HOMO–LUMO Gaps and Molecular Structures of Polycyclic Aromatic Hydrocarbons in Soot Formation

Abstract

A large number of PAH molecules is collected from recent literature. The HOMO-LUMO gap value of PAHs was computed at the level of B3LYP/6-311+G (d,p). The gap values lie in the range of 0.64–6.59 eV. It is found that the gap values of all PAH molecules exhibit a size dependency to some extent. However, the gap values may show a big variation even at the same size due to the complexity in the molecular structure. All collected PAHs are further classified into seven groups according to features in the structures, including the types of functional groups and the molecular planarity. The impact of functional groups, including –OH, –CHO, –COOH, =O, –O– and –CnHm on the bandgap is discussed in detail. The substitution of ketone group has the greatest reduction on the HOMO-LUMO gap of PAH molecules. Besides functional groups, we found that both local structure and the position of five-member rings make critical impacts on the bandgap via a detailed analysis of featured PAHs with unexpected low and high gap values. Among all these factors, the five-member rings forming nonplanar PAHs impact the gap most. Furthermore, we developed a machine learning model to predict the HOMO-LUMO gaps of PAHs, and the average absolute error is only 0.19 eV compared with the DFT calculations. The excellent performance of the machine learning model provides us an accurate and efficient way to explore the band information of PAHs in soot formation.

Light-Induced EPR Study of Polymorphic Acene-Stipulated Transition in P3DDT:PC61BM Composite

“…Besides, they should consider the influence of possible effects affecting the final result. For example, the value of band gap calculated for isolated benzene molecule by different methods can vary within 5.12–10.5 eV; ,,, however, its optical gap was measured to be around 3.6 eV, due to the effect of large exciton binding in small molecules equal, e.g., for this molecule 3.10 eV . Nevertheless, the data obtained allows us to hypothesize a close relationship of electronic properties of a composite with the band structure of an acene.…”

Backbone-controlled LUMO energy induces intramolecular C–H activation inortho-bis-9-borafluorene-substituted phenyl ando-carboranyl compounds leading to novel 9,10-diboraanthracene derivatives

“…For 3b, the higher energy of the comparable orbital (HOMO) can also be explained by the increase in PAH-HOMO energies with increasing number of aromatic carbon atoms in the system. [94][95][96][97] Both 3a and 3b exhibit yellow to orange, low energy emissions at 568 and 580 nm in hexane, respectively, showing a broad vibrational progression that is lost in solvents of higher polarity for 3a. For 3a, a bathochromic shi in the emission, depending on the solvent polarity, is observed from 568 nm in hexane to 596 nm in dichloromethane (Fig.…”

Optical Properties of Polycyclic Aromatic Hydrocarbon Clusters with Oxygenated Substitutions: A Theoretical Analysis

“…Soot particles have an adverse effect on the environment and are mainly emitted from industrial power generation and engine combustion. , Soot is composed of various polycyclic aromatic hydrocarbons (PAHs), − including planar/nonplanar PAHs, cross-linked species, and PAHs with oxygenated groups. Although the state-of-art experimental methods ,− are capable of shedding light on soot composition, PAH size distribution in soot is still under debate and has been for decades.…”

“…Compared with nonoxygenated aliphatic chain, these oxygenated functional groups have a much greater effect on the band structure and optical band gap of soot particles. In particular, the methyl groups only cause a negligible change as <0.1 eV in the bandgap of PAHs . Chen and Wang investigated the effect of functional groups on PAH monomers.…”

“…In particular, the methyl groups only cause a negligible change as <0.1 eV in the bandgap of PAHs. 7 Chen and Wang 22 investigated the effect of functional groups on PAH monomers. The −CHO group was found to reduce the gap value to a great extent.…”