Facile synthesis of alkylated carbon dots with blue emission in halogenated benzene solvents

The synthesis of carbon dots (CDs) has attracted much attention in recent years. In this article, alkylated, blue-emitting CDs are prepared from dodecylamine through pyrolysis in differently substituted benzene solvents. The influences of the solvent on the size, yield and fluorescence quantum yield of the CDs have been investigated in detail. Overall, eight solvents were probed. It was found that the presence of halogen atom(s) in the solvent structure is beneficial for the production of the CDs, as seen from the poor yields of the CDs obtained in methylated benzenes. For the three mono-substituted halogenated benzenes with Cl, Br, I, as the atomic number of the halogen atom increases, hence increasing the resulting polarizability of the medium, the quantity of CDs obtained after pyrolysis also increases. For the two dichlorobenzenes probed, only the ortho-substitution is advantageous for the production of the CDs. The major quantity of CDs was obtained in iodobenzene (∼26 %) while the highest fluorescence quantum yield was observed for CDs prepared in bromobenzene (∼24 %), which is almost 4 times and 3 times that of the CDs developed in the old strategy using chlorobenzene as a solvent. Not only the change of the solvent tunes the photophysical properties of the CDs, it also alters their chemical structure. While the mono-substituted solvent is conducive to CDs with enriched C = O, the di-substituted one facilitates instead the formation of C-O. The possible interaction between the alkylamine and the solvent during pyrolysis was probed by adding lewis base and lewis acid to the system. Evidence of the occurrence of a process similar to Friedel-Crafts alkylation has been found. Our research illustrates new synthetic pathways and mechanism analysis for the preparation of CDs.