Theoretical Study On Luminescence Mechanism Of Conjugated Small Organic Molecules

Conjugated small organic molecules have attracted extensive attention due to their diverse structures,easy modification,and high fluorescence quantum yield,and have great potential for applications in the fields of fluorescent probes,display lighting,medical treatment and so on.Considering the coordinated development of environment and energy,the research and development of low-pollution and high-efficiency conjugated small organic molecules have become a hotspot in contemporary research frontiers.Although many outstanding achievements have been made in the field of conjugated organic small molecules,the luminescence phenomena of some fluorescent dyes cannot be reasonably explained,which will greatly restrict the development and application of new and efficient luminescent materials.With the continuous development of computer technology and theoretical methods,the luminescence process of conjugated small organic molecule dyes can be explored through theoretical simulation,which allows in-depth analusis of the relationship between molecular structure and performance at the microscopic scale.Not only that,the design and prediction of conjugated organic small molecules using theoretical simulation methods can not only avoid the excessive consumption of reagents,manpower and funds caused by the complicated experimental "trial and error" method,but also guide researchers to design molecular structures on demand.Therefore,this thesis mainly studies conjugated organic small molecule dyes,and uses theoretical methods to study the luminescence mechanism of conjugated small organic molecule dyes for aggregation induced luminescence(AIE)and thermal activation delayed fluorescence(TADF).In addition,this thesis reveals the relationship between molecular structure and the luminescence performance,providing theoretical support for the development of efficient luminescent materials.As follows is idiographic research:1.The AIE mechanism of conjugated organic small molecules means that in the molecular aggregation state,the molecular motion is restricted due to steric effects,thereby suppressing the non-radiative energy dissipation path and improving the fluorescence efficiency.At present,the AIE mechanism has become a consensus,and provides theoretical support for the development of efficient small organic molecule fluorescent materials.In some reports,however,researchers still cannot reasonably explain the AIE mechanisms of certain molecular systems,such as boron difluorohydrazone(BODIHY)derivatives.So,we theoretically studied the decay process of the excited state of a series of BODIHY derivatives.Firstly,based on a more accurate theoretical method,one can find that the S1 state of the BODIHY derivative is a bright state,which can correct the previous misconception about the nature of the S1 state of the BODIHY derivative.Further analysis,it can be concluded that in low viscosity solvents,the molecular framework is prone to umbrella-like flipping motion,accompanied by the formation of a S1-S0 conical intersection,resulting in a high probability of non-radiative transitions.Based on this mechanism,the flipping motion is limited in the environment of molecular aggregation or large solvent viscosity,which effectively avoids the occurrence of non-radiative transitions and improves the luminescence efficiency.The new mechanism of the restriction of flip-flop motion to achieve AIE provides new ideas and guidance for the research of luminescent materials and photophysics.2.The photoluminescence mechanism of TADF is that triplet excitons formed by photoexcitation can undergo a rapid reverse intersystem crossing process at room temperature,so that triplet excitons are effectively arranged to the S1 state and then emits delayed fluorescence.Due to the wide application of TADF materials,many organic TADF dyes have been designed and developed.However,most of the organic TADF molecules reported so far are difficult to be directly applied in the field of biological and medical imaging due to their poor water solubility.Here,a water-soluble and organic TADF dye DCF-BXJ was developed by introducing a flexible propenyl group into a commercial traditional fluorophore DCF(2’,7’-dichlorofluorescein).The flexible propenyl group provides non-radiative rotational motion,resulting in an efficient energy level cross between the S1 state and the T2 state of DCF-BXJ.Results of transient absorption spectra and theoretical calculations supported that the non-radiative rotational motion of the flexible group can enhance intersystem crossing and bring out TADF.This work realizes the efficient regulation of TADF and provides theoretical guidance for the interpretation and design of TADF existing in organic molecules.3.In recent years,due to the advantage of high excitons utilization,more and more conjugated small organic molecules with TADF phenomena have been reported and widely used in the fields of photodynamic therapy,energy conversion,display and lighting,and time-resolved imaging.The currently proposed TADF mechanism mainly relies on the reverse intersystem crossing process mediated by the T1 state,while the TADF mechanism involving the high-level triplet state is less studied.In this work,based on the targets of 2’,7’-dichlorofluorescein derivatives DCF-MPYM and DCF-MPYM-Me,we report a novel TADF mechanism based on T2 excitons.The mechanism is implemented on the basis of the T2-T1 conical intersection point and the T2-S1 minimum energy cross point.The entire process requires only an energy barrier of less than 9 kcal/mol,so TADF can be achieved at room temperature.The new TADF mechanism in which high-energy triplet states participate in TADF will inject new vitality into the interpretation and development of novel TADF materials.