High-temperature-resistant long-afterglow carbon dot (at) g-C3N4 composite material as well as preparation method and application thereof

Abstract

The invention discloses a high-temperature-resistant long-afterglow carbon dot (at) g-C3N4 composite material as well as a preparation method and application thereof. The high-temperature-resistant long-afterglow carbon dot (at) g-C3N4 composite material realizes high According to the invention, chromophores and urea are subjected to pyrolysis in-situ conversion to form the carbon dot-coated g-C3N4 composite material so as to activate ultra-long high-temperature phosphorescence. The internal rigid structure of the carbon dots can be synergistically rigidized through the external rigid structure of the g-C3N4 matrix by means of the space confinement effect and multiple interactions. Under a high-temperature condition, the triplet excitons can be prevented from being quenched through shielding and confinement effects. When the temperature is 473 Kelvin, the service lives of the green phosphorescent composite material in a solid state and an N-methyl-2-pyrrolidone solution respectively reach 7.34 seconds and 4.91 seconds, and the persistence time of the green phosphorescent composite material in the solid state and the persistence time of the green phosphorescent composite material in an N-methyl-2-pyrrolidone solution respectively reach 44 seconds and 30 seconds. The red phosphorescent composite material lacking a B-N covalent bond shows a thermochromic phosphorescent phenomenon from red to blue due to the fact that low-energy-level triplet excitons are easily inactivated by heating. The carbon dot-coated g-C3N4 composite material expands the practical application of a phosphorescent material in a harsh high-temperature environment.

Description

Technical Field

[0001] This invention relates to the field of organic phosphorescent materials technology, specifically a high-temperature resistant long-afterglow carbon dot @g-C3N4 composite material, its preparation method, and its application. Background Technology

[0002] Organic phosphorescent materials have broad application prospects in fields such as bioimaging, sensing, anti-counterfeiting, and information storage due to their advantages such as long lifetime, low cost, easy functionalization, and good processing performance. However, triplet excitons are highly sensitive to environmental factors such as molecular oxygen, polar solvents, and thermal motion, which means that early organic phosphorescent materials could only be observed under low-temperature or inert gas conditions. To achieve efficient organic room-temperature phosphorescence (RTP) under environmental conditions, current research has developed strategies such as crystal engineering, carbon dot engineering, heavy atom doping, and rigid matrix encapsulation. These strategies effectively suppress intramolecular motion and protect triplet excitons, thereby significantly improving the lifetime and quantum yield of RTP. However, these materials still face a key bottleneck in high-temperature environments: the increased molecular motion at high temperatures promotes non-radiative decay pathways, leading to strong thermal quenching, which cannot meet the application requirements in extreme environments.

[0003] Currently reported high-temperature phosphorescence (HTP) materials generally suffer from performance defects: some polymer-based HTP materials can achieve an afterglow of up to 40 s at room temperature, but this drops sharply to 1 s at 433 K; most HTP materials can only achieve high-temperature luminescence in the solid state, and due to dissolved oxygen and solvent relaxation effects in the liquid phase, it is difficult to maintain stable high-temperature phosphorescence performance. In addition, existing HTP materials lack sufficiently rigid matrices and efficient synergistic strategies, resulting in short phosphorescence lifetimes and insufficient afterglow duration at high temperatures, which severely limits their practical application in harsh high-temperature environments.

[0004] Graphitic carbon nitride (g-C3N4), as a two-dimensional layered material with unique triazine ring structural units, possesses high physicochemical and thermal stability. Its abundant nitrogen-containing edges and defect sites can form various interactions, providing a good structural basis for anchoring luminescent materials. Matrix confinement has been successfully used to block the triplet exciton dissipation channels of carbon dots to achieve long-lifetime room-temperature phosphorescence (RTP). However, how to achieve effective composite of carbon dots and g-C3N4 through synergistic rigidification effects and endow it with high-temperature phosphorescence properties in both solid and liquid states remains a pressing technical problem to be solved in this field. Summary of the Invention

[0005] To address the technical problems of existing high-temperature phosphorescent materials, such as severe thermal quenching, short high-temperature luminescence lifetime, insufficient afterglow duration, and difficulty in stable luminescence in the liquid phase, this invention provides a high-temperature resistant long-afterglow carbon dot@g-C3N4 composite material, its preparation method, and its applications. Through the synergistic effect of the internal rigidity of the carbon dots and the external rigidity of the g-C3N4 matrix, stable long-afterglow luminescence is achieved at high temperatures. Simultaneously, a simple preparation method and multiple applications of this composite material are provided, expanding the application of organic phosphorescent materials in harsh high-temperature environments.

[0006] In a first aspect, the present invention provides a high-temperature resistant long-afterglow carbon dot@g-C3N4 composite material, comprising carbon dots (CDs) and a g-C3N4 matrix. The carbon dots are formed by in-situ co-pyrolysis of urea and a chromophore precursor and embedded in the g-C3N4 matrix. A hydrogen bond network is formed between the carbon dots and the g-C3N4 matrix. When the carbon dots are B-CDs or G-CDs, BN covalent bonds are also formed between the carbon dots and the g-C3N4 matrix.

[0007] Preferably, the chromophore precursor is one of 3,5-dicarboxyphenylboronic acid (DCBA), triphenyl-2-boronic acid (TPB), or rhodamine B (RhB);

[0008] When the precursor is DCBA, B-CDs@g-C3N4 is formed; when the precursor is TPB, G-CDs@g-C3N4 is formed; when the precursor is RhB, R-CDs@g-C3N4 is formed.

[0009] Preferably, the phosphorescence quantum yield of the B-CDs@g-C3N4 is 14.74%, the solid-state phosphorescence lifetime is 1.09 s at 298 K, the solid-state phosphorescence lifetime is 41.68 ms at 473 K, and the afterglow duration is 0.5 s.

[0010] Preferably, the phosphorescence quantum yield of the G-CDs@g-C3N4 is 48.44%, the solid-state phosphorescence lifetime is 11.92 s at 298 K, the solid-state phosphorescence lifetime is 7.34 s at 473 K, and the afterglow duration is 44 s; in N-methyl-2-pyrrolidone (NMP) solvent at 473 K, its phosphorescence lifetime is 4.91 s and the afterglow duration is 30 s.

[0011] Preferably, the phosphorescence quantum yield of the R-CDs@g-C3N4 is 10.44%, and the solid-state phosphorescence exhibits bimodal emission at 448 nm and 593 nm at 298 K, with lifetimes of 2.33 s and 1.33 s, respectively; at 323 K, its phosphorescence emission changes from red to blue, exhibiting thermochromic phosphorescence.

[0012] Preferably, the elemental composition of the composite material, expressed as a mass fraction, is as follows:

[0013] B-CDs@g-C3N4: C 45.84%, N 29.83%, O 23.20%, B 1.13%;

[0014] G-CDs@g-C3N4: C 41.72%, N 32.88%, O 24.46%, B 0.94%;

[0015] R-CDs@g-C3N4: C 36.72%, N 37.03%, O 26.10%, Cl 0.15%.

[0016] Preferably, the B-CDs have an average particle size of 2.61 nm and a lattice spacing of 0.207 nm; the G-CDs have an average particle size of 1.75 nm and a lattice spacing of 0.209 nm; and the R-CDs have an average particle size of 2.18 nm and a lattice spacing of 0.248 nm.

[0017] In a second aspect, the present invention provides a method for preparing the high-temperature resistant long-afterglow carbon dot @g-C3N4 composite material described in the first aspect, comprising the following steps:

[0018] (1) Add 3.0 g of urea and carbon dot precursor to 20 mL of distilled water and stir to form a homogeneous mixture;

[0019] (2) The mixture was heated at 200 °C for 3 hours.

[0020] (3) After the reaction is complete, the product is cooled, collected and ground to obtain the carbon dot@g-C3N4 composite material;

[0021] Wherein, when the carbon dot precursor is DCBA, its dosage is 25 mg; when the precursor is TPB, its dosage is 15 mg; when the precursor is RhB, its dosage is 1 mL of RhB aqueous solution with a concentration of 0.5 mmol / L.

[0022] Thirdly, the present invention also provides an application of the high-temperature resistant long afterglow carbon dot @g-C3N4 composite material described in the first aspect, for in-situ non-destructive microcrack detection at temperatures from 298 K to 473 K, for preparing high-temperature safety markings, for realizing multi-dimensional information encryption, or for temperature monitoring.

[0023] Furthermore, the in-situ non-destructive microcrack detection involves spraying an aqueous dispersion of G-CDs@g-C3N4 onto the surface of the component under test. After turning off the 302 nm ultraviolet light source, microcracks are identified by the afterglow generated, with an afterglow duration of no less than 39 s at 473 K. The high-temperature safety mark is made by dispersing G-CDs@g-C3N4 in the encapsulation material, with afterglow durations of 57 s, 43 s, 29 s, and 18 s at 298 K, 373 K, 423 K, and 473 K, respectively. The multidimensional information encryption involves placing B-CDs@g-C3N4, G-CDs@g-C3N4, and R-CDs@g-C3N4 in different areas of the anti-counterfeiting pattern, exhibiting blue and green phosphorescence under 254 nm ultraviolet light excitation, with only green phosphorescence remaining after more than ten seconds. It exhibits only red phosphorescence under nm visible light excitation; the temperature monitoring is based on the thermochromic phosphorescence effect of R-CDs@g-C3N4, which emits red phosphorescence when the temperature is not higher than room temperature, and turns into blue phosphorescence or quenches when the temperature is higher than 50 °C.

[0024] Compared with the prior art, the beneficial effects of the present invention are:

[0025] (1) This invention embeds carbon dots into the g-C3N4 matrix through an in-situ co-pyrolysis strategy, forming a synergistic rigid structure of “internal rigidity (carbon dots) - external rigidity (g-C3N4 matrix)”. The hydrogen bonds and BN covalent bonds (B-CDs / G-CDs system) between the carbon dots and the g-C3N4 matrix effectively restrict molecular thermal motion, suppress non-radiative transitions, significantly improve the high-temperature stability of triplet excitons, and solve the technical problem of severe high-temperature thermal quenching in existing materials.

[0026] (2) The composite material has excellent solid and liquid high-temperature phosphorescence properties. G-CDs@g-C3N4 has a solid afterglow of 44 s at 473 K and an afterglow of 30 s in NMP solution. Its performance far exceeds that of existing HTP materials, filling the gap in liquid-phase high-temperature phosphorescence materials.

[0027] (3) The composite material exhibits multicolor phosphorescence properties (blue, green, red), among which R-CDs@g-C3N4 can be excited by visible light and has water / heat stimulation-responsive color-changing phosphorescence, providing diverse options for multi-scenario applications;

[0028] (4) The preparation method adopts a one-pot co-pyrolysis process, which is simple in steps, easy to obtain raw materials, does not require complicated equipment, has low production cost, and is suitable for large-scale production;

[0029] (5) The application of composite materials in fields such as high temperature microcrack detection, safety marking, information encryption and temperature monitoring has expanded the application scope of organic phosphorescent materials. They are especially suitable for harsh high temperature environments and have important practical application value. Attached Figure Description

[0030] Figure 1 The following are examples of the ultralong high-temperature phosphorescence of CDs@g-C3N4 of the present invention: (a) Schematic diagram of the synthesis and synergistic rigidification of three CDs@g-C3N4 composite materials; (b) Comparison of afterglow time of G-CDs@g-C3N4 with previously reported high-temperature phosphorescence (HTP) materials; Afterglow photographs of G-CDs@g-C3N4 in solid state (c) and polar solvent (d) at different temperatures before and after turning off a 302 nm UV lamp (8 W);

[0031] Figure 2 Characterization of CDs@g-C3N4 of the present invention: Transmission electron microscopy (TEM) images and corresponding particle size distribution histograms (inset) of B-CDs@g-C3N4(a), G-CDs@g-C3N4(b) and R-CDs@g-C3N4(c); X-ray diffraction (XRD) patterns (d), Fourier transform infrared (FT-IR) spectra (e) and X-ray photoelectron spectroscopy (XPS) full spectra (f) of the three CDs@g-C3N4 composite materials; High-resolution C 1s (g), N 1s (h) and O 1s (i) XPS spectra of the three CDs@g-C3N4 composite materials and corresponding fitting results;

[0032] Figure 3 The photophysical properties of the CDs@g-C3N4 composite solid powder at room temperature of the present invention are as follows: (a) Afterglow photographs of the three CDs@g-C3N4 composite powders before and after turning off the 302nm UV lamp (8 W); (bd) Fluorescence and phosphorescence spectra of the three CDs@g-C3N4 composite powders (delay time: 3 ms); (eg) Time-resolved phosphorescence decay curves of the three CDs@g-C3N4 composite powders; (h) Density functional theory (DFT) calculation results for DCBA, TPB and RhB.

[0033] Figure 4 The photophysical properties of the CDs@g-C3N4 composite materials of the present invention dispersed in water are as follows: (a) afterglow photographs of the three CDs@g-C3N4 composite materials dispersed in water before and after the 302 nm UV lamp (8 W) is turned off; (bd) fluorescence and phosphorescence spectra of the three CDs@g-C3N4 composite materials dispersed in water (delay time: 3 ms); (eg) time-resolved phosphorescence decay curves of the three CDs@g-C3N4 composite materials dispersed in water.

[0034] Figure 5 The high-temperature phosphorescence (HTP) performance of the CDs@g-C3N4 composite material of the present invention in solid and polar solvents is as follows: phosphorescence spectra of G-CDs@g-C3N4 in solid (a) and polar solvent (d) at different temperatures (delay time: 3 ms); time-resolved phosphorescence decay curves of G-CDs@g-C3N4 in solid (b) and polar solvent (e) at different temperatures; comparison of phosphorescence lifetime of B-CDs@g-C3N4 and G-CDs@g-C3N4 in solid (c) and polar solvent (f) at different temperatures;

[0035] Figure 6 To investigate the high-temperature phosphorescence (HTP) mechanism of this invention: Scanning electron microscope (SEM) images of B-CDs@g-C3N4 (a), G-CDs@g-C3N4 (b), and R-CDs@g-C3N4 (c); thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) curves of B-CDs@g-C3N4 (d), G-CDs@g-C3N4 (e), and R-CDs@g-C3N4 (f); (g) Phosphorescence spectra of G-CDs@g-C3N4 and 1-CDs@g-C3N4 at room temperature; Comparison of phosphorescence lifetimes of G-CDs@g-C3N4 and 1-CDs@g-C3N4 in solid state (h) and polar solvent (i) at different temperatures;

[0036] Figure 7 Applications of the CDs@g-C3N4 composite material of the present invention: (a) in-situ non-destructive microcrack detection of aircraft engine turbine blades at different temperatures (298-473 K); (b) photographs of phosphorescent safety exit signs prepared by G-CDs@g-C3N4 at different temperatures (298-473 K); (c) photographs of a three-dimensional gradient fishtail anti-counterfeiting model prepared by dispersing B-CDs@g-C3N4 and G-CDs@g-C3N4 powders in an epoxy resin matrix; (d) photographs of anti-counterfeiting patterns under different excitation wavelengths; (e) temperature monitoring of R-CDs@g-C3N4 during pharmaceutical transportation.

[0037] Figure 8 High-resolution transmission electron microscopy (TEM) images of the B-CDs@g-C3N4(a), G-CDs@g-C3N4(b) and R-CDs@g-C3N4(c) composite materials of the present invention;

[0038] Figure 9 The Raman spectra of B-CDs@g-C3N4, G-CDs@g-C3N4 and R-CDs@g-C3N4 of the present invention are shown below.

[0039] Figure 10This is a high-resolution X-ray photoelectron spectroscopy (XPS) image of the B-CDs@g-C3N4 and G-CDs@g-C3N4 composite materials of the present invention.

[0040] Figure 11 The phosphorescence spectra (delay time: 3 ms) and corresponding decay curves (b) of B-CDs@g-C3N4 with different contents of 3,5-dicarboxyphenylboronic acid (DCBA) according to the present invention are shown.

[0041] Figure 12 Phosphorescence spectra (delay time: 3 ms) of G-CDs@g-C3N4 with different triphenyl-2-boronic acid (TPB) contents of the present invention (a) and corresponding decay curves (b);

[0042] Figure 13 Phosphorescence spectra (delay time: 3ms) and corresponding decay curves (b) of R-CDs@g-C3N4 with different Rhodamine B (RhB) contents according to the present invention are shown.

[0043] Figure 14 Phosphorescent quantum yield (Phos. QY) of B-CDs@g-C3N4(a) (delay time: 8 ms), G-CDs@g-C3N4(b) (delay time: 8 ms), and R-CDs@g-C3N4(c) (delay time: 3 ms) of the present invention;

[0044] Figure 15 The phosphorescence emission spectra of B-CDs@g-C3N4(a), G-CDs@g-C3N4(b) and R-CDs@g-C3N4(c) at different temperatures are shown below.

[0045] Figure 16 The UV-Vis absorption spectra and phosphorescence excitation spectra of B-CDs@g-C3N4(a), G-CDs@g-C3N4(b) and R-CDs@g-C3N4(c) of the present invention are shown.

[0046] Figure 17 The phosphorescence spectrum (excitation wavelength λ) of (a) R-CDs@g-C3N4 of the present invention is shown. ex = 472 nm, delay time: 3 ms); (b) time-resolved phosphorescence decay curve of R-CDs@g-C3N4; (c) afterglow of R-CDs@g-C3N4 before and after turning off the 450 nm visible light source (15 W);

[0047] Figure 18 This is a schematic diagram of the possible hydrogen bond network and BN covalent bonds between the carbon dots (CDs) and the g-C3N4 matrix of the present invention;

[0048] Figure 19 The images show transmission electron microscopy (TEM) images (a), high-resolution transmission electron microscopy (HR-TEM) images (b), and particle size distribution histograms (c) of U-CDs@g-C3N4 of the present invention.

[0049] Figure 20 The UV-Vis absorption spectrum of (a) Rhodamine B (RhB) of the present invention, and the phosphorescence spectrum of U-CDs@g-C3N4 (excitation wavelength λ) ex = 310 nm, delay time: 3 ms); (b) fluorescence spectrum of Rhodamine B, and phosphorescence spectra of U-CDs@g-C3N4 and R-Ds@g-C3N4 (delay time: 3 ms); (c) phosphorescence spectra of U-CDs@g-C3N4 and R-CDs@g-C3N4 with different Rhodamine B contents (delay time: 3 ms);

[0050] Figure 21 This is a schematic diagram of the fluorescence resonance energy transfer (FRET) mechanism between U-CDs@g-C3N4 and Rhodamine B (RhB) of the present invention;

[0051] Figure 22 The phosphorescence excitation spectrum of the U-CDs@g-C3N4 powder of the present invention is shown below.

[0052] Figure 23 The International Commission on Illumination (CIE) chromaticity coordinate diagrams of the phosphorescence of B-CDs@g-C3N4, G-CDs@g-C3N4 and R-CDs@g-C3N4 of the present invention are shown below.

[0053] Figure 24 The optical band gap energy difference of the phosphorescence spectra of B-CDs@g-C3N4, G-CDs@g-C3N4 and R-CDs@g-C3N4 of the present invention;

[0054] Figure 25 The following are examples of the phosphorescence emission spectrum (delay time: 3 ms) of U-CDs@g-C3N4 dispersed in aqueous solution and the time-resolved phosphorescence decay curve of U-CDs@g-C3N4 dispersed in aqueous solution.

[0055] Figure 26 The phosphorescence phenomenon of R-CDs@g-C3N4 after different treatments according to the present invention;

[0056] Figure 27The fluorescence intensity changes of B-CDs@g-C3N4(a), G-CDs@g-C3N4(b), and R-CDs@g-C3N4(c) of the present invention after continuous irradiation under a 254 nm ultraviolet lamp for 1 hour; the phosphorescence intensity changes of B-CDs@g-C3N4(d), G-CDs@g-C3N4(e), and R-CDs@g-C3N4(f) after continuous irradiation under a 254 nm ultraviolet lamp for 1 hour.

[0057] Figure 28 The phosphorescence emission spectra of B-CDs@g-C3N4(a), G-CDs@g-C3N4(b) and R-CDs@g-C3N4(c) of the present invention before and after one month of storage in air;

[0058] Figure 29 High-temperature phosphorescence (HTP) characteristics of solid-state B-CDs@g-C3N4 of the present invention: (a) afterglow images of solid-state B-CDs@g-C3N4 at different temperatures before and after turning off the 8 W, 302 nm UV lamp; (b) phosphorescence spectra of solid-state B-CDs@g-C3N4 at different temperatures (delay time: 3 ms); (c) and (d) time-resolved phosphorescence decay curves of B-CDs@g-C3N4 at different temperatures;

[0059] Figure 30 The high-temperature phosphorescence (HTP) characteristics of B-CDs@g-C3N4 in polar solvents of the present invention are as follows: (a) Afterglow photographs of B-CDs@g-C3N4 in water before and after turning off an 8 W, 302 nm UV lamp at 373 K; (b) Phosphorescence spectra of B-CDs@g-C3N4 in different polar solvents and at different temperatures (dispersed in water at 373 K, and in N-methyl-2-pyrrolidone (NMP) at 423 K and 473 K, with a delay time of 3 ms); (c) Time-resolved phosphorescence decay curve of B-CDs@g-C3N4 dispersed in water at 373 K; (d) Time-resolved phosphorescence decay curve of B-CDs@g-C3N4 dispersed in NMP at 423 K; (e) Time-resolved phosphorescence decay curve of B-CDs@g-C3N4 dispersed in NMP at 473 K.

[0060] Figure 31 The following images are for reference: (a) afterglow photographs of R-CDs@g-C3N4 powder before and after turning off an 8 W, 302 nm UV lamp at 323 K; (b) phosphorescence spectra of R-CDs@g-C3N4 powder at different temperatures (delay time: 3 ms); (c) time-resolved phosphorescence decay curves of R-CDs@g-C3N4 powder at 323 K.

[0061] Figure 32 The BET specific surface area of ​​(a) B-CDs@g-C3N4, G-CDs@g-C3N4 and R-CDs@g-C3N4 of the present invention; and (b) the pore size of B-CDs@g-C3N4, G-CDs@g-C3N4 and R-CDs@g-C3N4.

[0062] Figure 33 The following images represent the following: (a) fluorescence and phosphorescence spectra of 1-CDs@g-C3N4 powder at 298 K; (b) time-resolved phosphorescence decay curve of 1-CDs@g-C3N4 powder at 298 K; (c) phosphorescence spectra of solid 1-CDs@g-C3N4 at different temperatures (delay time: 3 ms); (d) time-resolved phosphorescence decay curves of 1-CDs@g-C3N4 powder at different temperatures; and (e) afterglow photographs of 1-CDs@g-C3N4 powder at different temperatures before and after the 8W, 302 nm UV lamp is turned off.

[0063] Figure 34 shows (a) the phosphorescence spectra of 1-CDs@g-C3N4 in polar solvents at different temperatures (delay time: 3 ms); (b) the time-resolved phosphorescence decay curves of 1-CDs@g-C3N4 in polar solvents at different temperatures; and (c) the afterglow photographs of 1-CDs@g-C3N4 in polar solvents at different temperatures before and after turning off the 302 nm UV lamp (8 W).

[0064] Figure 35 This is a non-destructive testing image of microcracks after the 302 nm ultraviolet lamp is turned off according to the present invention;

[0065] Figure 36 These are photographs of microcracks at different temperatures before and after the 8 W, 302 nm UV lamp was turned off, according to the present invention.

[0066] Figure 37 These are photographs of the safety exit signs at different temperatures before and after the 8 W, 302 nm UV lamp was turned off, according to the present invention.

[0067] Figure 38 This is a comparative graph showing the stability evaluation of safety exit signs under five high-temperature afterglow cycles according to the present invention.

[0068] Figure 39 This is a schematic diagram of the material composition of the three-dimensional gradient fishtail anti-counterfeiting model of the present invention;

[0069] Figure 40 This is a schematic diagram of the material composition of the multi-dimensional information anti-counterfeiting pattern of the present invention. Detailed Implementation

[0070] The present invention will be described in detail below with reference to specific embodiments. The embodiments are only used to explain the present invention and are not intended to limit the scope of protection of the present invention.

[0071] Graphitic carbon nitride (g-C3N4) is a two-dimensional (2D) layered framework similar to graphene, and has been applied in photocatalysis, energy conversion, and environmental remediation. Unlike common polymer matrices, g-C3N4, with its unique triazine ring structure, exhibits high physicochemical and thermal stability. Nanoparticles can be firmly embedded in the g-C3N4 matrix to enhance its catalytic performance. Notably, matrix confinement has been successfully used to block the triplet exciton dissipation channels of carbon dots to achieve long-lived RTP. Therefore, embedding carbon dots into the g-C3N4 matrix in this application can significantly improve HTP performance for the following reasons:

[0072] (1) During the precursor co-pyrolysis process, the chromophore can be converted into carbon dots in situ in the nanoporous g-C3N4 matrix;

[0073] (2) Carbon dots themselves can serve as internal rigid light emitters to achieve high-efficiency RTP;

[0074] (3) The extended layered two-dimensional g-C3N4 sheets with abundant nitrogen-containing edges and defect sites can form a variety of interactions, synergistically anchor and rigidify carbon dots, act as an external rigid matrix to suppress non-radiative decay, and protect carbon dots from thermal quenching and solvent quenching.

[0075] To clarify this concept, this application describes the one-pot co-pyrolysis synthesis of multicolor HTP materials using chromophores (i.e., 3,5-dicarboxyphenylboronic acid (DCBA), triphenyl-2-ylboronic acid (TPB), and rhodamine B (RhB)) and urea precursors. Figure 1 a) Carbon dots are generated in situ during the high-temperature sintering process to form the g-C3N4 matrix. Due to the increased conjugation degree of the precursor, the phosphorescence emission wavelength can be effectively tuned from blue to red. The samples were named B-CDs@g-C3N4, G-CDs@g-C3N4, and R-CDs@g-C3N4, respectively. The carbon dots are anchored and confined in the g-C3N4 matrix through in-situ reaction, forming various interactions, such as robust BN covalent bonds and hydrogen bonds. N, B, and O heteroatoms promote intersystem crossing (ISC) to fill triplet excitons. The synergistic rigidification effect of internal rigidity (carbon dot engineering) and external rigidity (matrix confinement) significantly suppresses nonradiative transitions, contributing to the ultralong HTP. As a result, G-CDs@g-C3N4 exhibits an ultralong phosphorescence lifetime (11.92 s) and a high phosphorescence quantum yield (48.44%) at room temperature.

[0076] Notably, G-CDs@g-C3N4 exhibits excellent thermal stability, superior to previously reported HTP materials. Figure 1 b and Table 1);

[0077] Table 1. Previously reported high-temperature phosphorescent materials

[0078]

[0079] At 473 K, the lifetimes of G-CDs@g-C3N4 in solid state and N-methyl-2-pyrrolidone (NMP) solution reached 7.34 s and 4.91 s, respectively, with afterglow durations of 44 s and 30 s, respectively. Figure 1 (c, d) B-CDs@g-C3N4 and R-CDs@g-C3N4 also exhibited varying degrees of HTP properties. Interestingly, R-CDs@g-C3N4, lacking BN covalent bonds, exhibited thermochromic phosphorescence ranging from red to blue, due to the high sensitivity of low-energy triplet excitons to thermal deactivation. This application opens new avenues for designing advanced HTP materials and significantly expands the application range of phosphorescent materials under harsh high-temperature conditions.

[0080] 1 Experimental Methods

[0081] 1.1 Materials

[0082] 3,5-Dicarboxyphenylboronic acid (DCBA, 99% purity), triphenyl-2-boronic acid (TPB, 99% purity), rhodamine B (RhB, 99% purity), urea (99% purity), triphenylene (99% purity), and N-methyl-2-pyrrolidone (NMP, 99% purity) were all purchased from Adamas. Distilled water was used in this study. All reagents were of analytical grade and used directly without further purification.

[0083] 1.2 Synthesis methods of B-CDs@g-C3N4 and G-CDs@g-C3N4

[0084] In a 250 mL beaker, 3.0 g of urea and 25 mg of 3,5-dicarboxyphenylboronic acid (DCBA) were added to 20 mL of distilled water and stirred vigorously until a homogeneous mixture was formed. The beaker was then covered with aluminum foil and heated in a 200 °C oven for 3 hours. After the reaction was complete, the beaker was allowed to cool naturally to room temperature. The resulting solid was collected and ground into a fine powder to obtain B-CDs@g-C3N4. G-CDs@g-C3N4 was synthesized using a similar method, except that the 25 mg DCBA used in the synthesis of B-CDs@g-C3N4 was replaced with 15 mg of triphenyl-2-ylboronic acid (TPB).

[0085] 1.3 Synthesis method of R-CDs@g-C3N4

[0086] In a 250 mL beaker, 3.0 g of urea and 1 mL of 0.5 mmol / L Rhodamine B (RhB) solution were added to 20 mL of distilled water and stirred vigorously until a homogeneous mixture was formed. Subsequent synthesis steps were the same as those for B-CDs@g-C3N4.

[0087] 1.4 Characterization

[0088] The morphology and structural properties of the three CDs@g-C3N4 composite materials were characterized using high-resolution transmission electron microscopy (HR-TEM, Tecnai G2 F30, FEI, USA), scanning electron microscopy (SEM, JSM-5600LV), and X-ray diffraction (XRD, Bruker D8 Advance, Bruker GmbH, Germany). The chemical composition and surface functional groups were analyzed using X-ray photoelectron spectroscopy (XPS, K-Alpha, Thermo Fisher Scientific, USA) and Fourier transform infrared spectroscopy (FT-IR, PerkinElmer, USA). The ultraviolet-visible absorption spectra were measured using a Shimadzu UV-3600 spectrophotometer to characterize their optical properties. The phosphorescence quantum yield (Phos. QY) was measured using an Edinburgh FLS1000 fluorescence spectrometer. Phosphorescence spectra and decay curves at room temperature were recorded using a Hitachi F-7000 spectrophotometer (Japan), while temperature dependence measurements were performed using an Edinburgh FLS1000 spectrometer (UK). Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were performed on a Netzsch STA449 F3 instrument (Germany), with a test temperature range from room temperature to 800℃, a heating rate of 20℃ / min, and a nitrogen atmosphere. For porosity analysis, after the samples were degassed under vacuum at 120℃ for 8 hours, nitrogen adsorption-desorption isotherms were determined at 77 K using a McMurray-Tictal ASAP 2020 instrument (USA). Subsequently, the specific surface area and pore size distribution were determined using the Brunauer-Emmett-Teller (BET) method and related calculations.

[0089] 1.5 Theoretical Calculation

[0090] Density functional theory (DFT) calculations were performed using Materials Studio 7.0 software. First, the molecular structure was initially optimized using the COMPASS force field in an implicit aqueous solvent environment, followed by final optimization at the GGA / BLYP / DND theoretical level.

[0091] 2 Results and Discussion

[0092] 2.1 Synthesis and Characterization of CDs@g-C3N4 Composite Material

[0093] CDs@g-C3N4 composites were prepared in situ via co-pyrolysis of urea and chromophore precursors. As shown in transmission electron microscopy (TEM) images, the carbon dots in all three composites exhibited a uniform spherical morphology with average diameters of 2.61 nm, 1.75 nm, and 2.18 nm, respectively. Figure 2 The lattice spacings of the B-CDs@g-C3N4, G-CDs@g-C3N4, and R-CDs@g-C3N4 composites are 0.207 nm, 0.209 nm, and 0.248 nm, respectively (e.g., ac). Figure 8 This is attributed to the crystal planes of graphitic carbon. Powder X-ray diffraction (XRD) patterns of the three CDs@g-C3N4 composites show diffraction peaks near 12.1° and 27.9°. Figure 2 d) These peaks are attributed to the (100) and (002) crystal planes of g-C3N4, respectively. Furthermore, peaks at 22.3° and 24.7° were observed, which can be attributed to residual urea and the (002) crystal plane of graphite, respectively. These results confirm that the CDs@g-C3N4 composite material possesses a highly crystalline, rigid structure.

[0094] In Fourier transform infrared (FT-IR) spectroscopy ( Figure 2 In e), the three CDs@g-C3N4 composite materials were in the range of 3100-3500 cm⁻¹. -1 and 762 cm -1 The absorption peaks at 1676 cm⁻¹ are attributed to the stretching vibrations of the OH / NH group and the triazine ring, respectively. -1 1607 cm -1 1464 cm -1 and 1060 cm -1 The peaks at 700 cm⁻¹ are attributed to the stretching vibrations of C=O, C=N, CN, and C-OH, respectively. The Raman spectra of the three CDs@g-C3N4 composites at 700 cm⁻¹ are... -1 A peak is shown at this point, corresponding to the stretching vibration of the triazine ring (e.g. Figure 9 ).

[0095] FT-IR and Raman spectroscopy further confirmed the XRD results, confirming the formation of the CDs@g-C3N4 structure. Furthermore, the study found that the synthesized carbon dots are rich in carboxyl (COOH), hydroxyl (OH), and amino (NH2) groups. These groups can form hydrogen bonds with the g-C3N4 matrix, constructing a rigid microenvironment, isolating dissolved oxygen, restricting molecular motion, suppressing non-radiative transition pathways, and stabilizing triplet excitons. X-ray photoelectron spectroscopy (XPS) analysis provides further insight into the chemical composition and functional groups of the three CDs@g-C3N4 composite materials. Figure 2 As shown in f, the elemental composition of B-CDs@g-C3N4 and G-CDs@g-C3N4 is mainly carbon (C), nitrogen (N), oxygen (O), and boron (B). The contents of B-CDs@g-C3N4 are 45.84%, 29.83%, 23.20%, and 1.13%, respectively, while the contents of G-CDs@g-C3N4 are 41.72%, 32.88%, 24.46%, and 0.94%, respectively. In contrast, R-CDs@g-C3N4 is mainly composed of carbon (36.72%), nitrogen (37.03%), oxygen (26.10%), and a small amount of chlorine (0.15%) (as shown in Table 2).

[0096] Table 2. Elemental composition of B-CDs@g-C3N4, G-CDs@g-C3N4 and R-CDs@g-C3N4

[0097]

[0098] High-resolution C1s spectra of three CDs@g-C3N4 composites confirmed the presence of CC / C=C (284.3 eV), CN (285.4 eV), C=O (288.8 eV), and NC=N (289.5 eV) bonds. Figure 2 g). It is worth noting that the spectra of B-CDs@g-C3N4 and G-CDs@g-C3N4 also show the presence of CB (283.5 eV) bonds, while the R-CDs@g-C3N4 does not have this bond (as shown in Table 3).

[0099] Table 3. C1s XPS spectral analysis of B-CDs@g-C3N4, G-CDs@g-C3N4 and R-CDs@g-C3N4

[0100]

[0101] After peak separation of the N 1s spectra of all three CDs@g-C3N4 composites, three distinct peaks were obtained. Figure 2h), corresponding to =CN (398.7 eV), CN=C (399.4 eV) and N-(C)3 (400.2 eV) bonds respectively, confirming the presence of the triazine ring (as shown in Table 4).

[0102] Table 4. B-CDs@g-C3N4, G-CDs@g-C3N4 and R-CDs@g-C3N4 N 1s XPS spectral analysis

[0103] The O 1s spectrum shows two peaks, attributed to the C=O (531.3 eV) and CO (532.3 eV) bonds. Figure 2 i and Table 5).

[0104] Table 5 O 1s XPS spectral analysis of B-CDs@g-C3N4, G-CDs@g-C3N4 and R-CDs@g-C3N4

[0105]

[0106] Furthermore, the high-resolution B 1s spectra of B-CDs@g-C3N4 and G-CDs@g-C3N4 show a characteristic peak at 190.5 eV, corresponding to the BN bond (e.g., ...). Figure 10 As shown in the figure, this indicates that BN covalent bonds are formed between the carbon dots and the g-C3N4 matrix. The above data suggests that the carbon dots are in situ embedded into the g-C3N4 matrix through various interactions.

[0107] 2.2 Photophysical properties of CDs@g-C3N4 composite material in solid state

[0108] The phosphorescence properties of the prepared CDs@g-C3N4 composites were investigated in detail. After the 302 nm UV lamp was turned off, the three composites emitted bright blue, green, and red phosphorescence visible to the naked eye, which lasted for several seconds or even more than 1 minute. Figure 3 a). Subsequently, the effect of chromophore doping concentration on the room-temperature phosphorescence (RTP) properties of CDs@g-C3N4 composites was investigated (e.g., Figure 11-13 Considering both phosphorescence intensity and lifetime, the optimal doping concentrations for B-CDs@g-C3N4, G-CDs@g-C3N4, and R-CDs@g-C3N4 were determined to be 0.83 wt.%, 0.50 wt.%, and 0.008 wt.%, respectively. The optimized phosphorescence quantum yields (Phos. QY) for B-CDs@g-C3N4, G-CDs@g-C3N4, and R-CDs@g-C3N4 were 14.74%, 48.44%, and 10.44%, respectively (e.g., ...). Figure 14 As shown). At their respective optimal fluorescence excitation wavelengths, the maximum fluorescence emission peaks of the B-CDs@g-C3N4, G-CDs@g-C3N4, and R-CDs@g-C3N4 composites are located at 429 nm, 379 nm, and 591 nm, respectively. Figure 3 The maximum emission peaks of B-CDs@g-C3N4 and G-CDs@g-C3N4 at the optimal phosphorescent excitation wavelength are located at 428 nm and 483 nm, respectively, with corresponding RTP lifetimes of 1.09 s and 11.92 s, respectively. Figure 3 e, f). Among them, the afterglow duration of the G-CDs@g-C3N4 composite material is particularly outstanding, lasting for more than 80 s ( Figure 3 a). Notably, R-CDs@g-C3N4 exhibits unique dual-emission afterglow characteristics ( Figure 3 d). Under 282 nm excitation, its phosphorescence spectrum exhibits double peaks at 448 nm and 593 nm, with lifetimes of 2.33 s and 1.33 s, respectively. Figure 3 g). Temperature-dependent phosphorescence spectroscopy showed a significant increase in phosphorescence intensity as the temperature decreased from 277 K to 77 K, ruling out the presence of thermally activated delayed fluorescence (TADF) (e.g.). Figure 15 (As shown). Since reverse intersystem crossing (RISC) cannot occur without thermal assistance at extremely low temperatures, phosphorescence emission becomes the only radiative transition pathway for triplet excitons in CDs@g-C3N4 composite materials.

[0109] The luminescence origins of the three CDs@g-C3N4 composite materials were clarified through ultraviolet-visible absorption spectroscopy and phosphorescence excitation spectroscopy analysis. Figure 16 As shown, the absorption peak of B-CDs@g-C3N4 at 284 nm overlaps with its phosphorescence excitation peak at 279 nm, indicating that its phosphorescence source originates from the π-π* transition of the C=C bond. G-CDs@g-C3N4 also exhibits a similar spectral overlap, confirming that the π-π* transition dominates its phosphorescence behavior. In contrast, R-CDs@g-C3N4 exhibits two excitation peaks at 282 nm and 472 nm, corresponding to the absorption bands in its UV-Vis absorption spectrum, indicating that the phosphorescence source originates from multiple emission centers associated with π-π* and n-π* (C=O / C=N) transitions. Therefore, R-CDs@g-C3N4 can be excited by both UV and visible light simultaneously. After stopping the 302 nm UV irradiation, R-CDs@g-C3N4 emits a visible red afterglow lasting for 18 s. Figure 3 a); and after the 450 nm excitation was stopped, a red afterglow lasting for 8 s could still be observed (the two emission peaks are located at 503 nm and 605 nm, respectively) (e.g. Figure 17 (As shown).

[0110] Fourier transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS) data showed that the three CDs@g-C3N4 composites were rich in carbonyl and nitrogen-containing functional groups, which promoted the electronic transition from the lowest singlet state (S1) to the lowest triplet state (T1), thereby improving the intersystem crossing (ISC) efficiency and enhancing room temperature phosphorescence (RTP) performance. Both B-CDs@g-C3N4 and G-CDs@g-C3N4 composites contained BN covalent bonds, indicating that the g-C3N4 matrix could anchor carbon dots (CDs) (such as...) through covalent and hydrogen bonds. Figure 18 (As shown). Furthermore, the empty p orbitals of boron atoms can interact with the lone pairs of electrons from donor heteroatoms such as nitrogen and oxygen. This interaction is crucial for enhancing RTP because it significantly increases spin-orbit coupling (SOC), thereby promoting efficient ISC and improving phosphorescence efficiency and lifetime.

[0111] To elucidate the phosphorescence mechanism of R-CDs@g-C3N4, a control sample U-CDs@g-C3N4 (e.g., Rhodamine B (RhB)-free) was prepared under the same conditions. Figure 19 As shown). Partial spectral overlap was observed between the phosphorescence emission of U-CDs@g-C3N4 and the absorption of RhB (e.g., ...). Figure 20 As shown in a), this indicates that there may be a Foster resonance energy transfer (FRET) between the two (as shown in a diagram). Figure 21 The solid-state phosphorescence spectrum of R-CDs@g-C3N4 exhibits dual emission peaks at 448 nm and 593 nm, which are attributed to the energy donor (U-CDs@g-C3N4) and the energy acceptor (RhB), respectively. Figure 20 b). With increasing RhB content, the intensity of the 593 nm peak increases and a redshift occurs. This phenomenon is attributed to the aggregation-induced redshift effect of RhB at high concentrations (e.g., Figure 20 c). Interestingly, R-CDs@g-C3N4 also emits red phosphorescence under visible light excitation at 450 nm. Given that U-CDs@g-C3N4 can only be effectively excited at 310 nm (e.g., ... Figure 22The red phosphorescence under 450 nm excitation cannot be explained by the FRET mechanism. Therefore, based on the experimental results, the red afterglow of R-CDs@g-C3N4 may originate from two independent luminescent centers: the red afterglow under 282 nm excitation is generated by the FRET pathway between the blue phosphorescence of U-CDs@g-C3N4 and the red fluorescence of RhB; while the afterglow emission under 472 nm excitation may originate from the phosphorescence of red carbon dots (R-CDs) formed in situ during the co-pyrolysis of urea and RhB. The in-situ generated R-CDs are fixed in the rigid g-C3N4 matrix, restricting intramolecular movement and protecting triplet excitons, thereby promoting red phosphorescence emission. B-CDs@g-C3N4, G-CDs@g-C3N4, and R-CDs@g-C3N4 (λ ex The CIE color coordinates of the afterglow at 472 nm are (0.150, 0.057), (0.122, 0.197), and (0.523, 0.430), respectively, consistent with the observed blue, green, and red afterglow (e.g., ...). Figure 23 ).

[0112] Density functional theory (DFT) calculations were then performed to elucidate the relationship between precursor conjugation and phosphorescence wavelength. The band gaps between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of DCBA, TPB, and RhB were 3.97 eV, 3.34 eV, and 1.13 eV, respectively. Figure 3 h). Generally, increased molecular conjugation leads to more significant electron delocalization within the conjugated system, thereby narrowing the band gap between HOMO and LUMO. The optimal phosphorescent emission optical band gaps for B-CDs@g-C3N4, G-CDs@g-C3N4, and R-CDs@g-C3N4 are 2.90 eV, 2.57 eV, and 2.09 eV, respectively (e.g., h). Figure 24 This trend is consistent with the change in the precursor band gap. The above results indicate that the RTP emission wavelength can be finely tuned by adjusting the degree of conjugation of the precursor.

[0113] 2.3 Phosphorescence properties of CDs@g-C3N4 composite material in aqueous solution

[0114] It is worth noting that the three CDs@g-C3N4 composite materials exhibit excellent afterglow luminescence in aqueous solution without any deoxygenation treatment. Figure 4a) B-CDs@g-C3N4 and G-CDs@g-C3N4 emit blue and green afterglows respectively in aqueous solution, similar to their luminescence in the solid state. The afterglow duration of G-CDs@g-C3N4 dispersed in water is particularly prominent, lasting for more than 70 seconds. In contrast, R-CDs@g-C3N4, which exhibits a red afterglow in the solid state, turns to a blue afterglow in aqueous solution, displaying a water-induced color-changing phosphorescence effect. Figure 4 As shown in b and c, aqueous dispersions of B-CDs@g-C3N4 and G-CDs@g-C3N4 exhibit fluorescence emission peaks at 410 nm and 378 nm, and phosphorescence emission peaks at 435 nm and 482 nm, similar to those in the solid state. However, the phosphorescence emission peaks of R-CDs@g-C3N4 in aqueous solution differ significantly from those in the solid state. Figure 4 d). When dispersed in water, its red phosphorescence is quenched and replaced by blue phosphorescence.

[0115] To elucidate this interesting phenomenon, the structural characteristics of the three CDs@g-C3N4 composites were compared and analyzed. Unlike B-CDs@g-C3N4 and G-CDs@g-C3N4, R-CDs@g-C3N4 is mainly confined within a rigid g-C3N4 matrix by hydrogen bonds, lacking stable BN covalent bonds. Figure 10 This structural defect leads to relatively poor stability. The low-energy triplet exciton, lacking strong BN covalent confinement, is highly susceptible to water influence. Furthermore, undecomposed rhodamine B (RhB) molecules gradually escape from the g-C3N4 matrix and diffuse into the solution, causing the distance between the donor (U-CDs@g-C3N4) and acceptor (RhB) to exceed the effective fluorescence resonance energy transfer (FRET) range (>10 nm), thus disrupting the resonance energy transfer pathway. This conclusion is supported by the fact that the phosphorescence spectrum and lifetime of R-CDs@g-C3N4 in water are extremely similar to those of pure U-CDs@g-C3N4. Figure 25 This confirmed that the blue afterglow emission observed in R-CDs@g-C3N4 originated from U-CDs@g-C3N4. Notably, after air-drying the aqueous dispersion of R-CDs@g-C3N4, the red afterglow could not be recovered, leaving only the blue afterglow emission. This indicates that even after solvent removal, the donor-acceptor distance remained greater than 10 nm, making it impossible to re-establish FRET. However, the red afterglow could be completely recovered through simple heat treatment: a small amount of water was added to the dried R-CDs@g-C3N4, and the mixture was heated at 200 °C for 3 hours. Figure 26This process induces RhB molecules to diffuse back into the g-C3N4 matrix. Subsequently, the donor and acceptor are brought close enough to reconstruct an effective FRET, restoring the red afterglow. Therefore, utilizing the distance-sensitive FRET process of R-CDs@g-C3N4 holds promise for developing stimulus-responsive RTP materials.

[0116] All three CDs@g-C3N4 composite materials exhibited ultra-long phosphorescence lifetimes in aqueous environments. Figure 4 (e.g.) The aqueous dispersion of G-CDs@g-C3N4 exhibits a phosphorescence lifetime of up to 9.06 s at room temperature. The high stability of the liquid phosphorescence can be attributed to the strong confinement effect of the g-C3N4 matrix, which also provides hydrogen bonds and / or BN covalent bonds, collectively constructing a stable and rigid microenvironment that effectively isolates dissolved oxygen and restricts molecular motion, thereby protecting triplet excitons. The three CDs@g-C3N4 composites also exhibit excellent photostability and storage stability. After continuous irradiation with a 254 nm UV lamp for 1 hour, their fluorescence and phosphorescence intensities remained almost unchanged. Figure 27 After one month of storage in air, the phosphorescence intensities of B-CDs@g-C3N4, G-CDs@g-C3N4, and R-CDs@g-C3N4 retained 83.1%, 85.7%, and 79.2% of their initial values, respectively, demonstrating good long-term stability. Figure 28 ).

[0117] 2.4 High-temperature phosphorescence properties of CDs@g-C3N4 composite material

[0118] Both B-CDs@g-C3N4 and G-CDs@g-C3N4 exhibited excellent high-temperature phosphorescence (HTP) performance in both solid-state and polar solvent dispersion systems. However, as the temperature increased from 373 K to 473 K, the phosphorescence intensity and lifetime of both B-CDs@g-C3N4 and G-CDs@g-C3N4 powders gradually decreased. Figure 5 ac and Figure 29 At 373 K, the solid-state phosphorescence lifetimes of B-CDs@g-C3N4 and G-CDs@g-C3N4 reached 1.06 s and 10.23 s, respectively, retaining 97% and 86% of their room-temperature phosphorescence (RTP) lifetimes. Figure 5 c). When the temperature was further increased to 473 K, its lifetime decreased to 41.68 ms and 7.34 s, respectively. Impressively, even at the extreme high temperature of 473 K, the blue phosphorescence of the B-CDs@g-C3N4 powder was still observable to the naked eye for about 0.5 s ( Figure 29 a) The green phosphorescence of G-CDs@g-C3N4 powder can last for about 44 s ( Figure 1c). The HTP properties of G-CDs@g-C3N4 far exceed those of previously reported HTP materials. Figure 1 (b and Table 1).

[0119] In high-temperature polar solvents, this composite material also exhibits excellent phosphorescence properties. When the temperature increases from 373 K to 473 K, the phosphorescence intensity and lifetime of B-CDs@g-C3N4 and G-CDs@g-C3N4 dispersed in the polar solvent both decrease. Figure 5 df and Figure 30 At 373 K, the blue phosphorescence of B-CDs@g-C3N4 dispersed in water is visible to the naked eye for approximately 3.5 s ( ). Figure 30 a) The lifetime is 0.98 s, retaining 85% of its room temperature phosphorescence lifetime; while G-CDs@g-C3N4 exhibits a significantly longer green phosphorescence duration in water, approximately 41 s ( Figure 1 d) The lifetime is 8.61 s, retaining 95% of its room-temperature phosphorescence lifetime. Even at 473 K, the green phosphorescence of G-CDs@g-C3N4 dispersed in N-methyl-2-pyrrolidone (NMP) remains visible to the naked eye for approximately 30 s, with a lifetime of 4.91 s. To our knowledge, achieving HTP in the liquid phase remains a significant challenge due to the presence of dissolved oxygen and solvent relaxation effects. The CDs@g-C3N4 composite material in this work exhibits excellent solid-state and liquid-state HTP properties, highlighting the significant advantages of the synergistic effect of the rigid internal structure of the carbon dots and the external rigid confinement of the g-C3N4 matrix.

[0120] As for R-CDs@g-C3N4, due to its lack of BN covalent bonds, its stability is relatively poor, and its high-temperature stability is inferior to that of B-CDs@g-C3N4 and G-CDs@g-C3N4. When heated to 323 K, its red afterglow turns blue, visible to the naked eye for about 4 seconds, exhibiting a thermochromic phosphorescence effect. Figure 31 a). Analysis of its phosphorescence spectra at different temperatures revealed that the emission peak intensities at 448 nm and 593 nm both decreased with increasing temperature, with the quenching of the 593 nm peak being more pronounced; the red afterglow peak almost disappeared at 348 K. Figure 31 b). At 323 K, its lifetime decreases from 2.33 s to 1.15 s (λ). em = 448 nm), shortened from 1.33 s to 0.43 s (λ em = 593 nm) ( Figure 31 c). As mentioned earlier, due to the lack of stable BN covalent bonds in R-CDs@g-C3N4, its triplet excitons are highly susceptible to thermal deactivation.

[0121] 2.5 High-temperature phosphorescence mechanism of CDs@g-C3N4 composite material

[0122] The remarkable high-temperature phosphorescence (HTP) properties of the CDs@g-C3N4 composite material prompted us to further investigate the intrinsic mechanism of the synergistic stiffening effect between carbon dots (CDs) and g-C3N4. Scanning electron microscopy (SEM) revealed the unique morphological characteristics of the CDs@g-C3N4 composite material. Figure 6 (ac). Among them, B-CDs@g-C3N4 exhibited a loose and porous layered structure, while G-CDs@g-C3N4 and R-CDs@g-C3N4 formed a dense or smooth layered structure, indicating that the precursor has a key influence on the crystallinity and formation process of the rigid matrix. BET analysis ( Figure 32 This further confirms the viewpoint, as the results show that G-CDs@g-C3N4 has the smallest specific surface area, consistent with its dense structure. The dense and compact structure of G-CDs@g-C3N4 plays an important role in its HTP performance in solid state and polar solvents, because this structure effectively protects triplet excitons from high-temperature quenching through strong shielding and confinement effects.

[0123] Thermogravimetric analysis (TGA) further confirmed the excellent thermal stability of G-CDs@g-C3N4. Figure 6 The g-CDs@g-C3N4 composite exhibited only a 5% mass loss at 246°C, significantly higher than B-CDs@g-C3N4 (185°C) and R-CDs@g-C3N4 (181°C). The initial mass loss was attributed to incomplete pyrolysis of urea in the composite. An endothermic peak at approximately 70°C in the differential scanning calorimetry (DSC) curve corresponded to the breaking of hydrogen bonds, confirming extensive hydrogen bonding between the g-C3N4 matrix and the carbon dots (CDs). Therefore, excellent thermal stability is equally crucial to the superior HTP performance of g-CDs@g-C3N4.

[0124] To investigate the influence of BN covalent bonds, 1-CDs@g-C3N4 was synthesized using triphenylene instead of triphenyl-2-boronic acid (TPB) following the same procedure. Its HTP performance in solid state and polar solvents was also studied (e.g., Figures 33-34 As shown). Direct comparison revealed that, under the same conditions, the phosphorescence intensity of 1-CDs@g-C3N4 was only 16.8% of that of G-CDs@g-C3N4 (as shown). Figure 6 g).

[0125] Correspondingly, at all test temperatures, the phosphorescence lifetime of 1-CDs@g-C3N4 in both solid state and polar solvents was significantly shorter than that of G-CDs@g-C3N4. Figure 6(h, i). The above results indicate that the BN covalent bonds in G-CDs@g-C3N4 effectively restrict molecular motion at high temperatures. However, although the HTP performance of 1-CDs@g-C3N4 is inferior to that of G-CDs@g-C3N4, it still exhibits excellent HTP: in the solid state, the lifetime is 9.71 s at 298 K and 2.39 s at 473 K; in polar solvents, the lifetime is 8.16 s at 298 K and 2.39 s at 473 K. The planar rigid conjugated structure of the two precursors (TPB and triphenylene) endows the CDs@g-C3N4 composite with internal rigidity. This internal rigid structure effectively suppresses molecular vibration and rotation, thereby increasing the radiative rate constant while suppressing the non-radiative rate constant. In summary, the superior HTP performance of G-CDs@g-C3N4 stems from a synergistic mechanism: the internal rigidity effect of TPB-derived carbon dots, and the external rigidity effect provided by the g-C3N4 matrix through strong confinement, covalent bonds, and hydrogen bond networks. Figure 1 a).

[0126] 2.6 Application of CDs@g-C3N4 composite materials

[0127] B-CDs@g-C3N4, G-CDs@g-C3N4, and R-CDs@g-C3N4 composite materials possess a variety of advantageous properties, including high thermal stability, multi-color phosphorescence, visible light-excited red phosphorescence, multi-stimulus responsive color-changing phosphorescence, and ultra-long afterglow lifetime. These properties make them suitable for in-situ non-destructive testing of microcracks in high-temperature components, high-temperature emergency marking, multi-dimensional information encryption, and temperature monitoring during pharmaceutical transportation or storage. As a proof of concept, an experiment was conducted using simulated cracks on an evaporating dish to replace microcracks in aircraft engine turbine blades: G-CDs@g-C3N4 aqueous dispersion was sprayed onto the surface of the evaporating dish and excess liquid was removed. After turning off the 302 nm UV lamp, a bright green afterglow at room temperature made the microcracks clearly visible and remained visible for approximately 60 seconds. Figure 7 a and Figures 35-36 This in-situ nondestructive testing method can detect microcracks in a temperature range of 298 K to 473 K, and even at a high temperature of 473 K, a crack signal lasting for 39 s can still be observed. Thanks to the excellent HTP properties of G-CDs@g-C3N4, this method has broad application prospects in real-time in-situ nondestructive testing of microcracks in high-temperature components. Figure 7 As shown in b, safety exit signs were prepared by dispersing G-CDs@g-C3N4 in silicone encapsulation material. After the 302 nm UV lamp was turned off, the signs emitted visible green phosphorescence at room temperature for about 57 s. Impressively, an afterglow of about 18 s could still be observed at a high temperature of 473 K. Figure 37Furthermore, the sign retains its phosphorescent properties even after repeated thermal cycling and exposure to environmental conditions, making it ideal for emergency scenarios such as dense smoke or fire. Figure 38 ).

[0128] To construct a three-dimensional anti-counterfeiting model, B-CDs@g-C3N4 and G-CDs@g-C3N4 were dispersed in epoxy resin and cast into different parts of the fish tail mold. Figure 39 After turning off the 302 nm UV lamp, the model exhibited a blue-green gradient fishtail effect: the blue phosphorescence disappeared after 11 seconds, leaving only the green fishtail visible. Figure 7 c). In addition, multi-dimensional dynamic anti-counterfeiting patterns based on excitation wavelength, color, and afterglow time were developed: B-CDs@g-C3N4, G-CDs@g-C3N4, and R-CDs@g-C3N4 were embedded into the beach / seagull, coconut tree, and sun areas of the mold, respectively. Figure 40 Under 254 nm ultraviolet light excitation, the beach / seagulls and coconut trees emitted blue and green phosphorescence, respectively; after 13 seconds, only the green phosphorescence of the coconut trees was observed. However, under 450 nm visible light excitation, only the sun emitted red phosphorescence. Figure 7 d). A four-leaf clover pattern was prepared using the thermochromic phosphorescence effect of R-CDs@g-C3N4. Figure 7 e) When the 302 nm UV lamp is turned off at room temperature, the pattern emits red phosphorescence. When the temperature exceeds 50°C, the red phosphorescence turns into blue phosphorescence or is quenched. Based on this principle, a conceptual temperature monitoring label for drug transportation or storage is proposed: if the storage temperature is below room temperature, the label emits red phosphorescence; if the temperature exceeds 50°C, the label turns into blue phosphorescence or is quenched, indicating that the drug may have deteriorated.

[0129] 3 Results and Discussion

[0130] In summary, this application successfully prepared multicolor high-temperature phosphorescence (HTP) materials by in-situ embedding carbon dots (CDs) into a rigid graphitic carbon nitride (g-C3N4) matrix. The high-temperature phosphorescence performance of the CDs@g-C3N4 composite material originates from a synergistic multiple rigidification effect, which is driven by two key factors: first, the intrinsic rigidity of the CDs due to the rigidity of the precursor molecules and the planar conjugated structure; second, the external rigidity provided by the g-C3N4 matrix, which has strong interactions with the CDs, including a tight confinement effect and a hydrogen bond / BN covalent network. These effects effectively restrict the thermal motion of the luminescent centers and suppress non-radiative transition channels, thereby activating the high-temperature phosphorescence performance. Notably, due to the lack of BN covalent bonds, relatively low structural rigidity, and the variation in donor-acceptor distance in the fluorescence resonance energy transfer (FRET) system, R-CDs@g-C3N4 exhibits multi-stimulus responsive color-changing phosphorescence properties. Based on its advantages such as high thermal stability, multicolor phosphorescence, visible light-excited red phosphorescence, multi-stimulus responsive color-changing phosphorescence, and ultra-long afterglow lifetime, CDs@g-C3N4 composite materials have shown broad application prospects in several cutting-edge fields, including real-time in-situ non-destructive microcrack detection of high-temperature components, high-temperature emergency marking, multi-dimensional information encryption, and temperature monitoring during pharmaceutical transportation or storage. This application can become a key milestone in the field of high-temperature phosphorescence research, promoting the application and development of organic phosphorescent materials in harsh high-temperature environments.

[0131] The specific embodiments of the present invention have been described in detail above with reference to the accompanying drawings. However, the present invention is not limited to the above embodiments. Within the scope of knowledge possessed by those skilled in the art, various changes can be made without departing from the spirit of the present invention.

[0132] References:

[0133] 1.Chen, K. et al. Twofold rigidity activates ultralong organic high-temperature phosphorescence. Nat. Commun. 15, 1269 (2024).

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Claims

1. A high-temperature resistant long-afterglow carbon dot@g-C3N4 composite material, characterized in that, It includes carbon dots (CDs) and a g-C3N4 matrix. The carbon dots are formed by in-situ co-pyrolysis of urea and chromophore precursor and embedded in the g-C3N4 matrix. A hydrogen bond network is formed between the carbon dots and the g-C3N4 matrix. When the carbon dots are B-CDs or G-CDs, BN covalent bonds are also formed between the carbon dots and the g-C3N4 matrix.

2. The high-temperature resistant long-afterglow carbon dot @g-C3N4 composite material according to claim 1, characterized in that, The chromophore precursor is one of 3,5-dicarboxyphenylboronic acid (DCBA), triphenyl-2-boronic acid (TPB), or rhodamine B (RhB); When the precursor is DCBA, B-CDs@g-C3N4 is formed; when the precursor is TPB, G-CDs@g-C3N4 is formed; when the precursor is RhB, R-CDs@g-C3N4 is formed.

3. The high-temperature resistant long-afterglow carbon dot @g-C3N4 composite material according to claim 2, characterized in that, The phosphorescence quantum yield of the B-CDs@g-C3N4 is 14.74%, the solid-state phosphorescence lifetime is 1.09 s at 298 K, the solid-state phosphorescence lifetime is 41.68 ms at 473 K, and the afterglow duration is 0.5 s.

4. The high-temperature resistant long-afterglow carbon dot @g-C3N4 composite material according to claim 2, characterized in that, The phosphorescence quantum yield of the G-CDs@g-C3N4 is 48.44%, with a solid-state phosphorescence lifetime of 11.92 s at 298 K, a solid-state phosphorescence lifetime of 7.34 s and an afterglow duration of 44 s at 473 K, and a phosphorescence lifetime of 4.91 s and an afterglow duration of 30 s in N-methyl-2-pyrrolidone (NMP) solvent at 473 K.

5. The high-temperature resistant long-afterglow carbon dot @g-C3N4 composite material according to claim 2, characterized in that, The phosphorescence quantum yield of the R-CDs@g-C3N4 is 10.44%. At 298 K, the solid phosphorescence exhibits a double peak emission at 448 nm and 593 nm, with lifetimes of 2.33 s and 1.33 s, respectively. At 323 K, its phosphorescence emission changes from red to blue, exhibiting thermochromic phosphorescence.

6. The high-temperature resistant long afterglow carbon dot @g-C3N4 composite material according to claim 2, characterized in that, The elemental composition of each composite material, expressed as a mass fraction, is as follows: B-CDs@g-C3N4: C 45.84%, N 29.83%, O 23.20%, B 1.13%; G-CDs@g-C3N4: C 41.72%, N 32.88%, O 24.46%, B 0.94%; R-CDs@g-C3N4: C 36.72%, N 37.03%, O 26.10%, Cl 0.15%.

7. The high-temperature resistant long-afterglow carbon dot @g-C3N4 composite material according to claim 2, characterized in that, The B-CDs have an average particle size of 2.61 nm and a lattice spacing of 0.207 nm; the G-CDs have an average particle size of 1.75 nm and a lattice spacing of 0.209 nm; and the R-CDs have an average particle size of 2.18 nm and a lattice spacing of 0.248 nm.

8. A method for preparing the high-temperature resistant long-afterglow carbon dot@g-C3N4 composite material according to any one of claims 1-7, characterized in that, Includes the following steps: (1) Add 3.0 g of urea and carbon dot precursor to 20 mL of distilled water and stir to form a homogeneous mixture; (2) The mixture was heated at 200 °C for 3 hours. (3) After the reaction is complete, the product is cooled, collected and ground to obtain the carbon dot@g-C3N4 composite material; Wherein, when the carbon dot precursor is DCBA, its dosage is 25 mg; when the precursor is TPB, its dosage is 15 mg; when the precursor is RhB, its dosage is 1 mL of RhB aqueous solution with a concentration of 0.5 mmol / L.

9. The application of the high-temperature resistant long-afterglow carbon dot @g-C3N4 composite material according to any one of claims 1-7, characterized in that, It can be used for in-situ non-destructive microcrack detection at temperatures from 298 K to 473 K, preparation of high-temperature safety markings, realization of multi-dimensional information encryption, or for temperature monitoring.

10. The application according to claim 9, characterized in that, The in-situ non-destructive microcrack detection involves spraying an aqueous dispersion of G-CDs@g-C3N4 onto the surface of the component under test. After turning off the 302 nm ultraviolet light source, microcracks are identified by the afterglow generated. The afterglow duration at 473 K is no less than 39 s. The high-temperature safety mark is made by dispersing G-CDs@g-C3N4 in the encapsulation material. Its afterglow duration at 298 K, 373 K, 423 K, and 473 K is 57 s, 43 s, 29 s, and 18 s, respectively. The multidimensional information encryption involves placing B-CDs@g-C3N4, G-CDs@g-C3N4, and R-CDs@g-C3N4 in different areas of the anti-counterfeiting pattern. Under 254 nm ultraviolet light excitation, they exhibit blue and green phosphorescence, with only green phosphorescence remaining after more than ten seconds. At 450 K, the afterglow duration is... It exhibits only red phosphorescence under nm visible light excitation; the temperature monitoring is based on the thermochromic phosphorescence effect of R-CDs@g-C3N4, which emits red phosphorescence when the temperature is not higher than room temperature, and turns into blue phosphorescence or quenches when the temperature is higher than 50 °C.

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