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 (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.