Photochemical Synthesis and Spectroscopy of Polycyclic Aromatic Hydrocarbon Dimers

Laser desorption time-of-flight mass spectrometry (LD-ToF-MS) experiments on pressed-pellet samples of polycyclic aromatic hydrocarbons (PAHs) produce covalently-bonded dimers at masses (m/z) of 2M-2 and 2M-4 (where M is the parent mass). Through replication of these LD-ToF-MS conditions at higher throughput, PAH dimers have been produced and collected in milligram quantities. For collected samples of pyrene, perylene, and coronene, differential sublimation has isolated the dimer sample from residual monomers. After confirmation of the purification using LD-ToF-MS, the samples are analyzed using a variety of spectroscopic methods, including UV-Vis, IR, and Raman spectroscopy. Computational studies on dimer samples have been done using density functional theory with the CAM-B3LYP method at the def2TZV level of theory. These calculations were calibrated and checked by comparison with the monomer samples and used to determine the lowest energy dimer structures. The comparison of simulated and experimental spectra allows determination of the most prevalent structures for the PAH dimers.