Controlling the Oligomeric and Mesogenic Composition of Pyrene Pitches via 1-Bromopyrene

Pyrene oligomers are proposed as a model system for petroleum pitches because they can be synthesized in high purity and exhibit properties representative of petroleum pitches. In particular, dimer is isotropic, trimer is mesogenic, and tetramer and higher oligomers (denoted as X-mer) are nonmelting (but still contribute to mesophase formation). Furthermore, these oligomers can each be isolated as pure components via supercritical extraction (SCE), enabling their combination to create pitches of well-defined and controllable composition. However, our inability to synthesize significant quantities of the key mesogenic species, trimer, has limited our ability to create dimer–trimer–X-mer composition diagrams for exploring the effect of oligomeric composition on pitch properties, including char yield, softening point, and liquid crystallinity. By substituting 100% 1-bromopyrene for pyrene and reducing the reaction time from 120 to 30 min at the baseline temperature of 400 °C, the trimer wt % in the pitch was more than doubled from 7.8 to 17.2 wt %; however, the largest detectable oligomer increased from hexamer to decamer. Fortunately, a tripling of the trimer content to 25.2 wt % was achieved by reducing the reaction temperature for 1-bromopyrene to 250 °C at 120 min. Particularly intriguing was the fact that this surge in trimer content was created solely at the expense of monomer and dimer, as the X-mer content was essentially the same for the two pitches, 27.0 vs 28.8 wt %. An additional benefit of the lower temperature was the reduction in the number of X-mers to essentially two (the fewer the components, the better-defined our system), with tetramer and pentamer comprising 93% of the observed MALDI mass spectrometer intensities for X-mer.