New Fluorescence Domain Excited Multimer Formed upon Photoexcitation of Continuously Stacked Diaroylmethanatoboron Difluoride Molecules with Fused π-Orbitals in Crystals

The crystal-packing structures of seven derivatives of diaroylmethanatoboron difluoride (1a-gBF(2)) are characterized by no overlap of the p-conjugated main units of two adjacent molecules (type I), overlap of the benzene ring pi-orbitals of two adjacent molecules (type II), and overlap of the benzene and dihydrodioxaborinine rings p-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 pi-orbitals generates new FL domains, referred to as excited multimers, which possess allowed S-0-S-1 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 p-orbitals in the type III crystals leads to excited multimer domains with forbidden S-0-S-1 electronic transitions and longer FL lifetimes at similar wavelengths as that in type I crystals.