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

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.