Stacking structure in liquid polyaromatic hydrocarbons.

We present a neutron total scattering study of the polyaromatic hydrocarbons phenanthrene and pyrene, analysed by simulation based data refinement. We compare the liquid structure in terms of the degree and relative position of π-stacked pairs, to measurements of benzene and naphthalene. We find that pyrene has the highest level of parallel stacking, with 46% of molecules in stacked pairs. We observe a trend of decreasing displacement in the stacking with increasing aromatic core size, and find that for pyrene no offset is observed. The results have important implications for the fundamental understanding and modelling of polyaromatic hydrocarbon structure, and for applied fields including optoelectronics, astrochemistry and crude oil phase behaviour.