Exact roles of individual chemical forms of nitrogen in the photoluminescent properties of nitrogen-doped carbon dots

The chemical and structural complexity of doped carbon dots impedes the understanding of their photoluminescence and hence rational design of doped carbon dots with tailored properties. We present an ensemble approach for modeling the absorption and fluorescence spectra of nitrogen-doped carbon dots that reveals how the type and content of nitrogen, as well as chromophore variability, affect the optical properties. We combined time-dependent density functional theory calculations and Boltzmann averaging of distributions of doped carbon dots models containing nitrogen in the form of amine, pyridinic, and pyrrolic moieties. The method was shown to be robust, provided reliable spectra and has applications in the systematic exploration of doped carbon dot optical properties. Amine groups caused red-shifts in the absorption and fluorescence maxima of nitrogen-doped carbon dots, while pyridinic and pyrrolic doping had the opposite effect, shown by blue-shifts in the absorption and fluorescence maxima. (C) 2017 Elsevier Ltd. All rights reserved.