ExBox: A Polycyclic Aromatic Hydrocarbon Scavenger

A template-directed protocol, which capitalizes on donor–acceptor
interactions, is employed to synthesize a semi-rigid cyclophane (ExBox4+) that adopts a box-like geometry and is
comprised of π-electron-poor 1,4-phenylene-bridged (“extended”)
bipyridinium units (ExBIPY2+). ExBox4+ functions as a high-affinity scavenger of an array
of different polycyclic aromatic hydrocarbons (PAHs), ranging from
two to seven fused rings, as a result of its large, accommodating
cavity (approximately 3.5 Å in width and 11.2 Å in length
when considering the van der Waals radii) and its ability to form
strong non-covalent bonding interactions with π-electron-rich
PAHs in either organic or aqueous media. In all, 11 PAH guests were
observed to form inclusion complexes with ExBox4+, with coronene being the largest included guest. Single-crystal
X-ray diffraction data for the 11 inclusion complexes ExBox4+⊂PAH as well as UV/vis spectroscopic data for
10 of the complexes provide evidence of the promiscuity of ExBox4+ for the various PAHs. Nuclear magnetic resonance spectroscopy
and isothermal titration calorimetric analyses of 10 of the inclusion
complexes are employed to further characterize the host–guest
interactions in solution and determine the degree with which ExBox4+ binds each PAH compound. As a proof-of-concept,
a batch of crude oil from Saudi Arabia was subjected to extraction
with the water-soluble form of the PAH receptor, ExBox·4Cl, resulting in the isolation of different aromatic compounds
after ExBox·4Cl was regenerated.