Systematic Investigation of π-π Interactions in Near-Edge X-ray Fine Structure (NEXAFS) Spectroscopy of Paracyclophanes.

NEXAFS spectroscopy has potential for study of packing and order in organic materials but only if intermolecular effects are understood. This work studies how π-π interactions between adjacent unsaturated groups affect their NEXAFS spectra, with a broader goal of building a general understanding of the role of intermolecular effects in NEXAFS spectroscopy. These effects are examined using paracyclophane (PCP) molecules, in which the benzene-benzene separation distance is controlled by varying the length of the alkyl groups separating the benzene rings. NEXAFS spectroscopy and density functional theory (DFT) simulations are used to examine spectroscopic changes related to the strength of these π-π interactions. A characteristic red shift is observed as adjacent benzene rings get closer together. This shift is attributed to Coulombic and orbital interactions between the adjacent benzene rings, mediated through overlapping π/π* orbitals.