Preparation And Application Of Boron And Carbon-based Triplet State Photoluminescent Materials

Phosphorescent and delayed fluorescent materials have shown promising applications in chemical sensing,bioimaging,light-emitting diodes and information protection due to their properties such as triplet state luminescence,long lifetime and even naked-eye recognizable luminescence,becoming active photoluminescent materials at present.Although numerous breakthroughs have been made in research related to phosphorescent and delayed fluorescent materials,their preparation and applications are still very limited compared to fluorescent materials.This is mainly due to the limited variety of phosphorescent and delayed fluorescent materials,the vast majority of which are based on inorganic materials containing rare earth elements,as well as conventional organic compounds based on molecular design.Recently,it has been reported that carbon-based materials and non-traditional organic molecular materials（boron-based materials）can also achieve phosphorescence and delayed fluorescence,which are new types of photoluminescent materials involving triplet states.For carbon-based materials,most of the research focuses on the synthesis mechanism and luminescence mechanism,the application of these materials is scarce.And there are disadvantages such as more cumbersome preparation process,shorter lifetime,lower stability,and extremely sensitive to oxygen/water vapor.The study of boron-based materials is mainly based on the combination of boron and electron-rich elements（O,N,F,etc.）as luminescence,but there are relatively few reports on such materials.And there is no unified and feasible theoretical basis yet;especially,the synthesis of materials with both aqueous phase and dual-emission properties faces a great challenge.Based on the above analysis,this thesis focuses on the development of novel carbon-based and boron-based room-temperature phosphorescent and delayed-fluorescence materials,from the preparation process,combined with morphological analysis,structural characterization and photophysical property testing,to investigate their luminescence mechanisms and apply them to bioimaging,sensing and anti-counterfeiting fields.The main researches are as follows:（1）Room temperature phosphorescence of carbon dots@silica（CDs@silica）composites in aqueous solution for cellular imaging and anti-counterfeitingRoom-temperature phosphorescent composites with ultra-long lifetime in aqueous solution were prepared by a one-step hydrothermal method using tetraethoxysilane as the raw material.Investigation shows that the C=O group on the surface of carbon dots is the main luminescence center;the silica matrix shell acts as the domain-limiting layer;the covalent and hydrogen bonds formed between carbon dot surface and silica act as immobilization,further limiting the motion of carbon dots.These factors work in concert to serve to immobilize the luminescence center with the suppression of non-radiative transitions,thus achieving ultra-long-lived room temperature phosphorescence in aqueous solution.CDs@silica exhibit ultra-long lifetime up to 2.19 seconds in aqueous solution and remain stable for several months.Based on the excellent room-temperature phosphorescence properties of this material in aqueous solution,we have applied it to cellular imaging,as well as to advanced anti-counterfeiting related to humidity.（2）Room-temperature phosphorescence of nitrogen-doped CDs@silica composites for Fe³⁺detection and anti-counterfeitingBased on the previous chapter,ultra-long-lived nitrogen-doped carbon dot@silica composites（N-CDs@silica）were prepared from tetraethoxysilane and triethylenetetramine by combining the many advantages of nitrogen atom-doped carbon dots.Triethylenetetramine was used both as a basic catalyst for the hydrolysis of tetraethoxysilane and as a nitrogen source.Combining morphology,structural characterization,photophysical properties and a series of control experiments,the effect of N doping on the luminescence performance was confirmed,mainly affecting the degree of cross-linking within the carbon dots,protecting the luminescence center and thus the luminescence performance of N-CDs@silica.Finally,N-CDs@silica were explored as solid-state RTP strip probes for visual and quantitative detection of Fe³⁺;and as advanced anti-counterfeiting optical inks for the protection of valuable items and important information.（3）Covalent immobilization enhanced dual delayed fluorescence of borate solution for cellular imaging and anti-counterfeitingBoroxylated delayed fluorescence materials（APB@CC）capable of stable presence in aqueous solutions were prepared by a simple hydrothermal method using ammonium pentaborate and cyanuric chloride as raw materials.The material has double-emission delayed fluorescence properties.Different blue and green delayed fluorescence can be observed by changing the excitation wavelength.Analysis shows that during the hydrothermal process,cyanuric chloride is first hydrolyzed to cyanuric acid,and then the-OH of cyanuric acid condenses with the-OH of ammonium pentaborate to form C-O-B covalent bonds.In addition,a large number of hydrogen bonds exist in this molecular structure,and the combined effect of these hydrogen bonds and covalent bonds can reduce the vibration and rotation of the molecule and effectively suppress the non-radiative transitions;the presence of characteristic n-π*andπ-π*transitions in the B-O alternating hexameric ring may be responsible for the delayed fluorescence double emission feature.Finally,the application of APB@CC in cell imaging and multi-stimulus response safety protection is demonstrated.In summary,this thesis focuses on the synthesis of new boron-based and carbon-based triplet state photoluminescent materials.Through the screening of suitable precursors and the construction of a one-step synthesis strategy,we have achieved the controlled preparation of long-lived carbon-based room-temperature phosphorescent materials and boron-based delayed fluorescent materials,explored their luminescence mechanisms,and revealed the relationship between material structure and luminescence performance.These research works will provide reference and guidance for the preparation of long-lived boron-based and carbon-based triplet state photoluminescent materials,and explore their applications in anti-counterfeiting,sensing and imaging for the research of multifunctional photoluminescent materials.