Theoretical Studies On Luminescent Properties Of Delayed Fluorescence Molecules

Organic lighting emitting diodes?OLEDs?show wide applications in the field of organic lightings and displays by virtue of its light weight,ultra-thin and flexible characteristics.Recently,thermally activated delayed fluorescence?TADF?materials which can realize 100%exciton utilization in OLEDs have attracted great attation and this kind of material is also known as the third generation luminescence materials.Different from the traditional fluorescent and phosphorescent materials,triplet excitons for TADF materials can up-convert to singlet excitons through an efficient reverse intersystem crossing?RISC?process,which effectively improves the triplet exciton utilization.In this thesis,the optimal Hartree-Fock method is adopted to calculate the energy gap between the lowest excited singlet state?S₁?and triplet state?T₁?.Furthermore,we investigate the influence of geometry changes on TADF molecular properties especially for the S₁-T₁ energy gap,and some efficient design strategies for TADF molecules are provided.Moreover,based on the combined quantum mechanics/molecular mechanics?QM/MM?method,we investigate luminescence properties of molecules in solid phase,and further reveal the aggregation induced emission?AIE?,thermally activated delayed fluorescence and hot exciton mechanisms.The main contents and results are as follows:1.Influence of geometry changes on S₁-T₁ energy gap for TADF moleculesBased on the optimal Hartree-Fock method,we calculate the experimental synthesized TADF molecules and the calculation parameters are determined by comparing with the experimental results.Furthermore,a variety of TADF molecules are theoretically designed with different electronic donating units,and we explore the relationship between electron donating ability,delocalization of frontier molecular orbitals and S₁-T₁ gap.Meanwhile,we study the effect of different connections between donor and acceptor units on S₁-T₁ gap.Results show that increasing the electron donating ability decreases the S₁-T₁ gap and this energy gap can be further reduced through enlarging the delocalization of highest occupied molecular orbital?HOMO?.Moreover,the connection through acetylene rather than benzene can effectively decrease the intramolecular steric hindrance and bring large fluorescence rate with also unchanged the S₁-T₁ gap.Thus,the luminescence efficiency can be improved.2.Influence of solid surrounding on luminescent properties of TADF moleculesAt present,investigations on TADF molecules are mainly focused on single molecules and rare studies are conducted to explore the luminescence property of TADF molecules in solid phase.The application of TADF molecules is mainly in solid phase.Thus,it is necessary to theoretically study the luminescence efficiency for TADF molecules in solid phase.Based on the QM/MM method,we calculate the energy landscapes,radiative and non-radiative decay rates and further investigate the emitting properties of 10-?7-fluoro-2,3-diphenylquinoxalin-6-yl?-10H-phenoxazine?FDQPXZ?in solid phase.Results indicate that the rotation motion of dihedral angle in low frequency regions is hindered when molecule is in solid phase,and this brings decreased Huang-Rhys factor and reorganization energy.Thus,the non-radiative decay rate is decreased and the excited state energy consumption process is hindered.Meanwhile,the additional energy levels between S₁ and T₁ are involved in the intersystem crossing and reversed intersystem crossing processes,and this further improves the delayed fluorescence efficiency of FDQPXZ.3.Influence of solid surrounding on AIE and TADF propertiesMaterials with AIE feature are regarded as ideal materials for developing non-doped OLEDs,which can greatly improve the stability and efficiency of device.Thesynthesizedmoleculedibenzothiophene-benzoyl-9,9-dimethyl-9,10-dihydroacridine?DBT-BZ-DMAC?which combines the AIE and TADF properties together,can improve the efficiency of OLEDs reported by Zhao et al.Based on the QM/MM method,we investigate the excited state properties of DBT-BZ-DMAC in solid phase.Results show that the rotations of DMAC and DBT units are restricted in solid phase,and the non-radiative energy consumption process is hindered.Thus,the efficiency is increased from 0.01‰in gas phase to 20.5%in solid phase,this confirms the AIE property of DBT-BZ-DMAC.Thus,our theoretical investigation reveals the AIE and TADF features,and this is important to design and synthesize more efficient emitting molecules.4.Influence of solid surrounding on AIE and hot exciton propertiesPrevious investigations show that the hybridized local excited and charge transfer state?HLCT?molecule2,3-bis?4'-?diphenylamino?-[1,1'-biphenyl]-4-yl?fumaronitrile?TPATCN?with hot exaction emission possesses high exciton utilization and luminescence efficiency.Based on the QM/MM method,the luminescence property and exciton transfer mechanism of TPATCN in solid phase are theoretically investigated.Results show that the rotation motion in low frequency region is restricted in solid phase,and the non-radiative energy consumption process is hindered.Thus,the luminescence efficiency is increased from 0.16%in gas phase to52.1%in solid phase,this verifies the AIE property of TPATCN.Through analyzing the energy landscape and excited state dynamics with the calculated rate parameters,we reveal the hot exciton transfer and emitting mechanisms.5.Influence of molecular stacking on intermolecular charge transfer and luminescence propertiesBalanced hole and electron transfer can effectively improve the composite ratio of exciton,and help to obtain efficient exction utilization and external quantum efficiency.Based on the Marcus equation and Monte Carlo simulation method,we obtain the charge mobility of 2,6-diphenylanthracene?DPA?and 2,6-diphenyl-9,10-bis?phenylethynyl?anthracene?DP-BPEA?respectively,and the relationship between mobility and temperature is also investigated.Thus,the influence of molecular stacking on intermolecular charge transfer property is illustrated.Moreover,we investigate the photophysical property of DPA and DP-BPEA in solid phase by QM/MM method,and the influence of molecular stacking on luminescence property is also revealed.Results show that DPA and DP-BPEA crystals are proven to be p-type semiconductors under the hopping transfer mechanism.Calculation confirms that the hole mobility of DPA is larger than that of DP-BPEA due to the number of effective transition pathways of DPA?four?being more than that of DP-BPEA?two?.However,a more balanced charge transfer can be found for DP-BPEA.In addition,DPA possesses aggregation induced enhancement emission?AIEE?.This thesis consists of eight chapters as follows.The first chapter includes an introduction of OLED and current research of emitting layers.In chapter two,we introduce the optimal Hartree-Fock method and the tuned range-separated functionals method,which are used to calculate the S₁-T₁ gap of TADF molecules.Meanwhile,we present the QM/MM method to investigate molecular luminescence properties in solid phase,and further illustrate the electroluminescence process as well as the theoretical method to calculate radiative and non-radiative decay rates.Based on the abovementioned methods,our investigations and corresponding results are presented in chapter three to chapter seven.In chapter three,we use the optimal Hartree-Fock method to investigate the influence of geometry changes on S₁-T₁ gap and further propose the strategy to design high efficient TADF molecules.In chapter four,we use the QM/MM method to study the luminescence property of FDQPXZ in solid phase,and illustrate the TADF mechanism.In chapter five,we investigate the excited state properties of TADF molecule with AIE feature,and reveal the AIE and TADF mechanisms.In chapter six,we investigate molecular luminescence property with HLCT feature,the AIE and hot exciton mechanisms are illustrated.In chapter seven,we study the charge transfer and luminescence properties for two systems,the influence of molecular stacking on intermolecular charge transfer and luminescence properties is illustrated.In the last chapter,we summarize the whole work of this thesis and provide a prospect on future work.