Remarkable difference in fluorescence lifetimes of the crystalline states of dibenzoylmethanatoboron difluoride and its diisopropyl derivative

The fluorescence (FL) and structural properties of dibenzoylmethanatoboron difluoride (1aBF2) and its diisopropyl derivative (1bBF2) in the crystalline state were investigated. Both 1aBF2 and 1bBF2 in their crystalline states have FL spectra that contain similar emission in the ca. 450–600 nm region, which correspond to emission of green light. However, the FL lifetimes (τFL) of these substances in their crystalline states are markedly different. Based on the results of wave deconvolution and decay profile analyses, we tentatively supposed that the FL band of 1aBF2 with average τFL = 47.7 ns is mainly comprised of FL band from an excited multimer, an emitting domain consisting of three or more molecules. In contrast, an excimer with average τFL = 5.3 ns is likely the emitting species in the crystalline state of 1bBF2. The results of X-ray crystallographic analyses support the conclusion that these contrasting properties derive from a difference in molecular overlap, especially between the benzene (B) and dioxaborinine (D) rings in the crystalline states of 1aBF2 and 1bBF2. Specifically, molecules in crystals of 1aBF2 are aligned having B-on-B and B-on-D overlap with respective short face-to-face distances (3.50 and 3.42 Å) while those in 1bBF2 are aligned in a single B-on-B overlap mode with a longer face-to-face distance (3.82 Å).