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The crystal-packing structures of seven derivatives of diaroylmethanatoboron difluoride ( 1 a- gBF2) are characterized by no overlap of the π-conjugated main units of two adjacent molecules (type I), overlap of the benzene ring π-orbitals of two adjacent molecules (type II), and overlap of the benzene and dihydrodioxaborinine rings π-orbitals of adjacent molecules (type III). The crystal-packing structures govern the fluorescence (FL) properties in the crystalline states. The FL domain that is present in type I crystals, in which intermolecular orbital interactions are absent, leads to excited monomer-like FL properties. In the case of the type II crystals, the presence of intermolecular overlap of the benzene rings π-orbitals generates new FL domains, referred to as 'excited multimers', which possess allowed S0-S1 electronic transitions and, as a result, similar FL lifetimes at longer wavelengths than the FL of the type I crystals. Finally, intermolecular overlap of the benzene and dihydrodioxaborinine ring π-orbitals in the type III crystals leads to 'excited multimer' domains with forbidden S0-S1 electronic transitions and longer FL lifetimes at similar wavelengths as that in type I crystals. [ABSTRACT FROM AUTHOR]

Boron difluoride (BF 2 ) complexes of N -(5-phenyl-2-pyrazinyl)pivalamide and its derivatives having a substituent on the phenyl group were prepared and their spectroscopic properties were investigated. It was found that the BF 2 complexes show fluorescence in solution and the solid state. The origins of the fluorescence properties are discussed on the basis of the fluorescence yields and lifetimes, crystal structures and DFT calculations. The present work would provide a guide to design BF 2 complexes of amidopyrazines exhibiting solid state fluorescence. [ABSTRACT FROM AUTHOR]

[Display omitted] • A crystal of the [2.2]paracyclophane-containing organoboron complex shows PFC. • Inter- and intra-molecular π-stacking distances are reduced under high pressure. • The intermolecular π–π interaction is still the important factor leading to PFC. Piezofluorochromism (PFC) of crystals of a tert -butyl and [2.2]paracyclophane-containing diaroylmethanatoboron difluoride (p CP- t Bu), which is expected to possess both intermolecular and intra molecular π–π interactions, was investigated. p CP- t Bu 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 p CP- t Bu molecules in the crystal at atmospheric pressure exist in the form of an intermolecular π-stacking dimer through back-to-back π–π interactions. Moreover, the crystallographic data under high pressure showed that increasing isotropic pressure causes a reduction in both the intermolecular π-stacking distance of the dimer (D INTER) and the intra molecular π-stacking distance between two benzene rings in the [2.2]paracyclophane moiety (D INTRA). Density functional theory calculations, using the Cartesian coordinates obtained by X-ray analysis, suggest that the origin of PFC of p CP- t Bu crystals is the reduction of the LUMO energy of the π-stacking dimer and that the intermolecular π–π interaction is still the important factor leading to PFC of p CP- t Bu crystals. [ABSTRACT FROM AUTHOR]