The knowledge gap on how different types of nitrogen centers affect the optical properties of N-doped carbon dots (CDs) hinders the rational design and synthesis of these nanostructures. We present a systematic theoretical study of 1 nm small CD models containing nitrogen and oxygen functional groups designed to explore the effects of various nitrogen centers on the absorption characteristics of CDs. Graphitic nitrogen is shown to have an electron-doping effect that alters the systems’ electronic energy levels and causes pronounced red-shift of their absorption spectra. Other kinds of nitrogens including pyridinic, pyrrolic, and amino centers had no appreciable effects on the CDs’ absorption properties.

Carbon dots (CDs) belong to a class of materials considered technologically important for their tunable absorption and emission properties and a huge application potential in cell labeling, theranostics, and optoelectronic technologies including LED diodes. Although improvement of their properties relies on a fundamental understanding of the underlying photophysical processes, this is currently far from complete. Here, we analyze the absorption spectra of nontrivial multilayer graphitic oxygen-functionalized CD models. The results suggested that the experimentally observed broad bands around 300 nm originated from n → π* and π → π* charge transfer transitions, whereas the effects of structural/energetic disorder, water environment, deprotonation, and excitonic coupling only weakly contributed to the spectra when compared to their monolayer counterparts. Owing to their weak interlayer interactions and thermal accessibility of low-energy conformations, the graphitic CDs are prone to structural disorder and consequent spectral-line broadening.

A synthetic route to achieve high phosphorescence quantum yield in a purely organic material was achieved by doping a crystal containing heavy bromine atoms with a molecule that contains a triplet producing aromatic carbonyl group. The enhanced phosphorescence originated from intermolecular nonbonding interactions between the bromine and the carbonyl oxygen. In this study we employ a computational approach to design molecules containing both structural motifs, which exhibit enhanced phosphorescence through intramolecular nonbonding interactions between bromine and carbonyl groups.