The room-temperature long-lived light-emitting materials are widely used in a new generation of photovoltaic devices, optical anti-counterfeiting, chemical/biological sensing, time-resolved imaging, and the like due to the unique light-emitting process. However, room-temperature long-lived luminescent materials (mainly organic small molecules, transition metal complexes, and rare earth-based long-lasting phosphors) that have been developed in the past few decades generally have complicated preparation and purification processes, require expensive raw materials, and have potential biological toxicity or Harsh long life creates conditions and other disadvantages. Therefore, the development of materials that are simple to prepare, cost-effective, low-toxic, and have long-life emission under conventional environmental conditions is an urgent problem that needs to be solved in the research field.
Carbon-based nanoluminescent materials (carbon dots) are a new type of luminescent materials that have been developed in recent years because of their simple preparation and purification processes, stable photophysical and chemical properties, adjustable emission characteristics, ease of functional modification, water solubility, and biocompatibility. Good advantages, etc., have attracted extensive attention from researchers since its discovery in 2004, and have shown great application prospects in many fields such as chemical/biosensing, bioimaging, medical diagnosis, photocatalysis, and optoelectronic devices. However, researchers in recent years have mainly focused on the regulation and preparation of fluorescent properties, the mechanism of luminescence, and potential applications of such materials, and the study of their long-life luminescence properties has been limited.
In order to further expand the application of carbon dots and solve the problems in the current research field of long-lived luminescence materials at room temperature, since 2015, the long-life of the carbon temperature around the carbon point of the Ph.D. student Kai Kai of Lin Hengwei, the Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences Launch control and application carried out a series of work.
Previous research of the research group showed that carbon dots prepared based on phenylenediamine as a carbon source have fluorescence and two-photon emission characteristics (Angew. Chem. Int. Ed., 2015, 54, 5360-5363). In 2015, they compounded polyvinyl alcohol (PVA) and used the hydrogen bonding interaction between PVA molecules and carbon dots to suppress the rotation and vibration of the carbon spot radiation center and the non-radioactive transition of triplet excitons to stably excite the triplet state. A long-life phosphorescence emission of the carbon dots at room temperature is achieved. Combining the fluorescence and two-photon luminescence properties of the carbon dots themselves, the triple emission characteristics of carbon dots (up-down conversion fluorescence and phosphorescence) and their potential application as triple anti-counterfeiting inks have been reported for the first time (see Figure 1). Relevant work was published in the journal VIP and cover papers in the German Journal of Applied Chemistry (Angew. Chem. Int. Ed. 2016, 55, 7231-7235).
Although the above work achieved a long-life phosphorescence emission of carbon dots at room temperature, it can be observed only in a dry solid state. This is because hydrogen bonds are easily damaged by the influence of water molecules, and therefore long-life emission of carbon dots in the dispersion liquid cannot be obtained. In order to expand the application range of long-lived emission carbon points at room temperature, they proposed innovatively in 2016 the use of covalent bonds (instead of the usual hydrogen bonds) to stabilize the excited state. The idea is to fix carbon dots in nanosilica that is easily dispersed in water. (nSiO2) surface. Since the covalent bond has a stronger interaction with the hydrogen bond, the binding of the carbon point to nSiO2 is less likely to be destroyed, and the long-life luminescence of the carbon point under the condition of the aqueous dispersion system is obtained for the first time (see FIG. 2 ). In addition, due to the stronger covalent interactions, the band gap (ΔEST) between the excited singlet and triplet states of carbon dots is reduced, further proving that the long-life emission exhibited by this material system is delayed. Fluorescence dominates, but contains a mixture of partially phosphorescent light. Finally, the long-life luminescence performance of the system is less affected by water vapor, and the combination of carbon dots and PVA compound hydrogen bonds are easily destroyed by water vapor, thus realizing water vapor-sensitive information multiple encryption applications. Related work was published in the journal Chemical Materials (Chem. Mater. 2017, 29, 4866?4873).
Although the previous work achieved the long-life emission of carbon dots in the solid and aqueous dispersion environment, it is essentially based on the compounding with other materials, which to a certain extent limits the scope and flexibility of their practical application. It is of great significance to develop carbon dots that have long-lived emission characteristics at room temperature. Since 2017, combining the research results of traditional room-temperature phosphorescent materials, they speculated that the prepared carbon dots would be expected to achieve long-life luminescent properties if the following conditions are met: 1 Carbon dots have an amorphous or polymer structure, and such structures may serve as substrates. The luminescent center contained in it is effectively isolated and fixed, and the non-radiation process is suppressed; the 2 carbon point is rich in oxygen (C=O and OH), nitrogen (C=N and NH2) or halogen (Br, I) functional groups, These groups can be used as potential luminescent centers on the one hand, and can also form effective hydrogen bonds or halogen bonds to further stabilize the excited triplet state; 3 carbon dots contain doping of elements such as B, N, P, or halogen, which can be Induced stronger spin-orbit coupling, enhanced inter-system crossing of the excited state, and thus promoted the formation of more triplet states.
Based on the above ideas, the research group used microwave irradiation heat treatment method of ethanolamine and phosphoric acid aqueous solution, and obtained carbon dots with long life (1.46 seconds, visible to the naked eye more than 10 seconds) room temperature phosphorescence emission. Further studies have shown that the amorphous structure of carbon dots, the presence of intramolecular hydrogen bonding groups, and the doping of N and P elements may be responsible for the long-lived room temperature phosphorescence of the carbon dots. This work achieved high efficiency (about 70% conversion), long-life (5 minutes microwave heating), and gram (2.8 g) preparation (see Figure 3). The results of the relevant studies were recently published in the German Journal of Applied Chemistry (Angew. Chem. Int. Ed. 2018, DOI: 10.1002/anie.201802441).
At the same time, in order to clarify the causes of long-lived room temperature phosphorescence in the above carbon point system, the conversion of the prepared carbon point material from fluorescence to long-life phosphorescence was achieved by stepwise heating methods (180° C. and 280° C.), and further research was conducted accordingly. The structural changes of this type of carbon dots during the preparation process and possible causes of long-lived room temperature phosphorescence. According to the characterization results of the products obtained by the two-step heating, it is presumed that at a relatively low heating temperature (180° C.), the raw material molecules (ethylenediamine/ethanolamine and phosphoric acid) mainly undergo chemical reactions such as dehydration condensation and cross-link polymerization, and pass through. The cross-linked enhanced fluorescence principle produces a fluorescent carbon dot with a polymer structure (without long-life phosphorescence emission); this fluorescent carbon dot undergoes further dehydration and carbonization at a higher temperature (280 °C). The chemical reaction of Lianhe produces carbon dots with phosphorescence emission characteristics. They speculate that the phosphorescence emission is mainly due to the formation of a denser structure after high-temperature treatment (280°C), favoring the formation of hydrogen bonds within the particles, and further stabilizing the excitation by inhibiting the free rotation and radiative transition of the luminophores contained therein. Triplet, resulting in efficient phosphorescence emission. In addition, comparative experiments show that N, P doping plays a key role in the long-life emission of this type of carbon dot system.
This work not only further explained the generation process of this type of carbon dot system and possible sources of long-life phosphorescence emission, but also obtained for the first time a material with a heat stimulus response to generate phosphorescence. Using this specificity feature (the fluorescent material is converted into a long-lived room-temperature phosphorescent material by heating), its potential applications in the field of advanced anti-counterfeiting and information protection are discussed (see Figure 4). The relevant research results were recently published in the magazine "Adv. Mater., 2018, 1800783".
The above work has been supported by the National Natural Science Foundation of China, the Natural Science Foundation of Zhejiang Province, the Ningbo City Foundation, the Wang Kuancheng Education Fund, and the Chongqing Postgraduate Innovation Project.
Fig. 1 (Left) Preparation of Carbon Dots and Up-Down Conversion Fluorescence and Phosphorescence Triple Emissions of PVA Composite Films; (Right) Potential Application of Triple Anti-counterfeiting Inks
Fig. 2 Preparation of carbon dots and their covalent bonding with nSiO2 to achieve long-life luminescence that can be dispersed in water (the luminescent properties of RhB aqueous solution as a control)
Fig. 3 High-performance, gram-preparation and long-life luminescent properties of ultra-long-lived room temperature phosphorescent carbon dots
Figure 4 Carbon dots with thermal response to phosphorescence and their potential applications in the field of information protection and anti-counterfeiting (decryption of encrypted information by authenticating hair dryers (approximately 300 °C, 30 seconds) and genuine/counterfeit goods, etc. Identification)
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