Manipulation Of Aggregation Structures And Luminescent Properties Of Pure Organic Room-Temperature Phosphorescence Crystals

Pure organic room temperature phosphorescence(RTP)materials have attracted more and more attention in many fields such as organic photoelectric devices,optical information storage,anti-counterfeiting,oxygen sensing,biological imaging and so on due to their unique advantages such as low cost,long lifetime and adjustable luminescence performance.The generation of RTP depends not only on the intersystem crossing(ISC)of the triplet excitons,but also on the excitons returning from the lowest triplet excited state to the ground state by an effective radiative transition.Therefore,it is a difficult task for pure organic materials to emit RTP.On the one hand,spin orbit coupling(SOC)in pure organic materials is generally small due to the absence of heavy metal atoms,while effective ISC usually requires a larger SOC between the singlet and triplet excited states.On the other hand,in atmospheric environment,the oxygen quenching and thermal vibration of triplet excitons are obvious,that is,the triplet excitons in pure organic materials have an unavoidable non-radiative deactivation process.Therefore,the design of effective molecular structure and material system is particularly important.Crystal materials play an important role in the system of pure organic RTP materials because of their simple preparation,definite packing structure and effective restriction of non-radiative transition process such as molecular motion.At present,research on RTP crystal materials has made some progress,but the relationship between the internal structure of the aggregated states and the luminescence performance still needs to be further explored.This not only affects the further design of RTP crystal materials,but also hinders the development of research on triplet excited state modulation based on the structure-property relationship.The luminescence properties of crystal materials are not only related to the molecular structure itself,but also inextricably linked to the stacking pattern of the aggregated states and intermolecular interactions.Therefore,this thesis takes pure organic RTP crystals as the research object,and explores the relationship between molecular structure,aggregated state structure and corresponding RTP properties from multiple angles by constructing organic single crystal and cocrystal systems,changing molecular conformation and intermolecular interaction from the perspective of molecular design,and regulating molecular stacking dynamically and statically by using external environmental stimuli,and combining experiment and theory to the luminescence mechanism at the molecular level was investigated,which laid a solid foundation for the future development of new pure organic RTP crystal materials.The main research contents are as follows:1.Three thianthrene(TA)derivatives,1TA1 TA,1TA2TA and 2TA2 TA were designed and synthesized,and the luminescence properties of the crystals of the four compounds,including TA,were studied in detail.The TA,1TA1 TA and 1TA2 TA crystals all exhibited dual emission of fluorescence and RTP.In contrast,2TA2 TA crystals did not exhibit RTP emission,and only fluorescence was observed.The crystal structures showed that the 2TA2 TA molecule exhibited a near planar molecular conformation in the crystal,while the remaining three compound molecules exhibited a distorted conformation.The analysis of theoretical calculations for different dihedral and single bond rotation angles shows that the planar molecular conformation makes the SOC ineffective and greatly increases the energy gap between the singlet and triplet states,so that the exciton cannot emit RTP through the ISC process,while the distorted molecular conformation not only reduces the energy gap between the singlet and triplet states,but also increases the SOC due to the different electronic grouping characteristics of the single and triplet states,so that the ISC process is opened in multiple channels and bright RTP is obtained.The experimental and theoretical results show that the molecular conformation plays an important role in the structure of the aggregated states and the corresponding RTP properties,which can provide scientific guidance for further broadening the molecular conformation tunable RTP material system.2.Cocrystal systems OPTM-O and OPYM-Y with charge transfer interaction were designed and cultivated with phenothiazine derivatives(OPTM)with RTP properties as donor molecules and tetracyanide benzene with strong electron absorption ability as acceptor molecules.OPTM-O cocrystal exhibits thermallyactivated delayed fluorescence(TADF)properties with increasing temperature,while OPTM-Y cocrystal exhibits RTP properties with decreasing temperature.The results of crystal structure,photophysical properties and theoretical analysis show that the different stacking modes and molecular interactions are the direct causes of the different photoluminescence effects of the two cocrystals.Theoretical calculations show that combining with acceptor molecules into cocrystal can greatly reduce the excited state energy level of the system,while the abundant non-covalent interactions between the donor and the acceptor molecules induce the formation of charge-transfer excited states between the molecules,effectively reducing the energy gap between the singlet and triplet states,providing the possibility of modulation between TADF and RTP linked based on the energy gap relationship.The differences in the SOC constants between the two crystals results in different luminescence properties.This study regulates the structure of the aggregate state and the corresponding excited state properties of organic cocrystal from the perspective of intermolecular interactions to achieve different luminescence effects,and establishes relationship between the aggregation structure and luminescence properties at the molecular level,which provides a new idea for the design and development of new organic materials with TADF and RTP properties.3.The RTP cocrystals PTL-O and PTL-Y were designed and cultivated with the fluorescent molecule phenanthroline(PTL)as the main molecule and 1,4-diiodotetrafluorobenzene(DIFB)containing heavy atom I as the ligand molecule through halogen bonding interactions,and the two cocrystals achieved the RTP of the triplet excited state of the main molecule by intermolecular interactions and heavy atom effects.Furthermore,by means of external stimuli,the cocrystal aggregation structure,especially the halogen bonding interaction,was dynamically regulated,and the RTP properties were adjusted.Comparative experimental and theoretical calculations show that the abrasive shear force of anisotropy and the acid fumigation process will destroy the halogen bonding interaction in the phosphorescent cocrystal,and make the main molecule get rid of the heavy atom effect,so the phosphorescence is substantially quenched.The dynamic adjustment of halogen bonding interactions was achieved under the stimulation of isotropy hydrostatic pressure,and the RTP exhibited a significant relative enhancement phenomenon as the interaction distance increased and the intensity of the interaction increased.In this study,from the design of static molecular system to the dynamic regulation of the aggregation structure by external means,the regulation of the RTP properties was realized.The relationship between the molecular aggregation structure and the properties were studied from various angles,and a new method of in situ regulation of the aggregation structure was established and developed.It provides new ideas for further understanding the relationship between the structure of aggregates and luminescence behavior,and also lays a solid foundation for the development of new concepts of responsive pure organic RTP materials.