Regulation And Investigation Of Triplet Excitons In Organic Luminescent Materials

Organic luminescent materials with triplet characteristics show long excited-state lifetimes and excellent photosensitization capabilities,thus enabling potential applications in a variety of different fields,such as optoelectronic devices,chemical sensing,anti-counterfeit encryption,optical imaging and photodynamic therapy.Once excited by the external energy,the excited electron will undergo intersystem crossing process and then change its spin direction to realize the population of triplet state.This spin-forbidden transition process could be significantly facilitated by enhancing the spin-orbit coupling and narrowing the single-triplet energy gap.At present,organic luminescent materials with triplet characteristics,including triplet luminescent materials and triplet photosensitizers,have made remarkable progress in molecular design and functional exploration.Nevertheless,the mixed singlet-triplet excited state dynamics process has raised tremendous difficulties to fine-tune triplet properties of organic luminescent materials,because excited molecules often involve simultaneous singlet and triplet states,and the interconversion between them.In addition,materials are often presented in forms of powders,thin films,crystals or nanoparticles in practical applications.The complex spatial stacking and intermolecular interactions in molecular aggregates also cause important effects on the excited-state properties of molecules,further increasing the difficulty of regulating the pathway of triplet excitons.Therefore,realizating tunable triplet properties of organic luminescent materials through rational molecular designs and dynamic regulation strategies will offer significant revelations for a better understanding of structure-property relationships and intrinsic excited-state dynamics in molecular aggregates,further optimizing the material properties and broadening their applications in advanced optics.This paper focuses on the regulation of triplet excitons in organic luminescent materials by the combination of structural optimization and dynamic stimulus.The detailed spectral analysis and theoretical calculations have offered in-depth investigation of triplet photosensitization and triplet luminescence of materials.The specific studies are as follows:1.In the second chapter,we proposed a "quadrupolarization" strategy by employing the aggregation-induced luminescent molecule tetraphenylethylene as the electronic donor and the dicyanoethylene as the electronic acceptor to design and synthesize two molecular models with dipolar and quadrupolar structures.Ultrafast spectroscopic analysis and theoretical calculations have revealed that quadrupolar molecule underwent a unique symmetry breaking process of excited states compared to dipolar molecules,which effectively facilitated the intramolecular charge separation and significantly reduced the single-triplet energy gap.Subsequently,the electron went through radical ion pairs intersystem crossing(RP-ISC)process to realize efficient population of triplet state.Due to the limitation by weak spin-orbit coupling,quadrupolar molecules cannot achieve triplet luminescence process,whereas they exhibit excellent triplet photosensitization capabilities with a singlet oxygen quantum yield up to 97% and maintain high performance in the photodynamic therapy of cancers.2.In the third chapter,to further enhance the spin-orbit coupling of electrons for achieving triplet luminescence of molecules,we selected the rigid dibenzofuran molecules as luminophores and fine-tuned intramolecular charge transfer properties by peripheral modification of different electronic acceptors.Meanwhile,electronic n-π~* transition induced by aromatic heteroatoms also enhanced molecular spin-orbit coupling,thus activating the triplet luminescence of materials in solid state.In addition,the conjugated structures and excited state properties of molecules could be regulated by protonation and deprotonation reactions under external stimulis of hydrochloric acid and trimethylamine,which realized reversible photoswitching between room temperature phosphorescence and fluorescence.3.In the fourth chapter,we have optimized and improved the way of regulating molecular triplet luminescence by adopting mild photo-stimulation.Photo-stimulusresponsive room temperature phosphorescent molecules were designed and synthesized by combining phosphors with photoresponsive units.Photoresponsive molecules could undergo intermolecular 2+2 cycloaddition reaction under continuous UV irradiation,resulting in the formation of dimeric molecules and photoresponsive room temperature phosphorescence properties.When we doped CZ-O-BT molecule into different polymer matrixes,the prepared polymer film showed a unique photoactivated triplet luminescence,showing potential applications in optical memory.4.In the fifth chapter,we constructed a novel phosphorescent photoswitch molecule with fast photoresponsive rate and reversible photoresponsive properties by integrating photoswitch molecule diarylethene with a phosphor through aromatic carbonyl linker.Photoswitch molecule not only exhibited excellent photochromic properties and optical modulation capabilities,but also enabled reversible fluorescencephosphorescence switching of materials in solid state by inducing reversible structural transformation under alternating UV and visible light.The molecular modification strategy based on photoswitches also provides a new approach to the regulation of triplet excitons in organic luminescent materials.