The Role Of Charge Transfer State On Triplet Exciton Dynamic Processes In Purely Organic Luminescent Materials

As one of the fundamental electronic processes in organic materials,charge transfer（CT）plays an important role in applications such as organic light-emitting diodes（OLED）,organic photovoltaics,organic field-effect transistors,purely organic room temperature phosphorescence（RTP）and organic long persistent luminescence.In-deep comprehension of CT state is the cornerstone for the future development of luminescent materials.The CT state with separated frontier orbitals can regulate the exciton dynamic process,including singlet and triplet state energy,excited state characters,fluorescence and phosphorescence emission,and intersystem crossing,which finally controls the luminescent properties of the materials.Currently,purely organic luminescent materials exhibiting fluorescence,thermally activated delayed fluorescence（TADF）,phosphorescence and long-persistent luminescence hold great promise for future applications,and specific photo-physical parameters are required for different applications.Among them,the special long-lived triplet state in organic semiconductor determines the key parameters such as luminescent efficiency and lifetime.Therefore,recognizing the role of CT state and optimizing the photo-physical properties by CT state modulation is significant to satisfy the steric requirements from varied practical applications.In this thesis,we focus on investigating the role of charge transfer state on triplet exciton dynamic process in purely organic luminescent materials by means of materials developments,theoretical simulations,photo-physical property measurements and analyses,and optoelectronic device characterizations.The research scope of this thesis involves the influences of molecular conformation,conformation relaxation,conformation distribution and intermolecular interactions on luminescent mechanism and photo-physical processes of triplet excitons,hoping to provide new insight of CT state for RTP,OLED and afterglow luminescence applications.Firstly,based on four donor-acceptor type phenothiazine-nitrogen-heterocyclic molecules with twist intramolecular charge transfer（TICT）character,we investigated how do the conformation relaxation and CT process influence the singlet and triplet exciton dynamic process.In systems with weak CT characteristic,long-lived RTP and single molecular white emission can be realized because of the large energy gap between singlet and triplet states(ΔE_ST).For the TICT molecules,strong coupling between singlet and triplet state can be found and they give TADF or short-lived RTP emission.Through controlling the CT strength,the emission mechanisms can be regulated,and the role of conformation relaxation and CT on triplet exciton dynamic process was revealed,which paves ways for future applications.Subsequently,the scope was extended to the conformation distribution effect in amorphous film state.Centering on the key excited state lifetime issue in CT-type TADF materials,we unveil the influence of conformation distribution in amorphous emission layer in OLED on the excited state lifetime.By comparing the photo-physical properties of TADF materials with different type of donors,we found that the rigidity of the donor controls the conformation distribution in amorphous film,which finally determines the excited state lifetime.Acridine-type flexible donors have a broad conformation distribution or bimodal distribution,in which some conformers feature large singlet-triplet energy gap,leading to long excited state lifetime.Utilization of rigid donors with steric hindrance can restrict the conformation distributions in the film to achieve degenerate singlet and triplet states,which is beneficial to efficient reverse intersystem crossing（RISC）.Based on this principle,three prototype TADF emitters with confined conformation distributions were developed,achieving high RISC rate constants greater than 10⁶s^-1,which enable highly efficient solution-processed OLEDs with significantly suppressed efficiency roll-off.In the third section,in view of long-lived triplet excitons,we investigated how does the CT intermediate state between host and guest influence the triplet exciton dynamics and RTP emissions.Host-guest CT intermediate state can be found in host-guest combinations with frontier orbital energy off-set.Through regulating the host-guest CT state energy,highly efficient（phosphorescence quantum yield of 27.5%）or long-lived（phosphorescence lifetime of 595 ms）RTP emission can be realized.Moreover,the key role of the host-guest CT intermediate state was disclosed:On one hand,the CT state with degenerate singlet and triplet state is advantageous for triplet state population to improve the phosphorescence quantum yield;on the other hand,the CT state will reduce theΔE_ST,which will quench the long-lived triplet excitons and shorten the phosphorescence lifetime.Therefore,intermolecular CT interactions should be considered carefully when developing novel RTP systems.Finally,we found that the CT interactions between host and guest can also affect the long-lived TADF emission.By incorporating heteroatom into aπ-conjugated fused ring molecule,we developed a highly emissive aggregate system with strongπ-πinteractions,which exhibits unprecedented afterglow TADF emission in OLED.The intermolecularπ-πinteractions can fine-tune theΔE_ST for moderate RISC process,which enables efficient afterglow TADF.Moreover,the afterglow emissions can be subjected to host-guest interactions.The host-guest CT interactions can boost the intersystem crossing and RISC processes,and more delayed component can be obtained.In addition,the host-guest CT interaction is also the driving force to charge separation,which can further enhance the afterglow emission by slow recombination of charges.Combining organic aggregate with robust luminescent properties and host-guest CT interaction optimization,efficient afterglow OLED can be achieved with maximum external quantum efficiency of 14.7%along with afterglow lifetime of 157 ms.