Theoretical Exploration On The Phosphorescence Mechanism Of The Host-guest Doped Materials Containing Phenothiazine Units

Pure organic room-temperature phosphorescent（RTP）materials have potential application in lighting,medicine,display and other fields due to their advantages of low toxicity,rich synthetic methods and low cost.Compared with traditional organic metal complex materials with phosphorescence,organic RTP materials cannot generate triplet excitons efficiently because of their smaller spin-orbit coupling（SOC）.At the same time,the smaller radiative decay rate of triplet excitons induce the difficulty to compete with non-radiation decay processes,which makes the achievement of high-efficiency RTP emission very challenging.In recent two years,to obtain high-performance RTP materials,researchers proposed the host-guest doping strategy,which improves structural rigidity to suppress non-radiative processes and thus reduce the loss of triplet exciton.However,the complicated photophysical process leads to the difficulty in understanding their intrinsic phosphorescence mechanism.Therefore,it is meaningful to investigate the luminescence mechanism of host-guest doping systems for the study of RTP materials.In this thesis,density functional theory（DFT）/time-dependent density functional theory（TD-DFT）were employed to investigate the luminescence mechanism of host-guest doping systems containing phenothiazine units,and try to design host-guest doping systems based on the long-lived RTP materials,hopping to provide some theoretical guidances for obtaining high-performance host-guest doping systems in experiment.The detailed contents are listed as follows:1.Based on the experimental reports on the host-guest doping system M-CH₃,in which the phenothiazine dioxide derivatives CS-CH₃ and the phenothiazine derivatives Cz S-CH₃were selected as host and guest,respectively.We employed DFT/TDDFT method to investigate the photophysical properties including geometric and electronic structures,absorption spectra,emission and intersystem crossing（ISC）processes,of two doping models in detail after we demonstrated the tetrameric structure could be an appropriate computational model.The results show that the doping manner,in which the guest molecule affects the nearest neighboring host molecule through strongπ…πinteraction,could enhance the ISC ability mainly by reducing the energy gap between S₁ and T₁ states and increasing their SOC constant.Meanwhile,the triplet state luminescence of the guest molecule itself in aggregation model induces red-shifted phosphorescence spectrum.While the doping manner,in which the guest molecule affects the nearest neighboring host molecule through non-π…πinteractions,promotes the ISC process mainly by increasing the number of ISC channels,although the energy gap between S₁ and T₁states is also reduced to some extent.Besides,this doping manner maintains the luminescence properties of the host material.Moreover,the latter doping model has the close energy levels of T₁ state with the host model,which is in favor of the recyclable energy transfer of triplet excitons between the host molecules and doping model,which may prolong the RTP lifetime in the cocrystal.This work provides a new way to understand the luminescence mechanism of host-guest doping systems.2.Referring to above work,we designed two groups of host-guest doping systems according to the reported pure organic RTP materials containing phenothiazine unit,and further investigate their photophysical properties by employing DFT/TDDFT methods.The results show that for the DOPEO-PEO doping system,when the guest molecule PEO replaces the host molecule DOPEO at the edge of the aggregation model,the doping model possesses reduced energy gap between the single and triplet states and improved SOC constants,and thus generates more triplet excitons through enhancing ISC rate;when the guest molecule replaces the central host molecule of the aggregation model,the S₁-T₁ energy gap of the doping model is almost equal to 0,which would greatly enhance ISC ability,although the effective ISC channels are decreased.For the DOPPMO-PPMO doping system,the doping of the guest molecule PPMO in host aggregation model causes a red shift in the phosphorescence spectrum,and when the guest molecule PPMO replaces the host molecule DOPPMO at the edge of the aggregation model,the high-efficiency ISC could be obtained through reduced S₁-T₁ energy gap;while when the guest molecule replaces the central host molecule of the aggregation model,more triplet excitons would be generated through reduced S₁-T₁ energy gap and enhanced SOC constants.Therefore,we hope this work could provide some theoretical guidances for designing high-performance host-guest doping materials.