Theoretical Study On Design,Tunation And Light-emitting Properties Of The Hybridized Local And Charge-transfer Excited State In Organic Materials

Organic light emitting diode（OLED）materials have been one of the research hotspots in the optoelectronic field.Right now,the materials which are well developed and manufactured are mainly phosphorescent complex with heavy metals.Because of the direct usage of triplet excitons in luminescence,their OLED devices show obvious advantages on efficiency.But due to their shortcomings of high cost and difficulties in fabrication etc,efforts are gradually transferred to the development on the organic fluorescent materials because of their variable structures,easy synthesis,low cost,rich color etc.For fluorescent materials,if the spin statistic could be broken by utilizing 75%triplet excitons,they will probably replace phosphorescent materials to be the main aspect in OLED materials.And this is also the critical scientific problem for the new generation organic fluorescent materials to achieve high electroluminescent efficiency.Four methods have been established to overcome this problem:（1）triplet-triplet annihilation（TTA）method;（2）thermally-activated delayed fluorescence（TADF）method;（3）hybridized local and charge transfer（HLCT）state accompanied with“hot exciton”method;（4）doublet emissive method.What is in common in these four methods above is the excited state which is the key determinant to influence the luminescence process.In another word,the features and composition of excited state largely determine the light-emitting properties（color,efficiency and lifetime etc）.Commonly,the excited states can be divided into two categories:locally-excited（LE）state and charge transfer（CT）state.For donor-acceptor molecular systems,the former is the excited states whose electronic transition is localized on donor（D*-A）,acceptor（D-A*）or throughout the whole conjugated molecular backbone（D-A）*with a large overlap between wavefunctions of hole and electron.The latter is the charge transfer transition from donor to acceptor with small ovelap between wavefunctions of hole and electron.According to the overlap extent,on one hand,the oscillator strength of LE state is always larger than that of CT state,corresponding to the high photoluminescence efficiency;on the other hand,due to the small energy gap between singlet and triplet states(ΔE_ST≈0),CT state is a practical way to achieve the reverse intersystem crossing（RISC）from triplet state to singlet state.Therefore,in order to maximize the efficiency of OLED devices,one ideal emitting state should be combined with both high photoluminescence efficiency and high exciton utilization.Based on this,our strategy is to develop a seris of emitting states with both LE state and CT state properties,in other word,LE state is responsible for high photoluminescence efficiency while CT state participates in high exciton utilization.In this dissertation,combined theoretical with experimental methods,we deeply discuss the formation,quantitative composition and tunation of HLCT state in D-A systems to reveal the essential cause of high efficiency of HLCT state,perfecting the HLCT state concept in order to guide molecular design of new OLED materials.Then we further extend the concept and investigate the intermolecular HLCT state in excimer systems.The research contents of this dissertation can be mainly summarized as follows:1.The excited state properties have been studied in a seris of D-A systems with different distance between donor and acceptor.The hole-electron distributions of two splitting HLCT states show“same shape,opposite phase”characteristic from each other.With the enlargement of the D-A distance,the HLCT state in the system undergoes three processes:“partial hybridization”,“fully hybridization”and“de-hybridization“,and the degree of hybridization is depend on the coupling and energy gap between intrinsic LE and CT states.Electron-hole pair 2D color-filled maps are used to roughly quantify the ratios of the state components in HLCT states.2.The excited state properties have been studied in a pair of isomers with different substitutional sites.The emitting state of systems transforms from mixing state to HLCT state with the decrease of twist angle between donor and acceptor.The coupling between intrinsic LE and CT states is the main force to drive this change,which is inversely proportional to the twist angle between donor and accepter.The calculated rates of certain photophysical processes indicate that HLCT state can effectively inhibit the internal conversion processes compared with pure LE and CT states,resulting in high exciton utilization in OLED materials.3 The HLCT systems are expended into intermolecular systems.Taking an isolated cofacialπ-πstacking dimer in unilateral substituted anthracene crystals as a model,the geometry,composition and formation of excimer state are systematically investigated.Excimer state is found to be an intermolecular HLCT state which largely enhances the rigidity of dimer,leading to the high photoluminescence efficiency of excimer state by inhibiting the non-radiative process effectively.The potential energy surfaces along three different directions illustrate that the interplanar distance between two monomers is the main force to the formation of excimer and the shift along long axis of the anthracene controls the species of formed excimer.The CT content in excimer state is the essential cause of the excimer emitting wavelength,rather than overlap extent between monomers,and it can be regarded as a standard to determine whether effective excimer is formed or not.