Molecular Design Strategy Of Normal Fluorescence And Thermally Activated Delayed Fluorescence: Controllable Switching Of Luminescence Mechanism And Realization Of White-light Emission

In the past few years,thermally activated delayed fluorescence(TADF)materials have grown tremendously and are regarded as promising next-generation organic electroluminescent materials due to their potentials for obtaining nearly 100% internal quantum efficiency through noble metal free pure-organic molecules.Compared with the first-generation organic electroluminescent materials,normal fluorescence(NF),TADF not only shows significantly upgraded luminescence efficiency,but also has great potentials as oxygen,temperature sensors and time-resolved fluorescence biological imaging,because of the harvesting of long-lived triplet excitons.Combining above unique features,we believe that this material may possess significance in fundamental researches and practical applications.White-light-emitting organic materials and devices have attracted widespread attention because of the great potential for innovative applications to next generation displays and light sources.Most organic white-light emitters reported so far rely on the combination of different emitters(red/blue/green or blue/yellow)to fully span the entire visible spectrum,which may cause problems such as color aging or difficult requirements for device fabrication.Compared with the multi-component materials,use of a single-component emitter would enable easy fabrication with perfect color reproducibility and stability.Limited by the Kasha's rule,organic compounds generally exhibit only one type of emission from the lowest excited state.Thus,achieve single-component organic white-light material is a great challenge in both synthetic chemistry and photo-physics field.In this thesis,we focus on exploring the influence of molecular structure on the regulation and reversible switching behavior among luminescence mechanism of organic light-emitting materials,as well as developing new single-component organic white-light materials.In chapter 2,the diphenyl acetylene unit was first used as electron acceptor and two novel D-A type organic light-emitting materials were designed and synthesized.The luminescence mechanism of two organic light-emitting materials with similar structures was regulated by changing the donor moiety.In addition,DPE-DDMAc exhibited dual-emission characteristics in powder state and tetrahydrofuran and water(THF/H2O)mixed solvent.The photoluminescence(PL)intensity of two complementary colours was effectively adjusted by changing the water content of mixed solvent.In the end,DPE-DDMAc exhibited white light emission in the mixed solvent with water fraction of 77%,77% and 80%,and corrponding Commission Internationale de l'Eclairage(CIE)chromaticity coordinates was(0.29,0.29),(0.33,0.33)and(0.35,0.35),respectively.In chapter 3,we designed and synthesized a D-A molecule PTZ-MDC,combining 10H-phenothiazine(PTZ)as a donor and phenyl(pyrimidin-5-yl)methanone as an acceptor.PTZ-MDC breakthrough the limitation of Kasha's rule,and can emit pure deep blue NF or yellow TADF in different conditions.Reversible switching behaviors between deep blue NF and yellow TADF can take place under thermal and mechanical activation.Based on these two complementary colors,white-light emission combining NF and TADF from a single compound can be achieved in various states.In chapter 4,the reversible switching behavior between deep blue NF and yellow TADF was continuing to study in depth.Experimental results and density functional theory calculations proposal that deep blue NF and yellow TADF of PTZ-MDC were derived from two conformers that possess distinctly different photophysical and photochemical properties.Conformational relaxations in both ground state and excited state were first affirmed in organic compound and contributed to above uniquely reversible switching behaviors both in emission colors and luminescence mechanism.Two compounds with only two atom replacement hold back the switch of conformational relaxation,contributing to our understanding of the subtle difference in chemical structure that may govern the on/off of switching behavior,and this understanding could be employed advantageously to further design and studies of new materials which are capable of reversible switching behavior between NF and TADF.This implausibly reversible switching behavior provided a new paradigm in developing “smart” materials and white-light-emitting emitters from a single organic compound,and also has great potentials to be adopted as amorphous organic semiconductor for developing stimuli-responsive devices benefiting from the morphology-independent virtue.