Theoretical Studies On The Excited States Manipulating Of Organic Conjugated Molecules

Developing organic optoelectronic materials with desired photophysical properties has always been at the forefront of organic electronics and the excited states are the main factors affecting the photophysical properties of organic photoelectric materials.Thus,how to achieve effective regulation of the excited states of the organic photoelectronic materials has been the core issue of organic electronics research.The present work carried out a series of theoretical studies on the manipulation of excited state of conjugated organic molecules.The geometric and electronic structures in the ground-state were studied with density functional theory(DFT),and the nature of the excited states were investigated by time-dependent density functional theory(TD-DFT).The main contributions of the present work are summarized as follows:A widely-tuned ?EST in a range from ultralow(0.0003 eV)to extra-large(1.47 eV)via a subtle symmetric control of triazine molecules was realize for the thermally-activated delayed fluorescence(TADF),triplet-triplet annihilation(TTA)and singlet fission(SF)materials.A convenient and quantitative approach to relate ?EST to the frontier orbital overlap and separation distance via a set of newly developed parameters using natural transition orbital analysis to consider whole pictures of electron transitions for both the lowest singlet(S1)and triplet(T1)excited states.These critical parameters revealed that both separated S1 and T1 states leads to ultralow ?EST;separated Si and overlapped T1 states results in small ?EST;and both overlapped Si and T1 states induces large ?EST.A systematic study of luminescence mechanism of TADF materials,hybrided local charge-transfer(HLCT)excited state materials and the organic ultralong room-temperature phosphorescence(OURTP)materials was realized through an in-depth study on the electron-hole distribution,electron-hole distribution,transition configuration and energy level of the excited states.It is found that for TADF materials,the S1 state has charge transfer(CT)character,and the T1 state has the localized excited(LE)character.But the S1 and T1 states do not have the same transition configuration.What's more,the higher triplet states with the CT character have the same transition configuration with the S1 state,but the higher triplet states are close to the T1 state,thus they will transform into Ti state in a fast rate by internal conversion.The reverse intersystem crossing from Ti state to Si state will be realized by thermally activation.For HLCT materials,the Si state has the HLCT character to ensure high efficiency of radiative transition,and S2 and T2 states have CT features;the S1 and S2 states have the same transition configuration state with the T2,which has a large energy level difference with Ti energy to prevent the formation of the T1 state.Therefore,the T2 state may intersystem cross into S1 and S2 states for the fluorescence.The S1 states of the OURTP materials states have CT characteristics,and can intersystem cross into the triplet states which are close to S1 state in energy,and have the same transition configuration with S1 states.Then the triplets can be further stabilized and emit.Compared with the heterofluorenes,the regulation of the excited states and the photoelectric properties of bis(heteroatom)-bridged biphenyls(BXF)were studied systematically,including aromaticity,frontier molecular orbital,ionization energy,electron affinity,reorganization,triplet energy and absorption and emission spectra.It is found that the introducing of heteroatoms has successfully tuned the energy levels of excited states,so that the absorption and emission spectra redshift.BNF is the potential hole transport material;BCF,BSiF,BGeF,BPF and BAsF could be the potential material for excellent host materials.BPF,BGeF,BOF,BSF and BSeF are the potential electron withdrawing group.The polymer of BSiF and BGeF are promising photovoltaic materials.