One-step solvothermal synthesis of Na-doped carbon quantum dots and concomitant photochromic small molecules

Hydrothermal method is a common way to synthesize carbon quantum dots (CQDs), but perplexing by-products are non-negligible shortcoming. The small molecules associated with CQDs synthesis have drawn more attentions because of their luminescent property. In this work, Na-doped CQDs with excellent photoluminescent property as well as small molecules 4,4′-diaminoazobenzene with photochromism characteristic were prepared based on p-phenylenediamine and sodium hydroxide via one-step solvothermal method. The photoluminescence of as-synthesized Na-doped CQDs is tremendously influenced by H+ ions, and CQDs-O- Na+ structure existing in Na-doped CQDs was verified. At the same time, photochromism which was owing to by-product 4,4′-diaminoazobenzene with photochromic was observed and investigated. The absorption spectrum of 4,4′-diaminoazobenzene overlaps markedly with the photoluminescence spectrum of Na-doped CQDs and thus result in fluorescence internal filtering effect, by which the fluorescence causes interesting photochromism phenomenon. Furthermore, compared with conventional synthetic route of 4,4′-diaminoazobenzene, this method has the advantages of simplicity and metal-free.