Graphene is a π-conjugated sheet of sp2-hydridized carbon atoms with exceptional physical and chemical properties. However, the available preparation methods hamper the large-scale isolation of graphene and nanographenes in a controlled fashion. To overcome these challenges, molecular nanographenes, or graphene cutouts, as well as polycyclic aromatic hydrocarbons (PAHs) can be utilized as good models of the 2D p-surface. Molecular nanographenes with well-defined compositions and dimensions can be generated by available synthetic methods, allowing for reliable isolation of uniform materials for direct structure/properties correlation studies. To further understand the influence of charge on solid-state packing and intermolecular interactions, comprehensive redox studies of nanographenes are necessary. Specifically, through controlled oxidation of PAHs, a unique bonding interaction called pancake bonding can be induced and then investigated through structural and theoretical methods. Prior to this work, partial oxidation of PAHs was largely achieved through electrochemical oxidation, while chemical oxidation methods remained notably underdeveloped. To address these synthetic deficiencies, we chose two different oxidants, namely gallium(III) chloride (GaCl3, 0.4 eV) and triethyloxonium hexachloroantimonate (Et3O+SbCl6-, 0.87 eV), for the chemical oxidation of several small planar polyarenes. The chemical oxidation of pyrene (C16H10), triphenylene (C18H12), and chrysene (C18H12) with GaCl3 resulted in the isolation and full characterization of three new products comprised of partially oxidized PAH dimers or trimers with various charge distribution patterns. Each cationic product was crystallized with one of two forms of gallium(III) chloride anions, namely Ga2Cl7- and Ga3Cl10-. While each crystalline product exhibits different composition and solid-state packing, some structural features remain consistent across the series, such as the interplanar contacts below the van der Waal (vdW) radii of carbon (3.4 Å) within the dimeric or trimeric PAH units. Subsequent computational investigation allowed in-depth theoretical analysis of the charge distribution and bonding interactions in the partially oxidized products. In addition, the chemical oxidation of perylene (C20H12) and coronene (C24H12) with Et3O+SbCl6- allowed the isolation and characterization of partially oxidized perylene and coronene dimers crystallized with a SbCl6- anion. The crystalline products of perylene and coronene exhibit the same structural characteristics, most notably the π-stacks with alternating interplanar contacts above or below 3.4 Å, illustrating distinct dimeric units within 1D columnar arrangements. An in-depth theoretical investigation modeled the pancake bonding interaction in the perylene dimer, which further inspired a broad analysis of 24 crystallographically characterized perylene-based radical cations available in the crystal structure database. Subsequently, the first chemical oxidation of a mixture of two PAHs, namely coronene and perylene, with GaCl3 has been investigated for the construction of a novel hybrid π-stacked heteromolecular system. The new crystalline product exhibits asymmetric trimeric units of perylene-perylene-coronene, arranged in continuous 1D columns connected through short interplanar p-p contacts and charge-separated by the monomeric gallium(III) chloride anions, GaCl4-. Subsequent computational investigations revealed the positive charge is concentrated mainly on the perylene units leaving the coronene unit essentially neutral, which is in line with UV-Vis absorption and structural data analysis. In summary, we have successfully isolated several new partially oxidized PAH products through chemical oxidation with GaCl3 or Et3O+SbCl6-. All products are characterized through single crystal X-ray diffraction, UV-Vis and EPR spectroscopy, and conductivity measurements, when allowed by the products stability. All products share similar structural features, such as short interplanar contacts below 3.4 Å, an atom-over-atom overlap between neighboring PAHs, and a slight loss of planarity in the π-stacked systems. These crystallographic features are consistent with the possibility of pancake bonding interactions. As such, these products were used as candidates for in-depth investigation of charge distribution and bonding interactions through DFT calculations. Overall, these studies promote the further development of efficient and controlled chemical oxidation methods for the preparation of new PAH radical-based products, which hold promise for applications in electronic and optoelectronic devices.
Megan E. McCormack (Wed,) studied this question.