The Role of a [2.2]Paracyclophane Moiety in Piezofluorochromism of Crystalline Organoboron Complexes

Fluorescence (FL) properties of crystals of [2.2]paracyclophane-containing organoboron complexes (pCP-H and pCP-iPr) were investigated under high pressure using a diamond anvil cell to evaluate the effects of intramolecular interactions in the [2.2]paracyclophane moiety on piezofluorochromism (PFC). Crystals of both pCP-H and pCP-iPr were found to display remarkable PFC with redshifts of more than 100 nm under high pressures up to ca. 8 GPa. However, the pressure-sensitivities of FL of pCP-H and pCP-iPr crystals differed. The results of X-ray crystallography studies under ambient and high pressure revealed that PFC of the pCP-H crystal mainly originates from intermolecular interactions taking place in a pi-stacked dimer. Density functional theory calculations also showed that intermolecular orbital interactions in the pi-stacked dimer play an important role in the PFC of pCP-H. In contrast, it was found that pCP-iPr does not form a pi-stacked dimer in the crystal state. Therefore, PFC of the pCP-iPr crystal is mainly controlled by intramolecular interactions in the [2.2]paracyclophane moiety, making it less sensitive than pCP-H to pressure changes.