From Intermediate Epoxy Group to Stable Ether Bridge: Insights from DFT Study on Graphene Quantum Dots

This study investigates the mechanism of ether bridge formation on the edges of graphene quantum dots (GQDs) and evaluates its impact on their structural, electronic, and optical properties. Using density functional theory (DFT) coupled with Clar's aromatic sextet rule, we analyzed different edge functionalization sites on a model nanographene. The kinetic parameters evaluated via the Eyring-Polanyi equation demonstrate that the stability of functional groups is fundamentally governed by the retention or migration of aromatic sextets. While epoxidation at thermodynamically favorable edge sites that form stable epoxy intermediates exhibits high kinetic stability with substantial activation barriers, alternative configurations directly relax during geometry optimization to minimize aromaticity disruption. Moreover, highly metastable epoxy intermediates convert to ether bridges via nearly barrierless pathways at ambient temperature. Simplified time-dependent DFT (sTD-DFT) calculations show that oxygen functionalization narrows the energy gap, yielding a distinct bathochromic shift into the visible range. Ultimately, Clar's rule is established as a predictive tool for ether bridge formation, enabling the rational design of GQDs with tailored stability and optical properties for bioimaging and optoelectronic applications.