| In the past decades, the rapid development of OLEDs has come to the stage ofapplication in industry production. Organic light-emitting materials is the coretechnology of OLEDs, is also the focus of international competition in this field.OLEDs materials have through two generations since found a revolutionarydevelopment.The first generation of OLEDs materials is tris (8-hydroxyquinoline)aluminum (Alq3) as the representative fluorescent material. However, such materialsare subject to the spin statistical limit, and their exciton utilization efficiency can’tbreak25%. In1998, Prof. Ma first proposed the75%of triplet exciton energy use upthe principle of improving the efficiency of OLED devices, namely if the singlet andtriplet luminous efficiency (same) at the same time, the efficiency will be three timesas much as the entire device and completed the world’s first electricelectrophosphorescent devices. The second-generation OLEDs materials are based oniridium (Ir) complexes as the representative of phosphorescent materials. Though theenciton utilization efficiency can be attained close to100%in these materials, somedrawbacks remain as expensive cost, chroma insufficiency (lack of bluephosphorescent material), iridium resource constraints and other issues. So developednew pure organic electroluminescent materials what low cost, high luminousefficiency, high utilization of exciton is imminent.Compared with metal phosphorescent materials, pure organic fluorescent materialis low cost advantage, but is limited to25%of singlet exciton as the spin statistical limit. In current stage, there are two roads to improve exciton utilization of organicelectroluminescent devices. The first is pi-conjugated molecule or polymer withdelocalized electronic state, a breakthrough in the singlet exciton ratio was alsoobserved, which is explained by larger singlet forming probability in weakly bindingstate or triplet transferring to singlet in the high energy inter-chain CT state. Thisdirects a concept for OLED material design to break through the spin couplingstatistics (singlet/triplet=1/3) in strongly bound excited state by employing molecularexciton state with weak binding energy. The second is the delayed fluorescence bytransferring triplet excitons to singlet for employing more than25%excitons throughTTA or TADF methods have been reported. TTA process is two triplet excitonscombine to form a singlet exciton through triplet–triplet annihilation, which results indelayed electroluminescence. The theoretical maximum value of the TTAcontribution to singlet excitons can be estimated as37.5%, there is still a big waste ofenergy.The TADF materials always possess a small S1and T1state energy gap (ΔEST)by constructing CT material with separated electron and Hole wavefunctions, whichallows most of the T1excitons converting to the radiative S1excitons through thethermally activated RISC route, causing an increase in the Χs. In principle, the abovetwo ways are established on the basis of the exciton statistics, a large proportion of T1excitons converting to the radiative S1excitons. However, it may be ascribed to thatthe RISC rate is generally very low in fluorescent devices, and the accumulated T1excitons will be wasted through triplet-triplet or singlet-triplet quenching process.Therefore, with high exciton utilization non delay fluorescence materials may becomepreferred materials with completely independent intellectual property rights of a newgeneration of organic electroluminescent materials.In this thesis, we adopt the quantum chemical calculation to describe theproperties of the ground state and the excited state which starting from the moleculargeometry and electronic structure. Combining with the experimental phenomenon,Expounds the discovery and the basic principle of the “Hot†exciton reverseintersystem crossing; the key controlling factors of the new theory, and we present ournew strategy of “Hot†exciton to enhance exciton utilization efficiency and HLCTnew principle for material design. On the basis of this idea, a series of D-A material systems were designed and synthesized, and their devices all exhibit high excitonsutilization efficiency and high stability.Among a series of our twisted D–A molecules, an ultra-high exciton utilizationefficiency of≈93%can also be achieved in the triphenylamine-thiadiazole moleculeTPA-NZP, in spite of a very large singlet–triplet energy splitting (ΔEST≈1.20eV)between S1and T1for an impossible thermal activation process. This has significantdifference with TADF phenomenon. In order to rationalize the difference between4CzIPN and TPA-NZP, the electronic structures of the ground and excited states ofboth4CzIPN and TPA-NZP were calculated and analyzed. By virtue of both CTfeatures of S1and T1states,4CzIPN has a very small ΔEST, under the condition ofthermal activation is easy to reverse intersystem crossing, improve the excitonutilization. Thus, this mechanism can be defined as “Coldâ€, exciton mode whichmeans that the RISC proceeds along the channel of “Cold†exciton T1→S1. As a sharpcontrast, there is a large ΔESTin the TPA-NZP, this indicates that this is a class ofmaterial completely different to TADF mechanism. An important finding is thatTPA-NZP has a very large energy gap (1.63eV) between T1and T2(ΔET1T2), it leadsto the blocked relaxation of T exciton Tm→Tm-1, at the same time, the strong CTcharacteristic state with a small energy gap at high-energy excited states (S2/T2) werefound, so RISC (T2→S2) along “Hotâ€exciton CT channel can also result in a highsinglet exciton yield in TPANZP. The “Hot†exciton mechanism possesses theseparated channels between exciton conversion and fluorescent radiation, which ismore feasible to realize a simultaneous maximization between the exciton utilizationefficiency and the pHotoluminescent efficiency. Overall,“Hot†exciton is an idealelectronic energy diagram to design the next generation OLEDs materials combininghigh exciton utilization efficiency with high exciton radiation efficiency.We further study and calculation the origins of the huge ΔET1T2and the contributionof spin orbit coupling between the states in the “Hot†exciton mechanism, Firstly,through separate calculation of TPA-NZP molecules determine the huge ΔET1T2comes from the electron acceptor (NZ), and then to NZ’s NTOs and excited state andtransition dipole moment calculation, found the huge ΔET1T2may have originatedfrom the adjacent excited states of transition in a different direction. To test and verify the correctness of inference, a series of one dimensional growth benzene moleculehave calculated, and through the transition dipole moment calculation, compared theconfigurations and properties of excite-states, confirm the huge ΔET1T2indeedoriginated from adjacent excited states of different warp direction. Next, the couplingconstants are calculated, discuss the state of the excited state and the size of thecoupling constant, the relationship between the final appropriate hybrids isadvantageous to the coupling of the preliminary conclusion. But in pure organiclight-emitting materials, spin orbit coupling is not the key factor, the small ΔESTis themost important of the RISC.By using the principle of the “Hot†exciton channel theory to carry on the design oforganic light-emitting materials, verifying the rationality and correctness of “Hotâ€exciton channel theory, and at the same time discuss the necessity of the huge ΔET1T2for “Hot†exciton channel. And compared with the experiment concluded anothernecessary conditions of building efficient “Hot†exciton channel is existence theappropriate higher excited states, with characteristics of CT, is the importantguarantee of constitute effective “Hot†exciton channel. In addition, combined withefficient HLCT state and “Hot†exciton channel theory, through the optimization ofD-A between the structure parameters, reveals the possibility of the high excitonutilization and high luminous efficiency. We take TPA-NZP as a model, change theangle between the D and A, the distance and the number of D and A to test theluminous efficiency and energy level, and the utilization ratio of exciton effect. Ourresults suggest that that fluorescent molecule with “Hot exciton†mode and HLCTstate may be an ideal strategy to design the next-generation, high-efficiency andlow-cost OLED materials. |
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