Molecular Design Strategy And Application Exploration In Organic Electroluminescence Device For Purely Organic Room-temperature Phosphorescent Materials Containing Sulfur Heterocycles

Room temperature phosphorescence(RTP)derived from the lowest triplet excited state and its related photophysical processes have broad application prospects in optoelectronics and biosciences such as photonic up-conversion,organic light-emitting diodes(OLEDs),photodynamic therapy,and bioimaging.In particular,the development of complex phosphorescent materials containing rare metals iridium and platinum in 1998 accelerated the development of OLEDs and achieved 100% electro-optical conversion efficiency in devices,demonstrating the great potential of phosphorescent materials in organic electroluminescent devices.In contrast to the phosphorescent complexes containing rare metals,it is difficult for purely organic materials to achieve efficient RTP.This is mainly attributed to the slow radiation process of purely organic materials,rendering the long-lived triplet excitons easily deactivate from thermal motion of surrounding molecules and oxygen diffusion.To achieve efficient RTP emission of pure organic materials and advance their practical applications in different fields,this thesis will design novel RTP materials to explore and demonstrate the triplet exciton behavior of purely organic materials in aggregated state for efficient RTP emission,and further realize their application in OLED devices,mainly including:1.Considering the positive effect of sulfur atoms on enhancing spin-orbit coupling(SOC),the RTP property of phenoxthione in the aggregated state was explored,and the different RTP properties of molecules after boronic ester substitution at different sites were further investigated to reveal the effect of molecular packing modes on RTP properties.With simultaneously enhanced SOC and ordered molecular close packing,one molecule achieved significantly improved phosphorescence quantum efficiency after substitution.In this study,the changes of aggregate structures after boronic ester substitution and the effect on phosphorescence properties were investigated in detail,and the structure-property relationship of purely organic RTP compounds was further explained.2.Based on the purely organic RTP molecule phenoxthione in Chapter 2,three groups of isomeric RTP materials with intramolecular or intermolecular hydrogen bonding interactions were further designed and developed,and the effect of intramolecular and intermolecular hydrogen bonds on regulating different phosphorescence properties was explored in detail.Further,taking advantage of the host-guest system in constructing enhanced RTP properties,a rigid phosphorous-containing host material was developed.By utilizing the rigid environment of the host,the external heavy atom effect,and the dispersion on the guests,significantly improved RTP performances were achieved in the host-guest system,relative to that of singlecomponent crystals.This work provided an in-depth understanding on the enhanced RTP emission induced by host-guest chemistry,and indicating the potential of rigid phosphorouscontaining host for efficient organic long-persistence luminescence.3.Based on the investigation of the luminescence properties of the materials in the aggregated state in the previous two chapters,and considering the role of the heavy atom effect and the n-π~* transition characters of sulfur atoms for developing high-efficiency RTP materials,we further developed a group of sulfur-decorated polycyclic compounds in the fourth chapter.Accordingly,a molecular design concept for highly efficient and mechanism-tunable thermally activated delayed fluorescence(TADF)and RTP OLEDs are proposed.The introduction of sulfur atoms can break the strong planarity of polycyclic hydrocarbons and avoid severe concentration quenching in aggregated state.In addition,the strong electron donating ability of sulfur atom makes the molecular orbitals distribute on the short-range multi-site sulfur atoms instead of the bonding orbits,which also helps to suppress nonradiative deactivation.In view of the highly efficient triplet exciton utilization ability of the sulfur-decorated polycyclic emitters,the corresponding TADF and RTP-OLED achieved maximum external quantum efficiencies up to 25.1% and 8.7%,respectively.Therefore,the great potential of sulfur-bridged polycyclic frameworks in achieving efficient devices is demonstrated.The versatile molecular engineering in this research shed light on the broad prospects of sulfur-bridged polycyclic frameworks for highly efficient devices and will guide the molecular design of new and efficient TADF and RTP emitters.Considering the n-π~* transition character and heavy atom effect of sulfur atom,the feasible strategy is even more advantageous in realizing purely organic RTP devices.