Remarkable Piezofluorochromism of an Organoboron Complex Containing [2.2]Paracyclophane

Piezofluorochromism (PFC) of crystals of a tert-butyl and [2.2]paracyclophane-containing diaroylmethanatoboron difluoride (pCP-tBu), which is expected to possess both intermolecular and intramolecular – interactions, was investigated. pCP-tBu crystals were found to exhibit remarkable PFC associated with an over 150-nm redshift of fluorescence (FL) wavelength occurring under increasing isotropic pressure applied by using a diamond anvil cell. X-ray crystallographic analysis showed that pCP-tBu molecules in the crystal at atmospheric pressure exist in the form of an intermolecular pi-stacking dimer through back-to-back pi–pi interactions. Moreover, the crystallographic data under high pressure showed that increasing isotropic pressure causes a reduction in both the intermolecular -stacking distance of the dimer (DINTER) and the intramolecular pi-stacking distance between two benzene rings in the [2.2]paracyclophane moiety (DINTRA). Density functional theory calculations, using the Cartesian coordinates obtained by X-ray analysis, suggest that the origin of PFC of pCP-tBu crystals is the reduction of the LUMO energy of the pi-stacking dimer and that the intermolecular pi–pi interaction is still the important factor leading to PFC of pCP-tBu crystals.