Electronic properties of polycyclic aromatic hydrocarbons and their derivatives

The HOMO-LUMO gaps of polycyclic aromatic hydrocarbons (PAHs) with and without functional group substitution were computed using the B3LYP methods with the 6-311++G(d,p) basis set. The functional groups considered are ethyl (-C₂H₅), phenyl (-C₆H₅), hydroxyl (-OH), vinyl (-C₂H₃), carboxyl (-COOH) and aldehyde (-CHO) group. The purpose was to explore the impact of substitution on the electronic structure of PAHs and provide rational explanations for the optical band gap values measured for nascent soot particles formed in premixed stagnation flames. Results suggest a high sensitivity of the HOMO-LUMO gap on the type and number of functional groups in a PAH molecule. It was found that single -OH, -COOH and -CHO groups on PAHs can reduce the HOMO-LUMO gap by roughly 0.2, 0.4 and 0.5 eV, respectively. Further functionalization would lead to a reduction in the HOMO-LUMO gap by as much as 1 eV. We also demonstrate that dimerization of substituted PAHs results in an additional drop of the HOMO-LUMO gap. It is concluded here that functional groups, such as -OH, -COOH and -CHO, cannot be neglected while determining the optical bandgap of nascent soot particles.