Revealing the optical properties of polycyclic aromatic hydrocarbon clusters with surface formyl groups

Abstract

Soot particles from flames were found to have quantum confinement behavior in recent UV–visible absorption measurements. However, the impact of surface functionalization was not considered in the previous interpretation. In this work, polycyclic aromatic hydrocarbon (PAH) clusters was selected as a model system of soot to explore the impact of surface functionalization on the optical properties. The HOMO–LUMO gap of PAH clusters with formyl group (–CHO) substitutions was computed using the B3LYP method with the 6-31G(d) basis set. The results showed that the HOMO–LUMO gaps of PAH clusters strongly depend on the surface coverage of –CHO groups on particle, and a higher coverage always yield a reduced gap value. Combining the observation from X-ray photoelectron spectroscopy measurements, the impact of functional groups was computed as 0.27–0.71 eV. Detailed analysis on the electronic structures of HOMO and LUMO reveals that the –CHO groups lower the LUMO energy to a great extent, while they have limited impact on the HOMO energy. Furthermore, we use the LUMO composition as a key descriptor to evaluate the impact of functionalization on LUMO energy, and a theoretical limit of gap reduction was calculated as 1.1 eV for both pyrene and coronene clusters.