-plane of the benzene has an effect on the molecular p-πconjugation,it makes the composition of the n-π*transitions vary significantly when the intersystem crossing process from S₁ to T₁ states occurs,which causes the change in the spin-orbit coupling.Taking phenothiazine as a model molecule,we have studied the regular curve of the spin-orbit coupling as a function of molecular bending degree,and it is found that the bending degree corresponding to the significant enhancement of the spin-orbit coupling.We have designed a series of phenothiazine derivatives with different bending degrees,and performed a detailed analysis of the molecular geometric structure,the transition properties of S₁ and T₁ excited states,spin-orbit coupling,and intersystem crossing process between singlet and triplet states.The calculation results show that with the increase of the molecular bent,the difference in n-π*transition properties between S₁and T₁ states is larger,and the spin-orbit coupling is significantly enhanced,which effectively promotes the intersystem crossing process from S₁to T₁ states.The increase of bending degree is also beneficial to improve the phosphorescent radiation transition process from T₁ to S₀ states and suppress the non-radiative transition process,thus increasing the phosphorescence efficiency.This work systematically elucidates the influence of molecular structure with different bending degrees on room-temperature phosphorescence performance,and according to theoretical calculations,designs actual molecule with near-optimal bending degree,which provides a new design strategy for the development of efficient organic room-temperature phosphorescent materials.2.The heavy atom effect can effectively improve the spin-orbit coupling of organic small molecules.The room-temperature phosphorescence properties of single-bridged and double-bridged heterofluorene molecules have been systematically studied by introducing different heteroatoms into these molecules.From a theoretical perspective,we have explained the reason why the phosphorescence efficiency of double-bridged heterofluorene molecules is significantly higher than that of single-bridged heterofluorene molecules.Based on the existing double-bridged heterofluorene molecules（As-Si,As-Ge）,we have designed the heterofluorene molecules of homo-atomic double bridge（As-As,P-P,N-N）and hetero-atomic double bridge（As-P,As-N,P-N）in the same main group by transforming different atomic combinations.We have investigated the geometric structure,the transition properties of S₁ and T₁ excited states,spin-orbit coupling,phosphorescent radiation and non-radiation transition process of double-bridged heterofluorene molecules.The calculation results show that the our designed heterofluorene molecules of hetero-atomic double bridge（As-P,As-N）can achieve efficient room-temperature phosphorescence,and the heterofluorene molecule of hetero-atomic double bridge（As-N）with the larger atomic radius difference improves the phosphorescence efficiency more obviously.This work provides a theoretical foundation for the development of efficient double-bridged heterofluorene organic molecules as room-temperature phosphorescent materials.