The room-temperature long-lived luminescent materials are widely used in new-generation optoelectronic devices, optical anti-counterfeiting, chemical/biosensing, and time-resolved imaging because of their unique luminescence processes. However, room-temperature long-life luminescent materials have been developed in the past decades. (Predominantly organic small molecules, transition metal complexes and rare earth-based long afterglow materials) have the disadvantages of complicated preparation and purification processes, need of expensive raw materials, potential biological toxicity or severe long-life generation conditions, etc. Therefore, development is simple and easy to prepare. The cost-effectiveness, low toxicity, and long-life emission material under conventional environmental conditions are the most pressing problems in this field.
Carbon-based nanoluminescent materials (carbon dots) are a new type of luminescent materials that have been developed in recent years. Due to their simple preparation and purification processes, stable photophysical and chemical properties, adjustable emission characteristics, and ease of functional modification, water solubility and biocompatibility Good advantages, such as being well received by researchers since its discovery in 2004, show great application prospects in many fields such as chemical/biosensing, bioimaging, medical diagnosis, photocatalysis and optoelectronic devices. However, scientific research In recent years, people have focused mainly on the regulation and preparation of fluorescence properties of such materials, the luminescence mechanism and potential applications, 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 existing in the research field of long-lived luminescence materials at room temperature, since 2015, the long-life of the carbon atom at room 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 regulation and application carried out a series of work.
Previous studies of the research group showed that carbon dots 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 adopted Polyvinyl alcohol (PVA) compounding, using hydrogen bonding interactions between PVA molecules and carbon dots, suppresses the rotation of carbon spot radiation centers, vibrations, and non-radiative transitions of triplet excitons to stably excite triplet states, achieving a carbon point at room temperature Long-life phosphorescence emission. Combined with the fluorescence and two-photon luminescence properties of carbon dots themselves, the triplet emission characteristics of carbon dots (up-down, down-conversion fluorescence and phosphorescence) and their potential application as triple anti-counterfeiting inks have been reported for the first time (see Figure 1). Related 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 only be observed in a dry solid state. This is because the hydrogen bonds are easily damaged by the influence of water molecules, and thus carbon dots cannot be obtained in the dispersion. In order to expand the long-life emission carbon emission range of the room temperature, they innovatively proposed in 2016 that the use of covalent bonds (instead of the usual hydrogen bonds) to stabilize the excited state of the idea of the carbon point in the easily dispersed in Nanosilica (nSiO2) surface in water. Since the covalent bond has a stronger interaction with the hydrogen bond, the bond between the carbon point and nSiO2 is less likely to be destroyed, and the long-life luminescence of the carbon dot in the aqueous dispersion system is obtained for the first time. (Figure 2) In addition, due to the stronger covalent bond interaction, the band gap (ΔEST) between the excited singlet and triplet states of the carbon dots is reduced, further demonstrating that the material system exhibits The long-life emission is based on delayed fluorescence, but contains a mixture of partial phosphorescence. Finally, the long-life luminescence properties of the system are less affected by water vapor, combined with carbon dots and PVA composite hydrogen bonds. Properties damaged water vapor, to achieve a moisture sensitive information multiple encryption applications. Related work published in the journal "chemical materials" (Chem. Mater. 2017, 29, 4866? 4873).
Although the previous work achieved the long-life emission of carbon dots under the environment of solid and water dispersions, it is essentially based on compounding with other materials, which to a certain extent limits the scope and flexibility of their practical application. Developing carbon dots with long-lived emission characteristics at room temperature is of great significance. Since 2017, combined with the research results of traditional room-temperature phosphorescent materials, they speculated that if the prepared carbon spot meets the following conditions, long-life luminescent properties are expected to be achieved: 1 The carbon dots have an amorphous or polymer structure. Such a structure may be used as a matrix to effectively isolate the luminescent center contained therein, fix it, and inhibit the non-radiative process; 2 Carbon dots are 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 triplet states; The doping of elements such as B, N, P or halogen can induce stronger spin-orbit coupling and enhance the intersystem-to-system crossing of the excited state, thereby promoting more triple The production of states.
Based on the above ideas, the research group used microwave irradiation to heat the ethanolamine and phosphoric acid aqueous solution, and obtained carbon dots with long life (1.46 seconds, more than 10 seconds visible to the naked eye) room temperature phosphorescence emission. Further studies showed that the carbon dots were not available. The shape structure, the presence of intramolecular hydrogen bonding groups and N, P element doping may be the cause of the long-lived room temperature phosphorescence of this carbon point. This work achieves the long-life luminescence performance of carbon dots (conversion rate is about 70%), Convenient (5 minutes microwave heating), Grams (2.8g) Preparation (Figure 3). Related research results 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 spot material from fluorescence to long-life phosphorescence was achieved by a stepwise heating method (180° C. and 280° C.), and was further studied accordingly. The structural changes in the preparation of this type of carbon dots and the possible causes of long-lived room-temperature phosphorescence. According to the characterization results of the products obtained by 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-linking polymerization, and a fluorescent carbon dot having a polymer structure (without long-life phosphorescence emission) is produced by cross-linked enhanced fluorescence principle; Under high temperature treatment (280 °C), the dehydration carbonization of the polymer structures involved, cross-linking and other chemical reactions generate carbon dots with phosphorescence emission characteristics. They speculate that the phosphorescence emission is mainly due to the high temperature treatment (280 °C The resulting denser structure favors the formation of intramolecular hydrogen bonds, which in turn inhibits free rotation and non-radiative transitions of the luminophores they contain. Stable triplet excited state, resulting in efficient phosphorescent emission. In addition, comparative experiments show that N, P doped emitter plays a key role in the long-life type of carbon-point system.
This work not only further explains the generation process of this type of carbon dot system and possible sources of long-lived phosphorescence emission, but also has for the first time obtained a material with a heat-stimulus response that produces phosphorescence. Use of this specificity feature (fluorescent material The conversion of heating to long-lived room-temperature phosphorescent materials explores potential applications in the field of advanced anti-counterfeiting and information protection (Figure 4). The results of this study were recently published in the magazine "Advanced Materials" (Adv. Mater., 2018). 1800783)
The above work was 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 Graduate Innovation Program.
Figure 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
Figure 2 Preparation of carbon dots and their covalent bonding with nSiO2 to achieve long-life luminescence that can be dispersed in water (RhB aqueous solution Luminescent properties as a control)
Fig. 3 High-efficient, 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 information protection and anti-counterfeiting
