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 (1aBF(2)) and its diisopropyl derivative (1bBF(2)) in the crystalline state were investigated. Both 1aBF(2) and 1bBF(2) 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 (tau(FL)) 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 1aBF(2) with average tau(FL) = 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 tau(FL) = 5.3 ns is likely the emitting species in the crystalline state of 1bBF(2). 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 1aBF(2) and 1bBF(2). Specifically, molecules in crystals of 1aBF(2) are aligned having B-on-B and B-on-D overlap with respective short face-to-face distances (3.50 and 3.42 angstrom) while those in 1bBF(2) are aligned in a single B-on-B overlap mode with a longer face-to-face distance (3.82 angstrom). (c) 2013 Elsevier Ltd. All rights reserved.