Theoretical Study Of The Heterocycle-Containing Small Organic Molecules With AIE And TADF Properties

Organic light-emitting diodes（OLEDs）show promising prospects in display and illumination fields due to their bright color,compact shape and good portability.As the core component of OLED,organic luminescent materials have attracted the attention of many researchers.Under the pressure of scarce energy sources and serious environmental disruption nowadays,a consensus has been reached to seek for novel light-emitting materials with less pollution and higher efficiency.Consequently,pure organic materials with aggregation induced emission（AIE）and thermally activated delayed fluorescence（TADF）have become a research hotspot.The AIE materials improve the efficiency mainly by inhibiting the non-radiative process in the aggregation state,and the TADF materials improve the utilization rate of the excitons through the reverse intersystem crossing（RISC）process,where the triplet excitons convert to singlet excitons,to improve the efficiency.By means of the calculation and simulation based on quantum mechanics and molecular mechanics,we can have a further insight into these two luminescence phenomena and guide the synthesis of the experimental molecules reasonably.In this paper,a series of AIE and TADF molecules were investigated by using density functional theory/time dependent density functional theory（DFT/TDDFT）.The geometrical and electronic structures of the molecules at both ground and excited states,and the parameters which determine the non-radiative rate of the excited state were calculated.Thorough analysis and demonstration on mechanism were carried out to explain how some typical groups and some combination forms of the donor and the acceptor influence the luminous efficiency.We also designed new molecules from the structure-performance relationship.The specific content is as follows:1.The huge influence of aggregation on the structure and fluorescent property of 5-（4-（1,2,2-triphenylvinyl）phenyl）thiophene-2-carbaldehyde（P₄TA）,an AIE compound reported in the experimental work,was detailedly investigated here by employing both quantum mechanics and molecular mechanics.Besides the isolated molecule,the aggregated molecule both in water and crystal state was studied by focusing on the comparison of photoelectronic properties,including the geometrical and electronic structures at ground and excited states,emission and internal conversation properties.For the aggregation state,the intermolecular interaction was employed to explain the difference in structure,emission color and intensity of different polymorphs.The noticeable contribution from low-frequency region,corresponding to the four phenyl rings twisting vibration,to the Huang-Rhys factor and reorganization energy,as well as the possible potential energy surface crossing between S0 and S1 states for isolated P₄TA were considered as the reasons of its AIE performance.Importantly,the aggregation process in water simulated at the same time helps us to have a deeperunderstanding of the AIE behavior of P₄ TA,which also provides another perspective to explore the AIE phenomenon in theory.2.A series of small organic molecules with phenoxazine or carbazole as donor and triphenyltriazine or diphenyltriazine as acceptor were calculated based on DFT/TDDFT and the reason why the similar organic molecules exhibit different luminescence types was investigated.The deep analysis of the geometrical and electronic structures shows the structure,especially for the donor-acceptor dihedral angle,is the root cause which determines the singlet-triplet energy gap and the property of excited states.The explorations on the electron-hole pairs of natural transition orbitals and the contribution of the key heteroatom（N）to different molecular orbitals reveal the distinct electron transition processes of excitation to singlet and triplet statess,explain the reason for different energy-level distributions of excited states,and identify which pairs have more favorable intersystem crossing for these molecules,while the calculations of spin-orbit coupling and reorganization energy display the efficiency of the different luminescence types.Meanwhile,we also respectively modified the phosphorescent molecule and the prompt fluorescent molecule through the introduction of methyls to increase the steric hindrance and realize the perpendicular orientation of donor and acceptor unit,and screened the TADF molecules theoretically and conducted experimental verification.3.Two o-carborane derivatives 1 and 2 exhibiting both AIE and TADF properties were investigated by combining density functional theory with molecular dynamics simulation.The reason for their TADF property was explored.Besides,taking 1 as an example,the AIE phenomenon was also probed by simulating the aggregation process and comparing the photophysical properties of the molecules in different aggregation states（including crystal,water,and films）with the isolated molecule.The results manifest that the separated orbital occupations of the HOMO and LUMO at the respective donor and acceptor units result in a small singlet-triplet energy gap and a favorable TADF feature.In the AIE phenomenon,a much smaller structural change of donor and acceptor units between S₁ and S₀ states was experienced for the molecules in all aggregate states compared with that for the isolated molecule,decreasing the energy dissipation substantially and enhancing the luminescence efficiency.In addition,o-carborane is conjectured to help in the free rotation of donor and acceptor units for the isolated molecule,which may contribute to AIE.What’s more,on the basis of 1 and 2,we designed 3-6 by means of replacing the triphenyltriazine group with other electron-withdrawing groups having lower LUMO energy levels,aiming to obtain this kind of red light emitting material.The results proved our assumption that 3-6 not only present the feature of red light emission but also have both TADF and AIE properties in films.