Theoretical Exploration On The Mechanochromic,Room-temperature Phosphorescence And TADF Properties In Organic Nitrogen-containing Heterocyclic Compounds

As organic light emitting diodes（OLEDs）have the advantages of wide viewing angle,self-luminous,high contrast and low energy consumption in the field of lighting display,the researches and applications of organic optoelectronic materials have developed rapidly.The development of OLED materials has mainly gone through three stages: traditional fluorescent materials,phosphorescent materials containing heavy metal elements and delayed fluorescent（DF）materials of pure organic molecules.In recent years,a variety of organic materials breaking conventional luminescence phenomena have been reported,such as the luminescent mechanochromic materials,the room temperature phosphorescent（RTP）materials,and the anti-Kasha emission materials.These new organic optoelectronic materials have injected new vitality into the development of OLED devices,thus broadening their applications.However,comprehensive understanding on the luminescence mechanisms of these new organic photoelectric materials at present are still required.In this thesis,the luminescence mechanisms of some typical mechanochromic materials,room temperature phosphorescent（RTP）and thermally active delayed fluorescence（TADF）materials were theoretically explored by density functional theory（DFT）,which would provide theoretical support for the applications and developments of OLED materials.This thesis mainly includes the following two research works:1.The luminescence mechanism of the mechanochromic 9-anthryl gold（I）isocyanide complex in different crystalline phases has been investigated based on the experimental results.The quantum mechanics/molecular mechanics（QM/MM）model was employed to calculate the geometrical and electronic structures and simulate the absorption and emission properties of monomers and dimers selected from different crystal phases.The results show that the weaker intermolecular interactions in α phase crystal have slight influence on the photophysical properties,illustrating that the monomer is enough to describe its spectral property.Meanwhile,the results manifest that the emission of α monomer arises from S₂ state which breaks the Kasha’s rule.While the intermolecular interactions in β and γ phase crystals are so strong that at least the dimer should be selected to simulate their photophysical properties.The phosphorescence emission of β phase crystal is found to be contributed by the larger spin orbital coupling between excited singlet and triplet states of dimer.In addition,the distance between two adjacent Au atoms in γ phase is much small and the aurophilic interaction appears which may induce the decreased energy gap and red-shifted emission wavelength.2.An interesting experimental report that two isomers OPM and OMP have RTP and TADF properties,respectively,attracts our attention.We tried to explore the reason of their very different photophysical properties from the theoretical perspective.The QM/MM model was employed to calculate the geometrical and electronic structures and the photophysical properties of OPM and OMP.The calculation results show that the difference in dihedral angle between the donor and acceptor fragments leads to their significantly different front molecular orbital distributions,and thus resulting in the huge difference in the singlet-triplet energy gap(ΔE_ST).For OMP molecule,it has an extremely small ΔE_STand low spin orbit coupling（SOC）value between the S₁ and T₁ states.While the decrease of the ΔE_STby an order of magnitude can increase the rate of reverse intersystem crossing（RISC）by some orders of magnitude.So the extremely small ΔE_STis the key factor determining TADF properties of OMP.For OPM,the SOC of S₁ and T₁,T₂ are large and the energy level distribution is reasonable,so it is easy to realize the dual phosphorescence emission of T₁ and T₂,which is further verified by the Frank-Condon vibration spectrum.