HOMO-LUMO gaps of polycyclic aromatic hydrocarbon clusters with formyl groups

The HOMO-LUMO gap of polycyclic aromatic hydrocarbon (PAH) clusters with formyl group (-CHO) substitutions was computed using the B3LYP method with the 6-31 G(d) basis set. The purpose was to explore the impact of functional group substitution on the electronic structure of PAH clusters. Our results show that the HOMO-LUMO gaps of PAH clusters strongly depends on the number of -CHO groups on particle surface. The detailed electronic structures of HOMO and LUMO reveal that the -CHO groups lower the LUMO energy while they have limited impact on the HOMO energy. Overall effect causes the HOMO-LUMO gap decrease with the increase in the number of -CHO groups, and the maximum reduction could reach as large as 1.03 eV in all cases. It is further concluded that the impact of functional groups in PAHs cannot be neglected while interpreting the optical band gap measured for soot particles.