Background and Originality Content
Supramolecular materials are dynamic materials with its components connected by a variety of non-covalent interactions, such as hydrophobic interactions, hydrogen bonds, π-π stacking, metal ligand coordination, and host/guest complexation [1-2]. The material could be imparted with intriguing properties such as self-repairing and chirality via incorporation of supramolecular recognition motifs. 2-ureido-4[1H]-pyrimidinone (UPy) is a well-explored supramolecular building block that can form robust dimer through quadruple hydrogen bonds. Owing to the high binding constant and ease of synthesis, UPy has been widely used to construct supramolecular polymers with self-healing, shape memory properties as well as enhanced mechanical performances[3-4]. However, there are less examples to use UPy units for achieving room-temperature phosphorescence (RTP), which requires the well-organization of recognition motifs and emission centers.
Carbon dots (CDs) [5], a new type of carbon nanomaterial, have attracted extensive attentions due to its unique optical properties as well as inherent advantages of superior stability, good solubility and low toxicity[6-9]. CDs has widely used in sensing, in vitro/in vivo imaging, cancer therapy, catalysts and optoelectronic devices[10-17]. Despite the well documented fluorescent properties, CDs with RTP have also been reported for several groups[18]. Lin and co-workers reported the production of effective RTP by aggregation-induced of CDs in trimellitic acid[19]. Shan and co-workers achieved UV phosphorescence by restriction of CDs in NaCNO crystals[20]. Yang and co-workers synthesized a series of CDs based RTP materials via incorporation of polymers[21]. Qiu and co-workers synthesized full-color long lifetime RTP by pyrolysis of the citric acid and boric acid precursors with various mass ratios at different temperatures[22]. Despite the progress achieved, matrices are commonly required in these works to produce effective RTP for the CDs by stabilization of the triplet state [23-25]. However, the use of matrices not only complicates the preparation process,
but also reduces the emission stability and uniformity. Hence, development of matrix-free RTP CDs materials is highly desirable[26,27].
In this work, o -phenylenediamine (o -PD) and ethylenediamine tetraacetic acid (EDTA) were employed as precursors to synthesize EDTA-CDs with abundance of surface functional groups (e.g., -NH2, -OH). Then, UPy was covalent attached to the surface of EDTA-CDs via isocyanate reaction to obtain UPy-CDs. Thermogravimetric and Fourier Transform Infrared Spectrometer (FT-IR) characterizations demonstrated the successful decoration of UPy on the surface of UPy-CDs. The strong hydrogen bonds between UPy moieties causes the self-assembly of UPy-CDs, as revealed by the obvious aggregation from Transmission Electron Microscope (TEM) images. Owing to the rigidifying effect induced by interparticle restriction, UPy-CDs not only inherent the fluorescence of EDTA-CDs, but also exhibit RTP without the aid of matrix. The afterglow emission after remove of UV light source could be observed by naked eyes, with a phosphorescence lifetime of 33.4 ms. On this basis, the application potential of UPy-CDs in the fields of anti-counterfeiting and information encryption were further exploited.
Results and Discussion
Results
Firstly, EDTA-CDs was prepared via hydrothermal method with o -PD and EDTA as precursors[28]. During the synthesis of EDTA-CDs, the polymerization and carbonization of o -PD were suggested to form the carbon core, while various functional groups (such as hydroxyl and amino groups) derived from the residue of precursors were decorated on the surface. These surface groups could react with isocyanate functionalized UPy (UPy-NCO, molecule 1 )[29], enabling the attachment of UPy to the surface of EDTA-CDs to obtain UPy-CDs (Scheme. 1). The as-