Clar model modified for nanographenes

Size Isn't Everything: Geometric Tuning in Polycyclic Aromatic Hydrocarbons and Its Implications for Carbon Nanodots

Recent developments in light-emitting carbon nanodots and molecular organic semiconductors have seen renewed interest in the properties of polycyclic aromatic hydrocarbons (PAHs) as a family. The networks of delocalized π electrons in sp2-hybridized carbon grant PAHs light-emissive properties right across the visible spectrum. However, the mechanistic understanding of their emission energy has been limited due to the ground state-focused methods of determination. This computational chemistry work, therefore, seeks to validate existing rules and elucidate new features and characteristics of PAHs that influence their emissions. Predictions based on (time-dependent) density functional theory account for the full 3-dimensional electronic structure of ground and excited states and reveal that twisting and near-degeneracies strongly influence emission spectra and may therefore be used to tune the color of PAHs and, hence, carbon nanodots. We particularly note that the influence of twisting goes beyond torsional destabilization of the ground-state and geometric relaxation of the excited state, with a third contribution associated with the electric transition dipole. Symmetries and peri-condensation may also have an effect, but this could not be statistically confirmed. In pursuing this goal, we demonstrate that with minimal changes to molecular size, the entire visible spectrum may be spanned by geometric modification alone; we have also provided a first estimate of emission energy for 35 molecules currently lacking published emission spectra as well as clear guidelines for when more sophisticated computational techniques are required to predict the properties of PAHs accurately.

Origins of the peroxidase mimicking activities of graphene oxide from first principles

Graphene-based nanomaterials, including graphene oxide (GO) and reduced graphene oxide (rGO), play key roles in the nanozyme field. GO and rGO carrying various oxygen-containing functional groups, including epoxy, hydroxyl, ether, endoperoxide, carbonyl, carboxyl, and ester, have been reported to display peroxidase mimicking activities. However, the active center and the underlying mechanism responsible for its peroxidase mimicking activities still remain unclear. Herein, taking the oxidation of 3,3,5,5-tetramethylbenzidine (TMB) by H2O2 as the model reaction, we investigate the possible catalytic mechanisms using DFT calculations. The results indicate that the carbonyl groups are the active centers. The activation of the C[double bond, length as m-dash]O bond is the key step in the catalytic cycle. The results will help realize the rational design of carbon-based nanozymes.