Non-Contact Atomic Force Microscopy (NC-AFM) Imaging of Model Compound Bispyrene Molecules

Understanding the exact chemical structure of tremendously diverse molecules in hydrocarbon mixtures presents a formidable challenge. State-of-the-art tools such as ultra-high resolution Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS) yield exact chemical formulae for molecules in complex mixtures but shed little light on the bond structure within molecules. Fortunately, the non-contact atomic force microscopy (NC-AFM) technique recently championed by IBM (Schuler et al., J. Am. Chem. Soc., 2015, 137, 9870-9876) demonstrates that exact molecular structure of single organic molecules can be determined for coal and petroleum asphaltenes.

This 2015 asphaltene study observed predominantly the ‘island’ structure motif. The scarcity of ‘archipelago’ motifs raised questions on what types of chemical bonds could survive sample preparation by flash heating and sublimating the hydrocarbons onto the imaging substrate. One hypothesis for the apparent absence of ‘archipelago’ motifs was the susceptibility of weak C-C bond linkages between the multi-core molecules to cracking during the heating/sublimating process. We tested this hypothesis with a series of a,w-substituted bispyrenyl alkane model compounds designed to mimic multi-core asphaltene structures. The successful imaging of these model compounds demonstrates that C-C bonds with bond strengths as low as 65 kcal/mol can survive sample preparation. Further experiments, such as a 50:50 mixture of these bispyrenyl molecules and model compound ‘island’ molecules, would increase our understanding of potential impediments to definitive observation of ‘archipelago’ motifs in petroleum asphaltenes.