Photochemical Synthesis and Spectroscopy of Covalent PAH Dimers

Abstract

Laser photochemistry of pressed-pellet samples of polycyclic aromatic hydrocarbons (PAHs) produces covalently bonded dimers and some higher polymers. This chemistry was discovered initially via laser desorption time-of-flight mass spectrometry experiments, which produced masses (m/z) of 2M-2 and 2M-4 (where M is the monomer parent mass). Dimers are believed to be formed from photochemical dehydrogenation and radical polymerization chemistry in the desorption plume. Replication of these ablation conditions at higher throughput allowed PAH dimers of pyrene, perylene, and coronene to be produced and collected in milligram quantities. Differential sublimation provided purification of the dimers and elimination of residual monomers. The purified dimers were investigated with UV–visible, IR, and Raman spectroscopy, complemented by computational studies using density functional theory at the CAM-B3LYP/def2-TZV level. Calculations and predicted spectra were calibrated by comparison with the corresponding monomers and used to determine the lowest energy dimer structures. Infrared and Raman spectroscopy provided few distinctive signatures, but UV–visible spectra detected new transitions for each dimer. The comparison of simulated and experimental spectra allows determination of the most prevalent structures for the PAH dimers. The work presented here provides interesting insights into the spectroscopy of extended aromatic systems and a new strategy for the photochemical synthesis of large PAH dimers.

Experimental and Theoretical Structure Elucidation of the [2 : 1] Complex Ion of Carbo[n]helicene with n=6, 7 and 8 and Ag+

“…[58] However, the aggregation of intact coronene molecules by LDI is generally believed to lead to noncovalent oligomers, while covalent aggregation is accompanied by the loss of hydrogen atoms. [79][80][81] Again, for the Cor 3 Ag + ion of the present study, a CorÀ Ag + À CorÀ Cor structure is proposed in which the third coronene molecule attaches only very weakly via π-π stacking to a Ag + -bound dimer. This structure is fully consistent with the observed twofold coronene loss with increasing collision energy.…”

“…Cross‐linking and fusion of unsaturated hydrocarbons is a common occurrence in laser desorption ionization (LDI) experiments [58] . However, the aggregation of intact coronene molecules by LDI is generally believed to lead to noncovalent oligomers, while covalent aggregation is accompanied by the loss of hydrogen atoms [79–81] …”

Synthesis of Clean Hydrogen Gas from Waste Plastic at Zero Net Cost

“…Polymerization of polyaromatic hydrocarbons has been previously demonstrated under high energy density conditions, such as in stars or under laser irradiation. [40,41] The detected polyaromatic hydrocarbons formed during FJH can be considered the seeds of eventual turbostratic graphene sheets, which grow through the aromatic coupling of other mobile carbon species.…”

Synthesis of clean hydrogen gas from waste plastic at zero net cost

“…Polymerization of polyaromatic hydrocarbons has been previously demonstrated under high energy density conditions, such as in stars or under laser irradiation. (40,41) The detected polyaromatic hydrocarbons formed during FJH can be considered the seeds of eventual turbostratic graphene sheets, which grow through aromatic coupling of other mobile carbon species.…”