| With the development of science and technology,phosphorescent materials are more and more extensively applied in daily life and has become an important part of the field of optoelectronics and biotechnology.They have been widely used in light-emitting diodes,display,biological imaging,anti-counterfeiting,information encryption,etc.The extensive development of traditional phosphorescent materials has benefited from the introduction of precious metal elements(such as iridium,platinum,etc.).But due to the lack of precious metal resources and high material cost,the commercial development space and application prospects of traditional phosphorescent materials are limited.Although researchers have made great progress in the field of luminescence,designing efficient metal-free phosphorescent materials still faces great challenges.In recent years,ultralong organic phosphorescence(UOP)in pure organic molecular crystals attracts the attention of researchers because of its large Stokes shift,low cost,easy availability of raw materials and non-toxicity.However,there are many controversies about the UOP emission mechanism.It is very important to study the UOP emission mechanism deeply for the design and synthesis of efficient UOP materials.Many-body Green’s function theory is excellent in studying the excited-state properties of crystals with high accuracy.In this dissertation,the 2,4,6-trimethoxy-1,3,5-triazine(TMOT)crystal,the carbazole crystal doped by 1H-benzo[f]indole(Bd-doped Cz)and boric acid(BA)crystal,which are widely discussed in the experiments,are taken as the research objects.The mechanisms of their ultra-long room temperature phosphorescence are studied by density functional theory and many-body Green’s function theory(including GW and Bethe-Salpeter equation),which can provide a theoretical basis for further design and synthesis of effective ultra-long organic room temperature phosphorescence materials.The main research contents and results are as follows:Firstly,UOP phenomena in the TMOT crystal and carbazole crystal,which have been widely discussed experimentally,are studied by the first-principles many-body Green’s function theory.The UOP phenomenon has been attributed to the H-aggregation of TMOT molecules or the formation of charge-transfer excitons between carbazole and impurity molecules.In this study,using the first-principles many-body Green’s function theory,energies of the excitons induced by H-aggregation and charge-transfer transition are too high to explain the UOP phenomenon measured in the experiments.The lowest triplet states of the two crystals were actually derived from the localized defect states of twisted TMOT molecules and impurities respectively.The UOP phenomenon is mainly caused by the little orbital overlap between the localized defect state and the delocalized band edge state of the crystal,together with inhibition of the non-radiative decay of triplet excitons localized on the defect as the result of strong intermolecular interactions.Secondly,taking boric acid crystal as the research object,geometries and optical properties of the BA molecule and crystal were investigated by density functional theory and many-body Green’s function theory.It is proved that the pure BA crystal does not emit phosphorescence,and the phosphorescence phenomenon obtained in the experiment may be due to some unknown impurities in the commercial boric acid.This work proposed a new angle for the study of phosphorescence emission from BA crystals:In the process of absorbing energy to the excited state,BA molecules react with each other to form water molecules or hydrogen molecules,thus forming products containing boron-oxygen-boron(B-O-B)bonds or boron-oxygen-oxygenboron(B-O-O-B)bonds.The phosphorescence of BA crystals may be due to the local defect states produced by these products. |
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