Singlet-Triplet and Triplet-Triplet Transitions of Asphaltene PAHs by Molecular Orbital Calculations

摘要

Two important features of the molecular structure of asphaltenes remain unresolved; the size distribution of the asphaltene polycyclic aromatic hydrocarbons (PAHs) and the number of PAHs per asphaltene molecule. The relatively small molecular weight of asphaltenes restricts the PAH size; if there are several PAHs per asphaltene molecule, they must be rather small. Optical spectroscopy especially when coupled with molecular orbital (MO) calculations is an excellent probe of asphaltene PAH populations and thus asphaltene molecular architecture. Previously, singlet-singlet transitions for asphaltenes were analyzed using both experiment and MO theory. Here, we describe MO calculations performed to treat triplet-triplet transitions from the ground triplet state for 103 PAHs. Qualitative comparisons with corresponding triplet-triplet transition measurements for asphaltenes are discussed. In addition, spin-forbidden transitions between the singlet and the triplet states, corresponding to phosphorescence, are calculated and discussed in terms of the probability of intersystem crossing of PAHs in asphaltenes and crude oils. Conclusions obtained here are consistent with the corresponding study of singlet-singlet transitions and support the model of a single, relatively large PAH per asphaltene molecule as the predominant asphaltene molecular architecture: the island model. This is consistent with a most probable asphaltene PAH of seven fused aromatic rings (7FAR) with a width of four to ten fused aromatic rings (4FAR-10FAR). This molecular architecture is a central feature of the Yen-Mullins model of asphaltene nanoscience.