Photoluminescence mechanism of red emissive carbon dots from a diaminobenzoic acid isomer

Red emissive carbon dots (RCDs), as nanoscale luminescent particles, hold great promise for bioimaging and full-color displays. However, the luminescence principle of RCDs remains rather elusive due to the poor understanding of the structure-function relationship. To address this, green to red emissive CDs were synthesized from o-phenylenediamine (OPD) as well as isomers of 2,3-diaminobenzoic acid (23DBA) and 3,4-diaminobenzoic acid (34DBA), respectively, by a simple one-step solvothermal method. The similar conjugated structure and pure composition help to reveal the luminescence mechanism of CDs. Experimental and computational studies confirmed that molecular states should be responsible for the fluorescence of CDs. The emission color variations originated from the different molecular fluorophores. Quinoxalino[2,3-b]phenolizine-2,3-diamine (QXPDA) was dominant for the red emission of OPD-based RCDs. Similarly, for DBA-based RCDs, carboxyl groups can provide an acidic environment and act as a catalyst, and new molecular fluorophores like QXPDAA-1 and QXPDAA-2 with different carboxyl group positions were demonstrated to be formed. Meanwhile, the different positions (para- and ortho-position) of -COOH on QXPDA affected the intramolecular charge transfer (ICT), thereby lowering the photon transition band gap, and further promoting red emission. Finally, transient absorption (TA) spectra confirm that the multiple-peak emission of the DBA-based RCDs originated from the single luminescence center. This study is a potential reference for the controlled synthesis of RCDs from other aromatic precursors. A carboxylic acid moiety realizes the transition of carbon dots from yellow to red emission.