Theoretical Study On The Delayed Fluorescence Mechanism Of Organic Molecules With Excited State Intramolecular Proton Transfer

Organic light-emitting diodes have attracted wide attention due to their advantages including good image quality,low blue light-emitting,no pollution,low production costs etc.in display.Thermally activated delayed fluorescence molecules have been seen as the third-generation light-emitting materials in OLEDs,since they have the probability to realize nearly 100%exciton usage efficiency.They have also achieved great development and most of them are composed of donors?D?and acceptors?A?.Actually there are some contradictory for the design of D-A type based TADF molecules.In addition,the self-absorption problem also exists.Therefore,new strategies are needed for the design of TADF molecules.The molecules with excited-state intramolecular proton transfer?ESIPT?property could avoid the drawback in D-A type TADF molecules and also the self-absorption problem by differing the absorption spectra and the emission spectra.In this thesis,the light-emitting mechanism of TADF molecules with ESIPT property is studied,which would help one better understand the light-emitting phenomenon and provide the reference for the design of new TADF molecules.The main contents of this thesis are as follows:?1?The light-emitting mechanism of planar TADF molecule TQB with ESIPT property is studied.It is found that there are four isomers for TQB:TQB-TA?the molecule without proton transfer?,TQB-TB?the molecule after one-proton transfer?,TQB-TC?the molecule with two-proton transfer?and TQB-TD?the molecule with three-proton transfer?.We found that the TQB-TB has close relationship with the light-emitting.The TADF and ESIPT mechanisms were systematically studied in both solution and solid phase using the polarized continuum model?PCM?and the combined quantum mechanics and molecular mechanics?QM/MM?method.By analyzing the potential energy surface,the energy structure and the decay rates of excited states,we found that the proton transfer in the lowest energy singlet excited state?S₁?for TQB is quite easy in both DMF and solid phase.The proton transfer in the lowest energy triplet excited state?T₁?or the second energy triplet excited state?T₂?in DMF is a little difficult,while the barrier in T₁ for TQB in solid phase is comparable with room temperature energy.The ^T2TQB-TA?^S1TQB-TB up-conversion path plays crucial role in TADF of TQB in solvent,while ^T2TQB-TB?^S1TQB-TB shows dominate contribution to the TADF in solid phase.Based on the investigation of TQB,we designed two molecules:TQB^*with single hydrogen bond and TQB^**with two hydrogen bonds.Our calculation results indicate that TQB^**molecules could realize dual emission by ESIPT and is a potential light-emitting material for white light.No ESIPT can be realized in TQB^*,while large energy gaps are found between higher excited states and lowest excited states.Our investigation provides new perspective for ESIPT and TADF mechanisms,which could help one understand the light-emitting properties of TQB and provide some insights on the design of new functional luminescent molecules.?2?The light-emitting mechanism of D-?-A-?-D type based TADF molecules with ESIPT property is studied.The reason for the quantum efficiency differences of two kinds of molecules are explained by comparing the light-emitting properties of PXZPDO and DMACPDO with PXZDMEPDO and DMACDMEPDO.There are all significant degeneration for the energy levels of four molecules.There is also obvious crossing for the energy surfaces of singlet states and triplet states for PXZPDO.When the proton moves to the center of two oxygen atoms,the energy surfaces for S₁ and T₁ totally overlap with each other.The similar phenomenon can also be seen in the energy surfaces for S₂ and T₂.By analyzing the electron-hole distribution for the crossing points in the energy surfaces,we find that both S₁ and T₁ are charge-transfer states.According to the El-Sayed rule,the conversion from T₁ to S₁ is forbidden.For S₁?or S₂?and T₂?or T₁?,the distribution for electrons is the same,while holes are located at different units.This phenomenon may favor the conversion between T₂ and S₁ as well as T₁ and S₂.In addition,the energy difference between S₁?or S₂?and T₂?T₁?are as small as 0.005 eV when the proton moves to the center of two oxygen atoms.It is also predicted that the conversion may be realized here.It is found that the proton transfer could result in the energy surface crossing and also favor the conversion between excitons with different spin multiplicity.This is the reason that the exciton usage efficiency is highly enhanced for the systems with ESIPT properties.In this thesis,there are five chapters.In Chapter one,the state-of-the-art of OLEDs is introduced.The development of light-emitting materials in OLEDs and TADF molecules is also presented.The ESIPT phenomenon is introduced at last.In the second chapter,the theoretical methods used in this thesis including the density functional theory?DFT?,the time-dependent density functional theory?TD-DFT?and the combined quantum mechanics and molecular mechanics?QM/MM?are introduced.In addition,some analysis methods and also the theoretical methods about decay rates of excited states are also presented.Chapter three and Chapter four includes the main work about the study on the light-emitting mechanisms of TADF molecules with ESIPT property.In Chapter three,a planar ESIPT-TADF molecule TQB is studied where the ESIPT and TADF mechanism in both solvent and solid state are presented.Based on the study,two new molecules are designed and their light-emitting properties are predicted.In the fourth chapter,two D-?-A-?-D type based TADF-ESIPT molecules are studied.It is found that ESIPT could result in the crossing of energy surfaces for the states with different spin multiplicity,which can favor the conversion between triplet excitons and singlet excitons.This is also the reason that the ESIPT-TADF molecules could achieve high quantum efficiency.In the last chapter,a summary is given for the works done in the thesis,and we are also inspired.The outlook in this area is presented at last.